3.5. Operation and Maintenance Phase

3.5.1. Methodology

  1. The overall operation and maintenance strategy will be finalised once the operation and maintenance base location and technical specification of the Array are known, including wind turbine type, electrical export option and final project layout. Therefore, this section provides an overview of the potential scheduled and unscheduled operation and maintenance activities within the Array which are reasonably foreseeable.
  2. Routine operation and maintenance works will be conducted using SOVs, CTVs, and/or ROVs. Divers and DSVs may be utilised if required, although it is anticipated that diverless operations will be utilised as far as practicable. For infrequent major operation and maintenance works, including major component replacements, wind turbines will be decoupled from their mooring and anchoring systems and towed to a suitable port facility. Jack-up vessels will be used for infrequent major maintenance campaigns associated with the OSPs. ROVs will be used to inspect foundations, mooring and anchoring systems, and cabling. A summary of the reasonably foreseeable operation and maintenance activities is provided in the following sections.
  3. Offshore operation and maintenance will comprise of both preventative and corrective activities.

Floating foundations (including mooring and anchoring systems)

  1. The following operation and maintenance activities are expected to occur in relation to the floating foundations:
  • routine inspections;
  • geophysical surveys;
  • repairs or replacements of navigational equipment and other ancillary equipment including condition monitoring equipment;
  • removal of marine growth;
  • repairs or replacements of corrosion protection anodes;
  • removal of fishing debris;
  • painting;
  • replacement of access ladders and boat landings;
  • modifications to/replacement of ancillary structures;
  • repairs or replacement of any buoyancy aids and/or clump weights; and
  • replacement of scour protection.
  1. It is assumed that the majority of these activities will be carried out using uncrewed surface vessels (USVs), SOVs, CTVs, ROVs, CSVs, DP1 vessels, survey vessels, and tug vessels, with appropriate equipment for the activity to be undertaken. Divers and DSV may be required if necessary. Although it is assumed that the majority of these operation and maintenance activities will be routinely scheduled throughout the lifetime of the Array, repairs and replacements of navigational equipment, corrosion protection anodes and access ladders and boat landings, removal of marine growth and fishing debris, and painting are expected to be unscheduled. The frequency of these unscheduled activities will be dependent on the findings of routine inspections and will be carried out during other works as and when required.

Floating wind turbines

  1. The following operation and maintenance activities are expected to occur in relation to the floating wind turbines:
  • replacement of consumables;
  • routine inspections;
  • blade coatings/repairs;
  • minor repairs and replacements within the wind turbines;
  • major component replacement;
  • painting or other coatings; and
  • statutory inspections.
  1. It is assumed that the majority of these activities will be carried out using SOVs and CTVs. ROVs, CSVs, tow vessels, cable vessels and anchor handler vessels may be used in the case of major component replacement which is anticipated to occur on an unscheduled basis (i.e. as and when required).
  2. It is currently anticipated that any large operation and maintenance activities, including major component replacements will take place at a local operation and maintenance port or harbour facility. The process would follow a reverse of the installation approach. It is anticipated that the following indicative steps will be followed to undertake any major operation and maintenance works:
  • Disconnect and unhook the inter-array cables, and wet store on the seabed.
  • Deballast the floating foundation, if required.
  • Disconnect the mooring lines from the floating foundation and wet store on the seabed.
  • Tow the turbine to a suitable operation and maintenance facility using up to two anchor handling tugs, or similar. It is expected that a quay side mounted crane, or a suitable alternative, will be used to undertake any major component replacements. Ballasting and de-ballasting at the quayside may also be required.
  1. Following completion of operation and maintenance works, the wind turbine will be towed back to the turbine location within the Array. Mooring lines would be reconnected, the turbine foundation would be reballasted (as required) and the inter-array cable will be pulled into the turbine and reconnected.
  2. Other operation and maintenance strategies would be considered including novel solutions which do not require towing to port. Temporary floating structures may also be used to and connected to mooring lines and dynamic cables to reduce the need for lowering to and recovery from the seabed.

OSP jacket foundations

  1. The following operation and maintenance activities are expected to occur in relation to the OSP jacket foundations:
  • routine inspections;
  • geophysical surveys;
  • repairs and replacements of navigational equipment and other ancillary equipment including condition monitoring equipment;
  • removal of marine growth;
  • replacement of corrosion protection anodes;
  • painting;
  • replacement of access ladders and boat landings;
  • modifications to/replacement of J/I-tubes; and
  • replacement of scour protection.
  1. It is assumed that the majority of these activities will be carried out using USVs, SOVs, CTVs, ROVs, CSVs, and DP1 vessels, with appropriate equipment for the activity to be undertaken. Unscheduled maintenance activities as the same as described in paragraph 138 (with the exception of fishing debris removal which is not anticipated to be required for OSP jacket foundations), the frequency of which will be dependent on the findings of routine inspections and carried out during other works as and when required.

OSP topsides

  1. The following operation and maintenance activities are expected to occur in relation to the OSP topsides:
  • routine inspections;
  • removal of marine growth;
  • replacement of consumables and minor components;
  • major component replacement; and
  • painting or other coatings.
  1. It is assumed that the majority of these activities will be carried out using SOVs and CTVs. Jack-up barges and/or heavy lift vessels may be required in the case of major component replacement. Although it is anticipated that the majority of these operation and maintenance activities will be routinely scheduled throughout the lifetime of the Array, replacement of consumables and minor components is an unscheduled activity which will occur as required, dependent upon the findings of routine inspections.

Inter-array and interconnector cables

  1. The following operation and maintenance activities are expected to occur in relation to both the inter-array cables and interconnector cables:
  • routine inspections;
  • geophysical surveys;
  • inter-array cable/interconnector cable repair;
  • inter-array cable ancillary equipment repair;
  • inter-array and interconnector cables reburial or installation of cable protection (if required);
  • removal of marine growth and/or fishing debris;
  • modifications to/replacement of J/I tubes;
  • replacement of scour protection; and
  • repairs or replacement of buoyancy modules and/or clump weights.
  1. It is assumed that the majority of these activities will be carried out using USVs, SOVs, CTVs, ROVs, CSVs, DP1 vessels, survey vessels, and cable vessels, with appropriate equipment for the activity to be undertaken (including burial equipment). Divers and DSV may be required if necessary. It is anticipated that the majority of these operation and maintenance activities will be routinely scheduled throughout the lifetime of the Array.

3.5.2. Operation and Maintenance Vessels

  1. Table 3.33   Open ▸ presents the maximum design envelope for vessels involved in operation and maintenance activities for the Array.

 

Table 3.33:
Maximum Design Envelope: Vessels Required During the Operation and Maintenance Phase

Table 3.33: Maximum Design Envelope: Vessels Required During the Operation and Maintenance Phase