ZEEPIPE CONSTRUCTION PROGRESSES TOWARD 1993 START-UP

Kai Killerud, Leif Solberg
Statoil
Stavanger

First-phase construction on the longest and largest pipeline ever laid in the North Sea will peak this year with completion of pipelaying and tie-ins.

Phase I of the Zeepipe transportation system (ZTS) targets full gas delivery to the Belgian port of Zeebrugge on Oct. 1, 1993. Start-up and partial delivery of first gas to the terminal will occur somewhat earlier, based on current negotiations with buyers.

More than 390 miles of 30 and 40-in. pipeline were laid between Apr. 12 and Sept. 16, 1991. This work included 0.93 miles of 40-in. at the Sleipner field, 366.8 miles of 40-in. between Sleipner and Zeebrugge, and 24.2 miles of 30-in. to connect Zeepipe at Sleipner to the Statpipe system at the nearby 16/11-S platform (Fig. 1).

Pipelay contractors are currently at work on 143 miles of 20-in. line to connect Sleipner to Karsto and on the final leg between Sleipner and Zeebrugge, 135.4 miles of 40 in.

When pipelaying operations are completed this year, 670 miles of 20, 30, and 40-in. pipeline will have been laid since April 1991.

Total original (1987) cost estimate for the entire project was put at $2.2 billion (OGJ, June 27, 1988, p. 54). Phase 1 is expected to cost $1.9 billion.

The Sleipner A platform delay (OGJ, Sept. 2, 1991, pp. 30 and 49) has not significantly influenced the project because the system will be ready as planned to deliver contractual gas volumes. Other gas, however, may have to substitute for Sleipner gas during initial months.

A separate riser platform will be installed in the Sleipner area by the Sleipner group to provide for a starting and tie-in point for Zeepipe to accommodate the changed plans.

Last year's work overcame significant engineering and construction obstacles. Chief among these were construction of the landfall at Zeebrugge, pipelaying through severe sandwave areas in the southern North Sea, and the crossing of the Scheur Channel near Zeebrugge. Fig. 2 shows the route profile from Sleipner to Zeebrugge.

NORWEGIAN GAS TO EUROPE

The development plan for Zeepipe was approved by the Norwegian parliament in 1986. The project organization, Zeepipe Development Project (ZDP), under operator Den norske stats oljeselskap AS, was formed in late 1988 and charged with building Phase 1 to transport natural gas from Sleipner to Belgium.

The project organization is further responsible for the offshore part of the 143-mile, 20-in. Sleipner-to-Karsto rich-gas and condensate pipeline.

The Zeepipe group is a joint venture consisting of the Troll and Sleipner owners.

Statoil is operator for the group and responsible for construction and operation of Zeepipe.

Owners are Statoil (70%), Norsk Hydro (8%), Shell (7%), Esso (6%), Elf (3.3%), Saga (3%), Conoco (1.7%), and Total (1%).

Snamprogetti (Fano, Italy) is the engineering contractor, and European Marine Contractors Ltd. (EMC), London, is the main offshore construction contractor of the project.

ZTS was conceived as part of an historic package consisting of Troll Phase I and Sleipner East field developments in addition to Zeepipe.

Zeepipe, together with the Statpipe-Norpipe chain (Fig. 1), was to make transportation capacity available for gas under the Troll gas-sales agreement (TGSA).

The Norpipe gas pipeline, which formed the basis for this historical package, originates at the Ekofisk field and delivers dry gas to the Emden (Germany) terminal. Norpipe is owned by the Ekofisk owners and Statoil (50/50) through Norpipe A/S as the formal operator with Phillips performing the operations.

The Statpipe system has several sections. A dense-phase pipeline transports rich gas from Statfjord to a terminal at Karsto (north of Stavanger).

At Karsto, the rich gas is separated into dry gas and heavier fractions. The latter are fractioned into commercial LPG products shipped by tankers. The dry gas is exported via the riser platform 16/11-S to Ekofisk.

At 16/11-S, dry gas from Heimdal also arrives to be joined with the gas from Karsto.

Statpipe is owned by a joint venture consisting of the major owners of Statfjord, Heimdal, and Gullfaks field. It was primarily designed to serve these fields. Statoil operates Statpipe.

In 1986, the TGSA called for firm sales of about 635.4 bcf/year at plateau level and a total contract period of about 30 years.

