Damage Stability s461q

Damage Stability •Damage due to collisions, subdivided

into 3 types; •High energy- These are increasingly rare, but ships due occasionally collide at right angles, going full ahead. •Moderate Energy- penetration no more than 1/5 the beam transversely or penetration of 10' + 3% of LWL or a vertical penetration of the vessels depth. •Low Energy- minor damage such as an allision with a dock or tugboat working between frames too hard.

Damage Stability • It is the moderate energy collision that is

of most concern to the ships officer. (WHY?) • In this situation the survivability of the ship is based in large part upon the sea state and the vessels stowed condition in addition to the damage sustained.

Damage Stability • Effects of hull damage must include an investigation of the following;

• Loss of Reserve Buoyancy • Loss or Gain in Transverse Buoyancy • Loss in Longitudinal Hull Strength • Is it possible for a flooded vessel to increase "Initial Stability?"

Effects of flooding on Transverse Stability

Two methods are used to approach the problem of flooding on Transverse Stability. •Lost Buoyancy Method •Added Weight Method

Damage Stability • Lost Buoyancy Method- Water that

enters the vessel is considered to be still part of the sea, buoyancy of the flooded space is lost. This method is used by ship designers and MARAD. • Imagine that the flooded space was never built. Only the intact portions of the vessel adjacent to the flooded compartment are contributing to the vessels buoyancy.

Lost Buoyancy Method • Since buoyancy has been lost it must be

regained by an increase in draft, therefore the vessel will sink until it displaces the volume of the flooded compartment. • This will increase KB and decrease BM. (WHY?)

• With the increase of Draft KB is increased. • In the Equation BM=I/V, “V” remains constant or may increase but “I" will decrease because the area of the waterplane is decreased

Damage Stability There are no hard and fast rules that apply to the effects of flooding on Transverse Stability. However broad conclusions can be made by reviewing certain situations that can occur which affect the change in the value of BM and KB. Such as the amount and location of

“Intact Buoyancy”

“Intact Buoyancy” • Spaces within the flooded compartment which exclude water.

(See LaDage Page 200

Figure 85)

• The location of the intact buoyancy is

extremely important. For example if a hold is flooded, but the adjacent double bottoms remain intact and are empty the loss of reserve buoyancy would be less, therefore limiting both an increase in draft and limiting the rise of “B”.

Intact Buoyancy

Intact Buoyancy • The decrease of BM will remain the same

as before because the loss of the AWP is the same. There is an increase of KM when intact buoyancy is below the surface of the flooded area, such as intact double bottoms. • When the intact buoyancy is at the surface of the flooded area, the loss of the AWP is reduced, thus decreasing the loss of “I”,”BM”, “KM”, and “GM” (See LaDage Figure 85 center diagram of 3 on page 200.)

Permeability of Flooded Surface • Permeability of Flooded Surface-

percentage of the total surface area of the flooded compartment which can be occupied by water. For example if a compartment were externally framed so as to be free of frames, beams, etc. and contained no cargo the permeability of the area would be %100.

Permeability of Flooded Surface

• Any reduction in permeability of the space will reduce the loss of waterplane area, resulting in a reduced loss of “I”, “BM”, “KM”, and “GM”.

Damage Stability Lost Buoyancy Method

Damage Control Whether or not a vessel will suffer lost GM after flooding depends upon;

1. Extent of lost waterplane area. 2. Location of intact buoyancy. 3. Permeability of flooding surface.

Damage Control • The ships officer may be able to mitigate

the loss of GM by; 1. Reducing the area of the flooded surface. 2. Flooding intact spaces below the surface. 3. Reducing the permeability of the flooded surface.

Damage Control

It must be emphasized that any change in the value of GM depends upon the individual situation and the individual vessel !

Added Weight Method

• The added weight method assumes the

water entering the vessel to be added weight thus affecting the position of “G”. • If the compartment does not have free communication with the sea, for instance a rupture which DC crews have repaired, or flooding from firefighting, the only possible method of approaching the problem is through the added weight method.

Free Communication with the Sea • If the compartment does have free

communication with the sea either method may be used to make damage stability calculations.

Added Weight Method General Assumptions 1. G will almost always move down. The only 2. 3.

exception being when there is considerable intact buoyancy in the lower part of the vessel. There will be a virtual rise in “G” from free surface that is increased with permeability and beam. There will be an increase in draft with a corresponding rapid decrease in KM for merchant vessels at lighter drafts and slight increases of KM at or near load draft. (See hydrostatic Tables)

Added Weight Method