Composite tethers provide five-fold tensile strength increase for TLPs

Technology adopted from CFRP bridge cables

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Cross-section of a CFRP tether with 241 wires.
Click here to enlarge image

The use of composite tethers offers a cost-effective solution for deploying tension-leg plat forms in water depths in which the use of steel tethers becomes impracticable. Umoe Oil and Gas has developed a small TLP design with composite tethers for application in deepwater off West Africa.

The company is working with carbon fiber reinforced plastic (CFRP) which displays a range of properties suitable for use as deepwater tethers. It has good longitudinal stiffness and outstanding fatigue capacity, does not corrode, and can be spooled for ease of installation, according to Jan Johnsrud, head of structural section in concept and technology development at Umoe Oil and Gas.

A further benefit is that the cost of carbon fiber is falling as its applications and market grow. From around $10/lb in 1994, it is now down to about $8, and is expected to fall further.

The TLP design developed by the company is an unstaffed, minimum facilities wellhead platform with typically 50,000 b/d production capacity, 12-16 wells, four columns, and an overall weight of 10,000 tons. It is designed for installation in water depths of 1,000-2,000 meters in benign conditions, such as those in the Gulf of Guinea. The project is one of Umoe's portfolio of initiatives aimed at developing offshore technology capable of meeting the $8/bbl challenge.

Tensile strength

The company has a cooperation agreement with Swiss cable supplier BBR, which in 1996 supplied two CFRP cables for a suspension bridge in Switzerland, the first CFRP stay cable application. The BBR carbon cable, which consisted of 241 parallel rods, each 5 mm diameter, is suitable with only minor modifications for use with the TLP, says Johnsrud.

The cable has great tensile strength, which according to Umoe calculations is about 2,500 MPa. This compares with less than 500 MPa for the steel grade under consideration for tether use. A weight comparison also shows distinct advantages for CFRP cable, which has a density of just under 1.6 tons/cu meter. Steel density on the other hand is 7.85 tons/cu meter.

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Cross-section of a CFRP tehter with 241 wires.
Click here to enlarge image

Under its agreement with BBR, Umoe also has access to the patented anchorage system developed by the company for use with its cable. Anchorage has been one of the sticking points in developing composite tethers, Johnsrud says. As the number of rods in the cable, and hence its diameter, increases, it becomes more difficult to design a good end-termination. Other companies have adopted the solution of splitting the cable and making multiple terminations, but this involves a more complex design and adds to the cost.

Terminations tested

BBR has developed a method which enables the cable to be anchored as a single whole. The solution, which involves generating a stiffness gradient in the load transfer media, which creates a more favorable shear stress distribution along the full length of the anchorage, has been shown to achieve a breaking load of up to 94% of total wire capacity. During qualification of the system for use in the suspension bridge, the termination underwent fatigue tests involving eight million load cycles without suffering any damage.

For use as a TLP tether, some additional qualification of the cable, and redesign of the anchorage, would be required, Johnsrud says. The anchorage uses well known materials - epoxy, ceramics, steel and carbon fiber - so it should be a relatively straightforward matter to qualify it for offshore use.

To apply the BBR cable and anchorage to the TLP, 16 tethers are preferred, rather than eight, as in the case of the steel tether option. Although this involves a higher cost in terms of the greater number of anchorages, there are also benefits from using smaller diameter tethers, as they are exposed to a lesser loading from water currents and are less susceptible to vortex induced vibrations.

In water depths beyond 1,000 meters, on the other hand, neutrally buoyant steel tethers begin to run into problems. To avoid the danger of buckling due to increasing external water pressure, the designer must choose one of the following options:

  • Increase the wall thickness in steps
  • Partly flood the tether
  • Use foam or pressurized air inside the tubular
  • Vary both the diameter and wall thickness over the tether height.

The latter two options have obvious cost penalties, Johnsrud says, while the former two alternatives require additional buoyancy. In this case, the submerged weight of the tether system increases significantly - from around 13,000 tons in 1,200 meters of water to about 3,900 tons in 2,000 meters.

Such a weight increase would have to be compensated for either by increasing the platform displacement or by placing buoyancy elements on the tethers. Umoe has calculated that on the assumption of a weight penalty of $4/lb, the difference in wet tether weight for a steel tether system in 2,000 meters of water would amount to an additional cost of $32 million.

Reeling tethers

CFRP tethers can be reeled, which makes for ease of handling, transport, and field installation. Deepwater steel tethers, on the other hand, may be wet-towed to the field - a process which involves some fatigue exposure and a demanding upending operation - or assembled in sections with the aid of a crane-barge.

Carbon fiber is also virtually maintenance free. One important advantage it has over steel is that it does not corrode, whether in seawater or not. Its life-cycle costs should be very favorable, according to Johnsrud.

Umoe has now taken its in-house carbon fiber tether development program almost as far as it intends to go, and is looking for oil company sponsorship to help share the costs of testing.

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