Module 2 : Ship Stresses Flashcards
Types of stress
- Tensile
- Compressive
- Shearing
- Tensile stress
Pulling apart
- Compressive stress
Pushing (crushing) together
- Shearing stress
Material moving (push or pull) in non-parallel plane
Static forces (overview)
Occur when the vessel is motionless, either in still water or in drydock
Includes:
- The effects of weights throughout the ship’s structures
- Water pressure on the hull structure
- Effect of the weight of the hull when the vessel is in dry dock
- Thermally induced stresses
Dynamic Forces (overview)
Occur when vessel is in motion. These forces are constantly changing and can be transient in nature. They can vary in magnitude and duration. Dynamic loads could amplify or reduce the effects of static loads.
Include:
- Sea action causing bending, shearing, twisting, pounding, and fluctuations of pressure on the hull
- Vibrations
Corrosion
Chemical or electro-chemical metal wastage process
Reasons on board of ships:
- Corrosiveness of the marine environment
- Chemicals used on board
- Types of cargoes being carries
- Microbial reactions (microbes leave an acidic residue)
- Electrical potential of the hull
- Construction materials
Erosion
Mechanical metal wastage process (physical abrasion)
Takes away protective coating and leaves space for corrosion
Abnormal loading conditions
Collision and grounding.
Result from loss of engine control or loss of steering
Static loads: Water Pressure
Hydrostatic pressure is dependent on the depth and density of the liquid: the deeper the submersion, the higher the pressure; the greater the density, the higher the pressure
Counter: ship is stiffened by the use of appropriate sized and spaced framing, adequate shell thickness, the use of transverse members.
Static Loads: Uneven distribution of weights
Balance between hydrostatic pressure & weight of cargo.
Weight on the ship generates downward forces that are countered by upward forces from the buoyance of the water. Localized loads result in significant longitudinal bending.
Localized loads
Forces can cause the structure of the ship to vibrate and create local flexions and deformations, transmitting the stressors to other parts of the structure
Thermally-induced Stresses
- Caused by variation of temperatures within or outside the vessels inducing expansion and contraction within the hull and setting up tensile, compressive and shearing stresses in the structures and components
- Can also be caused during construction or repair due to incorrect welding procedures (improper heating and/or cooling). Don’t want to push steel past it’s yield point
Fractures
Brittle fracture failure result from poor stress relieving practices & other factors (high load area, external thermally induced stress, etc). Occurs suddenly and without warning, resulting in complete sections of the ship breaking away
Drydocking
No longer have buoyance forces, so weight of vessel must be handled by the structure of the ship in localized areas.
Want ship as light as possible.
Keel blocks, bilge blocks, bilge shores and breast shores are used to support the weight of the vessel and reduce the amount of deformation of the structure.
Positioning of blocks is determined prior to drydocking and proper docking procedures must be followed.
Examples of dynamic loads
- Longitudinal bending (hogging & sagging)
- Transverse deformation (ie. racking, panting, transverse bending)
- Pounding
- Drydocking
- Vibration
- Wave induced hull pressure
- Hull pressure due to transient sea state and ship motions
- Inertial reactions from acceleration/deceleration of the mass of the ship and content
- Hydrodynamic loads induced by propulsion
- Rotating equipment loads passed to the hull
- Longitudinal bending
IMPORTANT
Sagging: center is down, ends are up. Deck under compression, hull in tension. Too much buoyance
Hogging: center is up, ends are down. Deck under tension, hull in compression. Too much weight
- Transverse deformation (bending)
Similar to longitudinal, but on the breadth of the vessel
Can occur simultaneously with longitudinal bending
- Transverse deformation (racking)
IMPORTANT
Twisting the hull shape due to extreme loads primarily on the corners of the structure. As ship rolls, different areas are accelerated or decelerated at different rates due to local loads.
Mitigate with bulkheads, brackets, tanks.
- Transverse deformation (panting)
Occurs primarily in the forward section of the vessel but a small amount could be seen in the aft area
Stress which occurs due to variations of water pressure on the shell as the vessel pitches in the sea. Manifested by an “in-and-out” movement or vibration of the plates - flexing like ribs
- Pounding
AKA slamming
Forward bottom area as the vessel is pitched in and out of the water. Most pronounced in the flat bottom section of the hull from 5% to 30% of the ship’s length from forward perpendicular. Not a common occurrence but felt as a shock or shuddering motion, causing vibrations throughout the length of the vessel
- Drydocking
As the vessel is removed from the water, buoyant forces are reduced. The first portion of the vessel to touch the blocks is the after part of the keel. The initial force on the keel is quite high. As the ship settles along the length, the load becomes more distributed
- Vibration
Noise and vibration mostly result from machinery and propulsion equipment or rotating devices.
Proper care can greatly reduce vibrations. Or add stiffening and/or noise absorption/dampening arrangements.
- Rotating equipment loads passed to the hull
Thrust of the propeller. Varying force can be extremely high at times. The thrust is absorbed by heavily constructed thrust block arrangement, secured to the hull by way of heavy construction.
Places material under tensile, compressive and shearing stress.
