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Marine Facilities | The Anchor Block cap is hollow to accommodate a splay chamber where the bridges cable strands are anchored to the cap.
The piles are battered at a 4:10 slope in the bridge longitudinal direction. The piles are approximately 65 m long, determined from borehole 34, and are driven through the overburden material down to bedrock. All of the piles are anchored into the bedrock with rock sockets with an embedment of 6.0 m. Out of the 64 total piles, there are 32 tension piles each anchored with 31-strand rock-anchors embedded 25 m into the bedrock. This foundation design is only preliminary and will change prior to the completion of a final design that is ready for construction.
The depth to rock at nearby boreholes B-33 and B-34 was determined as 70 m and 44 m, respectively, from the mudline. The bedrock is primarily overlain by poor quality material with only limited depths (~5 m) of more competent soil immediately above it. Further details are available in the Draft Geotechnical Recommendations for Trestle and Berthing Area of Pacific Northwest LNG (PNWLNG) Project by Fugro dated October 11, 2013. |
Marine Facilities | Construction Access
Providing adequate access to the marine foundations is a primary consideration. This is especially so, considering the marine foundations for the bridge are fairly sizable, and will require not only a significant amount of working area for construction, but also a fairly significant portion of the overall bridge construction schedule.
Several potential options for providing the required amount of temporary construction access have been considered including methods such as:
Self-elevating platforms (jack-up barges);
Temporary working platforms supported on temporary piles;
Temporary working platforms supported on the permanent piles;
Marine-based methods using floating equipment; and
Temporary/permanent Man-Made Islands.
A discussion of each option is provided below; challenges for each exist and their viability would need to be confirmed during the detailed design process. Once in place, each access option would need to be supplied, to varying degrees, with crew and materials by barges and vessels subject to tidal and other environmental restrictions such as wind, sea state and visibility. |
Marine Facilities | Jack-Up Barge
A jack-up barge comes equipped with legs or spuds which can be lowered to sit on the seabed and used to lift the hull of the barge up above the sea level providing a large fixed working platform unaffected by tides and swells as shown in Figure 2-1. Jack-up barges are fairly common in marine construction and can be located at each foundation location.
The barges can be equipped with large cranes sufficient to support the anticipated pile driving, cleanout, and concrete foundation installation anticipated for the bridge marine foundations. The challenge at this site is the significant depth of weak materials above the bedrock. The bearing capacity of the seabed may be insufficient to support the concentrated loads from the spuds when the barge is elevated, causing the spuds of the barge to sink deep into the mud. This will make extending and retracting the spuds very difficult if not impossible. Also many jack-up barges do not have legs of sufficient length or size to be utilized at this location. Although there are rigs with legs up to and in excess of 80 m in length, none are currently believed to be within local contractor fleets.
As given in Section 2.1.1.4 the overall schedule duration for construction of the SW Anchor Block foundation is 24 months. One or two Jack-up barges could be deployed to conduct construction operations on the foundation. With the exception of extreme sea states or weather conditions, it is anticipated that the jack-up barge(s) would stay in position for most of the foundation construction. The jack-up barges would need to be supplied by one or two supply barges carrying equipment or various construction materials. To resupply the work front and to accommodate the tidal windows, the supply barges would transit every day between an onshore staging area and the work site. For the 6 month period where concrete would be poured for the Anchor Block, at least three concrete supply barges would be required to work in rotation to make on average six deliveries per day in order to meet the quantities of concrete required. In practice the EPC contractor will attempt to maximize the concrete deliveries for any given work shift to provide a somewhat continuous concrete placement. The actual number of daily concrete supply trips may be as high as eight or more, depending on tidal restrictions. The EPC contractor may have to work double shifts during this 6 month period to accommodate tidal windows and meet the schedule. |
Marine Facilities | Temporary Working Platform Supported on Temporary Piles
With this method, the temporary pile supported working platform provides a fixed structure upon which a crane and equipment can be placed to install the bridge foundations. The platform would be supported on a series of additional temporary piles and would be required to extend around the perimeter of the foundations in order to provide adequate access for constructing the foundations. An example of a temporary work platform supported on temporary piles is shown in Figure 2-2.
The platform(s) would allow the construction work to continue independent of the tides. The cranes and equipment would remain on the platform for the duration of the foundation construction. Similar to the Jack-up Barge method the crews on the working platform would need to be supplied by one or two supply barges carrying equipment or various construction materials. To resupply the work front and to accommodate the tidal windows, the supply barges would transit every day between an onshore staging area and the work site. For the 6 month period where concrete would be poured for the Anchor Block, at least three concrete supply barges would be required to make up to six deliveries per day.
For this option the working space would likely be kept to the absolute minimum in order to limit the amount of construction required for the temporary platform. The restricted space would inhibit the ability to store material and equipment which may lead to more frequent barge supply trips.
Completely removing the temporary piles installed for the platform after foundation construction would be challenging and potentially unrealistic. Consideration for leaving them in place or potentially cutting them off at the mudline would need to be evaluated |
Marine Facilities | Marine-Based Floating Equipment
For the floating marine-based construction method, the cranes, equipment and materials are all brought to the work front on floating flat deck barges. The primary piece of equipment required for marine-based construction is the marine derrick, which is essentially a flat deck barge that has a duty-cycle crawler crane mounted on it. A typical marine derrick is shown in Figure 2-3.
It is expected that there will be two to four barges positioned at each foundation location during construction; one or two marine derricks and one or two flat deck scows for material storage and acting as a work platform to perform miscellaneous tasks. Marine derricks and scows operated by local marine contractors vary in size and can be as large as 60 m long and 20 m wide. To resupply the work front and to accommodate the tidal windows, the marine derricks and scows would transit every day between an onshore staging area and the work site. Alternatively one or more supply barges could also be used for transporting materials from the mainland to the work front depending on construction requirements.
For the 6 month period where concrete would be poured for the Anchor Block, at least three concrete supply barges would be required to make up to six deliveries per day to meet the quantities of concrete required. To maintain somewhat continuous concrete placement, the EPC contractor may have to work double shifts during this 6 month period to accommodate tidal windows and meet the schedule.
The shallow water depths adjacent to the bridges marine foundations necessitate any marine-based construction to be limited to tidal windows and weather delays. This would extend the construction period significantly as well as drive up construction costs, and therefore floating marine-based methods have a significant disadvantage compared to other options which are not tidal-dependent. This is especially so, when considering the sizable efforts required for constructing the bridge foundations. Additionally, the risk of equipment grounding and unintentionally disturbing sensitive habitat is of concern. Also if the marine derricks are spud barges, the use of spuds to anchor the barge into position will add to the habitat disturbance.
One advantage that marine-based-methods have over other methods is that additional crews and equipment can be readily deployed on multiple work fronts to mitigate delays. |
Marine Facilities | Temporary/Permanent Man-Made Islands
Due to the shallow water depths at the proposed bridge foundations, the construction of a man-made island at each foundation location was considered as a means to provide uninterrupted construction access. Following construction of the bridge, the islands could be removed, in case of temporary islands, or left in place permanently. The amount of material removed and the desired final elevation of the islands would be dependent on the intent of any offsetting measures such as the creation of additional inter-tidal habitat or the planting of additional eelgrass beds or riparian vegetation.
However, the islands would require placement and possibly reclamation, of a significant volume of material in the marine environment. The resulting footprint of the man-made islands on the marine floor would be considerable and at the SW Tower foundation would likely extend onto Flora Bank. Due to this large habitat disturbance footprint, the man-made island option was discounted and withdrawn from further consideration. |
Marine Facilities | Construction Safety Zone and Navigation Protection
Regardless of the method selected to gain access for constructing the bridge foundations, there should not be an impediment to marine traffic. The work faces will occur in relatively small areas in open waters away from or adjacent to approach channels and will be surrounded by a cofferdam. A safety zone would be enforced around the cofferdam, and work site in general which would prohibit vessels from approaching closer than 50 m. Work site restrictions would be promulgated through a variety of channels, such as Notices to Mariners, in accordance with the Marine Communication Plan. As a participant in the Port of Prince
Ruperts Construction Coordination Committee for de-confliction with other users, PNW LNGs
Marine Communications Plan will be used to alert mariners to project activities, hazards and safety measures such as enforced safety zones and marine route closures. |
Marine Facilities | Construction Process
Once equipment access and a working area at the foundation locations have been provided either through the use of a Jack-up barge, temporary work platforms, etc., the construction of each marine foundation is presumed to generally follow a similar process. This consists of cofferdam installation, pile installation, concrete work on the Anchor Block or pilecap, concrete pedestal/tower base construction, cofferdam removal, and placement of scour protection. Each activity is described in more detail below. |
Marine Facilities | Cofferdam installation is required around each of the marine foundations to allow dewatering and excavation to the depth of the concrete pile cap soffit to facilitate concrete works and construction. It is anticipated the cofferdams would be constructed of steel sheet piles driven to depth in the soft sediments and extending up beyond the high water level. Internal bracing within the cofferdam would be used to support the sheet piles against external pressures. If the size of the foundations, particularly at the SW Anchor Block location, and the poor soil at the site preclude the use of a single sheet pile cell, utilizing a ring of self-supporting sheet pile cells may be an option. After sheet pile installation, a steel template is installed inside the cofferdam to correctly position the piles. The bottom of the concrete foundations will be located to sit approximately at or slightly above the existing mudline to avoid the need for any overburden excavation from inside the cofferdam.
After the piles are installed, the steel template can be removed. Finally a concrete seal is constructed, via tremie pouring, at the bottom of the excavation to allow for dewatering.
After the cofferdam is dewatered the piles are cut off at the bottom of the Anchor Block. |
Marine Facilities | Pile installation commences after the cofferdam is excavated and the steel pile template has been installed.
The proposed construction method for pile installation involves the following steps:
? Set the pile into the sediments by gravity;
? Vibrate the pile open-ended though the sediment until the tip reaches bedrock;
? When the pile reaches bedrock, seat the pile into the bedrock with an impact hammer;
? Extract sediments inside pile with a grab hammer or airlift methods;
? Drill a hole in the bedrock with the drill inside the pile which is acting as a casing; and
? For a rock doweled pile, advance pile into under-reamed hole and then grout pile; or alternatively for a rock socket pile, do not advance pile but insert rebar cage and pour tremie concrete into bottom of hole.
Pile driving and drilling including clean out and socketing, for the proposed 1800 mm diameter pipe piles is similar, only on a larger scale, to the trestle foundations as outlined in Sections 3.4 and 3.5. Any overburden material removed from inside the piles will be stockpiled on a barge and disposed of on land as discussed in Section 3.4.
