15.6 Commercial Traffic Routeing (Cumulative)

15.6.1    Methodology

Given the complex nature of the cumulative scenario (in terms of the influence of cumulative developments and number of affected main commercial routes), each of the leading routeing options is considered individually on a cumulative level. This allows full consideration of the various options available.

Consultation feedback with regards to likely vessel behaviour (see section 4.2) has been incorporated where appropriate including specific behaviour by vessel type and size.

This approach has been applied to the relevant cumulative risk assessment scenarios where a quantitative approach is required, i.e., Scenario 1 and Scenario 2 developments (see section 3.3), noting that no developments were screened into Scenario 2.

15.6.2    Main Commercial Routeing Between Forth Ports and Northern Ports

This subsection considers main commercial routeing between ports located within the Firth of Forth and UK ports located north of the Firth of Forth and Firth of Tay, such as Aberdeen and Invergordon. Figure 15.2   Open ▸ presents two example routes for the pre wind farm scenario together with the baseline, Scenario 1 and Scenario 2 developments.

The route passing south of NnG is Route 14 from section 11.2. The route passing north of NnG is not represented in the analysis of pre wind farm routeing for the Proposed Development in isolation since it is located outside the Proposed Development array area study area and is therefore not characterised by the vessel traffic survey data nor affected by the presence of the Proposed Development in isolation. However, from Anatec’s ShipRoutes database it is known that this coastal route, transited by an average of one vessel per day, exists and as discussed below vessels on this route are considered potential users of the navigation corridor between the Proposed Development array area and Inch Cape.

Figure 15.2:
Examples of Pre Wind Farm Main Commercial Routes Between Forth Ports and Northern Ports

Figure 15.2 Examples of Pre Wind Farm Main Commercial Routes Between Forth Ports and Northern Ports

Based on Anatec’s ShipRoutes database, the majority of commercial vessels associated with the example routeing passing north of NnG are commercial vessels less than 150 m in length. Based on a review of the vessel traffic data, the vessels passing south on the associated routeing largely comprise tankers in the 100 to 200 m range.

With the presence of Scenario 1 developments (Inch Cape) in addition to baseline developments and the Proposed Development, a deviation will be required for north-south routeing out of the Firth of Forth which passes north of NnG. There are three clear options as illustrated in Figure 15.3   Open ▸ and detailed in the text that follows.

Figure 15.3:
Cumulative Options for Main Commercial Routes Between Forth Ports and Northern Ports Passing North of NnG

Figure 15.3 Cumulative Options for Main Commercial Routes Between Forth Ports and Northern Ports Passing North of NnG

  • Pass inshore of Inch Cape (option A) – this option would involve vessels making a change in course to pass between Bell Rock and Inch Cape, resulting in a small deviation.
    • Vessels in transit on the relevant routes include commercial vessels; however charted water depths (30 to 40 m contours) could be considered suitable for such a deviation, noting the following factors that would make such navigation unsuitable for vessels carrying hazardous cargoes:
      • Proximity to shore and shallows;
      • Potential for failures; and
      • The approaches to the Firth of Tay and subsequent interaction with other vessel traffic.
    • The need for additional course adjustments and proximity to surface piercing features to both port and starboard result in this option increasing collision and allision risk for passing vessels.
    • This option would not be suitable for vessels that are not already passing Bell Rock at this proximity.
    • Forth Ports noted that they may also have to contact vessels asking for intentions if vessels shifted to this option (see section 4).
    • This option may increase encounters with recreational vessels, noting that it was raised during consultation (see section 4) that the presence of coastal fishing pots may displace recreational vessels into deviated commercial vessels.
  • Utilise the navigation corridor between the Proposed Development array area and Inch Cape (option B) – this option would involve vessels passing closer to the northern boundary of NnG and then steering a course through the navigation corridor, resulting in a moderate deviation.
    • The proximity to surface piercing features to both port and starboard result in this option introducing increased collision and allision risk for passing vessels. However, the width and shape of the navigation corridor results in the number of additional course adjustments required being similar to the other options and the navigation corridor is MGN 654 compliant.
    • The presence of small craft operating within or in proximity to the navigation corridor may increase potential collision risk with commercial vessels (noting the associated routeing included large commercial vessels), noting this may discourage small craft from operating in proximity to the navigation corridor.
  • Pass offshore of the Proposed Development array area (option C) – this option would involve vessels passing around the south and east of the Proposed Development array area, resulting in a very large deviation.
    • Vessels using this option would be more exposed to adverse weather given the greater distance from the UK east coast. However, there is sufficient available sea room to the south and east of the Proposed Development array area and Seagreen to ensure that a safe distance can be maintained from wind farm structures, and so collision and allision risk is considered to be low.

