National Transportation Safety Board

Marine Accident Brief

Grounding of Mobile Offshore Drilling Unit Kulluk

Accident no. DCA13NM012

Accident type Grounding
Vessel Mobile offshore drilling unit Kulluk
Location Near Ocean Bay, Sitkalidak Island, Alaska
Date, time December 31, 2012
2040 Alaska standard time (coordinated universal time − 9 hours)
Damage Substantial
Injuries Four minor
Environmental None
damage
Weather and At departure, moderate winds and seas with scattered precipitation, expected to
sea conditions continue during first days of voyage; several days later, low pressure moved into
Gulf of Alaska area. By December 31, recorded winds reached 40−55 knots, with
seas more than 20 feet.
Waterway Gulf of Alaska lies south of the state of Alaska in the northern Pacific Ocean
information

The ice-class mobile offshore drilling unit (MODU) Kulluk, owned by Shell Offshore,
Inc., and operated by Noble Drilling, grounded in heavy weather near Ocean Bay on the eastern
coast of Sitkalidak Island off Kodiak Island, Alaska, about 2040 local time on December 31,
2012. The Kulluk, under tow by the ice-class anchor-handling tow supply vessel Aiviq, departed
Captains Bay near Unalaska, Alaska, 10 days earlier for the Seattle, Washington, area for
maintenance and repairs.1 Four crewmembers on the Aiviq sustained minor injuries as a result of
the accident.

Tow vessel Aiviq, left, tows Shell’s conical drilling rig Kulluk before the Kulluk
grounding in December 2012. (Photo from www.pbase.com)

1
Affiliates of Royal Dutch Shell plc are referred to as “Shell” in this report.

Note: This report was reissued on June 16, 2015, with correction to page 1. NTSB/MAB-15/10
CORRECTED COPY
Grounding of Mobile Offshore Drilling Unit Kulluk

Accident and Investigation ............................................................................................................................................2

Kulluk. .....................................................................................................................................................................2
Aiviq ........................................................................................................................................................................3
Tow plan .................................................................................................................................................................3
Warranty survey .....................................................................................................................................................5
Accident voyage. ....................................................................................................................................................6
Discussion ....................................................................................................................................................................11
Probable Cause ............................................................................................................................................................12
Vessel Particulars .........................................................................................................................................................13

The MODU sustained substantial damage. A postaccident inspection conducted by a third

party before the vessel was refloated found underwater portions of the hull were extensively
damaged although it retained watertight integrity. Widespread damage also was noted to areas
including the vessel’s superstructure, electrical equipment, lifesaving and safety equipment, and
interior and engineering spaces.
No environmental damage was found as a result of the grounding. The vessel was
carrying about 143,000 gallons of low-sulfur diesel oil and 12,000 gallons of other petroleum
products at the time of the accident.
The US Coast Guard convened a formal hearing on the accident in Anchorage, Alaska, in
May 2013. This accident brief is based on the National Transportation Safety Board’s (NTSB)
participation in the Coast Guard hearing and investigation and its analysis of evidence. Parties to
the NTSB investigation were Shell, owner of the Kulluk; Offshore Service Vessels LLC,2 owner
of the Aiviq; and the Coast Guard. In accordance with the International Maritime Organization
(IMO) Casualty Code, the Republic of the Marshall Islands, the flag state of the Kulluk,
participated in the Coast Guard’s investigation of this accident as a substantially interested state
and provided the NTSB with technical assistance throughout its investigation of the accident.

Accident and Investigation

Kulluk. The ice-class Kulluk, originally built in 1983, had a conical shape and
strengthened hull to increase its capability for navigating in ice-covered waters. Shell acquired
the MODU in 2005 and undertook extensive refurbishing for oil drilling operations north of
Alaska. Without its own propulsion, the rig required towing to move from one location to
another.
During normal operations, the Kulluk could accommodate 108 persons, but on the
accident voyage, a skeleton crew of 18 was on board.3 The tow master was from Offshore Rig
Movers International, and the remaining marine crewmembers were employed by oil and natural
gas drilling contractor Noble Corporation. Key positions and their responsibilities included:
 Tow master, on board the Kulluk―overall responsibility for controlling the
movement of the Kulluk and operation of all assisting vessels during the tow, from
unmooring in Alaska to mooring in Washington state, as well as interfacing with the
Kulluk crew and other personnel
 Offshore installation manager (OIM), on board the Kulluk―in command of the
Kulluk

2
A division of Edison Chouest group of operating companies.
3
The number of crew on the Kulluk exceeded the eight crewmembers required by its Minimum Safe Manning
certificate of the flag state. Nonetheless, if requested, the flag state administrator would have granted a waiver to this
requirement, as occurred in a previous 2012 tow.

