Enclosure 2 to the NC PARS: Likelihood of Vessel Incident Comparison
Contents
Figures	1
Tables	1
Analysis Overview	2
IWRAP Software and Model Overview	2
Model Inputs	4
Algorithm	4
Geometric Frequency	5
Causation Factor	5
Outputs	5
NC PARS Model Development	5
Alpha Model	5
Bravo Model	5
Alpha and Bravo Models' Leg Configuration	5
Results	8
Results Breakdown and Discussion	8
Appendix 1: Additional Model Images	9
Appendix 2: Detailed Alpha and Bravo Case Results	11
Alpha	11
Bravo	14

Figures
Figure 1: Study Area Including Wind Areas	2
Figure 2: NC PARS IWRAP Alpha and Bravo Leg Configuration	7
Tables
Table 1: IWRAP Vessel Incident Types	4
Table 2: Alpha and Bravo Models' Legs	6
Table 3: Alpha and Bravo Results	8
Table 4: Alpha Overall Results	11
Table 5: Bravo Overall Results	14

Analysis Overview
This report shows calculated vessel incident frequencies for the NC PARS study area. This includes the North Carolina Wilmington East and West, and the South Carolina Grand Stand areas. The vessel incident frequencies were calculated using the IALA Waterways Risk Assessment Program (IWRAP) software. Figure 1 shows the relevant areas. 

                   Figure 1: Study Area Including Wind Areas
IWRAP Software and Model Overview
IWRAP is a tool that predicts the frequency of vessel collisions, allisions, and groundings (vessel incidents) in a specified geographic area. IWRAP evaluates and estimates the annual number of allisions, collisions, and groundings in a modeled area using Automatic Identification System (AIS)-derived vessel traffic data. The specific breakdown and definitions of types of incidents that IWRAP evaluates are detailed in Table 1. 
                                 Incident Type
                                  Definition
                               Powered Grounding
Powered groundings occur when a vessel fails to maneuver to avoid shallow water or shoals and strikes the ocean floor while under power.
                              Drifting Grounding
Drifting groundings occur when a vessel is no longer under the control of the mariner or has lost power and the vessel strikes the ocean floor.
                               Powered Allision
Powered allisions occur when a vessel fails to maneuver to avoid a structure or stationary object and strikes that object. Such objects include bridges or wind turbines.
                               Drifting Allision
Drifting allisions occur when a vessel is no longer under the control of the mariner or has lost power and strikes a structure or stationary object. Such objects include bridges or wind turbines.
                             Overtaking Collision
Overtaking collisions take place when two vessels are on the same leg moving the same direction.
                                       
                               Head-On Collision
Head on collisions take place when two vessels are on the same leg moving in reciprocal directions.
                                       
                              Crossing Collision
Crossing collisions take place when two legs cross at a waypoint.
                                       

                                 Incident Type
                                  Definition
                               Merging Collision
Merging collisions take place when several legs merge at a waypoint.
                                       
                                Bend Collision
Bend collisions take place when ships on the same leg make a turn at a waypoint.
                                       
