Loss of ROV after umbilical termination failure and damage to ROV during recovery

The ROV and its TMS were lost during launch of the ROV when the umbilical detached from the TMS bullet.

The launch operation was supported by 3 ROV technicians, one operating the winch from a remote controller and another the handling platform using manual hydraulic valve handles. The third person (the ROV supervisor) monitored the operation from the ROV control room.

To be able to unlatch and disengage from the docking head, the operator needed to lift weight off the docking head latches. This was qualified visually by the operators. Once the bullet was lifted off and latches disengaged, the umbilical became detached from the TMS bullet. The ROV and TMS descended to the seabed and landed sideways at 298 metres water depth in soft sediments.

Another vessel was able to assist in the recovery of the ROV. The ROV was inspected on the seabed and based on that information and the sea conditions (3m significant wave height), a risk assessment and toolbox talk was carried out between the two vessels, and recovery was considered safe and possible. A soft sling was used and attached into the bullet. The sling was routed through the legs in the bullet, and doubled back. The TMS and ROV were then recovered to deck.

After breaking the splash zone it was discovered that the ‘tools’ were loosened from the manipulator. When landing on the seabed, the front of the ROV frame was on top of the ‘tools’. The crane driver held the ROV with 2 T tension while a tag line was attached to the ‘tools’. When lifting off again, approximately 1 metre, the soft sling snapped. The ROV frame collapsed from the weight of the TMS (2.9 tons). There were no injuries to personnel but the ROV, TMS and umbilical were damaged.

Our member had the parted umbilical analysed by a third-party consultant, who drew the following conclusions:

• Three main factors combined to cause the umbilical to fail.
  
  
• The overall effect of these factors was a failure to wedge the potting properly into the bullet.
  
  
• If one or more of the below factors had not occurred the break point would more likely to have been above the bullet.
  
  
• The factors were:
  
  
  1. The lack of seating was hindered by the rough internal surface of the bullet. The cast had not been seated properly in the socket cone, limiting the strength of the termination to the cohesion/friction between the steel wires and the cast.
     
     
  2. The reported use of a booster pack, at temperatures higher than recommended, dramatically shortened the curing time, causing the wirelock to gel rapidly. It is not known whether or not this caused the insufficient filling at the bottom part of the socket, but considered as likely. The use of a booster pack at temperatures above 9°C is not in accordance with manufacturer’s recommendations and is strongly advised against.
     
     
  3. Un-axial loads were imposed on the termination occurred, which caused snagging. This was confirmed by several witnesses onboard. This is a result of the design of the bullet/socket allowing the bullet/socket to catch/stop when entering the docking head receptacle. Un-axial loads are likely to have been a contributory factor. It was reported by the crew that the socket occasionally got caught on the edge of the receptacle of the snubber. Normally such a scenario would be more likely to cause overload of the wires than snagging should the termination have been arranged correctly.
     If, as argued above, the socket was not properly seated or if sufficient filling was not achieved at the bottom of the cone, un-axial loading is likely to have instigated the failure. Un-axial loading is a possible explanation for the fact that the termination failed at a significantly lower load than the load test. Snagging may have broken the bonding between the cast and the steel wires and the wires could have worked themselves out of the cast over time until the length of wires remaining in the cast were insufficient to support the weight of the ROV and TMS.

Our member’s investigation revealed that:

• Sharp edges on the bullet cut through the soft-sling and caused the ROV and TMS to drop to the deck during landing.
  
  
• The edge inside the bullet grade was sharp and not polished. An emergency recovery strop was not installed on the TMS.

Our member drew the following lessons from the incident:

• Umbilical re-termination procedures to be revised:
  
  
  • Prevent flat edge on the cone and use machine tools to prevent surface roughness inside the socket, all edges to be rounded and polished.
    
    
  • Use of a booster pack at temperatures above 9°C not in accordance with manufacturers recommendations and is strongly advised against.
    
    
• Emergency strop to be mounted on TMS systems.
  
  
• Bullet design to be approved and quality checked before use;

Members may also wish to refer to:

• IMCA R004 - Guidance for the safe and efficient operation of remotely operated vehicles
• IMCA R011 - The initial and periodic examination, testing and certification of ROV launch and recovery systems