Optical fiber technology for SHM of offshore wind turbines monopiles
24Sea and Com&Sens teamed up in various projects over the last years all focusing on foundation monitoring.
Typically, up to 10% of the foundations in offshore wind farms are equipped with a Foundation Structural Health Monitoring (FSHM) system, mainly consisting out of installing accelerometers and strain gauges in the Transition Piece.
In recent projects, 24SEA and Com&Sens developed with success an innovative robust monitoring technology to instrument monopiles with optical strain gauges (FBGs) over the entire length of the monopile . This innovative monitoring set-up allows to validate new design methodologies for the soil structure interaction and in addition, the sensors can be used to directly have access to the consumed lifetime near the critical welds.
In both projects, three monopiles in the farm were selected based on their position in the fleet, the soil conditions and their dynamic properties. The monopiles were instrumented during fabrication. Each optical fiber line is imprinted with several Bragg gratings (FBGs). These FBGS act as local strain sensors on a single optical fiber line and are positioned above and below the mudline of the monopile foundation. Note that the sensors had to be installed prior to the actual pile-driving. To assure the sensors survived this process the subsoil sensors were covered with protective sheeting and a driving shoe.
The first results show the success of optical strain sensors on the monopile for offshore wind turbines. The measurements allow to record the bending moments over the entire length of the monopile for fatigue assessment. The measurements allow to validate design assumptions and monitor the fatigue progression of the structure.