Ship strength

The subject of Ship Strength deals with the assessment of the ship's structural design to withstand the service loads she will confront during her lifetime. It is of profound importance for the design and usability of the ship. During the design phase of the ship, the naval architect has to account for the type and service conditions of the ship under consideration in order to engineer its structural arrangement accordingly.

As is the case in many applications, the engineer has to compromise conflicting requirements. For seagoing ships, this conflict occurs usually between service requirements, targeted service conditions, ship life cycle and structural strength. For example, a crude oil tanker has to have the least structural weight (increase DWT) in order to increase profitability, sail under any conditions, survive on standard preventive maintenance for 30 years and yet have a structure that can deliver this. For a coast liner on the other hand, requirements may include among others high speed and increased passenger safety.

Additionally, the structural arrangement has to provide for convenience and usability of the ship. For example, the compartmentation of the cargo and ballast area of the ship has to comply with strength criteria, pump/piping availability, ship stability and operations requirements. For example a tanker might have to receive and deliver particular quantities of oil. Thus the grouping of the cargo and ballast tanks along with the piping and pump arrangements should be such as to make it more convenient to unload such quantities as fast as possible. The remaining quantities however, will have to fulfill both stability and strength criteria. From the strength point of view, non-uniform loading conditions might result in complex loads on the structure, including transverse shear, bending and torsion. Moving cargo around is not a choice, as this will at least blow up the operations' scheduling: having to pump cargo out of all tanks takes much more time in the harbor and as usual time is money. But even more important is that half-filled tanks, may result in excessive sloshing, which in turn can pose a considerable safety thread.

Apart from this "high level" structural design aspects, the structural arrangement has to provide for other usability issues that are more local. A ship comprises a constellation of different pieces of machinery: engines, power units, turbines, pumps, cranes, derricks, mooring equipment and so on. The structural arrangement must consider all these local loads and foresee for adequate foundation and support. Though this may seem it can be handled at the local level, it is not always so straight-forward: consider that a lot of the machinery lies on the main deck or even higher ones that are usually of smaller rigidity than the inner bottom for example.

The purpose of the above discussion is merely to illustrate few issues the Naval Architect has to face when confronted with the task of designing the structure of a ship. Consider that these are trivial issues: special types of craft might - and probably will - need much more elaboration at the design stage. However this introduction also outlines the different levels of structural design that the designer has to consider. Ships, and in particular large ones, are hollow structures composed of very small elements. Consider the midship section of a large double hull tanker: it might be 40 m in breadth and 20 m in depth. However, the actual area of the elements comprising the midship section, might be less than $5m^{2}$! That is, if you could squeeze it so that there are no gaps left, it wouldn't be larger than your carpet. The challenge is clear: design one of the biggest structures ever made by human beings.

Load types exerted on marine structures

There are different ways to classify the loads that are exerted on a marine structure. As a first approach, a ship is considered along with all its equipment, cargo and fluids as the system under consideration. The loads exerted to this system could be classified into the following two categories:

• Standard Loads

Standard or operational loads are the ones that the ship will experience during most of her lifetime. These loads act to the ship as a whole as concentrated loads. Such loads include:

• • Static still water and wave buoyancy: these are the hydrostatic forces that act to the ship hull when the ship is afloat.
  • Dynamic lift loads: for semi-displacement and planning hulls.
  • Wind pressure: especially for ships with large superstructure area.
  • Drydock loads: when a ship lays on the drydock platform.
  • Mooring lines and anchors: these act as concentrated loads.

• Extreme Loads

This class of loads occurs when the ship sails in harsh weather conditions. The naval architect should keep in mind that these loads may occur rarely. But they will occur and therefore they should never be neglected. Such loads include:

• • Ship-Ship and ship-obstacle collisions.
  • Grounding.
  • Slamming.
  • Green sea.

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