SHIP LOADS AND STRESSES DISTINGUISH BETWEEN STATIC AND DYNAMIC LOADS

INTRODUCTION The size and principal characteristics of a ship are determined primarily by its mission, intended service, and cost. In addition to basic functional considerations there are requirements such as stability, low resistance, high propulsive efficiency, good seakeeping, and various navigational restrictions on draft or beam, all of which influence the choice of dimensions and form. The ship's structure must be designed, within the basic constraints, to sustain all the loads expected to arise in its seagoing environment. In contrast to land structures, the ship does not rest on a fixed foundation but derives its entire support from buoyant pressures exerted by a dynamic and ever changing ocean environment, which plays both roles of friend and foe for the ship.

The loads that the ship structure must be designed to withstand have many sources. There are static components which consist principally of the weight and buoyancy of the ship in calm waters. There are dynamic components caused by wave induced motions of the ship, and by slamming in waves, as well as vibratory loads by the propeller and machinery, all of which are of different frequency ranges.

CLASSIFICATION It is convenient to divide the loads acting on the ship structure into two main categories : Static Force Dynamic Force

We will now look into each of these forces in detail.

STATIC FORCE Static loads are loads that change only when the weight of the ship or its weight distribution changes. These include: Weight of the ship and its contents. Static buoyancy of the ship at rest or moving. Thermal loads resulting from temperature gradients within the hull. Concentrated loads caused by dry docking or grounding.

They are due to: Internal forces result from structural weight, cargo and machinery weight. External static forces include the hydrostatic pressure of the water on the hull.

Consider a ship floating in calm water Consider a ship floating in calm water. Two different forces will be acting upon it along its length. The weight of the ship and its contents will be acting vertically downwards. The buoyancy of vertical component of hydrostatic pressure will be acting upwards. In total, the two forces exactly equal and balance one another such that the ship floats at a particular draft. The centre of buoyancy force and the centre of the weight will be vertically in line.

A transverse section of a ship is subjected to static pressure from the surrounding water in addition to the resulting loading from the weight of the structure, cargo etc. although transverse stresses are of lesser magnitude than longitudinal stresses, considerable distortion of the structure could occur, in the absence of adequate stiffening.

The structures which resist transverse stresses are transverse bulkheads, solid floors in the double bottom, deck beams, side frames and the brackets between them and adjacent structure such as tank top flooring or margin plates.

DYNAMIC FORCES Dynamic loads are loads that vary in time with periods ranging from a few seconds to several minutes. They may also be time varying loads of sufficiently high frequency that may induce vibratory response of the ship structure

These can be broken dowin into the following components: Wave induced hull pressure variations. Hull pressure variations caused by transient ship motions. Inertial reactions resulting from the acceleration of the mass of the ship and its contents

Hydrodynamic loads induced by propulsive devices. Loads imparted to the hull by reciprocating or unbalanced machinery. Hydro elastic loads resulting from interaction of appendages with the flow past the ship. Wave induced loads due primarily to short waves whose frequency of encounter overlaps the lower natural frequencies of hull vibration, called springing.

They are due to: The ship’s motion in the sea The action of the wind and the waves The effects of operating machinery.

A ship is free to move with 6 degrees of freedom, 3 linear and 3 rotational. The various forces acting on a ship are constantly varying in degree and frequency. Consider a ship moving along waves. Distribution of buoyancy will vary as a result of the waves. The movement of the ship will also introduce dynamic forces. The ship’s structure will be subjected to constantly fluctuating stresses as waves move along the ship’s length.

The hull girder bending in waves takes place continuously throughout the seagoing lifetime of any ship. It has been estimated that in a twenty ship lifetime, a typical ship undergoes on the order of hundred million such bending reversals.