Captain got under way without sufficient freeboard at the stern and without ensuring watertight
The National Transportation Safety Board (NTSB) has issued the report of its investigation on the sinking of tug Nalani on 22 January 2015 off Oahu.
About 1510 local time on January 22, 2015, the uninspected towing vessel Nalani began taking on water and sank in 2,200 feet of water while conducting sea trials off the southwest coast of Oahu, Hawaii. All 11 persons on board were rescued after abandoning the vessel. No one was injured. An oil sheen was observed by responders and crewmembers. The vessel was not salvaged due to the water depth and was declared a total constructive loss.
The Coast Guard’s Marine Safety Center Salvage Engineering Response Team (SERT) conducted a stability evaluation of the Nalani to determine the vessel’s pre-accident lightship condition after the addition of the auxiliary boat platform and emergency generator. The SERT also calculated the stability of the vessel with both the starboard aft peak tank and steering space flooded.
To estimate the lightship displacement and centers of gravity, SERT used the values documented on Nalani’s 1981 stability letter, (the most up-to-date stability information provided). The SERT then added the modifications to the vessel that were made during the 2014 dry docking, including the addition of the emergency generator, the rescue boat and davit, and the two liferaft rails.
It was also reported that several of the vessel’s stiffeners were either increased in size or added in the shipyard. Marisco Shipyard provided a sketch of the completed metalwork, but it was unclear which modifications were replacements in kind and which were new structural members. The impact of the structural modifications on the overall lightship displacement was small, and therefore those modifications were not included in the analysis.
The report concluded that because the Nalani was registered in Panama, the vessel was not subject to US stability regulations. If the Nalani had been a US vessel assigned an international load line through its classification society, it would have been required to meet the stability criteria specified in 46 CFR Subchapter S, specifically sections 170.170, 173.095, and 174.145.
These criteria were therefore used as an objective reference standard for evaluating the stability characteristics of the Nalani at the time of the accident. The analysis indicated the Nalani, in its calculated condition, would have met the intact stability standards had the opening in the starboard aft main deck been sealed before getting under way. However, with the opening unsecured and included as a downflooding point, the vessel would not have met the stability standards.
In addition to assessing intact stability, the SERT analyzed the impact of progressive flooding on the vessel. The calculations assumed that flood water entered through the starboard aft void opening in the main deck and spread to the steering room.
The analysis indicated that the aft deck edge would have been completely submerged and the Nalani would have had minimal righting energy. In a static condition without the effects of wind, waves, water on deck, or additional flooding, the vessel may have remained afloat. However, in a dynamic environment, any or all of these factors could have led to sinking.
NTSB determines that the probable cause of the flooding and eventual sinking of the Nalani was the captain’s decision to get under way without sufficient freeboard at the stern and without ensuring proper watertight integrity |
Please click below to view the NTSB Investigation Report
Source: NTSB