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Derivation of Vessel Motions as Operational Limits and Motion Forecasting With 2D Wave Spectra: Case Study of Inter-Array Subsea Cable Installation in the North Sea


 The conventional approach for estimating vessel operability for offshore installation operations relies on numerical results from discrete sea state simulations (tabulated operability) and wave forecasts simplified into a set of statistical parameters (Hs, Tp, direction, etc.), leading to an important loss of operability and reliability. Recently, the potential of using vessel motions as cable installation limits has been explored due to the evidence of strong correlations between the vessel motions and the dynamic response of the cable or pipe. This paper presents an end-to-end analysis methodology for calculating operational limits of cable installation operations based on vessel motion criteria using statistical modeling and estimating vessel operability using 2D wave spectra for motion forecasting. This response-based forecasting method mainly benefits from more accurate and realistic wave forecast input, more reliable probabilistic model of the system response (vessel and cable) and avoids the conservative sea state discretization. The method is then applied to the entire sequence of inter-array cable installation in a North Sea offshore windfarm using 2D wave spectral hindcast data, including a weather downtime comparison with the conventional method. Heave velocity was found to be a great and simple predictor of cable response in shallow water applications. Response-based method results showed a significant reduction in vessel weather downtime for array cable installation. This approach could help to reduce the cable installation costs in offshore windfarms considerably, while also increasing the confidence and understanding of the risks involved in the operations.