Winch shaft failures diagnosed
Pipelay barges achieve their forward motion and control their position by means of a spread of anchors. These are regularly relocated by workboats and the barge pulls itself along by means of large winches connected to the forward anchor cables whilst at the same time slacking off the after cables. TWI was called in when, after less than a week's operation, the 300mm diameter shafts of two of the fourteen winches on a brand new state-of-the-art barge suffered brittle fracture. The barge was forced to abandon the pipe which it had started to lay and return to sheltered waters.The investigation showed that the failure was the consequence of two factors:
- the forged steel was extremely brittle
- there was a keyway cut in the shaft with a very sharp corner radius, leading to a high stress concentration.
For the client, a more important aspect of TWI's work was that, based on the failure investigation, it was able very rapidly to recommend materials selection and heat treatment procedures, which would prevent a recurrence of the problem. The speed of response was imperative. The charge rate for the barge during pipelaying was about £250,000 per day and the costs in interest charges and crew wages whilst it was unable to operate were running at £85,000 per day.
TWI was also involved in other aspects of the winch design, and since the various modifications which were made during its early life as a result of these investigations the barge has given sterling service.
Copyright © 2001, TWI Ltd