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

UPWIND WP4: Offshore Support Structures and Foundations
The results of the convergence in the present study are evaluated through the error norm defined by:
The grid converge ratio (R) is a useful quantity to identify the behaviour of the convergence. This quantity defined
by:
And the solution is classified in terms of the value of R as:
• Oscillatory divergence, if R < -1
• Oscillatory convergence, if -1 < R < 0
• Monotonic convergence, if 0 1.
The uncertainty (Uk) associated with the computations finer discretisation ( ) is given by:
where Fs is a safety factor which may vary between 1 and 3. The present study considers a very conservative
approach and takes Fs = 3.
Convergence studies for the OC3-Hywind platform
The geometry of OC3-Hywind spar buoy was discretised for three grid sizes shown in Figure 8.4. Following
Cruz in [113], the grid cell size (hi) is related to the WAMIT higher order panel size parameter associated to an
automatic discretisation of the geometry. Panel sizes equal to 10 m, 5.0 m and 2.5m were considered and the
hydrodynamic quantities were evaluated for five wave periods selected randomly between 3 s to 16 s.
Figure 8.4: Three discretisations of the geometry of the OC3-Hywind used for the convergence tests. The panel size parameter
equal to (a) 10.0 m, (b) 5.0 m and (c) 2.5m.
The convergence ratio (R) (Equation 3-6) associated with the discretisation triplet used in this study computed
for the linear excitation forces is shown in Table 8.4. The values of (R) depend on the hydrodynamic quantity
being evaluated and on the frequency. Most of the values are between -1 and 1 and the solution for the majority
of evaluated cases can be classified as convergent.
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