On August 11, 2022, a containership operating in the North Pacific Ocean experienced a loss of propulsion due to a controllable pitch propeller (CPP) system failure. The National Transportation Safety Board (NTSB) investigated the incident involving significant mechanical damage, though no injuries were reported.
What Happened
While en route from Tacoma, Washington, to Anchorage, Alaska, the vessel’s engine crew detected a hydraulic oil leak from the CPP hub lubricating system. In response, the crew reduced the propeller pitch. It replenished the oil in the CPP hub head tank to maintain propulsion. Suspecting a blade seal failure, they continued the voyage to Anchorage, where shore-based technicians recommended emergency drydocking. The vessel then proceeded toward a shipyard in Oregon.
During the transit, the hydraulic oil loss worsened, prompting the crew to use fresh water to supplement the fluid. The situation deteriorated further as water began contaminating the stern tube lubricating system. To prevent complete propulsion failure and potential seawater ingress into the machinery space, the main engine was shut down, and the vessel was towed the remaining distance to the shipyard.
Subsequent underwater inspection revealed fractures in two CPP blades. One blade had a large fracture that extended through the hub area and was confirmed to be the source of the hydraulic oil leak. Post-incident testing verified that potable water used during the voyage had leaked from this damaged blade.
Why It Happened
Post-incident analysis determined that the loss of propulsion was caused by a crack in one of the CPP blades that developed into a complete fracture due to high-cycle fatigue. The blade did not meet several of the manufacturer’s design specifications, including the required counterbore radius at the bolt hole, material impact toughness, tensile and yield strength, and chemical composition. These deficiencies made the blade more prone to fatigue and cracking. The failure originated at the bolt hole counterbore, an area critical to structural integrity.
Actions Taken
In response to the findings, the CPP blade manufacturer revised the internal radius requirements for all seven bolt hole counterbores to enhance fatigue resistance. A finite element analysis of similar CPP systems did not identify comparable issues, suggesting the incident was an isolated manufacturing defect.
Lessons Learned
- Critical components such as CPP blades must conform strictly to engineering and material specifications to ensure fatigue resistance and operational reliability.
- Early identification and monitoring of fluid loss in propulsion systems are essential for managing risk during a voyage.
- Substituting incompatible fluids, such as water for hydraulic oil, can lead to further system degradation and should only be considered in extreme circumstances with clear contingency plans.
- Effective coordination between crew, shore-based engineers, and manufacturers is vital in managing emergent technical failures at sea.
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Source: NTSB
