The hull forms used for FPSOs and trading tankers look very similar. So similar in fact that it may appear that one could use the same design and design methods for both vessels. Both vessels will be subject to extreme and fatigue loading and both will have a similar design life. But each will have different design requirements and therefore different designs.
The trading tanker spends most of its life moving oil from one location to another, in either one of two draft conditions, ballast or laden. They are dry-docked at regular intervals for maintenance, inspection and if necessary repair.
FPSOs are stationary, i.e. they have zero forward speed, the vessel's draft is continuously changing, the maintenance and inspection is done in situ, and they are not usually dry-docked.
These apparently subtle differences, see Table 1, in design requirements are not so subtle with respect to the load differences, both short term and long term for the two designs. This difference is the driver for the FPSO Integrity Joint Industry Project.
The JIP was established to obtain fatigue load information on an FPSO through full-scale measurements. The main goals were as following.
Develop of a computational model for fatigue loading
Evaluate current practice.
Have the Classification Societies conduct a comparative study, and
Recommend a fatigue method for use in design of FPSOs.
The work in this JIP is being further enhanced through a sister JIP "FPSO Fatigue Capacity" being administered by DnV. By combining the results of the two JIPs, the industry is gaining a better knowledge of the fatigue loads and fatigue resistance on FPSOs. Predictive methods are being developed to calculate the fatigue lifetime of FPSOs and this is leading toward more reliable systems.
In the upstream oil industry it can be generally said that all ideas are bred, birthed and reared from within. In order to maintain this axiom it often means that nothing gets implemented in the upstream sector unless they have thoroughly analyzed and tested it and meticulously prepared proper standards and specifications. The idea of using an oil trading tanker as a floating production, storage and offtake unit is one solution that breaks the rule. It is a crossbreed, a marriage between upstream and downstream, a child of unsuspecting parents. It arrived in time to feed a hunger in the industry to expand operations into the seven seas. It looked positively prefect; it was a solution waiting on a problem. In the enthusiasm to apply the solution not all areas of design and design methods were assessed. The industry has now reached a stage where they are asking the question whether or not shipping standards are applicable to FPSO designs.
The design rules for trading tankers developed through time by trial and failure. In the past, the rules did not recognize fatigue was not a primary issue in ship structure design. In many FPSO cases this is not suitable.