Dynamic Response Behaviour of Stiffened Triceratops Under Regular Waves: Experimental Investigations

Abstract

Among the various compliant type offshore structures, offshore triceratops is one of the recent innovative designs that is developed for exploration and production of oil and gas fields in deep and ultra-deep waters. Triceratops consists of a deck, which is supported by the Buoyant Leg Structures (BLS). Buoyant legs are connected to the deck by ball joints and held down in position by taut-moored tethering system. As buoyant legs are independently flexible, they need to be stiffened with the intermediate stiffeners to integrate them for counteracting the encountered lateral loads. Presence of ball joints restrains the transfer of rotational response from the BLS to the deck and vice-versa; only translational motion is transferred to the deck. Stiffened buoyant legs impose sufficient rigidity in the vertical plane, making it similar to that of a taut-moored Tension Leg Platform (TLP). The objective of present study is to show the reduction in transverse motions of offshore stiffened triceratops in operational sea state by increase in tethers stiffness (pretension). Hence, the study is focused on experimental investigations of a scaled model of a triceratops with stiffened buoyant legs under regular waves. Results show that the free-oscillation characteristics conform to a stable behavior of the platform under calm sea state. Deck exhibits lesser roll response under the chosen sea states, highlighting the advantage of ball joint between the deck and BLS units. However, the surge response is relatively high, which is controlled by stiffening the buoyant legs. This type of behavior is advantageous to upkeep more facilities on the deck and ensure comfortable operation during moderate sea states.