December 15, 2020
Feature article – NjordGuard
Critical to the successful operation of offshore wind turbines, subsea power cables have the essential function of transmitting generated power from the turbines to the substations, the electric hubs of windfarms, and then onward to shore. Protecting these power cables from excessive movement or bending that could potentially cause fatigue damage, is of utmost importance.
For dynamic bend protection applications where the power cable is exposed to frequent motion, such as in offshore wind, CRP Subsea (formerly Trelleborg Offshore UK) NjordGuard cable protection system maintains cable integrity both during installation and throughout use.
A cable laying vessel facilitates the installation of power cables on fixed-bottom offshore Wind Turbine Generators (WTGs). A messenger line pulled through the mono-pile turbine’s aperture hole installs the cables onto the Transition Piece (TP), which sits on top of the monopile foundation. During the installation process, NjordGuard shields the power cable, preventing over bending and abrasion.
The power cable inserts into the NjordGuard cable protection system and fits on the cable laying vessel’s firing line. Safely protected inside the NjordGuard, the power cable has a pulling sock fitted over its end, before it is connected to the pull-in tool (PIT).
NjordGuard, together with the protected power cable inside, pulls off the end of the installation vessel and the cable pays out. This process continues until the NjordGuard connector latches into the monopile aperture hole and the weak link in the PIT breaks. The over-pull operation completes when the power cable pulls all the way up to the hang-off location on the TP.
CRP Subsea’s NjordGuard utilises dynamic bend stiffeners for fixed wind applications. A bend stiffener comprises of a homogeneous elastomeric cone with geometry and material properties designed to provide a gradual and tailored transition in stiffness from its tip to its base.
This crucial feature removes the previous sharp change in stiffness at the connection point, protecting the cable from over-bending and excessive fatigue. The bend stiffener provides continuous support to the cable at the connection point, ensuring the overall curvatures of the cable inside and adjacent to the bend stiffener are significantly reduced when under load.
Therefore, for any given wave or current load cycle there is significant reduction in the stress range acting on the cable and associated fatigue. By protecting the wind turbine power cable at its most vulnerable points, the integrity can be maintained during installation and for the entire design life expectancy of the field and beyond.