Ship Structure

Question 1

What is shear force?

Answer 1

Unaligned forces acting on an object in opposite directions.

In a ship, this is unequal weight and buoyancy forces along the length of the ship

Question 2

What is the bending moment?

Answer 2

The area under a shear force curve. Where the shear force curve crosses the x axis to go negative, the area under the curve is at its highest and so the bending moment will be at its highest.

Question 3

What are the causes of structural failure in a ship?

Answer 3

1. Rupture - The strain is too great for the material
2. Fatigue - Repeated strain on a material
3. Instability - The material is not strong enough and unable to hold the weight required so may bow or buckle
4. Vibration
5. Racking - distortion of the transverse section of the ship

Question 4

What are the three main causes of stress and strain in a material?

Answer 4

1. Overall loading of the ship girder from vessel movement and bending
2. Local stresses from vessel loading or impacts
3. Stresses due to manufacturing processes

Question 5

What ships require hull stress monitoring systems installed?

Answer 5

bulk carriers over 20,000 dwt

Question 6

What are the component parts of hull stress monitoring systems?

Answer 6

1. Sensors along the main deck length of the ship
2. Accelerometer on the bow to measure vertical accelerations
3. Two accelerometers on the centreline to measure roll and sway
4. Microprocessor to interpret the sensors and give audible/visual warning of approaching limits
5. Visual display
6. Data storage capacity

Question 7

What is a hull stress monitoring system?

Answer 7

Provides the Master and Officers with a visual display showing motions of the ship and global stresses it is experiencing then compares them with allowable limits to give warnings.

Question 8

What additional measures are in place for bulk carriers in SOLAS Chapter 12?

Answer 8

1. Additional damage stability criteria
2. Additional strengthening of the hull
3. Double skin design to allow regular inspection of shell plating and structural members
4. Density of the cargo should be declared before loading and some vessels have a density restriction.
5. Loading instrument to provide hull girder shear forces and bending moments to the bridge
6. Hold and ballast tank water ingress alarms on the bridge
7. Easily accessible means of pumping ballast and bilge water from spaces
8. Shall not sail with a cargo hold empty when the vessel is at full deadweight condition.