Ships are filled with increasingly sophisticated equipment, which should it go wrong, can leave those in charge of the vessel facing serious problems. One of the items which repeatedly seems to feature in accident reports is the controllable pitch propeller; an excellent and useful addition to a ship’s manoeuvrability, but offering something of an easy passage to a very expensive accident should it malfunction.
A particularly spectacular accident, made even more graphic by a series of photographs showing the sequence of events, is given due prominence in the latest UK Marine Accident Investigation Branch Safety Digest. In this sequence of pictures, we see a general cargo ship entering a lock with half ahead pitch to counter a sheer. Unfortunately, those on the bridge found it impossible to stop the ship which, accelerating up to nearly four knots in the length of the locks, crashed through the outer gates, sinking one of the gates and wrecking the other.
The gates astern of the ship were closed although the head of water in the lock was such that they crashed shut, damaging them and closing the lock for traffic. The crew, which had made valiant efforts to stop their ship using a backspring, were unable to do so and while nobody was hurt, the cost of this accident, which also damaged the ship’s bulbous bow, was clearly considerable.
But it is the repetitive nature of accidents involving uncontrolled CPPs which the MAIB notes as something which the industry needs to address. Linkspans, berths, other ships and locks have all fallen victim to ships which could not be controlled after a CPP went badly wrong. The MAIB also points out that there is invariably difficulty in isolating and identifying the precise problems which caused the accident and the near impossibility of replicating the failure in safe circumstances.
Control mechanisms appear to be the root of many of these problems, involving mechanical, electrical or hydraulic issues which appear to be intermittent and are difficult for even specialist technicians to isolate. The inspectors point out that “modern CPP systems are generally complex affairs, yet still often incorporate “basic potential single points of failure”. In this particular case it was not found possible to identify the root cause, although “stickiness” of the single hydraulic control valve was thought a “possible” culprit.
The MAIB recommends that efforts are made to identify “safety critical” systems which, should they go wrong, can menace the safety of the ship and all around her. It also suggests that “robust” pre-departure checks are employed to identify potential problems and give some leeway to sort them out. What the MAIB is really suggesting is an intelligent use of the “what if....” scenario, with critical issues identified, the margins to remedy the situation, and the possible strategies which can recover the situation and mitigate the consequences thought through. In the case of the ship which crashed through the lock, those aboard had no more than 1.5 minutes to save the situation. The case begs the question “...just how prepared would you and your crew be to quickly respond?” It is a not unreasonable question!
Author: The Watchkeeper Source BIMCO