Understanding Vortex-Induced Vibration

Vortex-induced vibration occurs when turbulent wind flows interact with cylindrical structures, forming vortices in repeating patterns that cause structural vibrations, as shown in schematic below. In the realm of offshore wind turbines, VIV is especially pronounced during installation, where high wind speeds prevail. Offshore locations often experience wind speeds of up to 25 m/s, significantly limiting the weather windows available for safe and efficient turbine installation. After the installation of the nacelle and blades, VIV is usually disrupted due to damping from blades.

Challenges and Implications

The manifestation of VIV presents a myriad of challenges for offshore wind projects. Chief among them is the constraint imposed on installation weather windows due to the prevalence of high wind speeds. With wind speeds reaching up to 25 m/s, the operational window for turbine installation becomes narrower, leading to increased downtime for installation vessels and potential project delays. Moreover, the structural integrity of wind towers and monopiles can be compromised by excessive vibration, posing safety risks, and threatening the overall viability of the installation.

Wind Tower VIV Assessment

To minimize the tower installation cost and risks, the tower’s VIV response should be evaluated in the early stages of the project. We assessed a series of wind turbines to determine their susceptibility to VIV during their installation, investigating sensitivity of tower VIV response to water depth, tower diameter and stiffness, soil strength, and monopile stiffness. We performed finite element simulations for 5MW, 10MW and 15MW wind turbines installed at 30m water depth with monopiles with tower outside diameters varying from 6m to 10m. We also investigated 10MW wind turbine at 10 and 50m water depths.  

The Difference Between Onshore and Offshore Installation

The assessment showed that standalone wind towers installed onshore are not susceptible to VIV for wind speeds below 25m/s.  as shown in the plot below, however with the addition of a monopile and when installed in 30m water depth with a 10m extension above mean seawater, all three wind turbines become susceptible to VIV at lower wind speeds of 14.0 m/s, 18.7m/s, and 15.6 m/s for the 5MW, 10MW, and 15MW wind turbines respectively.

 The Impact of Soil Strength on Tower VIV Response

A sensitivity to investigate impact of soil stiffness on VIV was conducted for all three wind turbine generator (WTG) sizes and considered very stiff soil (i.e. monopile fixed at the seabed with no soil springs) and minimum wind speeds to excite VIV increasing by 1 m/s to 2 m/s. We concluded that impact of soil on wind tower VIV is negligible.

 The Impact of Monopile Stiffness on Tower VIV Response

The impact of monopile stiffness on wind tower VIV was investigated to see if the onset of VIV can be pushed away from the expected installation wind speeds. By stiffening the monopile, it is possible to increase minimum VIV wind speeds by 6 m/s to 7 m/s which can improve the installation weather windows significantly. However, this would require monopiles with extremely thick wall thicknesses which increases the weight and cost of the foundation. Increasing the monopile wall thickness and stiffness to eliminate VIV is not a feasible option for a temporary installation issue.

Tower VIV Response During Service

Once the WTG is installed, the minimum wind speed to excite wind tower VIV decreases significantly to 5 m/s - 7 m/s, however due to additional damping from the blades and disruption of the vortices by the presence of blade motions, VIV of the wind tower for fully installed wind turbines are not deemed to be a concern.

Water Depth Sensitivity

We then investigated the impact of water depth on VIV of a 10MW wind turbine at 10m and 50m water depths in addition to the base case at 30m. Even though monopile stiffness was adjusted for the varying water depths to produce similar stresses at the critical regions, as the water depth increases, wind towers become susceptible to VIV at lesser wind speeds as shown in the plot below. At 30m water depth, the expected minimum VIV excitation wind speed is 18.7m/s which reduces to 15.1m/s for 50m water depth and increases to 23.0 m/s for 10m water depth.

 Impacts on Installation Operations

The impact of VIV on offshore wind installation operations cannot be overstated. As wind speeds at installation height approach and exceed 14 m/s, VIV is expected to start Installation activities must be carefully planned and executed within limited weather windows to ensure the safety of personnel and equipment. However, the presence of VIV exacerbates this challenge, further constricting the already narrow operational window and increasing the likelihood of downtime due to adverse weather conditions.

Remediation Strategies

Addressing VIV is imperative for the success of offshore wind projects and various remediation strategies have been developed to mitigate its adverse effects. Temporary VIV suppression devices, including strakes, motion dampers, and ventilated trousers, can be strategically installed on wind towers and monopiles to disrupt vortex shedding, dampen oscillations and facilitate the WTG installation. These solutions not only alleviate fatigue concerns due to VIV but also extend the operational lifespan of offshore wind infrastructure, enhancing safety and reliability.

Conclusion

As the global demand for renewable energy continues to escalate, offshore wind remains a critical component of the clean energy transition. Vortex-induced vibration poses a significant challenge for offshore wind turbines, particularly during installation in high wind speed environments. 2H can assist in addressing these challenges by evaluating the tower's VIV response during the initial design phases and implementing specific VIV mitigation strategies for offshore wind towers susceptible to VIV. Proactively addressing VIV and similar challenges is essential for harnessing the full potential of offshore wind resources and driving global efforts towards a cleaner, more sustainable world.