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 Shell’s Alaska marine manager―Shell shoreside manager, located in Anchorage;

oversight responsibility for planning and managing Kulluk rig moves
 Aiviq master—in command of the dedicated tow vessel and its crew; legally
responsible for the tow once the towline was made fast and the vessels were under
way

Aiviq. The US-flagged Aiviq is an ice-class, anchor-handling tow vessel designed to

operate in extreme weather and sea conditions. The tow vessel is capable of 15 knots in open
water and 5 knots in ice-covered water, and its bollard pull, or towing power, at the time of
departure was 208 tons, which exceeded the requirement for towing the Kulluk set in 2010.
According to information Shell provided to the Coast Guard, the Aiviq had towed the Kulluk in
three previous transits in 2012 before the accident voyage, the last trip concluding a month
before the December 2012 departure.
The Aiviq experienced mechanical difficulties on earlier tow voyages. In a statement to
the NTSB, Offshore Service Vessels noted that on August 31, 2012, a low fuel pressure alarm
alerted for main engine #3. The fuel pressure gauge was found to be faulty and the fuel meter
was found to be fouled. The fuel filter was replaced and the meter cleaned, and the problem was
corrected. On November 10, 2012, an 11-minute blackout occurred, during which time the main
engines continued to run but their cooling water pumps shut down due to the loss of electrical
power to the grid. Four minutes into the blackout, main engine #4 shut down following an oil
mist alarm. The crew was unable to restart the engine after the blackout was resolved. It was
determined that two of the rod bearings had failed. They were replaced by the engine
manufacturer under the provisions of the warranty, and the engine was tested by the
manufacturer and the class society and functioned normally as a result.

Tow plan. Shell began planning early in 2012 for the Kulluk’s voyage from Unalaska to
the Seattle area in December. A coastal route was chosen to maintain a distance less than
200 nautical miles from the shore of southern Alaska to allow easier access by search and rescue
(SAR) personnel should the need arise. The 1,773-mile transit was expected to take 24.6 days at
an average speed of 3 knots, 21.1 days at an average of 3.5 knots, or 18.5 days averaging 4 knots.
Shipyard capabilities and equipment in the Seattle area were deemed more suitable for
performing the Kulluk’s planned maintenance and repairs than facilities available in Alaska, a
primary factor in the decision to tow the Kulluk to Washington to enable the MODU to be fully
operational for the planned 2013 oil exploration drilling season. In addition, there was a question
as to whether the Kulluk would be subject to Alaska state property tax, anticipated to reach
millions of dollars, had the vessel remained in Alaska beyond the end of the year.

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Approximate course taken by the Aiviq towing the MODU Kulluk between Unalaska, Alaska, and
the site of the grounding near the southeast coast of Kodiak Island. The Seattle, Washington,
area was the intended destination. (Background by National Geographic MapMaker Interactive)

The towing plan was developed by Shell’s Alaska marine manager and reviewed by the
Aiviq master, the Shell Alaska operations manager, Alaska drilling manager, logistics team lead,
health safety and environmental team lead, emergency response specialist, tow master, Noble
Drilling Kulluk rig manager and operations manager for Alaska, and the GL Noble Denton
warranty surveyor.4 The Shell Alaska operations manager was the final approval authority.
Because he was on vacation at the time the tow plan was approved, a subordinate whom he had
designated approved the tow plan in his place.
The tow plan recognized historical climate data for the route and season. If adverse
weather were encountered, the tow was to “turn into weather if possible and create as much sea
room as possible, then proceed at a speed conducive with conditions.” The tow master and Aiviq
master were authorized to alter the course in response to weather conditions. Shell commissioned
a private meteorological service to conduct a meteorological and oceanographic (metocean) tow
route simulation study that provided hindsight weather forecasting. The study was based on
30-year global wind and wave hindcast data to determine the maximum wave height and wind
speed that might be encountered along the possible routes of the planned tow.
Shell contracted with a weather forecasting service to provide daily forecasts tailored
along the route of the towing operation. This weather information was to be e-mailed to the
Kulluk tow master, Aiviq master, and Shell Anchorage management each day. If the weather

