Designing a sacrificial Cathodic Protection (CP) system is not simply a matter of attaching anodes to the steelwork and expecting long-term protection. The effectiveness and longevity of the system depends heavily on the quality and robustness of the design, and the design itself is influenced by a wide range of factors. These include:
- Applicable Standards and Codes of Practice – CP systems must be designed in accordance with recognised international and industry standards such as DNV, NACE, and ISO to ensure compliance and reliability.
- System Design Life – Offshore assets are often expected to remain in service for 20–30 years or more, which means anodes must be sized and distributed to maintain protective current throughout this lifespan.
- Environmental Conditions – Seawater temperature, salinity, oxygen levels, flow rates, and seabed conditions are some of the factors which have a direct impact on how the anodes will perform and how quickly they will be consumed.
- Coating Quality and Breakdown Factor – A well-applied coating significantly reduces current demand, but the design must account for coating deterioration over time.
- Installation, Monitoring and Maintenance Constraints – Structural geometry, access for welding or clamping, and the practicality of installation during fabrication or offshore operations must all be considered.
The Question
How do we know the CP system will operate correctly once installed?
The Challenge
The sacrificial anode CP design is based on very specific input criteria – assumed coating breakdown, current density requirements, and environmental parameters. These values are used to calculate the type, size, and number of anodes required. However, the design itself cannot be proven during the engineering design phase. True performance of a cathodic protection design can only be validated once the system is in place, connected, and operating in the real-world environment. Computer based predictive modelling is a tool to aid design and should never be relied upon to confirm real-world operation.
How do we know?
To confirm the system is functioning as intended, a post-installation CP survey should always be carried out. It should never be assumed that a sacrificial anode system is fit-and-forget and will function satisfactorily. These commissioning checks involve measuring potentials and current distribution across the structure to verify that the system is providing adequate cathodic protection to all areas of the structure the system is intended to protect. If the potential measurements indicate that the structure is not yet fully polarised, it can still be acceptable provided that the structure is trending towards achieving protective criteria and further checks are planned to update/confirm the trend.
At Subsea Corrosion Ltd., our engineers specialise in:
- Independent CP design reviews to ensure calculations are robust and compliant with relevant standards.
- Commissioning surveys post-installation to validate system performance in the real-world.
- Regular monitoring surveys to update and confirm trends and assess for any unforeseen changes, for example detached anodes, inadvertent CP system drains and interference, third party structural damage.
- Later-life assessments and system upgrades to extend protection beyond the original design life.
With over twenty-five years of proven experience in the offshore and subsea corrosion industry, we provide assurance that your CP system is not only designed correctly but also continues to perform effectively throughout the assets designed operational life and beyond.
