Maintaining a sufficient distance between an FPSO and a shuttle tanker is critical for the safe offloading of hydrocarbons. Catenary anchor leg mooring (CALM) buoy-based offloading systems with 2-km long, large-diameter lines provide a quick and safe offloading solution and have been considered a preferred choice for offloading operations, especially for remote and deepwater fields.
Because the weight of offloading and mooring lines attached to the buoy are comparable to the CALM buoy's displacement, the dynamic coupling effects between the CALM buoy and its attached offloading and mooring lines cannot be ignored in the offloading line design. But reports of fully coupled dynamic analysis for CALM buoy-based steel offloading system with an FPSO are rare in the public domain. This paper presents the results of fully-coupled and semi-coupled dynamic analyses of offloading lines connected to a CALM buoy and an FPSO to examine the dynamic coupling effect on the design of the offloading lines.
Different types of CALM buoy mooring systems, such as chain, polyester and steel cable, were considered in the study. The semi-coupled analysis proposed in this paper achieved similar results as the fully coupled analysis with more conservative predictions. It was found that the dynamic coupling effect on the strength of the offloading line is important to the CALM buoy-based offloading system. It also was observed that the polyester mooring system is prone to produce larger tensions on offloading lines and larger buoy pitch and heave motions compared to the other two types of considered mooring systems in the intermediate water depth. Finally, both the buoy hang-off point and mid-water buoyancy section of the offloading line proved to be critical locations for strength assessment of the offloading line.