4
Noble Denton and Germanischer Lloyd (GL) merged in 2009, and in 2012, Det Norske Veritas (DNV) and GL
merged to form DNV GL.

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deteriorated, weather reports would be more detailed and furnished more frequently. The Kulluk
and Aiviq also had online access to additional weather prediction sources. Although rough
weather was anticipated, according to the Kulluk OIM, “None of us expected to have seas as
rough as we had.” The 5-day forecast beginning the day of departure was considered by Shell
officials and onboard managers to be acceptable for the vessel’s planned departure. Coast Guard
investigators determined:
The Metocean study conducted for the December 21, 2012 tow of the Kulluk
contained wind and wave statistics that, even at the least conservative 10%
exceedance rate, were significantly higher than the wind and wave criteria
established for the bollard pull requirements in the 2010 study conducted for the
Kulluk. . . . After analyzing the data, the [Coast Guard determined that the] Aiviq
would be capable of generally maintaining position with the Kulluk in tow during
Beaufort 8 weather in 5 meter seas and 40 knot winds while encountering a 1 knot
current, with all forces acting against the tow into the wind and seas. In weather
conditions in excess of this standard, as predicted by the Metocean study, the
Aiviq could not be expected to maintain position, and would be pulled astern by
the forces acting on the Kulluk.
The potential hazards facing the transit were known. The day after departure, the Aiviq
master wrote an e-mail to the Kulluk tow master stating, in part, “I believe that this length of tow,
at this time of year, in this location, with our current routing guarantees an ***kicking.”

Warranty survey. A warranty survey assesses whether the vessels and the planned route,
equipment, manning, and other aspects of a planned voyage will provide vessel and crew safety.
According to a Shell representative, a Shell finance specialist advised that a warranty survey was
not needed for insurance underwriting for towing the Kulluk, but the company decided to have
the surveys carried out regardless. For earlier tows, Shell contracted with marine risk assessment
specialists MatthewsDaniel or Noble Denton to provide warranty surveys. The December 2012
voyage was the Kulluk’s first transit of the Gulf of Alaska in winter.
In November 2012, the Kulluk transited from the Beaufort Sea to Dutch Harbor, near
Unalaska, ultimately to continue to the Seattle area. The Noble Denton warranty surveyor who
conducted the December 2012 survey had conducted the surveys and approved two previous
Kulluk voyages, including its November 2012 trip.
The warranty surveyor arrived in Anchorage prior to the December tow to meet with
Shell shoreside personnel. The surveyor reviewed the tow plan and attended planning meetings
with Shell’s management team before proceeding to Dutch Harbor. The warranty surveyor
completed his inspection of the towing equipment for compliance with the requirements of the
Shell towing plan on December 21, the day of departure. His review included a visual
examination of the tow gear components, examination of vessel records, and meetings with the
tow master, the Aiviq master, the chief mate and the chief engineer, and an additional Aiviq
engineer. The chief mate showed the surveyor around the Aiviq. The Aiviq’s chief engineer
testified that he met with the surveyor for about 30 minutes before the departure from Dutch
Harbor, and the surveyor asked him one question―“Do you have any major engineering
issues?”―to which the chief responded, “No.” The chief engineer was not asked about, nor did
he provide information regarding, the previous incidents involving the Aiviq’s main diesel
engines and the temporary repairs undertaken to the vessel’s winch room covers and safe deck
areas.

