In the first of a three-part series, Paul Shillito, engineering director at Oliver Valves, looks at how manufacturers can avoid subsea valves corroding by using cathodic protection.
Subsea valves play a crucial role in offshore drilling and production operations and any failure or malfunction by a unit poses not only a major safety concern, but also a serious commercial setback.
Sitting in depths of up to around 3,000m of water, component replacement requires the use of remotely operated vehicles (ROVs) which means the replacement of even a single valve can become a very expensive outlay.
So how can manufacturers be certain that their valves will perform reliably once in position throughout their long lifespan?
The most frequently used material to manufacture valves is stainless steel. However, this material does not have a resistance to dissolving in seawater. Some materials such as super duplex alloys and nickel based alloys have a natural resistance, but there are high costs associated with using these materials.
The way to avoid corrosion of the stainless steel valves is to connect them to a cathodic protection system.
In addition a coating, often a polymer, provides a further line of defence from direct contact with water, but it’s not the full solution. Over extended periods of time, the water will inevitably find a way through the coating and come into contact with the metal below.
Ensuring each subsea valve is cathodically protected is critical. This works by making the whole valve unit operate as the negative element, or cathode, of an electrolytic cell as no loss of material can occur under these conditions.
We achieve this by placing the valve in electrical contact with a block of exposed material that is more easily corroded. As this block dissolves, it protects the valve by ensuring that it remains negatively charged, creating a sacrificial anode. Anodes are typically made from Aluminium / Zinc.
This is a highly effective means of protecting valves over the long term. Although the anodes may need to be replaced periodically, it’s far more manageable than replacing the valve and it requires no hold ups to production.
For the valve’s components to be protected by the cathodic effect, there needs to be a clear electrical path from them to the anode. Any metal component on the valve that is not connected to the rest of the valve – for example, any bolts securing plastic components – will not be protected if they aren’t connected to other metal parts of the construction and will quickly dissolve as a result.
This is known as electrical continuity and it is an important concern for those designing any subsea equipment.
In our next instalment, we’ll look at the importance of protecting against hydrogen-induced stress cracking which results from the pressures at which subsea equipment is required to operate.