prepared by: nuka research & planning group, llc. version ... · scenarios where on-scene...

September 2014 | version 1

Brief to the Aleutian Islands Risk Assessment Advisory Panel & Management Team

Prepared by: Nuka Research & Planning Group, LLC.

Estimated Response Times for TUGS OF OPPORTUNITY

in the Aleutians

Estimated Response Times for Tugs of Opportunity in the Aleutians

2 version: September 2014

Large ships regularly transit through the Aleutian Islands on the great circle route between the west coast of North America and East Asia. Historically, there has not been a dedicated emergency tow vessel in the Aleutian Islands to assist a distressed ship. However, tugs of opportunity – tugs that are available in the region but not dedicated to rescue services - may be able to aid a distressed ship if they are willing and able to divert their activities.

Nuka Research and Planning Group, LLC developed this study to supplement the Aleutian Islands Risk Assessment (AIRA)-sponsored study entitled, “Tug of Opportunity Study” (2013a) which estimated the amount of time it would take a potential tug of opportunity to reach six hypothetical incident locations and whether the tug would have sufficient bollard pull to control a large container ship once it reached the location (The Glosten Associates, 2013a and 2014). This study analyzes actual tug location data for a one-year time period and extrapolating this information to illustrate the availability and capability of towing vessels in service in the Aleutian Islands region to arrive on-scene and assist disabled vessels in various locations.

This study estimates how long it would take potential tugs of opportunity to reach six hypothetical incident locations in the Aleutian Islands considering different wind and sea conditions. Travel time estimates are derived from actual towing vessel locations based on a weekly sampling of AIS data in 2012. A containership was chosen since their high profile makes them more challenging to control, particularly in high wind conditions.

In addition to the AIS data used to assess tug locations, data from National Data Buoy Center buoys in the Aleutian Islands region was used to estimate wind and sea state. Data about tug capability was derived from the Tug of Opportunity Study (The Glosten Associates, 2013a). Two sets of analysis were performed. The first estimated response times for the first tug on-scene without consideration of the tugs capability. This addressed scenarios where on-scene conditions were favorable or the vessel in distress was small enough to be controlled by a tug regardless of its capability. A second set of scenarios applied tug capability thresholds, and considered the arrival time of tugs capable of controlling the hypothetical distressed vessel under a range of wind and wave conditions.

Eighty-six tugs were included in the analysis, plus two additional vessels, which were treated as special cases – the US Coast Guard cutter Alex Haley and the tug Resolve Pioneer. For all scenarios, the tugs most likely to reach a distressed vessel in the Aleutian Islands first are overwhelmingly located in the eastern Aleutians (near Dutch Harbor), or even farther east, near Kodiak or in Bristol Bay. Of the 86 tugs identified in the AIS data, 23 of them were not useful in any scenario because for each incident site, in each sea state, in each week it was present, a more capable tug would have arrived first. Of the remaining tugs, most of them were useful in only a handful of weeks, with only one tug--the James Dunlap--showing up as a potential responder in more than 12 weeks.

Tug availability was not consistent for the one-year period analyzed. A fully functional tug

Report to the Aleutian Islands Risk Assessment Advisory Panel & Management Team

version: September 2014 3

with greater than 80 MT bollard pull was present in about half the weeks of the analysis. More tugs were available in late July and August 2012; tugs with bollard pull greater than 100 MT were most available in July--August and again in November--December, but were rare during the rest of the year. One reason for this may have been the positioning of tugs in the region to support oil exploration activities in the Chukchi Sea.

Even in extreme weather, a tug of opportunity could usually be expected to reach a distressed vessel within 12 hours near Unimak Pass, but a distressed vessel in the western Aleutian Islands area would likely wait two or more days for a tug of opportunity rescue. Adequate emergency towing assistance could be delayed or impossible in very stormy weather, especially if relatively large tugs, present in the region in 2012 to support oil exploration activities, were not available.

Cover Photo: The tug Gyrfalcon participates an emergency towing exercise in Unalaska Bay, September 2009. Photo credit: Tim Robertson.