The tow gear between the Aiviq and the Kulluk had been used on three previous voyages
to tow the Kulluk. According to Offshore Service Vessels, heavy weather was encountered on
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two of the tows. The warranty surveyor stated that he visually inspected the tow gear with the
tow master and found it to be in good shape with no damage. He also stated that nondestructive
testing is not required between uses unless the integrity of the equipment is in doubt. The tow
master for the December 2012 tow, who also visually inspected the tow gear, stated that the
industry standard for the life of towing gear is 5 years and, based on the age of the gear, a visual
inspection was appropriate. Data were not available, nor were such data required, that would
have enabled a determination of the remaining strength of the tow gear after its use towing the
Kulluk in previous tows.
No regulations or class rules govern the demonstrated strength, useful life, number of
cycles, or repeated stresses to which the tow line and tow gear could be subject, although the
IMO has established guidelines and several countries have promulgated regulations to that effect.
Shell testimony and documentation provided to the Coast Guard hearing demonstrated that the
company followed industry practices with regard to manufacture, use, and inspection of the tow
gear, and as noted, the warranty surveyor visually inspected the tow gear. The Aiviq’s towing
gear consisted only of a 3.5-inch tow wire connected to the Kulluk’s gear with a segment of
surge chain followed by the towing pennant. This pennant was connected to shackles and then to
the towing plate. Shackles connected the towing plate to chain bridles, which were secured on
the Kulluk with Smit brackets.5

Configuration of Kulluk and Aiviq tow gear. (Drawing adapted from Coast Guard)

Noble Denton publishes guidelines for marine transportation including towing, but the
Noble Denton surveyor was not requested nor required to analyze or review the towing
configuration to ensure it met these guidelines prior to approving the tows. The components were
evaluated to ensure that they were in compliance with the towing plan as provided to him by
Shell.

Accident voyage. Following oil drilling operations in the Beaufort Sea north of Alaska
during the summer of 2012, the Kulluk was returned to Unalaska in November and remained
moored until its December 21 departure for Washington state. On December 20, the Aiviq was
refueled in Dutch Harbor with 443,000 gallons of fuel that were fed from a gravity drain shore
side tank.

5
Smit brackets are fittings welded on deck to enable towing connections to be made quickly and securely; the
bracket consists of a large sliding pin to receive the eye of a tow line or chain; Smit brackets are often found in pairs;
it was named after the inventor, Dutch towing specialist company, Smit International
(http://www.m-i-link.com/dictionary/default.asp?term=Smit+bracket. , accessed May 15, 2015)

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December 21−24―The Aiviq left Captains Bay, Unalaska, Alaska, at 1325 on

December 21 with the Kulluk under tow, initially traveling about 3.5 knots, and the first few days
were uneventful. On the second day of the voyage, the National Weather Service predicted winds
increasing to 35 knots over the next 2 to 3 days with seas reaching 17 feet in 5 days. The Kulluk
tow master, Aiviq master, and the OIM discussed altering their course to reduce the overall
distance and transit time, but they decided to continue on the planned coastal route to remain
within the 200-mile SAR limit.
December 25―Four days into the voyage, the National Weather Service issued gale
warnings for the following day, forecasting wind velocity up to 40 knots and seas up to 15 feet.
The Aiviq master and Kulluk tow master again discussed a course change, this time to the east to
attempt to avoid the low pressure system approaching from the southwest, and they informed the
Shell marine manager of their concerns.
December 26―Winds increased rapidly, from 15−20 knots late on December 25 to
35−40 knots by 0000 on December 26, and seas reached 10–15 feet. By the end of the day on
December 26, the Kulluk was about 35 nautical miles southeast of the Trinity Islands at the
southernmost point off Kodiak Island and encountering significant swells.
December 27―The Aiviq master changed to a more southerly route, away from the
worsening weather. His decision was made in consultation with the OIM and the Kulluk tow
master and authorized by Shell’s marine manager.
The manufacturer of the Aiviq’s towing winch system, examining data from the vessel’s
winch system after the accident, revealed that on this day the vessel’s alarm for “wire tensile
strength overload on tow drum” activated 38 times between 0534 and 1129, each alert indicating
that the Aiviq’s tow load exceeded 50 percent of the wire’s preset breaking load of 600 tons, that
is, a strain of at least 300 tons. The Aiviq paid out an additional 360 feet of tow line between
0500 and 1100 hours to ease the tension on the tow wire.
At 1135, when the vessel was roughly 50 miles southeast of Sitkalidak Island, the tow
connecting the Aiviq to the Kulluk failed. Winds were recorded from the west-southwest at
15−20 knots, and sea swells were 20−25 feet, occasionally reaching 30 feet. Shell put its incident
management team in Anchorage on notice immediately after the tow failure and was regularly
informed of the Aiviq’s and Kulluk’s situation.
Following the failure of the tow gear, Shell shoreside personnel had two other vessels
under long-term charters to Shell, the tug Guardsman and the oil spill response vessel Nanuq,
moved to be in position to assist the Kulluk if needed. Shell shoreside personnel also located the
tow vessel Alert and requested that it be moved to assist the Kulluk. Shell personnel also notified
the Coast Guard of the situation, and the Coast Guard dispatched its cutter Alex Haley from
Kodiak, Alaska, to the Kulluk’s location.
At 1430, the Aiviq’s emergency tow line was connected to the Kulluk. The tow master
advised the Aiviq that the tension on the emergency tow gear should not exceed 60 tons. About
1900 that evening, according to engine data, one of the Aiviq’s engines shut down, and the other
three engines also failed over an approximate 5-hour period starting about 2235. Offshore
Service Vessels told investigators that, despite these engine failures, the Aiviq was capable of
maintaining sufficient power to put a considerably greater force on the emergency tow line than
what the tow master had advised. Shell called for the entire incident management team to be in
place upon being informed of the Aiviq’s propulsion loss, and it implemented a 12-hour watch
rotation for all incident response center personnel.

