For deep-water floating structures, mooring system is essential for the safe operation of the facility. Historical accidents show that mooring failures can put serious threats to the environments, asset, and even human lives. First, detailed numerical analysis is performed for the mooring system of a deep-water platform. Then, the response characteristics and effects of mooring failure on platform performance are studied. Finally, a mooring monitoring and early warning system are proposed based on the numerical analysis results.
Mooring system is an indispensable component of deep-water floating structure, which responsible for station-keeping under extreme weather conditions. Mooring system affects not only the rigid body motions of the floating structure, but also the performance of riser system.
A mooring system normally comprises mooring lines, anchors and connectors. A mooring line connects the floating structure to an anchor on the seafloor. The typical materials employed for mooring line include chain, wire, synthetic fiber rope, or a combination of the three. Water depth and environmental condition determine which materials make up the mooring system. Due to its lightweight, low cost, and high horizontal restoring efficiency, polyester rope becomes more and more popular for the permanent mooring system of deep-water floating structures (Chen, 2013).
During the past two decades, there are numerous mooring incidents around the world for offshore oil and gas industry. In 2008, the mooring incident for Kikeh Spar in Southeast Asia was due to its improper heat treatment of the shackle (Gordon, 2014). In 2011, the multiple mooring line failure of Gryphon Alpha FPSO in UKCS was caused by the severe weather (Christou, 2012 & Bhardwaj, 2017).
Based on the historical accident data, the main reasons for the mooring failure include 1) several environments, 2) component failure due to manufacturing defects, corrosion, fatigue, etc., and 3) human error during installation and operation.
Mooring system failure will reduce the floating structure's station-keeping capability, and may cause progressive failure of neighboring lines, and thus results in large offset and even riser rupture, production shutdown.
