DESIGN, TECHNIQUES BRING BIG LINE ON TO CONGESTED SHOPE

Keith Shotbolt
Bechtel Ltd.
London

The shore approach for one of the world's largest offshore gas pipelines was successfully achieved in an area congested with other pipelines and mooring berths.

The 46-in. line from Umm Shaif field to Das Island in the Arabian Gulf off Abu Dhabi (Fig. 1)(81796 bytes) was designed in 1992 and installed in 1993 (OGJ, Apr. 19,1993, p. 17).

Trenching, pull-in, and safe stabilization of the line's shore approach used submersible tracked trench-cutters and suction dredger; a lay-barge winch pulling around a shoreline pulley-block and back to the barge ramp; and large "saddle-bag' shaped cast in situ grout bags over the entire 800 m length.

UMM SHAIF DEVELOPMENT

The Abu Dhabi Marine Operating Co. (ADMA-OPCO) Umm Shaif gas-development seeks to increase the gas-production capacity of the Umm Shaif field to meet the extra demand for the third LNG train on Das Island.

Several subsea pipelines already deliver oil and gas from Umm Shaif, Abu Al Bokoosh, and Zakum fields, and there are major oil, LNG, and sulfur loading facilities on the eastern side of the island making access very difficult.

The engineering, survey, route selection, trench excavation, shore pull, stabilization, and shoreline tie-in phases were successfully completed in 1992-93.

Preliminary engineering work recommended that the pipeline should be 46 in. OD and should be trenched to provide 0.6-m cover in nearshore water depths to 6 m lowest astronomical tide (LAT), corresponding to a distance of 400 m from shoreline.

Further hydraulic analysis confirmed this pipe size is required to meet the target production rate with the inlet and outlet pressure constraints set by existing process facilities at Umm Shaif and Das Island.

A detailed engineering study was required to show that installation of a 46-in. line was feasible with respect to on-bottom stability and

pipelay stresses over the range of environmental conditions along the entire route.

Review of admiralty charts indicated a possible northern alternative to the proposed route, which would avoid careful anchor handling along the existing pipeline corridor that contained four main lines.

An extra 200-m length over the 34.5 km proposed route was more than compensated for by the improved pipelay rate associated with less critical anchor handling; maximum water depth for both options was 31 m LAT.

Approach access to the new gas-treatment platform was improved by use of the northern route. Maximum deviation between the two options was 5 km with similar seabed soil conditions, and the approach to Das Island was similar over the final kilometer.

Water depth, seabed soil, wave, and current data including angles of incidence along the selected northern route allowed calculation of required concrete weight coating to provide lateral stability according to an accepted recommended practice.1

Lay stress analysis following review of available laybarge, stinger, and tensioner capacities indicated that a 46-in. OD x 0.938-in. W.T. line from Grade X-60 steel with the required submerged weights (up to 5-in. concrete coating) in various depths could be safely installed.

Detail design of the shore approach could now proceed using 46-in. OD pipe.

LANDFALL

Land reclamation for a shoreline road at the east side of the island has resulted in a rapid change in elevation from a nearshore water depth of -2.0 m to +6.0 m at the top of a seawall. A drainage outfall from onshore process facilities is located only 8 m north of concrete protection over the existing 30-in. main gas line.

It was agreed that the tie-in spool to bring the 46-in. line from below seabed to aboveground ashore would bisect the 5-m gap between the southern corner of the seawall and the northern corner of the drainage outfall (Fig. 2)(76772 bytes).

The quadrant between the outfall and the seawall was filled with a rock-armored 30 slope which formed an approximate 20-m radius at seabed level reducing to 0 radius at the +6 m level.

Rocks on this sea defense slope averaged I cu m in size, and the tie-in spool was designed to be set below the 30 slope with 1 m of cover to top of pipe.

Onshore and offshore surveys were performed in spring 1992 to allow preparation of detailed drawings of the pipeline route. Soil borings were taken at the top of the slope and at 100-m intervals along the proposed nearshore route.

The result of the boreholes (BH-1; Fig. 2)(76772 bytes) showed sand and gravel fill with occasional boulders (up to 300-mm grain size) and density from 1.9 to 2.1 Te/cu m.

The water table at 4-6 m below ground level coincided with sea level which had maximum variation up to +2 m LAT. Soil down to 7.5 m belowground (at elevation +6.4 m) was similar and assumed by the surveyors to be fill.

