Classification
| Field | Value |
|---|---|
| Incident type | Grounded |
| Severity | Medium |
| Region | United States |
| Location | Ocean Bay, Sitkalidak Island, Alaska, United States |
Vessel Details
| Field | Value |
|---|---|
| Primary vessel | Kulluk |
| Type | Mobile offshore drilling unit, ice class |
| Flag state | Marshall Islands |
| IMO/official number | 802785 |
| Owner and operator | Shell Offshore, Inc. / Noble Drilling |
| Year built | 1983 |
| Length overall | 81 m |
| Gross tonnage | 27,968 |
| Persons on board during tow | 18 |
| Tow vessel | Aiviq, US-flagged ice-class anchor-handling tow vessel |
| Aiviq gross tonnage | 12,892 |
| Aiviq persons on board | 18 |
Incident Details
| Field | Value |
|---|---|
| Date and time | 2012-12-31 20:40 Alaska Standard Time (UTC-9) |
| Voyage | Captains Bay near Unalaska to the Seattle area for maintenance and repairs |
| Operation | Kulluk under tow by Aiviq and later assisting vessels |
| Coordinates | 57.0000, -153.1000 |
| Fatalities | |
| Injuries | 4 minor injuries to Aiviq crewmembers |
| Vessel outcome | Kulluk sustained substantial damage; the official NTSB brief does not state a total-loss determination |
| Direct damage | Extensive damage to underwater hull areas, superstructure, electrical equipment, lifesaving and safety equipment, and interior and engineering spaces. |
| Environmental impact | None found by NTSB |
Key facts
- The tow plan was for a late-December transit through Alaskan waters, where severe weather was known to be possible.
- The tow line between Aiviq and Kulluk failed during the voyage, Aiviq later lost engine power, and additional vessels were brought in to assist.
- The Kulluk crew was airlifted off on 29 December after worsening weather and continuing tow problems.
- On 31 December, Alert continued towing Kulluk toward shore until the unified command ordered the tow released about three miles from shore; Kulluk grounded near Ocean Bay about 40 minutes later.
- NTSB determined that Shell's inadequate assessment of the risk for the planned tow led to a tow plan insufficient to mitigate that risk.
Narrative
Kulluk was an ice-class mobile offshore drilling unit without its own propulsion. Shell owned the unit and Noble Drilling operated it. Kulluk departed Captains Bay near Unalaska on 21 December 2012 for the Seattle area, under tow by Aiviq. The unit had a skeleton crew of 18, and Aiviq was an ice-class anchor-handling tow vessel.
The tow plan recognized historical climate data and allowed the tow master and Aiviq's master to alter course for weather. The NTSB report states that the December voyage was Kulluk's first transit of the Gulf of Alaska in winter. As the weather deteriorated, tow gear failures and Aiviq engine problems reduced the ability of the towing system to keep the unit clear of shore. The tow line between Aiviq and Kulluk failed on 27 December, and Aiviq later lost engine power. Alert, Guardsman, Nanuq, and the Coast Guard cutter Alex Haley were brought into the response.
The 18 people aboard Kulluk were evacuated by Coast Guard helicopter on 29 December. On 31 December, Alert and Aiviq again towed Kulluk in tandem, but the combined wind and seas continued to set the vessels toward shore. Aiviq's tow line parted at 16:26. Alert continued towing Kulluk in winds as high as 55-60 knots and seas of 30-35 feet. When the tow was about three miles from shore, the unified command ordered Alert to release it. Kulluk grounded near Ocean Bay about 40 minutes later.
The NTSB found extensive damage to the Kulluk's underwater hull and other systems, while retaining watertight integrity before refloating. It found no environmental damage from the grounding. Four Aiviq crewmembers sustained minor injuries. The NTSB's probable-cause finding was that Shell inadequately assessed the risk of the planned tow and implemented a tow plan insufficient to mitigate it.
Sequence of events
| Date/time | Event |
|---|---|
| 2012-12-21 | Kulluk departed Captains Bay near Unalaska under tow by Aiviq for the Seattle area. |
| 2012-12-27 | The tow connection failed during severe weather; additional vessels were mobilized to assist. |
| 2012-12-28 | Aiviq experienced loss of engine power. Guardsman connected to assist, and the Coast Guard cutter Alex Haley responded. |
| 2012-12-29 | Coast Guard helicopters airlifted the 18 people from Kulluk. |
| 2012-12-31 07:00 approximately | Aiviq and Alert had reconnected tow arrangements and attempted to move Kulluk toward Port Hobron. |
| 2012-12-31 16:26 | The tow line between Aiviq and Kulluk parted again. |
| 2012-12-31 18:15 to 18:35 | Alert increased its engine setting, then reduced it after engine alarms while the tow continued toward shore. |
| 2012-12-31 approximately 20:00 | The unified command ordered Alert to release the tow when the vessels were about three miles from shore. |
| 2012-12-31 approximately 20:40 | Kulluk grounded near Ocean Bay. |
Contributing factors
| Factor | Evidence status | Description |
|---|---|---|
| Inadequate tow-risk assessment | Confirmed finding | NTSB determined that Shell inadequately assessed the risk of the planned tow, resulting in an insufficiently mitigated tow plan. |
| Insufficient safety margin in the tow plan | Confirmed finding | The plan scheduled a winter Gulf of Alaska tow despite the known likelihood of severe weather and did not provide enough margin for the conditions encountered. |
| Lack of towing redundancy | Confirmed finding | NTSB found that additional tow vessels could have reduced the likelihood of catastrophic consequences after a failure of Aiviq or its tow gear. |
| Aiviq loss of engine power | Confirmed finding | Aiviq lost engine power at a critical point in the tow, reducing the towing system's ability to maintain position. |
| Cause of fuel contamination leading to power loss | Reported | Coast Guard investigators believed the Aiviq fuel tank vent and overflow design contributed to seawater contamination, while the operator attributed the contamination to fuel taken on in Dutch Harbor. NTSB did not determine the source. |
Investigation
The National Transportation Safety Board investigated the casualty with the US Coast Guard, which convened a formal hearing in Anchorage in May 2013. NTSB adopted Marine Accident Brief MAB-15/10 on 22 May 2015. Its probable-cause finding assigns the cause to Shell's inadequate risk assessment and the resulting tow plan. The official brief also records the Coast Guard investigation's conclusions concerning tow redundancy, weather avoidance, and engine-power loss.
References
- NTSB investigation page: Grounding of Mobile Offshore Drilling Unit Kulluk
- NTSB Marine Accident Brief MAB-15/10
- US Coast Guard Report of Investigation on the Kulluk accident
- Source note: NTSB Kulluk grounding brief