SunStation™ - New solar-powered CP tool helps reduce corrosion risks
by Brian Gibbs
The increasing demands on subsea infrastructure to work at greater depths, in harsher conditions, and for longer design lives requires a new look at the tools and techniques used for cathodic protection (CP) monitoring. This is increasingly important in today’s world, where regulatory oversight and the need to effectively manage risks is paramount.
The conventional way of verifying that a CP system is working effectively is to measure the electrochemical potential, referred to commonly as CP readings. This technology was developed for use on shallow-water fixed platforms and other large structures, such as ship hulls, where the technology works well. Complex subsea equipment presents major challenges for gathering good CP data when using the tools developed for large simple structures. Yes, CP readings are recorded, but their value can be limited if they are not taken consistently and at the locations where the corrosion risk is greatest. The consequence of poor data can result in additional inspections, premature retrofit of anodes, or unexpected failure.
Permanently installed monitoring tools have been in use for decades and generally these require cables to be brought up above water to electrically powered meters. This approach is totally impractical for remote deepwater infrastructure. Deepwater Corrosion Services has addressed this impediment by incorporating solar panels into the CP monitoring system. This new product – the SunStation – is a solar-powered monitoring tool that has many advantages over the traditional, and often difficult, approach to taking contact probe CP readings.
The illumination provided by the ROV lights is sufficient to activate the solar panels. This compact, stand-alone monitoring system requires only the ROV’s lights and video camera to gather the vital CP data from the strategically positioned CP measurement probes.
This solution to CP monitoring provides the operator and the regulatory bodies with consistent and reliable data, which is vital for an effective risk mitigation plan. Additionally, it reduces ROV operational time and, therefore, operating costs. The technical advantages, the reliability of the data, and the reduced operational cost by far outweighs the cost of supplying and installing the monitoring system.
The goal here is to show how bad corrosion data hampers the ability of the integrity engineer to offer sound judgement on the condition of the subsea infrastructure, and how new technology can help mitigate this risk.
The key to successful integrity management is to demonstrate that risks are managed to acceptably low levels. Key corrosion data provides this assurance. For the integrity management authority, the challenge is dealing with the uncertainty of verifying that the materials of fabrication are performing well over the expected service life; often, even longer because of the need to continue extending the operating life of fields.
In complex subsea equipment, we mix a variety of metals to meet specific design and operating needs. We often build increasingly complex systems, many of which need protection from the natural process of corrosion. In doing so, we mix corrosion resistant alloys and a range of carbon steels, with the possible addition of other materials such as titanium. These materials are electrochemically incompatible, meaning that one metal will cause another to corrode. While this approach creates an effective mechanical design, it presents challenges from an integrity management standpoint.
Carbon steels require CP to limit the effects of seawater corrosion. The effective solution to address the corrosion issues and electrochemical imbalance is, and probably will always be, sacrificial anodes. This generates two main challenges for the designer: where to attach the anodes, and how to monitor their effectiveness.
Attachment of the anode is usually confined to structural components, such as the frames, and the design relies on a series of mechanical (bolted) connections to create the electrical continuity required for the anodes to protect safety-critical components.
In deepwater environments, verifying the ongoing in-service effectiveness of CP comes down to two simple techniques. The first is visual inspection to see that the anodes are present, functioning, and to look for signs of corrosion. The second is CP measurement, which typically employs a simple probe that contacts steel somewhere on the perimeter of the equipment.
This simple CP measurement probe, intended for checking CP on offshore structures, only tests the location where the measurement is made. It does not examine the safety-critical (usually pressure containing) components, and it can be concluded that taking CP measurements at non-critical locations does not provide the sound data needed to verify that corrosion risks are being well managed.
The primary purpose of CP inspection is to confirm that the metals at risk of corrosion are protected by cathodic protection at the time of inspection. The secondary purpose is to predict whether the CP system will continue to perform adequately for some period (the next three to five years, for example), or when retrofit would be necessary for continued service.
Good data is essential to making this judgement. The classic approach to CP inspections is to take measurements on accessible parts of the exterior envelope. This is of limited value, however. These test points are typically on secondary items, such as structural frames and not on, or even near, the safety critical components. This happens because of limited ROV dexterity in these complex and confined spaces on subsea equipment.