Structural Health Monitoring (SHM)

In general: OWI-lab offers data-acquisition systems, dedicated sensors and analysis services to Identify the current state of the offshore wind turbine support structure and foundation. This allows gaining the insights that are crucial to minimize construction and installations costs, extend the lifetime of offshore structures and reduce their operation and maintenance costs.

Why Structural health monitoring in offshore wind farms?

For O&M purpose:

  •           a) Identify scour and resonance behaviour
  •           b) Early indication of possible structural damages
  •           c) Corrosion and cracks assessment
  •           d) Structural assessment of extreme events (storms)

For Design purpose:

  •            a) Design verification of new concepts
  •            b) Data base for possible design optimization

 

© OWI-Lab

A way to minimize O&M costs is early detection or even prediction of a reduced structural integrity. Structural health monitoring is a key-expertise within the OWI-lab. Structural health monitoring involves damage detection and characterization of structures through the observation of the system over time using measurements from an array of sensors. Damage in this context can be a change in material and/or geometric properties of the structural system. When it comes to offshore structures, scouring, corrosion and reduction in foundation and grout integrity over time can be problematic. Continuous monitoring will therefore help to prevent failures and make better decisions on when to plan maintenance activities.

Dynamic monitoring:

An offshore wind turbine (specifically a monopile base offshore turbine) has low system damping properties and low inertia. As a result of that, it can be excited very quickly with low energy input causing resonance.

In order to assess the resonance behavior of such a system continuous monitoring of the vibration levels, resonant frequencies and damping values of the fundamental modes can be applied on the turbine tower and its foundation structure. Modal analysis provides an insight into the dynamic response of the structure and determines the natural eigen frequencies at different mode shapes.

© OWI-Lab

Scour monitoring:

The phenomenon of scour is the process where water current accelerates around a support structure and due to its acceleration picks up and transports soil particles (fine sand) away from the support structure . This effect which is typically for shallow water, causes a lower seabed around the support structure which makes to structure longer. A result of this is a lower natural frequency which can have an impact on fatigue damage. Next to that, a large scour hole can also be harmful cable coming out of the j-tube. In the worst case scenario the turbine structure could tilt if scour protection is not applied.

The North Sea is relative sensitive to this phenomenon because the seabed consists mainly out of sand and silts. Lessons learned from the oil and gas industry reported 2-4 meter deep scour around steel structures installed at depths ranging from 10m to 30m. In very deep water (>70m depths), the effect is almost negligible. Not only monopiles are subjected to scour, lessons learned from Alpha Ventus shows scour of 4.5m – 6m at jacket foundations and tripods due to several storms. 

Observation of the system over time using sampled dynamic response measurements from an array of sensors on the structure of the wind turbine can give better insights in this potential problem. Structural Health Monitoring, in particular 'scour monitoring', will lead to better understanding of this phenomenon during operations, and will determine if this scour effect even leads to critical changes in natural frequency of the turbine.

Grout integrity monitoring:

Consists of the continuous monitoring of the loads taken by the grout connection and the relative displacement between the monopile and the transition piece. Grout monitoring is on the roadmap of OWI-lab to look into the coming months.

OWI-Lab measurement campaign services:

Our offering:

Example of a possible outcome of a long term monitoring campaign using state-of-the-art automated operational modal analysis techniques.

Cases:

Ongoing Monitoring Campaigns at Belwind

Currently OWI-lab is performing several monitoring campaigns at the offshore wind farm Belwind, which consists of 55 Vestas V90-3MW wind turbines placed on monopile foundations. The wind farm is located in the North Sea on the Bligh Bank, 46 km off the Belgian coast. All monitoring cases mentioned above are currently being carried out since 2011.

More information about continious dynamics monitoring of an offshore wind turbine

More information about damping estimation on an offshore wind turbine