Gale-force winds, high waves led to tilting of liftboat in Gulf of Mexico, NTSB report reveals

Gale-force winds and high waves from a storm led to the listing (tilting) of an elevated liftboat in the Gulf of Mexico in an incident that occurred in late 2022, the National Transportation Safety Board (NTSB) said recently.

On November 20, 2022​, the liftboat L/B Robert was reported to be listing (tilting) alongside a stationary oil platform in the Gulf of Mexico about 80 miles (128 kilometres) southeast of Lake Charles, Louisiana.

The crew had evacuated two days prior due to forecasted adverse weather in the area, leaving the vessel elevated out of the water and unattended.

No injuries or pollution were reported, though damage to the vessel and cargo was estimated at US$6.9 million.

The liftboat captain decided to evacuate the vessel on November 17, 2022, because forecasted waves would exceed both the boat’s eight-foot (2.4-metre) underway and 15-foot (4.5-metre) jacked up operating limits. Investigators concluded the decision to evacuate rig personnel and liftboat crewmembers from the vessel, rather than attempt a transit to an area of refuge or remain on board, was appropriate for the forecasted conditions and decreased the risk to those on board.

When the crew evacuated the liftboat they left an air gap about 25 feet (7.62 metres) above the water’s surface per guidance from shoreside management personnel to stay just at the maximum wave heights predicted by the weather forecasts to maintain the vessel’s greatest resistance to overturning from the combined forces of wind and waves. The vessel likely experienced waves as high as 30 feet (9.1 metres), exceeding the air gap and the forecasts at the time of the captain’s decision to evacuate.

Overturning forces from the wind and waves transferred down the vessel’s legs to the pads, causing the seabed foundation around them to deteriorate. Investigators determined it is likely the deteriorated seabed foundation under the port leg gave way, causing the leg to slide into a can hole and the vessel to tilt to port, submerging its deck edge.

Marine Investigation Report 23-28 is available online here.