Dynamic response of offshore triceratops under regular waves with tether postulated failure

Abstract

Triceratops are innovative offshore platforms that consist of a deck, three buoyant legs, and ball joints. Their deck is supported by buoyant legs, which, in turn, are restrained on the sea bed using taut-moored tubular tethers. Ball joints partially isolate the deck from the buoyant legs by restraining the transfer of rotations from the legs to the deck. Triceratops platforms exhibit a monolithic structural action in the vertical plane and a hybrid dynamic behavior resembling tension leg platforms and spar buoys. The present study investigates their dynamic response in regular waves. Response amplitude operators (RAOs) are estimated for intact and postulated failure conditions of the tethers. Based on detailed numerical investigations, it is seen that the pitch response of the deck increases significantly under the postulated failure of tethers, which are otherwise absent due to the presence of ball joints. Marginal change in surge response, even under tether failure, confirms the suitability of triceratops for ultra-deepwater installations. The fatigue life of tethers before and after postulated failure conditions is compared using the S-N curve approach and noticed that the adjacent tethers’ fatigue life reduces significantly under postulated failure conditions of tethers.