Case Studies

There are strong economic drivers to increase the effective life of the steel structures, which support the offshore wind farms, to 40 years. To achieve this, the turbines require foundations that withstand splash zone corrosion without maintenance. Designers currently "manage" corrosion by material allowance and organic coatings, but these methods are inadequate for extended life due to prohibitive increases in structural mass and limited coating durability, exemplified by high-profile coating failures on several recently-constructed wind farms. Current multi-layer, splash & tidal zone organic coating formulations are vulnerable to mechanical and environmental damage, commonly resulting in coating failure and structural corrosion well within the 20-year structural design life. Standards require additional structural steel thickness allowance of 0.3mm steel per year of service. Splash zone maintenance requires expensive coffer dams. The challenges are to develop improved, rapidlydeposited, low-cost coatings and to develop service-validated, quantitative corrosion rate measuring techniques for these coatings, which can predict with confidence extended 40-year coating lifetimes.

The successful outcome of the project will bring technical and economic benefits to the industrial partners, ultimately the wider offshore wind turbine construction community in the North of England, and energy consumers nationally through:

• Reduced on-site coating repair and maintenance costs and lower life cycle costs
• Extended corrosion design life with extended maintenance intervals
• Reduced structural mass and lower associated material costs
• Reduced coating costs during fabrication
• Increased foundation production rates
• Lower electricity unit prices

Twenty-five years of oil sector experience indicates thermally-spayed aluminium (TSA) coatings provide longer term protection. Switching to TSA coatings could reduce coating application and maintenance costs significantly, whilst reducing coating application times. Improved coatings based on TSA, modified-TSA compositions and novel sealants will be developed in this project. Demonstration of the compatibility of TSA with regulation high-visibility topcoat formulations is also critical. In addition, quantitative coating corrosion measurement techniques will be employed and a ruggedized coating performance monitoring unit will be validated at an offshore site. The data will provide confidence that coatings can be applied with a 40-year life expectancy. The results will be applicable to:

• All top-side, subsea and seabed structures susceptible to corrosion, including large monopiles, turbine support towers, and steel generator elements (e.g. stators)
• Other foundation designs e.g. piled tripod and lattice jackets
• Foundation appurtenances e.g. access ladders, platforms and J-tubes

EurIng David Harvey (MEng CEng FWeldI)
Technology Manager, Surface Engineering

TWI Ltd, Granta Park, Great Abington
Cambridge, CB21 6AL, UK

Tel: +44 (0)1223 899000
Email: dave.harvey@twi.co.uk
Website: www.twi.co.uk