Synthetic mooring line, when stretched taut, extends from a rig in almost a straight line, while the weight of wire causes it to form a catenary curve. (Tension Technology International)
Fifteen years ago there was much reluctance to use polyester and other fiber ropes in deep-water mooring systems for oil exploration and production platforms. The tendency was to continue to use and adapt wire-rope mooring systems into deeper water.
That skepticism has now been overcome. Polyester deep-water mooring lines are now used on platforms in the Gulf of Mexico, Brazil and elsewhere. The experiences have been favorable. Several permanent polyester platform moorings survived the recent hurricanes without incident.
Fiber ropes have a number of advantages over steel-wire rope in deep-water platform mooring systems, including weight savings, durability and reduced platform offset.
The principal advantage is weight. Wire rope in the form of a catenary is the traditional way of mooring platforms. In deeper water, the weight of suspended wire rope increases and the downward angle of the catenary at the platform becomes steeper. This results in a large downward pull on the platform, which decreases payload or increases required buoyancy.
The use of lightweight fiber rope mooring line eliminates these problems. It is a misnomer to call this a catenary mooring. The fiber rope extends essentially as a straight taut line from the platform down to the anchor point. Thus this is called a taut-leg mooring.
The slope of the synthetic line at the platform is much less than with a wire-rope catenary. The weight of suspended mooring line is much less. These features reduce the downward pull on the platform. More payload can be carried by the platform. Or alternatively, platform buoyancy can be decreased, thus reducing wave and current forces.
Wire rope must be protected against corrosion, either by coating or by cathodic protection or both. Synthetic fiber does not corrode. This is an obvious advantage.
Tension cyclic-load tests on polyester rope, for as many as 40 million load cycles and for wide tension ranges, indicate that polyester rope will not lose significant strength even after hundreds of years of cycling at the tensions and over the tension ranges typical of deep water mooring applications. This is much longer than the potential fatigue lives of either wire rope or chain in seawater.
The steep slope of the wire-rope catenary at the platform exerts only a small horizontal restoring force on the platform. A significant horizontal restoring force is only produced after a large platform offset.
As a result, the force-deflection characteristic of the wire-rope catenary mooring system is very nonlinear, and a large force can produce a substantial platform offset.
The polyester taut-leg mooring slope at the platform is about 45°, and its force-deflection characteristic is essentially linear. The platform offset is thus much less than for a wire mooring system. This reduces the watch circle, which minimizes the necessary length of drill pipe and production risers. This feature is especially important in the Gulf, where hurricanes and loop currents can exert large horizontal forces on the platform.
Petrobras, the Brazilian national oil company, pioneered the use of polyester ropes in deep-water moorings in the mid 1990s. The first applications of polyester rope were short sections installed in one mooring leg of each of two platforms in 1995. After one year of service, these sections were removed, examined and tested. The residual strengths were the same as new.
The first deep-water mooring system to employ polyester in all mooring legs was installed in 1997 at a water depth of 4,660 feet. This mooring system operated for 18 months at one site and was then reinstalled at another site.
Twenty-four polyester moorings have been installed in Brazilian waters to date. Some of these were mobile offshore drilling units (MODU) and others were permanent or semi-permanent oil production moorings.
The first polyester deep-water moorings employed in the Gulf were on MODUs.
The Ocean Victory semi-submersible used a single polyester mooring line, along with conventional wire mooring lines, to drill in 5,300 feet of water for five weeks in 1999. The single polyester mooring line was recovered and found to be in good condition.
The next MODU to employ polyester mooring lines was the Transocean Marianas in 2000. 5,900-foot sections of polyester rope were installed in two of the eight mooring legs. The vessel drilled in 6,180 feet of water.
Marianas again began using a polyester mooring system in January 2007 at a water depth of 7,000 feet.
The Ocean Confidence MODU used a complete polyester mooring system employing suction anchors in 2001. Anchor depths ranged from 6,090 to 6,380 feet. Each of the eight legs of the taut-leg system included 8,990 feet of polyester rope.
This use of polyester lines with suction piles allows the mooring lines to be preset by an anchor-handling vessel, thus reducing the time required to moor the MODU. Other advantages compared with conventional wire-rope mooring systems are that small MODUs can be employed in deeper water, the polyester mooring system has a smaller mooring pattern and the holding capacity is greater.
The MODU Deepwater Nautilus also began using an eight-leg polyester mooring system with suction piles in 2001. Deepwater Nautilus has used that system a number of times and has set a series of depth records for polyester preset water moorings. The first of these records was set in 2001 in 7,790 feet of water, with the deepest anchor point at a depth of 8,400 feet. The current record, held by Deepwater Nautilus, was set in 2004 at 8,950 feet, with the deepest leg at 9,205 feet.
The first permanent polyester deep-water platform mooring systems were installed in 2004. The Mad Dog floating production system (FPS) was the first permanent deep-water polyester mooring to be installed in the Gulf. The mooring system installation operation was completed in March 2004. It is located in about 4,420 feet of water. The operator is BP.
The Mad Dog mooring consists of 11 mooring lines. The polyester rope is 10.6 inches in diameter with a rated minimum breaking strength of 2,003 tons.
At the time, this was the largest and strongest polyester rope manufactured and the largest quantity of polyester rope produced for a single project.
The Red Hawk platform mooring system installation was completed in April 2004. It is moored in 5,300 feet of water. Anadarko (formerly Kerr-McGee) is the operator and Devon Energy is a partner in the facility.
The Red Hawk mooring arrangement is six equally spaced, taut mooring legs. The total length of polyester mooring line in the installation is 41,980 feet, excluding spares. The polyester rope is 8.5 inches in diameter.
The Gomez FPSO was installed in 2005 in 3,000 feet of water with a taut-leg polyester mooring system. This was a relatively shallow application of such a system.
Five other permanent polyester mooring systems are now under contract for the Gulf: Independence Hub, Blind Faith, Tahiti, Mirage and Perdido.
Polyester mooring systems have also been installed in deep water off Angola, the Ivory Coast, Nigeria and Mauritania in Africa. Over 50 temporary and permanent polyester mooring systems have been installed in deep water since 1995. The total length of polyester rope used in deep-water moorings is currently at least 500 miles.
As oil and gas development moves into even deeper water, other synthetic fiber rope materials — aramid (Kevlar, Twaron, Technora), high-modulus polyethylene (Spectra, Dynema) and liquid crystal polyester (Vectran) — are being considered. Such ropes are stronger and can be smaller in diameter, thereby reducing the volume of rope that will have to be shipped and handled. Such ropes are also stiffer and thus will provide more desirable mooring-system characteristics in very deep water.
This article is based on the paper “Polyester Mooring Lines on Platforms and MODUs in Deep Water” presented at the 2007 Offshore Technology Conference in Houston.
John F. Flory is president of Tension Technology International LLC, in Morristown, N.J. He conducted research and development of offshore moorings for Exxon for 20 years before becoming one of the founders of Tension Technology International (TTI) in 1986. He has written and advised on many rope standards and guidelines, including the ABS and API deep-water fiber rope mooring guidelines. He can be reached at Flory@TensionTech.com.
Stephen J. Banfield is managing director of Tension Technology International Ltd., in Eastbourne, Sussex, U.K. He was a researcher with Marlow Ropes for 15 years before joining TTI in 1989.Â
Chris Berryman is a consultant with Tension Technology International Ltd. Before joining TTI in 2005, he worked for Marlow Ropes for 26 years, where, as design manager, he worked on the development of the polyester rope used on Mad Dog.