                     Table 1: IWRAP Vessel Incident Types
Model Inputs
Model inputs include the data required by IWRAP in order to calculate probabilities of vessel incidents. The inputs include AIS data, bathymetry, and structures.
AIS Data
For this analysis, the AIS data set is all AIS equipped vessel traffic in the study area in calendar year 2019. Figure 1 shows this area.
Bathymetry
Bathymetry comes from ENC data (encdirect.noaa.gov) and is uploaded directly into IWRAP. 
Model Structures
Structures are modeled by creating a georeferenced representation of size and shape, and are uploaded directly to the software. The structures used in this analysis were created by NAVCEN in ArcGIS Pro, assuming fully developed wind areas, as monopile wind turbine generators with twelve-meter diameter bases. The turbines were spaced evenly in a one nautical mile grid pattern.
Algorithm
Given the model inputs, the fundamental calculation used by IWRAP is: 
      Incident Frequency = Geometric Frequency x Causation Factor
Geometric Frequency 
Geometric Frequency is the number of incident candidates that exist in an AIS data set. After an AIS data set is uploaded, a color-coded line density plot is used to visually identify the most frequented routes. A network is then developed by an analyst who assigns legs connected by waypoints to the highest density routes based on a visual evaluation. IWRAP then assigns statistical distributions of vessel traffic to legs and waypoints based on the AIS data. The distributions are used to determine the total number of opportunities for a collision to take place. 
Causation Factor
The Causation Factor is a thinning probability applied to the geometric incident candidates. In other words, the causation factor allows IWRAP to determine how many of the candidates for collisions, allision, or grounding actually had an incident occur. The causation factor accounts for the tendency of vessels to avoid other vessels, structures, and grounding. A set of universally applicable causation factors, based on Bayesian statistics and rooted in studies of past incidents, was developed by International Association of Marine Aids to Navigation and Lighthouse Authorities (IALA) in cooperation with the academic community. These values are the default for the IWRAP incident frequency calculations and were not modified from the default for this report.
Outputs
The resulting incident frequencies that are derived from the model are reported for each incident type described in Table 1 with the units years between incidents. These values represent the probabilities a certain type of incident will take place in any given year assuming the traffic makeup is similar to the sample year. 
NC PARS Model Development
In this analysis two models were considered, referred to as Alpha and Bravo.
Alpha Model
The Alpha model is the baseline model in which no structures exist, and represents the probability that incidents will take place in the sample year. Since there are no structures in this model, allision frequencies do not exist.
Bravo Model
The Bravo model assumes that traffic does not alter patterns and adds in the fully developed wind areas. Bravo is the most congested scenario; Bravo shows the maximum allision frequencies the projects may present.
Alpha and Bravo Models' Leg Configuration
The network of legs for the model was developed in cooperation with CG District 5 Waterways based on the 2019 all vessels traffic density and includes 32 legs. The full model is shown in Figure 2 (additional larger scale screenshots are included in Appendix 1). Also in Figure 2, vessel traffic density for 2019 is represented on a yellow, orange, purple, and black scale. Yellow is the highest density areas and black the lowest density. The four colors represent quantiles with cutoffs at 99 (yellow), 95 (orange), 66 purple, and 0 (black) with a continuous gradient in between. The proposed ACPARS fairways as well as the wind areas are also shown in this figure, to provide context. The black lines represent the numbered legs connected by waypoints. Each leg has two distributions assigned, representing the two directions of travel on that leg, which were extracted from the AIS data. The legs used in this model are tabulated in Table 2. 

                                      Leg
                                  Description
                                                                    coastwise_1
                          Legs for coastwise traffic.
                                                                    coastwise_2
                                       
                                                                    coastwise_3
                                       
                                                                    coastwise_4
                                       
                                                                    coastwise_5
                                       
                                                                    coastwise_6
                                       
                                                                    coastwise_7
                                       
                                                                          TSS_1
        Legs near or branching off the TSS approaching Cape Fear River.
                                                                          TSS_2
                                       
                                                                    TSS_split_1
                                       
                                                                    TSS_split_2
                                       
                                                                    TSS_split_3
                                       
                                                                    TSS_split_4
                                       
                                                                    TSS_split_5
                                       
                                                                        inlet_1
        Legs near inlets. Some of these legs cross through wind areas.
                                                                        inlet_2
                                       
                                                                        inlet_3
                                       
                                                                        inlet_4
                                       
                                                                        inlet_5
                                       
                                                                        inlet_6
                                       
                                                                        cross_1
                         Legs that cross a wind area.
                                                                        cross_2
                                       
                                                                        cross_3
                                       
                                                                        cross_4
                                       
                                                                        cross_5
                                       
                                                                        cross_6
                                       
                                                                        shore_1
                    Legs near the shore in the study area.
                                                                        shore_2
                                       
                                                                        shore_3
                                       
                                                                        shore_4
                                       
                                                                         exit_1
                                  Other legs.
                                                                   connection_1
                                       
                                       
                                       
                     Table 2: Alpha and Bravo Models' Legs


                                  Wind Areas
                           ACPARS Proposed Fairways
                                     Legs
           Figure 2: NC PARS IWRAP Alpha and Bravo Leg Configuration

Results
Table 3 shows the results of the Alpha and Bravo models and the percentage change between results ( New Model Value-Old Model ValueOld Model Value*100). The units are years between incidents. Appendix 2 contains a detailed presentation of these data by model and individual leg. 

