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

UPWIND WP4: Offshore Support Structures and Foundations
MDD can produce any of the following outputs:
• Graphical plots of the steady-state shape of the mooring;
• Movie sequences showing the changing shape of the mooring in response to a time-varying current;
• Tension in each segment of the mooring;
• Final steady-state position of each component (horizontal and vertical);
• Final angle of each component to the vertical;
• Anchor mass required;
• Adequacy of buoyancy elements in maintaining vertical position of the mooring.
Key findings - MDD
MDD is designed as a stand-alone, self-contained mooring design package. It is not designed to interact dynamically
with other software. In principle, as the MDD m-files are freely available, it would be possible to modify
it so that it could be used as a mooring-force calculation module for a wind turbine package, but this would
not be straightforward. Also it only produces what are effectively steady-state solutions, which are suitable for
slowly-varying forces like those from wind or currents, but would not be appropriate for wave-induced forces.
MDD as it currently exists is not suitable as a mooring analysis module for interfacing with wind turbine codes.
Its most serious drawback is its inability to carry out a true dynamic analysis of moorings subjected to rapidlyvarying
(wave-induced) loads, which would require a considerable extension. The principles of the approach
are similar and some components (e.g. drag force calculation algorithms) and some of the post-processing
capabilities may be useful (in both the frequency and time-domain).
ROMEO
ROMEO is a mooring and riser analysis package produced by GL Noble Denton. It can perform both static and
dynamic (frequency-domain) analysis in response to wind, wave and current loadings. Systems of up to 16
mooring lines are supported. Riser calculations are ignored here.
ROMEO carries out three types of analysis on a mooring system: static analysis, dynamic analysis (both wavefrequency
and low-frequency) and then a quasi-static analysis which is a combination of the first two. The inputs
for all analyses include the details of the moored device or ship, “no-load” mooring geometry, and environmental
conditions.
Static analysis
The environmental inputs for static analysis are steady wind and current forces. It is possible to enter current as
a depth-dependent velocity profile. The algorithm uses Morison’s equation to calculate the drag on the mooring
lines. This equation gives the inline force (i.e. force in the direction of the flow) on the mooring line element as
1
= ρVu + ρCaV
( u&
− v&
) + ρC
( u − v)
u − v
2
& [8-13]
F d
(i.e. the sum of Froude-Krylov force, hydrodynamic mass force, and drag force). It also calculates wind and current
forces on the ship (or the WEC) using a simple drag equation as a function of cross-flow area, height coefficients
and shape coefficients for the body in question. The coefficients are loaded in as part of the vessel
definition file, so they have to be obtained externally. They are typically obtained either by manual estimation or,
if higher accuracy is needed and there is no published data available for similar-shaped objects, they would be
scaled-up from model tests.
The static analysis outputs consist of:
• steady-state shapes of mooring catenaries.
• steady-state (mean) position of the moored device.
Dynamic and quasi-static analysis
Dynamic analysis is carried out in the frequency-domain only. The following parameters are used to characterise
the wave environment:
• significant wave height.
• zero up-crossing period.
• spectral shape factor (peak enhancement factor, γ).
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