When Shell U.K. Exploration & Production announced development approval for its U.K. North Sea Mallard field last week, it faxed a press release and development schematic to journalists.
The development plan looked conventional enough, with two subsea wellheads connected by a 15 km long pipeline to Kittiwake platform, except for a thing like a giant spring in a cage between the wellheads. The thing was labeled 'cooling coil'.
Having never seen a cooling coil in a development plan, I called to find out more from Chris Gunstone, principal engineer on Shell Expro's Gannet E and F development, and also involved in the Mallard concept.
Gunstone told me the cooling coil idea was suggested by Allseas Marine Contractors SA of Chatel-St. Denis, Switzerland, which later won the contract to provide the coil and pipeline.
The coil was conceived for high temperature/high pressure (HT/HP) developments such as Mallard, and has not yet been built, but is expected to hold no major technical problems.
Strain threshold
The cooling coil concept is brilliantly simple: hot oil leaving the well, at 150° C. in the case of Mallard, swirls through the coil, to be cooled by surrounding sea water as it travels. By the time it exits the coil and enters the export pipeline, the oil has cooled, to 105° C. in this case.
"The cooling coil will bring the pipeline entry temperature of the oil below the threshold for 'strain-based' design," said Gunstone. "At higher temperatures, steel begins to lose its strength, so you need increasingly thicker pipelines to overcome the added strain."
Mallard is the first HT/HP field to be developed by Shell Expro, the U.K. offshore operating combine of Shell U.K. Ltd. and Esso Exploration & Production U.K. Ltd.
Because Mallard development required technology unfamiliar to Shell/Esso, Gunstone explained that it faced a dilemma over the bidding process: "We felt we didn't know all the technical answers, so we couldn't tell bidders how to approach it."
Problem specific
So Shell/Esso decided to specify the problem to be solved, and pay contractors to develop concepts, rather than as usual specifying the equipment to be built.
"In normal tendering, contractors can misunderstand the requirements," said Gunstone. "This way a contractor can offer technical solutions the operator may not have thought of, which may be better and cheaper."
Shell/Esso helped bring the various ideas to an equal level of development, so it could then choose one of the options based on price. Mallard development is costed at £ 100 million ($150 million).
Production from Mallard is expected to start in October 1997, and to reach 16,000 b/d of oil and 11 MMcfd of gas. The field has estimated reserves of 25 million bbl of oil and 17 bcf of gas (see other article on this page).
An added benefit of this tendering process is that concepts which are developed and not suitable for one project can be implemented on another, Gunstone said one contractor's ideas were rejected for Mallard, but are now being pursued for Gannet E and F (OGJ, Aug. 26, p. 28).
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