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How do you take a subsea cable from concept to shovel-ready?

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As the Australian Government fast-tracks the approval process for a new sub-sea cable route, the Marinus Link, Brett Allen asks 'How do you take a subsea cable from concept to shovel-ready?'

As the Australian Government fast-tracks the approval process for a new sub-sea cable route, the Marinus Link, Brett Allen asks 'How do you take a subsea cable from concept to shovel-ready?'

There are many reasons to welcome the announcement that the Australian Government has fast-tracked the approvals process for a new sub-sea cable route, known as Marinus Link. Not only will this major project help to drive economic recovery in the wake of COVID-19, it will also form an important piece of a future grid that can provide reliable, affordable and clean electricity across the Australian National Electricity Market. This proposed 1500 MW link between Tasmania and the Australian Mainland (2 x 750 MW cables) is expected to be shovel ready by the mid-2020s. Meanwhile in the far north of the country, the ambitious Sun Cable project continues to garner attention and action, with the goal of moving Australian sunshine to Singaporean electricity consumers.

But successfully delivering subsea cable takes so much more than a line on a map would suggest. While submarine power cables appear strong and robust, they are a very delicate product. Correct handling is critical to their longevity. If installation happens in rough seas, the cable’s integrity may be affected. Other issues that can cause failures are production issues with the factory joints or contamination entering the production process.

The consequences of falling short are costly and serious. The 2200MW, 600kV, 400km Western Link between Scotland and Wales, commissioned in 2018, has since experienced outages where the cause is unknown. At 600kV, it is the first submarine cable of this type at this voltage level. Getting the product right, from the design, requirements and tendering stages, is the first and most critical line of defence in securing a successful sub-sea cable project.

Using Marinus Link as an example, what does the journey from now to ‘shovel-ready’ look like? Certainly, the cable will use DC transmission technology for the subsea route as the capacitive current required for an AC connection would be too high. We might assume the cable will be 525kV or thereabouts to maximise the power transmission. How do you choose a reliable manufacturer of Extra High Voltage (EHV) cable?

Firstly, the type of insulation needs to be understood. For EHV there are four types of insulation that may be offered for Marinus Link. 

Cross-Linked Polyethylene (XLPE) – EHV DC XLPE is mature technology and thus well understood. But that does not automatically make XLPE the best option. We know Basslink has had some issues with the XLPE cable, but on the other hand, an XLPE manufacturer has successfully passed a type test and pre-qualification test to the Cigré TB496 at a record-breaking 640kV. My own very careful specification of XLPE cable now connects Kangaroo Island to the South Australian mainland. So, it is a clear candidate, but careful consideration is required.

Mass Impregnated Insulated Cable (MI) – MI is paper impregnated with a highly viscous oil, wrapped many times around the core. While this is old technology, it is still manufactured for EHV cables by many trusted manufacturers. A 525kV MI cable, to be manufactured by Prysmian at their Arco Felice plant in Italy, has recently been selected for the Viking link between England and Denmark. MI would be the most mature technology available and a safe choice for the Viking link.

Paper Polypropylene Laminated – Mass Impregnated (PPL or PPLP) – This type of insulation is a layer of polypropylene film sandwiched between two layers of oil impregnated kraft paper which is then applied in many layers around the core. The Western Link between Scotland and Wales is a PPL insulated cable. Compared to a MI cable, PPL has a higher breakdown strength and can provide lower dielectric losses.

High Performance Thermoplastic Elastomer (HPTE) – This new technology has recently qualified at 525kV. HPTE has low dielectric loss and excellent electrical and physical properties for power Transmission. Would Marinus link be so bold to be the first EHV submarine cable to use HPTE? Maybe; the advantages could be increased performance.  

The specification

So, how would you specify the insulation for an EHV DC cable?

A project could conceivably begin with a decision to specify that the insulation shall be (for example) of type Mass Impregnated, or be even more precise and state Borlink LS4258DCE (XLPE) as a mandatory requirement of the tender. Either option would cut out some experienced players at the starting gate. Specifying HPTE, for example may leave a project with just one compliant tender. 

The drawback of such a prescriptive approach is obvious. So, at this voltage the specification needs to be general in terms of technology choices, while being tightly and skilfully crafted in terms of performance requirements – including requirements that consider ease of installation and repair. This will allow the largest pool of manufactures to offer their most reliable and cost-effective options for consideration, while protecting the project in terms of procuring reliable, high quality and fit-for-purpose product.

The early work for the purchaser’s engineering team is therefore in developing a specification of the necessary quality, and then the analysis of the tenders in assessing and grading each cable proposal for:

  • Reliability of the proposed insulation
  • Past history of the manufacturer with the proposed insulation
  • Quality and reliability of factory joints – a critical quality concern and a factory visit is highly recommended as part of the assessment
  • Ease and reliability of off-shore jointing
  • Ease and cost of installing a repair joint post fault
  • Ability to locate faults
  • Robustness of the cable design
  • Total cost of ownership.

Getting the specification right is critical, not only for the cable reliability but for the success of the installation and commissioning. Short-cuts might seem tempting but must be avoided. This may be an open specification with respect to the insulation technology, but it needs to be tight with a respect to performance requirements. When the vendors send in the responses, the real work begins, and the better the specification, the more confident and certain that process will be. Strong specifications prevent the most painful cuts of all – cuts to power supply, and cuts to faulty subsea cables. 

About the author

Brett Allen is an expert in high voltage cables, his exemplary technical knowledge, stringent delivery and absolute integrity offered procurement confidence and protection for SA Power Networks for over 10 years. Frazer-Nash offers expert consultancy services in low and high voltage power cables.