Use of carbon fiber composites for tension leg platform tethers

The interest of the oil industry in deepwater production facilities has greatly increased in recent years.

The interest of the oil industry in deepwater production facilities has greatly increased in recent years. The latest exploration in such regions as the Gulf of Guinea and the Gulf of Mexico indicates the most promising production in 4,000 ft water depths. In deep water, the classical solutions for platform installations require several technical trans formations that increase their cost considerably. The need to reduce investment and operating costs for deepwater developments has led the industry to evaluate new technologies which promise potential savings.

Freyssinet, IFP (Institut Français du Pétrole) and Doris Engineering have carried out a study to identify the best tendon technology for deep water tension leg platform (TLP) tethers on a quality/price basis. In 1998, this study proved that of the technologies available for carbon fiber composite tendons provided the best economic solution, compared with traditional steel tendons and other composites for the construction of TLPs in deep water. The Freyssinet carbon fiber composite cable and its anchorage is designed for this typical application.

Costing of carbon fibre

The study has concentrated on two independent projects which most realistically simulated the design of TLPs with classical and alternative materials such as steel tubes, steel cables, and composite cables. The first was an installation in the west Shetlands in 1,600 meters and 2,000 meters water depths, and the second in the Gulf of Guinea in 1,000 meters and 3,000 meters water depth.

In addition, Freyssinet and IFP, with their partners, have conceived a specific TLP, making full use of the mechanical benefits of carbon fiber composite cables. These cables are remarkable for their low weight, corrosion resistance, high modulus, high mechanical strength, and the high fatigue resistance essential to the TLP tether.

Thus for each tether technology costs were calculated from the start of the TLP installation through to the exploitation: cost of tethers, transport, seafloor anchorages, platform construction, and tether installation.

The work clearly shows that a TLP fitted with composite tethers is the most cost effective solution in water deeper than 1,600 meters. In addition, the benefits are sensibly greater for a TPL specifically adapted to the characteristics of the Freyssinet carbon fiber composite cable.

Composite cable

Freyssinet is renowned for their expertise and leading position in a wide variety of techniques using high-capacity steel cables in building and bridge construction. The company also designs, develops and produces composite materials. For the TLP application, Freyssinet has designed and developed a carbon fiber composite cable specifically adapted to the offshore environment.

Beyond the required mechanical properties, the cable has been conceived to provide flexibility in assembly, transport and adaptability to the TLP design. As can be seen from an accompanying figure, the cable is modular. Each module consists of a sub-assembly of 19, 6-mm diameter pultruded carbon fiber/epoxy resin composite rods in a hexagonal grouping.

The necessary number of hexagonal modules are assembled in parallel in a compact grouping to provide the required mechanical characteristics of the cable. This modular conception offers several advantages: safer cable, better durability, ease of construction, and modular anchorages that improve reliability and application to the whole cable anchorage.

Modular anchorage

Unlike conventional carbon fiber composite cable anchorages, which are big and unique, Freyssinet has developed and patented a new compact individual anchorage. This type of anchorage allows multiple modules to be assembled into a cable. Thus, for a TLP tether containing multiple modules, each sub-element works individually, and so the safety increases with the number of modules in the cable.

Conventional carbon fiber cable anchorages are unique and very large. They are different for each size of cable, and so each cable size must be subjected to several tests to prove the fatigue and static properties. The modularity of the Freyssinet cable means that:

  • Each module is identical and anchored individually in a small anchorage.
  • Each module in the cable assembly is individually protected against environmental effects.

Thus, there is no limit to the size of cable which can be assembled, and small scale testing at module level is statistically representative of the whole cable assembly. The modular protection provides a high level of safety Freyssinet, IFP, and Doris are continuing the study of the economic advantages of carbon composite cables for TLPs in deep water. The technology demonstrates real benefits, essentially because of the remarkable mechanical properties which require a TLP hull. Carbon composite tethers also provide good economic advantages in installation and reduce the number of sea-floor anchorages. In addition, this new cable technology could be of interest for catenary cables on others structures in deepwater, such as spars or floating production vessels. A cost study is in progress.

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