AUTHOR Nuka Research & Planning Group, LLC

TABLE OF CONTENTS Executive Summary ........................ 2

Overview ............................................ 4

1. Introduction ................................. 6

1.1 Background ................................. 6

1.2 Purpose and Scope .................... 6

2. Methodology ................................. 6

2.1 Overview ...................................... 6

2.2 Data Sources ............................... 7

2.3 Data Characterization .............. 7

2.4 Analysis ...................................... 10

2.5 Assumptions .............................. 12

3. Results ......................................... 12

3.1 Tug Locations ........................... 12

3.2 Tug of Opportunity Availability and Capability ... 13

3.3 Tug Arrival Times ................... 16

4. Conclusions ........................................ 21

References ....................................... 22

Appendix A ...................................... 23

Appendix B ...................................... 24



1. INTRODUCTION 1.1 Background

Tankers, container ships, and other deep draft vessels regularly transit through the Aleutian Islands on the great circle route between the west coast of North America and East Asia. The time required for an assist vessel to arrive on-scene can be the deciding factor between vessel casualty and serious accident or oil spill.

Historically, there has not been a dedicated emergency tow vessel in the Aleutian Islands to assist a ship that has lost power or propulsion.1 However, tugs of opportunity – tugs that are available in the region but not dedicated to rescue services – may be able to aid a distressed vessel if they are willing and able to divert their activities. Effective emergency towing requires that a tug be available, able to reach the distressed vessel in the conditions at the time, and able to control the vessel once on scene.

1.2 Purpose and Scope

Nuka Research and Planning Group, LLC developed this study to supplement the AIRA-sponsored, “Tug of Opportunity Study,” (2013a) which estimated the amount of time it would take a potential tug of opportunity to reach six hypothetical incident locations and whether the tug would have sufficient bollard pull to control a large container ship once it reached the location. This study analyzed actual tug location data for a one-year time period and extrapolated this information to illustrate the availability and capability of towing vessels in service in the Aleutian Islands region to arrive on-scene and assist disabled vessels in various locations.

This study is part of a series of reports developed to inform the recommended Optimal Response System for the Aleutian Islands, which is summarized in a key findings report (Nuka Research, 2014) for Phase B of the Aleutian Islands Risk Assessment.

2. METHODOLOGY 2.1 Overview

This study first considers how long it would take any tug of opportunity to reach six hypothetical scenario locations in the Aleutian Islands area considering different wind and sea conditions. It then considers how long it takes for a tug of opportunity with sufficient bollard pull of capable of saving a hypothetical distressed vessel under the same regime of wind and sea conditions. The hypothetical distressed vessel is 7,500 twenty-foot equivalent unit (TEU) containership.2 Travel time

1 In 2013, Resolve Marine stationed the M/V Resolve Pioneer in Dutch Harbor, which can provide emergency towing services. 2 This hypothetical containership represents the 75th percentile vessel, by tonnage, of containerships transiting Unimak Pass in 2012.



estimates are derived from actual towing vessel locations based on a weekly sampling of AIS data in 2012

Response times depend on the location of the tug of opportunity relative to the distressed vessel scenario, speed of the tug, whether the tug had to first go to port to drop a tow, and wind and sea conditions. Response times were calculated using the same method applied in Glosten Associates, 2013a. The stricken vessel scenario locations were chosen based on DNV and ERM, 2011.

2.2 Data Sources

This study uses three sets of data:

• Automated Identification System (AIS) data from the Marine Exchange of Alaska was used to determine the location of known towing-capable vessels within the Aleutian Islands Risk Assessment study area at 12:00 noon each of the 53 Wednesdays in 2012.

• Data from National Data Buoy Center buoys in the Aleutian Islands region (as summarized in Nuka Research, 2013) was used to for wind and sea state measurements. See Figure 1.

• A data set produced for the AIRA Tug of Opportunity study (The Glosten Associates, 2013a), with estimates of response times for the tug fleet to the six scenario locations. Raw data from the file “TugsOpportunity3.csv” was used with minor corrections.

2.3 Data Characterization

2.3.1 Characterization of Tug Fleet

We summarized the existing tug fleet based on the bollard pull of each tug and its availability as documented in the 2012 AIS dataset. Eighty-six tugs were included in that data, plus two special cases.

We separated two vessels from the larger analysis as special cases:

• The USCGC Alex Haley and the Resolve Pioneer. The USCGC Alex Haley is sometimes stationed in the Aleutians, but lacks a towing winch. It has limited utility as a rescue vessel currently, but could hypothetically be retrofitted for towing. We estimated its response time assuming it was a fully capable tow vessel with a towing winch.