                                     Alpha
                                     Bravo
                                       
                            Years Between Incidents
                          Percent Change (From Alpha)
                            Years Between Incidents
Powered Grounding
                                     5.02
                                    0.040%
                                     5.022
Drifting Grounding
                                     49.61
                                    0.020%
                                     49.62
Total Groundings
                                     4.559
                                    0.044%
                                     4.561
Powered Allision
                                      ---
                                      ---
                                     104.7
Drifting Allision
                                      ---
                                      ---
                                     2,937
Total Allisions
                                      ---
                                      ---
                                     101.1
Overtaking
                                     3,596
                                      0%
                                     3,596
Head On
                                     1,892
                                      0%
                                     1,892
Crossing
                                   1.19E+05
                                      0%
                                   1.19E+05
Merging
                                   3.91E+05
                                      0%
                                   3.91E+05
Bend
                                     8,604
                                      0%
                                     8,604
Total Collisions
                                     1,071
                                      0%
                                     1,071
                       Table 3: Alpha and Bravo Results
Results Breakdown and Discussion
The probabilities from the models provide a basis to compare the relative likelihood of vessel incidents in the study area in the present configuration (Alpha) as well as in some projected future configuration (Bravo). While Alpha shows no allisions since there are no structures in the model, Bravo does show allisions due to the addition of the WTGs. However, some vessels would likely route around the WEAs depending on their size. In this model, it is anticipated that cargo and tank ships would re-route from cross_5 to another area. These vessel types make up 91% of the total traffic on cross_5. The other legs that cut through wind areas do not have any or have very little cargo and tank ship traffic, and smaller vessels are presumed to continue to route through the WEA. 
Any vessels that re-route around the WEA will reduce the number of candidates for allision. However, when vessels re-route to other areas, they transit in already congested spaces. More vessels would then be present on certain legs in the model, increasing overtaking and head on collisions. Thus, it is anticipated that actual allisions would be less and actual collisions be greater in a real-world scenario than are represented in the Bravo case.

Appendix 1: Additional Model Images 
                                       
                                       
                                       

Appendix 2: Detailed Alpha and Bravo Case Results
For models Alpha and Bravo, overall results are shown followed by tables containing groundings and collisions by leg. Alpha and Bravo collisions are shown only under Alpha since the collision results are the same in each case. Bravo also contains allision results. All units are years between incidents.
Alpha
The Alpha Case is the base case with current traffic based on 2019 data and no developed wind areas. No allisions are contained in the base case as there are no structures in the model.
           Alpha Case  -  Overall Results (Years Between Incidents)
Powered Grounding
5.02
Drifting Grounding
49.61
Total Groundings
4.559
Powered Allision
---
Drifting Allision
---
Total Allisions
---
Overtaking
3,596
Head On
1,892
Crossing
1.19E+05
Merging
3.91E+05
Bend
8,604
Area
---
Total Collisions
1,071
                        Table 4: Alpha Overall Results
                            High Incident Frequency
 Low Incident Frequency                             Low Incident Frequency
                             Low Incident Frequency
                            High Incident Frequency
          Alpha Case  -  Groundings by Leg (Years Between Incidents)
                                      Leg
                                    Powered
                                   Drifting
                                     Total
coastwise_1
                                      ---
                                   1277589.1
                                   1277589.1
coastwise_2
                                  19449778.4
                                    30470.4
                                    30422.8
coastwise_3
                                    8346.0
                                    1638.7
                                    1369.7
coastwise_4
                                     50.6
                                     193.8
                                     40.1
coastwise_5
                                    1055.7
                                    6452.5
                                     907.2
coastwise_6
                                     297.8
                                    1139.8
                                     236.1
coastwise_7
                                    1182.5
                                    38393.6
                                    1147.2
connection_1
                                   139990.3
                                   545795.6
                                   111413.9
cross_1
                                1224523814189.5
                                    70182.0
                                    70182.0
cross_2
                                      ---
                                    87081.6
                                    87081.6
cross_3
                                      ---
                                   7955819.7
                                   7955819.7
cross_4
                                     423.2
                                    11353.0
                                     408.0
cross_5
                                      ---
                                    20982.5
                                    20982.5
cross_6
                                    4367.3
                                    38897.8
                                    3926.4
exit_1
                                      ---
                                  26801195.3
                                  26801195.3
inlet_1
                                    5754.6
                                   891122.5
                                    5717.7
inlet_2
                                      ---
                                   2973906.1
                                   2973906.1
inlet_3
                                    1033.8
                                    10500.1
                                     941.1
inlet_4
                                    3733.7
                                    36185.9
                                    3384.5
inlet_5
                                     762.3
                                    7008.0
                                     687.5
inlet_6
                                     160.7
                                     901.3
                                     136.4
shore_1
                                    1248.7
                                    80059.0
                                    1229.5
shore_2
                                     255.8
                                    9621.5
                                     249.2
shore_3
                                   110436.6
                                   2146373.3
                                   105032.4
shore_4
                                     316.3
                                    3409.4
                                     289.4
TSS_1
                                     21.0
                                     227.3
                                     19.2
TSS_2
                                      9.4
                                     152.9
                                      8.8
TSS_split_1
                                      ---
                                    65612.7
                                    65612.7
TSS_split_2
                                 32297878736.4
                                    25316.7
                                    25316.7
TSS_split_3
                                      ---
                                   2106130.9
                                   2106130.9
TSS_split_4
                                      ---
                                   141958.5
                                   141958.5
TSS_split_5
                                    4825.3
                                    3143.2
                                    1903.4
Overall
                                      5.0
                                     49.6
                                      4.6


