Mooring System ULS Analysis Based on Empirical QTFs of Wave Drift Loads

Abstract Ultimate limit state (ULS) analysis for the mooring system of floating structures is performed for the most severe seastate conditions. For many structures, horizontal wave drift loads and related slow drift oscillations are simultaneously the environmental component with the largest contribution to the extreme line loads and the component with the largest uncertainty in numerical predictions. It is acknowledged that state-of-the-art hydrodynamic design analysis tools, namely radiation/diffraction potential flow codes, tend to underestimate wave drift loads in severe seastates for floating structures with large waterplane area. This is often the case for floating, production, storage and offloading vessels (FPSOs). While correction methods have been proposed recently, this paper presents a different approach. The proposal is to use empirical quadratic transfer functions (QTFs) of horizontal wave drift loads directly in the mooring analysis time domain simulations. The empirical QTFs are identified from model test data in severe seastates with current by a second order signal analysis technique known as cross bi-spectral analysis. The paper describes the method and validates with a case study consisting of a FPSO operating in the North Sea. Standard hydrodynamic analysis tools provide correct first order motions in extreme seastates with current, as compared to model test data. However, it significantly underestimates the wave drift loads and the related low frequency (LF) horizontal vessel motions. Use of empirical QTFs significantly improve the LF motion predictions, both in terms of mean offset and in terms of LF standard deviation and energy spectra. Furthermore, the limitations of Newman’s approximation versus use of full QTFs are highlighted. The later provides a much better phasing of the LF motions and also better prediction of the LF peaks.