• The Resolve Pioneer is an offshore supply vessel with 73 MT bollard pull that was stationed in Dutch Harbor as a salvage vessel in 2013. Although it was not technically available during the 2012 study period, it was included in this analysis because it is currently available for emergency towing services. Because of the special circumstances for each of these two vessels, their arrival times were considered in separately from the tugs of opportunity; they did not compete with those vessels to be first on-scene.



2.3.2 Bollard Pull Requirements based on Wind and Wave Conditions in the Aleutians

The tug capability analysis considered whether a tug had sufficient bollard pull to control the hypothetical container ship in the observed wind and wave conditions based on the relationship between wind and waves in the Aleutians Islands area. Actual conditions vary greatly around these typical values as shown in Figure 1. In this figure, wind and wave conditions are averaged across four buoys in the North Pacific and Bering Sea (Nuka Research, 2013), moderating extreme conditions that may exist any one location at a given time. The Glosten Associates (2014) used 98th percentile wind and wave conditions as marked with the yellow diamond to determine that a tug would require 109 MT rated bollard pull to turn and control the hypothetical distressed vessel in conditions that are far worse than usual, but not unheard of in Aleutian Islands.3

Figure 1: Variability in wind and wave conditions relative to conditions modeled for this study. Values from Table 1 fall on the blue line.

Table 1 shows the minimum bollard pull required to turn and tow the hypothetical containership in a variety of conditions.

3 This is rounded to 110 MT in for the purposes of the Best Available Technology study (The Glosten Associates, 2013b).



Table 1: Environmental pairing between wave height and wind speed used to identify tugs capable of responding in particular wave height conditions (The Glosten Associates, 2014). Highlighted rows are reproduced directly, while other rows are interpolated from the original analysis.4

WAVE HEIGHT (m) WAVE HEIGHT (ft) WIND SPEED (kt) BOLLARD PULL (MT)

04 04 N/A 04

1 3.3 11 5

1.5 5 13 10

2 6.6 15 11

3 9.8 18 19

3.0 10 18 19

4 13.1 21 28

4.6 15 23 34

5 16.4 24 38

6 19.7 28 49

6.1 20 28 50

7 23.0 31 61

7.6 25 33 68

8 26.2 34 75

9.1 30 38 91

10.1 33 41 109

Figure 2 presents the minimum bollard pull required to turn and tow the containership for three different wind speeds recorded in the Aleutian Islands, and the number of times those speeds were observed. As shown, wind speed in the Aleutians is usually at least 10 knots, winds over 20 knots are quite common, and larger storms bring winds more than 40 knots. The size of tug needed to rescue the study containership vessel is highly dependent on these wind conditions. In calm conditions, even 5 MT of bollard pull could be sufficient. In 24-knot winds, the force required jumps to 38 MT. In extreme, 98th percentile, conditions of 41-knot winds, at least 109 MT bollard pull is required (The Glosten Associates, 2014).5

4 Wind speeds fit an exact linear function, while bollard pull is approximately fit by a quadratic function. No value was provided for zero wave height; zero was assumed for wave height and bollard pull for the purposes of approximating the relationship with a quadratic function. 5 Bollard pull is used here to simplify the analysis by using only one vessel characteristic factor, and because it is generally accepted to be the most representative of these for indicating a vessel’s ability to implement rescue towing operations. Other characteristics of the vessel and its equipment and the capabilities of the crew will influence the success of the mission as well. The Glosten Associates describes other vessel features applicable to effective operations in the Aleutian Island region in the Best Available Technology study (2013b).



Figure 2: Rated bollard pull required to turn and tow a hypothetical containership based on wind speeds and observations of those wind speeds averaged across four weather buoys in the Aleutian Islands (based on The Glosten Associates, 2014). Percent of conditions that are equal to or below a given wind speed and associated bollard pull are reported in parentheses.

2.4 Analysis

The process used to estimate travel times for tugs of opportunity followed several steps where data sets were compiled and analyzed, a series of assumptions applied, and calculations performed.

2.4.1 Any Tug Arrival Time Analysis

The Glosten Associates calculated the time required for each tug to travel from its initial location to each of the incident locations (The Glosten Associates, 2013a). They make the following assumptions:

• All tugs are towing barges unless it is specifically known that they do not tow barges in these waters.