For north-south routeing out of the Firth of Forth which passes south of NnG, the available options are considered equivalent to those outlined above for north-south routeing out of the Firth of Forth which passes north of NnG, as illustrated in Figure 15.4   Open ▸ and detailed in the text that follows.

Figure 15.4:
Cumulative Options for Main Commercial Routes Between Forth Ports and Northern Ports Passing South of NnG

Figure 15.4 Cumulative Options for Main Commercial Routes Between Forth Ports and Northern Ports Passing South of NnG

  • Pass inshore of Inch Cape (option A) – this option would involve vessels passing north of the Isle of May and inshore of Inch Cape, resulting in a large deviation.
    • This option is considered less feasible in this instance since passing north of the Isle of May requires navigating in shallower water where fishing pots are known to be present or making a sharp turn once beyond the two special marks located east of the Isle of May.
    • Vessels on this route include gas carriers with flammable cargoes that may not wish to use this route due to the potential for machinery failure closer to shore. Their own risk assessments (standing orders) may also prevent them passing closer to the shore.
    • From consultation with the RNLI, the increase in commercial vessel movements inshore of Inch Cape could increase the occurrence of incidents between Arbroath and Anstruther, especially in relation to leisure craft.
  • Utilise the navigation corridor between the Proposed Development array area and Inch Cape (option B) – this option would involve vessels steering a course through the navigation corridor, resulting in a small deviation.
    • This option is considered more feasible in this instance since vessels will have better alignment with the navigation corridor after making a turn to port around NnG, with limited further course adjustments required.
    • The proximity to surface piercing features to both port and starboard result in this option introducing increased collision and allision risk for passing vessels. However, this may be mitigated by the navigation corridor being MGN 654 compliant.
    • The note on gas carriers for option A is again relevant here.
  • Pass offshore of the Proposed Development array area (option C) – this option would involve vessels passing around the south and east of the Proposed Development array area resulting in a large deviation.
    • As per the equivalent option for north-south routeing out of the Firth of Forth, noting that a Regular Operator on this route indicated during consultation that their vessels would opt for this option given the navigational safety risks associated with the other options.

In summary, a safe deviation is available for vessels routeing between the Firth of Forth and northern ports. This can be achieved either by passing offshore of the Proposed Development array area or utilising the MGN 654 compliant navigation corridor. Although use of the navigation corridor will result in some increases in collision and allision risk, the size of the deviation associated with this routeing option is small compared with the very large deviation associated with passing offshore of the Proposed Development array area.

15.6.3    Main Commercial Routeing Between Forth Ports and Eastern Ports

This subsection considers main commercial routeing between ports located in the Firth of Forth and mainland European ports located in Norway and the Baltic region. Figure 15.5   Open ▸ presents two example routes for the pre wind farm scenario together with the baseline, Scenario 1 and Scenario 2 developments. These are based on Routes 5 and 10 from section 11.2.

Figure 15.5:
Examples of Pre Wind Farm Main Commercial Routes Between Forth Ports and Eastern Ports

Figure 15.5 Examples of Pre Wind Farm Main Commercial Routes Between Forth Ports and Eastern Ports

Based on a review of the vessel traffic data, the majority of commercial vessels associated with the example routeing are commercial vessels in the 100 to 200 m range. However, larger vessels are present including passenger vessels and tankers in excess of 200 m.

With the presence of Scenario 1 developments (Inch Cape) in addition to baseline developments and the Proposed Development, a deviation would be required for east-west routeing out of the Firth of Forth. There are three clear options as illustrated in Figure 15.6   Open ▸ and detailed in the text that follows. This figure also shows the worst case pre wind farm route.

