Experimental Investigation on Hydrodynamic Performances of a Spar FPSO

Abstract

A deepwater Spar Drilling Production Storage Offloading (SDPSO) floating system or Spar FPSO that integrates Spar dry-tree production, oil storage and offloading has been proposed for offshore oil exploitation. Benefits of the deep drafted first generation classic Spar, such as excellent stability and hydrodynamic performances that allow dry tree drilling and production, large capacity of the mid hull section for oil storage, cost efficiency for construction and save in-service operations, are combined to provide a competitive solution from moderate deep water of 300m to ultra-deep water of 3,000m. The present Spar FPSO is proposed for the potential deepwater oil field development in South China Sea. It uses the hull mid-section with large capacity for wet oil storage, utilizing the density difference between the oil and the sea water for wet storage and oil-water displacement in the storage tank. To demonstrate the feasibility and to investigate its hydrodynamic performances of the Spar FPSO, comprehensive experimental investigations, including the in-place model tests in the deepwater offshore basin and the VIM model tests in the towing tank, have been undertaken. For the in-place model tests, both the stand alone and the tandem offloading conditions were considered. The classic Spar hull, the taut mooring system and the top tensioned risers (TTRs) were all included to represent the coupling effects of the whole system. Various metocean environments, such as the 100-year storm, the 1-year storm and the offloading condition, were considered to give a complete assessment. Different mooring configurations were also studied to adapt to the deep and moderate water depths correspondingly. The global responses, such as the hull six degree of freedom motions, top tensions of mooring lines and risers, possible green water occurrences, were monitored and recorded in real time. Additionally, a preliminary VIM experiment was also carried out in the towing tank since the vortex induced motions (VIM) is one of the most concerned issues for the deep drafted Spar. The VIM characteristics with different heading angles and current velocities were obtained. It is demonstrated that the Spar FPSO has satisfactory hydrodynamic and VIM performances in metocean environmental conditions in South China Sea. It gives a feasible and competitive alternative with integrated dry-tree drilling, production, oil storage and offloading functionalities for offshore oil field development in deep water depth.