Ship Structural Failure Modes

 

Ship Structural Failure Modes

Cracks can occur due to fatigue and brittle fracture. Buckling is due to in plane compressive stresses during hogging and sagging of the ships. An Indent occurs during cargo handling. Any dent in the plate due to lateral hitting of the ship by external vessels like tugs is in Indent. Corrosion is one of the main failure modes that presents always in the maritime field.

1.       CRACKS:

Most of the cracks that occur in ship are due to fatigue. Fatigue is the failure of a material by cyclic loading. A large number of complicated welded plate joints can be found in an ocean- going ship. During the service time of the vessel these plates are exposed to time-varying loads caused by the irregular seaway, the propulsion system, and changes in the loading conditions. Hard spots, such as connections between longitudinal stiffeners and web frames, where local rises in stress intensity can be found, may therefore be prone to fatigue cracking. Sometimes, the acceptable level of fatigue cracking is exceeded when significant structural changes are introduced in the ship designs, e.g. when high-tensile steel on a large scale was adopted in the building of the so-called second-generation of very large crude oil carriers (VLCCs). The higher structural strength of the high-tensile steel is not reflected in the same improvement of the fatigue strength of the material. Therefore, the reductions of the scantlings based on structural strength resulted in increased frequency and severity of fatigue cracking in many new tankers

 2.       BUCKLING

Buckling is permanent deformation that occurs when structure is subjected to excessive compressive forces. Shear buckling occurs when structure is subjected to excessive shear forces. Hull Girder buckling occurs when the load gets too high or the steel is worn too thin due to corrosion, the deck or bottom may buckle across the entire breadth of the ship. Hull girder buckling is a sudden failure mode and may lead to total loss of the ship. When the vessel is in sagging mode, buckling occurs in the deck panels. When the vessel is in hogging mode, buckling occurs in bottom panels. Figure 5 shows the buckling failure in the bottom structure of the ship.

3.       INDENTS

Indents are caused by lateral forces on the structure. Yield strength of the material exceeded resulting in permanent deformation of the structure.  the indents on the tank top of cargo hold due to cargo cranes operation. 

Consequences of indents depend on the extent of damage caused. The permissible indent is S/12 times as per the classification regulations . Where, S is the frame spacing. Figure 7 shows the severe indents caused due to lateral hits on the ship side shell structure.  permissible indent as per the classification society regulations.

4.       CORROSION

Corrosion may occur in several forms.

a. General Corrosion

b. Local Corrosion

c. Pitting Corrosion

d. Edge Corrosion

 e. Grooving Corrosion

 f. Bacterial Corrosion

g. Galvanic Corrosion