Foundation Monitoring – Resistive vs. Fiber-Optical Strain Gauge Systems

The role foundations play in the deployment of larger, heavier, more powerful wind turbines, in deeper waters and further from shore, has perhaps not received the attention it deserves. But it is crucial in driving down offshore wind’s levelised cost of energy.

Technology and Costs Compared

Fixed foundations, whether monopile, jacket or gravity base designs, support towers and turbines that weigh well over 1,000 tonnes and have a tip height of more than 200 metres. The foundations needs to do this, in high winds and heavy seas, for at least the 25 years of the turbine’s anticipated operating life. 

Monitoring the structural integrity of the foundations, especially in the critical areas of greatest strain – just above and below the seabed – requires high expertise and state of the art test and measurement technology. This is a field where industry development has outstripped design standards - There is no easy guide for offshore developers. In this report, technology leader HBM Test and Measurement compares resistive and fiber-optical gauge systems for offshore wind foundations.

To help design verification, or as a system to provide long-term assurance that the foundations in offshore wind turbines are in the intended condition, measurement in the foundations of offshore wind turbines is vital. The place of greatest bending in the foundation typically lies where the structure meets the seabed. Measurements above and even below the seabed are possible, with the correct design and protection, using strain gauge technology.

In monopiles this may consist of concentric rings of protected strain gauges placed around the circumference, inside the foundation. In gravity base structures and jackets, there are other ‘hotspots’. These temperature-compensated measuring points route to a dedicated DAQ system for logging and − combined with readings from accelerometers, inclinometers and other sensors in the structure − provide a complete measurement system. This system then either works with the operator’s SCADA network or reports remotely via GSM or Satellite.

The Cost Comparison

Fiber-optical technology lends itself well to large structures with many measurement points. In addition to the technical benefits there are cost advantages, too. HBM’s first installation in an offshore wind turbine was in 2003 and there have been many since, from jackets to gravity base to monopile structures. In the graph below, we look at a typical installation: A monopile with a condition monitoring system in the foundation, designed primarily for short-term (a few years) measurement of the critical areas of the foundation. The same system will continue to operate well beyond this and, in any case, the costs are the same regardless of the length of the measurement campaign. 

The DAQ hardware cost is, on its own, typically more for fiber-optical gauges than the equivalent channel count DAQ for resistive strain gauges. Fiber-optical technology employs laser technology, for example. Conversely, HBM’s fiber-optical strain gauges can be spot-welded into place, a technique which, with training, takes roughly half the time to install as a resistive gauge via a gluing method. 

Considering the large amount of strain gauge positions sometimes required in an offshore wind turbine, the reduced installation cost (through time saved) per position outweighs the comparatively more expensive DAQ hardware required. There is a tipping point; of course, as the cost benefits of complex hardware lends itself to bigger systems. Training costs should be considered, too. It could be argued that the skill (and, therefore, employee cost) of spot welding a strain gauge is lower than that of a glued strain gauge. There is no soldering and less cable handling. The protective covering over the measurement position needed for strain gauges located in the foundation and, in particular, below the seabed, is the same regardless of strain gauge type. Also, the factory and site acceptance tests and set-up times are broadly the same for both. 

In Summary