Prysmian Case Study – Grimista

Bringing Renewable Energy to Shetland

Working in collaboration with client SSEN Transmission and principal contractor Morgan Sindall Infrastructure, Prysmian is responsible for the supply and jointing of 99 drums of 132 kV cable to one of the most remote clean energy projects in the UK.

The project comprises a new Grid Supply Point (GSP) at Gremista connected to the Kergord substation which will connect Shetland to the Scottish mainland power grids for the first time. The project will reduce reliance on local diesel-powered energy generation and support the decarbonisation of Shetland.

Bespoke cable design

Prysmian is a trusted supplier to SSEN transmission, with whom it has built a reputation for its ability to design cable systems to meet project-specific system requirements. In this instance, the geology of the islands is a major factor. The peat substrate provides a cold and damp environment, so the thermodynamics of the cable and duct insulation needed to be designed to cope with these particular ground conditions. The soil structure is also prone to sinking, so the cable route was designed with snaking to allow for ground movement.

These considerations meant that the first stage of the design for the cable system was to focus on the geologists whose initial task was a detailed analysis of the terrain. This analysis is required by the system design engineers who design the cable for each installation. They will calculate the best option of conductor metal (aluminium in this case) and the design of the conductor itself.

In this instance, this phase of the programme was carried out by main contractor Morgan Sindall Infrastructure who then provided a detailed specification for the bespoke cable and system design to Prysmian.

Logistics and completion

Once the cable design was agreed, all the manufacturing and logistics was managed by the Prysmian team from the UK. For this project the cable was manufactured in the Prysmian plant in China. Before the cables were shipped, factory acceptance tests (FATs) were conducted to ensure the cable met the client’s design brief. Representatives from both the principal contractor and client were invited to Prysmian China to witness the exhaustive tests which took place over several days.

The UK team takes responsibility for all the logistical challenges around moving 99 cable drums (each weighing up to 15 tonnes) across the world for delivery to Shetland on a specific time scale. This was no mean feat, involving shipping initially to Grangemouth terminal port, followed by overland transport to Aberdeen from where the cable was transported by sea to Lerwick on Shetland.

Each section of this journey required detailed research. For example, the overland route from Grangemouth to Aberdeen needed to be checked to make sure the roadways (and bridges) could take the weight and dimensions of this load.

Each drum is loaded with cable to the specific length required for each section of the cable system, with directional guidance included on the drum for the installation team. There is limited storage on site, so the cable drums need to arrive in the right order to co-ordinate with the cable pulling programme.

The cable drums are stored at Grangemouth and dispatched to match the timescale of the installation teams. Constant communication between the installation and logistics teams is required to make sure everything is in the right place at the right moment – with particular consideration for the weather conditions that also need to be factored in.

For this project the Morgan Sindall Infrastructure team is completing all the civil engineering and cable pulling. However, given the bespoke nature of the cable design, Prysmian has provided its own specialist high voltage jointers to complete each circuit using Prysmian cable accessories such as joints and terminations. This is no problem for Prysmian, whose jointing team works in shifts on Shetland for the seven-month duration of the project.

An international perspective

In this project Prysmian UK is also responsible for the supply of the cable accessories such as bonding cables from the Prysmian factory in Wrexham, England, link boxes and pillars, manufactured by Prysmian in Delft, Holland and Eastleigh, England, plus joints and switch gear.

This project is using the Prysmian CLICK FIT connectors which make for fast and uncomplicated jointing on site. They are dry joints so there is no potential for oil leakage over time and removes the need for ongoing maintenance.

In projects of this kind, Prysmian can take complete control of the design, manufacture and testing of the cable system or, as in this instance, work in close co-operation with the main contractor, manufacturing cable systems to their design.

The Gremista to Kergord connection project involves creating a 22km connection between the new Gremista Grid Supply Point (GSP) and the Kergord Substation using a combination of overhead line and underground cabling. Once complete, the Kergord-Gremista link will provide a connection to Shetland’s local electricity distribution network, connecting the islands to the GB energy system for the first time and helping to secure Shetland’s future electricity supply.

Once the system is complete, the Prysmian mobile high voltage testing equipment will be taken to Shetland to complete the on-site commission testing. The first phase of commission testing is scheduled for May 2025.