Newbuild FPSO Corrosion Protection - A Design and Operation Planning Guideline
Authors: MacMillan Adrian , Fischer Karl Petter , Carlsen Henning , Goksoyr Oystein 
Published: October 2008

Corrosion control design and management for a newbuild Floating Production Storage and Offloading installation (FPSO) operating in certain benign regions, such as West Africa, China and Brazil, can provide significantly increased challenges compared to their North Sea counter parts. This is primarily driven by a number different environmental factors, such as relatively high ambient temperatures, humidity and cargo temperatures. Therefore, it is difficult to select a cost effective corrosion control design that addresses both the fabrication and operational aspects. This paper provides guidance on how to address the key corrosion protection design and fabrication issues and their corresponding impact on inspection, maintenance and repair during operation.

Introduction

Eventhough there are over 100 FPSOs operating worldwide, designing and implementing a cost optimal Inspection, Maintenance and Repair (IMR) system for a 20 year service still remains a major challenge. Primarily this is due to the fact that there is limited information available to facilitate the corrosion control design for a 20 year continuous service.

Newbuilding FPSOs have traditionally been purpose built for harsh environments, such as the North Sea, with converted FPSOs dominating the benign regions, such as Asia, Australia, Brazil and West Africa. As a result previous newbuildings have been designed with a major focus on fatigue and ultimate strength. However, it is forecast that 90% of future FPSOs will be installed in benign regions, with 60% of these being newbuildings/1/. Although these regions may be benign from a wave climate perspective they can impose significant corrosion control challenges relative to their North Sea counterparts. This is primarily a function of increased ambient temperatures and humidity.

Many of the current newbuilding 'mega' FPSOs will have production capacities in excess of 200,000 bopd and, therefore, any lost production time can have very significant economic consequences. Therefore, it is very important to design, implement and manage a proper corrosion protection design.

This paper provides some background and guidelines to facilitate the cost effective corrosion control design for newbuilding FPSO hulls. Specifically the focus is on:Identifying and quantifying the dominating factors related to corrosion control,How to select a cost effective corrosion protection system based on a combination of corrosion margins, coating systems and cathodic protection,Fabrication inspection related to the desired corrosion protection system,Providing an operation inspection, maintenance and repair strategies for selected corrosion protection system,Introducing a new RBI tool for IMR control of coated areas. Corrosion problems for converted vessels operating in the same regions, e.g. Brazil and West Aftrica, are well documented and selecting a cost-effective corrosion protection system can be equally difficult. Although this paper does not address converstions specifically, many of the basic concepts are still applicable.

Basic aspects of FPSO corrosion control

For carbon and low alloy steel FPSO hulls there are three principal types of corrosion to be considered/2/:General corrosion,Pitting corrosion, including "in-line pitting attack" and "grooving corrosion", andGalvanic corrosion (e.g. at welds).