Question

The vessel upon which you are serving as Chief Engineer is shortly to be dry-docked;(a)describe the areas on the external hull that should be examined for cracking.(b)the nature of any repairs to be carried out.

Answer.

Cracks may be initiated from a number of mechanisms that may act singly or in unison viz:

• Collision, grounding, tug handling, anchor operation.

• Through severe weakening of scantlings by accelerated corrosion.

• From stress concentrations.

The first category are self explanatory and depending on the soundness of undamaged adjacent sections repair is by replacing shell plates, stringers, frames etc. that are severely indented or cracked.

In the second category we must first attempt to ascertain and eliminate the cause of the corrosion, prior to replacement/welding of the material.

The weakening may be from internal sources, external sources, or a combination of both.

An examination of the protective mechanisms should be carried out i.e. protective coatings and the cathodic protection systems.

Severe expansion/contraction of surfaces due to heating of cargoes etc. may increase corrosion by removing protective stable oxide films.

The stern sections are particularly susceptible owing to the presence of the propeller causing enhanced galvanic action.

The final category may further be sub-divided viz. uneven cargo distribution wave action.

poor material/design/construction.

i) Nowadays the use of a load indicator to examine for excessive bending and shear is mandatory, nevertheless failure may occur.

Not only is the final distribution critical but the loading/unloading programme is important if unnecessary stresses are to be avoided.

The cracks emanating from this source will be more likely to occur in the midship 40% of the length, in continuous material, furthest from the neutral axis of the vessel. (Ref. Energy Concentration).

ii) Wave action, particularly in the panting and pounding regions of the fore end.

The alternating stresses are generated from the deflections of the members due to varying hydrostatic and air pressures (panting,) and the impact strode (pounding). The presence of corrosion (inside and outside) together with the cyclic stresses can cause rapid degradation. (Ref. Energy Endurance).

iii) The material of ships hull material may vary. This is a consequence of 'sample testing', i.e. The sample may be satisfactory, but this does not guarantee the quality of the steel used. Also, the modern practice uses higher tensile steels of thinner section means that a particular corrosion rate may have more significance than with thicker 'mild steel'.

The design of the structure should be to minimise discontinuities that lead to stress concentrations. During construction all structure should be aligned correctly and welded joints should not be excessively open. All welded seams that are suspect should be examined. Cracks may start from inadequate welds due to a variety of faults, inclusions, undercuts, poor penetration, stress, corrosion etc.

As stated some of these mechanisms may act in unison and in addition a vibratory stress may be imposes upon the structure. The investigation will take account of the position and orientation of the cracks, the nature of the cracks (% of cleavage present). If vibration is considered to be a serious component then methods should be employed to reduce it.

(b) No repairs to be undertaken before the following are ascertained (i) the cause (as far as practicable); (ii) the nature of the material is known.

The ends of any cracks may be examined by magnetic particle to ascertain its extent. A 'Crack arrester' hole may be drilled at the root of the crack to provide a temporary prevention of propagation.

Small cracks may be ground out with an electric grinder which will also allow edge preparation to be made. Large cracks may be gouged out by pneumatic chisel.

The crack is welded and a good smooth head should be achieved. If the type of steel requires low hydrogen electrodes to be used, the electrodes are to be preheated in a heater.

The greatest of care should be taken with any repair in high stress areas such as the deck stringer plate, sheer strake, bilge strake (and bilge keel). A poorly made repair in these areas can result in catastrophic failure. (Ref. Kurdistan - see M 1059)