Ship Stress: Static and Dynamic Forces

6 Degrees of Freedom

• Large motions can generate significant forces, causing structural vibration and stress transmission.

Fundamental Causes of Ship Stresses

• Ship and cargo weight contribute to stress
• Ship buoyancy is a cause of stress
• Ships' movement is a cause of stress
• External forces, like wind, waves, and current, contribute to stress
• Equipment operations create stresses.

Static Forces and Resulting Ship Stresses

• Ship's weight is a static force, which leads to stresses like sagging in still water
• The action of water pressure (buoyancy) leads to hogging in still water
• Distribution of Dead Weight Tonnage (DWT) leads to hydrostatic pressure
• Equipment weight leads to concentrated loads and docking forces

Dynamic Forces and Resulting Ship Stresses

• A ship's movement, action of wind/waves, and operating machinery are dynamic forces
• Dynamic forces lead to stresses such as sagging and hogging in waves
• Dynamic forces lead to racking, torsion, pounding/slamming, panting, and vibrations

Hydrostatic Pressure

• Water pressure increases with depth, acting perpendicularly on the ship's shell
• Frames, bulkheads, floors, and girders resist the effect of water pressure
• P = ρ.g.h (Pressure = density x gravity x depth)
• Pmax = ρ.g.d (Maximum Pressure = density x gravity x total depth)
• P x A = Force → Buoyant Forces

Concentrated Loads

• Localized heavy loads can cause distortion in the transverse section
• Examples of localized heavy loads
• Main engine and Aux machines
• Propeller shaft / Propeller / Rudder
• Cranes / Winches / Concentrated cargo

Docking

• Vessels docked on keel blocks tend to sag transversely
• Including additional rows of blocks outboard can reduce transverse sagging

Hogging and Sagging in Still Water

• Longitudinal bending stresses can be caused by the difference between buoyancy and local loading weight

Hogging and Sagging in Waves

• When the ship is in waves, different stress conditions apply compared to still water

Sagging in Waves

• A ship being supported at the ends by wave crests while the middle is unsupported is an example of this
• Occurs in heavy seaways
• Buoyancy increases at the ends and decreases mid-ship
• Results in a bending moment causing the ship to sag

Hogging in Waves

• Increased buoyancy at mid-ships with wave troughs at the ends causes a bending moment, leading to hogging
• A wave crest at mid-ships can cause this
• Wave troughs positioned at the ends reduce buoyancy
• Creates a significantly increased bending moment
• Exposes a ship structure to extreme stress

Racking

• Accelerations during rolling cause distortion in the transverse direction
• The deck moves laterally relative to the bottom structure
• One side of the shell moves vertically relative to the other side
• Transverse bulkheads primarily resist transverse deformation

Torsion

• Navigating a wave train at an angle subjects the ship to a twisting moment (torque)
• Decks with large openings are most affected decks
• A heavy torsion box girder, including the upper deck, accommodates torsional stresses

Slamming (Pounding)

• Heavy pitching combined with heaving can subject the forepart to severe impact from the sea
• The worst effects are experienced in lightship conditions

Whipping

• Ship whipping involves rapid flexing of the hull girder due to wave impact
• Typically occurs from impacts on the flat bottom or bow flare

Panting

• Stress at the ends of a ship is caused by variations in water pressure when pitching is combined with rough conditions
• The shell plating undergoes in-and-out movement
• Occurs as the ship pitches in a seaway
• Bow areas are most affected when making headway
• Additional stiffening in the form of panting beams and stringers is needed to combat this