Michael SmithBased on a presentation to Offshore Europe 95, Aberdeen, Sept. 5-8.
Land & Marine Engineering Ltd.
Bromborough, Merseyside, U.K.
Landfall at Point of Ayr, North Wales, for BHP Petroleums Liverpool Bay development pipeline was accomplished with minimal disturbance to seabed and beach.
The 2.7 mile, 20-in. gas pipeline consisted of a series of 600-m pipe strings fabricated at a Point of Ayr construction site. Piggybacked onto the line were twinned 3-in. condensate pipelines.
The strings were pulled into position by bottom-pull with an 800-metric ton (mt) capacity winch spread on a marine barge. When the pulling was complete, the pipelines were buried in the seabed with a submarine trenching machine operated from two work barges. Use of a trenching machine was preferred to extensive dredging and backfill.
Feasibility study
In 1991, BHP Petroleum (BHP) awarded Land & Marine Engineering Ltd., Bromborough, Merseyside, a pilot study to investigate and propose a potential landfall site for the pipelines. The brief was to find a suitable site between Llandudno on the North Wales coast and the Ribble estuary on the Lancashire coast (Fig. 1)(29851 bytes).
The studys conclusions led to a feasibility study for a landfall at Point of Ayr. This study established that, subject to satisfactory results from proposed survey and site investigation work and successful consultation with the authorities, a landfall at this location could be satisfactorily constructed.
Over the following months, the survey and site investigations and consultations with the local authority proved successful. BHP therefore issued invitations to tender for the offshore pipeline contract, which was to include the landfall.
McDermott-ETPM was awarded the offshore pipeline contract in 1993 for pipelines from the Douglas complex to Point of Ayr and for pipelines from Hamilton, Hamilton North, and Lennox to Douglas (Fig. 2)(29492 bytes).
Land & Marine received a subcontract from McDermott-ETPM to construct the landfall section.
Welsh Channel area
The selected landfall construction site was adjacent BHPs new terminal at Point of Ayr.
The site runs through low agricultural pasture and the Gronant Dunes and the Talacre Warren Site of Special Scientific Interest (SSSI) consisting mainly of coastal dunes and associated scrub land.
The sandy beach that stretches seaward from the dunes to the edge of the Welsh Channel is approximately 1 km long.
The Welsh Channel, for navigation serving the port of Mostyn, is approximately 1.4 km wide and a maximum of 8.5-m deep below mean sea level. The channel bed consists of a surface layer of sand approximately 2-m thick overlaying stiff clay.
Beyond the Welsh Channel is the West Hoyle Spit which is 1.2 km wide and dries out at low water. The level at its highest point is +3.0 m above mean sea level. Geological make up of the spit is pure sand. The spit was instrumental in the decision to adopt the post-installation burial technique for the pipelines in preference to predredging and backfilling.
Seaward of the spit at chainage +4,150 m is the termination point of the landfall. At this position, the pulling head of the pipeline was to be laid down for later recovery by McDermott-ETPMs laybarge 1601. The 1601 was then to lay the pipelines out to the Douglas complex.
The water depth at the termination point was shallow, 5 m below mean sea level. McDermott-ETPM therefore dredged a channel along the route of the pipeline from offshore to the laydown position to provide access and adequate clearance below the hull of the laybarge.
Planning; construction
The local planning authority, through consultation with the Countryside Council for Wales (CCW), required that a detailed baseline ecological study of the sand dunes be undertaken. This study was presented, along with other environmental impact information, through an environmental statement.
Planning permission was granted, provided detailed construction working practices within the SSSI were agreed upon in advance with CCW, together with a reinstatement plan and restoration monitoring program.
Site working hours were also strictly controlled, through the use of noise-level criteria, to minimize disturbance to the local population. A temporary noise screen fence was erected around the main construction site (Fig. 3), and noise levels were measured regularly. Noise-level reports were sent to the local and county councils.
The construction work was scheduled to minimize both disturbance to the internationally important wintering wader population and the local summer tourist industry.
Work commenced in early January 1994 on site preparation. The first activity was to strip topsoil and construct temporary access roads, together with requisite drainage for the construction area. Temporary offices were also erected on site.
The next major activity was to construct the temporary pipe-support plinths from reinforced concrete: 53 plinths were constructed at 12-m centers along the length of the stringing site. Each plinth was surfaced with a thin steel plate later greased to aid pipestring handling.
The concrete plinths were to support the 12-m pipes before they were welded into 600-m pipeline strings and to store the strings prior to offshore pulling.
