Tendons

    Tension leg platforms (TLPs) are floating structures that are moored to the sea floor with vertical tethers or tendons that limit heave or vertical motion. Because heave is minimized, wellheads can be situated on the platforms (“dry” trees) rather than on the sea floor. This significantly reduces well maintenance over the life of the field. Full-size TLPs can support complete drilling and processing operations, while smaller versions, called mini-TLPs or tension leg wellhead platforms (TLWP), are favored for deepwater developments because of their reduced hull displacement. TLPs are used in a number of offshore fields, with 11 located in the Gulf of Mexico.

    Suspended vertically between each corner of the TLP platform and anchorages on the sea floor, the tendons traditionally have been made with watertight, hollow steel pipe to reduce their submerged weight. As the water depth increases, however, the tube walls must be thickened to withstand the greater hydrostatic pressure. The significant increase in tendon weight, eventually causes the tether to sag, reducing the axial stiffness that is crucial to its ability to hold the TLP on station. Buoyancy strategies can compensate for sag, but they increase dynamic mass and displacement. At depths greater than 6,000 ft/1,800 m, the weight of the steel and the sheer volume of material required makes TLP solutions particularly challenging.

    Lightweight carbon fiber composite tethers have the potential to substantially improve TLP performance and cost effectiveness in deep water. A composite components stiffness-to-weight ratio is more than three times that of steel in air, and up to eight times that of steel when submerged in water. The benefits of reduced weight impact the entire TLP design: Lighter, spoolable composite tendons reduce TLP hull size requirements (including pontoon and column size). A smaller hull puts less tension on the foundation, leading to smaller seabed anchorages. And composite tendons reduce installation costs to such a degree that composite-tethered TLPs are cost competitive with steel-tethered platforms in depths as shallow as 2,600 ft/800m for benign conditions and 1,000 ft/300m in harsh conditions. When composite tethers are combined with composite risers, pipes and topside components, the benefits are amplified.