One difference between the trestle piles and the Anchor Block piles is that the Anchor Block piles are filled with concrete over their full length. Thus, instead of stopping the concrete after the rock socket is complete, the concrete pour will continue until the pile is filled to the top. Since the pile driving operations are within the confines of the cofferdam, the effects to the surrounding marine life will be minimal. The piles will be approximately 65 m long and will require welded field splices. Field splices can either be done on a barge with the pile in a horizontal position or in-situ, depending on the size of crane used.
In addition to the rock sockets, rock anchors need to be installed for the 32 tension piles.
The rock anchors consist of 31-strand tendons that are embedded 25 m into the bedrock and anchored within the Anchor Block. After drilling for the rock sockets, a smaller diameter drill bit is used to drill into the bedrock to the required depth. The strand anchors and an inner casing are lowered to the bottom of the drilled shaft. Concreting for the rock sockets is completed and the piles are filled with concrete. The rock anchors are completed by injecting grout from the bottom of the shaft upwards to a depth required to fully develop the anchor force. The strands are stressed from within the Anchor Block and the remaining length of the shaft is grouted to protect the anchors from corrosion. |
Marine Facilities | Concrete foundation construction would progress similar to onshore construction after completing pile installation. The connection between the piles and pile caps consists of a reinforced concrete plug inside each pile head with the reinforcement cage extending up into the mass concrete of the pilecap. Construction of the pilecaps and other concrete elements would consist of placing reinforcement, constructing formwork, and pouring concrete in a series of lifts separated by construction joints. Supply of the large volumes of concrete required for the foundations will need to be conducted via barge. As a result, concrete supply and hence concrete placement will be affected by tidal restrictions and may impose longer construction times. Considering the large volumes of concrete required for the bridge foundations, there are two possible methods for supplying concrete to the work face; one involves concrete produced by an onshore batch plant and delivered to the work face via concrete trucks on a barge, and the other involves using an actual floating batch plant located adjacent to the work site. For the first method, the barges will be loaded with a pump truck and several concrete trucks and will travel from the construction batch plant located on the mainland to the work front at approximately 1-hour intervals.
Figure 2-5 shows a typical barge loaded with a pump truck and several concrete trucks. This method is generally slower and is restricted by a delivery window of about 2-hours which represents the time the concrete is allowed to sit in the trucks. As mentioned previously, considering the volume of concrete required, it is estimated that at least six barge runs carrying at least four 10 m3 concrete trucks and one concrete pump truck would be required to meet the needed concrete volumes and meet the schedule.
The floating batch plant method on the other hand is more efficient at supplying concrete to the work face, however it is more susceptible to tidal and weather related down times.
The contractor will need to the weigh the pros and cons of either method or may choose to use a combination of both, depending on seasonal weather conditions
Prior to casting the top portion of the Anchor Block, the suspension cable anchorage assemblies and splay saddles are installed and secured into the Anchor Block. These are large assemblies of prefabricated structural steel that will be transported to the foundation on barges and installed with the barge crane. Finishing works on the Anchor Block will be completed after the main suspension cables have been installed and adjusted. |
Marine Facilities | Cofferdam removal can commence once the required foundation concreting works are above the high water level. Excavated material can be placed back within the cofferdam allowing for the removal of internal bracing layer by layer. The steel sheet piles can be removed with a vibratory hammer. Consideration for leaving the sheet piles in place below the top of the footing to help with scour prevention should be made. |
Marine Facilities | Placement of scour protection around the perimeter of the Anchor Block would be conducted soon after removal of the cofferdam. Due to the shallow waters around the Anchor Block, the scour protection would be placed via marine-based methods using tidal windows and a flat deck barge equipped with an excavator or front end loader and a stockpile of riprap. The excavator or front end loader would place the scour protection to the appropriate thickness. |
Marine Facilities | Construction of the SW Anchor Block will consume a considerable amount of the construction schedule and will be on the critical path. It is estimated to take approximately 24 months to construct the Anchor Block, with the following breakdown:
3 months to construct the cofferdam
10 months to install the piles and rock sockets
6 months to install the concrete Anchor Block mass concrete
3 months to install the rock anchors (work can overlap with constructing the mass concrete Anchor Block)
2 months to install the suspension cable anchorage assemblies and splay saddles |
Marine Facilities | 2.1.2.1 Overview
The preliminary foundation design for the South West (SW) tower consists of 4 rows of 7 piles each (total of 28). The piles in the outer periphery of the pile group are battered at a 1:10 slope. The piles are 1800 mm diameter steel pipes, filled with concrete with rock socketed ends. The rock bed at the SW tower location is approximately 100 m below Low Water Level.
The approximate mud line and rock bed top level have been taken from Bore Hole BH 33. The approximately 100 m long piles will be vibrated through the overburden material into the bedrock. A drilled rock socket of 6 m length will be provided. The pile is filled with concrete over its entire length and connected rigidly to the pile cap. The cap provided is 36.4 m × 20.2 m × 4 m deep. A 13 m high concrete tower base extends from the pile cap to the underside of the superstructure. The tower base consists of two 4 m x 8.5 m columns connected with a 1 m thick infill wall for transverse rigidity. This foundation design is only preliminary and will change prior to the completion of a final design that is ready for construction. |
Marine Facilities | Construction Access
Similar to the SW Anchor Block, there are several potential options for providing the required access to construct the tower foundation. The south edge of the footing cap however, is directly adjacent to Flora Bank and therefore there will be limited to no access on the south side of the foundation to avoid disturbance to Flora Bank. |
Marine Facilities | Construction Process
The SW Tower foundation will be constructed in the same manner as the SW Anchor Block, by installing a cofferdam around the perimeter of the foundation, installing piles and rock sockets and finally concreting the pile cap. Scour protection would be placed shortly after removal of the cofferdam once the concrete foundation is complete. The equipment used will be the same as that for the Anchor Block and the number of daily barge transits will be similar. The volume of concrete required for the tower foundation however, is significantly less than the Anchor Block, so concreting will take less time and require less barge traffic. For the 4 month period where concrete would be poured for the SW Tower foundation, at least two concrete supply barges would be required to work in rotation to make, on average, one delivery per day in order to meet the quantities of concrete required. In practice the EPC contractor will attempt to maximize the concrete deliveries for any given work shift to provide a somewhat continuous concrete placement. The actual number of daily concrete supply trips may be as high as eight or more, depending on tidal restrictions. Concrete placement for the SW Tower should be coordinated to not overlap with concrete placement for the SW Anchor Block. This will allow for the least amount of concrete supply equipment required for the bridge foundations.
Similar to the Anchor Block, the concrete caisson method is a possible alternative to the use of cofferdams. However for the same issues and concerns surrounding the use of concrete caissons for the Anchor Block foundation, use of this method for the tower pilecap was not given much consideration past the initial concept development.
The concrete tower base will be constructed on top of the pile cap within the cofferdam using the same concreting methods as the pile cap. The pile cap will be used to anchor a tower crane that will be used to construct the steel portion of the tower. As a result, the cofferdam should be left in place at least until the tower crane base is installed. |
Marine Facilities | Construction Schedule
Construction of the SW tower foundation will be on the critical path. It is estimated to take approximately 16 months to construct the foundation, with the following breakdown:
3 months to construct the cofferdam
9 months to install the piles and rock sockets
2 months to install the concrete pile cap
2 months to install the concrete tower base |
Marine Facilities | Overview
The 128 m long portion of the towers above the concrete base are composed of structural steel legs connected with cross-bracing. The towers are 125 m high above deck level. The thick walled steel boxes are 2.2 m wide and vary in length from 5.5 m at the top to 7.0 m at the bottom. Each segment is approximately 5.0 m in height. The connections between the leg segments as well as the connections of cross members to the main tower legs are bolted splice connections detailed for repetition and ease of on-site installation. Saddles that carry the main suspension cables are positioned on top of the towers. Typically of cast steel, the saddles can also be custom fabricated using built up plates. They will be equipped with rollers to allow the main cables to shift/adjust under construction and normal loads. |
Marine Facilities | Construction Access
Construction of the steel tower is done mainly from a tower crane and temporary work platforms supported off the permanent tower legs at required locations. The tower crane is supported on the permanent footing cap and is self-climbing (i.e. it increases in height as the tower height increases). The tower crane is braced to the permanent tower legs as its height increases. Figure 2-6 below shows a typical tower crane used for bridge tower construction |
Marine Facilities | Temporary work platforms are installed to the tower segments before they are erected to provide access to the bolted connections. Work platforms are installed at the top of the tower legs to facilitate installation of the suspension cable saddles. The work platforms at the top of the towers typically remain in place for future maintenance and inspection of the cable saddles. Temporary stair scaffolding is typically provided the full height of the tower to provide access to the temporary work platforms. |
Marine Facilities | Construction Process
Construction of the steel tower segments consists mainly of lifting prefabricated structural steel segments off the barge and onto previously installed segments using the tower crane.
There are two tower legs for the SW tower braced to one another by structural steel bracing.
Tower segments are spliced together with bolted connections. Bracing members are bolted to the tower leg segments as the tower erection progresses. Steel segments are typically trial assembled at the fabrication facility to ensure member fit-up and ease of installation on site.
During trial assembly, adjacent segments are assembled using the final splice plates and about half the number of bolts required in the final connection. Corrections are made so that the segment fit-up and finally geometry is met. Finally, the pieces are marked, disassembled and shipped to site. Temporary working platforms will be pre-installed to the tower segments on site. This work will likely take place on the mainland to avoid having to install the platforms on a barge or high in the air. |
Marine Facilities | Construction Schedule
The tower segments, with work platforms already installed, will be delivered using material supply barges with an anticipated delivery frequency of one barge per week. Construction of the SW Tower is estimated to take approximately 6 months. |
Marine Facilities | Construction Schedule
The tower segments, with work platforms already installed, will be delivered using material supply barges with an anticipated delivery frequency of one barge per week. Construction of the SW Tower is estimated to take approximately 6 months. |
Marine Facilities | Overview
The main suspension cables are fabricated from numerous small diameter (5 to 6 mm) high strength parallel wires that are galvanized. Cable clamps are provided every 10 m and serve two purposes. First the cable clamps are used to compact the parallel wires into a tight bundle of circular shape and minimum diameter. Second, the vertical hanger cables used to support the deck are connected to the cable clamps.