In 1987, this volume was increased to about 706 bcf/year. Further, the TGSA included options (30% and 50%) amounting to about 406 bcf/year at plateau level. The 30% options could be taken by the buyers before 1993, and the 50% options are to be taken before 1995.

ZEEPIPE'S ROLE, PHASES

The demands on Zeepipe may be summarized as follows:

together with the second chain, Statpipe-Norpipe facilities, the system should provide capacity for the TGSA volumes including part of the options.
A third transportation system was envisioned should a major part of the optional volumes be taken.
At present all 30% options have been taken, and significant additional volumes have been sold under Troll gas-sales agreements, giving total volumes of about 1,182.6 bcf/year.
Consequently, implementation of the third system, Europipe (Fig. 1), is under way.
Zeepipe should accommodate the gas from Sleipner East under the TGSA from the start of Oct. 1, 1993, and then from Troll starting 3 years later.
Zeepipe should provide an interconnection between the initial two transportation chains, Zeepipe itself and Statpipe/Norpipe, to provide flexibility.
Zeepipe should be designed for easy connection to Frigg for potential export to the U.K. market, utilizing the Frigg pipelines).
Zeepipe should provide a landing point for the gas, which could give strategic advantages considering the landing point of the existing chain (Emden, Germany) and the different buyers under the TGSA and potential new buyers.

Buyers under TGSA include the major German, Dutch, Belgian, and French gas companies. Later gas has been sold to Spain and Austria and to another Dutch buyer (SEP). Thus both mid and western continental buyers are represented.

The concept to satisfy these demands on the system evolved into a three-phase development plan (Fig. 1).

Zeepipe Phase I will connect Sleipner East with Zeepipe terminal at Zeebrugge, and with Statpipe at the riser platform 16/11-5. This will enable gas export from Sleipner East to both Zeebrugge and Emden from 1993.
Zeepipe Phase 2 (two legs) will connect the Troll processing plant onshore at Kollsnes to Sleipner and Heimdal. The first leg must be ready for gas export to both Emden and Zeebrugge in 1996. The other leg will be installed to be ready when required, probably 1 year later.
Zeepipe Phase 3 consists of compression capacity to be installed as and when required, at one or more of the following points: Sleipner East field, subsea connection point (SCP) at the hydraulic mid-point, or Zeebrugge terminal.

Without compression, Zeepipe will have a capacity of about 423.6 bcf/year and with maximum compression about 670.7 bcf/year.

The SCP provides for a connection point for compression capacity at the hydraulic mid-point. The original concept assumed an unmanned riser platform for this provision.

Substituting the SCP for the unmanned riser platform yielded significant savings; Fig. 3 provides a rendering of the SCP.

For additional flexibility, a hot tap T-connection has been installed in a location to suit a branch-off to the Bacton area in U.K.

Today the daily flow in Statpipe is managed from the Statpipe Control Centre at Bygnes (west of Karsto). The Bygnes control center will in the future also manage the daily flow through ZTS, including the terminal and Europipe, in addition to management of the gas storage at Etzel.

Such centralized management will ensure the flexibility of the transport and production facilities together with a downstream storage capacity in order to obtain high reliability of gas deliveries.

1991 CONSTRUCTION

In late April 1989, Statoil awarded the engineering contract to Snamprogetti following a bid to which nine major world engineering organizations were invited. Pipelay operations began almost exactly 2 years later, on Apr. 10, 1991.

LOGISTICS, EQUIPMENT

Zeepipe crosses five offshore sectors, Norwegian, Danish, German, Dutch, and Belgian, and ends ashore in Belgium.

It has been a major task for Statoil together with Snamprogetti, to perform the authority engineering, acquire necessary approvals, and achieve a uniform design and similar operational practices, considering that the different countries have different laws and regulations.

In addition, Belgium has no laws and regulations for and no experience with offshore pipelines.

In weight, 1.4 million tons of pipe were transported; more than 230,000 miles were navigated. Welding wire used totalled 450 tons; X-ray film reached more than 192 miles; and total mastic used weighed 35,000 tons.

Table 1 shows elements of the pipe used on the project. Pipe was supplied by Mannesmann Rohren-Werke, Hickingen and Muhleheim, Germany; by GTS Industries, Dunkirk and Belleville, France; by Nippon Steel Corp. (Kimitsu Works), Japan; and by Sumitomo Metal Industries (Kashima Works), Japan.

Under the concrete weight coating, all offshore joints received an asphalt enamel anticorrosion coating. The shore portions of the line received a polypropylene abrasion coating.

