Offshore wind infrastructure in stratified seas: Interactions between waves and wakes

Project Description:

The offshore wind sector is exponentially growing to meet global energy demand and net-zero carbon emission targets. By 2030 the UK offshore wind capacity will grow from the current 13 GW operational to 50 GW operational, further increasing to 140 GW by 2050. Due to spatial requirements this exponential growth is now focused on deep waters facilitated by floating infrastructure technologies. These deeper waters differ from the shallows of current operational sites. Deeper waters are subject to seasonal stratification, where, in the spring-summer months, temperature and salinity gradients are a key control of mass, momentum, energy and nutrient transport through the water column. This is due to thermal heating at the surface which enables the formation of a two-layer water column with deep cold water lying beneath warmer shallow water, separated by a thin region of steep density gradient (thermocline). The stratified shelf seas are a vital part of marine ecosystems. They control biological activity through primary production, support 90% of global fish landings, and act as a carbon sink for atmospheric CO₂. The targeted large-scale deep-water expansion of the offshore wind sector will have, currently unquantified, impacts on shelf sea physics and subsequent shelf sea functioning, due to the wakes shed by tidal flows past infrastructure.

While much is known about wakes generated by “bluff bodies” in unstratified (shallow water) flows, wakes in stratified flows are poorly understood, particularly at the scale of offshore wind infrastructure. Complications arise due to the strong buoyancy forces that suppress turbulence and mixing at the thermocline, and enable propagation of internal wave (IW) fields. IWs arise when strong density gradients are perturbed by, for example, tidal flow over topography. The waves that are produced can travel for hundreds of kilometers beneath the water surface, before eventually breaking and dissipating their energy. Wakes shed by wind turbine foundations in such environments are, at best, poorly understood. Yet they may have profound impacts on marine ecosystems as a source of turbulence, and therefore mixing. For the first time, we are now developing wind farms in stratified shelf sea regions. By locally mixing the stratified waters, offshore wind infrastructure will impact shelf sea functioning, yet the extent of such water column mixing is currently unknown.

The project aim is to address the open research question: How are wakes shed by offshore wind infrastructure affected by the presence of internal waves, and how do these modified wakes affect water column mixing? This aim will be achieved through the following objectives:

• Develop an experimental model that will generate internal waves in a stratified shear flow, and quantify the background flow processes.
• Experimentally investigate the effects of internal waves in a stratified shear flow on wakes shed by scaled fixed-bottom offshore wind structures.
• Experimentally investigate the effects of internal waves in a stratified shear flow on wakes shed by scaled floating offshore wind structures.

Delivery of these objectives will provide offshore wind engineers and oceanographers a detailed understanding of how infrastructure induced wakes are affected by the presence of internal waves, and the subsequent impact on local water column mixing.

Methodology:

Experiments will make use of a globally unique stratified flow facility, at the University of Hull. A stratified shear flow will be created in a ducted flow using individually controlled vertically stacked layers of hot/cold water inlets, each with a specified flow rate. Internal waves will be generated in this flow using 3D printed obstacles placed in the thermocline, upstream of the test section. These waves will then convect downstream, with the background shear flow. The first set of experiments aim to understand this complex background flow processes, replicating the flows often observed in stratified seas, such as waves perturbed by ship keels crossing the thermocline, or topography.

The second set of experiments will place 3D printed fixed-bottom offshore wind infrastructure downstream of the internal-wave-generating obstacle. Wave-wake interactions will be investigated over a wide parameter regime by varying the background flow properties (flow velocity and temperature gradient). Finally, floating infrastructure will be investigated in the third set of experiments. Here, 3D printed structures will be fixed in the channel but will be allowed to pivot in the spanwise axis, and vertically travel, replicating heaving motion.

Dynamics will be quantified using a state-of-the-art suite of simultaneous optical measurement techniques: Laser Induced Fluorescence (LIF) and tomographic Particle Tracking Velocimetry (PTV). These two systems enable measurements of instantaneous 3D velocity and density (temperature) in a volume, situated downstream of the 3D printed infrastructure. The use of this equipment and analysis/interpretation of data will be supported by an academic, Magda Carr at the University of Newcastle, and an industry partner, David Hollis at LaVision. Their expertise will facilitate measurement of internal waves and stratified wakes at unprecedented quality.

Timeline:

Year 1: Student will carry out PGDip training at the University of Hull.

Year 2: Experiments to investigate background flow processes

Year 3: Experiments investigating wave-wake interactions with fixed-bottom structures

Year 4: Experiments investigating wave-wake interactions with floating structures. Student will also disseminate and write up work.

Training and Skills:

The PhD student will be trained to carry out experimental research using state-of-the-art measurement equipment. In addition, the student will learn how to efficiently collect, manage, and analyse large datasets. The student will be part of a strong, internationally leading team working across two institutions (Newcastle and Hull). The student will get exposure to international collaborators and opportunity to become part of a global network. In addition, the team will consult regularly with industry through Dr David Hollis (Technical Director of LaVision UK Ltd.). As such the student will gain invaluable experience of working with non-academic partners.

This project will position the student for a career in research, particularly fluid dynamics (mathematics, engineering, oceanography). In addition, careers requiring a candidate experienced with analysing and handling large datasets would be well suited.

References and further reading: (5 references)

Dorrell RM, Lloyd CJ, Lincoln BJ, Rippeth TP, Taylor JR, Caulfield CCP, Sharples J, Polton JA, Scannell BD, Greaves DM, Hall RA and Simpson JH (2022) Anthropogenic Mixing in Seasonally Stratified Shelf Seas by Offshore Wind Farm Infrastructure. Front. Mar. Sci. 9:830927. doi: 10.3389/fmars.2022.830927

Schultze, L. K. P., Merckelbach, L. M., Horstmann, J., Raasch, S., & Carpenter, J. R. (2020). Increased mixing and turbulence in the wake of offshore wind farm foundations. Journal of Geophysical Research: Oceans, 125, e2019JC015858. https://doi.org/10.1029/2019JC015858

Carr M, Sutherland P, Haase A, Evers K-U, Fer I, Jensen A, Kalisch H, Berntsen J, Parau E, Thiem O, Davies PA. Laboratory Experiments on Internal Solitary Waves in Ice-Covered Waters. Geophysical Research Letters 2019, 46(21), 12230-12238.