Module 2 : Ship Stresses

Question 1

Types of stress

Answer 1

1. Tensile
2. Compressive
3. Shearing

Question 2

1. Tensile stress

Answer 2

Pulling apart

Question 2

2. Compressive stress

Answer 2

Pushing (crushing) together

Question 3

3. Shearing stress

Answer 3

Material moving (push or pull) in non-parallel plane

Question 4

Static forces (overview)

Answer 4

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

Question 5

Dynamic Forces (overview)

Answer 5

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

Question 6

Corrosion

Answer 6

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

Question 7

Erosion

Answer 7

Mechanical metal wastage process (physical abrasion)
Takes away protective coating and leaves space for corrosion

Question 8

Abnormal loading conditions

Answer 8

Collision and grounding.
Result from loss of engine control or loss of steering

Question 9

Static loads: Water Pressure

Answer 9

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.

Question 10

Static Loads: Uneven distribution of weights

Answer 10

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.

Question 11

Localized loads

Answer 11

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

Question 12

Thermally-induced Stresses

Answer 12

• 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

Question 13

Fractures

Answer 13

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

Question 14

Drydocking

Answer 14

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.

Question 15

Examples of dynamic loads

Answer 15

1. Longitudinal bending (hogging & sagging)
2. Transverse deformation (ie. racking, panting, transverse bending)
3. Pounding
4. Drydocking
5. Vibration
6. Wave induced hull pressure
7. Hull pressure due to transient sea state and ship motions
8. Inertial reactions from acceleration/deceleration of the mass of the ship and content
9. Hydrodynamic loads induced by propulsion
10. Rotating equipment loads passed to the hull

Question 16

1. Longitudinal bending
   IMPORTANT

Answer 16

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

Question 17

2. Transverse deformation (bending)

Answer 17

Similar to longitudinal, but on the breadth of the vessel
Can occur simultaneously with longitudinal bending

Question 18

2. Transverse deformation (racking)
   IMPORTANT

Answer 18

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.

Question 19

2. Transverse deformation (panting)

Answer 19

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

Question 20

3. Pounding

Answer 20

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

Question 21

4. Drydocking

Answer 21

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

Question 22

5. Vibration

Answer 22

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.

Question 23

10. Rotating equipment loads passed to the hull

Answer 23

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.