Figure 15.6:
Cumulative Options for Main Commercial Routes Between Forth Ports and Eastern Ports

Figure 15.6 Cumulative Options for Main Commercial Routes Between Forth Ports and Eastern Ports

  • Pass inshore of Inch Cape (option A) – this option would involve vessels passing between Bell Rock and Inch Cape, resulting in a large deviation.
    • Water depths are suitable for such a deviation including for the large commercial vessels present.
    • Since these vessels do not currently pass in proximity to Bell Rock it is unlikely to be favoured by mariners.
    • Since this option passes in proximity to both Inch Cape and Seagreen there is also an increased collision and allision risk, which could increase due to the presence of coastal cruising routes for small craft.
  • Utilise the navigation corridor between the Proposed Development array area and Inch Cape (option B) – this option would involve vessels passing around the south of NnG and then steering a course through the navigation corridor, resulting in a large deviation.
    • The proximity to surface piercing structures to both port and starboard result in this option introducing increased collision and allision risk for passing vessels. However, the width and shape of the navigation corridor results in the number of additional course adjustments required being similar to option A and the navigation corridor is MGN 654 compliant. However, this option is more complex to navigate than option C.
    • The presence of small craft operating within or in proximity to the navigation corridor may increase collision risk with commercial vessels (noting the associated routeing included large commercial vessels), noting this may discourage small craft from operating in proximity to the navigation corridor.
  • Pass offshore of the Proposed Development array area (option C) – this option would involve vessels passing south of the Proposed Development array area, resulting in a large deviation.
    • Vessel schedules could be compromised, although given that the route heads into the open Northern and Central North Sea there is likely to be sufficient opportunity to make up time and soften the extent of the deviation.

In summary, a safe deviation is available for vessels routeing between the Firth of Forth and eastern ports. This can be achieved either by passing offshore of the Proposed Development array area or by utilising the MGN 654 compliant navigation corridor. Given that the size of the deviation associated with these options is similar, it is more likely that mariners will choose to pass offshore of the Proposed Development array area, noting that this option also minimises increases in collision and allision risk.

15.6.4    Main Commercial Routeing Between Forth Ports and Southern Ports

This subsection considers main commercial routeing between ports located within the Firth of Forth and European ports located in the Southern North Sea, such as Antwerp and Hamburg. Figure 15.7   Open ▸ presents two example routes for the pre wind farm scenario – both on a straight east/west course out of the Forth and passing south of the Proposed Development and NnG – together with the baseline, Scenario 1 and Scenario 2 developments. These are based on Routes 2 and 8 from section 11.2.

Figure 15.7:
Examples of Pre Wind Farm Main Commercial Routes Between Forth Ports and Southern Ports

Figure 15.7 Examples of Pre Wind Farm Main Commercial Routes Between Forth Ports and Southern Ports

The majority of vessels on the associated example routeing were tankers. This included large tankers in excess of 300 m in length, however the majority were less than 200 m.

With the presence of Scenario 1 developments (Inch Cape) in addition to baseline developments and the Proposed Development there is not expected to be any deviation required for routeing out of the Forth and headed into the Southern North Sea, owing to the distance from Scenario 1 developments.

15.6.5    Main Commercial Routeing North–South Following UK East Coast

This subsection considers main commercial routeing between ports located on the UK east coast, such as Aberdeen and ports in the Humber. Figure 15.8   Open ▸ presents two example routes for the pre wind farm scenario – one on a relatively straight north-south course passing towards the eastern extent of the Proposed Development array area and one on a curved course following the UK east coast passing west of the Proposed Development array area and through Inch Cape – together with the baseline, Scenario 1 and Scenario 2 developments. These are based on Routes 1 and 4 from section 11.2, although also bear similarities to Routes 3 and 9.

Figure 15.8:
Examples of Pre Wind Farm Main Commercial Routes North-South Following UK East Coast

Figure 15.8 Examples of Pre Wind Farm Main Commercial Routes North-South Following UK East Coast

The majority of vessels on the associated example routeing were commercial vessels of less than 100 m in length, however larger vessels were also present on the routeing further offshore.

With the presence of Scenario 1 developments (Inch Cape) in addition to baseline developments and the Proposed Development a deviation will be required for more westerly routeing. There are three clear options as illustrated in Figure 15.9   Open ▸ and detailed in the text that follows.

Figure 15.9:
Cumulative Options for Main Commercial Routes North-South Following UK East Coast

Figure 15.9 Cumulative Options for Main Commercial Routes North-South Following UK East Coast

