Summary:
On September 6, 2016, about 00:05 local time, the tanker Aframax River allided with two mooring dolphins in the Houston Ship Channel (HSC) near Deer Park, Texas. The allision punctured the ship’s hull plating, and about 88,000 gallons of low-sulfur marine gas oil spilled into the water. The oil ignited and burned for about 45 minutes. The two onboard pilots sustained minor burns, and the property damage exceeded $1.5 million.
Analysis:
Investigators and the shipping company representatives focused their efforts on identifying the potential sources that caused the engine to ramp up to 80 rpm and not respond to EOT command inputs from the bridge. After interviewing the pilots, ship and tugboat crews, it became apparent that the engine’s governor and printed circuit board, which controls the engine speed, may have played a role in the accident.
The Diesel United – Sulzer Type 7RTAC2 is a single-acting, slow-speed, two-stroke, reversible marine diesel engine. The engine was directly connected to the propeller, and the engine was designed to come to a complete halt when changing propeller direction from astern to ahead. In the presence of investigators and ClassNK surveyors, technicians from Diesel United, Ltd. inspected and tested the mechanical components of the governor input, fuel control linkage, and other related components. All items were determined to be in good working condition.
Technicians from Nabtesco Marine Control Systems Company (the manufacturer of the main engine control system and governor) troubleshooted the system and tried to identify the fault. In the presence of Coast Guard personnel and a marine surveyor from ClassNK, the technicians examined the system’s electrical and mechanical components and conducted tests from both the bridge and the ECR. No failures or causes were identified during the test. The technicians concluded that the governor actuator system had encountered a momentary abnormality, one which they could not replicate.
Nabtesco technicians subsequently replaced the governor actuator and as a precautionary measure, they also replaced the printed circuit board in the ECR, which mechanically supports and electrically connects the governor motor actuator to the bridge’s EOT command inputs. The technicians then carried out a series of calibration and simulation tests, as well as started and stopped the main engine ahead and astern several times, confirming that the system was functioning as designed.
The ClassNK surveyor concluded in his report: “The main engine started without any delay, ahead and astern, on the command input from the EOTs. The main engine rpm was in accordance with design, dead slow 30 rpm, and slow 38 rpm. This surveyor believes the main engine is ready to answer all orders.”
According to the manufacturer’s technicians, the governor’s actuator system experienced a momentary abnormality. Investigators believe that the abnormality likely resulted from an electrical and/or mechanical failure of the system, which led to loss of engine control while the vessel was engaged in astern propulsion.
Further, due to the oil spill that resulted from this accident, investigators also reviewed ship design standards pertaining to fuel oil tank protection. The International Maritime Organization’s Regulation 12A (an amendment to MARPOL Annex I) applies to all vessels delivered on or after August 1, 2010, and requires that individual fuel oil tanks aboard ships with a fuel oil capacity of less than 500 cubic meters be located at least 760 mm inboard of the molded line of the hull plating, thus helping prevent tank breaches and oil spills in case of collision or grounding. However, because the Aframax River was delivered in May 2002, Regulation 12A did not apply to its construction. If the Aframax River had been built after the new standards took effect, fuel would not have spilled in this accident because the maximum inward deformation of the hull plating at the damaged fuel tank was only about 300 mm.
The actions of the Aframax River crew, the tugboat crews, and the pilots successfully prevented the spread of fire to other vessels and structures.
Probable Cause:
The National Transportation Safety Board determines that the probable cause of the Aframax River’s allision with mooring dolphins and the subsequent fire in the waterway was a momentary abnormality of the tanker’s main engine governor actuator system in responding to command inputs from the bridge.
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Source: NTSB