Coating was applied by Ncoat AF, Rotterdam (for the southern sector), and by Bredero Norwegian Coaters, Leith, Scotland (for the northern sector). Pipe joints were transported from the coating yards of Leith and Rotterdam directly to the pipelaying vessel. Four 40-in. and two 20-in. (by-pass) subsea ball valves have also been installed.

Major equipment used during the 1991 season is listed in Table 2.

In addition to this key equipment, a large fleet of other auxiliary vessels was required (survey vessels, diver service vessels, smaller dredgers, barges, pipecarriers, tugs, and supply boats).

In 1991 more than 90 vessels were deployed, and during several months, 50-60 vessels were in operation at the same time. Semac 1 and Castoro Sei were deployed in the northern and southern part of the project, respectively, for optimum use of their different features. The two lay vessels underwent extensive upgrading, which included the following:

Marine and safety equipment and documentation to comply with Norwegian standards and obtaining Letter of Compliance by NMD (Norwegian Maritime Directorate)
Upgrading of tensioning equipment to 225 tons for Semac 1
General upgrading of cranage and pipe-handling equipment for both semisubmersible lay vessels
Modification and extension of the double-joint system on Semac 1 to improve productivity
Improved welding systems and sequences onboard
Fitting of new davits on Semac 1 to perform abovewater tie-in.

Castoro Due was used for the beachpull operation at Zeebrugge and subsequent laying of approximately 11 miles (including crossing of the Scheur channel). Also, this barge had to go through some refitting and upgrading before being deployed on the project.

The cutter-dredgers Leonardo da Vinci and Vlanderen XIX were used in the Belgian sector for dredging and crossing the Scheur channel and the barge access at the Zeebrugge landfall, respectively. The two cutter dredgers were complemented by six split-hopper spoil carriers varying in capacity from 2,340 to 6,604 cu yd.

Three large hopper dredges were only used for the sandwaves leveling work (to be discussed presently) in the Dutch sector, owing to pipeline profile optimization by Statoil, and in consideration of the work efficiency.

In view of the large quantity of trenching work required for the southern part of the pipeline and the scheduling of the work to the late part of the 1991 season, both of EMC's jet barges were mobilized in order to meet the high project standards.

New working procedures were also developed in order to improve the accuracy of the trenching work in the sandwave area.

PIPELAYING; TERMINAL PROGRESS

Pipelaying operations started on Apr. 10, 1991, with Semac 1 at the northern end of System P53 (Fig. 1) 10 days later than originally planned. The lay vessel registered slow progress during the first month of operations, then managed to recover all the lost time and finish a few days earlier than originally planned.

In 150 days, approximately 230 miles of 40-in. pipe were laid, yielding an average lay rate of more than 1.5 miles/day, while the peak production was close to 2.5 miles in 1 day.

The barge also managed to lay pipe for 144 days without interruption for weather downtime.

Castoro Sei started pipelaying operations in the northern sector on June 1, 1991, exactly as planned.

The planned work was completed several days ahead of schedule and the barge continued the 40-in. pipeline southward for an additional 52 miles. The average lay rate on the 40-in. pipeline was almost 1.6 miles/day, while the peak production was more than 2 miles/day.

Castoro Due arrived in the North Sea from India approximately 2 weeks later than originally planned.

At Zeebrugge, the pipeline was pulled to shore with no major problem, and the more than 11 miles of pipelay completed in 36 days. Semac eventually tied in the two sections of the System P53 pipeline above water and laid it down in 6 days.

After completion of landfall tie-in and post-trenching work, the System P53 pipeline was flooded, gauged, and successfully hydrotested from shore.

The northern sector pipelines were successfully gauged and hydrotested soon after completion of pipelaying from EMC's DSV Bar Protector.

The gas-receiving terminal at Zeebrugge is currently under construction with a precommissioning target set for the end of February 1993.

SPECIAL CHALLENGES

It goes without saying that carrying out the largest offshore pipeline project in the North Sea involves many challenges. Although new technology has not been required, the laying and tie-in activities are challenging simply because of the large scope.

The tie-in contract calls for 14 hyperbaric welds at pipeline intermediate and terminal ends in addition to the installation of the SCP. The work will start early this summer and is based on the new Sleipner plans.

SANDWAVES

Zeepipe had to cross almost 125 miles of sea-bottom with sandwaves through the southern part of the Dutch sector and the Belgian sector (Fig. 1). The height of the sandwaves varies from a few yards to 26-33 ft. The distance between the crests is typically 495-990 ft.