     Alpha and Bravo Cases  -  Collisions by Leg (Years Between Incidents)
                                      Leg
                                    Head On
                                  Overtaking
coastwise_1
                                    87527.2
                                   482723.8
coastwise_2
                                    55840.8
                                   227632.3
coastwise_3
                                    6743.7
                                    28510.9
coastwise_4
                                    5018.2
                                    6069.4
coastwise_5
                                    75118.3
                                   282806.1
coastwise_6
                                    17183.8
                                    92416.8
coastwise_7
                                    52606.1
                                   217271.5
connection_1
                                  45448006.7
                                  153746483.1
cross_1
                                   650861.4
                                   2379371.2
cross_2
                                   561210.3
                                   2190186.4
cross_3
                                   1511264.2
                                   5798121.4
cross_4
                                   431161.3
                                   2205434.4
cross_5
                                   2165910.7
                                   4690012.5
cross_6
                                   553058.5
                                   1682771.6
exit_1
                                  100348786.8
                                  277390906.1
inlet_1
                                   5318024.9
                                  33002607.1
inlet_2
                                  414948050.4
                                  561962622.5
inlet_3
                                   2193414.8
                                   3128068.6
inlet_4
                                   116857.2
                                   753813.0
inlet_5
                                   100958.3
                                   345872.1
inlet_6
                                    38547.8
                                   174872.7
shore_1
                                  67820977.0
                                  122140969.8
shore_2
                                   2732888.2
                                   5987298.7
shore_3
                                   1346093.2
                                   2232914.4
shore_4
                                   848575.9
                                   2493469.9
TSS_1
                                   644115.1
                                    71842.1
TSS_2
                                   470177.5
                                    43087.6
TSS_split_1
                                   6212914.7
                                   1717660.9
TSS_split_2
                                   2561992.1
                                   900791.7
TSS_split_3
                                   2086319.7
                                   7135776.0
TSS_split_4
                                756072450703.1
                                  20057330.8
TSS_split_5
                                   1705359.5
                                   4701581.4
Overall
                                    1891.6
                                    3595.7


Bravo
The Bravo Case represents the fully developed scenario with no re-routing of vessel traffic around the wind farms. Allisions are expected to increase from zero. Collisions are expected to remain the same as the Alpha Case.
           Bravo Case  -  Overall Results (Years Between Incidents)
Powered Grounding
5.022
Drifting Grounding
49.62
Total Groundings
4.561
Powered Allision
104.7
Drifting Allision
2,937
Total Allisions
101.1
Overtaking
3,596
Head On
1,892
Crossing
1.19E+05
Merging
3.91E+05
Bend
8,604
Area
---
Total Collisions
1,071
                        Table 5: Bravo Overall Results
                            High Incident Frequency
 Low Incident Frequency                             Low Incident Frequency
                            High Incident Frequency
          Bravo Case  -  Groundings by Leg (Years Between Incidents)
                                      Leg
                                    Powered
                                   Drifting
                                     Total
coastwise_1
                                      ---
                                  1619446.453
                                  1619446.453
coastwise_2
                                  19811304.8
                                    31466.4
                                    31416.5
coastwise_3
                                    8346.0
                                    1642.3
                                    1372.3
coastwise_4
                                     50.6
                                     193.8
                                     40.1
coastwise_5
                                    1055.7
                                    6452.5
                                     907.2
coastwise_6
                                     297.8
                                    1139.8
                                     236.1
coastwise_7
                                    1182.5
                                    38393.6
                                    1147.2
connection_1
                                   139990.3
                                   545795.6
                                   111413.9
cross_1
                                1293254338168.3
                                    70709.5
                                    70709.5
cross_2
                                      ---
                                    88478.2
                                    88478.2
cross_3
                                      ---
                                   8220996.2
                                   8220996.2
cross_4
                                     433.1
                                    11358.7
                                     417.2
cross_5
                                      ---
                                    21179.7
                                    21179.7
cross_6
                                    4376.6
                                    38904.9
                                    3934.0
exit_1
                                      ---
                                  27034759.1
                                  27034759.1
inlet_1
                                    6042.2
                                   891375.5
                                    6001.6
inlet_2
                                      ---
                                   3000955.0
                                   3000955.0
inlet_3
                                    1037.8
                                    10506.7
                                     944.5
inlet_4
                                    3733.7
                                    36185.9
                                    3384.5
inlet_5
                                     762.3
                                    7008.0
                                     687.5
inlet_6
                                     160.7
                                     901.3
                                     136.4
shore_1
                                    1248.7
                                    80059.0
                                    1229.5
shore_2
                                     255.8
                                    9621.5
                                     249.2
shore_3
                                   110436.6
                                   2147334.9
                                   105034.7
shore_4
                                     316.3
                                    3409.4
                                     289.4
TSS_1
                                     21.0
                                     227.4
                                     19.2
TSS_2
                                      9.4
                                     152.9
                                      8.8
TSS_split_1
                                      ---
                                    67369.5
                                    67369.5
TSS_split_2
                                 32297878736.4
                                    25935.0
                                    25935.0
TSS_split_3
                                      ---
                                   2153759.5
                                   2153759.5
TSS_split_4
                                      ---
                                   144144.9
                                   144144.9
TSS_split_5
                                    4825.3
                                    3143.2
                                    1903.4
Overall
                                      5.0
                                     49.6
                                      4.6




