Structural monitoring for offshore wind turbine foundations.
In April 2010 a fundamental flaw in the design of offshore wind turbine foundation structures was publicised.
This concerned the grouted connection between turbine structures and their monopile foundations. Grout adhesive is an integral part of the foundation structure, fitted within the annular gap between the monopile and a transition piece. This arrangement enables the transition piece to be installed completely level, ready to accept the main turbine on top it.
The problem was identified on certain structures, where grout failure consequential slippage of the transition piece was discovered. This had caused the turbine tower structure to slip downwards by up to 25mm, until jacking brackets used during construction came into contract with the top of the monopile. Whilst no immediate safety issues were identified, a number of wind farm development embarked upon a program of monitoring to investigate the displacement and potential fatigue problems arising from this arrangement.
By January 2010, James Fisher Strainstall (JF Strainstall) had worked for 6 different organisation at 8 separate offshore wind farms and had fitted monitoring systems to 13 turbine foundations, with a further 22 ordered and to be installed. Using a team of specially trained staff, JF Strainstall installed an array of strain gauges, displacement sensors and accelerometers, along with bespoke PC monitoring equipment.
Each wind farm had a different requirement for data collection and transfer, with some facilitating a communications connection to land via the existing networks. Other relied upon local collection using removable hard drives. JF Strainstall has also set up an office based server to receive and process data collected by these monitoring systems and to automatically send it on to the client and their engineers.
The following key information was provided:
- The stresses present at the specified strain gauge/sensor positions
- A record the variation in stress peaks
- A record of the cumulative fatigue damage
- Data stored and reported in pre-defined customized format