To categorize the level of corrosion internally in uncoated offshore wind turbine structures a combination of different online monitoring systems has successfully been utilized combining findings from online corrosion monitoring, continuous water level monitoring and online water quality monitoring.
Online water level monitoring is being used to identify leakage of the J-Tube seal and evaluating corresponding corrosion risks as not all leakages can be identified by manual measurements, which are typically only performed over a few hours. The findings of the water level monitoring are supplied with water quality measurements as different levels of leaking will result in different levels of ingress of fresh seawater with resulting different levels of dissolved oxygen in the water column. These parameters characterizing the environment are then correlated to the corrosion rates measured by online corrosion monitoring ultimately characterizing the associated corrosion risk of the measured structures.
This approach allows offshore wind turbine structures to be precisely categorized into different populations with different associated corrosion risks, which is the base for evaluating mitigation activities. Positions with a high corrosion risk and high level of ongoing corrosion can be recommended for retrofit with anodes and positions with a lower risk can be followed using corrosion monitoring, which hence serves as basis for evaluating if mitigation is needed at a later point in time. Further, Electrical Resistance (ER) sensor technology can be used to document arrest of corrosion, where cathodic protection is used as mitigation measure.
Steel monopile structures are widely used in the offshore wind industry as foundations for wind turbines. [1,2] The structures are exposed to a highly corrosive marine environment and therefore subject to corrosion protection [3-6]. While most recent structures are fully coated both externally and internally earlier structures have been manufactured uncoated internally since internal corrosion protection was not required by standards and guidelines such as DNV-OS-J101 "Design of Offshore Wind Turbine Support Structures (2014)" initially. The internal environment was assumed to be controlled by a fully closed compartment with no oxygen replenishment. In an airtight structure the dissolved oxygen is assumed to be quickly consumed by uniform corrosion of the steel surface and corrosion rates will decrease after the initial consumption. Hence corrosion allowance for internal zones with frequent contact with seawater was accepted in the design to account for initial corrosion [7].
