Dynamic amplification of drag dominated structures in irregular seas

Offshore bottom-fixed structures, such as jackets, are typically designed with a natural period of 3 seconds or less. This means that there is limited dynamic excitation from wave loads at design sea-states. The benefit of this is reduced uncertainties from the dynamic effects in the design phase. However, some jacket structures in deep water or soft soil-conditions may have natural periods exceeding 4 seconds, which implies large dynamic effects. For a jack-up, industry guidelines are developed to account for the dynamic structural response in both ULS and FLS conditions. Although advanced methods are available to determine the dynamic amplification factors (DAF), they are unsuitable to estimate the dynamic response in the concept design phase. The current simplified method is based on a single-degree-of-freedom (SDOF) system. However, the estimator is not very accurate for the DAF of a structure in irregular seas, and must be used with caution. The most correct method for estimating the extreme static and dynamic responses is stochastic time-domain simulation. In this paper, it is investigated whether an analytical formulation is appropriate for DAF in irregular seas, as a function of natural period. The models used are a slender cylinder and a dynamically sensitive jacket, in an extreme sea-state. Non-linear quasi-static and dynamic time-domain analysis are carried out with USFOS, a computer program especially designed for space frame offshore structures, using linear wave theory and the Morison equation.