Pipestring fabrication
Weld procedures were qualified according to BS 4515 and BHPs welding specification at the Land & Marine plant yard, Bromborough, and on site.
The pipeline for the Liverpool Bay development would transport natural gas containing a high proportion of hydrogen sulfide (H2S) to the shore terminal.
Charpy impact testing was specified with a test temperature of 30 C., and low-hydrogen weld consumables were selected for all on-site girth welding.
The 12-m joints were delivered to the landfall site with a 100-mm thick reinforced-concrete coating. Linepipe was unloaded and placed directly onto the concrete support plinths.
Linepipe ends had been bevelled at the pipemill and the reinforced concrete coating set back from the ends to allow access for clamp line-up and welding. When sufficient linepipe had been delivered, welding operations commenced according to approved weld-procedure specifications.
Following visual examination, 100% radiographic examination of the welds using internal crawlers was performed at the end of each working shift.
The overall weld-repair rate for the landfall project was less than 0.5%.
Following acceptance of each radiograph, the girth weld and adjacent sections of linepipe were prepared for field-joint coating by wire brushing of the pipe and application of a shrink-wrap sleeve.
The concrete coating was completed by application of a reinforced steel mesh and a thin-sheet steel shutter around the joint and filling of the annulus with concrete.
Each completed pipestring was lifted from the pipe-support plinths, and a polyurethane saddle inserted between the underside of the pipestring and the plinth.
Additional polyurethane saddles were banded to each pipe string at regular intervals. These saddles provided a fixed support for the twin 3-in. condensate lines. Following saddle attachment, the 3-in. pipelines were transferred from storage on the plinths and into the support grooves in the saddles (Fig. 4). Top caps were then bolted to the saddles to secure the 3-in. pipelines.
A half-round steel protection plate was lifted onto the saddles, supported on the saddle caps, and fastened to the pipestring with welded girth straps. The joints between adjacent protection plates were butt welded to provide continuous support along the entire length of the string. The protection plates were fitted to allow the trenching machine to be used.
The fitting of piggyback pipelines to a main pipeline during a pull tie-in is a labor-intensive operation at a critical time. It was therefore imperative to devise a method of transferring pipe strings to the launch line with the piggyback pipelines and the protection plate already fitted.
On previous contracts involving piggyback pipelines, the main pipeline was rolled over the support plinths onto the launch line then the saddles fitted. The piggyback lines were then lifted onto the support saddles and secured.
A new method was developed to skid completed strings across the support plinths on greased plates and onto the launch line to minimize pull-barge downtime during tie-in between strings.
Dune, channel
excavation
During pipestring fabrication, a corridor through the coastal dunes was excavated.
There are several theories on construction treatment for successful restoration of dune areas. After much debate, it was agreed that a combination of measures would be used to aid restoration.
Initially, a minimal cut through the dunes was utilized. This cut varied depending on the height of the dunes, with a maximum width of 60 m. This has led to a ragged edge effect, helping the restoration blend back into the rest of the dune system.
A second measure was to remove vegetation, largely marram grass, in whole clumps and store it in a nursery area where it was kept damp for replanting. Throughout the rest of the dunes area, the top layer of marram was stripped and stockpiled separately for respreading on the final contoured dunes.
Bulk excavation to achieve the desired cross-section and profile for the pipe-pulling corridor through the dunes was undertaken with backhoe excavators, dumptrucks, and bulldozers.
The excavated sand material was stored adjacent to the dunes in stockpiles. These and the slopes of the dune excavation were sprayed with a biodegradable bitumen compound to prevent the material being windblown.
For channel excavation, it was proposed that the pulled pipeline from chainage +1,000 m to +4,150 m would be buried into the seabed using the Land & Marine trenching machine TM4. This was in preference to predredging and backfill operations.
TM4 has two pumps which fluidize granular material in the vicinity of the machine. It also has two dredge pumps which eject the fluidized granular material out of the trench as the machine traverses the pipeline, thereby burying the pipeline into the seabed.
The TM4 will not operate in cohesive materials with a strength greater than 10 kPa, such as clay. Its tubular jetting frame projects some 2 m below the rollers which run on the top of the pipeline.
It would therefore act as a bulldozer if it were unable to fluidize the seabed material in front while operating along the pipeline. There must therefore be granular material present from original seabed level to 1.5 m below the final required bottom of pipe profile.
Across the Welsh Channel, the specified depth of cover for the pipeline was 2 m. There was therefore a requirement for the bottom of the machine frame to traverse the Welsh Channel some 3.5 m below original seabed level during the final passes of the trenching machine.
