Corrosion Experience with Low Carbon Steel R4 Grade Mooring Chain

Abstract

Abstract Equinor owns and operates a fleet of floating installations on the Norwegian Continental Shelf (NCS). As the installations are getting older, the importance of knowing the actual condition of the mooring systems has increased, and continuous mooring system integrity evaluation is important for the safekeeping of the assets. As a part of the mooring system integrity management, seabed chains have been replaced for condition evaluation, as in situ inspection techniques so far has not been able to determine the actual conditions of longer lengths of seabed chain. Thorough onshore inspection and full-scale fatigue and break load tests has been performed on retrieved chains, to map the actual condition and effect to integrity. After retrieving seabed chains from approximately half of the floating installations, Equinor now has collected experience on chain degrading mechanisms and corrosion on different locations, water depths and different chain deliveries. The inspections have revealed several corrosion phenomena, where microbiologically influenced corrosion (MIC) due activity of sulphate reducing bacteria (SRB), is observed to have a significant effect on fatigue capacity. An important finding is also that corrosion mechanisms and severity change along the seabed chains. In the Equinor fleet mooring chain grade R4 has usually been used. In total four different vendors have supplied chains, but the vast majority is supplied by two vendors. One of the mooring chain vendors have supplied low carbon (LC) steel chains for the larger chain diameters (larger than 114mm). A significant difference in corrosion is found between the low carbon steel R4 grade chains and other chains. Differences are found both for general corrosion, light surface corrosion and MIC/SRB corrosion. This paper presents and discusses findings on MIC/SRB for seabed chains, in connection with the type of steel used in the mooring chain, demonstrating limited corrosion on low carbon steel chains. Also, special corrosion phenomena found only on low carbon steel chain is presented. These are found to have no or limited effect to integrity of the chains.