Polyethylene and carbon steel combine
corrosion-resistance, strength for West of Shetland field
(Left) The Foinaven Field is a subsea completion connected to a host platform through catenary risers. (Right) A three-dimensional cutaway view of the lined pipeBP's Foinaven Field is in some of the most challenging waters of the UK North Sea and as such has demanded radical innovations in subsea technology, diverless installation and maintenance. The field is on the Atlantic frontier, 119 miles west of the Shetland Islands in a 1,300-ft to 2,000-ft deep trough. Besides water too deep for divers, severe currents made complex by the interaction of warm Gulf and cold Arctic streams, extend nearly to the seabed.
Water depth has caused an emphasis on the reliability of the subsea equipment designed for diverless installation and up to 25 years of maintenance-free service life. Although every effort was made to use methods and products which had previously been proven in the North Sea, a different approach was used for the water injection flowlines.
A new type of water injection flowline
Early in the design phase of the Foinaven project, it was thought traditional water injection lines might not remain maintenance-free for 25 years. Inconel and super duplex stainless steels have the necessary strength and corrosion resistance but are expensive.
The McDermott Marine Construction Ltd. (MMCL) design team decided to test the concept of lining standard carbon steel pipe with thin-walled, non-pressure rated, medium-density polyethylene (MDPE) pipe. The steel pipe, it was reasoned, provided the requisite strength and the PE pipe protected the steel from internal corrosion.
Swagelining was developed by British Gas as a means to replace existing gas distribution mains without digging. It has also been used successfully in such distribution systems as potable water and forced sewer mains. In previous applications polyethylene pipe was rated for direct burial and did not rely on existing pipe for strength. In effect, the existing pipe is a conduit for the PE pipe whose sections are usually joined with conventional electrofusion fittings.
The Swagelining process has also been used for many years to install PE pipe in existing onshore gas and industrial pipelines. However, in these installations the lined steel pipe has been rejoined with flanged fittings unacceptable in the Foinaven oil field because of their leak potential. British Gas solved the problem by developing a welding technique to join lined sections without flanges.
About 50,000 ft of new 10-in. steel pipe and 9,300 ft of eight-in. pipe has been lined in lengths of 1,650 ft at the Ardersier, Scotland spoolbase of MMCL subsidiary, McDermott Subsea Constructors (MSCL). These lines were then joined together using welded fittings and the pipeline coiled onto a 21-ft diameter drum. Each reel holds between 32,000 ft and 39,000 ft of lined pipe. MSCL's specially designed reel ship, Norlift, transported the reels to the Foinaven field and laid the pipe on the seabed. When production begins in the fourth quarter of 1996, three water injection lines will operate at pressures as high as 4,350 psi, injecting 165,000 b/d of water back into the reservoir.
The swagelining process
Swagelining uses PE pipe whose outside diameter (OD) is slightly larger than the inside diameter (ID) of the pipe to be lined. PE pipe is pulled through a die to temporarily reduce its OD by about 7%, allowing it to be pulled easily through the outer pipe. When the pulling force is disconnected, the PE pipe begins to return to its original diameter and within hours the internal pipe presses tightly against the inside of the outer pipe.
Polyethylene-lined pipe sections are welded and reeled onto a 21-ft diameter reel on the Norlift.
New compression fittings
Fittings were fabricated from sections of the carbon steel pipe being lined. Each fitting was machined to enlarge its ID and welding inlay of corrosion resistant alloy (CRA) used to replace the removed material. Once the CRA had been deposited, the inner surface of the section was again machined to the specified profile. Each compression fitting was then fully inspected for inlay thickness and CRA chemistry.
Charlie Tighe, Director, QA Weld Tech, Middlesbrough, England, was responsible for the fabrication of the new compression fittings, jointly designed and developed by British Gas and MMCL, which made subsea use of PE-lined pipe possible.
"The process we employed was state-of-the-art techniques of weld cladding using hot wire, fully automatic T.I.G. welding," Tighe said. "We are able to program the welding parameters to ensure that the procedures which have been developed and tested for this critical application are repeated exactly."
The interior of the finished piece had the same ID as the original pipe except for raised ridges in the center of each section. A fitting was welded to each end of the 1,650-ft lengths of steel pipe and once the PE had been installed inside the steel pipe, a compression ring made of the same CRA material was inserted to press the PE pipe into the ridges. This formed a permanent connection between the PE pipe and the outer pipe. Since the PE pipe stopped short of the ends of the steel pipe, sections of lined pipe could be welded together without melting the PE.
The final staging point
A compression fitting was welded onto each end of the 1,650-ft pipe sections before they were cleaned for the Swagelining operation. A cleaning pig was pulled through the pipes to remove any sharp edges from the weld root. A short section of PE pipe was machined to the final Swagelining size, pulled through the steel pipe, and examined for any signs of excessive scoring.
Jack Ross was the project manager for Swagelining contractor, A Hak of West Lothian, Scotland. He said MSCL decided to assemble the steel pipe in 1,650 ft sections simply because that was the length of the on site pipe storage racks.
"The pipe was delivered in 39-ft lengths," he said. "So we could weld 42 of them together before we ran out of room."
A winch pulled the PE pipe through the steel pipe with a set pulling force not to exceed the half-yield of the PE pipe. After tension was removed from the PE pipe and it stabilized, a compression ring was inserted into each fitting, forcing the PE pipe into the ridges inside the fitting to grip the PE pipe and create a permanent seal between it and the steel pipe. Excess PE pipe is then carefully cut away and each pipe section is preliminary air tested at 100 psi. The 1,650-ft sections were welded together as they were wound onto the reel aboard the Norlift.
"The project was very successful," Ross said. "Now we know we can Swageline and weld long lengths of pipe in a very, very short period of time. We averaged 1,650 ft a day."
Installation on the seabed
When the reel ship is properly positioned at sea, the outer end of the pipe is connected via cable to a weight on the seabed. The ship then moves in the desired lay direction, paying out pipe under a carefully monitored range of parameters. At the end of the line, the pipe is again laid down on the seabed via a cable.
Despite variable, unpredictable currents that ran up to four knots throughout the water column in the pipelay area, the 2.2-mile sections of lined pipe were laid to an accuracy of 3 ft in water more than 1,600 ft deep.
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