Corrosion and biofouling of offshore wind monopile foundations
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Date
03/07/2020Author
Canning, Claire
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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.