The vertical hanger cables that support the bridge deck from the main suspension cable consist of high strength wire ropes that connect to the cable clamp at the top and will utilize a stressable anchor socket at the lower connection to the superstructure. The stressable anchor sockets at the lower end will provide for adjustability to address fabrication and erection tolerances. The anchor sockets are connected to the superstructure steel girder by a steel pin with the adjustability coming from the threaded anchor socket itself. |
Marine Facilities | Construction Access
The majority of the construction for the main suspension cable happens from a hanging catwalk system suspended from the suspension cable itself. Hence, there is very little marine based traffic required while constructing the main suspension cable. Barges will only be required for supplying materials for the catwalk system. |
Marine Facilities | Construction Process
The main suspension cable starts by stringing a small diameter pilot cable from the SW Anchor Block to the NE Anchorage. The pilot cable can be carried from one anchorage to the other by helicopter or by boat and then hoisted to the top of the towers using the tower cranes. Once the pilot cable is installed, it is then used to pull successively larger cables over the towers to the opposite anchor block. Through this means, multiple cables will be installed to allow for catwalks to be erected about 1.5 m below the proposed suspension cables over the full length of the bridge. The catwalks are an access walkway used by ironworkers to facilitate main suspension cable installation. After completion of the catwalks, the main suspension cables are installed by continuously pulling individual wires back and forth from anchorage to anchorage until the required number of wires is achieved (aerial spinning). The reels of wires are delivered to Lelu Island and will be fed to the spinning process from immediately behind the NE Anchorage. Figure 2-7 shows a suspension cable during the spinning process.
The cable spinning is done using equipment supported by the cables themselves, thus no floating construction equipment is required during the cable installation process. After the requisite number of wires is installed, the bundles of wires are compacted into a circular pattern and the cable clamps are installed. The cable clamps serve to maintain the circular shape of the cable and for attaching the vertical hanger cable. A typical cable |
Marine Facilities | With the main suspension cable and cable clamps completed, cable cranes that ride on top of the suspension cable are installed. The cable craned will be used to install the vertical hanger ropes and the bridge deck segments.
After the superstructure has been constructed, finishing work on the main suspension cable is done. This includes final cable compaction, installation of cable wraps for corrosion protection and catwalk removal. As shown in Figure 2-9 this work is done from the catwalks and the newly installed superstructure, thus requiring no support from marine equipment. |
Marine Facilities | Construction Schedule
It is expected that installation of the main suspension cables lasts approximately 8 months and must be completed before the superstructure segments can be installed. Cable finishing works is expected to last approximately 8 months and will begin after the superstructure is complete. During work on the main suspension cables there will be very little construction marine traffic, although crews will be continuously working above any marine traffic navigating below the suspension bridge. |
Marine Facilities | Overview
The suspended portion of the deck extends from the NE Tower to the SW Anchor Block (total length of 1490 m). Structurally the deck is composed of an orthotropic steel box girder supported on either side with the two suspension cable planes. The deck is suspended with four hanger cables spaced approximately 10 m apart along the longitudinal axis of the bridge.
The orthotropic steel box is aerodynamically shaped for wind stability. Each segment is approximately 20 m long and is connected to adjacent segments by fully bolted splice connections. Each segment weighs approximately 200 metric tons. |
Marine Facilities | Construction Access
The steel box deck segments are lifted into position using a cable crane that travels along both suspension cables. The deck segments are delivered on a barge directly under their intended final position. Since the main span of the suspension bridge spans over the Flora Bank, all deck segments within the main span will need to be barged into position during high tides (water depths greater than 4 m). Once the segment is lifted off the barge (approximately 1 hour), the barge is no longer required and can be taken away from the Flora Bank. All other work for installing the deck segment can be done above water without any tidal restrictions.
A below-the-deck gantry will be used to facilitate installation of the bolted connections along the underside of the deck. This gantry will travel along the deck maintaining its position at the leading edge of deck construction. |
Marine Facilities | Construction Process
The cable crane will hoist each segment from the barge into its final position where four hanger ropes will be attached. The segment is bolted to the previously installed segment and the cable crane is released. The cable crane walks itself up the suspension cable and sets itself into position for the next segment and the process repeats until all segments are installed. Figure 2-10 shows installation of a deck segment of similar shape as that proposed for this bridge. |
Marine Facilities | Construction Schedule
It is expected that construction of the suspended superstructure will last approximately 9 months. With an estimated 75 deck segments, it is anticipated that each deck segment will take approximately three to four days to install, with only one of these days involving marine traffic to transport the segment to its final position. Only those areas which are within a safety radius of the deck section being lifted will be restricted from traffic underneath the bridge. The balance of the passage underneath the bridge should remain open to marine traffic throughout the duration of the suspended superstructure construction. The erection of deck sections will be conducted at high tide to mitigate the possibility of grounding the supply barge onto Flora Bank. The erection of deck sections will also need to be timed to occur during minimal wind conditions.
There is an estimated 8 months of additional finishing work to take place on the suspension cables and bridge deck once the suspended superstructure is complete. However, all of this work will be within the confines of the new bridge and will pose minimal interruption to normal marine traffic. |
Marine Facilities | The construction methodologies described in this section are suitable for pile-and-deck structures as required for the LNG Jetty Trestle and Berth Structures component. Pile-and deck structures are commonly used for marine facilities in British Columbia. For this type of structure, individual support piles are either driven into overburden sediments, if the overburden sediments are deep enough, or drilled into bedrock. The piles provide support to a pile cap and deck superstructure.
The trestle structural configurations as given in the Environmental Assessment submission and the various FEED contractor bid submissions are all pile-and-deck type structures with some variations with respect to materials and member sizes. In general, the various designs for the trestle can be described as a deck superstructure supported on steel pipe pile bents installed at 36 m spacing. The pipe piles vary from 914 mm to 1219 mm in diameter and are either seated on the bedrock or socketed into the underlying bedrock. Each bent has at least 4 piles, including vertical and battered piles, and has a steel box beam or precast concrete pile cap which spans perpendicular to the trestle alignment. The pile caps in turn support steel box beams or precast concrete girders which span between the bents and form the main trestle superstructure and vehicle roadway. The pipe racks consist of steel truss assemblies spanning between the bents and are installed adjacent to the roadway. A typical LNG trestle structure is shown in Figure 3-1.
The two principal methods proposed for constructing the LNG Jetty Trestle and Berth
Structures are:
The Marine-Based Method, which uses floating equipment; and
The Cantilever Method, which uses a mobile construction platform that progresses above the water supported by the very trestle foundations that it installs.
Although either method may be used to construct the entire Trestle, as discussed in the following sections, the floating Marine-Based Method is more effective in deeper water, whereas the Cantilever Method is more efficient and quicker in shallow waters, since marine based equipment will be restricted to tidal windows and hence longer construction times. |
Marine Facilities | Floating Marine-Based Method
Where the water depths are sufficient enough to not limit construction to tidal windows, marine derricks and floating equipment may be used to install the Trestle and Berth Structures. There is less likelihood of barges grounding in deeper waters where the seabed is sub-tidal. Also sub-tidal habitat is less sensitive to disturbance. In shallow waters, the work would be limited to tidal windows and the risk of equipment grounding and disturbing the sensitive inter-tidal habitat is greater. Also the cost of construction will be higher and take longer due to the constant interruption of work to accommodate tidal windows.
For this construction method, the cranes, equipment and materials are all brought to the work site on floating flat deck barges. In general, the methods and techniques used to install and drill piles are similar to the Cantilever Method with the exception that the crane is on a floating platform and not a fixed one. As such, the construction activities are more susceptible to delays due to sea state and environmental conditions. In addition, temporary piles and false work may be required to support templates for accurately installing the permanent piles.
Placement of scour protection around the perimeter of each pile for either the Trestle or the Berth Structures would be conducted soon after installation of the piles and before the installation of the main deck elements to allow easier access for floating equipment. For structures located in shallow waters, the scour protection would
be placed via marine-based methods using tidal windows and a flat deck barge equipped with an excavator or front end loader and a stockpile of riprap. The excavator or front end loader would place the scour protection to the appropriate thickness around each pile. For structures in deeper waters, in lieu of an excavator, a marine derrick fitted with a clamshell grab may be used for more accurate placement of the scour protection. |
Marine Facilities | Construction Equipment
The primary piece of equipment required for marine-based construction is the marine derrick.
A marine derrick is essentially a flat deck barge that has a duty-cycle crawler crane mounted on it. In order to drive piles and drill into rock the crane must be able to accommodate various driving and drilling rig equipment. Both vibratory and impact hammers will be required for the initial setting of the piles. The drill rig will typically be a diesel powered drill unit capable of accommodating various types of drill bits.
The size of crane is dictated by the capacity requirements of the heaviest pile and by the length of crane boom required for properly handling and pitching the longest pile. Because the floating marine equipment can be brought relatively close to the work face, the crane will have a smaller lift radius than other construction methods and therefore the capacity requirements will be less onerous. Local marine contractors have duty-cycle cranes with capacities ranging up to 350 tons which, depending on boom length, can easily pick loads ranging up to 60 to 100 tons based on a 15 m to 10 m lift radius.
For the heaviest or longest piles, in-situ field splicing is an alternative, if cranes with the necessary capacity or boom length are not readily available. Although field splicing is not uncommon, it should be limited due to the time and expense required to make field welds over open seas as well as associated quality issues. Field splicing may also be difficult to execute, if the piles sink into the weak seabed material under their own weight. |
Marine Facilities | Marine-Based Construction Schedule and Equipment Profile
With marine-based construction, the basic sequence for installation of the pile foundations and the trestle superstructure is similar to the Cantilever Method. For the most part the actual installation techniques will be the same except the equipment is based on floating platforms instead of a fixed platform. Although mobilization and demobilization will be quicker with marine-based methods, the possible need for erecting false work and temporary templates for installing the pile bent foundations may reduce overall productivity. For those piles that need to be socketed into the rock, the critical activity will be the drilling operation which may take slightly longer with floating equipment than it would with the Cantilever Method. Also for those pile foundations that are not socketed, the productivity of the marine-based crane operations may be reduced due to weather and sea state delays. However, unlike the Cantilever Method, overall productivity can be easily improved by just adding additional crews and equipment. Multiple crews and equipment can advance on multiple work fronts. The productivity rate will be directly proportional to the number of additional crews and equipment deployed.
The overall estimated schedule duration for the construction of the trestle and berth structures is 14 months and 18 months respectively. Since neither of these structures is on the critical path, it is possible to construct them either simultaneously or sequentially. As a possible mitigation measure, constructing the two components sequentially may help minimize construction equipment requirements and reduce possible cumulative noise effects from multiple work sites.
For the LNG Jetty Berth Structures it is expected that there will be one or two marine derricks and one or two flat deck scows mobilized to conduct the work. For the Trestle it is expected that one marine derrick and one flat deck scow would be mobilized. Since these construction sites are in open water, there is space for additional crews and equipment to be mobilized as required to improve productivity. Although the construction vessels are not subject to tidal restrictions in the deep waters of the LNG Berth Structures and the outer Trestle, it is unlikely the EPC contractor will allow the equipment to stay in open water overnight. Thus the marine derricks and scows will likely be moved between the construction site and a safe anchorage or staging area daily. The anchorage or staging area could be located in Port Edward.
