A key aspect in the design of a mooring system for a floating production unit is the estimation of the extreme mooring line loads for a specified short-term sea state of typical duration equal to 3 hours. Commonly used design approaches today are based on time-domain simulations whereby each 3 hour sea state is run a number of times (typically 10–30 times) to represent the randomness of the sea. A maximum response is recorded from each simulation. Particular statistic of the maxima data (e.g. mean, most probable maximum or a percentile) is used to represent the extreme mooring load for which the lines are designed. This paper studies and assesses the accuracy of obtaining design value from a population of maxima with reference to the mooring line load of a large ship-shaped floating production vessel. A coupled model, including all mooring lines and risers, has been developed, validated and used to generate responses for 100yr extreme condition and 10,000yr survival condition. To establish an accurate benchmark against which the results are compared, the time-domain analyses (duration 3 hours) are repeated 170 times, for each sea state, to represent different random realisations of each environment. It is examined how the accuracy of predicting the design mooring line load, from a sample of response maxima, improves as the number of simulations is increased progressively from 10 through to 170. The assessment is performed across different statistics of maxima that are usually chosen to represent the design response. Besides the mooring line load, other response parameters such as heave and turret excursion, are examined in this paper. The paper examines whether the severity of the response (100yr vs 10,000yr storm) or the response variable affect the number of maxima required to achieve statistical stability. The results indicate fitting a Gumbel distribution to the maxima from about 30–40 simulations can yield results that are statistically stable and accurate and are recommended as preferred methods of estimating the design response.

Floating liquefied natural gas (FLNG) facilities are a new type of offshore structure, which have been developed as a game changer in offshore hydrocarbon development for unlocking stranded gas reserves. One of the key challenges associated is offloading from FLNG facilities to LNG carriers. Offloading may proceed with vessels in a side-by-side configuration, which allows offtake by un-modified vessels and minimizes requirements for new hardware or procedures (e.g. compared to a tandem operation). Significant challenges remain, however, and reliable offloading is critical for successful FLNG implementation. In this scenario, the two vessels are separated by a narrow 4 m wide gap. The resonant response of the sea surface in the gap has been predicted by numerical simulations [1] to be a few times that of the incident waves at particular frequencies. As a consequence, the gap resonant response may play a role in determining the operational window for side-by-side offloading operations, and thus has attracted a lot of attention recently. There have been studies on this topic both numerically and experimentally. However, many of these studies are in 2 dimensions (2D), for relatively large gaps and relatively shallow water depth, which may pose difficulties when extending the results to a real project. It is unclear what will happen for a gap resonance if the gap width gets narrower (say 4 m in full scale) and the water depth gets deeper (say 600 m in full scale). In this study, we conducted a series of model tests at a scale of 1:60 in a large wave basin, and focused on deep water and, crucially, narrow gaps, which are closer to a real project geometry. To facilitate future numerical simulations, we used two identical fixed bodies in the model tests and the vessels were simple barge-like shapes. Using white noise waves as the excitation, which covers a broad brand, the response of the fluid in the gap has been measured at several points. In these experiments, different modes of the gap resonance have been observed. Response amplitude operators (RAOs) of the gap resonance have been obtained through spectral analyses, which provide valuable information for the design of side-by-side operations and will benefit future numerical simulations. Test runs in white noise waves with different significant wave heights were also performed, to study the nonlinearities of the gap resonance phenomenon.