One of our members has informed us of the following incident. An ROV system was providing subsea intervention support working from a drill rig in depths up to 2500msw. Whilst conducting routine operations, to a depth of 2300m, the ROV lift umbilical winch system was stopped. After approximately five minutes, the umbilical winch started to ‘creep’ and quickly gather speed. The operator attempted to arrest the load by operating the winch in an upward direction. This had no effect and by the time the operator had activated the emergency stop the load could not be stopped and the winch suffered an uncontrolled payout. Although nobody was injured, the ROV together with 3300m of steel armoured umbilical was lost from the rig and the winch suffered serious damage.
The initial investigation conducted by the company concerned has shown that the mechanical brake system had failed due to excessive wear on the brake plates.
The particular winch involved was a CORMAC unit manufactured by Norlau. It appears that only certain types of this winch have this characteristic, as the design appears to have changed over the life of the model.
Actions to be taken in the event of an Uncontrolled Payout
In the incident described above, by attempting to arrest the load by operating the ‘pay-in’ command, the operator unwittingly aided the uncontrolled payout. This is because when the winch command lever is in neutral, the mechanical brake is fully engaged. When a command is given to the winch, the mechanical brake is released as the system pressure builds in response to demand. In this situation, with the winch already starting to gather speed, any release of the mechanical brake will aid the uncontrolled payout before the hydraulic drive can counter this release.
The company concerned has instructed its personnel that if a similar incident is experienced whereby a CORMAC winch starts to ‘creep’ under load, no action should be taken and the winch allowed to continue to ‘creep’ until it stops. Should this occur the company has advised that immediate advice be sought in order to ensure safe recovery of the load.
The company concerned has identified a torque test with the motor against the brake to determine the amount of wear on similar winch systems. This simply tests the brake against the maximum torque induced by the motor and thereby gives an indication of brake condition.
In the unlikely event that winch drum movement occurs during the test, the company has instructed its personnel that all operations using the winch unit under test should be suspended until the extent of wear to the brake system has been evaluated.
The company concerned has noted that actions described above are designed to prevent re-occurrence of this serious incident by testing the integrity of the mechanical brake and advising what to do in the event of a similar uncontrolled payout. However, it notes that these actions are considered to be short term and a long term solution is being investigated, possibly as a modification to be agreed in conjunction with the manufacturer.
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