At present conventional steel wire is preferred for deepwater deployments. Its properties and performance are well know and there are a clear set of standards and guides for steel wire’s use in offshore applications. In addition, it has good over-sheave fatigue resistance, heat and cut tolerance. However, as the water depth increases, the ratio of the weight of the cable to the weight of the payload becomes critical. At 3,000m the weight of a 5” wire rope is about the same as its 170t payload at a depth of about 6,000m the safe working load (SWL) of the steel wire rope is entirely used up by its self-weight, leaving zero payload capacity.
Faced with the self-weight limitations of steel wire rope, HMPE (high modulus polyethylene) synthetic rope is a popular alternative due to its high strength, excellent mechanical properties, low density and, most important of all, neutral buoyancy.
Winch System – Keep It Simple
Of course steel wire or synthetic rope selection is a bit more complicated than this. The other deciding factor is the type of winch: single drum or traction winch. A single drum is preferred wherever possible. However, using fibre rope on a single drum is problematic, the tendency of the fibre rope to ‘knife’ down into underlying layers on the drum causing spooling problems and an increased risk of rope abrasion.
Synthetic fibre ropes on traction winches must offer adequate bending fatigue life and be resistant to heat generation under repeated rapid cycling through a heave compensator and bending, or slipping, on sheaves, and exhibit high axial stiffness and strength even after thousands of loading cycles.
Recent developments to address concerns over the fatigue life of synthetic fibre, and the inability to visually inspect the rope through the outer braid, have concentrated on using computer models to build up a predictive picture of usage over the length of the rope. By recording the section of rope used on the traction winch and the load sustained, it is able to show which parts of the rope have been worked and compare the wear with cyclic bend testing. The rope is then reversed to expose a fresh section of rope to the traction winch.
Steel-like Rope Required
Resolving the synthetic rope issues, the industry by developing new types of winches fails to address the fundamental problem – the rope. A better approach to deepwater deployment with synthetic ropes advocated by Caley Ocean Systems is to tackle the problem at its root – use a fibre rope with ‘steel-like’ properties. Such a synthetic rope is, in fact, available now. It exhibits tight dimensional tolerance, retains its roundness under compression rather than flattening, exhibits high resistance to surface abrasion, and has excellent MBL and friction characteristics.
Caley and high performance rope manufacturers, Hampidjan, Iceland have formed a strategic alliance to bring the benefits of suitable designed, tested and certified steel-like fibre ropes to the offshore market. While the general benefits of fibres ropes have long been appreciated concerns over handling, inspection, qualification and reliability have limited their application offshore. Caley and Hampidjan are able to demonstrate that much of this concern is ill founded. By exploiting the performance characteristics of Hampidjan’s patented DynIce rope construction, it is possible to both avoid specialist handling techniques and to even change-out wire rope on cranes and winches without significant modification.