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December 28―With the engines providing no propulsion, the Aiviq maintained a safe
distance from the Kulluk by using its thrusters. The Coast Guard cutter Alex Haley arrived on
scene about 0130 on December 28. The Coast Guard crew attempted unsuccessfully to rig a tow
line to the Aiviq, and in the process, the tow line fouled the Coast Guard vessel’s port propeller.
The Alex Haley departed for Kodiak for repairs later that morning.
By the afternoon, the Guardsman arrived on scene, and at 1329, personnel connected a
tow line from the Guardsman to the Aiviq while the Aiviq remained connected to the Kulluk, and
both vessels, in tandem, then began to tow the MODU. Because of the worsening weather and
continuing problems with tow lines and the Aiviq engines, a Shell representative contacted the
Coast Guard about 1800 that evening to request evacuation of the 18 persons on the Kulluk.

Coast Guard MH-60 Jayhawk helicopters from Air Station Kodiak airlifted 18 crewmembers from
the Kulluk on December 29. The oil spill response vessel Nanuq is in the background. (Photo by
Coast Guard)

The Aiviq chief engineer stated that because all four engines failed, he believed the fuel
system was likely contaminated. Aiviq engine room logs indicate that numerous injector failures
occurred, with “trash and grit” found in the fuel strainers and “unidentified ‘sludgeʼ” blocking
the engine filters. On the afternoon of December 28, personnel on the Aiviq contacted Offshore
Service Vessels, the owner/operator of the vessel, to report that replacement fuel injectors were
needed to restore engine function. Offshore Service Vessels contacted the engine manufacturer,
Caterpillar, which had fuel injectors for the engines in stock at its Peoria, Illinois, manufacturing
facility and one other location. Offshore Service Vessels dispatched its company airplane to
those locations to pick up the fuel injectors and transport them to Kodiak, the airport nearest the
Aiviq.
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At 2250 that night, a Coast Guard helicopter arrived at the Kulluk to evacuate personnel.
The wind velocity was then about 50 knots, however, and the pilot determined that both wind
and sea conditions precluded a safe evacuation. The pilot returned to base, along with a second
Coast Guard helicopter that had also been dispatched.
December 29―The Offshore Service Vessels airplane arrived in Kodiak and transferred
74 new fuel injectors to a Coast Guard helicopter. The helicopter delivered the fuel injectors and
associated parts to the Aiviq about 0400 that morning, with a second delivery about 1000.
About 0510, the Guardsman’s tow line to the Aiviq parted. After engineers replaced the
fuel injectors, the first of the Aiviq’s four engines was restarted at 0640, the second at 0715, the
third at 1020, and the fourth at 1335. About 0630, the Nanuq arrived on scene, and at 1145, its
tow line was connected to a 1,800-foot mooring wire from the Kulluk. Both the Aiviq and the
Nanuq then towed the Kulluk. That afternoon, the Coast Guard again dispatched two helicopters
to the MODU to evacuate personnel. In three flights during daylight, with six individual lifts per
flight, all 18 personnel were evacuated from the Kulluk.
Because of the increasing severity of conditions near the Kulluk, Shell’s incident
management team was upgraded to a unified command under the authority of the Coast Guard’s
on-scene commander and including representatives of Shell and Offshore Service Vessels and,
