Warren R. True
Pipeline/Gas Processing Editor
Start-up occurred last month on the $2.1 billion Central Graben project in the North Sea.
Led by Amoco (U.K.) Exploration Co., the project consists of the development of North Everest and Lomond fields and construction of the Central Area Transmission System (CATS) and of a new gas terminal and processing plant near Teesside.
The pipeline is designed to provide the backbone for the long-expected infrastructure for natural gas produced from central North Sea fields.
Additionally, the project-field development, pipeline, terminal, and processing plant-along with the gas-fired power plant taking the gas has been one of the largest ever in the U. K.
Other partners in the field are Amerada Hess Ltd. (16.7%) and British Gas Exploration & Production Ltd. (61.1%).
Estimated recoverable reserves in the two fields, located 36 miles apart and 120 miles east of Aberdeen, are 1.5 tcf of gas and 50 million bbl of condensate. Lomond was discovered in 1973; Everest in 1983. Analyses of the fields' gas streams are shown in Table 1.
Driving the project were plans by Teesside Power Ltd. to build at Teesside the largest gas-fired electric power generating plant in Europe-1,875 mw. Sales contracts committed the first 300 MMcfd to the new plant (OGJ, April 19, p. 23).
Teesside Power is a consortium of Enron Power (U.K.) Ltd., ICI Chemicals & Polymers Ltd., and four regional U.K. electricity companies.
JACKETS, DECKS INSTALLED
Fabrication of the Everest and Lomond platforms and the riser 'platform (Fig. 1) took place at Highland Fabricators of Nigg Bay on the Cromarty Firth in the north of Scotland.
For the platform jackets, Amoco and prime engineering contractor Brown & Root Vickers chose an X-braced tower jacket configuration that offered substantial weight savings, says Amoco, together with significant economies and other benefits in construction, load out, and installation.
The design of the jackets also utilized the concept of leg piles grouted to form composite sections with the jacket legs. These lightweight jackets were installed in approximately 300 ft of water over predrilled templates in single lifts.
The integrated production decks for Everest and Lomond, each weighing some 9,000 tons, were installed as single lifts. The design of the decks allows for the skidding on of a drilling package from a harsh environment jack up drilling rig operating in tender-assist mode.
Contracts for the production decks were placed with two other Scottish yards: UIE (Scotland) Ltd., Clydebank (near Glasgow), which built the Lomond integrated deck; and RGC Ltd., Methil, Fife, which built the Everest integrated deck.
Jackets for the three platforms along with the riser deck were installed in April-July 1992 in single lifts by Saipem U.K. Ltd.'s crane barge M7000.
The production platform jackets weigh approximately 3,000 tons each; the riser jacket 2,700 tons. The riser platform topsides weigh 2,880 tons.
Tie-backs of the predrilled wells at Everest and Lomond have been accomplished successfully with the jack ups West Omikron and Santa Fe Magellan.
The Everest and Lomond integrated production decks were installed in November-December 1992 in single lifts by the crane barge DB102. Installation contractor was Hereemac.
Processing and compression on each platform were designed to be 200 MMscfd from two compressors per platform. CATS is designed to operate its entire length in dense phase.
With daily contract quantity at 300 MMcfd, the system can therefore rely on compression from only three of the four compressors, allowing for routine downtime and maintenance.1
OFFSHORE LINES LAID
From Lomond to North Everest production platform run two field pipelines: a 36 mile, 20-in. gas line and an 8-in. condensate line. A 40 mile, 14-in. line carries condensate to an interconnection with BP's Forties oil line to Cruden Bay.
Installation of the three Lomond/North Everest lines was by Allseas Marine Contractors in 1991.2
The 255 mile, 36-in. CATS was installed in 1991 and 1992 and will initially carry 300 MMcfd to meet demand of Teesside Power's plant.
But design capacity is 1.6 bcfd, far exceeding what is needed to develop the two fields. The system was in fact planned to provide transportation for other nearby fields as yet, individually, too small for commercial development.
Six valved Tees were installed to accommodate third-party gas. One may soon be connected to the Phillips Petroleum Co. U.K.-operated Judy and Joanne fields (U.K. Blocks 30/7a and 30/12a).
In its run from producing fields to shore, CATS crosses three other major pipelines: the Fulmar gas pipeline and the Gannet and Ekofisk oil lines.
The system's metallurgy was chosen to handle sour gas: It will tolerate H2S levels of 150 ppm (vol) and CO2 of 25 mol %.1
Line pipe was supplied by Thyssen (U.K.) and manufactured by Mannesmann in Germany. The pipe is a modifier API 5L Grade X-65 steel with 28.4 mm (1. 12 in.) W.T. for the offshore portion and 33.9 mm (1.34 in.) W.T. for other sections such as the riser, tie-in spools, shore approach, and the 4.6-mile shore line to the terminal.
The pipeline was externally coated with 0.24-in. coal-tar epoxy corrosion coating and 51 mm (2 in.) weight coating. In the shore approach area, the weight coating was increased to 76 mm (3 in.).
All external coatings were applied by British Pipe Coaters, Leith, Scotland. The pipeline is internally coated with Copon EP 2306.
The basic pipeline engineering was by Brown & Root (U.K.) while detailed engineering for fabrication and installation for the offshore components was carried out by McDermott-ETPM in Rotterdam.
These offshore components included Tee assemblies and protective structures, pipeline crossings, subsea intervention valve (SSIV), and tie-in spools.