However to mitigate possible marine traffic congestion in Porpoise Channel, the working vessels could transit to a location within the inner harbour of Prince Rupert.
Since the marine derricks and scows would be transferred to and from a staging area every day, they could be used to resupply the work front without the need for additional supply barges. Alternatively one or more supply barges could also be used for transporting materials from the mainland to the work front depending on construction requirements.
In general the construction of the Trestle and Berth Structures via marine-based methods should not be an impediment to marine traffic. The work faces will occur in relatively small areas in open waters away from or adjacent to approach channels. A safety zone would be enforced around the work site which may prohibit vessels from approaching closer than 50 m.
Work site restrictions would be promulgated through a variety of channels, such as Notices to Mariners, in accordance with the Marine Communication Plan. |
Marine Facilities | Cantilever Construction Method
The Cantilever Construction Method, also known as the cantitravel, over-the-top or cantilevered bridge method, is a common method of constructing pile-and-deck trestles which has been used successfully around the world. This method uses a span by span approach for building the trestle using a mobile work platform that is supported above tidal waters. The pile guides and templates are cantilevered out from the main work platform to facilitate a work front that is located a full span away from the last finished bent. This method produces a minimum amount of disturbance to the existing ground or seabed, while still allowing work to proceed from the elevated construction platform which is supported on the same pile foundations used to support the final structure.
To accommodate the Suspension Bridge construction, the working platform would be required to progress from the LNG berths towards the SW Anchor Block to avoid interference between work fronts. In this situation the first few Trestle bent foundations at the LNG berths would need to be installed using marine-based methods. Once these initial bents were erected, the working platform and crane for the Cantilever Method would be assembled and erected on these bents. The working platform would then complete the rest of the trestle advancing towards the bridges SW Anchor Block.
When cantilever construction is used, the construction methodology must be taken into account during the design phase to make the construction process as efficient as possible. |
Marine Facilities | The structural design is typically optimized to suit the construction methodology in terms of the selected member sizes, spans, materials, connections, detailing, etc.
As mentioned above, the working platform, also known as a cantitraveler or cantilevered bridge is a primary piece of equipment required for cantilever construction. A photo of a typical cantitraveler platform is shown in Figure 3-2. A cantitraveler or cantilevered bridge is basically a customized mobile working platform which provides:
A support platform for a crawler crane and associated equipment such as vibratory hammers, impact hammers, and drilling equipment;
Integral guides and templates used to locate, drive and install foundation piles; and
A platform which facilitates the installation of superstructure elements and the finishing of connections between foundations and superstructure elements.
For cantilevered construction, the exact sequence of construction operations will vary from project to project depending on various factors such as trestle layout and overall construction methodology. Regardless of the actual construction sequence however, a main benefit for this type of construction is the repeatability of the construction cycle for each pile foundation. |
Marine Facilities | In general terms the construction cycle for any given pile foundation starts with the installation of the piles. The installation of the piles is followed by the erection and assembly of the transverse pile cap which essentially completes the pile bent foundation. Depending on the construction process, once a pile bent is completed, trestle deck elements may be erected or temporary bracing installed to provide longitudinal stability to the bent. Upon completion of the cycle, the platform is advanced forward one span to begin a new cycle and initiate construction on the next pile bent. Possible construction sequences and techniques for pile installation and drilling which may be used for the PNW LNG trestle are described in more detail in the following sections. |
Marine Facilities | Construction Equipment
Selection and design of the required construction equipment is integral with both the trestle design and the chosen construction methodology. The design of the trestle itself will typically be developed with consideration of the intended construction equipment, sequencing and overall process. Likewise the design of the construction platform will also be developed to facilitate an efficient construction process. Both design of the trestle and the construction platform will be based on the optimization of various interrelated parameters including trestle span length, construction platform dimensions, crane size, weights of components, structural interferences, structural connections, etc.
The two main pieces of equipment for cantilevered construction is the construction platform and the crane. The construction platform can come in various shapes and sizes, but there are two basic types which are most commonly used consisting of:
Cantitraveler platform; and
Cantilevered bridge.
Both types of construction platforms and the crane requirements are described below. |
Marine Facilities | Cantitraveler Platform
A cantitraveler platform as shown in Figure 3-2 is made up of a main carriage frame that supports a work deck for the crawler crane, equipment, and personnel. The main frame of the cantitraveler is mounted on steel wheel bogies which ride on rails attached to the top flanges of heavy rail girders.
The rail girders are separate components designed to bear on the trestle pile caps and take the large construction loads from the cantitraveler and transfer them directly to the pile foundations. The rail girders are necessary to support the heavy construction loads, since it would be uneconomical to design the deck elements of the final trestle structure to support such loads. The rail girders are typically individual simple span girders which can be picked up by the cantitraveler crane and leapfrogged from the back span to the fore span allowing the cantitraveler to index forward as each pile bent foundation is completed. Because the main carriage of the cantitraveler is usually positioned over a single span it also allows the crane to install deck elements in the back span behind the cantitraveler or in some cases the fore span in front of the cantitraveler once the next pile bent is complete. A cable system comprised of winches, pulleys and sheaves that are restrained in the trestle superstructure is used to advance the cantitraveler.
The guides or templates used for positioning, setting and installing the piles are typically located at the ends of truss frames cantilevered out from the side of the main carriage frame.
The pile guides or templates are customized to suit the trestle layout and can be adjusted or dismantled as required. |
Marine Facilities | Cantilevered Bridge
Another type of platform which may be used in cantilevered construction is the cantilevered bridge as shown in Figure 3-3. A cantilevered bridge is comprised of a steel through-truss which extends over multiple spans of the trestle. The bridge has a platform on its upper chords for supporting a crawler crane and has a rail mounted gantry crane running between the main trusses. The functionality of the cantilevered bridge is similar to that of the cantitraveler in that the bridge supports a crawler crane for installing the foundation piles, has built-in piling templates for positioning the piles, and also facilitates the installation of trestle deck components. A cable system with winches, pulleys and sheaves is employed to advance the cantilevered bridge from span to span, similar to the cantitraveler system.
Unlike the cantitraveler system however, the cantilevered bridge does not use wheels and rail girders to facilitate movement. Instead, the cantilevered bridge bears directly on the foundation pile caps and is typically slid forward on guides and bearing pads attached to the pile caps.
Another primary difference between the two platform types is the method by which trestle deck elements are erected and installed. Since the cantilevered bridge covers multiple spans and the top platform and bracing prevents the crawling crane from gaining access to the trestle deck below, the bridge is equipped with an internal gantry crane that can be used to erect and install trestle deck elements in the spans covered by the bridge. |
Marine Facilities | Crane
The required crane size is directly related to the trestle span and weight of the structural members required to be installed by the crane. As the spans increase, the crane reach increases and a larger crane is required to lift the same weight at the extended reach. Also, as the span increases, in general, so will the pile size and the size of hammers required for the installation. The crane size quickly limits out and sets the maximum span length.
The Trestle section of the LNG Jetty is located further from the shore well past the outer edge of Flora Bank. In this area the bedrock is much deeper than it is near Lelu Island and it is estimated that pile lengths may be as long as 80 m to 100 m or greater with corresponding weights of 60 to 75 tonnes or more. To lift a 60-tonne pile to accommodate a 36 m trestle span would require a 600-tonne capacity crane. This is a very large crawler crane size and may be impractical for the Cantilever Method. However, it is also likely not possible to pitch and drive such long piles since the required size of crane boom would be excessively high.
Therefore it may be necessary to splice the piles in-situ, which will not only reduce the length of piles required to be handled and pitched but will also bring down the required lifting capacity for the crane.
Another method of keeping the crane size to a minimum is to reduce the size of the trestle spans and hence the required lifting radius of the crane, or alternatively use temporary spud piles to allow the crane to get close to the work front. |
Marine Facilities | The maximum size of crawler crane that local marine contractors have in their fleets is 350 tonnes. Although the trestle designs currently call for 36 m spans, as discussed above, it is likely that the trestle span will need to be reduced or temporary spud piles used to accommodate the Cantilever Method. Past projects in BC have been limited to spans between 12 and 16 m using locally available equipment. Clearly, with the scope of this project, the crane equipment will be optimized for the installation method and we expect a span length of 25 to 30 m can be reached. |
Marine Facilities | Construction Sequencing
The basic construction cycle for building a trestle span, as shown in Figure 3-4, consists of the following general construction steps:
1. Installation of piles;
2. Installation of pile cap;
3. Installation of temporary longitudinal bracing;
4. Transfer of rail girders;
5. Movement of cantitraveler platform; and
6. Installation of longitudinal girders and deck elements. |
Marine Facilities | Depending on the design and geotechnical conditions, the pipe piles may be either driven into the seabed overburden material, or driven through the overburden material and then drilled into the underlying bedrock to achieve the required bottom fixity. Various techniques for driving and drilling piles are described in more detail below in Sections 3.4 and 3.5 |
Marine Facilities | After all the piles for a particular bent have been installed and cut-off to the proper elevation, the pile cap is erected and attached to the pile heads thus completing the pile bent foundation.
The connection between the pile heads and the pile cap varies depending on the type of materials used in the design:
1. For a steel pilecap, a basic field weld would be used to directly connect the pile cap to the steel pipe piles.
2. If a cast-in-place concrete pilecap is used, reinforced concrete plugs would be cast into the pile heads with the reinforcement extending up into the pilecap to provide the connection. Formwork would be set up above the water, supported on the pipe piles, reinforcement for the pile cap placed, and finally the pile cap would be poured. The formwork would be removed after several days of curing time.
3. If a precast concrete pile cap is used, concrete infill plugs would be cast in the pile heads similar to the cast-in-place construction. The infill plug reinforcement cages would extend up through holes in the precast units to provide the connection to the pilecap. The pilecap is complete when infill concrete is poured into the precast unit to monolithically join the elements together. The precast option does not require formwork like a regular cast-inplace pilecap would. But, regardless of which concrete option is used, those elements that have some amount of cast-in-place concrete will require at least 7 days curing before loads may be placed on the structure.
Upon completion of a pile bent, the crane on the cantitraveler will install bracing to stabilize the bent in the longitudinal direction. Once the new pile bent is stabilized, the rail girders are advanced to the next position and the cantitraveler is moved to the next span to initiate the next construction cycle.
Throughout the above process, there are several options for installing the trestle superstructure and deck elements. One option is to install the trestle girders or deck elements with the crane on the cantitraveler before it is moved into the next position. The trestle girders may either be installed in the fore span in front of the cantitraveler or the back span depending on the selected construction methodology. Another option is to allow the cantitraveler to be moved forward and immediately begin on the next bent foundation while a second smaller crane follows behind on the finished trestle deck and provides a second work front by installing the trestle superstructure and deck elements on the back spans behind the cantitraveler.