A subsea core taken at DSA-01 location showed weak calcirudite with strength of 1-2 MPa down to pipeline trenching depth, increasing to 6 MPa at 6 m below seabed. Cores DSA-02 to DSA-05 at 480 in from shore showed similar low strength carbonate rock with strength up to 10 Mpa.

At 200 in from shore, however, a surface layer of coral limestone with average thickness 0.4 in was overlying loose sand and gravel down to trench depth of 2 m.

CONTRACTORS' INFLUENCE

In 1991, the trenching and pipelay contractors had installed a 36-in. oil line approach to a gradually shelving beach 250 m further south. This trench was specified as 2 in wide x 1.5 in deep x 400 m long.

In addition to his bid to excavate a new trench with 2.3 m width and 2 m minimum depth but similar length, the trenching contractor offered an alternative to extend the trench to a maximum 22 in water depth at bottom. Trench length would increase to 800 m and the pipe length to be pulled along it would also increase.

The pipelay contractor requested the trench to be as straight as possible to ease pull-in, which resulted in a straight length of 750 in but gave only 25-m clearance from the southern dolphin of the existing LNG jetty.

Pipelay would continue on a 1,700-m radius through a 15 arc to the 72.5 bearing of the selected route and would pass the existing 30-in. gas line with a minimum separation of 25 in at 1.2 km from shore.

Consideration of lateral stability of the pipeline between the 6-m LAT contour at 400 in from shore to 22 m at 800 m from shore resulted in an initial preference for post-lay stabilization mattresses rather than increased concrete weight coating. The latter was less practical due to excessive thickness and very high pipe joint weight.

During the course of bidding for the trench-excavation contract to 6 m depth, the trenching contractor proposed an extra cost to extend trenching into 22 in water. This would eliminate concern over lateral stability but would still require attention to vertical stability after back-filling.

The trenching contractor's offer to double the trench length, although more expensive than stabilizing mattresses above seabed, was accepted because of increased safety, increased protection from propeller cash created by ships moving to the LNG jetty, increased draught for shipping, and elimination of the possibility of disturbance by dragging anchors.

TRENCH PROFILE

The design trench profile is shown in Fig. 3 (59011 bytes).

A combined stress calculation showed that remaining within 90% specified minimum yield strength (SMYS; 54,000 psi) dictated the minimum vertical plane bend radius should be 1,250 m.

For vertical stability, coated-pipe specific gravity should be a minimum of 1.60 for the pipe to resist flotation. This would require 170-mm thickness of concrete and give a submerged weight of 1,032 kg/m.

Pull force (PF) can be calculated by F1 + F2 + F3 + F4; where:

F1 = WPS X fsb x Lsb

F2 = Wps x fst x Lst

F3 = Wpa X flb x Llb

F4 = Hold-back tension

where:

NS = Submerged weight of pipe

IN = pa Weight of pipe in air

fsb = Seabed longitudinal friction coefficient (assume 1.0)

fst = Stinger roller friction coefficient (assume 0.25)

flb = Lay-barge roller friction (assume 0.25)

Lsb = Length of pipe on seabed

Lst = Length of pipe on stinger

Llb = Length of pipe on lay-barge

The most significant of these forces is F1, which with a proposed trench length of 800 m and pipe having 170 mm concrete thickness will be (1,032 x 1.0 x 800)/1,000 = 826 Te. Pull force could be greatly reduced by applying less concrete and by adding post-lay stabilization in the form of concrete mattresses.

Discussion with the pipelay contractor resulted in a decision to keep pull force at less than 100 Te due to a winch force limitation of 125 Te. Pipe submerged weight during the pull would need to be less than 120 kg/m. Design concrete thickness was reduced to 60 mm, with 75 mm for anode joints, which were installed one in every four joints.

Although a shore-mounted pull winch was used for the 36-in. pipeline in 1991, the rapid change in elevation at shoreline made this option unattractive.

Alternatives all required installation of a seabed pile and pulley block with a wire extending back to a winch positioned either on the laybarge deck, on top of the pile, or on the deck of a jack up platform near the shoreline.

Although the second and third have potential to reduce the lateral force applied to the pile, the pipelay contractor decided to use the first option on the basis of minimum cost, ample space for mounting the winch and cable reel, full control of pull and holdback tension at one location, and confidence that a drilled and cemented pile would resist the force of up to 250 Te which may be applied by the 125 Te winch acting around a pulley block (FIG. 4)(58338 bytes).

TRENCHING, INSTALLATION

Design dimensions of the trench plus an allowance for

seabed undulations meant that more than 3,500 cu m (6,500 Te) of soil needed to be excavated before the shore pull could begin. The trenching contractor used his crawler-mounted cutting and suction dredges which are guided by a taut wire.