  • Pass inshore of the Proposed Development array area and Inch Cape (option A) – this option would involve vessels passing between Bell Rock and Inch Cape, resulting in a moderate deviation.
    • Water depths are suitable for such a deviation for the relevant vessels.
    • Since these vessels do not currently pass in proximity to Bell Rock it is unlikely to be favoured by mariners.
    • Since this option passes in proximity to both Inch Cape and Seagreen there is also an increased collision and allision risk, which could increase due to the presence of coastal cruising routes for small craft.
    • Forth Ports would have to contact vessels asking for intentions if vessels shifted to this option (see section 4.2).
    • This deviation may increase encounters with recreational vessels, noting that it was raised during consultation (see section 4.2) that the presence of coastal pots may ‘push’ recreational vessels into deviated commercial vessels.
  • Utilise the navigation corridor between the Proposed Development array area and Inch Cape (option B) – this option would involve vessels steering a course through the navigation corridor, resulting in a small deviation.
    • The proximity to surface piercing features to both port and starboard result in this option introducing increased collision and allision risk for passing vessels. However, the width and shape of the navigation corridor results in the number of additional course adjustments required being similar to the pre wind farm route and the navigation corridor is MGN 654 compliant.
    • There is a risk for larger vessels utilising this option in adverse weather associated with under keel clearance from the shallow Marr Bank although given that the associated depths are similar to those navigated on the pre wind farm route the additional risks are considered minimal.
    • The presence of small craft operating within or in proximity to the navigation corridor may also increase collision risk with commercial vessels, which may discourage small craft from operating in proximity to the navigation corridor. However, during consultation it was indicated that recreational vessels operating a north-south transit may choose to navigate internally within the eastern portion of Inch Cape (depending on layout), thus avoiding the navigation corridor and reducing collision risk involving recreational vessels.
  • Pass offshore of the Proposed Development array area (option C) – this option would involve vessels passing around the east of the Proposed Development array area, likely resulting in a decrease in the route length.
    • Vessels using this option would be more exposed to adverse weather given the greater distance from the UK east coast. Given the curved course of this route in the pre wind farm scenario, it is assumed that using the UK east coast as shelter is a key justification for the choice of longer passage and therefore use of this option would not be preferable.
    • This option would likely involve passing offshore of Kincardine, potentially increasing adverse weather exposure.

In summary, a safe deviation is available for vessels routeing north-south following the UK east coast. This can be achieved either by passing offshore of the Proposed Development array area or utilising the MGN 654 compliant navigation corridor. Given that the offshore option results in a shorter passage distance and minimises collision and allision risk it is more likely that mariners will choose to pass offshore of the Proposed Development array area in standard weather conditions, noting that this option also minimises increases in collision and allision risk. However, some vessels may prefer to utilise the navigation corridor given that this option retains a passage closer to the coast, particularly in adverse weather conditions.

For more easterly routeing in a north-south direction, there is not anticipated to be any deviation in addition to those anticipated for the Project in isolation scenario, owing to the distance from Inch Cape and the assumption that such vessels will make a small deviation east of the Proposed Development array area.

15.6.6    Main Commercial Routeing Between Dundee and Eastern Ports

This subsection considers main commercial routeing between Dundee and mainland European ports located in the Baltic region. Figure 15.10   Open ▸ presents an example route for the pre wind farm scenario passing north of the Proposed Development array area, together with the baseline, Scenario 1 and Scenario 2 developments. This is based on Route 11 from section 11.2.

Figure 15.10:
Example of Pre Wind Farm Main Commercial Route Between Dundee and Eastern Ports

Figure 15.10 Example of Pre Wind Farm Main Commercial Route Between Dundee and Eastern Ports

The majority of vessels on the associated example routeing were commercial vessels of less than 100 m in length, however larger vessels were also present.

With the presence of Scenario 1 developments (Inch Cape) in addition to baseline developments and the Proposed Development, a deviation would be required. There is one clear option as illustrated in Figure 15.11   Open ▸ and detailed in the text that follows.

Figure 15.11:
Cumulative Options for Main Commercial Routes Between Dundee and Eastern Ports

Figure 15.11 Cumulative Options for Main Commercial Routes Between Dundee and Eastern Ports

  • Pass north of Inch Cape and Seagreen – this option would involve vessels passing around Seagreen, resulting in a large deviation.
    • Water depths are suitable for such a deviation and there is sufficient available sea room west and north of Inch Cape to ensure a safe distance can be maintained from the wind farm structures. Therefore, collision and allision risk is considered to be low.
    • Additionally, Regular Operator consultation feedback from HAV Ship Management indicated that when considering the cumulative scenario there are no safety problems foreseen for their vessels – which operate on such a route out of Dundee – including in adverse weather conditions.

In summary, a safe deviation is available for vessels routeing between Dundee and eastern ports. This can be achieved by passing north of Inch Cape and Seagreen. Although a longer passage, this option minimises collision and allision risk and, once the route reaches the open Central North Sea, there is likely to be sufficient opportunity to make up time and soften the extent of the deviation.