As shown in Figure 3-5 the basic construction sequence described above for a cantitraveler platform is essentially the same for a cantilevered bridge with some minor exceptions as follows:
1. Installation of piles;
2. Installation of pile cap;
3. Pull back cantilevered bridge and install longitudinal trestle girders in the new span;
4. Place guides and bearing pads on new pile foundation;
5. Move cantilevered bridge forward onto new pile foundation and install deck elements in span behind cantilevered bridge; and
6. Index the bridge forward to begin new cycle. |
Marine Facilities | Similar to the Marine-Based Method, placement of scour protection around the perimeter of each pile would be conducted soon after pile installation. The scour protection could be placed via marine-based methods using tidal windows and a flat deck barge equipped with an excavator or front end loader and a stockpile of riprap. This work could be conducted simultaneously as the cantilever platform advances along the trestle length. Alternatively the cantilever platform crane could be used to install the scour protection as each bent is completed prior to the erection of deck elements. |
Marine Facilities | Cantilever Construction Schedule and Equipment Profile
Mobilization and start-up includes assembly of the construction platform and crane over open waters. The initial pile bents will be installed using marine-based methods and then the cantilever work platform can be erected from barges using marine derricks so that the balance of the trestle can be completed with the Cantilever Construction Method.
The assembly of the construction platform and the initial start-up of the trestle construction may take a few weeks to a month depending on the geotechnical conditions and the number of initial trestle bents that need to be constructed.
Once the start-up process is completed, typical cantilever construction can achieve an average productivity rate of approximately 2 days for one complete span (including the foundation and superstructure). This productively rate however, is typical for projects where the spans are less than what is proposed for the PNW LNG trestle and where the piles are only driven into the overburden soils.
For the PNW LNG project, the spans may be up to 36 m and the local geotechnical conditions require the piles to be driven into the overburden soils and drilled into the underlying bedrock. Of the various construction activities that make up a typical production cycle, the pile drilling operations will be the most time consuming, especially if the piles are required to be socketed to significant embedment depths. It is estimated that pile drilling and socketing may take up to 3 to 4 days per pile, and since the piles must be drilled sequentially, a typical span with a 4 pile bent may require 14 to 18 days to complete. For pile bents with additional piles, add 3 to 4 days per pile to complete the given span. If the piles are not required to be socketed into the rock, such as for the spans in deeper water, the productivity rate will improve. |
Marine Facilities | Once the trestle is complete, disassembly of the heavy construction crane and travelling platform will be conducted using floating marine derricks which will lift the crane and platform components onto flat deck barges for demobilization. The disassembly and demobilization process is estimated to take several weeks.
The overall estimated schedule duration for the construction of the trestle structures is 14 months. However since the Trestle is not a critical path item, there is enough schedule float to allow for longer Trestle construction times without delaying the overall project. |
Marine Facilities | Although marine derricks and scows would not be required for the Cantilever Construction Method other than the initial mobilization, delivery of piles and other construction materials will be required using supply barges. In shallow waters the supply barge movements would be dependent on tidal windows while for those sections of the Trestle in deeper waters they will not. Although the supply barges are not subject to tidal restrictions in the deeper waters around the outer trestle, it is unlikely the EPC contractor will allow the equipment to stay in open water overnight. Thus regardless of the water depths, the supply barges will be moved between the construction site and a safe anchorage or staging area daily. The anchorage or staging area could be located in Port Edward. However to mitigate possible marine traffic congestion in Porpoise Channel, the supply barges could transit to a location within the inner harbour of Prince Rupert.
In general the construction of the trestle via Cantilever Construction methods should not be an impediment to marine traffic. The work face will occur in a relatively small area in open waters and only one round-trip transit per day would be expected from the supply barge. A safety zone would be enforced around the work site which may prohibit vessels from approaching closer than 50 m. Work site restrictions would be promulgated through a variety of channels, such as Notices to Mariners, in accordance with the Marine Communication Plan. |
Marine Facilities | Pile Driving Techniques
The pile installation techniques described below are applicable to either Cantilever or Marine Based Methods.
The steel pipe piles proposed for the trestle foundations will be supplied in approximately 12 m (40 ft) long sections and will require welding to be assembled and spliced into their final length. Because the seabed overburden material at the project site is predominantly comprised of weak marine clay, it is anticipated the steel pipe piles will sink some distance into the overburden material under their own weight, essentially setting themselves into the seabed. This can make in-situ field splicing of the pile sections difficult. As a result, the piles will likely need to be preassembled and spliced lengths that can be practically handled prior to being transported to site.
An efficient way of transporting long piles to site is on flat deck barges, which can be kept at the project site and used to stockpile the piles. When it comes time to install a pile, the barge can be brought into position during the tidal windows, and then the pile can be picked and pitched by a crane. For relatively long piles, the rigging may need to be adjusted in order the pick and pitch the pile without exceeding the bending strength of the pile. Once the pick is complete the barge can be moved away to a secure anchorage until it is required again.
Stockpiling the piles on a flat deck barge is also advantageous in that it avoids any requirements for a pile lay-down area on Lelu Island. |
Marine Facilities | Once a pile is positioned by a crane and set into the seabed, it needs to be driven through the overburden material until it reaches bedrock. A pile may be driven either closed-ended or open-ended depending on the design requirements and geotechnical conditions. However if the pile is required to be socketed into the bedrock, it will need to be driven open-ended, and cleaned out prior to rock drilling.
For the driving operation, either an impact or vibratory hammer may be used. Although impact hammers have the ability the drive piles through harder material, they are much noisier than vibratory hammers and do cause greater underwater pressure disturbances.
Fortunately it is anticipated that a vibratory hammer will be adequate enough to advance the pile through the weak overburden material until the pile tip is seated close to bedrock.
For the last few meters above the bedrock however, an impact hammer will likely be required to advance the pile through any harder till layers until the pile tip actually reaches bedrock. |
Marine Facilities | Once the bedrock is reached, the impact hammer will drive the piles approximately 1 m into the bedrock to properly seat the pile and/or seal it for socketing. Although an impact hammer will be noisier than a vibratory hammer, it will only be required for a short duration to drive the pile the remaining depth and tap it into the bedrock.
Measures such as the use of bubble curtains may be employed to mitigate underwater overpressures from the pile driving operations and keep underwater acoustics within allowable limits. Typically the overpressure limit for pile driving is set at approximately 30 kPa, but may vary at the regulators discretion.
If the pile is designed to be only seated on the bedrock, then the pile installation is complete.
However, if the pile is designed to be socketed into the bedrock, the overburden material left inside the pipe will need to be cleaned out prior to the drilling operation. A grab hammer will typically be used to clean out the pile, however if the material is weak and loose enough it may be possible to flush or airlift it out of the pile. The removed material is stockpiled on a barge for disposal. Filter cloth and other materials may be used on the barge to reduce turbidly caused by any remaining water runoff from the stockpile. |
Marine Facilities | Pile Drilling Techniques
The pile drilling techniques described below are applicable to either Cantilever or Marine Based Methods.
Driving piles into rock is not feasible, unless the rock is very soft and weak. Therefore for most types of rock, drilling equipment and techniques must be employed to fix piles into the rock. Depending on the design requirements and the degree of moment capacity and fixity required at the bottom of the pile, piles may be installed into bedrock either as rock doweled piles or rock socketed piles.
For a rock doweled pile, the pile itself is inserted down the entire length of a hole drilled into the bedrock. The pile can be fully grouted in the hole or advanced with a friction fit, thereby providing fixity at the bottom of the pile as well as development of the piles full moment capacity.
A rock socket is similar to a rock dowel, but in lieu of inserting the steel pile all the way into the drilled hole, the steel pipe pile is only embedded a certain distance into the rock and the balance of the hole is filled with a cast-in-place reinforced concrete core. The concrete core also extends a certain distance up into the hollow of the pile above the rock to facilitate load transfer. The reinforced concrete core acts like an extension of the steel pile into the rock and thereby provides anchorage and a degree of fixity at the bottom of the pile. The reinforced concrete core however, will not provide the full moment capacity of the steel pipe pile therefore a rock socketed pile is not as strong as a rock doweled pile. |
Marine Facilities | For installing a pile into rock, initially the pipe pile must be driven a certain distance into the bedrock in order to seal the hole and prevent overburden material from sloughing in as the hole is advanced past the tip of the pipe pile. Various drill bits and techniques may be employed depending on the type of rock being drilled and whether it is a rock socketed pile or rock doweled foundation. Some typical drill bits which may be employed include down-the hole hammers, churn drills, core barrels, rock augers, tri cone bits and other rotary bits. The contractor will use the most effective bit given the local rock conditions. Drill bits are typically attached to a rotating Kelly bar operated from a diesel-powered drill unit mounted on a frame or the leads of a crane.
The installation of a rock socketed pile, involves inserting the drill bit into the pile from the top and lowering it to the bottom. The drill unit rotates the bit, grinding the rock away at the bottom of the hole and advancing it with a diameter slightly smaller than the inside diameter of the pile. The hole is drilled to the desired embedment depth while the drill tailings are simultaneously airlifted to the surface through the kellybar. Once the hole is complete and cleaned, a reinforcement cage and tremie concrete can be placed, completing the socket. |
Marine Facilities | For a rock doweled pile, the hole must be drilled with a diameter large enough to allow the pile itself to be inserted. However, since the drill bit is generally smaller than the piles inside diameter (so that it may be inserted into the pile) special techniques must be used to drill an oversized hole.
One method is to use a down-the-hole hammer with undercutting capability. Although the overall hammer bit is small enough to fit inside the pile, once the tip of the bit is advanced beyond the pile end, under reaming cutter wings extend out from the bit, drilling a hole slightly larger in diameter than the pile.
Another method involves the attachment of a sacrificial cutting ring to the pile itself. The drill bit is used to drill a slightly undersized hole similar to a rock socket. The cutting ring on the end of pile is advanced by rotating the pile itself, typically in the opposite direction of the drill bit, and reaming out the hole thereby enlarging it and allowing the pile to be embedded. For this method the drill unit must have the capability to rotate the pile. |
Marine Facilities | A third method for installing a rock dowel involves the use of a large diameter pipe casing. Using the same techniques as for piles, the oversized casing is driven down, sealed into the rock and cleaned out. Using standard rock socket drill techniques, a hole is drilled in the rock that is smaller than the casing but larger than the pile. The pile is inserted into the casing from above and grouted into the hole. The casing is then removed and positioned for the next pile.
This technique has some disadvantages compared to the previously mentioned methods in that it requires multiple handling of large long pipes, including pile insertion and casing removal. This may require a crane with a very large boom. Another disadvantage is the greater volume of material required to be cleaned out of the casing as compared to other methods where just the pile is installed. Also, when the casing is removed there may be additional disturbance and soil sloughing around the pile. This is avoided with the other methods.
