Abstract

Mooring chains for permanently moored structures – including FPSOs - have shown rapid corrosion of the chain in warm waters. Cathodic protection of mooring lines is normally not provided due to the discontinuous nature of chain links and installation constraints. In recent years, corrosion allowance and pre-emptive replacements of mooring systems are the most common solutions against this corrosion phenomenon.

This study addressed CP of mooring lines with ROV installable clamp anode design. The study is divided into three different sections to examine possible issues and constraints. Including, 1) Mechanical & Electrical properties testing to examine function and continuity between clamp and chain links and between chain links. 2) Development of a computer-based CP model of the mooring chain to simulate CP attenuation with realistic contact resistances between chain links. 3) Sea trials using a section of mooring chain to measure actual contact resistances between chain links and the polarization level of mooring chain links with a single Al anode.

The test results indicate that the subject technology could support new and retrofit CP designs via ROV installable CP System, enhance the operational lifetime of the mooring chains and be beneficial for all other marine and offshore assets.

Introduction

Corrosion in Mooring systems for permanently moored floating production units has been identified as a problem area by authorities as well as industry. A Joint Industry Project (JIP) initiated by the Bureau of Safety and Environmental Enforcement (BSEE) with participation from major global oil and gas operators as well as equipment suppliers was established in 2014 to review the problem area.1 Studies performed as a part of this program have shown that especially mooring chains located in tropical waters have shown signs of rapid corrosion, both general and localized with corrosion rates significantly larger than those specified in design standards. Increased corrosion allowance, as well as increased inspection requirements, have been recommended and corrosion has been reported as the leading cause for pre-emptive replacement of mooring.

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