Using experience to execute flawless HV submarine cable repairs
Figure 1. Submarine cable overboarding during repair operations
Given the inherent risks associated with submarine power cables, it is essential for their operators to have a rock-solid contingency plan for dealing with the rare event of a cable failure. As those plans become increasingly common and repair timelines are condensed, the criticality of flawless execution of the repair operation itself is emphasized.
A repair preparedness plan primarily ensures that no time is wasted on tasks that could have been done in advance (e.g. preparing documentation). Furthermore, having a plan can also drastically improve collaboration between all parties involved in the repair and reduce the potential for conflict. Ultimately, a good plan prevents complications and unnecessarily high outage costs.
As a cable manufacturer, NKT is dedicated to helping its customers get the most out of their cable assets. On top of repair preparedness plans, NKT can also provide guarantees for the availability of critical resources and even a suitable repair vessel through its offshore services portfolio.
Still, it is essential to understand the complexity and challenges of an offshore cable repair operation. Even with excellent preparation, the circumstances can change from minute to minute – due to poor weather conditions or even unexploded ordnance (UXO). A typical repair can be divided into five main steps all of which can be prepared and executed by NKT. (Note: The illustrative example assumes over 10 meters water depth.)
Figure 2. Illustrative repair timeline (post-repair activities omitted)
1. Finding the fault location
Rough fault location – Immediately after notification of a failure, a team of experts is dispatched to perform initial measurements from the substation. This allows a rough estimation of the fault location and determination of next steps.
Precise fault location – A remotely operated vehicle (ROV), equipped with special fault detection instruments, is dispatched on a small vessel to find the exact fault location. Some faults are easy to find (e.g. anchor strikes with visible damage), but others can be tricky due to a ‘noisy’ measurement environment.
2. Preparing the cable for repair
Cut and seal – The cable must be cut and tested to confirm the fault location. If done immediately (using the fault-finding vessel) this allows the estimation of the required replacement cable length. After removing the faulty section, the cable ends are sealed, to prevent further water ingress, and floated off.
Deburial – Most submarine cables are buried, so a mass flow excavation (MFE) vessel is used to uncover a section for repair.
3. Mobilizing the repair vessel and crew
Vessel selection – The choice of repair vessel is affected by factors like water depth, cable specifications, weather, proximity to other offshore assets and vessel availability. Ideally, an advanced cable laying vessel (CLV), like the NKT Victoria, will be used. Unfortunately, such vessels are often otherwise committed and not readily available. To mitigate this, NKT developed a cable handling system that can be used on a wide range of offshore construction vessels (OCVs). Using this approach in the Marine Resource Plan, NKT can guarantee the availability and mobilization of a vessel within 10 days.
Vessel mobilization – Depending on the selected vessel, preparation can range from the loading of jointing equipment to completely converting the vessel with a cable handling system.
Figure 3. NKT Victoria – one of the world’s most advanced cable laying vessels
Loading of spare parts –The replacement cable and spare cable joints are loaded from storage.
Transfers – The repair vessel must first travel to the mobilization port, then the spare parts storage location and finally the repair site. Reducing transfers (e.g. by storing spares at vessel mobilization site) can significantly accelerate the repair.
Crew mobilization – While the vessel will already have a crew, a skilled cable repair team must be assembled. Availability and rates of such a team can be secured through a suitable service level agreement.
4. Cable repair work
On-site preparation – At the failure location, the first cable-end is recovered, tested for moisture ingress and placed in an omega formation back on the seabed. The second cable-end is then retrieved and prepared for jointing.
In-line jointing – The first joint connects one cable-end to the replacement cable. Part of the replacement cable is then laid down in-line with the original cable route.
Omega jointing – The other cable end is then recovered and jointed to the replacement cable. Now the repaired cable can be carefully overboarded (using a quadrant) and laid on the seabed in the characteristic omega or hairpin formation to accommodate the extra length.
Figure 4. Cable welding during a repair operation
Testing and energizing – When laid, the cable is ready to undergo final electrical tests, after which it can return to normal operation.
5. Post-repair activities
Demobilization – Unused replacement cable and accessories are returned to storage and all equipment is returned to its original condition (demobilized). Although the cable is now operating, speed is still paramount to minimize costs.
Burial – The cable should be buried again to reduce the risk of future damage.
A repair could be conducted quite rapidly (especially considering historic outage times which often exceeded 3 months) but it is crucial to be prepared and have a service partner with the right resources and expertise to help. NKT’s Service deartment builds on over 125 years of the company’s cable industry experience, which proves invaluable in preparing and flawlessly executing each step of such projects.
Note: The opinions, beliefs, and viewpoints expressed in this article do not necessarily reflect the opinions of Offshore WIND.