Protecting the parts of an offshore wind turbine in the splash zone is a tricky business – without adequate protection high-cost repairs can all too easily be required
The focus of corrosion control efforts in the offshore wind industry has been on primary structures such as foundations, the structural integrity of which is key to safe and efficient turbine operation.
But the marine environment also corrodes other steel structures associated with turbines. Maintaining secondary steel components and construction details such as boat landings, J-tubes, internal and external platforms, bolted connections, plug connections, earthing connections, access ladders and railings is also important and, if not carried out properly, can add to costs.
It was for this reason that the Carbon Trust in the UK recently issued a call for entries from companies and consortia interested in undertaking a study looking into all aspects of corrosion including identifying issues, available protection measures, mitigation options for secondary steel components and construction details on offshore wind substructures.
As Carbon Trust energy systems associate Elson Martins tells OWJ, the insights obtained from the study will be used to develop industry guidance on the best options available and recommendations on best practice strategies for OWA partners.
Mr Martins says there are already offshore corrosion protection standards developers and manufacturers can use, but the Carbon Trust would like to develop industry-specific guidance for offshore wind. Ideally, the best practice that comes out of the study will enable developers and the supply chain to design corrosion prevention systems that would last the lifecycle of an offshore windfarm – in excess of 25 years.
Mr Martins explains that the main objectives of the work the Carbon Trust wants to carry out are to explore above-water internal and external corrosion protection requirements and mitigation options; research the design and application of mitigation options, including detail on their in-service performance, reliability, repair needs and retrofit applicability; construct a corrosion protection and mitigation framework to inform the design of secondary steel components; and identify industry gaps and opportunities for further development. “We are looking at steel exposed to the outside environment as well as the secondary steel inside offshore foundations that is not exposed to the outside environment,” Mr Martins says.
“The scope includes all components located above the lowest astronomical tide or enclosed by the foundation’s primary steel. That means the relevant sections of J-tubes and boat landings should be considered. Any sections beyond this boundary should also be considered to the extent that their protection or damage is relevant within the scope of related protection/mitigation options.”
Mr Martins says the Carbon Trust has been looking at aspects of corrosion in the offshore wind industry for several years and turning its attention to secondary steel components is part of a process of addressing corrosion control as a whole, with a primary focus on structural integrity and efficient design, which can deliver cost reduction.
“The challenging thing about many secondary steel components on a turbine or other offshore structure is that they are located in the splash zone,” he explains. “This zone creates favourable conditions for corrosion. The industry needs state-of-the-art solutions to corrosion and better ways to protect those components and mitigate any issues with them should they develop.”
The splash zone is an area immediately above and below mean water level and has long been a concern to design and corrosion engineers in the offshore industry. It is one of the most aggressive marine environments, because of exposure to fully aerated seawater (and hence plenty of oxygen), UV radiation, repeated wetting and drying and possibly salt build up. If left unchecked, corrosion in the splash zone can occur at a rapid rate.
Components in the splash zone are also problematic from a protection point of view: the upper splash zone is not fully and continuously immersed, so cathodic protection of the type used on fully immersed structures such as foundations are not suitable. This means coatings and sacrificial steel thickness are usually the only barriers to corrosion. As Mr Martins puts it, “The basic problem with the splash zone is that structures get wet but not permanently. It is tricky. It means you have to apply coatings, allowing an extra margin to take account of the risk of corrosion or use different materials.” All of these solutions add to costs and, if not adequately designed, constructed and applied, increase the risk that corrosion will take place.
Should corrosion of offshore structures take place, the cost of repairs can be extremely high. For this reason, corrosion protection systems must be reliable and must be capable of protecting a structure for a sufficient period of time even if the corrosion protection system is mechanically damaged (such as due to mechanical damage to a boat landing).
Mr Martins says the study is due to start by the end of 2019, and the OWA hopes to have produced framework guidance by late 2020.
Ultrasonic technology could keep foundations clean
UK-based OES Group has proposed using ultrasonic antifouling systems for the offshore renewables sector. The company recently secured a contract from energy company Total to supply what it claims will be the world’s first Atex-rated ultrasonic fouling prevention system for the Elgin oil platform in the North Sea and says its technology is equally applicable to preventing marine growth in the offshore wind industry on structures such as foundations.
The technology developed by OES produces bursts of ultrasonic energy at a range of frequencies. These are transmitted through the material the transducer is attached to and produce a pattern of positive and negative pressure on the surface of the material being targeted. Microscopic bubbles are created during the negative pressure cycle, which then implode during the positive pressure cycle.
“This microscopic agitation has a cleaning effect that destroys surface algae, which are the first link in the marine food chain. Disrupting the food chain keeps the surface clean and makes it a much less inviting habitat for larger organisms that feed on algae. As well as inhibiting the creation of biofilm, the microscopic movement of the water prevents barnacle and mussel larvae from embedding themselves in the surface,” the company says.
OES believes its technology is particularly well-suited to the emerging floating offshore wind sector where access for operations and maintenance teams could be challenging.
In 2018, a consortium of organisations announced plans to develop similar technology as part of the CleanWinTur project, which aimed to develop an ultrasonic system capable of antifouling and continuous condition monitoring. The partners in the project – including NDT Consultants, the European Marine Energy Centre, 3-Sci, InnotecUK, Alpha Electro Technology and Brunel University London – claimed effective anti-fouling and predictive and/or condition-based maintenance could reduce operation and maintenance costs significantly and enable capex reductions by allowing less conservative substructure designs.
Like the technology proposed by OES, ultrasound would be used to prevent micro-organism growth by creating acoustic cavitation on the outer surface of a substructure. In condition monitoring mode, the system would monitor and detect changes in wall thickness.
Sheringham Shoal deal for Stowen
Stowen has been awarded a contract to deliver a cathodic protection system to Equinor’s Sheringham Shoal offshore substations. The cathodic protection systems were due to be installed in Q4 2019 and Q1 2020.
The installation will be undertaken within confined spaces, an area that Stowens’ project team has long experience in.