Final report for WP4.3: Enhancement of design methods ... - Upwind
Final report for WP4.3: Enhancement of design methods ... - Upwind
Final report for WP4.3: Enhancement of design methods ... - Upwind
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UPWIND WP4: Offshore Support Structures and Foundations<br />
The major difficulty associated with a fully nonlinear potential flow <strong>for</strong>mulation is related with the solution <strong>of</strong> the<br />
complicated nonlinear free surface boundary conditions which has to be satisfied over the instantaneous free<br />
surface which is not known a priori. Most <strong>of</strong> <strong>methods</strong> developed use a Mixed Eulerian-Lagrangian (MEL) time<br />
stepping technique <strong>for</strong> which the fully nonlinear boundary conditions are satisfied over the instantaneous free<br />
water and body surfaces. The unknowns <strong>of</strong> the linear equations which result from the discretisation <strong>of</strong> the geometry<br />
are distributed on the boundary <strong>of</strong> the whole computational domain and a new system <strong>of</strong> equations is<br />
generated and solved at each time step, since the free surface change and the body surface move to new positions.<br />
An advantage <strong>of</strong> second order <strong>methods</strong> when compared with fully nonlinear is that through the approximations<br />
involved the linear system <strong>of</strong> equations to solve are always the same and so are computationally more<br />
efficient.<br />
Vortex induced vibrations<br />
Another effect currently not accounted <strong>for</strong> in hydrodynamic analysis <strong>for</strong> floating wind turbines is vortex-induced<br />
vibrations. This effect is caused by steady currents or by velocities associated with long period waves, and refers<br />
to the dynamic loading which occurs as a result <strong>of</strong> fluctuations in pressure due to the motion <strong>of</strong> vortices in<br />
the wake <strong>of</strong> a body. If the frequency <strong>of</strong> excitation is near a natural frequency <strong>of</strong> the structure the interaction between<br />
the flow and the motion <strong>of</strong> the structure can cause the two frequencies to lock in to each other, which<br />
can result in large amplitudes <strong>of</strong> oscillation. The <strong>for</strong>ces due to vortex shedding are complex and predictions <strong>of</strong><br />
loading and response are not well understood; however the frequencies at which oscillations may occur can be<br />
predicted with more confidence. Vortex-induced vibrations are not generally seen in conventional fixed-bottom<br />
<strong>of</strong>fshore support structures, but are more likely to be experienced in mooring lines and can be critical <strong>for</strong> the<br />
stability <strong>of</strong> some <strong>design</strong>s.<br />
8.3 Mooring line dynamics<br />
Floating <strong>of</strong>fshore wind turbine structures are held in position by means <strong>of</strong> mooring systems, which have, depending<br />
on the type <strong>of</strong> the structure and the water depth, different levels <strong>of</strong> complexity. For floating WT applications<br />
a general distinction must be made between slack catenary, taut catenary and taut tension leg mooring<br />
systems. In slack catenary <strong>design</strong>s, <strong>of</strong>ten the lower part <strong>of</strong> the line is resting on the seabed, adding more complexity<br />
to the system. In the oil and gas industry, large floating drilling plat<strong>for</strong>ms are restored by up to 20 mooring<br />
lines with different geometrical and material properties, consisting <strong>of</strong> a combination <strong>of</strong> chains and cables<br />
made <strong>of</strong> natural or synthetic fibres (e.g. polyester, aramid, polyamide or polypropylene fibres). Submerged<br />
buoyancy tanks along the mooring lines are also common. Such complex mooring solutions will likely be implemented<br />
and specially adapted <strong>for</strong> future floating WTs as well, requiring the codes to have adequate capabilities.<br />
In addition to station-keeping, the mooring system also provides stability; <strong>for</strong> some plat<strong>for</strong>m <strong>design</strong>s such as the<br />
tension leg plat<strong>for</strong>m (TLP), the mooring system is the main contributor to the system’s stability, meaning a<br />
failure in this component would cause the likely destruction <strong>of</strong> the complete system. The mooring system <strong>of</strong><br />
floating WT plat<strong>for</strong>ms is there<strong>for</strong>e one <strong>of</strong> the most important components regarding the stability and the dynamic<br />
behaviour <strong>of</strong> floating <strong>of</strong>fshore wind turbines, making appropriate modelling <strong>of</strong> the mooring system highly<br />
critical during the <strong>design</strong> process.<br />
The central issue with regard to mooring line dynamics is whether or not it is acceptable to neglect the dynamic<br />
effects <strong>of</strong> mooring lines <strong>for</strong> floating wind turbines. For shallow mooring systems the total mass <strong>of</strong> the lines is<br />
negligible and the motion is small, so even though the drag <strong>for</strong>ce <strong>of</strong> the lines through the fluid may still be significant<br />
it is generally accepted that dynamics may be neglected. However <strong>for</strong> deeper water configurations<br />
mooring line dynamics become increasingly important. A number <strong>of</strong> studies have been per<strong>for</strong>med in the context<br />
<strong>of</strong> oil plat<strong>for</strong>ms, ships and semi-submersible vessels in order to determine the depth at which mooring line dynamics<br />
become significant. Polderdijk [101] proposed approximate analytical solutions to the line dynamic problem<br />
which can be used to give preliminary checks as to whether line dynamic effects are likely to be significant.<br />
Kwan and Bruen [102] analysed line dynamic tensions due to plat<strong>for</strong>m wave frequency motion <strong>for</strong> a range <strong>of</strong><br />
conditions using both dynamic and quasi-static <strong>methods</strong>, and showed that the ratio <strong>of</strong> maximum dynamic to<br />
quasi-static tension varied between 1.2 and 19.5 across the cases investigated. Their results can be used to<br />
help determine whether or not dynamic analysis is necessary <strong>for</strong> a given configuration. A Joint Industry Project<br />
managed by the Noble Denton Group on the dynamics <strong>of</strong> catenary mooring [103] studied a number <strong>of</strong> vessel<br />
types, mooring systems and water depths from both a theoretical and practical point <strong>of</strong> view. The conclusion<br />
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