‘Stunning’ test results offer longer lifetime hope for offshore wind foundations

Scanning technology at the Wikinger offshore wind farm in the Baltic Sea reveals the potential to ramp up the lifetime of offshore wind structures

Ramboll's digital twin technology offers the potential to increase lifetime of offshore wind structures
Ramboll's digital twin technology offers the potential to increase lifetime of offshore wind structures

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Details are emerging from a pilot test measuring the structural health of turbine foundations that could unlock the tools to significantly increase the lifespan of wind farm installations.

The pilot testing of Ramboll’s True Digital Twin technology at the 350MW Wikinger Wikinger (350MW) Offshorenorth-east of Rügen, Mecklenburg-Vorpommern, Germany, Europe Click to see full details offshore wind farm was based on a measurement campaign using structural health monitoring (SHM).

The technology aims to continually monitor the factors that can affect the structural integrity of a wind turbine, using SHM solutions, cloud computing and advanced mathematical calculations.

The testing came as part of the Romeo project, an initiative supported by the EU research programme Horizon 2020 that aims to boost energy transition and reduce the costs of offshore wind energy.

Ursula Smolka, Ramboll’s lead consultant for offshore wind asset management, said: “Our goal with this project is to demonstrate the feasibility and impact of data-driven O&M (operations and maintenance) strategies by having the True Digital Twin continuously deliver instantaneous insights on the physics of the assets that are being monitored.

“The results from this pilot test are stunning, revealing the significant potential for lifetime extension and cost reductions in operating and maintaining offshore wind structures.”

A Ramboll engineer working on the project told Windpower Monthly that simple, robust and cost-efficient sensors are being used to measure acceleration at “easily accessed locations” where the "patterns of movements" are captured along with a "full history of loads".

“The same measurements of acceleration are also used to identify structural integrity issues via modal properties. However, these measurements enable monitoring of rather severe damages like the failure of jacket braces, whereas small cracks are unlikely to be detected,” the engineer said.

An algorithm – known as modal decomposition and expansion – is then used to estimate displacements and stresses in all locations of the structure without the need for direct measurements, the engineer said.

The 350MW Wikinger wind farm

For offshore wind turbines, this provides figures on displacements and stresses in parts of the submerged jacket foundation, based on accelerations measured at multiple locations at the transition piece or tower.

The modal decomposition and expansion algorithm is based on the structural mode shapes derived from modelling.

By measuring structural responses, a complete picture of displacements and stresses can be evaluated at any point in time, the company stated.

Measurements of acceleration can be used to identify structural integrity issues.

Monitoring can take place using simple harmonic motion solutions, cloud computing and advanced mathematical calculations.

Extensive simulation studies showed that monitoring modal properties like natural frequencies and mode shapes not only can detect anomalies but can also identify the type when combined with a design model database.

Smolka added: “The concept makes detailed design models available for predicted lifecycle management and provides the framework to incorporate measurement findings of a specific turbine into the simulation world. We can track the history of exposure of an individual structure or detect damages and replace the extensive instrumentation of traditional methods with mathematical calculations.”

The firm and its partners anticipate a reduction of offshore time for annual inspections as well as the number of planned time base offshore visits.

Mark Paine, offshore asset integrity manager at Wikinger's owner Iberdrola, said: “The benefits of the True Digital Twin are very clear to us, and this pilot test has now reinforced our initial expectations.

“Ramboll has made great progress in the Romeo project and we look forward to collaborating with them during the next phases of the project where we will look at True Digital Twin’s potential of reducing human offshore time and number of offshore visits.”

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