On design criteria for a disconnectable FPSO mooring system associated with expected life-cycle cost

ABSTRACT

Some floating production, storage and offloading units (FPSOs) possess disconnectable systems to avoid harsh environments. According to a literature survey, the practice is based on perceptions and experiences of operators to judge disconnection; however, this paper offers a rational approach. A life-cycle cost model is proposed to optimise (1) the disconnection criteria and (2) the design of mooring lines under reliability format. Relevant ultimate limit states are considered in association with hull, moorings and green water failure. Effects of future failure costs are considered (downtime, environmental damage, reputation, etc.). Disconnection criteria are then formulated in terms of significant wave height and wind speed limits. Because a permanent mooring system may exhibit excessive resistance, it is possible to reduce the lines’ thickness until the cost is optimised for non-permanent service. Results for an example in the Gulf of Mexico show that important savings can be achieved by implementing the proposed optimisations.

KEYWORDS:

• Disconnection criteria
• FPSO
• life-cycle cost
• mooring system
• target reliability index
• ultimate limit state

Acknowledgments

This study was undertaken at the Korea Ship and Offshore Research Institute at Pusan National University which has been a Lloyd's Register Foundation (LRF) Research Centre of Excellence since 2008. This work was supported by a 2-Year Research Grant of Pusan National University. The authors are grateful to Dr. Alberto Omar Vazquez-Hernandez of the Mexican Petroleum Institute (IMP) for his valuable discussion on the design of mooring systems. The first author would like to acknowledge the scholarship from the Mexican National Council for Science and Technology (CONACYT). The second author wishes to thank the financial support of PRH-15 ANP program.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Patrícia Mika Sakugawa: PRH-15 ANP program.