Ultimate and accidental limit state analyses of mooring systems for permanent floating storage units (FSU) or floating production, storage and offloading units (FPSO) require calculations of extreme mooring line tension in severe storms. Return periods for such storms are 100 years for field locations in Norwegian waters. In such sea states there are load effects from waves alone or in combination with current which are not captured by standard numerical tools. Calibration and verification against experimental data from model test basins are therefore essential to ensure that complex physical effects are incorporated into the numerical models.

The extreme mooring line tension in severe sea states will often be underestimated if calibration against experimental data is not performed. The main goal of this paper is to demonstrate how to calibrate numerical models for large floating structures based on experimental data, in order to improve the estimates of extreme mooring line tension. This is illustrated by studying two permanent production units in the North and Norwegian Seas, which represent different physical phenomena that are not captured by standard numerical methods.

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