I.2.5 Cable pull-in

Function

For the array cable, the pull-in consists of the pulling of the cable into the offshore substation, floating offshore wind turbine or buoyant junction box.

For export cables, the pull-in consists of pulling the cable into the onshore transition joint pit as well as into the offshore substation.

What it costs*

About £5 million for a 450 MW floating offshore wind farm.

Who supplies them

The cable pull-in is usually provided by the offshore cable installation contractor.

Key facts

The installation of the export cable starts with the onshore pull-in at the beach, during which the CLV is anchored offshore. A pull-in head is attached and the cable winched on floats or through a pre-laid duct to the onshore transition joint pit, where it is eventually jointed to the onshore cable. The CLV then moves off, laying the cable as it goes. Depending on the landfall site, some projects require horizontal directional drilling which may extend to the first short length of burial offshore. In other cases, the cable may be transferred to a third-party shallow draft barge or amphibious vehicle to bring the cable to shore.

At the offshore substation, it is preferred to pull-in the export cable to the substation immediately after it has been laid by the CLV. It may however be necessary to wet store the cable if, for example, the substation is not yet installed or if the CLV is not equipped to conduct the second-end pull-in at the substation. The offshore pull-in process is normally as follows:

  • End-fitting and pull-in head is installed onto the cable end
  • Bend stiffener is installed onto the end-fitting and the cable
  • The cable is lowered into the sea
  • A messenger wire that was placed in the I-tube/J-tube is attached to the pull-in head using an ROV
  • Pull-in head is pulled into the I-tube/J-tube using a winch, and
  • End fitting is connected to the hang-off at the substation or the floating substructure.

The post-lay array cable installation process, where the array cable is laid after the floating offshore wind turbine has been installed, starts with the pull-in at the substation using a similar process to that described above. Pull-in at the floating substructure is carried out in a similar way. In addition, buoyancy devices and bend stiffeners are attached to the dynamic part of the cable before it is deployed. Once the cable has been pulled-in the CLV then moves off to the next location, laying the cable as it goes and pulling it in once it arrives at the following location. For second-end pull-ins, a quadrant is generally used.

For pre-lay array cable installation, the cable laid to rest on the sea bed for pull-in once the floating offshore wind turbine has been towed to site.

New methods for pre-and post-lay array cable installation are expected that will allow faster cable installation in a wider range of metocean conditions, for example:

  • Array cables can also be pulled-in, terminated and connected in a buoyant junction box or buoyant connector which is then connected to the floating offshore turbine. This solution allows for easier connection to, and disconnection from, the floating offshore wind turbine.

What’s in it

Guide to a Floating Offshore Wind Farm