Abstract
Floating spar platform has been proven to be an economical and efficient type of offshore oil and gas exploration structure in deep and ultra-deep seas. Associated nonlinearities, coupled action, damping effect and extreme sea environments may modify its structural responses. In this study, fully coupled spar–mooring system is modelled integrating mooring lines with the cylindrical spar hull. Rigid beam element simulates large cylindrical spar hull and catenary mooring lines are configured by hybrid beam elements. Nonlinear finite element analysis is performed under extreme wave loading at severe deep sea. Morison's equation has been used to calculate the wave forces. Spar responses and mooring line tensions have been evaluated. Though the maximum mooring line tensions are larger at severe sea-state, it becomes regular after one hour of wave loading. The response time histories in surge, heave, pitch and the maximum mooring tension gradually decreases even after attaining steady state. It is because of damping due to heavier and longer mooring lines in coupled spar–mooring system under deep water conditions. The relatively lesser values of response time histories in surge, heave, pitch and the maximum mooring tension under extreme wave loading shows the suitability of a spar platform for deep water harsh and uncertain environmental conditions.
Highlights
Offshore oil and gas exploration from shallow and intermediate water depths is traditionally carried out using the conventional jacket type fixed platforms
A spar platform is a compliant floating structure used for deep water applications of drilling, production, processing and storage plus off-loading of ocean deposits (Halkyard 1996; Islam et al 2011a)
Chen et al (1999) presented the response of a spar constrained by slack mooring lines to steep ocean waves by two different schemes: a quasi-static approach (SMACOS), and a coupled dynamic approach (COUPLE) to reveal the coupling effects between spar and its mooring system
Summary
Offshore oil and gas exploration from shallow and intermediate water depths is traditionally carried out using the conventional jacket type fixed platforms. A spar platform is a compliant floating structure used for deep water applications of drilling, production, processing and storage plus off-loading of ocean deposits (Halkyard 1996; Islam et al 2011a). Coupled dynamic behaviours of hull/mooring/riser of a spar platform have correspondingly been investigated by several researchers (Chen et al 2006; Kim et al 2005, 2001; Islam et al 2011b). Though application of spar platforms is rapidly increasing all over the world, there is a lack of precise modelling and nonlinear coupled response investigation in extreme sea environments. The main objective of this study is to idealize the spar mooring integrated system as a fully coupled structure; to study the damping effects of mooring lines and to investigate the nonlinear responses under extreme wave loading. Nonlinear coupled responses under the severe sea state have been evaluated in the form of translational motion at surge, heave and rotational motion in pitch direction along with the mooring line tension
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