Final report for WP4.3: Enhancement of design methods ... - Upwind

UPWIND WP4: Offshore Support Structures and Foundations
Excitation Force (roll) [MN.m]
unrestrained motion (surge) [m]
unrestrained motion (roll) [deg]
40
30
20
10
0
-10
-20
-30
-40
0 20 40 60
Time [s]
80 100 120
Excitation Force (pitch) [MN.m]
60
40
20
0
-20
-40
0 20 40 60
Time [s]
80 100 120
117
Excitation Force (yaw) [MN.m]
200
150
100
50
0
-50
-100
-150
-200
0 20 40 60
Time [s]
80 100 120
Figure 8.33: Comparisons between first and second-order excitation forces in surge, sway, heave, roll, pitch and yaw
modes for a Pierson-Moskowitz spectrum with Hs=5.0 m (Tp = 11.2 s).
1.5
1
0.5
0
-0.5
-1
-1.5
-2
0 20 40 60
Time [s]
80 100 120
0.4
0.3
0.2
0.1
0
-0.1
-0.2
-0.3
-0.4
0 20 40 60
Time [s]
80 100 120
unrestrained motion (sway) [m]
unrestrained motion (pitch) [deg]
0.2
0.15
0.1
0.05
0
-0.05
-0.1
-0.15
0 20 40 60
Time [s]
80 100 120
1.5
1
0.5
0
-0.5
-1
0 20 40 60
Time [s]
80 100 120
unrestrained motion (heave) [m]
unrestrained motion (yaw) [deg]
1.5
1
0.5
0
-0.5
-1
-1.5
0 20 40 60
Time [s]
80 100 120
1.5
1
0.5
0
-0.5
-1
0 20 40 60
Time [s]
80 100 120
Figure 8.34: Comparisons between first and second-order unrestrained motions for the semi-submersible platform for a
Pierson-Moskowitz spectrum with Hs=5.0 m (Tp=11.2 s).
Summary and Key Findings
A study is presented which compares the results obtained from linear and weakly nonlinear potential flow hydrodynamics
models applied to two floating offshore wind structures: a spar-buoy adapted to accommodate a
NREL 5MW offshore wind turbine called “OC3-Hywind” and semi-submersible platform with geometric dimensions
similar to the WindFloat platform concept.
All potential flow hydrodynamic models assume that the fluid is incompressible and inviscid and the flow irrotational,
allowing the fluid velocity to be described by a potential function required to satisfy the Laplace equation
in all fluid domain and certain boundary conditions at the fluid, solid and air interfaces. The full expression of
these boundary conditions is mathematically difficult to solve and computationally intensive as the numerical
methods developed require the redefinition of the problem conditions at each time step to fully cover the
changes of the free-surface of the fluid and describe fully the floating structure motions. It is usual however to
approximate the hydrodynamic solution of the problem to first or second order by assuming that the wave amplitude
of the incoming waves is small in relation to the wavelength. These approximations are computationally
more efficient as they avoid a time stepping solution by computing the hydrodynamic forces and motions over
the mean wet surface instead of the instantaneous wet surface of the floating structure.