The extreme dynamic response of bottom supported structures using an equivalent quasi-static design wave procedure

While there is great experience with the deterministic and quasi-static design wave method for the design of bottom founded offshore structures, the determination and inclusion of random dynamic effects in the design process is far less developed and only occasionally needed. An attractive procedure is to determine an 'inertial load set' that accounts for the effects of the dynamic response and to add this to the applied loading, after which the same quasi-static design wave method can again be used. This engineering method is described and critically reviewed, after which it is applied to a conceptual design of a compliant tower in 600 m water depth west of the Shetland Isles. This is a demanding test for the method and to verify the results these are compared with the results of random time domain simulations which serve as a reference. The 'inertial load set' is determined in two different ways: (1) based on increases in the global responses (base shear and overturning moment) and (2) based on increases in the level shears at various elevations over the height of the structure. It is shown that the second method provides markedly better results. (C) 1998 Elsevier Science Ltd. All rights reserved.