           Bravo Case  -  Allisions by Leg (Years Between Incidents)
                                      Leg
                                    Powered
                                   Drifting
                                     Total
coastwise_1
                                      ---
                                    3.5E+09
                                    3.5E+09
coastwise_2
                                    16979.8
                                    1.1E+05
                                    1.5E+04
coastwise_3
                                      ---
                                    4.7E+04
                                    4.7E+04
coastwise_4
                                      ---
                                    6.6E+12
                                    6.6E+12
coastwise_5
                                      ---
                                    4.5E+13
                                    4.5E+13
coastwise_6
                                      ---
                                    3.6E+13
                                    3.6E+13
coastwise_7
                                      ---
                                    2.9E+13
                                    2.9E+13
connection_1
                                      ---
                                      ---
                                      ---
cross_1
                                     405.8
                                    1.5E+04
                                    3.9E+02
cross_2
                                     519.5
                                    1.6E+04
                                    5.0E+02
cross_3
                                     910.9
                                    5.1E+04
                                    9.0E+02
cross_4
                                     568.4
                                    3.0E+04
                                    5.6E+02
cross_5
                                    2066.7
                                    4.9E+04
                                    2.0E+03
cross_6
                                    2773.5
                                    8.9E+04
                                    2.7E+03
exit_1
                                      ---
                                    4.5E+09
                                    4.5E+09
inlet_1
                                    1073.0
                                    7.4E+04
                                    1.1E+03
inlet_2
                                    9372.9
                                    2.1E+05
                                    9.0E+03
inlet_3
                                    2945.3
                                    1.2E+05
                                    2.9E+03
inlet_4
                                      ---
                                    1.3E+06
                                    1.3E+06
inlet_5
                                      ---
                                    3.8E+05
                                    3.8E+05
inlet_6
                                      ---
                                    5.9E+06
                                    5.9E+06
shore_1
                                    1.3E+12
                                    6.9E+09
                                    6.9E+09
shore_2
                                    1.4E+10
                                    3.7E+05
                                    3.7E+05
shore_3
                                      ---
                                    3.2E+05
                                    3.2E+05
shore_4
                                    5.6E+12
                                    7.3E+05
                                    7.3E+05
TSS_1
                                    1.1E+05
                                    6.3E+04
                                    4.0E+04
TSS_2
                                      ---
                                    7.2E+04
                                    7.2E+04
TSS_split_1
                                   3623552.2
                                    1.6E+05
                                    1.5E+05
TSS_split_2
                                    66987.0
                                    9.7E+04
                                    4.0E+04
TSS_split_3
                                  187689484.3
                                    1.3E+05
                                    1.3E+05
TSS_split_4
                                  27358711.7
                                    8.8E+05
                                    8.5E+05
TSS_split_5
                                      ---
                                    8.2E+05
                                    8.2E+05
Overall
                                     104.7
                                    2.9E+03
                                    1.0E+02

