Precise station keeping is a requirement of mobile offshore drilling units (MODUs) and production vessels operating in ultra-deepwater. DeepSea Engineering and Management in co-operation with Petrobras is undertaking a concept, feasibility development study and scale prototype testing of pultruded Carbon Fibre Reinforced Plastic (CFRP) rod for ultra-deepwater mooring line applications. For a given capacity this paper illustrates the potential the technology has to compete with polyester lines on weight per unit length and with steel lines on diameter.
This paper presents a Carbon Fibre mooring line for MODU applications as part of the PROCAP 3000 Program. It describes the suitability of CFRP as a reelable mooring system, presents a design approach and analysis process. One of the key aims is to use existing vessels, equipped to handle steel lines, for ultra-deepwater deployment of CFRP lines with no or minimal modification. The product undergoing testing was designed for 200 Te breaking load with a cross section identical to its steel equivalent and circa ¼ the weight. DeepSea Engineering working with Reading University in the UK successfully tested CFRP terminations to circa 80% efficiency using conventional rope open spelter sockets with a proprietary potting compound.
CFRP is being considered for ultra-deepwater MODU mooring lines because of its potential for substantial weight reduction and increasing payload for the same diameter as an equivalent steel wire. In addition, existing polyester mooring line systems for ultra-deepwater are significantly larger in diameter for a given load capacity when compared to steel lines. Dedicated handling vessels are required to facilitate synthetic mooring installation, increasing deployment costs. CFRP has the potential to utilize existing fleets equipped to handle steel lines with little or no modification. Figure 1 below illustrates the potential benefits of CFRP lines over steel and polyester lines by comparing specific strength and modulus.
Figure 1 Comparison of specific strength and modulus of existing rod, cable and tendon technologies1 (Available in full paper)
Figure 2 Comparison between the relative diameter and weight/unit length between the technologies for a 200 tonne line (Available in full paper)
1 Data obtained from appropriate sources such as company literature and web sites (1)
Figure 2 illustrates the relative diameters and weight per unit length between the competing technologies for a 200 Te line. CFRP shows a competing diameter with spiral strand steel and roughly half the diameter of polyester. CFRP is 20% of the weight per unit length of spiral strand wire rope and competes with polyester.
Current CFRP line technology is largely centered around stressed members; rods, tendons and stranded cables, for civil engineering applications such as pre and post-stressing concrete and cable stayed bridges. The majority of the products have been developed by the Japanese. Carbon Fibre Composite Cable (CFCC) was developed by Tokyo Rope, Nippon Steel Advanced Carbon Fibre Cable (NACC) developed by Nippon Steel in Figure 3 below. These products have been deployed in numerous civil applications thoroughout the world (1).
Figure 3 Left to right, CFCC & NACC, Examples of CFRP rope (Available in full paper)