United Kingdom

United Kingdom

Easy does it: the art of a gentle cable pull

Connecting the world's largest offshore wind project to the grid required a new approach and a delicate touch, writes Mike Morrison, global head of Sparrows Group Cable and Pipe Lay Solutions (CPLS).

The project was the largest AC offshore wind export cable system ever to have been installed (all pics: Sparrows Group)
The project was the largest AC offshore wind export cable system ever to have been installed (all pics: Sparrows Group)

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Ørsted's Hornsea One development, located 120km off the UK’s east coast, is poised to be the world’s largest offshore wind project when fully operational in 2020.

More than 170 turbines are expected to generate 1GW of clean energy to power well over one million homes.

Sparrows Group’s Cable and Pipe Lay Solutions (CLPS) division worked with Boskalis Subsea Cables & Flexibles, DEME Offshore and DeepOcean to deliver the nearshore, export and inter-array cables for the project.

It marked the longest AC offshore wind export cable system ever to have been installed.

Export cable lines used for offshore projects are often more delicate when compared to the more rigid cables and umbilicals common in the oil and gas sector.

This is due to the increasing inclusion of fibre optics that run inside the composite cables, which enable developers to monitor the turbine remotely.

As a result, it is a technical and logistical challenge for laying cables, as the lines are less capable of withstanding the squeeze pressures from the lay tensioner, which holds the lines during deployment.

Sparrows’ CPLS division was required to provide equipment for a cable laying system at Hornsea One, which was suited to cable deployment, while assuring the lower squeeze pressure limitations were not exceeded.

The company developed a solution for transferring the cables across a 3.6km beach site out to the landfall location.

Tools for the job

It was determined that three tensioners would be required for the two-month project — this was the first time Sparrows was tasked to deliver this quantity in a consecutive series.

A 15-tonne tensioner was stationed on a jack-up vessel located on the beach, while two 10-tonne cable tensioners were mounted on Linkflote floating pontoons.

Track pads were designed to be soft but durable enough to ensure cable clamping limits were maintained for the entirety of the project.

Linkflotes are steel pontoons that can be linked together to form larger platforms and stay above water level when the tide moves in.

Onshore piles were installed on to which the tensioners were positioned to ensure there was no lateral movement during the cable pulling activities.

Sparrows engineered a lashing system to hold the equipment down to the pontoons using rigging, chains and steel sections.

During cable pulling, the tensioner logged all activity by measuring speed, distance, grip and tension.

The data was stored locally on the programmable logic controller (PLC) memory and downloaded daily, to make sure that the tensioners were operating without comprising the integrity of the cables.

Flexible approach

The second phase of the project required the installation of over 350km of export cables from nearshore locations to a repeater station and three substations using two 36-tonne tensioners.

Before starting the project, Sparrows created 3D models of the tensioners so the equipment could be easily incorporated into the lay spread design.

The primary tensioner conducted the spooling of the cable on the vessel and the laying offshore with zero downtime.

Data from the tensioners was again logged and fed to the vessel's bridge by serial output to provide real-time monitoring of the equipment.

As the offshore wind industry continues to evolve, the demand for cable and pipe lay solutions will become increasingly important.

Offshore wind is moving into deeper waters, with more complex subsea cable systems required.

The growing sector needs a strong supply chain to ensure these vital cables are effectively installed and monitored.

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