For all of the above drilling techniques, the tailings from the drill process are typically extracted by airlift systems. The removed material can be run through a cyclone to extract water before it is stockpiled on a barge for disposal. Filter cloth and other materials may be used on the barge to reduce turbidly caused by any remaining water runoff from the stockpile. |
Marine Facilities | MOF Construction Methodologies
There are various design configurations for the MOF and as such the construction methodology will be tailored to suit the type of marine structure selected by the EPC contractor. Unlike the other marine facilities however, the MOF also has a requirement to dredge rock and sediment within the MOF cove and its approaches to provide a water depth that can accommodate the vessels expected to offload at the facility. Both the dredging and construction methodologies for the MOF are discussed in the following sections.
6.1 Dredging Methodology
Dredging between Lelu Island and Ridley Island (Porpoise Channel) is planned to provide adequate depth for the MOF berths. Approximately 790,000 m3 of material will need to be removed for the MOF of which approximately 590,000 m3 is expected to be rock (phyllite/schist). Further description of the MOF dredging methodology is detailed in Appendix A.
The methodology used in this study was based mainly on the geotechnical information provided by PNW LNG, tidal variations and Hatchs experience in rock drilling/blasting (D&B) and dredging operations. This is a preliminary assessment and final methodology should be selected by the contractor. Dredgeate and rock material not used for onshore purposes will be disposed at sea in Brown Passage as indicated in the EA submission.
Due to the important tidal variation in the area of study, the dredging area will be divided into two regions with distinct D&B and dredging methodologies:
Area below MSL to be performed with marine-based equipment;
Area above MSL to be performed by land-based equipment.
Most of the dredging area is located outside a natural navigation channel and is not expected to disturb navigation activities in the area.
The site conditions, geotechnical information, dredging volume, disposal options and productivity will determ |
Marine Facilities | An estimated 200,000 m3 of marine sediment will have to be dredged prior to the commencement of the drilling blasting activities. All sediment will be disposed at sea in Browns Passage.
The most common equipment for dredging marine sediments is a Trailing Suction Hopper Dredger (TSHD). The TSHD is a self propelled vessel that accumulates the dredged material into its hopper and disposes this material through doors located at the bottom of the vessels hull. TSHDs come in a wide range of hopper volumes. As the hopper volumes increase so too does the vessels draft. Therefore it is suggested that a small TSHD with a capacity up to 5,000 m3 be used so that a wider dredging area can be accessed. No auxiliary equipment is necessary for a TSHD operation other than a crew vessel.
Since 90% of the sediment volume is located below MSL it is expected that the TSHD will be able to tackle most of the marine sediment dredging. A 5,000 m3 TSHDs monthly dredge production rate is estimated to be approximately 150,000 m3. This means that approximately 2 months would be required to dredge 90% of the marine sediment. |
Marine Facilities | Drilling and Blasting
Drilling and blasting is used to fragment the rock layers with explosives. Drilled holes are filled with explosives and laid out in a grid pattern. A common pattern for blasting operations consists of drilling holes of about 130 mm in diameter in a 3 m x 3 m square grid. The blasting material most widely used in environmental sensitive areas, as a substitute to dynamite, is water-based gel explosives. |
Marine Facilities | Marine-Based Equipment
The most common equipment for drilling in offshore areas is a jack-up barge (also known as a self elevating platform) equipped with multiple drilling towers.
The jack-up barge consists of a floating pontoon with movable spuds, capable of raising its hull above water surface. An example of jack-up barge is shown in Figure 6-1. Its approximate dimensions are 40 m long x 30 m wide x 5 m moulded depth.
The main advantage of using a jack-up barge is its ability to withstand heavy seas, currents and the tidal range to maintain stability for drilling activities and positioning. The main disadvantage is the jack-up barge is not self propelled and is dependent on tugs for transportation and positioning.
The jack-up barge can be positioned in most of the drilling areas, up to an elevation of +5.40m CD. This is the upper access limit-based on an estimated jack-up barge draft, when floating, of about 2 m and barge deployment occurring when tides reach their highest levels.
Even though the time span for highest tides is short in a semi diurnal tide region, the jack-up barge could be positioned fairly quickly with the help of tugs and anchors positioned on shore. |
Marine Facilities | Land Based Equipment
The upland area will be drilled with mobile drilling rigs and will need road access and level terrain for stability. Drilling rigs can be deployed on wheels or on tracks as shown in Figure 6-2. Therefore, it will be necessary to fill some upland areas to create level access to the rigs. Shallow areas between +5.40 m CD (draft limited depth) and HHWL (+7.40 m CD) will also have to be filled in order to allow access for land-based drilling equipment. No auxiliary equipment is required for the drilling rigs. Fill and cut to provide level access of the land-based drill rigs will not be assessed in the study. |
Marine Facilities | Marine Based Equipment
The most suitable equipment for recovering blasted rock is the marine backhoe dredger (BHD), as shown in Figure 6-3. BHD is a stationary water-based excavator mounted on a dedicated dredging pontoon that has a rotating table. The position and stability is maintained by three spud poles, where two spuds are fixed to the front side of the pontoon near the excavator crane and one tilting spud at the aft side. The equipment can move with the tilting spud and direction can be adjusted with the two-part articulated arm. In the dredging position the front spuds are firmly planted in the seabed. The dredger then raises itself partially out of the water to further anchor the spuds. With the pontoon slightly lifted out of the water, part of the weight of the dredger is now transferred via the spuds to the seabed, resulting in an increase in anchoring.
Because of the hydraulic action of its stick and bucket cylinders, BHDs exert positive forces pushing the bucket into the material to be excavated |
Marine Facilities | BHDs are becoming the dredging equipment of choice as dredging operations and projects have expanded. This equipment can dig at greater depths and have greater installed power, therefore can be utilized cost-effectively in heterogeneous soils containing a mixture of clay, sand, cobbles and boulders. Because BHDs can generate reasonable cutting force, they are also suitable for digging heavy clays, soft stones, blasted rock and soil containing fractured rocks or rubble.
BHDs are equipped with accurate positioning systems and can operate with precise underwater profiles. The bucket position is followed by the operator on a computer screen in real time and the seabed elevation is updated automatically in the dredging software once the bucket removes material from the seabed.
The suggested minimum requirement for the BHD is specified as 3,000 kW power capacity, 10 m³ bucket volume and approximate pontoon dimensions being 60 m long x 18 m wide x 4 m moulded depth, which falls into the class of mega BHD. |
Marine Facilities | The main advantage of using a BHD is its ability to dig into hard soils containing boulders or debris due to its hydraulic power and can reach water depths up to 20 m. The main disadvantage of BHD is that it mainly relies on the pontoons buoyancy to support its weight as the pontoon spuds do not have the capacity to carry it. The BHD has to then work with enough under keel clearance in order to allow safe water draft for dredging operations. The semi diurnal tidal variation at the MOF site will restrict the area of influence for this equipment and force it to move a great deal to cover the intertidal area. Another disadvantage of BHDs is downtime due to waves and cross-currents. Since the MOF dredging area is protected it is not expected that climate conditions will cause equipment standby.
Dredging is expected to start only after blasting is complete and surveyed. The BHD bucket dumps the rock material into a self-propelled split barge moored alongside the dredger. The suggested minimum requirement for the split barge volume capacity is 700 m³ with bow thrusters.
Auxiliary equipment involved in marine dredging includes:
1 multicat barge (25 m long x 12.7 m wide x 2.3 m moulded depth) with 40 t crane capacity;
Self-propelled split barge;
1 survey and crew launch boat; and
1 operational launch boat. |
Marine Facilities | It is suggested that rock disposal site be positioned on intertidal area with access to landbased equipment once water level drops. According to client communication the dredged rock will be used for civil works. Therefore, the split barge would have to dump the rock disposal material as close as possible to shore at high tide.
Dredging is expected to start only after blasting is complete and surveyed. Due to safety reasons no vessels are allowed to operate near the blasting area.
Production rate for the marine-based BHD is estimated to be 37,500 m3/month considering an operation rate of 350 hours/month and a production rate of 85.8 m3/hour with a 10 m3 bucket. Higher productivities may be achieved with a larger bucket size. |
Marine Facilities | Land Based Equipment
The most suitable equipment to dredge the upland areas as well as the intertidal areas that are inaccessible by a marine-based BHD, is an elevated land-based backhoe excavator mounted on an under-carriage with spread tracks as shown in Figure 6-4.
This versatile equipment can operate at water depths up to 5 m and dig into hard soils containing boulders or debris due to its hydraulic power. The main objective of the land-based backhoe is to move the blasted and disposed rock towards the upland and make it available for civil works and earth moving equipment (not part of the scope of this study).
The elevated backhoe will gradually drag and move the blast rock material upland. The estimated production rate for land-based backhoe is 60,000 m3/month considering an operation rate of 350 hours/month and production rate of 200 m3/hour with a 10 m3 bucket.
Since the total volume to be dredged by land-based equipment is approximately 118.000 m3, the estimated schedule for one land-based backhoe is about 2 months.
Land-based dredging can be performed simultaneously with the marine-based dredging. |
Marine Facilities | Phasing and Schedule
Marine sediment dredging equipment will have to be mobilized prior to drilling and blasting activities and rock dredging. It is recommended that the faster dredging method be selected in order to avoid delays in the schedule.
Drilling and blasting activities will be performed from the same equipment, either a jack-up barge with drilling equipment, or alternative equipment such as a specialized drill vessel.
Upland drilling and blasting activities can proceed simultaneously with marine drilling and blasting activities. Equipment production will have to be assessed in more detail in order to determine the phasing of the works for the MOF Dredging.
To speed up production, dredging with a BHD can be supplemented with an elevated landbased excavator which has the capability to reach the seabed as deep as Elev. -8.0 m CD.
The land-based excavator would dredge the foreshore and shallow waters, while the BHD would dredge the deeper waters down to Elev. -12 m CD.
As per client information, the in-water blasting window for Porpoise Channel is a 2.5-month period from November 30 to February 15, subject to site-specific changes based on species use and discussions with DFO. Blasting should be performed only during low tide. However, the proposed schedule for MOF blasting and dredging activities is 7 months. In order to achieve the blasting schedule, drilling operations should be mobilized well in advance of the actual blasting window to prepare the blasting boreholes. Also, onshore blasting is not subject to the same restrictions as in-water blasting and can be conducted outside the 2.5-month period. |
Marine Facilities | Construction Methodology
The exact construction methodology for the MOF will be dependent on the type of marine structure selected by the EPC contractor. Based on the currently proposed FEED designs, the MOF is comprised of two wharf structures which, depending on the design concept, may be constructed using Concrete Caissons, Steel Sheet Pile Bulkheads, or Pile-and-Deck type structures. Despite the wide variety of possible structural configurations, the required marine construction equipment will generally be similar to the other marine facilities.
