OpenFOAM simulations of irregular waves and free surface effects around a monopile offshore wind turbine Ariel J. Edesess 1 4 th Year PhD Candidate Supervisors: Dr. Denis Kelliher 1, Dr. Gareth Thomas 1, Prof. Alistair Borthwick 2 Industrial sponsor: Alexis Billet 3 1 University College Cork, Ireland 2 The University of Edinburgh, UK 3 Resilience Energy, Ltd.
Introduction: Project motivation Major challenges currently facing offshore wind farms: Operations and maintenance (O&M) strategically difficult due to locations of turbines Prohibitively high cost of O&M ~ 50% of total cost of energy production Limited turbine access H s is only used parameter: not very reliable; no information wave conditions directly at the turbine Most importantly: Safety of repair workers: No available statistics on frequency of dangerous circumstances Economic loss: Length of time turbines go without repair due to unfavourable repair conditions leads to large monetary losses (approximated loss per year ~ 9.4 Million per year* for a typical wind farm with 100 5 MW turbines!!) Approximation includes lost production time, repair workers pay without repair, average downtime per fault *Approximation provided by Resilience Energy Ltd. consultation 10/01/17
2 Video courtesy of Alexis Billet, Resilience Energy 10/01/17
Problem set-up Crew Transfer Vessel u! Motor Force Transition Piece/Boat landing Monopile blocks the vessel from incident waves during landing operation implies that vessel lies within the wake of the turbine Vessel held in place through controlled thrust from motor and frictional forces between vessel and the boat landing no mooring lines Vessel must remain in place for the time it takes a worker to move from the vessel onto the turbine or from the turbine to vessel Implies the vessel must remain steady for a certain length of time, regardless of changes in the kinematics within the wake. 10/01/17
Governing equations Incident wave field sea state determined for an irregular wave field using linear superposition: η = a i cos k i x ω i t +ϕ i i u = i w = i ( ) ( ) cosh k a i ω i z + h i cosh k i h ( ) sinh k a i ω i z + h i cosh k i h cos( k i x ω i t +ϕ i ) sin( k i x ω i t +ϕ i ) (1) (2) (3) Flow around cylinder Flow around cylinder requires solution to unsteady Navier-Stokes equations for rotational and viscous flow t u = 0 ( ρu) + ρuu ( ) = p + µ 2 u + S Mi (4) (5) wavefoam uses volume of fluid (VOF) method to solve for both air and water flow with Reynolds averaged Navier- Stokes: ρ u t + i ρuut = p * g i x ρ (6) + i [ µ u + ρτ ]+σ T κ α α 16/01/17
6 Non-dimensional analysis of flow features Dimensionless quantities Reynolds Number Keulegan- Carpenter Number Frequency (Stokes) parameter Definition & Uses Describes flow regime (laminar or turbulent) Anticipates features in the flow Describes relative importance of viscous to inertial forces Anticipates vortex shedding When KC < 1, damping values change from typical Reynolds dependence Formula Re = U D ν KC = TU D β = Re KC = fd2 υ For small KC (or very large β), β is a better indicator of drag than Re 16/01/17
7 Computational Solution: CFD and OpenFOAM Role of OpenFOAM in this project is to provide approximate validation for analytical solutions and to simulate in situ data to the following problems: 1. Free surface effects within the near-wake (surface elevation that differs from significant wave height, H S ) 2. Turbine effect on the free surface flow, the affected flow is then incident on vessel 3. Suitability for using OpenFOAM to model in situ wave conditions: validate approximations by comparison to on-site data 10/01/17
8 Computational Domain: meshing with GMSH Length of element (m) Mesh Open (no monopile) min. Δx min. Δz max. Aspect Ratio (Δx/Δz) No. of elements Initial Delta T L min /6 H/7 4.7 3.4M 1*10-2 Mesh Type 1 With monopile L min /4 (L min /16 in monopile region) H/7 8.9 3.6M 1*10-2 Mesh Type 2 10/01/17
