16.3.3 Decommissioning Phase

  1. Since the numbers and types of vessel used to remove infrastructure are expected to be similar to those used for installation, this impact is expected to be similar in nature to the equivalent construction phase impact. In particular, project vessels will be managed by marine coordination, applications will be made for statutory safety zones, and decommissioning activities will generally be located within the buoyed decommissioning area.
  2. The impact will be present throughout the decommissioning phase which is expected to be of similar duration to the construction phase (i.e. maximum of 8 years).  With the embedded mitigation measures noted in section  18.1 implemented, it is considered unlikely that an encounter between a third-party vessel and a project vessel will occur. As per the equivalent construction phase impact, in the event that such an encounter does occur, collision avoidance action would be implemented by the vessels as per the COLREGs, thus ensuring that the likelihood of the encounter developing into a collision incident is very low.

16.3.3.1  Frequency of Occurrence

  1. The frequency of occurrence is considered to be extremely unlikely based on the available sea room, consideration of historical incident data, and the embedded mitigation in place to manage project vessel movements and activities.

16.3.3.2  Severity of Consequence

  1. The severity of consequence is considered to be serious.

16.3.3.3  Significance of Risk

  1. Overall, the frequency of occurrence is considered to be extremely unlikely and the severity of consequence is deemed to be serious. The risk will therefore be of tolerable significance.

16.3.3.4  Additional Mitigation and Residual Risk

  1. No additional shipping and navigation mitigation is considered necessary because the likely risk, in the absence of mitigation beyond the embedded mitigation measures outlined in section 18.1, is ALARP and not significant in EIA terms.

16.4 Vessel To Structure Allision Risk

  1. The spatial extent of the impact is considered small given that a vessel must be in close proximity to a structure in the Array during construction for an allision incident to occur. The forms of allision considered are:
  • powered allision;
  • drifting allision; and
  • internal allision.
  1. These are discussed separately for each phase, with a combined impact significance ranking provided.

16.4.1 Construction Phase

16.4.1.1  Powered Allision

  1. Powered allision risk may be caused by human/navigational error, unfamiliarity with the Array and/or a failure of an aid to navigation.
  2. Experience from previous under construction offshore wind farms indicates that Masters regularly choose to transit greater than 1 nm from construction works. In doing so, vessels are unlikely to navigate close enough to a structure to create an allision risk. There is a distance of 27 nm between the Array and Seagreen 1 Offshore Wind Farm, the closest baseline offshore wind farm, which provides notable sea room for safe navigation.
  3. Based on the modelling (see section 15.4.3), with the main commercial route deviations in place and assuming all structures are installed, the base case annual powered vessel to structure allision frequency is estimated to be 6.91×10-3, corresponding to a return period of approximately one in 145 years.
  4. The impact will be present throughout the construction phase which may last for up to eight years and will cover a greater spatial extent as more structures are installed. Safety zones of up to 50 m around partially completed or completed but not yet fully commissioned surface piercing structures will be in place and assist with ensuring that vessels are aware of the presence of structures. Where identified as necessary via risk assessment undertaken in advance of any given activity (which will include consideration of the other mitigation measures in place), a guard vessel may also be used, which will alert passing vessels to the presence of the ongoing construction. Furthermore, the use of lighting and marking as required by the NLB and the MCA (including for partially completed structures), charting of the buoyed construction area and promulgation of information will allow vessels to passage plan a safe route in advance. It should also be noted that commercial vessels are expected to comply with international and Flag State regulations (including the COLREGs and SOLAS). Consultation with the NLB to establish agreement on lighting and marking will be undertaken post-consent. With these embedded mitigation measures in place, it is considered unlikely that a powered allision incident will occur.
  5. From historical incident data, there have been no reported instances of a powered allision involving a third-party vessel with a pre-commissioned wind farm structure in the UK.
  6. The most likely consequences in the event of a powered allision incident are minor damage to property with the vessel able to resume passage and undertake a full inspection at the next port. However, this will depend on multiple factors including the energy of the impact, structural integrity of the vessel and the sea state at the time. Given the potential for a non-steel construction, commercial fishing vessels and recreational vessels are considered more vulnerable. Although considered less likely allision could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Applicant’s MPCP will be implemented to reduce the environmental effects should pollution occur.