• Tugs will proceed to Dutch Harbor or Adak at towing speeds to drop off tow and pick up emergency towing package.

• Tugs spend minimal time in port and do not refuel.

• Tugs proceed to stricken vessel at top speed, degraded by sea state and heading.

The travel time of the tug vessel is based on its top speed in calm water and its ability to maintain that speed in higher sea states. The speed reduction in waves was estimated based on tests of other vessels and Glosten’s judgment. Glosten provide a data set of their calculated transit times for each vessel to each incident location for every sea state for each of the 53 weeks. We then analyzed those data to investigate the availability of tugs of opportunity to assist a disabled ship.

For the purpose of this analysis, the “First Responder” was the tug calculated to be first on scene. This analysis assumes that all tugs were capable of rescue, regardless of the bollard pull rating. In weeks where a tug arrived on-scene before any other tug of equal or greater bollard pull in at least one scenario and wave



height, it was identified as a First Responder in the results. In many cases, a tug achieved this standard of usefulness in some weeks and but not in others.

2.4.2 Capable Tug Arrival Time Analysis

The tug arrival time analysis was extended to only consider those tug with a bollard pull sufficient to turn and tow the hypothetical containership in the wind/wave condition considered, based on the thresholds in Figure 3. This results in identifying the First Capable Responder for the conditions and containership studied.

2.4.3 Effects of Periods of High Waves

The Glosten Associates (2012a) estimates response times assuming that an entire rescue will be conducted under constant wave conditions, even when response times are several days long. To determine the extent to which persistent high seas are a concern, we looked at periods where continuously high seas occurred in buoy data (2008-2012). We calculated the portion of total hours occurring in a sequence of hourly observations wherein waves continuously exceed a given value, for a certain length of time. For example, at the North Pacific Buoy 46072, 15% of the measurements of wave height are found in periods of 3 m or greater waves lasting for more than 48 hours. (In other words, if a random hour were selected from the dataset, there is a 15% chance it would be part of a series of 48 or more observations that were all more than 3 m. See Figure 3.) This implies that 15% of the time there is at least 24 hours before seas fall below 3 m.6

Figure 3: Cumulative distribution of periods of 3 and 5 m + seas for North Pacific buoy 46072.

6 For the purpose of this analysis, we conservatively considered data gaps to end periods of continuously high seas. A single observation of seas below the threshold would also break the sequence.



Although extreme seas occur infrequently, they may also impact vessel operations and therefore the likelihood of an incident occurring in the first place. A complete table showing periods where seas were consistently over certain heights at the buoy is provided in Appendix A.

2.5 Assumptions

The following simplifying assumptions were applied throughout this study:

• In considering the impact of wave height and wind speed on vessel transit speed, it is presumed that both will be constant during the transit from the tug location to the disabled vessel.

• For all scenarios, it is presumed that the most capable tug (highest bollard pull and speed) with the shortest transit to the distressed vessel is the first on-scene.

• Our analysis of transit data showed that waves from zero to one meter had no effect on response time, and 2 m waves have almost no effect, so 2 m waves are used as the lower limit of conditions considered.

3. RESULTS 3.1 Tug Locations

Figure 4 shows the six scenario locations and the original location of the tug that was able to reach a scenario location first (the First Responder) in the any tug arrival time analysis. The tugs most likely to reach a distressed vessel in the Aleutian Islands first are overwhelmingly located in the eastern Aleutians (near Dutch Harbor), or even farther east, near Kodiak or in Bristol Bay.

USCGC Alex Haley is marked separately, because it was analyzed independently from the other tugs. The Resolve Pioneer (not marked) was also handled separately, and is always stationed in Dutch Harbor for our analysis. Scenario locations (yellow stars) included three in the eastern Aleutians, and three farther west. Vessels near the three eastern scenarios and the Attu Island scenario would be vessels following the "Great Circle Route" through Unimak Pass, while vessels nearer to Adak and Amlia are transiting the North Pacific south of the Aleutians.



Figure 4: Locations of tugs and scenarios used in this study. First Responder Tugs (blue dots) are those that for some wave height and scenario location were able to arrive ahead of all other tugs with equal or greater bollard pull in our analysis. They are shown at their original location.

