An ULS reliability-based design method for mooring lines using an efficient full long-term approach

•Importance sampling monte carlo simulation (ISMCS) is a powerful and efficient tool for assessing long-term extreme tension responses in mooring systems.•ISMCS enables the use of a multi-dimensional environmental parameters distribution that statistically describe wind and swell waves separately.•The use of ISMCS methodology coordinated with a detailed description of the environmental scenario can lead to more accurate extreme tension response estimates, without being overly conservative.•The use of ISMCS enables an accurate and precise calibration of a ULS LRFD design methodology. In the long-term scenario, the environmental actions to which floating offshore structures are subjected to, such as waves, wind and current, are non-stationary stochastic processes. However, this long-term behavior is usually modeled as a series of short-term stationary conditions. In a full long-term analysis approach, an estimate of the N-year response can be obtained through a multi-dimensional integration over expected short-term environmental conditions. An innovative and more efficient long-term integration approach based on the Importance Sampling Monte Carlo Simulation (ISMCS) method is presented, where the uniform distribution over an environmental contour is used as the sampling function. In parallel, a multi-dimensional joint environmental model that statistically describes all relevant environmental parameters is employed, contemplating linear and directional variables, and thoroughly accounting for the occurrences of wind waves and swell. The methodology is applied to two FPSOs systems installed in Brazilian ultradeep waters. Ultimately, a design-oriented procedure based on the developed methodologies is provided, using an Ultimate Limit State (ULS) reliability-based design with calibrated safety factors in an LRFD (Load and Resistance Factors Design) format. It is shown that the developed procedures can be powerful tools to account for the simultaneous occurrence of wind sea and swell waves in offshore system response evaluations required in the design and life extension analyses.