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 optimize (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 is 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 optimized for non-permanent service. Results for an example in the Gulf of Mexico show that important savings can be achieved by implementing the proposed optimizations.

Ship-Shaped Offshore Installations

“…These systems allow installations to move to face into the weather, thus minimising the environmental loads placed by winds, waves and currents. In addition, single-point mooring systems can be disconnected prior to storm conditions to enable ship-shaped offshore installations to be towed or sail to sheltered areas and then return to restart operations when the weather calms (Cabrera-Miranda et al 2018;Ma et al 2019).…”

Quantitative collision risk assessment of a fixed-type offshore platform with an offshore supply vessel

“…The calculation of repair costs for offshore structures has been extensively discussed in the literature [35][36][37]. Typically, the on-site repair operation of the offshore platform is costly compared to land-based structures due to various factors such as operating in dynamic sea conditions (such as underwater welding), the cost of transportation such as equipment, machinery, labours, and engineers.…”