Similar to the Pioneer Dock, the construction activities for the MOF can be divided into a marine scope and an upland scope. Both the marine and upland scope activities will vary depending on the selected MOF configuration.
If a Pile and Deck wharf structure is selected for the MOF, the construction methodology would be the same as for the other facilities requiring marine piles. Standard marine-based methods and equipment as described previously would be used to install the piles. Pile vibratory and drilling techniques, as described for the LNG Jetty Trestle, would be used to mitigate noise concerns during pile installation. Once the piles have been installed, the deck superstructure can be installed from the land, or from the waterside, or both, depending on the framing layout and the materials used. Typically a retaining wall is required to prevent scour or undermining of the landside abutment of the wharf. The landside abutment or interface would be constructed using land-based equipment and should not affect the marine habitat. If a revetment slope under the wharf structure is used for scour protection in lieu of a retaining wall, the riprap protection would be placed after pile installation and before the deck was constructed. |
Marine Facilities | For Steel Sheet Pile Bulkhead structures, a marine derrick and flat deck scow would be required similar to the Pile and Deck design. The marine derrick would use a vibratory hammer to install the sheet piles into the overburden sediments, thereby limiting noise levels.
The flat deck scow would be used to stockpile the sheet piles and equipment. The marine equipment is used primarily to install the steel sheet piles and once the bulkhead is complete, the balance of the work can be conducted by land-based equipment. When the bulkhead wall is near completion, the space between the bulkhead wall and shoreline would be filled and compacted in a series of lifts using land-based equipment. Since the activities occur behind the wall, there is less probability of generating turbidly in the water column. Anchor systems for the bulkhead wall, including walers, connections, tie-rods and concrete deadman anchors, would be installed in between lifts at the appropriate elevations. The tops of the steel sheet piles would be cut-off level and to the proper elevation. Once the backfill behind the bulkhead wall was complete a concrete cope beam and decking would be formed and installed from the land side. Finally, the marine derrick would use a clamshell grab to place scour protection in front of the bulkhead in the berth pocket to prevent undermining of the bulkhead wall. |
Marine Facilities | For the Concrete Caisson concept, again the basic marine construction equipment deployed would be a marine derrick and flat deck scow. In addition, a hopper barge may be used for the initial placement of the engineered fill for the caisson mattress pad. The installation of concrete caissons will require more dredging and rock blasting than the other concepts since enough space must be provided for the caissons and mattress pad upon which the caissons sit. After initial placement of the engineered fill, the marine derrick will use a special spreader bar to distribute and screed the engineered fill to produce a mattress pad with a flat level bearing surface. The concrete caissons would be fabricated in dry-dock and towed to site just prior to installation. Tugs and the marine derrick, which would likely be a spud barge, would be used for initial placement of the caissons and for ballasting the caissons to seat them on the mattresses. Once the infill panels are installed between the caissons and the caissons are fully ballasted into position, the balance of the backfilling behind the caissons and completion of the deck and cope beam on top of the caissons can be completed with landbased equipment. Finally, the marine derrick will use a clamshell grab to place scour protection in front of the caissons in the berth pocket to prevent undermining of the mattress pad. |
Marine Facilities | Construction Schedule and Equipment Profile
The overall estimated construction schedule for the MOF construction is 12 months.
Regardless of the configuration of the MOF wharf structures, the basic marine construction equipment deployed would be a spud barge type marine derrick and a flat deck scow. The activities performed by this equipment will be slightly different depending on the structure type being built. Work tugs would also be deployed to move the marine equipment from one work face to another. For the concrete caisson design a hopper barge may also be deployed for a brief period for placing engineered fill for the mattress pad.
The contractor may elect to mobilize two crews to work on two work faces simultaneously.
This essentially means that two sets of marine derricks and scows would be required. These construction barges would be located in the cove and would be staged so as to not block Porpoise Channel. Therefore the construction equipment should not be an impediment to marine traffic in Porpoise Channel. A safety zone would be enforced around the work site which may prohibit vessels from approaching closer than 50 m. Or the safety zone may be set up around the entrance to the MOF cove to correspond with a silt curtain deployment. In either case, work site restrictions would be promulgated through a variety of channels, such as Notices to Mariners, in accordance with the Marine Communication Plan. |
Marine Facilities | Progression of Marine Facilities Design
Although the marine facilities design has been advanced to a level sufficient for the purposes of planning and permitting, the design will be subject to change prior to the finalization of the design details. The current design details which have been submitted as part of the EA are only preliminary, and considerable engineering effort is still required to complete the designs to a level that is ready for construction. The designs will be received as they currently stand by the EPC contractor and will be modified to suit that contractors particular construction methodology as it fits into the framework of the EA and the projects overall Construction Management Plan. Factors which will influence the evolution of the design include:
Changes resulting from further geotechnical and geophysical investigations;
Changes resulting from further site and metocean investigations;
Changes to suit EPC contractors equipment, construction preferences, and schedules; and
Changes to suit regulated environmental restrictions and mandated mitigations. |
Marine Facilities | Construction Schedule and Equipment Profile
A summary of the estimated construction schedules and equipment profiles discussed above for each marine construction site is given in Figure 7-1. For the sake of conservatism, marine based construction methods have been assumed for all applicable construction sites, since this represents the largest floating equipment profile that may have an effect on marine traffic.
The minimum required equipment is identified for each construction site as well as the estimated frequency of floating equipment transits. Considering the estimated overall construction durations for each marine work site, a temporal profile is given for each piece of marine-based equipment. This allows a quick determination of the estimated construction fleet in operation at any given time.
As can be seen by Figure 7-1 at its peak, the construction fleet may consist of up to 4 to 7 marine derricks and 4 to 7 flat deck scows operating simultaneously over 3 different sites.
Also if the MOF dredging program lags, a jack-up barge working beside a marine backhoe dredger, and split hopper barge in the MOF cove may also be an ongoing operation during the peak time. The total construction fleet mobilized may include anywhere from 14 to 18 working barges and perhaps a dozen or so supply barges transiting from the various construction sites to staging and anchoring areas. This fleet will be supported by several crew boats, utility tugs and other small craft vessels.
The fleet will be distributed over a wide area of several kilometres from Lelu Island Slough to the LNG Berth locations out in open water. Much of the construction activities will occur in relatively confined or contained work sites. With a few exceptions, these work sites will be mainly located in coves and inlets away from regular marine traffic channels or in areas along the perimeter of Flora bank which are naturally probative to marine through-traffic. |
Marine Facilities | Daily transits are expected for much of the working vessels as they are deployed and recalled to accommodate tidal restrictions, weather conditions, material resupply, and work shift requirements. Staging areas and anchorages can be located to minimize disruptions to marine traffic. During construction activities much of the fleet will remain stationary while construction operations are being conducted and will only transit for the reasons discussed above.
In general the construction of the marine facilities should not be an impediment to marine traffic. Mitigations include enforcing a safety zone around each work site and promulgating work site restrictions through a variety of channels, such as Notices to Mariners, in accordance with the Marine Communication Plan. As a participant in the Port of Prince Ruperts Construction Coordination Committee for de-confliction with other users, PNW LNGs Marine Communications Plan will be used to alert mariners to project activities, hazards and safety measures such as enforced safety zones and marine route closures. |
Mining Concentrator Plants | The 40 ft (12.2 m) diameter 20 MW Cadia gearless SAG mill, was commissioned. This was a leap of over 40% above the largest operating SAG mill. A significant saving in capital cost gave the incentive necessary for a single mill line at Cadia, resulting in the selection of the 40 ft (12.2 m) diameter mill. Now, after 18 months of operation,
The grinding ball mills are important in the ore comminution circuits. Depending on numerous factors, such as, for example, inappropriate design, manufacturing, overloads, poor maintenance and inadequate operating procedures, flaws are developed in the structural components of this equipment. The structural components of a mill, basically, shell, heads and trunnions, besides high costs, have lead times that might reach three years. |
Offshore Design & Installation | A typical subsea oil and gas field consists mainly of Xmas trees, manifolds, termination units and pipelines. The central in a subsea field is the manifold. It is the link between the subsea field and the production facility. The manifold consists of a network of pipes and valves for gathering and distribution of the production flow. By using a manifold, the number of pipelines required in a subsea field is reduces, and it allows for a single pipeline for transportation to the production facility.
The Xmas trees are (normally) placed on the seabed, acting like satellites around the manifold. The trees are connected to the manifold with a pipeline called jumper or spool.
A termination unit can be called a PLEM or a PLET. These units are connection points between two or several pipelines. The Pipeline End Termination (PLET) comprises a single pipeline connection only, while the Pipeline End Manifold (PLEM) is supporting two or more pipeline connections.
In the subsea industry, the pipeline is a collective term for flowlines (pipelines transporting fluids and/or gas), spools, jumpers and risers |
Offshore Design & Installation | Platform solution
Per definition, in this context (thesis), a platform is all kinds of offshore surface units, like fixed platforms, floaters, FPSOs, etc. used in conjunction with offshore oil and gas production. If a subsea field is connected to a platform in such way that the produced oil and gas is transported to the platform for processing, it is called a platform solution. A common feature for this subsea solution is the riser which connects the subsea field to the platform. |
Offshore Design & Installation | Subsea-to-shore solution
For this type of subsea solution, all produced oil and gas is transported (tie-back) to an onshore facility for processing. The transportation is in a long export flowline. The subsea fields Snøhvit and Ormen Lange, which are well known in Norway, comprise the subsea-to-shore solution. |
Offshore Design & Installation | SUBSEA PIPELINE CONNECTIONS
Subsea pipeline connections can be differentiated between
pipe-to-pipe connections
Pipe-to-pipe connection is the definition when to pipelines are connected to operate as one pipeline
E.g If a long export flowline from shore is to be connected to a subsea facility, a spool is required between the flowline and the structure. The shape of the spool (L-shape or Zshape) makes the spool compensate for thermal expansion in the flowline to avoid application of heavy loading directly into the connection point on the structure. The connection between a flowline and a spool is a very common subsea pipe-to-pipe connection
pipe-to-structure connections. While pipe-to-structure connection is the definition when a pipeline is connected to a subsea facility such as a Xmas tree, a manifold or a PLEM. |
Offshore Design & Installation | PIPELINE END TERMINATION (PLET)
A pipe-to-pipe connection can take place when a riser is connected to a subsea installed pipeline, or when an export flowline is connected to a spool. These types of connections, compared to the pipe-to-structure connections, do not have a given substructure for support. For a pipe-tostructure connection, the Xmas tree, the manifold or the PLEM will provide the required support to the connection point, while a pipe-to-pipe connection requires a purpose-built substructure.
