Deployment of Subsea Equipment: Qualification of Large Diameter Fibre Rope for Deepwater Construction Applications

Abstract

Abstract This paper describes the SIRIUS (Safe Installation with Ropes In Ultradeep Sea) JIP full scale testing program conducted to determine the bend fatigue performance of large diameter fibre ropes with HMPE fibres (Dyneema®) on small diameter sheaves, together with a summary of the results and the impact the results have on installation operations. The test rig has previously been used for testing of Ø109 mm steel wire ropes and these results will be used as benchmark for the fibre ropes tested at similar Safety Factors. Steel wire rope together with active heave compensation has been used for subsea deployment applications for many years. However, with the requirement to install heavy subsea hardware in increasingly deeper water, there is a need to use large diameter fibre rope as part of the deployment system. As the water depth increases, the high self-weight of steel wire rope limits the maximum payload of conventional steel-based deployment systems. Fibre ropes with HMPE fibres such as Dyneema® are however buoyancy neutral and can therefore carry the same payload at ultra-deep water, whereas steel-wire ropes generally reach a self-weight limit at around 2000 meters with SF=5. During the deployment of subsea equipment, the fibre rope will be subjected to bend fatigue loading when it is bent over the heave compensated sheave. This failure mechanism is the dominating factor in the degradation process of the rope. At present no generally accepted design standard for bend fatigue of fibre rope exists. In the SIRIUS JIP, cyclic bend over sheave fatigue tests were performed within normal industry range of safety factors. The bend fatigue tests show a slightly better fatigue life for the engineered fibre ropes tested at higher safety factors when compared to multi-strand steel wire ropes. These promising results are illustrating that large diameter fibre ropes on sheaves have a potential to be used in subsea deployment systems containing active heave compensation. Hence, the work performed in the SIRIUS JIP is an important step on the way towards developing safe usage for fibre ropes during subsea installation in ultra-deep sea. Background The trend to produce oil in increasingly deeper water has led to development of subsea fields imposing challenges on the lifting appliances in terms of capacity, weight, size and space constraints on the construction vessels. The installation of the subsea equipment is often performed by hoisting the equipment by steel wire ropes running over a heave compensated sheave. In recent years, however, the use of fibre ropes for this application has been more and more accepted. During subsea installation, the fibre rope will be subjected to bend fatigue loading over the sheave. Present design routines for fibre ropes are based on safety factors related to the static breaking load and no generally accepted design standard for fatigue lifetime exists. The main objective of the SIRIUS JIP is to benchmark the large diameter fibre ropes against previously tested Ø109mm steel wire ropes under identical conditions and with the same sheaves (D/d of about 20), see ref Vennemann et al 2008-1, Vennemann et al 2008,-2 and Vennemann et al 2008-3. Cyclic Bend Over Sheave (CBOS) testing with fibre ropes with as large diameter as 100mm has never been conducted before and the tests presented in this paper are therefore ground breaking.