Corrosion and biofouling of offshore wind monopile foundations

Abstract

The impact of corrosion and biofouling on offshore wind turbines is considered to be a key issue in terms of operation and maintenance (O&M) which must be better addressed. Early design assumptions for monopile foundations anticipated low, uniform corrosion rates in a sealed compartment that would be completely air- and water-tight. However, operational experience has shown that in practice it is very difficult to maintain a fully sealed compartment, with seawater and oxygen ingress frequently observed within many monopiles across the industry. A key concern is that this situation may accelerate corrosion of the internal surfaces. On the external surfaces, the accumulation of biofouling is known to impede the safe transfer of technicians from vessel to transition piece (TP) and requires frequent cleaning. It is also likely to influence the dynamic behaviour of the foundation due to the added weight and the hydrodynamic loading due to thickness and surface roughness changes. There is sufficient evidence to suggest that the current offshore wind guidelines on biofouling could be improved to optimise the design margins. This thesis investigated the influence of internal monopile corrosion and external biofouling growth on the turbines at Teesside Offshore Wind Farm (owned and operated by EDF Energy). At Teesside, the primary drivers of internal monopile corrosion are identified as temperature, oxygen, pH and tidal variation. The influence of each of these parameters on the corrosion rate of monopile steel were investigated in a series of laboratory experiments and in-situ monopile trials. The experimental study was conducted at EDF laboratories in France using 186 corrosion coupons that were exposed to various treatments simulating internal monopile conditions. At Teesside, 49 coupons were suspended at various internal monopile locations across 5 foundations. In both cases, the weight loss measurement of coupons over time was used to determine the corrosion rates. Results suggest that tidal (wet/dry cycles) low pH and oxygen ingress have the greatest influence on the corrosion degradation of unprotected monopile steel. Internal tidal variations create a particularly aggressive corrosion environment. A decision tree matrix has been developed to predict corrosion rate classification (high/medium/low) under a range of environmental conditions. In parallel, a biofouling assessment was conducted at Teesside Offshore Wind Farm to determine the type and extent of marine growth on the intertidal and submerged zones of turbines. This has enabled a better understanding of the species diversity and community morphology but has also facilitated the development and testing of two sampling methodologies for the intertidal and subsea regions of offshore wind turbines; scrape sampling and remotely operated vehicle (ROV) surveying, respectively. The results of the assessment suggest a zonation pattern of marine growth with depth that is consistent with findings from other offshore wind farms and platforms. A super abundance of the non-native midge species T. japonicas at the intertidal zone has also been observed at other offshore wind farms in Belgium and Denmark, however, this is first evidence of its existence at a UK offshore wind farm. Removal of biofouling from the intertidal zones and jet-washing has now been optimised to coincide with peak settlement periods of mussels and barnacles. Image analysis and 3D mapping was conducted on the subsea ROV video footage to estimate thickness, roughness and added weight of biofouling. This research provides an initial investigation into the effects of internal corrosion and external biofouling on monopile foundations at Teesside Offshore Wind Farm. The methodologies developed for this investigation and the results are critically discussed in the context of asset life assessment and improvements are suggested in further work.