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Structural safety at sea

   
 
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A consortium of maritime classification societies and university research groups has developed an improved set of reliability-based rules for the structural design of ships. Unlike traditional rules based on empirical safety factors, the new rules are derived from detailed analysis of the uncertainties in the computer models used to design the hulls of ships, notably models for loading effects and structural strength. Already, a number of societies are considering the SHIPREL rules.

When we board a ship, we would like to think that the vessel is strong enough to withstand the sea conditions that may be encountered on the voyage. And so it is - designers always allow generous safety factors when calculating the structural strength of ships.
However, there is a modern approach to quantify structural safety. In principle, one could calculate the probability of a hull failing under the expected range of conditions at sea, and then design the ship so that the chances of failure were smaller than some minimum, acceptable value. In civil engineering, for example, designers have been using such reliability based methods since the late 1970s to replace traditional safety factors in the design codes. For each structure, such as a bridge, the probability of failure can be calculated during the planning stage and the design modified accordingly. This ensures that the bridge is safe without incurring unnecessary expense.
Unfortunately, the models currently used to calculate the strengths of ships' hulls, and the stresses encountered in operation, have not been sophisticated enough to allow reliability-based methods to be adopted by the world's classification societies. Established by the insurance industry to improve the safety of shipping, the societies lay down rules for the design of different kinds of vessels.
In SHIPREL, ship classification societies in France, Germany and Italy, together with university research groups in Portugal and Denmark, joined forces to develop new reliability-based methods for the structural design of ships.
Modern ships come in a vast range of shapes and sizes, so the partners decided to concentrate on just two types: oil tankers and container vessels. These represent opposite ends of a spectrum of characteristics, broad and slow on one end, and narrow and fast on the other.
Reliability analysis requires answers to many difficult questions about the nature of hull failure, the range of conditions expected at sea, the ultimate strength of the hull, and so on. The project concentrated on the primary strength of ships; that is, the strength of the hull under longitudinal bending and, in particular, the design of the midship section where stresses and strains are greatest.

Stresses and strains

The first part of the project was to develop new computer models to represent the stresses in the structures of a ship's hull. These stresses have several sources. First, stresses arise from the loading of the ship - the way cargo is distributed within it. These still water effects will change during a voyage, as cargoes are loaded and unloaded; there are large differences, for example, between full and empty oil tankers. As well as searching existing databases for loading information on tankers, SHIPREL has collected new data from voyages of 40 container ships to quantify the effects of different loadings.
A second source of stress, also tackled by SHIPREL, is a variety of effects caused by the motion of the sea waves. These range from gentle rolling to violent slamming, and vibrations course through the ship due to the impact of the waves. The partners developed an improved method for predicting the effects of large waves.
To test the models developed by SHIPREL, the partners fitted a container ship with instruments to record stresses, strains and temperatures. The ship was monitored for two years as it sailed the Atlantic between France and North America.

Modelling of structural strength

The models used at present for calculating the structural strength of ships suffer from many uncertainties. Ships are complex structures and the models have to be simplified to enable computers to handle the calculations.
In finite-element analysis, a well-established but complex method, the actual modelling process is time-consuming, requiring skilled and expensive personnel, so detailed modelling is often restricted to the midship section rather than the whole hull. A cheaper and simpler method is to represent the entire hull as a single beam subject to bending and twisting forces, but it is uncertain how closely this model represents a real ship. Also, while the distribution of wave-induced forces at each point on the hull should be taken into account, it is usually simplified for ease of computation.
Whichever method is used, designers must take care with interpreting the results - the calculated strengths may be very different to those of a real ship.
To get some idea of the practical uncertainties, and thus estimate the safety factors, the participants made several calculations using different models of the same ship. They also looked at the effects of fatigue on the strength of welded joints.
One notable success is a simplified procedure for calculating the collapse strength of a girder consisting of stiffened panels. The new method takes only a few seconds of computer time compared with several hours for non-linear finite element methods.

New rules for ship design

At the end of a long and complex series of computations, the SHIPREL team produced simplified procedures and formulae for use in ship design. By combining the uncertainties in the loading models and the strength models they have produced a set of safety factors based upon sound engineering principles. They propose that ships be designed so that there is one chance in 10,000 of a structural failure occurring in any one year.
The new rules will also be helpful in assessing the condition of existing ships and estimating how many years they can safely continue in service.
SHIPREL is arousing great interest in the world of shipping. Many research publications have stemmed from the work, and a special issue of the Marine Structures journal has been devoted to it. In autumn 1996 a presentation of the project was made to the Society of Naval Architects and Marine Engineers in New York.
Some classification societies are in the process of modifying their design rules in the light of the SHIPREL findings. But while the demonstration of reliability methods is of great interest to all those concerned with the safety of ships, the various software modules devised by SHIPREL are not yet at the stage where they can be used for routine engineering. A single, integrated software package for designers is the next step towards more reliable structures in the world's ships.

 

 

Project Title:  
Reliability methods for ship structural design

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: BE-4554

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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