An investigation into the effect of turret mooring location on the vertical motions of an FPSO vessel

Turret-moored floating production storage and offloading (FPSO) vessels hav e Sound application in several offshore oil and gas fields in Australia's N orth West Shelf (NWS). These vessels are either custom-built or converted t ankers, with an internal or external turret The position of an internal tur ret is decided based on a number of design considerations, primarily, avail able deck and interior space, and weathervaning capabilities. It is known t hat turret position can influence vertical motions and accelerations of a v essel, but this factor has not been given much importance, in comparison wi th the effects on the horizontal plane motions, primarily surge. This paper presents the results of a pilot study conducted at the Australian Maritime College, Tasmania, to study the vertical motions of a single-point moored FPSO model in waves, while systematically varying the mooring position acro ss the length of the model. The displacement of the vessel was held constan t at 50-percent-loaded condition. A single-point mooring system was designe d and implemented on the model to simulate the prototype turret mooring sys tem. Results show that the mooring location significantly affects the verti cal motions and accelerations of the vessel. Astern turrets were found to p roduce higher heave and pitch than other locations tested. Although turrets positioned close to the longitudinal center of gravity produced the lowest overall motions, it is suggested that turret position forward of midships be preferred, as it provides a balance between lowering vertical motions an d improving weathervaning characteristics.