Breaking wave slap loading on FPSO bows and shallow water cylinders
In 1989 the Schiehallion FPSO suffered bow damage from a steep fronted wave slap and the uncertainty in how to design for this type of loading became a concern to the oil industry and the regulators. The aim of this study is to research the insight of breaking wave impact on the bow of ship-type offshore structures experimentally and develop a methodology on how to design this type of loading. Steep wave impact pressures and the structural dynamic response on FPSO (shipshaped Floating oil Production Storage and Offloading vessel) bows are studied using 1180 scale instrumented models and time domain simulation with the funding from HSE and BP, a grant from EPSRC, associated in-kind industrial contributions, a University/Departmental Scholarship and an IMarEST Scholarship. This work has increased the understanding of the nature of the breaking waves that can cause large slap forces that are important for the design of offshore floating structures (and should also be relevant to ship design). Methods of generating model scale wave groups that should produce approximately the 1 in 3 hour maximum loads, when large waves break in unidirectional sea states prescribed by Hs and Tz, have been developed. These methods have been extended to spread seas and also to a 'partial' breaking wave in less steep seas, but no testing has taken place in spread seas or the longer period seas. In addition an empirical relationship has been determined that represents the steepening of a wave front based on the underlying linear wave. The forces and pressures from these waves have been measured on 1180 scale models of the Schiehallion FPSO and Loch Rannoch shuttle tanker. A time history simulation method of bow loading in random seas has been developed. It uses the wave front steepening relationship derived from the tests and a relatively simple slap force prediction based on velocity times rate of change of added mass. Incident wave pressure effects (with a non-linear correction) and added mass times acceleration forces are also included. Simple slam coefficient type formula has also been derived for easy application. The formula accounts for the effect of the size of the loaded area on the average pressure and the rise and decay times of the average pressure and, hence, the dynamic amplification of the response at the bow. The above experimental and theoretical work has considerably advanced the quantitative understanding of bow slap. Quantitatively we have some confidence in the most probable maximum slap force predictions in: long-crested seas with sea state steepnesses around 1114 - 1115 and when no air is trapped.