to shore. Protecting these critical power cables from excessive movement or bending that could potentially cause fatigue damage in this demanding offshore environment is of utmost importance. High voltage power cables are both expensive to install and replace, with replacement costs in the region of millions of Euros, even before factoring in wind turbine downtime and the huge loss of revenue from reduced output.
Figure 1. Monopile with scour protection.
Experience gathered from fixed wind technologies, along with the drive of floating wind into increasingly dynamic environments, has increased focus and highlighted the importance of cable protection. Careful consideration is essential in order to maintain the integrity of the power cables for the life of the field.
Why is cable bend protection needed? Whilst numerous kilometres of power cables are installed on every wind farm, they are vulnerable to damage at a number of critical locations – one of the key ones being the connection points into the turbine or substation foundation. With fixed wind turbine structures, the cable is typically protected through trenching and burial for the majority of its length, with exposure in the approach towards connection points in consideration of the connection arrangement and installation methodology. In some cases, the burial point can typically be as far as 30 m from its connection point, and can be even longer if there has been significant seabed scour around the foundation of the structure. In this exposed area, the cable becomes subject to loads and motion from the surrounding sea conditions. With floating wind platforms, the cable is exposed over longer lengths in the water column between the seabed and the floating foundation of the turbine, and will potentially experience even greater levels of dynamic load and motion. In the exposed areas described, the dynamic forces on the cable produce cyclical motions relative to the foundation. These motions and loads concentrate towards the rigid connection point where the cable experiences a sharp transition in stiffness. As the cables have relatively low stiffness, they are highly susceptible to both over-bending and fatigue damage at this point. To mitigate this, bend protection is needed.
Cable bend protection: key applications The critical locations where dynamic bend protection should be applied to power cables are illustrated in this article.
Fixed offshore wind: Figure 2. Monopile with scour pit.
>> Monopile with scour protection: inside and outside monopile (Figure 1).
>> Monopile with scour pit: inside and outside monopile (Figure 2).
>> J tube with scour protection (Figure 3). >> J tube with scour pit (Figure 5). Floating offshore wind:
>> Topside floating foundation connection (Figure 6). >> Tether clamp transitions (dependent on the level of motion at the tether clamp location) (Figure 6).
Cable bend protection analysis and design
Figure 3. J tube with scour protection.
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A cable’s bend protection requirements can be unique to each application. In order to analyse the application and develop a bend protection device that is fit for purpose, the following parameters are critical design considerations:
