With the gathering drive for global deepwater development, one of the less talked-about factors is the need to anchor the huge production vessels now coming into service.

As projects move into deeper waters, operators are paying closer attention to the mooring systems tethering these larger production vessels. The heavier mooring loads, generated by traditional chain and wire rope mooring systems in catenary configurations, are being replaced with lighter taut-leg systems that rely on synthetic rope solutions.

Catenary systems can limit production vessel design because of their weight and larger subsea footprint. Chain wire and rope combinations rely on the weight of the mooring line to provide the return force necessary to bring a floating production unit (FPU) back on station.

Taut-leg mooring configurations involve a smaller seabed footprint, which in a large subsea field spread, may be a distinct advantage. Elasticity within polyester ropes - often featured in taut systems - provides the return force to bring an FPU back on station. But taut systems also require both horizontal and vertical loads to be carried by the anchor at the end of the line.

Vryhof Anchors recently outlined how its Stevmanta vertically loaded anchor (VLA) is fulfilling the requirement for these taut-leg applications (Table 1).

Stevmanta VLA units are now in use in three Brazilian deepwater projects: the P27, where the water depth is 1,640 ft (500 m); the P40, where the water depth is 3,280 ft (1,000 m); and the floating production, storage and offloading vessel Brasil, moored in a depth of 4,920 ft (1,500 m).

Gradually, development of the VLA design has allowed the vertical loading on these units to be increased, relative to the installation loads they have to endure.

"Evaluating the performance of the Stevmanta VLA over the years, it can be concluded that the ratio between the ultimate pull-out capacity and the installation load has increased from 1.8:1 with the early prototypes to 3.5:1 with the correct model of the Stevmanta VLA," said Gijs Degenkamp, technical director at Vryhof Anchors.

His paper, co-authored by application engineer Roderick Ruinen, outlines advances with this type of anchoring device and was presented in December at IBC's Floating Production Conference in London.
Anchor holding capacity is highly dependent on the soil conditions in which it is embedded. With the availability of ample soil data at the mooring location, an anchor design can be optimized for the job in one or several locations.

"This can lead to two or three different anchor sizes and even types being required for the project," Degenkamp points out.

Installation of an anchor designed to take either vertical or horizontal loads will be driven by the applied installation load, he said. "This means that the anchor is self-adjusting to the soil conditions it encounters. If softer soil conditions are encountered at the intended anchoring location, the anchor will continue to penetrate until it reaches soils that provide sufficient resistance."

Typically, a 15-tonne anchor will penetrate to between 50 ft and 80 ft (15 m and 25 m) below the mudline to provide the necessary holding capacity.

Two types of anchor installation system are outlined by Degenkamp and Ruinen. One involves an anchor-handling vessel utilizing its bollard pull capability to tow an anchor to its position on the seabed with the necessary loading.

Another involves a subsea tensioning system, where two opposite anchors around the taut-leg mooring circumference are tensioned against one another. In this instance, an installation vessel applies a vertical load to both anchors, via a subsea tensioner, which results in a larger horizontal force on both anchors. Typically, the vertical force is between 40% and 50% of the anchor installation load.

With this technique, either an anchor-handling or a crane vessel can perform the subsea installation of the mooring anchor. But the downside, Degenkamp said, is that some mooring systems do not have anchors aligned opposite one another. An example is in a tri-point system where nine anchors are set in three clusters of three lines.

Working up the linear path from anchors to moorings, composite materials using carbon fiber have come into the equation as part of the overall cost-reduction effort for deep applications.

Work on composite tethers and risers has been discussed in various papers delivered at Offshore Technology Conference (OTC) events between 1999 and 2001.

In a paper presented at OTC in 2002, two Kværner Oil and Gas experts, Stig Bøtker and Thomas Johnannessen, looked at the potential for composite risers and tethers used on deepwater tension-leg platforms (TLPs). After a detailed assessment of the effect composite tethers can have on TLP design, including hydrodynamic optimization, the state of composite tether technology, certification, transportation and installation issues, they concluded that a TLP design could be optimized with composite tethers and risers to become an "attractive alternative for ultradeepwater applications."

