COMPLETE PLANNING OF A FPSO CONVERSIONTAKING INTO ACCOUNT STRENGTH AND FATIGUEASPECTS

Abstract

There is an ever-increasing trend to convert existing tank vessels into Floating Production Storage/Offloading Systems (FPSOs). These FPSOs, in many cases, are to be on site for upwards of twenty years. With an extended period of time on station, careful planning needs to take place before the actual shipyard conversion. Overall hull girder strength, local structural member strength, corrosion, and fatigue need to be taken into account in this planning process, not only for the current state of the vessel, but also in the projected condition to avoid costly repairs that would require time off site. Required structural member strength and hull girder strength can be significantly different than that required for unrestricted worldwide tanker service. Many of the wave environments in which FPSOs operate are usually more benign than those wave environments that classification Rules have historically considered in design. The reduction in environmental loads should be credited in determining the required strength on site. This paper will present a procedure and methodology using the ABS/SafeHull technology to determine the site specific structural strength requirements taking into account all of the relevant failure modes that may be experienced by the vessel. Once the strength for the specific operating site is known, the current state of the vessel can then be assessed to determine what areas of the structure will need to be reinforced to ensure adequate performance for the desired time on site. To ensure this longer span between shipyard visits, corrosion must be directly addressed and predicted in the planning process. The results of this predictive corrosion analysis must then be incorporated into determining the level of renewals required at the time of conversion. Transactions on the Built Environment vol 29, © 1997 WIT Press, www.witpress.com, ISSN 1743-3509 330 Offshore Engineering It is well known that fatigue is a cumulative damage phenomena, therefore it must be treated in a separate manner than that of strength. An assessment must be made to determine the number of fatigue cycles consumed during the ship phase (the time spent in tanker service before the conversion) and the number of fatigue cycles that will be consumed during the phase. The prediction of fatigue cycles consumed on site must take into account all of the operating parameters such as environmental data for the site, weathervaning of the vessel, zero speed, etc... Once the determination of these two components has been made, the predicted damage is compared against the fatigue resistance that is inherent in the detail under investigation. This comparison can then be used to plan repairs necessitated by fatigue. This paper will present a detailed methodology using the ABS/SafeHull technology to quickly ascertain past and future fatigue performance. As can be seen, it is very important to consider all aspects of the structure and failure modes when planning a FPSO conversion, both at the time of conversion and at the end of the desired service life. Proper conversion planning will alleviate any costly down time that could be required for structural repairs of the vessel.