Severe weather and wave action caused natural back-filling over the 3-month trench cutting period; the trench then needed to be redredged.

Before final clearance, the pipelay contractor drilled and cemented a 42-in. OD x I-in. W.T. 12-m long pile into the bottom of the trench. The upper 1.3 rn of this pile was 1.5-in. WT. and incorporated a pad-eye for attaching the 300 Te-rated pulley block.

Trench clearance along the entire 800 m length was completed and ready for the shore-pull operation to begin on May 4,1993.

The barge was set so that the maximum length of pipe to be pulled over seabed was approximately 600 m. Pipe welding used a semiautomatic process and initial setting-up problems caused some delays.

Every two or three joints, 3-Te buoyancy air bags were positioned, but divers reported that some attachments were breaking because of wave action.

By early morning on May 7, however, the pull head reached final location near the pile, needing a maximum pull force of only 68 Te. Diver inspection confirmed that the pipe was lying close to center of the trench as required (Fig. 5).

The lay-barge was then able to lay away from the jetty area and allow tanker loading immediately following the annual plant maintenance 2-week shutdown.

STABILIZATION, TIE-IN

At the design stage, precast, linked-block concrete mattresses were planned to be lowered over the pipe to bring the specific gravity of combined pipe plus mattresses to the required value of 1.60 to provide vertical stability.

The installation contractor, given the option to propose alternatives, preferred to add the required weight using special cast in situ grout bags. These specially designed bags were 5 m long each and had a volume of 2.0 cu m/m length.

When the bags were filled with neat cement grout with density of 1,940 kg/cu m, the assembly's specific gravity of mattress plus pipe could be calculated: [(vol x sp gr)mattress + (vol x sp gr)pipe]/vol mattress + vol pipe = [(2.0 x 1.94) + (1.39 x 1.1)]/3.39 = 1.596.

Where necessary, natural backfill sand was air-lifted out over a 30-50 ra length of trench and the "saddle" bags were placed and filled prior to further air-lifting. The operation to inject more than 3,000 Te of cement grout went smoothly and resulted in improved protection for the trenched pipeline.

Shoreline transition spool to pipeline-end connection was to be flanged, but before tie-in could begin the entire pipeline was flooded with seawater containing corrosion inhibitor, biocide, and leak-detection dye.

A train of two brush pigs and one gauging pig with intermediate 200-m long slugs of cleaning water were collected in the pull head. After removal of the head, the transition spool comprising the lower 60 "rolled" elbow and the 30 angled leg up to the +6-m level was lowered into position.

The 46-in. flange connection to the pipeline used a pressure-energized Laurent-type wedge seal which would allow up to 0.4 misalignment. Hydraulic tensioners preloaded 32 2.25-in. universal coarse threadform (UNC) flange studs to 40 Te.

A brush pig, with inlet/vent ports added to allow fore and aft communication, was installed at the top of the transition spool to allow filling of the spool with inhibited water up to the pig.

After final tie-in welds to the onshore line, the pig was pushed through the 400-m long shore section of the line to the pig receiver. Movement was obtained by more inhibited water being pumped into the line at the offshore riser, and the pig brush-cleaned and flooded the onshore pipe as it traveled.

Early in 1994, hydrotesting of the entire line, which included offshore platform pig launcher and five more subsea flanges, was successfully completed at 140 barg (2,030 psi). The line was dewatered, caliper-pigged, vacuum dried, and packed with nitrogen to 0.5 barg awaiting production gas.

The rock-armored slope required more excavation than planned to allow near-shoreline access for the underwater trench cutting and dredging machines.

Saddle-type grout bags used to stabilize the below-seabed section of the line were modified to allow fitting and grouting over the angled (30) leg of the shoreline spoolpiece.

These bags provided good protection to the coated pipe during installation of extra rocks and stabits to improve sea defenses around the 46-in. line landfall.

Major contractors on the project were, for survey and trenching, Subtec, Sharjah, U.A.E.; for pipeline installation, National Petroleum Construction Co. (NPCC), Abu Dhabi, U.A.E.; for stabilization-mattress design, ULO, Switzerland, and for 46-in. class 600 flange design, Taper-lok, Houston.

ACKNOWLEDGMENTS

Thanks are due to Bechtel which encouraged preparation of this article and to ADMA-OPCO and Abu Dhabi National Oil Co. which granted permission to publish this article.