Their conclusions followed a considerable amount of work within the CompRiser and CompTether research programs established in 1996 and 1998, respectively, by Kværner and what was then Conoco.
After examining composite manufacturing and installation costs, the Kværner paper suggests savings of up to US $21 million (NKr 185 million) on a composite tether system for a water depth of 8,220 ft (2,500 m) in a benign environment (Table 2). But in a water depth of 4,920 ft (1,500 m), the savings in both total tether and installation cost for composites is less dramatic: just $5.6 million (NKr 50 million).

But cost is not the only advantage in using composite solutions. A composite tether, which uses carbon fiber rods, can be bundled intro strands and then spooled. Composite tethers can use top and bottom connectors that are identical to steel tether connectors.

TLPs with steel tethers use either electromagnetic sensors or complex underwater cells to monitor the tether loading. These cells can be expensive and unreliable, said Bøtker and Johnannessen. Instead, the latest advances in fiber-optic sensing technology are proving much more reliable and cheaper than fiber-optic measuring to check on composite loads. The point is composite technology can be used for mooring systems without a huge divergence from existing manufacturing and installation procedures.

Kværner's Moss umbilicals plant in Norway was used to manufacture a composite tether body. "The successful use of existing fabrication facilities reduces the investment cost, and thus makes the composite tether more attractive from an economical point of view," said Bøtker and Johnannessen.

Extrusion of the outer jacket on a composite tether is identical to that used on existing umbilicals manufactured by Kværner Oilfield Products.

End terminations are added to the tether in the form of a steel cone filled with epoxy resin during a vacuum injection process, also similar to the technique used for steel wire connections.
Also, composite tethers can be spooled onto drums with relatively small diameters, between 14.4 ft and 16.4 ft (4.5 m and 5 m). Because the composite tether can be transported in smaller sizes (typically on spools commonly used to transport umbilicals and cables), the benefit is the lower cost of offshore installation vessels. Smaller offshore support vessels can carry more composite tether than a steel tether. And smaller installation vessels have lower day rates than the heavy lift vessels required for steel tether installation.

Kværner has considered reel ship installation of composite tethers, which can be loaded onto a turntable directly from the manufacturing plant such as that at Moss in Norway. It is possible to use a vertical lay system either through a moonpool or over a ship's side, allowing relatively quick installation. And, in the case of mooring a TLP, the Kværner paper suggests tethers can be put in place after the TLP is installed, which makes it unnecessary to use temporary buoyancy units to keep the tethers in tension prior to connection to the TLP.

Alternatively, Kværner has considered using a cargo barge for composite tether installation. In this instance, the cargo barge would need assistance from tugs for station keeping and a dynamically positioned vessel equipped with an A-frame for final tether positioning.

Finally, composite tether installation directly from a TLP is also possible, using spools fixed to the TLP columns.

Norway's Demo 2000 program for offshore technology development has been instrumental in advancing composite technology. In June 2002, the board awarded a $241,681 (NKr 1.6 million) grant for the second stage of a $722,007 (NKr 5 million) research program for Kværner's CompTether. The program will provide analysis and verification of manufacturing, transport and installation of composite tethers; Det Norske Veritas became involved in certifying both tethers and risers for the Kværner/Conoco research program after developing composite design guidelines in 2001.

But the current story of moorings and risers doesn't end there. Synthetic rope is part of the modern deep development, too, and lengths for these ropes are moving up - or rather down - all the time.
Marlow Ropes has been contracted to supply the highest strength polyester ropes ever supplied for deepwater mooring on BP's Mad Dog truss spar development in the Gulf of Mexico. The company is supplying a total of 11 lines with breaking strength of 1,932 tonnes for this project, in a water depth of 4,420 ft (1,350 m); the ropes are to be delivered by October.

But steel rope isn't completely out of the deepwater picture. Bridon Ropes supplied 20 strands of spiral strand mooring for Dominion's Devil's Tower project in a water depth of 5,608 ft (1,710 m) in the Mississippi Canyon area of the Gulf of Mexico. Devil's Tower, which was the deepest dry tree development at the time, uses lines designed for a minimum breaking load of 1,700 tonnes.
But the inevitable march to deeper water suggests synthetic materials such as polyester and composite fibers may push steel out of the mooring game as reachable depths are extended.

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