January 24 2019 0Comment

Blog post Cable lay

This blog post concerns the engineering aspect of subsea power cable installation operations. It consists of two parts: the first provides basic information regarding cable installation and the second provides a solution to significantly lower risks associated with installation.

A power cable is a delicate and crucial element of an offshore wind farm. Inter array cables connect the turbines   to one or two offshore sub-stations (OSS). From the OSS export cable(s) transfer the current to shore. Visualization is always better [1]:

 

Installation is performed with a cable lay vessel (CSV). The operation consists of a first end pull-in, followed by cable lay to the location where the second end is installed. Especially the second end installation is difficult. The cable needs to be pulled in from one side of the vessel’s chute, with the cable still on the other side of the chute. This is a helpful video to visualize the second end pull-in with the usage of a quadrant video.  The cable is commonly buried to protect it from any damage after installation.

Cable damage can occur in multiple stages: during manufacturing, installation and in operation.  Even though manufacturing of the cable takes place in the a controlled environment, failures occur. Especially jointing is prone to errors. A buried cable can still be damaged by anchor, significant scour or fishing equipment. This blog posts focusses on damages taking place during installation. At this stage faults in all stages have been found.

Current cable lay installation methods have a high level of risk, as insurance claim data show: approximately 80% of the claims related to a wind farms concern subsea cables [2]. About 8-12% of overall wind farm CAPEX costs are related to cables; a significant heap. It is expensive to repair a cable, it has to be unburied and a repair joint has to be made offshore. Besides the operational side, the related juridical settlement is just as expensive. There is a lot to gain by improving the ways of installing cables to prevent damage.

A cable failure differs from a broken sling or other damaged equipment as it is not clearly visible. Failures may only show after years of operation. Acceptance tests right after installation will likely not show any damages present in the cable.

Cables can be damaged easily. The thresholds for overloading and overbending are commonly low. Power cables should also not be compressed by only a small margin. As you can imagine, this makes installation in an offshore environment subjected to wind, waves and currents very difficult. Thorough engineering and operational monitoring are key aspects for successful operations.

Cable lay engineering is often done too simplified, and neglects important aspects. Guidelines such those of DNVGL provide too little support. Given the delicate nature of the cable this leads to likely cases of significantly exceeding cable criteria during installation. Our next post will go into more detail about this!

 

Author: Mark Paalvast

 

[1] https://infrastructure.planninginspectorate.gov.uk/wp-content/ipc/uploads/projects/EN010078/EN010078-000059-EAN2%20-%20Scoping%20Report.pdf

[2] https://www.carbontrust.com/offshore-wind/owa/cables/