Installation of subsea equipment- design and planning with focus on specification of operational limits

Abstract

In this thesis, an investigation of a 170 [t] dual Cap-X template launch through the splash zone has been conducted. The dual Cap-X solution is based on two suction anchors as the foundation in addition to a framework structure that holds subsea equipment and trawl pro- tection covers. Both a simplified method outlined in DNVs recommended practice for marine operations and a numerical time domain simulation in SIMO has been utilized to establish the operational limits based on acceptance criteria for dynamic tension in the lifting wire. Skandi Acergy is used as installation vessel with a crane capacity of 400 [t]. A numerical model of the operation has been analyzed in SIMO. The resulting hydrodynamic forces in such a case are highly dependent on the timing of the incoming waves and motions of the vessel. Therefore several simulations of the same realization has been performed, and by statistically fitting the extremes to a Gumbel distribution, a resulting characteristic load has been found for different sea states based on acceptance criteria from the regulations and recommendations given by DNV. Based on the results of the simplified method, the opera- tional limits where found to be HS = 2 [m] for a lowering speed of 0.5 m/s, and HS = 3 [m] for a lowering speed of 0.2 m/s. The simplified method was however found to be conserva- tive compared to the results from the time domain simulations, where non of the sea states investigated exceeded the acceptance criteria of the crane. Further, a proposed method of addressing the uncertainties in weather forecasts has been compared to the traditional alpha factor method. The proposed "forecast of response" take directly use of ensemble forecasts as input to the SIMO model and simulates actual responses prior to operation. The test case reveals that the "forecast of response" method contributes to less dynamic forces in the hoisting line compared to the alpha factor method. A shortcoming of the alpha factor approach is that it does not consider peak periods. The factors are only applied to significant wave heights of the deterministic wave forecast issued. One has to rely on hindcast data for the specified location of operation, and consider several peak period values in order to be sure whether or not an operation can be executed. The "forecast of response" method take the wave directions and possible swell seas into consideration, in addition to significant wave heights and peak periods.