The PLET is the required substructure for a pipe-to-pipe connection. It provides the support for the connection point. Figure 1.10 is an example of a PLET. A detail description of PLET is given in section 2.3 |
Offshore Design & Installation | Figure 1.11 (overleaf) illustrates a difference between a platform solution and a subsea to- shore solution with respect to the use of a PLET. For the subsea-to-shore solution, the PLET is positioned at the connection point between the export flowline and a spool as the flowline cannot be connected directly into the manifold. For the platform solution the PLET is positioned at the connection point between the riser and a spool as the riser cannot be connected directly into the manifold. |
Offshore Design & Installation | Generally, a PLET is positioned where a flowline is connected to a spool. Most of the flowlines in a subsea field are rigid pipelines (section 2.1), and spools are then required between the flowlines and the connection points on the subsea facilities to compensate the thermal expansions in the flowlines |
Offshore Design & Installation | SUBSEA PIPELINES
The most common way to fabricate a subsea pipeline is to weld a large number of pipe joints into each other on a special lay-vessel at the same time as the pipeline is lowered and installed on the seabed. Typical pipeline material is carbon steel or a type of alloy.
Pipelines are generally regarded as rigid or flexible. Rigid pipelines are made of steel and have limited bending capacity and flexibility. Export flowlines and spools are usually rigid pipelines. The rigid pipelines are generally less expensive than flexible pipelines. |
Offshore Design & Installation | Installation methods
S-lay
Pipe joints are welded to a pipeline on a lay-vessel. From the lay-vessel the pipeline appear as an S-curve to the seabed
J-lay
Pipe joints are welded to a pipeline on the lay-vessel. The welding is done with the pipe joints in vertical position
Installation of spools and jumpers to a subsea field is done by an installation vessel (notlay-vessel). The spool or jumper is fabricated onshore, and is then transported by the vessel to the offshore location. From the vessel, the spool or jumper is installed to seabed by vessel crane
Subsea connections are the connections completed on the seabed. As the industry has become driverless, subsea connections by welding and flanges are more or less non-existing. Subsea connections are completed by remotely operated mechanical connectors, also called clamp connectors. |
Offshore Design & Installation | PLET example 1
Figure 2.4 shows the PLET (fixed end), the termination (movable end) and the configuration before and after the connection operation. The termination is landed on the PLET, close to the Porch. A ROV operated connection tool executes the pull-in operation by pulling the termination towards the Porch. A torque tool operated by the ROV closes the clamp connector.
|
Offshore Design & Installation | Design challenges for flowline systems
As offshore oil and gas production is moving into deeper waters, the risk of hydrate plugging and wax formation in flowlines increases, as does the cost of remediating any such plugs [45].
Hydrate and wax formation in subsea flowlines will cause undesired fluid properties and even blocking of the wellstream, which implies shutdown and comprehensive reparations
There are several solutions to prevent formation of hydrates and wax during production. In the North Sea, the most common method combines thermal insulation and chemical injection. The disadvantage with this is that large amounts of chemicals are injected continuously into the well stream, then have to be removed again at the topside.
Electric heating is developed as a method for removing hydrates, and is also applicable for solving plug situations [46]. Electric heating may also prove effective in preventing or remediating wax plugging. In this case it may be possible to reduce capital costs by replacing conventional pigging loops with single heated flowlines.
Two systems are considered:
Direct electric heating (DEH), Figure2.4, with strap-on piggyback cable and current carrying pipeline
Indirect electric heating of pipelines using a) cables as ohmic elements or b) induction heating of pipeline wall. In both cases the cables are embedded inside the thermal insulation.
In a DEH system, the pipe to be heated is an active conductor in the electric circuit formed by the dynamic DEH riser cable, the armored feeder cables, the piggyback cable, and the flowline. The heating effect results from the fact that an electric current flowing in a metallic conductor generates heat.
AC current comes from the topside power system through the DEH riser cable. For safety and reliability reasons, the heating system is electrically connected to surrounding seawater (i.e. it is an "open system") through several sacrificial anodes. These anodes must be rated for both corrosion protection and for sufficient grounding of the system during the expected lifetime of the flowline |
Offshore Design & Installation | Risers
A riser is a pipe or assembly of flexible or rigid pipes used to transfer produced fluids from the seabed to surface facilities, and transfer injection or control fluids from the surface facilities to the seabed [50]. Riser connects subsea to topside [12].
There are different types of risers [12]:
? Drilling Risers - Typically top tensioned risers
? Production risers
Flexible risers
Steel catenary risers
Hybrid riser towers
Single Hybrid Risers SLOR
Grouped SLOR
? Export risers
Similar to production risers
? Water/Gas Injection risers
Similar to production risers |
Offshore Design & Installation | Flexible risers
Flexible pipes have been successful solutions for deep and shallow water riser and flowline systems worldwide. In such applications the flexible pipe section may be used along the entire riser length or limited to short dynamic sections such as jumpers. |
Offshore Design & Installation | Steel catenary risers
A Steel Catenary Riser (SCR) is a prolongation of a sub-sea pipeline attached to a floating production structure in a catenary shape. SCR lines are commonly subjected to fatigue loads, particularly in the touchdown zone, due to platform movements, Vortex Induced Vibrations (VIV) and sea currents.
In order to avoid any excessive stress concentration, the oil and gas industry pays particular attention to the welding process of SCRs in order to minimize any possible misalignment between pipe ends |
Offshore Design & Installation | Primary Functions of Umbilicals
An umbilical is defined as an assembly of fluid conduits, electrical and fibre optic cables, either on their own or in combination with each other, cabled together for flexibility |
Offshore Design & Installation | Glycol dehydration (absorption) is the most commonly used method of natural gas dehydration. Glycol has the advantages of having a high affinity for water vapor, is non-corrosive and is fairly easy to regenerate with low chemical losses |
Offshore Design & Installation | Operation
Wet gas is injected into the bottom of an Absorber Column, also known as a Contactor, while lean or dry glycol is injected near the top. The glycol then cascades over a number of trays within the contactor, comingling the glycol with the gas stream; the lean glycol has an affinity for water vapor, which bonds with the glycol. The now dry gas passes out the top of the contactor, generally through a heat exchanger to lower the temperature of the lean glycol before it is injected into the absorber column. Glycol has more affinity for water at lower temperatures.
The wet or rich glycol, which now includes the water vapor removed from the gas stream passes out the bottom of the contactor through a flash tank where gas entrained with the glycol is collected. It then passes through a heat exchanger to further cool the incoming lean glycol.
The rich glycol then flows into a regenerator or stripper column, which is mounted on top of the reboiler. The stripper column is packed with a ceramic packing. The rich glycol flows down the stripper column into the reboiler, where it comes into contact with rising hot glycol which has been heated in the reboiler to a temperature where the entrained water will vaporize and travel up the still column through an atmospheric reflux condenser before being released to the atmosphere.
For TEG glycol systems, the glycol can be heated to a maximum of 400° Fahrenheit (F) or 204° Celsius (C), which ensures that virtually all of the water contained within the rich glycol is vaporized. (98.6% purity) The now lean glycol can be stored or recirculated through the system.
Natural gas containing four parts per million (4 ppm) or less is generally considered to be pipeline quality sweet natural gas by U.S. gas transmission and distribution companies. Some jurisdictions, such as the Texas Railroad Commission, allow concentrations as high as 100 ppm to be considered as sweet gas. Natural gas with higher concentrations of H?S is referred to as sour natural gas. When H?S is combined with CO?, as is often found in natural gas well streams, the combination creates what is termed acid gas. Acid gas, in addition to being deadly as described above is very corrosive to steel.
The most common methods of H?S and CO? removal from natural gas are as follows
Amine Treatment Plant Processes
Iron Sponge Processes
Proprietary Processes
Claus Process for Sulfur Recovery |
Offshore Design & Installation | Amine Treatment Plant Processes
The alkanolamine treatment process is the most commonly used process to sweeten natural gas. These are commonly referred to as amine treatment processes or plants. The most common alkanolamines used are
Monethanolamine
Diethanolamine
Methyldiethanolamine (MDEA)
Other amines that are less often used include:
Diisopropanolamine
Aminoethoxyethanol (Diglycolamine)
Amines are used in an aqueous solution. The selection criteria and concentration level of the amine type is based on the ratio of H?S to CO? in the gas stream and the concentrations of each. Regardless of which amine is chosen for the gas stream conditions, the sweetening process is essentially the same |
Offshore Design & Installation | The sour natural gas normally runs through an inlet scrubber to remove any slugs of liquid, liquid HC, grit or sand from the gas stream. After the scrubber the gas is introduced into the bottom of an absorber or contactor tower. Lean amine (amine that has been stripped of H?S and CO?) is injected into the top of the tower. The tower includes a series of trays to increase the time for comingling the amine with the sour gas. Each tray has a liquid amine level of 2-3 inches (5-8 centimeters) maintained by a weir. The gas passes up from underneath the tray though openings in the trays causing it to bubble through the liquid forming froth. The H?S and CO? are absorbed by the lean amine in the trays. The gas flows out the top of the contactor tower as sweet gas. |
Offshore Design & Installation | The now rich amine (amine that has absorbed H?S and CO?) is run through a flash tank separator to remove any natural gas that may have carried over in the rich amine. Any liquid HCs are removed via conventional oil/water separation in this separator and the rich amine is drained from the bottom of the flash tank separator. Separation of the HCs also helps to prevent foaming due to HCs in the amine. |
Offshore Design & Installation | The rich amine is then preheated by a heat exchanger utilizing heat from the lean amine, which must be cooled before being reinjected to the contactor. The rich amine is injected into the top of a stripper column (regenerator) which is packed with trays. As the rich amine flows down through the regenerator column, it is met by a gas stream of steam, H?S and CO? that is generated in the reboiler. This lowers the partial pressure of the H?S and CO?, causing it to separate from the amine. Overhead gas is passed through a condenser to recover water and the small amount of amine that is vaporized by the regenerator. The condensed liquid and acid gas flow through a reflux drum from which it is pumped back into the regenerator column. |
Offshore Design & Installation | The acid gas containing the concentrated H?S and CO? are sent to a flare, incinerator, reinjection to a depleted well, sulfur recovery or other disposal method.
The lean amine is then sent through a filter followed by an activated charcoal filter to remove any particulate matter that may have been picked up during the process. Depending on which amine is used, the amine may also be sent though a reclaimer to further purify the amine. The lean amine is then cooled before being pumped back to the top of the contactor tower to begin the sweetening process again.
A typical layout of a 100 gallon per minute sweetening plant packaged by Zephyr Gas Services (currently constructed in Sheberghan, Afghanistan) is shown below in Figure 3-7 Typical Sweetening Plant Layout. |
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