16.4.1.2  Drifting Allision

  1. Drifting allision risk may be caused by mechanical or technical failure, adverse weather and/or a navigational system error. A vessel adrift may only develop into an allision situation if in proximity to a pre-commissioned structure. This is only the case where the adrift vessel is located in proximity to the buoyed construction area and the wind and/or tide directs the vessel towards a structure.
  2. As discussed in relation to powered allision risk, it is likely that commercial vessels will deviate to avoid the buoyed construction area. As such, it is likely that associated allision risk would be highest to pre-commissioned structures on the periphery of the Array. Smaller vessels may still choose to transit through, and as such may come in proximity to internal structures.
  3. Based on the modelling (see section 15.4.4), with the main commercial route deviations in place, the base case annual drifting vessel to structure allision frequency is estimated to be 2.16×10-4, corresponding to a return period of approximately one in 4,619 years.
  4. For drifting allision incidents, the adrift vessel would initiate its emergency response procedures to avoid a Closest Point of Approach (CPA) with a structure resulting in an allision. This may include emergency anchoring following a check of the relevant nautical charts (thus ensuring that the anchor deployment does not lead to other impacts such as anchor snagging on a subsea cable), noting this would depend on the vessel and water depths. These measures may also include the use of thrusters (depending on availability and power supply). Moreover, under SOLAS obligations (IMO, 1974), other nearby vessels including project vessels (via marine coordination) may be able to render assistance, depending on the type and size of vessel.
  5. From historical incident data, there have been no reported instances of a drifting allision involving a third-party vessel with a pre-commissioned wind farm structure in the UK.
  6. Should a drifting allision occur, the consequences will be similar to those noted for the case of a powered allision including the unlikely worst case of foundering and pollution. In the highly unlikely scenario of a drifting allision incident resulting in pollution, the implementation of the MPCP will reduce the environmental risk. Additionally, a drifting vessel is likely to be moving at a reduced speed compared to a powered vessel dependent on conditions, thus reducing the energy of the impact, including in the case of a recreational vessel under sail.

16.4.1.3  Internal Allision

  1. As noted in the discussion on third-party vessel to third-party vessel collision risk, it is likely that only smaller vessels (e.g. fishing, recreation) may choose to transit through the Array during the construction phase. On this basis it is considered very unlikely that a commercial vessel would be involved in an internal allision.
  2. Minimum spacing between structures of 1,000 m is considered sufficient for safe internal navigation, i.e. keeping clear of the structures in the Array. The final layout will be agreed with both NLB and MCA, noting these discussions will include consideration of ensuring safe internal navigation.
  3. As with any passage, any vessel navigating in or near the Array is expected to passage plan in accordance with SOLAS Chapter V (IMO, 1974), and promulgation of information will ensure that such vessels have good awareness of the works being undertaken. Charting of the buoyed construction area will further increase mariner awareness.
  4. The Applicant will apply for safety zones of radius 500 m around structures where construction is underway, with 50 m pre-commissioning safety zones applied for around structures where work is not underway during the construction phase. These safety zones would make clear to passing mariners the areas which should be avoided to reduce allision risk.
  5. Should an internal allision occur, the consequences will be similar to those noted in the discussion for the case of a powered allision, including the unlikely worst case of foundering and pollution. In the highly unlikely scenario of an internal allision incident resulting in pollution, the implementation of the MPCP will reduce the environmental risk.

16.4.1.4  Frequency of Occurrence

  1. The frequency of occurrence is considered to be extremely unlikely based on the available sea room, outputs of the modelling, and consideration of historical incident data.

16.4.1.5  Severity of Consequence

  1. The severity of consequence is considered to be serious.

16.4.1.6  Significance of Risk

  1. Overall, the frequency of occurrence is considered to be extremely unlikely and the severity of consequence is deemed to be serious. The risk will therefore be of tolerable significance.

16.4.1.7  Additional Mitigation and Residual Risk

  1. No additional shipping and navigation mitigation is considered necessary because the likely risk, in the absence of mitigation beyond the embedded mitigation measures outlined in section 18.1, is ALARP and not significant in EIA terms.