Subsea ball valves for the six Tee locations were supplied by T.K. Valves Ltd., Dunfermline, Scotland; and subsea check valves by Tom Wheatley, Houston.
Pipelaying was completed in July 1992 and tie-in to the Everest riser platform was completed in August 1992. The entire CATS system was successfully hydrotested in October 1992. The line is designed to operate under pressures of up to 2,200 psig, according to Amoco.
LAND STRATEGIES
The landward element of the pipeline's construction (Fig. 2), from its beach landfall at Coatham Sands to the CATS terminal, around 4 miles inland, also proved technically demanding, says Amoco.
The Coatham Sands landfall site is a dune area classified for environmental and ecological reasons as a "Site of Special Scientific Interest."
The Amoco project team, after extensive consultation with local conservationists developed a plan that minimized disruption to the landfall site during the construction phase and initiated a comprehensive program to restore the area to its original condition following completion of the operation.3
For various technical reasons, says Amoco, the line had to be landed south of the River Tees and then taken under the river in a specially constructed tunnel to the 72 acre terminal site. This was only the most significant of 11 crossings on the land section of the line.
For construction of the tunnel, the directional drill method was rejected because the pipe's heavy wall thickness reduced the pipe's flexibility and there was insufficient space to drill in one welded string.
The tunnel was therefore constructed by excavating two 6-m diameter concrete segmental shafts 40 m deep with a 2.4 m diameter tunnel 670 m long. The tunnel was driven with a shielded boring machine longer than 9 m.
Overall contractor for the shore pipeline and tunnel construction was AMEC Utilities which subcontracted the tunnel work to sister company AMEC Tunnelling. AMEC Utilities is based in Swindon, U.K.
The tunnel and shaft design was carried out by Brown & Root Civil, London. The line was installed in the tunnel by AMEC Utilities.
Shore pull for the CATS pipeline was carried out in June 1991. Land and Marine Engineering, Bromborough, U.K., executed the beach pull from the LB200 laybarge through a predredged trench.
TERMINAL; GAS PLANT
The CATS terminal is complete and pressured up, says Amoco. Primary contractor was Press Construction (AMEC), Darlington, U. K.
The terminal is a receiving and metering facility for accepting the gas transported from offshore through CATS. The major components are a pig receiver; two slug catchers; filters; gas meter; liquid pumps and meter; and flare. Operating pressure is 1,600 psig.
Any liquids which drop out in the two slug catchers will be metered and pumped to the Teesside gas-processing facility adjacent Amoco's terminal.
The terminal is operated by a distributed control system supplied by ABB Process Automation Ltd., Stevenage. The entire CATS line can be monitored from the terminal control room. That pipeline monitoring system was supplied by LIC Consult, Copenhagen.
Operated by Enron Corp.'s subsidiary Teesside Gas Processing Ltd. and completed in February, the gas plant at Seal Sands has an initial capacity of 300 MMcfd with expansion expected by 1996 to handle an additional 300 MMcfd.
The plant will produce approximately 7,500 b/d of C3+ liquids. Fig. 3 is a simplified process overview. Main contractor for the plant was Costain Oil, Gas & Process Ltd., Manchester.
Inlet-gas pressure from Amoco's CATS terminal can be as much as 2,000 psig, according to Teesside Power, but needs to be only 450 psig for the power station.
The gas is dehydrated in a glycol unit, passed through a series of Joule-Thomson pressure-reduction control valves, reheated, then moved down a 24-in. pipeline as fuel for the power station.
Liquids from the pressure-reduction process are run through a de-ethanizer, the ethane from which is blended into the stream fed to the power plant. Remaining C3S and heavier will be stored prior to further fractionation.
The fractionation plant will separate the product into propane, butane, and C5s and heavier which will be shipped by pipeline off-site for storage and sale.
Among major equipment suppliers were OPC Engineering, Houston, for the glycol dehydration and Atlas Copco Energas GmbH, Cologne, for the overhead gas compressors. Process pumps were supplied by Dresser Pump Division Ltd. and Ingersoll Rand Sales Co. Ltd., both U.K.-based.
Santaz Censa SA, Vigo, Spain, supplied the pressure vessels; Krueger Engineering & Manufacturing, Houston, and GEA Spiro Gills Ltd., Pulborough, England, supplied heat exchangers.
Process skids were fabricated by South Humberside Fabrication Services, Immingham, England; column dressing for on site fabrication was by Hartlepool Erection Co. Ltd., Hartlepool; and pipe racks were fabricated by U.K. Construction Ltd., Merseyside.
Foxboro USA, Houston, was responsible for the distributed control system; Jiskoot Autocontrol Ltd., Tunbridge Wells, England, the metering skids; and Sella Controls Ltd., Stockport, England, the emergency shutdown system.
Teesside Power also says that propane is being imported and stored at the gas plant in two 100-metric ton tanks to act as back-up/start-up fuel for the power station.
Additionally, natural gas supplied from British Gas through a Teesside gas plant-operated 10 mile, 24 in. pipeline is functioning as contingency fuel for the power station.
REFERENCES
- Haynes, M.D., "The Central Area Transmission System (CATS) and Central Graben Development," OTC No. 7165, Offshore Technology Conference, Houston, May 1993.
- "CATS purrs into life," Pipes & Pipelines International, September-October 1992, p. 12.
- Etter, C.K., and Bettelley, L.M. "Approval and Restoration of the Environmentally Sensitive CATS Pipeline Landfall," OTC No. 7170, Offshore Technology Conference, Houston, May 1993.
Copyright 1993 Oil & Gas Journal. All Rights Reserved.
