Case Study

Reducing carbon emissions by up to 250,000 tonnes a year with the world’s longest power link to an offshore platform

The Statoil Gjøa semi-submersible platform in the North Sea is the first ever floating platform to have power supplied by a direct link from shore. The cable, bringing electricity to the platform from the Mongstad refinery in Norway, has resulted in reduced carbon emissions by an estimated 210,000 – 250,000 tonnes a year.

Global Marine Systems Limited completed the installation works for the submarine power cable involving the lay of 99km of static cable and 1.5km of dynamic cable; at the time, the world’s longest power link to an offshore platform. Cable lay vessel NorthOcean 102 was used to install the power cable using a horizontal lay reel system. 

Global Marine was responsible for the complete installation of the cable in one section. This included static and dynamic cable sections, vortex-induced vibration (VIV) strakes, permanent buoyancy units on the lazy wave riser, cable anchors and various heavy joints.

Vessel conversion

All cable installation work was completed using the Cable Lay Ship NorthOcean 102.

Delivered from its build yard in early 2009 as an offshore construction vessel, the ship was taken on charter and converted into a cable lay vessel specifically for the purposes of the Gjøa project. This major conversion work, which was managed and supervised by Global Marine, included installation of a 7,000 tonne capacity horizontal reel carousel system and associated lay spread equipment.

The vessel before and after conversion

Techniques Used

Due to the requirement for a one piece installation, innovative installation techniques and systems had to
be developed to enable work to continue in greater than 4.0m Hs significant waves and to survive the worst sea conditions likely to be encountered.

When laying this type of heavy cable in deep water, control of the lay tension is vital to ensure the cable is not damaged. This was principally achieved by monitoring the point that the cable touched down on the seabed using ROVs. 

These cutting-edge techniques paved the way for future offshore interconnector cables and offshore wind farms to be installed in greater depths and more exposed sea areas for the next generation of developments.

Project Overview

Vessel: NorthOcean 102
Submersible: QUANTUM ROV
Cable length: 99km static cable, 1.5km dynamic cable
Maximum water depth: 544m

Project Challenges

The scope of the cable installation work on the Gjøa project presented a number of challenges, including:

  • Laying the cable down the underwater cliff at Mongstad, including cable anchoring and installation of Vortex-Induced Vibration (VIV) suppression strakes
  • Routing of the cable through an exposed area of sea. This required use of a novel Heave-Compensated (HC) Overboarding Chute and a buoy-based Mid-Water Survival System (MWSS) to mitigate the risks of adverse weather
  • Cable lay in water depths of up to 540m, including continuous Touchdown Monitoring (ROVs)
  • Safe handling & overboarding of heavy cable fittings
  • Installation of permanent buoyancy modules to create a ‘lazy wave’ riser to the platform
  • Development of an omega joint spread design capable of completing a cable repair at any point in the cable throughout the guarantee period

“The installation process needed to be highly controlled as waves can exceed 7 metres in height, even in Summer, and pose a significant challenge. Lay vessel motion would induce unacceptable stresses into the cable which were alleviated by use of a heave compensated lay chute - the first time this has ever been done.”

Project Manager, Global Marine

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