From soils data, Land & Marine was aware of a stiff clay layer some 2 m below the seabed. A dredging contractor was employed to excavate a trench 3.5 m deep by 6 m wide across the channel to prove the trench prior to installation of the pipeline.
The intention was that, once excavated, this trench would be allowed to infill naturally by sediments present in the Welsh Channel.
Wire laying
As stated, the method of pipeline installation was to be bottom pull. The pipestrings were constructed between chainage 790 m and 190 m, and the termination point for the pulling head was at chainage +4,150 m, hence a pulling length of 4,340 m.
The pulling was to be achieved by mooring the pulling barge LMBalder at chainage +5,500 m and placing four 22.5-mt Flipper Delta anchors on the seabed seaward of the LMBalder at chainage +7,500 m. The anchors would resist the 800 mt pulling force to be applied by the linear winches aboard the LMBalder.
A pontoon unit was installed between the anchors and the barge. It was to provide an interface between the static 89-mm diameter wires from the anchors and the four lanes of 64-mm diameter running wires from the 200-mt capacity linear winches on the LMBalder.
The pontoon unit was fitted with sheave blocks to provide the required mechanical advantage.
Twin 89-mm wires were laid from the beach to the LMBalder. This was achieved by towing a 64-mm diameter wire from the LMBalder with a tug. The operation was performed at high water to ensure sufficient water beneath the tug while it steamed over West Hoyle Spit and into the Welsh Channel.
A second 64-mm wire was towed out from the beach and made fast to the first 64-mm wire on the stern of the tug.
A pontoon unit was connected to the 64-mm wire on the beach. On the shoreward side of the pontoon, a pair of 89-mm wires was connected to a triangular link plate.
One of the Balder winches then pulled the pontoon out towards the LMBalder. Care was taken to ensure that the twin 89-mm wires were maintained on the pull centerline throughout the wirelaying operation.
The wires were kept on line by positioning a small tug alongside the pontoon which was able to push or pull the pontoon. The surveyor on the beach kept in radio contact with the skipper on the small tug.
When the pontoon reached the stern of the barge, the two eyes of the 89-mm wires were taken aboard the LMBalder and secured to a spine beam on the deck.
Pipe pulling
Before wire laying began, roller conveyors were laid out along the launch line from the rear of the stringing site, across the Warren, through the dune cut, and down the beach to the low water mark.
The pulling head was welded to the front of String No. 1. This was prelaunched along the conveyors with several backhoe excavators pulling on wire strops attached to the pipestring. Great care was taken to maintain the piggy back lines and cover plate in the 12 oclock position.
The prelaunch operation stopped when the tail end of the string reached a position in line with the front end of String No. 2. This string was skidded across the plinths, onto the launch line, and pulled forward with a small winch, aligned, and welded. The joints were completed and the cover plate fitted.
Twin buoyancy tanks were fitted to pipe Strings No. 1 and 2 at regular intervals along their length. The uplift provided by these tanks maintained the required pulling capacity below the available 800 mt. The tanks were attached with a system of saddles, wires, and turnbuckles.
The shoreward two free ends of the 89-mm wires were fed around a sheave in the pipe pulling head and joined with a torpedo shackle.
Pipe Strings No. 1 and 2 were now ready to be pulled by LMBalder. The barge was slowly winched seaward and the winches tensioned up the twin 89-mm wires until the pipeline on the beach began to move.
Pulling continued until the end of String No. 2 arrived adjacent to the front of String No. 3.
Several backhoes tracked along with the moving pipeline, ready to apply eccentric loading on the pipestring to counteract any tendency for it to rotate.
The pulling stopped for the tie-in of String No. 2 to No. 3. Meanwhile, the barge slacked off on the 64-mm pull wires and at the same time warped its way shoreward taking aboard some 600-m of the twin 89-mm wires.
The joining shackles were opened, these 89-mm wires were removed from the system, and the shoreward eyes secured aboard LMBalder. Buoyancy tanks were fixed to the pipe string on the beach, and when the tie-in was completed the pulling resumed.
The pulling progressed for 6 days with the marine and land-based spreads working two shifts for each 24-hr period (Fig. 5).
On completion of the pull, the buoyancy tanks were removed from the pipeline by divers. The recovered buoys were taken onto the beach and handed over to a shore crew.
The pipeline was flooded to provide additional stability and to assist the trenching operations.
Trenching
The usual method of providing required depth of cover to installed pipelines at landfall locations is to predredge a trench, then backfill over the pipelines on completion of the pull.
