Presentations

Abstract

As offshore wind farms enter the gigawatt scale, understanding wake interactions under realistic atmospheric conditions becomes essential for accurate energy yield prediction and next‑generation turbine design. This work quantifies wake losses in a modern giga‑scale wind farm using future large twin‑rotor platforms, simulated with realistic atmospheric boundary layer (ABL) inflow profiles and varying stability conditions. Within Vattenfall’s Offshore Wind Resource & Production department, we have developed an advanced RANS CFD framework tailored for offshore cluster‑scale simulations. Built on an enhanced in‑house version of OpenFOAM, the methodology has been validated against SCADA data from multiple sites, demonstrating strong capability in resolving turbine–ABL interaction. The inflow modelling captures velocity and turbulence variations across both the ABL and geostrophic layer using a Coriolis‑aware k–ε–fp turbulence model combined with the Apsley–Castro turbulence length‑scale limitation. Turbines are represented via an efficient actuator disc model suitable for large‑scale wind farm calculations. To investigate emerging multi‑rotor concepts for floating offshore wind, we compare two idealized platforms with equal rotor area and matched thrust and power curves: a single‑rotor 30 MW turbine (334 m diameter, 192 m hub height) and a twin‑rotor concept comprising two 15 MW turbines (236 m diameter, 143 m hub height). The twin‑rotor system has laterally separated blade tips (5% of diameter) and a total width 45% greater than the single‑rotor design. Both systems maintain 25 m water clearance, and the twin‑rotor platform yaws as a rigid unit. The study proceeds in two parts. First, isolated wakes are assessed at 8 m/s inflow under stable, neutral, and unstable stratification. Second, a 750 MW wind farm of 25 platforms in a staggered 5×5 layout is simulated every 6° across a 30° inflow sector under the same stability classes. Results reveal asymmetry in the twin‑rotor wake: at 5 diameters downstream, the right rotor shows the largest velocity deficit despite equal loading. By 10 diameters, individual wakes merge into a wider but lower‑height wake relative to the single‑rotor case. Under stable conditions, a low‑level jet around 190 m and an ABL height near 350 m cause the single‑rotor wake to partially extend above the boundary layer, reducing upper‑wake dissipation. At wind‑farm scale, the twin‑rotor array produces roughly 5% lower efficiency at 8 m/s across all stabilities, largely due to reduced free space between rows created by its larger platform width. Far‑wake deficits strengthen at hub height under stable and unstable conditions, while neutral cases behave oppositely. These findings highlight the complex coupling between turbine architecture, wake merging, and ABL stratification, informing the viability of future large twin‑rotor offshore platforms. Careful model fidelity assessment remains critical for energy‑yield applications, since dual‑rotor interactions may challenge assumptions of unchanged aerodynamic curves, and actuator disc models omit near‑wake mixing important for such configurations.