16 Collision and Allision Risk Modelling

16.1 Overview

To inform the risk assessment, a quantitative assessment of some of the major hazards associated with the Proposed Development has been undertaken. The following subsections outline the inputs and methodology used for the collision and allision risk modelling.

16.1.1    Scenarios Under Consideration

For each element of the quantitative assessment, both a pre and post wind farm scenarios with base and future case traffic levels have been considered. As a result, six distinct scenarios have been modelled:

  • Pre wind farm with base case traffic levels;
  • Pre wind farm future case with a 10% increase on base case traffic levels;
  • Pre wind farm future case with a 20% increase on base case traffic levels;
  • Post wind farm with base case traffic levels;
  • Post wind farm future case with a 10% increase on base case traffic levels; and
  • Post wind farm future case with a 20% increase on base case traffic levels.

The results of the base case scenarios are detailed in full in the following subsections, with the equivalent results for each future case scenario provided in section 16.4.

16.1.2    Hazards Under Consideration

Hazards considered in the quantitative assessment are as follows:

  • Increased vessel to vessel collision risk;
  • Increased powered vessel to structure allision risk;
  • Increased drifting vessel to structure allision risk; and
  • Increased fishing vessel to structure allision risk.

The pre wind farm assessment has been informed by the vessel traffic survey data (see section 10 and other baseline data sources (such as Anatec’s ShipRoutes database). Conservative assumptions have been made with regard to route deviations and future shipping growth over the lifetime of the Proposed Development.

16.1.3    Post Wind Farm Routeing

The methodology for the post wind farm routeing is outlined in section 15.5.1.

16.2 Pre Wind Farm Modelling

16.2.1    Vessel to Vessel Encounters

An assessment of current vessel to vessel encounters has been undertaken by replaying at high speed the vessel traffic data collected as part of the vessel traffic surveys (see section 5.2). The model defines an encounter as two vessels passing within 1 nm of each other within the same minute. This helps to illustrate where existing shipping congestion is highest and therefore where offshore developments, such as an offshore wind farm, could potentially increase congestion and therefore also increase the risk of encounters and collisions. No account of whether encounters are head on or stern to head are given; only close proximity is accounted for.

Figure 16.1   Open ▸ presents a heat map based upon the geographical distribution of vessel encounter tracks within a density grid. Following this, Figure 16.2   Open ▸ illustrates the daily number of encounters recorded within both the Proposed Development array area study area and the Proposed Development array area throughout the survey periods.

Figure 16.1:
Vessel Encounters Heat Map within the Proposed Development Array Area Study Area (28 Days, August 2022 and January 2021)

Figure 16.1 Vessel Encounters Heat Map within the Proposed Development Array Area Study Area (28 Days, August 2022 and January 2021)

Figure 16.2:
Vessel Encounters per Day within the Proposed Development Array Area Study Area (28 Days, July 2020 & January 2021)

Figure 16.2 Vessel Encounters per Day within the Proposed Development Array Area Study Area (28 Days, July 2020 & January 2021)

There was on average one encounter per day within the Proposed Development array area study area throughout the survey periods. The greatest number of encounters recorded in one day was four, on 23 January 2021, due to a number of cargo vessels and tankers transiting in the NW corner of the Proposed Development array area study area.

The most frequent vessel types involved in encounters within the Proposed Development array area study area were cargo vessels (31%) followed by tankers (28%) and commercial fishing vessels (19%).

16.2.2    Vessel to Vessel Collision Risk

Using the pre wind farm vessel routeing as input, Anatec’s COLLRISK model has been run to estimate the existing vessel to vessel collision risk within the Proposed Development array area study area. The route positions and widths are based on the vessel traffic survey data and have been validated using the long-term vessel traffic data and consultation with local stakeholders.

A heat map based upon the geographical distribution of collision risk within a density grid for the pre wind farm base case is presented in Figure 16.3   Open ▸ .

Figure 16.3:
Pre Wind Farm Vessel to Vessel Collision Risk Heat Map within the Proposed Development Array Area Study Area

Figure 16.3 Pre Wind Farm Vessel to Vessel Collision Risk Heat Map within the Proposed Development Array Area Study Area

Assuming base case vessel traffic levels, the annual collision frequency pre wind farm was estimated to be 8.49×10-4, corresponding to a return period of approximately one in 1,178 years. This is slightly above the average for UK offshore wind farm developments and is reflective of the relatively large area covered. It is noted that the model is calibrated based upon major incident data at sea which allows for benchmarking but does not cover all incidents, such as minor effects. Other incident data, which includes minor incidents, is presented in section 9.