Failure of Cable Protection Systems

“Cable Repairs are a major cost”, “Cable damages are source of major insurance claims”, “Submarine power cable losses totaling over EUR 350 million in claims”, “How to reduce risks for cable damages”, and the list goes on and on. Everybody in the offshore cable industry must have seen these re-occurring articles on LinkedIn and on websites. Cable damage and Cable Repairs are a very hot topic in the offshore cable industry (and rightly so!). Everybody talks about the damages of cables and how the risks are to be decreased as much as possible, which is unarguably a very good thing. However, it is not common to read anything about the failure of Cable Protection Systems (CPS). Unfortunately, CPS damage is not uncommon and it actually happens more than most people think. There are numerous reasons for CPS damage and there are ways to reduce the risk of this happening. In this article I will try to make people aware of this phenomenon and I will provide some insight in a few types of CPS damage and how to mitigate the risk of damage.

Generally, Cable Protection Systems are designed to be lasting the entire lifetime of the windfarm (usually between 25-30 years). This would mean that the cable is protected for all this time. Unfortunately, now that several windfarms have been in operation for multiple years, it has been observed that there are numerous Cable Protection Systems damaged and not up to the specifications which would be expected from the actual CPS design after 6-8 years of operation.

The most fragile part of the cable and CPS is the part that is suspended between the J-Tube Bellmouth/Monopile Entry Hole and the seabed/scour protection. It is exactly this part of the cable that needs the protection from the CPS throughout its lifetime. Since it is also the most fragile part of the CPS, it is also the part that is damaged the most, which means that the most delicate part of the cable is not protected anymore.

The majority of the damages observed throughout my years in the wind industry, consist of PU Bend Restrictors/Dynamic Restrictors breaking off at the suspended part the cable. Figure 1 presents a case where the PU bend restrictors have come off the CPS and exposed the submarine cable. This particular CPS has been installed approx. 8 years ago.

Figure 1: Exposed cable and broken CPS right after the Bellmouth entrance of a J-Tube

As can be seen in Figure 1 the CPS did not comply with the design lifetime of 25-30 years. When looking at the fatigue analyses of the components of the CPS, it is very important not to forget the connection bolts. For example, fatigue in PU shells is a lot less likely than fatigue in the bolts keeping the half shells together. It is therefore highly recommended to make sure the fatigue of the bolts is taken into consideration in the design acceptance process of the Cable Protection Systems.

During this design process, I have also encountered that in some occasions the expected scour is only analysed to the as-installed CPS instead of a CPS being installed and buried. The scour that happens after burial of the CPS creates additional stresses in the CPS and is not taken into account when doing a design loadcase with the CPS installed on the already scoured seabed. These stresses can also result into CPS damage and it is therefore recommended to perform not just the as installed load case on a scoured seabed, but also having the installed and buried CPS integrity verified during the scour process along the time the CPS has been installed.

The above mentioned phenomena are only a few of the things that have to be taken into consideration when purchasing and designing the CPS. If you are interested to find out more about CPS damage mitigation and what Tramontana Engineering can do for your organisation with regards to CPS procurement and design management, feel free to contact us using the contact from located here.

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