Vulnerability

A ship might be quite safe while it remains intact but be very likely to suffer extensive damage, or loss, as a result of a relatively minor incident. For instance, a ship with no internal sub-division could operate safely until water entered by some means. It would then sink. Such a design would be unduly vulnerable. This is why in the safety case the designer must consider all the ways in which the ship might suffer damage.

An incident may involve another ship, in collision say, or result from an equipment failure. Thus loss of the ability to steer the ship may result in its grounding. It can arise from human error, the crew failing to close and secure watertight doors and hatches. It will often be the result of several factors coming into play at the same time.

For each way in which a ship may be damaged, the outcome of that damage on the ship and its systems can be assessed. The aim is to highlight

28 SHIP DESIGN

any weaknesses in the design. Taking the steering system as an example, the various elements in the total system can be set out in a diagram showing the inter-relationships. There will be the bridge console on which rudder angles or course changes are ordered, the system by which these orders are transmitted to the pumps/motors driving the rudder and the rudder itself. If two rudders are fitted the two systems should be as independent as possible so that an incident causing one of the rudders to fail does not affect the other. If only one rudder is fitted the system would be less vulnerable if duplicate motors/pumps are provided. Wiring or piping systems and electrical supplies can be duplicated. Each duplication costs money, space and weight so it is important to assess the degree of risk and the consequences of failure. The consequences are likely to depend upon the particular situation in which the ship finds itself. Loss of steering is more serious close to a rocky coast than in the open ocean. It may be even more serious within the confines of a crowded harbour. Thus safety and vulnerability studies must be set within the context of likely operational scenarios.

It will be apparent to the student that probabilities play a major role in these studies and the statistics of past accidents are very valuable. For instance, from the data on the damaged length in collisions and groundings, the probability of the ship being struck at a particular point along its length and of a certain fraction of the ship's length being damaged in this way, what is likely to happen in some future incident, can be assessed. This is the basis of the latest IMO approach to merchant ship vulnerability. The probability of two events occurring together is obtained from the product of their individual probabilities. Thus the designer can combine the probabilities of a collision occurring (it is more likely in the English Channel than in the South Pacific), that the ship will be in a particular loading condition at the time, that the impact will occur at a particular position along the length and that a given length will be damaged. The crew's speed and competence in dealing with an incident are other factors. IMO have proposed standard shapes for the probability density functions for the position of damage, length of damage, permeability at the time and for the occurrence of an accident. There is a steady move towards probabilistic methods of safety and vulnerability assessment and passenger and cargo ships are now studied in this way.

It must be accepted, however, that no ship can be made absolutely safe under all possible conditions. Unusual combinations of circumstances can occur and freak conditions of wind and wave will arise from time to time. In 1973 the Benchruachen, with a gross tonnage of 12 000, suffered as a result of a freak wave. The whole bow section 120 feet forward of the break in forecastle was bent downwards at 7 degrees. When an accident does occur the question to be asked is whether the design was a reasonable one in the light of all the circumstances applying. No matter how tragic the incident the design itself may have been sound. At the same time the naval architect must be prepared to learn as a result of experience and take advantage of developing technology. For instance knowledge of 'freak waves' is improving and oceanographers are providing the data and tools for assessing the probability of meeting extreme waves.

How To Have A Perfect Boating Experience

How To Have A Perfect Boating Experience

Lets start by identifying what exactly certain boats are. Sometimes the terminology can get lost on beginners, so well look at some of the most common boats and what theyre called. These boats are exactly what the name implies. They are meant to be used for fishing. Most fishing boats are powered by outboard motors, and many also have a trolling motor mounted on the bow. Bass boats can be made of aluminium or fibreglass.

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