3.2 Tug of Opportunity Availability and Capability

Of the 86 tugs identified in the AIS data, 23 of them were not useful in any scenario because for each incident site, in each sea state, in each week it was present, a more capable tug would have arrived first based on the assumptions and data used in this analysis. It should be noted that many of the largest towing vessels in the region in 2012 were employed in an offshore exploratory drilling operation. These vessels are not considered to be regularly available in the region.

Of the remaining tugs, most of them were useful in only a handful of weeks, with only one tug--the James Dunlap--showing up as a potential responder in more than 12 weeks. This is in part because it does not have a tow, and can depart directly from Dutch Harbor towards an incident site. The James Dunlap can transit high sea states, but its capacity is limited by high winds, so scenarios where the James Dunlap was most likely to be successful to rescue a 7500 TEU container ship were during light winds.

If a tug is present in the region but not the first, most capable vessel to respond in a given scenario, it could still potentially provide support as a secondary responder. The Resolve Pioneer and the Alex Haley were analyzed separately--their arrival times did not affect whether the tugs in this chart were marked as useful or not.

Figure 5 summarizes the bollard pull of each vessel in the dataset, the number of weeks it was present in the region (light blue squares), and the number of weeks in



which the tug arrived on-scene to one of six scenario locations ahead of any more capable vessel in at least one sea state (dark blue squares). Tugs for which The Glosten Associates (2013a) assumed there was no tow involved are indicated with yellow diamonds.7

Figure 5: Towing vessels included in the study with bollard pull (red bars), number of weeks that the tug was present in the region (light blue squares) and the number of weeks in which the tug arrived on-scene to one of six scenario locations ahead of any more capable vessel in at least one sea state (dark blue squares).

7 The Gyrfalcon also would not typically be assumed to have a tow, though this was not represented as such in The Glosten Associates, 2013a.



Figure 6 presents the distribution of available tugs – and their bollard pull – throughout the year. More tugs were available in late July and August; winter months show relatively few emergency towing vessels. In 2012, tugs with bollard pull greater than 100 MT were usually available in July/August and again in November/December, but were rare during the rest of the year. This suggests that in addition to seasonal variability, there may be inter-annual variability not fully captured in our one-year dataset. This is supported by the fact that there were towing vessels in the region in the study year employed in offshore oil exploration. A fully functional tug with greater than 80 MT bollard pull was present in about half the weeks of the analysis. In the remaining 24 weeks, the only tug with more than 80 MT bollard pull was the USCGC Alex Haley (which lacks a towing winch). In 17 of 53 weeks, the USCGC Alex Haley was the only tug available with more than 60 MT bollard pull.

Figure 6: Temporal distribution of available tugs over the study period. Two tugs are flagged – the Alex Haley, which lacks a towing winch, and the James Dunlap, a smaller tug that was often an early responder. The Resolve Pioneer was not present in the area in 2012.



The majority of tugs in the dataset have a bollard pull of 40 MT or less. Few tugs were present for the entire study period, and most of the larger tugs (bollard pull >60 MT) were present for fewer than 15 weeks of the 53-week study period.

3.3 Tug Arrival Times

Tug arrival times were calculated for two sets of conditions – one where we assumed that the first tug to arrive on-scene was sufficient to execute a save regardless of its bollard pull, and one where we required the responding tug to have bollard pull above limitations outlined in Table 1 (The Glosten Associates, 2014; Table 5). Our full analysis is plotted in “TugsOpportunity3.csv.”

3.3.1 Any Tug Arrival Time Analysis

Table 2 presents the median response time for any tug to reach each incident location in six different sea states.

Table 2: Median response times in hours for each incident location and sea state for any tug.

Response Time (in hours) Sea State (m) 0-‐2 3 4 5 6 7

Scenario 1 (Attu Island) All other tugs 58 61 70 86 119 220 USCGC Alex Haley 69 72 83 101 140 260 Resolve Pioneer 49 52 59 72 99 184

Scenario 2 (Adak) All other tugs 31 33 38 46 63 116 USCGC Alex Haley 52 55 63 77 106 196 Resolve Pioneer 27 28 32 39 53 97

Scenario 3 (Amlia Island) All other tugs 24 25 28 34 47 86 USCGC Haley 47 49 56 69 95 175 Resolve Pioneer 20 21 24 29 40 72

