Replacement of Corroded Mooring Chain on an FPSO

After approximately 9 years in service out of their 30 year design life, excessive corrosion on FPSO mooring chains in West Africa was observed through detailed chain inspection and measurements. The offloading operation of the FPSO was adjusted proactively in response to the increasing risk of potential mooring failure. An extensive engineering study determined that the heavily corroded water column chain sections for each of the FPSO's anchor legs needed to be replaced by new chains of the same size but higher grade. Also, a detailed engineering study was conducted to determine the best replacement strategy for each anchor leg. During the mooring line replacement campaign, it was confirmed that the buried sections of chain did not experience significant corrosion and therefore were acceptable to remain in the FPSO's mooring system for the long term. With a well—planned installation procedure in place, replacement of all heavily corroded chains was successfully completed in 50% of the estimated time, without interrupting production and offloading operations, and more importantly without injury or incident. All class and Flag State restrictions have been lifted immediately and the FPSO's operation is restored to its original design condition. Upon recovery of the original chains, lab tests were conducted to investigate the residual strength of the corroded chains. The test results can serve as valuable input data for life continuation or life extension of an FPSO in the future.