Integrity Assessment of Mooring Systems Using Position Measurements

Floating Production Systems (FPS) remain on station many years without dry docking. The mooring lines, which ensure station keeping, are designed to last the entire service life of the FPS. However, many mooring line failures or premature replacements have occurred in the past. Most existing mooring line failure alarm systems utilize subsea load cells or inclinometers, which are expensive to maintain or replace and are not sufficiently reliable. Hence, this paper presents a new methodology to assess the mooring integrity by means of position measurements only, which is less expensive and potentially more reliable. This methodology for mooring line integrity assessment was developed as part of the LifeLine Joint Industry Project and focuses on application for a variety of FPSOs with different characteristics. The methodology contains two different approaches for the position data processing. One approach is an event-based method. It attempts to identify failure events by the semi-instantaneous shift in the mean static offset of the floater. The second approach is a status-based method. It assesses the integrity of the mooring lines by keeping track of the unit position in a longer time frame.

In this paper the methodology will be described in general and the validation results will be presented. Validation is done by using simulated blind tests as well as in-service measurements of several FPSOs. The validation results of blind tests and in-service measurements are presented for 3 floaters. It was concluded that all mooring line failures were identified by the LifeLine methodology within the target time of two to twelve days, and in all cases the correct bundle with the broken mooring line was identified. This research is a big first step towards realizing a more reliable, more sustainable and less expensive mooring line failure warning system than the existing state-of-the-art systems. Further validation efforts will be continued in the LifeLine JIP, and a first prototype of the LifeLine system will soon be implemented on an FPSO in the field.

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