Scenario 4 (North Unimak Pass) All other tugs 6 6 7 8 11 18 USCGC Alex Haley 28 29 33 41 56 102 Resolve Pioneer 6 6 7 8 10 17

Scenario 5 (Urila Bay) All other tugs 8 8 9 11 14 24 USCGC Alex Haley 26 27 31 37 51 94 Resolve Pioneer 8 9 10 12 15 27

Scenario 6 (Sanak Reef) All other tugs 6 7 7 8 11 19 USCGC Alex Haley 24 25 29 35 48 87 Resolve Pioneer 11 11 13 15 21 37

Figure 7 shows two contrasting scenario locations, assuming either 3-or 5-m seas, but no limit based on bollard pull. These two scenario locations capture the difference between eastern Aleutian incidents (e.g. at North Unimak Pass), near Dutch Harbor where many tugs can be found, and western Aleutian incidents (e.g. Attu Island), where no tugs are nearby. Three-meter seas are extremely common in



the Aleutians, present over 40% of the time, and persist for more than 48 hours 15% of the time. Five-meter seas are present 12% of the time, but rarely persist for more than a day.

Figure 7: Response times for any tug transiting to incidents at North Unimak Pass and Attu Island.

As shown in Figure 8, North Unimak Pass is close enough to Dutch Harbor that response times are relatively good – less than 8 hours except by USCGC Alex Haley when it's stationed in Kodiak, and in rare cases for other tugs when there happened to be none available nearby. However, very few responding tugs were available within 48 hours to the scenario on Attu Island.



3.3.2 Capable Tug Arrival Time Analysis

The Glosten Associates (2014) shows that the tug capability necessary to effect a rescue tow in the Aleutians varies greatly (5 to 109 MT) depending of the size and type of distressed vessel and the environment conditions at the scene. Table 3 and Figure 9 repeats the same analysis as shown above but requires the responding tug to have sufficient bollard pull to turn and tow the hypothetical containership, presuming sea state is related to wind speed as shown in Table 1 and Figure 2.

Including bollard pull limits does not change the response time for the USCGC Alex Haley or the Resolve Pioneer, which are both large enough to respond to our study case containership in up to 7 m seas. However, for the actual tugs of opportunity present during the 2012 study period, response times at 6 m seas or greater are significantly impeded, as some of the available tugs are incapable of responding in the winds corresponding to those sea states.

Table 3: Median response times in hours for each incident location and sea state for only tugs capable of turning and towing the hypothetical containership.

Response Time (hours) Sea State (m) 0-‐2 3 4 5 6 7

Scenario 1 (Attu Island) All other tugs 58 61 70 86 148 299 USCGC Alex Haley 69 72 83 101 140 260 Resolve Pioneer 49 52 59 72 99 184

Scenario 2 (Adak) All other tugs 31 33 38 46 90 187 USCGC Alex Haley 52 55 63 77 106 196 Resolve Pioneer 27 28 32 39 53 97

Scenario 3 (Amlia Island) All other tugs 24 25 28 34 73 139 USCGC Alex Haley 47 49 56 69 95 175 Resolve Pioneer 20 21 24 29 40 72

Scenario 4 (North Unimak Pass) All other tugs 6 6 8 9 17 31 USCGC Alex Haley 28 29 33 41 56 102 Resolve Pioneer 6 6 7 8 10 17

Scenario 5 (Urila Bay) All other tugs 8 8 10 13 20 47 USCGC Alex Haley 26 27 31 37 51 94 Resolve Pioneer 8 9 10 12 15 27

Scenario 6 (Sanak Reef) All other tugs 6 7 10 16 25 53 USCGC Alex Haley 24 25 29 35 48 87 Resolve Pioneer 11 11 13 15 21 37



All of our analysis thus far assumes constant weather conditions: if a tug is transiting to a distressed vessel in 3 or 5 m seas, it will conduct its entire response under the wind conditions associated with that sea state. Obviously, this is not always the case. In some cases, weather may improve while a tug is on its way. But in other cases, weather will get worse. A capable tug setting out for an incident site may become an incapable tug by the time it gets there. The "best available technology" for tug response is a tug that would be capable of responding during the most extreme weather conditions known to occur in the Aleutians. While these conditions are unlikely to occur continuously for multiple days, they do occur. These weather conditions have been defined as 41-knot winds and 10-m seas, and a tug would need a bollard pull of 109 MT to respond (The Glosten Associates, 2014). Few tugs in the Aleutians meet this requirement, and those that do were only sporadically present in 2012. As a result, response is impossible in these wind/wave conditions for the majority of weeks, for either the Unimak Pass or Attu Island incident site (Figure 8).8 On the left half of the figure transit conditions are assumed to be 5-m seas, but in the right side of the figure, tugs are required to effect their response in more extreme 7-m seas--necessitating the a tug with a minimum bollard pull of 109 MT (discussed in The Glosten Associates, 2014). With the 109 MT minimum most vessels are omitted so response is often impossible.