Sandwave dredging took place between April and September 1991; post-trenching activities, July to October.

The predredging was required to enable laying and post-trenching to achieve the final pipeline stability configuration.

More than 600 sandwaves had to be partially predredged. Total dredged volume, to achieve a minimum 33-ft corridor, was more than 35 million cu ft.

Subsequent to pipelaying, trenching operations further lowered the pipeline. The trench depth in the sandwave area varied continuously to remove only what had not been removed by the dredging operations.

The capability to perform such accurate trenching work with the jet barges produced a saving of more than 155.3 million cu ft of dredging work compared with the "all-dredged" solution to sand waves' presence.

To allow accurate monitoring of the progress of the sandwaves dredging, a network of computers was installed allowing communications of profile data from the survey vessels to the dredgers and from the dredgers (via satellite) to EMC's London offices.

There, the profile data were analyzed, recorded, and new instructions sent back to the dredgers in the form of revised cut lines; the as-dredged profiles could be considered acceptable only when predicted pipeline stress and free span agreed with the specified values.

Careful planning and preparation paid off when only 301 dredger days were used instead of the originally envisioned 425 days. The pipeline's as-laid survey eventually showed that, for more than 600 predredged sandwaves, only one pipeline free-span exceeded the specified requirement because of rough seabed information.

The free-span was immediately corrected.

The final lowering of the pipeline was accomplished with jet barges. In the most difficult areas, up to five passes were required to achieve the desired result in line with design specifications. Trenching operations amounted to 101 barge days, instead of the 119 originally foreseen.

SCHEUR CHANNEL CROSSING

North of Zeebrugge, Zeepipe had to cross the Scheur East shipping channel sen,inc, Antwerp.

Approval from governmental authorities was only obtained by a plan to dredge to a depth of about 33 ft below the current seabed level to allow for possible future channel deepening and a minimum 10 ft net cover on top of pipeline for protection (Fig. 4).

The dredging had to be performed in the open sea during March-June to be ready before pipelaying operations.

Although weather conditions were extreme in the early season and only one dredging vessel proved fit for the conditions, the work was concluded in time as a result of strict optimization of ditch geometry across the channel.

The longitudinal profile of the cross channel is shown in Fig. 4.

The scale of the operation is indicated by the following project parameters:

Length across channel: 1.12 miles
Maximum width: 396 ft
Maximum dredging depth: 31.4 ft
Material type: 10% sand/silt, 38% compact sand/clay, 52% hard clay
Siltation removed: more than 7 million cu ft.

After pipelaying, the channel crossing was completely backfilled by rock-dumping. North Sea weather and strong cross tides, soil conditions (presence of hard clay), and heavy shipping traffic along the Scheur (RD-20 large ships passing every day on average) presented special concerns for this job.

Dredging operations were therefore based on very large cutter dredgers. Allowing for environmental and traffic conditions, an expected duration of 21 cutter weeks was planned.

The work started in March 1991 with the two cutter dredgers. After a few weeks, the more weather-sensitive dredger was moved to the landfall area. Throughout March and April, progress was hampered by severe weather and poor visibility, by confined swing widths, and by continuous vigilance of ship movements in the channel. In May and June, as the weather improved, the expected efficiency was achieved. Eventually optimized operational procedures made 18 weeks of cutter dredging sufficient. Dredging for the Scheur crossing occurred between March and July 1991 with pipelaying at the end of July.

ZEEBRUGGE LANDFALL

At Zeebrugge, the man-made harbor break-water provided the only acceptable landing point, an approach with which governmental authorities agreed.

A directional drilling solution originally planned had to be abandoned because of unacceptable soil conditions (partly gravel and partly very soft soil). A second alternative, a concrete-lined tunnel, required a long construction time and thus limited contingency possibilities.

A third solution, a modified beachpull solution coming in parallel to the breakwater then turning 90 into it at a shallow depth, was finally selected.

Although engineering time available became very short and was done partly in parallel with the construction starting in the open sea in the middle of the winter, the landfall was completed according to plans during the 1991 season. The Zeebrugge landfall concept consists of a sheetpiled cofferdam into which the pipeline is pulled from a flat-bottom laybarge with winches on the beach. The cofferdam ends in an artificial temporary peninsula built against the breakwater of the Zeebrugge harbor because of environmental restraints (Fig. 5).

