Lifetime reliability based design of an offshore vessel mooring

Moored floating vessels offer an attractive solution to oil and gas recovery in a range of water depths. The design of moorings for these compliant vessels require careful prediction of the lifetime response extremes. Reliability based predictions offer more accuracy than prescriptive schemes but at a higher computational expense: existing tools are in need of some improvement. Here a frequency-domain response model is coupled with a pseudo-asymptotic integration scheme to provide a rapid estimate of lifetime reliability and hence the extreme response at a design target level. The potential lifetime environmental and system parameter variation is consistently included. Reliability bounds and an associated convergence criteria are also defined. This methodology is tested on a representative FPSO in a wave-dominated environment. A comparison with traditional single-storm design methods highlights a significant variation in the 100-year excursion response. The convergence of bounds on the lifetime reliability at target levels consistent with existing design codes is shown. The provision of additional design information as a byproduct of the pseudo-asymptotic analysis is demonstrated.