J R Still, J B Speck and G R Razmjoo Published in 'The Naval Architect', International Journal of the Royal Institution of Naval Architects , March 2000, London, pp 28-36
(Jack Still is at Amerada Hess Ltd; while Julian Speck and Reza Razmjoo are from TWI Ltd)
FPSO options
An oil producer, contemplating the development of an oil and gas field, has to select the most economical way to exploit a field's reserves. In a recent offshore development, an operator decided on the use of a floating production storage and offloading (FPSO) vessel as the most efficient and cost effective way to produce 'first oil', i.e. in the shortest possible time. A decision, based on the economics of the field, was subsequently required to select one of the following FPSO options:
- A purpose-built FPSO
- A conversion of an existing tanker
- An interception of a new build tanker (prior to, or during construction)
The 'purpose-built' and 'converted tankers' options are self-explanatory, whereas interceptions refer to an order placed with a shipyard for a tanker, where the future owner of the vessel is prepared to sell the slot in the shipyard's programme. Although such a vessel would be designed as a tanker, modifications can be made to meet the service requirements of an FPSO.
Conversion of a new-build tanker
In the case of an interception, with the exception of minor modifications, the vessel hull is designed and constructed in accordance with class rules. Structural alterations to the hull, such as the construction of a moon pool, through the bottom of the hull for the mooring system and risers, can either be incorporated into the shipyard building programme, or carried out at another location.
Several factors [1] must be taken into account to ensure that the vessel structure can operate successfully without suffering significant in-service fatigue damage. Although fatigue crack initiation does not discriminate between steels with different mechanical properties, suitable hull materials must be selected with improved fracture initiation toughness and crack arresting properties.
To ensure that any future fracture and fatigue programme can be implemented with confidence, consideration should also be given to additional inspection of critical areas during construction. Poor weld quality, Figure 1, can considerably reduce in-service fatigue life and promote fracture. It is advisable for the FPSO operator to agree with the shipyard to apply additional inspection during construction, i.e. undertaken either by the shipyard, or by the operator's on-site inspection team.
√( πa), where Y is a function of applied loading, crack size and the weld detail geometry. British Standard BS PD 6493 provides guidance on the use of this method for assessing welded structures, including the formulae for calculating ΔK for the weld detail, values of the constants m and C in the 'Paris law', and the method of numerical analysis
[13] .
Since a variable inspection interval is not practical for an FPSO, inspections must be carried out a regular times that can be scheduled well in advance. Using this approach, it is possible to optimise the planned period between surveys of the FPSO's critical hull welds. The main purpose of a fatigue crack growth analysis is to ensure that flaws will not grow to an intolerable size leading to failure between inspections, Figure 12. The precise methods for achieving this goal depend on the practical and economic circumstances surrounding the operation of the FPSO.
Fig. 12. Illustration of use of crack depth versus endurance curve predicted by fracture mechanics to estimate remaining fatigue life
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Conclusions
Ship-based FPSOs are relatively new concepts and unlike conventional tankers, they are semi-permanently moored and may experience rough sea conditions for much of their lives. Two factors have a bearing on the long-term structural integrity of the vessel's hull, i.e. the fracture properties of the outer hull's weldments, and the fatigue performance of fillet welded internal structural details. In any event, questions regarding material selection, fracture and fatigue performance and fabrication practices, with regard to the hull, must be resolved in order to ensure the vessel hull is suitable for its intended service. A fracture control and fatigue assessment programme is one method of establishing the structural integrity of an FPSO vessel's outer hull.
Acknowledgements
The authors acknowledge the guidance of Dr Mike Dawes (Principal Consultant, Fracture Testing, TWI) in preparation of this paper.
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