A pipeline and barge-access channel had to be dredged to facilitate the beach pull. After the pull, the cofferdam was plugged and dewatered. The pipeline was then tied in to a preinstalled spool piece through the Zeebrugge breakwater.

The site was then completely reinstated. The following list reveals the scope of the Zeebrugge Landfall:

Length of cofferedam: 1,386 ft
Weight of sheetpiles: 4,000 tons
Excavated depth: 21.5 ft LLWS
Peninsula protection stones: 90,000 tons
Peninsula sandfill (working platform): 3 million cu ft
Length of dredged channel: 1.1 mile
Maximum dredging depth: 37.3 ft
Dredged quantity: 24.7 million cu ft
Maximum pull expected: 400 tons.

Construction started in January 1991 and all the preparatory work was completed by the end of June. The pipeline was pulled to shore from Castoro Due on July 4; inside a cofferdam, in the center of an artificial peninsula, at -30 ft MSL, approximately 1 km away (on high water) from the natural coastline.

After installation of the 323-ft spool piece going through the Zeebrugge harbor breakwater, the L-shaped cofferdam was plugged and dewatered to allow for the tie-in between spool piece and pipeline. The dewatering system, installed into the peninsula, consisted of 42 deep well points having active suction depth of - 36 ft MSL to - 86 ft MSL. The water table was gradually lowered to -43 ft MSL and held there for 11 weeks.

The final tie-in inside the cofferdam was completed on Oct. 7, thus achieving a task force target for conception to completion in less than a year.

THIS YEAR'S WORK

During 1992, Semac 1 is laying 124.2 miles of 40-in. pipe from the SCP (Fig. 1) to a point 124 miles south of the Sleipner field.

Castoro 6 is laying 143 miles of 20-in. pipe from Karsto across the Norwegian trench in water depths of nearly 1,000 ft to the Sleipner field.

The Bar 331 will trench between 197 MP (mile point) to 259 MP, nearly 62 miles of trenching north of the SCP.

In July, the SCP (Fig. 3) will be installed by the Russian heavy lift vessel Stanislav Yudin. Total weight of the installed SCP will be approximately 750 tons.

The seabottom surface will be predredged and the structure piled after the installation is finished.

Several tie-ins will occur. The Seaway Pelican will perform a tie-in. at the 16/11-S platform. The Amethyst will perform a tie-in at the Sleipner riser platform. And the Seaway Condor will perform a midline tie-in at the SCP. Modifications to 16/11-S will also be conducted this year. These include installation of a small module (200 tons) and approximately 73,000 man-hr of offshore hookup.

Finally, the Trollness is set to perform rock dumping at the SCP, cable crossings, and span correction on the 20-in. Sleipner condensate pipeline.

BIBLIOGRAPHY

Bernetti, R., et al., "Theoretical and Experimental Analysis of Soil-Pipe Interaction at Free Span Shoulders for Oscillating Pipelines," Euroms, Trondheim, 1980.

Bernetti, R., et al., "Pipelines Placed on Enodible

Seabeds," OMAE Paper, Houston, 1990. Bijker, R., et al., "Scour Induced Free Spans,"

OTC Paper No. 6762, Houston, 1991.

Bruschi, R., et al., "Submarine Pipeline Design Against Hydroelastic Oscillations: The SVS Project," OMAE Paper, Houston, 1988.

Bruschi, R., "Design Criteria for pipelines crossing sand waves," to be published.

Carpeneto, R., and Tominez, M., "Risk Analysis of Subsea Pipelines hazards induced by human activities," lst Isopev Conference, Edinburgh, 1991.

Eide, A., et al., "Assignment of Coastal Processes for the Design and the Construction of Zeepipe Landfall in Zeebrugge," 23rd Int. Conf. on Coastal Engineering, Venice, 1992. Eide, L. O., et al., "The Experience from the Statpipe System on Free Span Development and Analysis," Offshore Pipeline Technology Seminar, Stavanger, 1987.

Jiao, G., and Bruschi, R., "Methods for Probabilistic Assessment of Environmental Hazards to Submarine Pipelines," 1st Isopet Conference, Edinburgh, 1991.

Hvam, C., et al., "Risk of Pipe Damage from Dragging Anchors," Euroms, Trondheim, 1990.

Wolfram, W.R., et al., "PIPESTAB Project: Improved Design Basis for Submarine Pipeline Design," OTC Paper No.5501, Houston, 1987.