The National Transportation Safety Board (NTSB) has released an investigation report in which the liftboat L/B Robert was reported to be listing (tilting) alongside a stationary oil platform in the Gulf of Mexico near South Marsh Island Block 137, about 80 miles southeast of Lake Charles, Louisiana, on November 20, 2022, at 1256 local time.
Analysis
On the morning of November 16, the liftboat L/B Robert was jacked up next to Platform A in SM 137 in the Gulf of Mexico when weather forecasts called for potential maximum wave heights greater than 20 feet. Concerned that waves would exceed the vessel’s elevated operating limits (15-foot waves while jacked up), the captain considered raising the vessel’s legs and transiting away from the storm—a decision the captain and vessel owner had made and executed as recently as 2 months prior. However, the immediate forecast called for seas greater than the operating limit of 8-foot waves while underway. Because it would take about 12 hours to liberate the vessel’s legs from the seafloor and get underway, and the nearest safe refuge area was 9 hours away, the captain—in consultation with shoreside vessel management personnel—decided instead to evacuate the vessel. In the 2 days that followed, the unattended L/B Robert tilted to port, submerging its deck edge. Therefore, the decision to evacuate personnel and 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 eliminated the risk to personnel from continued vessel operations on board.
When the crew evacuated the L/B Robert on November 18, leaving it unattended, they left an air gap about 25 feet above the water’s surface per guidance from shoreside management personnel to stay just at the maximum wave heights predicted by the weather forecasts, believing this would maintain the vessel’s greatest resistance to overturning from the combined forces of wind and waves. The vessel was jacked up to an elevation of 267 feet, in a water depth of 217 feet, with the leg pads averaging about 25 feet of penetration into the seafloor.
All these conditions were within the limits identified in the vessel’s operations manual. The vessel had been elevated and operating for 6 weeks without incident, and the base map of the seafloor produced by a third-party survey company indicated no obstructions, debris, or can holes within 25 feet of the L/B Robert’s three leg pads. Two days after the crew evacuated the vessel, CCTV showed the elevated vessel tipping to port, and a helicopter crew flying over the L/B Robert also reported the liftboat tilting to port. Based on observations from the closest observation points, gale-force wind conditions (greater than 34 knots) began to occur at the casualty site around midnight on November 17. These wind conditions continued to increase in strength through the evening of November 19, with peak winds likely near 45 knots (based on the observations at KGBK). Additionally, based on analysis completed by the Ocean Prediction Center, it is likely that the significant wave heights at the casualty site peaked early November 20 between 12 and 15 feet; thus, the vessel likely experienced waves as high as 24–30 feet, exceeding the 25-foot air gap, as well as the forecasts at the time the captain and shoreside personnel decided to evacuate the liftboat.
According to the captain, after the casualty, the vessel’s onboard position monitoring system recorded the vessel had moved 19 feet to port from its original
position. A postcasualty inspection of the legs and pads indicated no catastrophic failures that could have contributed to the vessel tilting—although the vessel’s legs were bent within the leg towers on the vessel, the legs and the leg pads were otherwise undamaged. In the time leading up the tilting, overturning forces from the waves striking the elevated liftboat and gale-force winds would have transferred down the legs to the pads, causing the seabed foundation under and surrounding the leg pads to deteriorate. Although the vessel was originally positioned 25 feet from the nearest can hole, the deteriorated seabed foundation under the vessel’s port leg likely gave way, causing the leg to slide into the can hole and the vessel to tilt to port, submerging its deck edge.
Conclusions
Probable Cause
The National Transportation Safety Board determines that the probable cause of the tilting of the elevated liftboat L/B Robert was gale-force winds combined with waves that exceeded the vessel’s air gap, causing vessel movement, which led to a leg sliding into an adjacent can hole in the seafloor.
