THE INFLUENCE OF ALTERNATIVE WAVE LOADING AND SUPPORT MODELING ASSUMPTIONS ON JACK-UP RIG RESPONSE EXTREMES

A study is conducted of the response of a jack-up rig to random wave l oading. Steady current and wind load effects are also included. The ef fects of varying the relative motion assumption (in the Morison equati on) and of varying the bottom fixity assumptions are investigated One ''fixity'' model employs nonlinear soil springs. Time domain simulatio ns are performed using linearized as well as fully nonlinear models fo r the jack-up rig. Comparisons of response statistics are made for two seastates. Hydrodynamic damping causes the mts response to be lower i n the relative Morison case. The absence of this source of damping in the absolute Morison force model gives rise to larger resonance/dynami c effects-this tends to ''Gaussianize'' the response. Hence, the relat ive Morison model leads to stronger non-Gaussian behavior than the abs olute Morison model. This is reflected in moments as well as extremes. The different support conditions studied are seen to significantly in fluence extreme response estimates. In general, stiffer models predict smaller mts response estimates, but also exhibit stronger non-Gaussia n behavior. The choice of the Morison force modeling assumption (i.e., the relative versus the absolute motion formulation) is seen to have at least a secondary role in influencing response moments and extremes .