PIPELINES Flexible/rigid pipelay system operable in waters below 2,000 meters

Schematic of the Fennica with modular flexible/rigid pipelay system (inset). Over the next few years exploitation of new oil and gas fields in existing offshore locations and deeper or remote areas will be increasingly accomplished through floating production systems and subsea developments.

Feb 1st, 1997

Over the next few years exploitation of new oil and gas fields in existing offshore locations and deeper or remote areas will be increasingly accomplished through floating production systems and subsea developments.

This will result in a proliferation of oil, gas and water injection flowlines combined with service lines and control umbilicals. In combination, the lengths of pipeline/flowline systems and allied flexible/umbilical and riser systems amount to a substantial expansion of this part of the subsea construction industry.

In response to this development, DSND has awarded a design and build contract to Norson Power of Glasgow, UK, for a new integrated flexible and rigid pipelay system. This modular system will be capable of installation on numerous offshore construction vessels and therefore has the potential for use worldwide.

The system will be designed to install the following range of flowlines/riser/umbilicals in water depths exceeding 2,000 meters for the smaller diameter lines:

  • Flexible pipeline and risers - 4-19-in.
  • Rigid pipelines - 2-12-in.
  • Piggyback lines/umbilicals - 3-6-in.

    Components

    The chief items of equipment are a fully featured pipelay ramp located at the stern of the construction vessel and pipe storage reels located midship on the deck. For laying flexible pipelines and risers the ramp will normally be vertical and the flexibles will be deployed from under the main reel, up and over the top chute and down through the pipe tensioner into the sea.

    In the case of rigid pipe the ramp angle may be as shallow as 20° to provide a satisfactory arrangement for spooling pipe onto the vessel, but will then be elevated from, say, 35° in shallow water up to 90° in deepwater to facilitate pipelay with the correct tension and catenary.

    With an angle close to 90°, the system will effectively be performing J-lay and using this technique, installation of steel pipe in up to 2,000 meters or more of water will be possible, depending on the diameter, wall thickness and grade of pipe.

    With respect to flexibles, most flowlines and risers will come into the 3-1/2-in. ID category at a variety of operating pressures. However, the advent of much larger diameter flexibles for export risers on planned floating production developments has also been taken into account by DSND.

    There are a number of innovative features in the system that should promote more efficient and safer working practice. One is the auto-leveling work platform which permits easy transfer of essential pipelay ancillary equipment, such as pig launchers/receivers, pipe anodes, buoyancy modules and other items ranging typically from 0-5-5 tons in weight from the deck into the sheltered habitat on the ramp.

    A key aspect of the system design is the use of very high tensile steel (E=700N/mm2). Only by this means has it been possible to achieve a transverse fleeting ramp, with full reel utilization and no lateral bending restrictions; the ability to lay all common steel flowline sizes and extra-large flexibles; and to accommodate all the sophisticated features on the ramp, without compromising the support vessel's stability. No other pipelay system has eliminated all these problems in such a compact and weight-efficient design.

    The initial support vessel will be the dynamically-positioned MSV Fennica. The pipelay ramp will simply sit on the stern of the vessel and be attached to the two existing A frame foundations and two new anchorages at the forward end of the base grillage.

    Power, controls

    The 550kW containerized hydraulic power packs and control cabin will sit on the base grillage and the only interfaces with the vessel will be electrical power for the prime movers, seawater for cooling and video communications and data transfer cables from the control cabin to the vessel bridge. The modular nature will permit rapid mobilization/demobilization (within two to three days).

    MSV Fennica's high thrust and excellent station-keeping, combined with its fully redundant (NMD-Class 3) DP system, will enable pipelay to be carried out in conditions of high lateral wind and current loads: fore and aft loads are insignificant in relation to the bollard pull of around 240 tonnes. Pipelay or umbilical and cable lay could even be performed in icebound conditions during the winter in northern latitudes, by virtue of the vessel also being an icebreaker.

    Without compromising the Fennica's intact and damage loading/stability criteria it will be possible to transport and lay 1,000 tons of pipe on the main reel along with 250 tonnes of pipe or umbilical on the piggyback reel. In real terms this equates to approximately 20 km of six-in. pipe, 15 km of eight-in. pipe, 11 km of 10-in. and 6.5 km of 12-in. pipe. Despite this payload there is still sufficient reserve stability to accommodate 150 tonnes of pipelaying equipment including single or dual work class ROV(s) for pipelay support and/or diverless tie-in after pipelay.

    Norson's £5 million contract was awarded last September and the system, to be built to DNV rules, will undergo final acceptance testing on the Fennica and handover to DSND this spring. A first contract has already been secured, to abandon Lasmo's Staffa Phase II 9.5 km, eight-in. carbon steel line in the UK North Sea.

    Copyright 1997 Offshore. All Rights Reserved.

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