Distributed CP Sensor - Design and Operation Sequence Optimization

Abstract

Focus on reducing cost and improve efficiency has been key aspects in the oil and gas industry the recent years. New technology and new ideas that can improve efficiency and reduce cost should be more than welcome for companies within this industry.

Sacrificial anodes are widely used for corrosion protection of subsea structures. The sacrificial anodes are inspected at given intervals using of Remotely Operated Vehicles (ROV) or divers. These inspections are often time consuming and expensive. GE Oil&Gas has come up with an idea to have a sensor network that would be an integrated part of a subsea structure. This sensor network will have capabilities to measure the protection potential and the current output from the sacrificial anodes at different locations of the structure. A mapping of both protection potential and current output will give the operator valuable information of the integrity of the structure. By using the already installed equipment, as the subsea control module (SCM), data can be transferred to a land organization that can initiate maintenance actions when needed.

Control boxes and sensor assemblies were made to be used in experiments. The sensor assembly had four different sensing elements (Super Duplex Stainless Steel, titanium, Nickel-Aluminium Bronze and 254SMO) and a zinc reference electrode. The control boxes can control duration of coupling between sensing elements and a sacrificial anode. Experiments were conducted in SINTEF SeaLab at Brattøra, Trondheim, where natural seawater was available. Experiments were conducted in 8°C and 55°C seawater with different sacrificial anode configuration to simulate different stages of an anode lifetime.

Results from the experiments were evaluated and a conclusion was taken to recommend the best suited material to act as the sensing element. Important parameters for the sensing material were to represent the anode potential and the current output when connected to the sacrificial anode. When the sensing element was exposed at open circuit potential (OCP) the potential should not change over time as a stable potential represent a sensor surface that does not change over time.

The conclusion based on the experiment work and analyze of the results was that the best suited material to act as a sensing element for monitoring sacrificial anodes were Nickel-Aluminium Bronze (NAB).