A Floating Production, Storage and Offloading vessel, or FPSO, presents a corrosion challenge unlike almost any other marine asset. It combines a large steel hull, extensive seawater and firewater systems, and pump caissons, and it stays on station for years or decades without the dry-docking that conventional vessels rely on for inspection and anode replacement. Corrosion protection and marine growth prevention systems therefore have to be designed to last, monitored continuously, and capable of being maintained or extended in situ.
Corrpro Europe brings decades of marine cathodic protection experience to FPSO integrity, through its Wilson Walton heritage and a combined offering of sacrificial and impressed current systems, marine growth prevention and remote monitoring. This article sets out how these technologies protect an FPSO and support life extension.
Why FPSOs are a distinct corrosion challenge
Most ships return to dry dock on a regular cycle, which provides a scheduled opportunity to inspect the hull and renew sacrificial anodes. An FPSO does not. Once on station it remains in place, continuously exposed to seawater, for the duration of a field’s production life. Several consequences follow:
- Anode systems must last far longer, or be designed so they can be supplemented without dry-docking using diver replaceable solutions.
- Protection cannot be verified by dry-dock inspection, so continuous monitoring becomes essential.
- Seawater systems run continuously, making biofouling control critical to the reliability of cooling and firewater circuits.
- Life extension is common, with many FPSOs operating well beyond their original design life and requiring their corrosion protection to be reassessed and upgraded.
Hull and structural cathodic protection
The submerged hull, ballast tanks and external structure are protected cathodically, in combination with coatings. Two approaches are used, often together:
Sacrificial anode systems
Aluminium and zinc alloy anodes protect the hull, ballast tanks and submerged structures. For an FPSO, anode mass is designed for a long service period, since replacement in the water is far more difficult than in dry dock. Corrpro Europe manufactures aluminium and zinc anodes at its UK foundry, cast to the dimensions and output a given design requires.
Impressed current systems
Impressed current cathodic protection uses inert MMO type anodes powered by a controlled current source, and it is well suited to large hulls and to long-duration protection because output can be adjusted over the asset’s life. Corrpro Europe’s Aquamatic ICCP systems provide this controllable protection for hulls, and we supply a wide range of MMO hull‑mounted anodes—including elliptical, circular and linear formats—to match different outputs, installation locations and operational requirements. ICCP is also valuable in life extension, where it can supplement or replace a depleted sacrificial system without removing the vessel from service.
Protecting pump caissons and internal systems
Seawater and firewater lift pump caissons are a critical and often overlooked part of FPSO integrity. The submerged internal surfaces of a caisson, including the pump bowls, riser pipe and submersible motor, are protected by sacrificial anode bracelet assemblies, typically aluminium or zinc alloy, positioned to distribute current across the assembly. These are designed to hold the caisson at a protective potential, commonly minus 0.80 volts or more negative with respect to a silver and silver chloride seawater reference electrode, at which corrosion is controlled.
Because caissons cannot be inspected easily in service, a monitoring system using permanent reference electrodes at points between the anodes gives an indication of protection levels and advance warning when anodes approach the end of their design life and need replacement. This can be monitored remotely, removing the need for manual intervention to confirm caisson protection.
Marine growth prevention for seawater systems
An FPSO’s seawater cooling and firewater systems draw raw seawater continuously, and without treatment mussels, barnacles and other marine growth settle inside the pipework, restricting flow and threatening critical cooling. Corrpro Europe’s Anfomatic marine growth prevention system applies a controlled current to copper anodes, releasing copper ions that create an environment in which marine life does not settle or develop. Aluminium anodes release a floc that reduces corrosion of ferrous components, and iron anodes are used for cupro-nickel pipework.
On FPSOs and platforms, a common life-extension upgrade is the conversion of obsolete chlorination systems to copper dose. Where older electrochlorination antifouling has reached the end of its life, an Anfomatic copper-dose system provides a proven, lower-maintenance alternative that keeps seawater systems clear without interrupting operations.
Monitoring and life extension in service
The defining requirement of FPSO corrosion protection is that it must be managed without dry-docking, which makes monitoring central. Remote monitoring of cathodic protection potentials, transformer rectifier output and caisson protection levels gives operators a continuous view of asset integrity and early warning of any decline, so intervention can be planned rather than forced by a failure.
For FPSOs approaching or exceeding their original design life, Corrpro Europe provides asset life extension through retrofit design and installation, bespoke CP anode design and manufacture, computer-aided CP design and verification, and ongoing performance monitoring, inspection and maintenance. This allows an operator to demonstrate continued integrity and safely extend the productive life of the asset.
Frequently asked questions
How are FPSOs protected from corrosion?
FPSOs are protected by a combination of coatings and cathodic protection. The submerged hull, ballast tanks and structure use sacrificial anodes, an impressed current system, or both, while pump caissons use sacrificial anode bracelet assemblies. Seawater systems and pumps are protected from biofouling by a marine growth prevention system, and protection levels are monitored continuously.
Why do FPSOs need different cathodic protection from other ships?
Most ships dry-dock on a regular cycle, which allows inspection and anode replacement. FPSOs stay on station for years or decades without dry-docking, so their cathodic protection must be designed for a much longer life, monitored continuously in service, and capable of being supplemented or upgraded in the water.
Can an FPSO’s cathodic protection be upgraded without dry-docking?
Yes. Although it can be challenging but, impressed current systems can supplement or replace a depleted sacrificial system in service, and antifouling systems such as chlorination units can be converted to copper dose. Corrpro Europe provides retrofit design and installation for asset life extension without removing the vessel from station.
What is the role of marine growth prevention on an FPSO?
A marine growth prevention system keeps the FPSO’s seawater cooling and firewater pipework clear of mussels, barnacles and other biofouling by dosing the water with copper ions. This protects critical cooling and firewater circuits, which run continuously on an FPSO and cannot be allowed to become restricted.
To discuss cathodic protection, MGPS or a life-extension programme for an FPSO, contact our marine department or explore our Marine Cathodic Protection page.
