Laying a gas pipeline across the V-shaped cross-section of the typical Norwegian fjord is far from easy. Most available options suffer from serious drawbacks. Faced with a number of such crossings on its contract to lay 30km of 1m diameter pipeline across the rugged terrain of south westNorway, contractor Per Aaslef looked at new ways of dealing with fjords up to 84m deep.
The pipeline is part of the massive £6876M Asgard Transport scheme for Norwegian state-owned oil and gas producer Statoil, which will connect the Asgard offshore gas field in the North Sea with Germany. Aaslef is part of the German/Norwegian jv Kaarstoe Pipeline Contractors, and it has dubbed the new technique it has developed 'off-the-bottom pulling'.
The contractor uses the same basic set-up at each fjord launch location. Several cast in situ concrete beams are constructed in a series of long traversing steps from near the waterline and up the sloping shore, which at one location has a 15degrees gradient. Each section of pipe, average length just over 12m, is delivered to the stepped ramp and welded together into strings of four to six pipes weighing up to 130t.
Once welds are checked and joints sleeved, a final infill coating is applied. In the shoreline splash zone locations the pipe is covered with a special polychloriprene coating. Other areas are coated with polyurethane and the entire line is covered with ballasting concrete which varies in thickness from 44mm to 100mm, depending on location on the seabed.
The first string is rolled across the sloping ramp onto a series of air bags located in line with a set of launching support guide rollers. A large hold back bracket is welded to the rear of the string and connected by a two fall wire rope to a holdback winch higher up the shore. A similar pulling head is welded to the front of the airtight string. This is connected to the main 77mm diameter pull winch rope which is trailed across the fjord to the opposite shore, through a 300t test pull gripper-type jack linear winch and onto a large diameter storage drum. The single line pull winch is freely supported by large cables looped over a fixed sheave anchored into the bedrock.
The holding winch takes the strain and holds the pipe which is then lowered onto the guide rollers by deflating the air bags. The restraining winch is fitted with a rope tension indicator, providing the operator with a constant readout of the holding load. The main winch on the opposite shore is activated, tensioning the pipe as it overcomes the restraining load. As this first string is pulled into the fjord, pressurised buoyancy tanks are attached to the top of the pipe with web straps. Ballasting and stabilising chains are shackled to the straps beneath the pipe. Tanks and chains are added until the launch is complete and the load released from the pull winch.
After pulling, the pipe is temporarily held by a rubber-coated friction clamp bolted round the outside of the pipe. This is allowed to butt up and rest against a buffer frame anchored into the shoreline rock bed. The backhead is cut off the pulled pipe and the second string is rolled across. The backhead is re-attached to the rear of the second pipe which is then positioned and butt welded to the first. The welded joint is heat sealed, coated in concrete as before and buoyancy tanks fixed to the top of this pipe string before pulling. The load on the friction clamp is released by pulling back on the holding winch, and removed from the pipe ready for the pull to continue. Ballasting chains are attached to the buoyancy tanks as before.
This operation is repeated until the entire crossing is complete. Spacing and combination of the tanks and chains has been calculated so that the chains just drag along the bottom, keeping the pipe string about 4m to 5m off the bed of the fjord. Holdback and pulling winches are also balanced during pulling to adjust the tension in the pipe string and avoid touchdown and to prevent the pull-head nose diving into the seabed. Pulling force is comparatively small, just enough to overcome the resistance of the chains dragging along the bottom. It increases during the last pull to overcome friction as the string is winched up the shore to mate with the land-based section.
When pulling and subsequent welding to the land line are completed the RUV is sent down to cut the webbing straps and release the tanks and chains which float to the surface for collection and re-use. The ballasted pipeline slowly sinks to its final position on the seabed and any long hollows are backfilled.
To design, build, test and learn how to use the off the bottom equipment and technique in a comparatively short time has been very demanding, but successful, says Per Aarsleff construction manager Hans Havbro. 'Working two shifts round the clock we are able complete a cycle of assembling a pipe string and pulling it on average every 30 hours with the actual pulling sequence only taking about an hour.'
Per Aarsleff worked very closely with consulting engineer Cowi Consult to produce a working sequence. Cowi developed a computer model, using the actual weight of the pipes provided by the site, which determined the correct spacing of the buoyancy tanks and amount of stabilising ballast needed. Cowi is able to analyse the system for tension and corresponding distances from the seabed at all stages of the pull and advise Aarsleff to adjust the sequence as necessary.
Aarsleff used the same system on its first three crossings. But on the widest and deepest dog leg crossing of Karmsund, between Karmoy and Fosen, the team adapted the successful technique and split the operation into two separate shorter pulls. For this they used a specially prepared pontoon anchored at the top of a short peninsula at Hogevarde on Karmoy where the pipeline changes direction.
The flat deck pontoon is equipped with the holdback winch, friction brake, pipe string welding and assembly equipment together with the buoyancy tanks and ballasting chains transferred from the previous shore based operations.