Degradation of wire rope mooring lines in SE Asian waters

Steel wire rope has been used for mooring floating offshore production systems since their introduction nearly 30 years ago. In the majority of systems, unless fully sheathed in plastic, the life of the rope is less than that of the installation. This requires a policy of life prediction, inspection and replacement. The efficacy of various options such as zinc wires are discussed. Case studies are described illustrating how service life can be influenced by the local conditions. It is shown that primary factors in determining long term life are the weight of zinc coating on wires, the effectiveness of lubricant as a blocking compound, and especially the rate of zinc dissolution. There are clear indications from experience that this critical last issue is accelerated considerably by sea water temperature. This is a major difference between some SE Asia and other tropical locations and the N Sea. Experience in cold northern waters has been the basis for recommendations which are seen as inappropriate here. This paper was originally presented at “Offshore Asia” in Kuala Lumpur, in March, 2008, but was not made available in the public domain. It seems appropriate therefore to present these findings here, which it is planned will be the subject of further rigorous investigation in the future.

Author(s): C.R. Chaplin, A.E. Potts and A. Curtis