Scope of work

Our client asked us to perform a comprehensive installation analysis for all the subsea array cables at a proposed offshore windfarm in the North Sea. The scope included determining the following:

  • The limiting sea states for installation
  • The minimum bend radius to maintain cable integrity during installation
  • The maximum tension and compression the cable would experience during the lay-phase
  • The forces applied to the cable by the trencher. This is essential in preventing damage to the cable or the trencher

The effect of wave incident angle on the forces exerted on the cable during the lay. This was investigated to allow the client to explore different cable lay schedules to match the environmental conditions.

The resolution

Jee started by creating a comprehensive case matrix designed to ensure that the cable behaviour was fully understood across a wide range of environmental conditions. The matrix included assessment of the full range of tidal levels, as well as a fi nely incremented range of wave heights and periods.

Jee also analysed a number of lay techniques against the specifi ed parameters. This included different vessel confi gurations, lay angles and methods of trenching on the sea bed. Free lay on the sea bed was also analysed in the event that the trencher was nonoperational.

Using the latest version of OrcaFlex simulation software, we were able to deliver intricate and detailed visuals and models of the vessel and the cable to our client. The sea bed was created as a static boundary and the motions and properties of seawater were also added to the model. These motions are not assumed to be simply sinusoidal, instead they were obtained through careful study of historical MetOcean data for the area and time of year. All the relevant stresses in the cable were then recorded through a series of waves in the model and saved for post-processing. Wave and vessel incident angles were varied to allow for the best and worst case scenarios to be explored.

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