later, personnel from Noble Corporation, operator of the Kulluk; the state of Alaska; and Kodiak
Island Borough.
December 30―About midnight on December 29–30, the unified command directed the
Nanuq and Aiviq to head east and then north in an attempt to bring the Kulluk to a safe
harbor―Marmot Bay on the northeastern side of Kodiak Island. This route would take the
vessels around the eastern side of Sitkalidak Island. At 0856 on December 30, the Aiviq master
wrote in an e-mail to Offshore Service Vessels operations personnel that he doubted the Aiviq’s
emergency tow could withstand more pressure, adding, “The long-range forecast doesn’t give me
confidence that this situation will change within the next three days.”
About 1310, the tow line connecting the Nanuq to the Kulluk parted but was still
connected to the Kulluk. About 1330, the Aiviq’s tow line separated as well. The tow vessel Alert
arrived at 1325 in response to Shell’s request for assistance. Around that time, wind velocity was
estimated to be 35 to 45 knots, with 20- to 25-foot seas.
The Kulluk was about 30 miles from the coast. Aiviq personnel determined that the
vessel’s heavy steel grapple anchor was suitable for retrieving the mooring wire that had been
used by the Nanuq to tow the Kulluk. This anchor would need to be retrieved from a vessel
storage area and secured to the Aiviq towing equipment on its main deck―an operation deemed
best undertaken in more protected waters. For this reason, the Aiviq departed at 1930 for a
sheltered area off Sitkinak Point, about 20 miles away, where Aiviq personnel deployed the
grapple and then returned the vessel to the Kulluk’s location the next morning.
December 31―At 0110 on December 31, Alert crewmembers retrieved the Kulluk’s
emergency towline from the water and connected it to a shackle on the Alert’s tow line. The
Aiviq returned by 0400, and crewmembers grappled and retrieved the Kulluk’s mooring wire and
connected it to the Aiviq’s towing wire by about 0700. Nevertheless, the two vessels towing the
Kulluk in tandem were unable to withstand the combined effects of the wind and the seas, which
pulled the vessels toward the shore. The unified command then decided to move the Kulluk to a
closer safe harbor―Port Hobron, about 74 miles away on the north side of Sitkalidak Island.
At 1626, the tow line between the Aiviq and the Kulluk parted again. The Alert continued
to tow the Kulluk about 7 miles offshore with its engines at 85 percent power. Weather
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conditions continued to worsen, and with winds as high as 55–60 knots and seas 30–35 feet, the
Alert, with the Kulluk in tow, continued to be pulled toward shore. At 1815, the master of the
Alert ordered that the engines be set at 100 percent power. They were brought back to 85 percent
power at 1835 due to the alerting of engine alarms at the 100 percent setting. When the vessels
were 3 miles from shore, the unified command ordered the Alert master to release the tow, and
the Kulluk grounded off Ocean Bay, Alaska.

Vessel positions when the Alert released the Kulluk and location of the Kulluk grounding about
40 minutes later on December 31, 2012, near Ocean Bay, Sitkalidak Island, Alaska.
(Excerpt from NOAA chart 16592)

The broken component of the Kulluk’s tow gear, a 120-ton shackle that was connecting
the towing plate to the 100-foot-long, 3-inch pennant, was lost at sea during the initial
December 27 tow gear separation, and investigators were unable to determine the cause of its
failure.