Dredging a trench for the landfall over the spit, however, would have involved removing massive quantities of material. Opening such a wide channel for dredger accessibility would be cause for considerable environmental concern.
Land & Marine offered an alternative to BHP: use of the post-installation trenching machine TM4 which would follow the profile of the existing seabed and bury the pipeline to the required depth of cover. The volume of material to be moved during this operation would be minuscule when compared with the dredging option.
This alternative was accepted by BHP, and the trenching machine TM4 was mobilized to the barge LMBalder.
LMBalder moved into the Welsh Channel and was set up for trenching operations. A 100-mt capstan winch was positioned and anchored in the dune area. A 52-mm wire was connected from this winch to the TM4 on the LMBalder.
The trenching machine was deployed over the pipeline by the LMBalder crane. It was jetted onto the pipeline by operating the fluidizing and dredge pumps.
Once the trenching machine was positioned on the pipe, the bottom rollers were engaged. While the machine jetted and dredged the granular material from around the pipeline, the shore winch pulled the TM4 towards the beach on the rising tide.
The machines speed of travel was approximately 2 m/min, which controlled the rate of embedment of the pipeline into the seabed.
Once the machine reached its required inshore transition, its direction of travel was reversed, and a second pass was made with the falling tide, the LMBalder providing the pull force.
Operations continued 24 hr/day, yo-yoing the TM4 along the pipeline. The depth of cover was regularly monitored by an echo sounder aboard the survey vessel.
Once this section of the work was complete, the wire from the capstan winch was disconnected from the TM4. It was recovered from the pipeline onto the deck of the LMBalder, and the vessel turned around in the Welsh Channel.
A second pull barge, LMNjord was mobilized offshore the West Hoyle Spit. It was anchored and a pull wire run out across the spit and connected to the TM4 aboard the LMBalder.
Trenching operations resumed using the winch aboard the LMNjord to pull the trenching machine towards the peak of the spit from the Welsh Channel. The now familiar yo-yoing operation was repeated until the pipeline achieved the specified depth of cover.
The positions of the LMNjord and the LMBalder were then reversed and the pipeline offshore the West Hoyle Spit was trenched.
All sections of the trench formed during the trenching operations were allowed to infill naturally.
Concurrent with the first section of trenching with the TM4, a fleet of backhoes and bulldozers worked on the beach to lower the pipeline to attain the required depth of cover. This was achieved by excavating along both sides of and beneath the pipeline simultaneously as the backhoes traversed the beach following the tide.
The pipeline lowering operation was regularly monitored using land survey techniques to ensure that the pipeline did not become overstressed. The section of pipeline through the dunes was lowered by this same method.
Reinstatement
Following successful survey of the lowered pipeline, the trench formed during this operation was backfilled with bulldozers and the beach compacted to allow full access to the general public.
The dune cut was then infilled by returning the sand from the stockpiles. The finished surface of the dunes was landscaped to resemble the original contours as closely as possible.
On the beach, an inland gravel source was identified which matched the beach gravel and a small ridge was deposited to prevent erosion.
Storing marram grass in the nursery area proved largely unsuccessful because of the crumbling nature of the sand; the clumps would not hold together. The stored top layer of marram, however, was laid over the contoured dunes in the frontal dunes area. This layer has successfully re-established itself.
The high points of the dunes were covered with a biodegradable jute mesh to help stabilization, and the whole area replanted with marram taken from outside the working area but within the planning approval area.
Detailed monitoring of the reinstatement will be ongoing for several years to come.
Acknowledgments
Thanks are due BHP Petroleum and McDermott-ETPM for their permission to publish this article and Land & Marine Engineering Ltd. (a wholly owned subsidiary of the Costain Group) for its support. Thanks are also due Sally Stevenson, BHP Petroleum environmental officer, and Henry Scolley, Land & Marine chief civil engineer, for helpful reviews and advice. n
Noise screens were erected around much of the pipe string makeup site (Fig. 3).
Two 3-in. condensate pipelines were piggybacked onto the 20-in. gas pipeline (Fig. 4).
The pipeline is pulled by LMBalder. Point of Ayr lies in the background (Fig. 5).
The Author
Michael R. Smith is operations manager in the construction department of Land & Marine Engineering Ltd., a wholly owned subsidiary of the Costain Group. He has 21 years experience with Land & Marine in project management of submarine pipeline landfalls and outfalls in 11 countries. He is a chartered engineer and a member of the Institution of Civil Engineers. Copyright 1995 Oil & Gas Journal. All Rights Reserved.