The make-up of a line can be chains, steel wire ropes, synthetic fiber ropes, or a combination of these.įor the three-column semisubmersible, such as WindFloat, an asymmetric mooring pattern may be applied. The spar FOWT can be kept in position by a catenary or a taut mooring system. įor spar platforms, such as Hywind, the mooring system may comprise (3×2, three clusters with two lines per cluster) six legs for a design with redundancy or (3×1) three legs for a design without redundancy.Typical spread mooring design with catenary lines. These effects have to be accounted for using a more sophisticated finite element or computational fluid-structural dynamics software.įigure 15.5. Nevertheless, in practice there are some other effects such as the mean drag force and its direction owing to the combination of wave and current, marine growth and installation imperfection (eg, due to the water absorption or ingress to a buoyancy module) which can lead to variable or uncertain hydrodynamic (normal and tangential) drag coefficients to be used in the design of non-uniform cable sections in comparison with the bare sections. These parameters should be systematically verified through a cable shape sensitivity study.
Depending on the hang-off angle, water depth, platform offset, footprint and soil stiffness, areas of local maximum static tensions typically occur at the hang-off point and the two ends (ie, lift and drag points) of the distributed buoyancy portion, whereas areas of local maximum static bending moments correspond to the sagging, hogging and touchdown areas where maximum curvatures occur.
Since the W or wave cable configuration is significantly controlled by the change of additional buoyancy, designers must accurately estimate the buoyancy particulars such as the diameter, submerged weight (density) and spacing: these should be maintained throughout the lifetime operations by avoiding any buoyancy loss and/or slippage.įig. 13.9 displays the effect of buoyancy change on the overall configurations of lazy wave cables manifesting different hang-off, arch bend and touchdown catenaries. The use of such distributed buoyancy is mainly aimed at minimizing the overall cable dynamic responses and accommodating the cable dynamic tension or curvature changes in harsh weather conditions, by decoupling the platform motion from the cable touchdown point at the seabed. Secure fastening and precise positioning of buoyancy modules are ensured by using internal clamps. In any case, there is a requirement for a design of buoyancy modules (typically made of synthetic foams) to be distributed along a certain mid-water part of the cable. The W-type cable may be used for connecting between the two floating wind turbines or between the floating substation and wind turbines, whereas the lazy wave cable may be used as an interarray, interplatform and export cable. Dynamic power cable concepts: (a) interarray cable (b) export cable. In terms of the design, this requirement can be translated into a floating support structure minimum rotational stiffness ( Section 11.2.2).įigure 13.8. It is important to remember that this is the total angle of inclination, the sum of the static and the dynamic angles of oscillations, due, respectively, to the average value (mainly due to the wind) and the oscillation amplitude (mainly due to waves) of the inclining moments.
The exact value of this maximum inclination angle is still open to discussion, but according to the literature a good starting value is 10 degree. Taking also into account the fact that many of the sub-systems of an offshore wind turbine (bearings, gearbox, generator, etc.) have been designed to operate close to the upright condition, it is necessary to impose a maximum roll/pitch inclination angle. As a consequence, there is very little experience in estimating the performance of wind turbines at large inclination angles, and relatively few data have been presented in literature. While FOWT systems can experience relatively large inclination angles (in roll and/or pitch), onshore and fixed-to-seabed offshore wind turbines do not experience such angles. Borg, in Offshore Wind Farms, 2016 11.2.1.2 Maximum inclination angle