9 New boundary condition To follow on from single linear wave solutions, inputting the raw displacement data was desired. Method for inputting raw data as boundary condition: 1) Find power spectral density (PSD) of raw heave (surface elevation) data 2) Calculate a simulated random sea state from raw data PSD using linear superposition for a range of frequencies/wave numbers 3) Check random sea state calculation ( ) σ η 2 vs. m 0 4) Implement new section of code to read in spectral values, calculate amplitudes and velocity for boundary cells 5) Following simulation, compare simulated results to original data for validation 10/01/17
10 Spectral Checks Spectral parameters: σ η 2 = m 0 = 1 (N 1) 2 ˆη i and: 2 G η ( f )df = σ η 0 Where: G η is spectra of surface elevation ˆη i is discretised surface elevation *The variance of surface elevation (η) should be equal to the zeroth moment (m 0 ) (i.e. the area under the spectral curve). Finally: H s = 4 m 0 10/01/17
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13 Results: Summer (June 2016) Raw data Calculated sea state data Simulated data σ η 2 S( f )df = m 0 0 H s = 4 m 0 0.01485 0.01479 0.01222 0.01484 0.01479 0.01222 0.48735 m 0.48645 m 0.44224 m Simulated modal period (Tp): = 8.35 sec Raw data modal period(tp): = 9.30 sec ----------------------------------------------------- Variance error: 21.531759 % Energy error: 21.442806 % Hs error: 10.201092 % 12/01/17
14 Results: Spring (March 2016) σ η 2 S( f )df = m 0 0 H s = 4 m 0 Raw data Calculated sea state data Simulated data 0.04988 0.04985 0.04694 0.04987 0.04984 0.04690 0.89323 m 0.89303 m 0.86624 m Simulated modal period (Tp): = 8.20 sec Raw data modal period(tp): = 7.50 sec -------------------------------------------------------------------------- Variance error: 9.524096 % Energy error: 9.513267 % Hs error: 4.875485 % 12/01/17
15 Results: Autumn (September 2015) σ η 2 S( f )df = m 0 0 H s = 4 m 0 Raw data Calculated sea state data Simulated sea state data 0.01763 0.01762 0.01948 0.01762 0.01762 0.01948 0.53103 m 0.53096 m 0.55825 m Simulated modal period (Tp): = 8.94 sec Raw data modal period(tp): = 8.70 sec -------------------------------------------------------------------------- Variance error: 9.52 % Energy error: 9.51% Hs error: 4.88% 12/01/17
16 Results: Winter (December 2015) σ η 2 S( f )df = m 0 0 H s = 4 m 0 Raw data Calculated sea state data Simulated data 0.01807 0.01799 0.01695 0.01806 0.01799 0.01696 0.53758 m 0.53647 m 0.52087 m Simulated modal period Peak 1 (Tp): = 2.69 sec Simulated modal period Peak 2 (Tp) = 7.25 Raw data modal period Peak 1 (Tp): = 2.61 sec Raw modal period Peak 2 (Tp) = 8.11 sec ------------------------------------------------------------------- Variance error: 6.582305 % Energy error: 6.518812 % Hs error: 3.207951 % 12/01/17
17 Velocity spectra Spring Summer Autumn Winter 12/01/17
Post processing: Obtaining data and Probe Locations Probes located at intervals of π 8 at 10 positions from 0.1*r to L vessel 18 Average of FS elevation at the ten locations around the monopile is found and spectra of averages L vessel ~16 m 10/01/17
19 Transfer function derivation Simulations that are inclusive of the monopile will show a spectra of the diffracted surface elevations. In order to simulate the motion of the vessel against the transition piece, a transfer function, H T should be found, using: ( ) H 2 T = S exp iϕ diff_data diff S exp( iϕ ) S diff_general = H T 2 S Where S diff is the spectra from the diffracted surface elevations. The transfer function can be used to determine the diffracted spectra for any input sea state, and the transfer function, H T, will be used as input for equations of vessel motion. 10/01/17
20 ( ) H 2 T = S diff_data exp iϕ diff S exp( iϕ ) S diff_general = H T 2 S Transfer function Transfer function found for each of ten locations along the length of the vessel Autumn data transfer function 12/01/17
21 Additional full-scale validation Using acoustic wave and current meters (AWAC) on-site to measure the surface elevation around a monopile Transfer function validation: future project Future work and challenges 12/01/17
22 Thank you! QUESTIONS? 10/01/17