Corrosion and fatigue health monitoring systems appraisal for offshore steel structures

Offshore structures are submitted to aggressive environmental conditions as it is in salty environment, submitted to important bio-colonization, and higher mechanical fatigue due to cyclic wave loading. Because of these conditions, the maintenance of these structures plays a key role in their reliability over time. The use of monitoring in such a case could allow drastic decrease of maintenance costs provided safe and reliable systems may be applied on site. In this context, IFSTTAR is currently working with several partners in a regional project called SURFFEOL (Survey and reliability of offshore wind turbines jackets) in order to provide to offshore wind farms developers such solutions. One of the main objectives of this project is to carry out health monitoring of a wind turbine steel foundation to determine damage factors in relation more particularly with corrosion and fatigue degradations. The presented work introduces our laboratory investigations concerning different electrochemical and nondestructive techniques used to monitor these damage mechanisms. Firstly, accelerated corrosion tests by artificial tidal cycle with natural seawater are described on carbon steel coupons with and without protective coating in order to monitor the evolution of corrosion versus time. Several parameters are measured during the test such as corrosion intensity, pH, corrosion potential and environmental parameters (temperature, seawater salinity, O2 dissolved). Secondly, fatigue sensors and more classical strain gauges have been used before and during fatigue to monitor damage under normalized fatigue cycles. The tests were carried out on T welded joints until failure. For each case, the comparison of the different techniques allows concluding on their suitability. Further investigations in laboratory are currently done to ensure such a conclusion, especially on durability issue. The final validation of these techniques will then be done on site with the help of a reduced-scale prototype at sea near French Atlantic coast.