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MODU Kulluk aground off Ocean Bay, Sitkalidak Island, Alaska. (Photo by Coast Guard)

Discussion
No single error or mechanical failure led to this accident. Rather, shortcomings in the
design of a plan with an insufficient margin of safety allowed this accident to take place. The
plan was created to move the MODU at a time of year with a known likelihood of severe weather
conditions for reasons unrelated to operational safety.

Shell had retained warranty surveys on all five previous tows of the Kulluk. No
regulatory requirement existed for a warranty surveyor to review and approve, or suggest
modifications to, the tow plan and its components. The surveyor that Shell retained for the
accident voyage approved the tow plan in its entirety. This was the only external review of the
tow plan and the equipment that was to be used in the planned tow.

The Coast Guard was not required to oversee the tow. Rather, the Coast Guard’s role, a
critical one given the circumstances, was limited to its response to this accident―delivering the
needed engine components to the Aiviq, unsuccessfully attempting to tow the Kulluk, and, most
important, rescuing Kulluk personnel when their lives were endangered, an operation that itself
risked the lives of the Coast Guard rescuers.

Many maritime regulators in countries with operations in environments with the potential
for severe weather actively oversee tow operations. For example, Norway addresses many of the
shortcomings in the oversight of tow gear and severe weather avoidance that were evident in this
accident.6 Canada similarly addresses safe towing operations, including specifying parameters
for tow gear strength, by recommending that operators adhere to IMO towing guidelines (IMO

6
Norwegian Maritime Authority, Regulation 17, December 1986, No. 2319, “Concerning field moves and
towing of mobile offshore units and concerning towing system and mooring of supply ships at such units.”

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MSC Circ. 884, 21 December 1998).7 IMO and the Norwegian Maritime Authority provide
guidance for oceangoing tows with margins of safety for encounters with adverse weather.

The Aiviq lost engine power at a critical point in the Kulluk’s tow. Coast Guard
investigators believe that the design of the fuel oil storage tanks’ common vent and overflow
system was flawed and that these flaws led to the seawater contamination of the fuel tanks during
the Aiviq’s transit during rough sea conditions. Offshore Service Vessels contends that fuel
contaminants were present in the fuel taken on by the Aiviq in Dutch Harbor and that this
contamination, rather than seawater that entered the system later, led to the engine power loss.
Regardless, the source of the fuel contamination was outside the scope of the NTSB’s
investigation of this accident and, therefore, was not determined.
Given the risks associated with this transit, including the likelihood of the tow
encountering severe weather, Shell and its contractors, particularly Offshore Service Vessels, the
operator of the Aiviq, who reviewed and approved the tow plan should have either mitigated
those risks or departed at a time of year when severe weather was less likely. For example, Shell
and its contractors could have included additional tow vessels to the entire transit to reduce the
likelihood of catastrophic results from a failure of the Aiviq or its tow gear. Redundancy is a
necessary element of safety-critical transportation systems, and given the hazards of operations
in Alaskan waters, those involved in the tow plan should have recognized and addressed the lack
of redundancy.
The series of failures that led to this accident began when Shell failed to fully address the
risks associated with a late December tow in Alaskan waters, and ended with the grounding of
the Kulluk. Although multiple parties were involved in the review and approval of the tow plan,
the ultimate decision to approve and implement the tow was Shell’s. The dynamics of a single
entity approving a go/no-go decision in the face of risks, with multiple parties involved, have
been addressed in studies of previous catastrophic events.8 This research demonstrates that, even
with formal review processes involving multiple entities, the ability of parties involved in a
decision to articulate and draw attention to risks is limited when a single entity bears ultimate
decision-making responsibility and at the same time favors a particular outcome of the decision.
For this reason, Shell, as the organization responsible for designing, approving, and
implementing the tow plan, is considered to be ultimately responsible for this accident.

Probable Cause
The National Transportation Safety Board determines that the probable cause of the
grounding of the mobile offshore drilling unit Kulluk was Shell’s inadequate assessment of the
risk for its planned tow of the Kulluk, resulting in implementation of a tow plan insufficient to
mitigate that risk.

