Dynamics & Vibration Research Group

Depletion of reserves at shallow water depths has seen the oil and gas industry move towards floating rather than fixed offshore structures in order to facilitate the extraction of hydrocarbon. Reserves are often located within exposed conditions, subjecting the mooring system of the floating structure to the actions of considerable environmental force. The potential environmental and economic consequences of a mooring system failure demands careful consideration of the most probable strength and motion extremes. With a gradual move towards ultra-deep water sites and the depth limitations of existing test facilities, there is a need to re-evaluate the reliability of current design methods and promote the use of a statistically consistent framework in future design effort.

Project Details

By developing an efficient code to accurately compute the statistics of floating structure motions, the short-term reliability of mooring system components may be determined. Equivalently the most probable extreme of any design action may be computed for a specified, rather than inferred, reliability level. By accurately combining the low and wave frequency motion components, this rigorous approach permits the empirical methods of existing design codes to be directly questioned.

In order to compensate for system uncertainty, existing design codes specify a series of generic safety factors, which are unlikely to be appropriate in all possible design cases. By introducing a rigorous, consistent framework to include potential parameter uncertainty in the calculation of component reliability, the adequacy of these factors of safety may be challenged. Where the uniqueness of the system prevents specific application of a traditional design approach, the efficiency of this method provides rigorous calculation at a minimal expense.

Design code methods are statistically inconsistent in that the selection of a single critical design storm is separated from the calculation of design action extremes. A statistically consistent measure of system reliability may be found from a long term design approach. While usually too expensive to compute, the development of an efficient algorithm provides rapid long term reliability assessment within a time frame commensurate with conventional design. Such efficient, accurate tools may further promote the current shift in design codes to the inclusion of reliability methods as a viable design alternative.

The fatigue life of critical mooring system components is traditionally specified as a discrete application of Miner's law, where damage from a set of 'appropriately selected' storms is summed. By re-casting the equations in integral form and summing over all possible conditions, a more accurate fatigue damage assessment may be made. The introduction of an efficient calculation method not only makes this computationally feasible but also alleviates the need to choose a sufficient number of 'appropriately contributory' storm conditions.

	Figure 1: Example Floating Production Storage and Offload vessels (FPSO's)
	Figure 2: FPSO Mooring system schematic
	Figure 3: Project overview

Relevant/Recent Publications

1. Andrew J Grime, Extreme Response Prediction of FPSO Motions using Realistic Uncertainty Models, First year report, CUED, 2002
2. Andrew J Grime and R S Langley, On the efficiency of crossing rate prediction methods used to determine extreme motions of moored offshore structures, Applied Ocean Research, Accepted for publication, 2003
3. Andrew J Grime and R S Langley, On the reliability of empirical methods used to calculate extreme motions of moored offshore structures with uncertain design inputs', Applied Ocean Research, Submitted, 2003
4. Andrew J Grime and R S Langley, On the rapid assessment of long term reliability and mooring line fatigue damage for floating offshore structures, In progress, 2003

Principal Investigator & Researchers

• Professor R. S. Langley
• Andrew J Grime

Funding Bodies

• duration of funding: 2001-2005
• Cambridge Australia Trust
• ORS
• EPSRC