8 The captain of the tug attempting – unsuccessfully – to tow the ill-fated Selendang Ayu in 2004 reported winds of 40-55 knots and seas of 20-25 feet (NTSB, 2006).



Distribution of modeled response times for tugs transiting in 5 m and 10 m waves.

Figure 8: Response times for capable tugs transiting to incidents at North Unimak Pass and Attu Island.



4. CONCLUSIONS Tug of opportunity availability is not expected to be consistent. In our dataset, there were far more tugs present in the later half of the year than the earlier half, consistent with the increase in oil exploration activities in the Chukchi Sea. There were also more tugs of opportunity present in summer than during the winter months. The oil exploration fleet is also the source of most emergency tow vessel with high bollard pull ratings, capable of responding in high wind conditions. As with other tugs of opportunity, these oil industry vessels were more available during the summer months when the wind/wave conditions are favorable. Oil exploration activities vary seasonally and year to year according to economic and political factors separate from those driving Aleutian shipping traffic, making reliance on these tugs problematic.

In the scenarios considered for this study, distressed vessels in the western Aleutians must wait days for a tug rescue. In the eastern Aleutians, distressed vessels can usually be rescued within less than half a day, but if the combination of distressed vessel size and environmental conditions require a tug with high bollard pull, availability of a suitable tug of opportunity is not guaranteed. Although the tugs of opportunity in the region are capable in the conditions that are common and persistent in the Aleutians, very few tugs could respond in the most extreme weather. Rescue may be delayed in rough weather, especially in the absence of larger tugs involved in oil exploration, which were present during the 2012 study year.



REFERENCES Det Norske Veritas and ERM-West, Inc. (2011a). Task 3-4 consequence analysis

report. Aleutian Islands Risk Assessment Phase A.

National Transportation and Safety Board. (2006). Marine accident brief: Grounding of Malaysian-flag bulk carrier M/V Selendang Ayu on north shore of Unalaska Island, Alaska, December 8, 2004. NTSB/MAB-06/01.

Nuka Research and Planning Group, LLC.(2014). Recommending an optimal

response system for the Aleutian Islands: Summary report. Aleutian Islands Risk Assessment Phase B.

Nuka Research and Planning Group, LLC. (2013). Characterizing environmental

conditions in the Aleutian Islands. Aleutian Islands Risk Assessment Phase B.

The Glosten Associates. (2014). Minimum required tug for the Aleutian Islands. Report No.12127.03.1. Aleutian Islands Risk Assessment Phase B. Nuka Research and Planning Group, LLC.

The Glosten Associates. (2013a). Tug of opportunity study. Report No.12127.01.12e.

Aleutian Islands Risk Assessment Phase B. Nuka Research and Planning Group, LLC.

The Glosten Associates. (2013b). Best available technology. Report No.12127.02.12c.

Aleutian Islands Risk Assessment Phase B. Nuka Research and Planning Group, LLC.



APPENDIX A HOURS of CONTINUOUS MEASUREMENTS ABOVE the WAVE THRESHOLD



APPENDIX B Given six different sea states from 2 to 7 m, six scenario locations, and four response vessels (Resolve Pioneer, James Dunlap, USCGC Alex Haley, and all other tugs) we generated 144 histograms of response times. Each histogram cuts off at 6 days, and longer response times (including non-response) are lumped into a final bin (Figures B1, B2).

Figure B1: Histograms of response times for tugs of opportunity. All tugs capable of transiting in given weather conditions are plotted, regardless of bollard pull.



Figure B2: Histograms of response times for tugs of opportunity. Only capable tugs with bollard pull sufficient to control the hypothetical container ship are plotted.

prepared by: nuka research & planning group, llc. version ... · scenarios where on-scene...

Documents