Adopted: May 22, 2015

7
Transport Canada, TP 13585 E, Policy Regarding Ascertaining the Safety of International Towing Operations
of Deadships Departing Canada, 1 September 2013.
8
Janis, I. L. (1982). Groupthink: Psychological studies of policy decisions and fiascoes. Boston: Houghton
Mifflin; Vaughan, D. (1996). The Challenger launch decision: Risky technology, culture, and deviance at NASA.
Chicago: The University of Chicago Press.

12 NTSB/MAB-15/10
Grounding of Mobile Offshore Drilling Unit Kulluk

Vessel Particulars
Vessel Kulluk Aiviq Nanuq Alert Guardsman Alex Haley
Owner/operator Shell Offshore/Noble Offshore Service Vessels Offshore Service Vessels Crowley Maritime Corp. Crowley Maritime Corp. US Coast Guard
Drilling

Flag Marshall Islands United States United States United States United States United States
Type Mobile offshore drilling Anchor-handling tug OSRV oil spill response Tug Tug Medium-endurance
unit (ice class) supply (ice class) vessel cutter
Builder, date Mitsui Engineering & North American North American Dakota Creek Industries McDermott Shipyard Brooke Marine,
Shipbuilding, Shipbuilding Fabricators February 2000 May 1976 Lowestoft, UK
Tamano, Japan November 2011 June 2007 1967
1983
IMO/official 802785 9579016 9382841 9214381 7506003 ―
number
Construction Steel Steel Steel Steel Steel Steel
Length overall 265.7 ft (81 m) 360 ft (109.7 m) 301.8 ft (92.0 m) 140 ft (42.7 m) 136 ft (41.5 m) 283 ft (86.3 m)
(length between
perpendiculars)
Breadth (conical hull) 80.0 ft (24.4 m) 60.0 ft (18.3 m) 42.0 ft (12.8 m) 36.5 ft (11.1 m) 50.0 ft (15.2 m)
Draft 35 ft (10.7 m) 28 ft (8.5 m) 17 ft (5.2 m) 20 ft (6.10 m) 16 ft (4.9 m) 17 ft (5.2 m)
(at sailing)
Gross tonnage 27,968* 12,892* 3,575* 845* 538* 3,100 long tons
Propulsion/main n/a Four 21,776-hp Two 7,268-hp Two 10,192-hp Two 7,200-hp 6,800-hp (5,001.4-kW)
engines (16,240-kW) diesel oil (5,422-kW) diesel oil (7,500-kW) diesel oil (5,148-kW) diesel oil diesel engine
engines, geared drive engines, geared drive engines, geared drive engines, geared drive
Service speed n/a 16 kts 14 kts 16 kts 14 kts n/a
Towing capabilities n/a Low pressure towing 250-ton anchor winch Markey TDS-40 towing Markey TDSDW 36C Towing bit on fantail
winch with auxiliary winch double drum winch
wire, chain drums, and
a computer monitoring
system
Bollard pull (tons) n/a 208 90 150 75 ―
Persons on board 18 18 10 7 7 104
*Tonnage according to International Tonnage Convention
n/a not applicable
― not available

For more details about this accident, visit www.ntsb.gov and search for NTSB accident ID number DCA13LM012.

13 NTSB/MAB-15/10
Grounding of Mobile Offshore Drilling Unit Kulluk

The NTSB has authority to investigate and establish the probable cause of any major marine casualty or
any marine casualty involving both public and nonpublic vessels under 49 United States Code 1131. This
report is based on factual information either gathered by NTSB investigators or provided by the Coast
Guard from its informal investigation of the accident.
The NTSB does not assign fault or blame for a marine casualty; rather, as specified by NTSB regulation,
“[NTSB] investigations are fact-finding proceedings with no formal issues and no adverse parties . . .
and are not conducted for the purpose of determining the rights or liabilities of any person.” 49 Code of
Federal Regulations, Section 831.4.
Assignment of fault or legal liability is not relevant to the NTSB’s statutory mission to improve
transportation safety by conducting investigations and issuing safety recommendations. In addition,
statutory language prohibits the admission into evidence or use of any part of an NTSB report related
to an accident in a civil action for damages resulting from a matter mentioned in the report. 49 United
States Code, Section 1154(b).

14 NTSB/MAB-15/10