This paper presents the results of a case study regarding corrosion data from 14 No. internal wind turbine structures within the flooded sections. The study analyzed 342 No. test coupons used to determine the corrosion rates within different locations within the monopiles under the air tight deck over a period of several years and timescales. Coupons were inserted below the air tight deck (also referred to the gas tight floor) at various elevations from top in the atmospheric region to the seabed. The coupons were carefully removed and treated to avoid further corrosion occurring, cleaned and weighed. Corrosion rates were established from this and further analysis of pitting rates were carried out. The results were compared to published literature and standards.
In the UK a huge effort was made in the mid to late 2000's to minimize carbon emissions and the country had seen a rapid increase in wind-turbine generators being installed onshore and increasingly in offshore waters, nearly 2000 were operating in September 2018 and many more are expected in the coming decades.1 One operator took the challenge to install a number of wind-turbines in the southern sector of the North Sea, just off the coast of south east England. These wind-turbines are constructed using the monopile foundation type principle.
Whereas great strides have been made by the corrosion industry to coat the upper sections internally and externally, one area often overlooked is the "sealed" compartment under the air tight hatch floor between the transition piece (TP) and monopile foundation, where a monopile structure is left flooded with seawater, as shown in figure 1. The corrosion industry has reported on misgivings in this section of the monopie and much discussion was reported in the early 2010's.2,3
The owner/operator of the wind-turbine field, which is the subject of this paper, carried out an initial assessment on two structures below the sealed platform or airtight deck around the same era other operators were finding unexpected conditions in this zone. On seeing the conditions, the operator wanted a further understanding of how concerned they should be on the corrosion risk and whether any remedial works were necessary. It should be noted at this point that published mechanical fatigue curves at the time, relied on one of two curves in seawater, corroding and non-corroding (i.e. in the case of submerged conditions this would be for example with or without cathodic protection). Having established the internal monopile was corroding, with no cathodic protection, the owner wanted a better understanding of the extent of corrosion that was occurring. Subsequent to this study, further improvements to the design standards have been implemented with lessons learned based on this and other studies carried out by a number of windfarm owner/ operators. 4,5,6