Answer Construction Question 11
Question
11: The Hull of a vessel in way of the purifier room requires extensive
welding repairs and as Chief Engineer you are requested to supervise.
A. Suggest a suitable type of welding process.
B. State, With reasons, FOUR common welding defects.
C. State what tests may be carried out for the hull repair before
returning the vessel to service.
Answer: (a) The possible damages include buckling, grooving, fracture or widespread areas of wastage over the allowable limits.
Assume the purifier room is adjoining the ship side and ship side plating in way requires renewal. The ship is in dry dock when this repair has to be carried out and completed.
Precautions: The following checks to be carried out prior to commencing the repairs The room should be checked for hot work and suitable permit obtained from ship yard The oil pipelines within the room should be dismantled so that necessary hot work can be safely carried out.
Tools and Rigging: – Apart from the regular welding tool, electrodes transformer and gas cutting torch extra rigging and gear required are.
(a) Sufficient pad eyes and lifting hooks
(b) Wedge brackets.
The above two items are fabricated on spot with extra steel available
(c) Chain blocks
(d) Slings
(e) Staging to be erected as required
Stage 1:- The portion to be renewal by an insert plate of similar original thickness and same quality, is identified by U.S gauging (if wasted) or marked out by surveyor (if damaged). The identification is roughly marked by white paint
Stage 2:- The wasted or damaged portion of plating is cut by oxy-acetylene flame in bits (only if frames are not required to be renewal). The excess metal of plating sticking on frames is gouged out. The corners are properly rounded off and finally the edges are bevelled by grinding the edges with the portable disc grinder. The bevelling angle is 30 dge to 400 deg
Stage 3:- A suitable strake conforming to the material specification and scantling is selected by the workshop from a steel stockyard. If the material specifications are in accordance with national standards organization, Such material is acceptable to the classification but they have to undergo a UTS, % elongation and impact test, provided the chemical composition of the material is broadly in accordance with the original steel. If so a test portion is marked out of the strake and duly stamped for identification of test pieces. The original specification is got from the shell expansion plan.
This strake is then used for cutting out the insert plate, edges are bevelled by grinding to the corresponding angles as made on the dressed cut on ship side sufficient penetration allowance is given on the edge for proper welding. The corners are suitably rounded to the corresponding radius of curvature.
The insert is lifted up by chain blocks rigged at proper support. Supports are made by pad eyes with lifting plate hooks. The insert is positioned in place, supported by wedge brackets welded on Hull along the cut edges. This insert is adjusted to ensure that the welding gap or penetration allowance is uniform all along the edges. Welding can proceed from outside and inside after correct setting of the penetration gap. The welding is done in two or three continuous runs with proper wire brushing and chipping after each run to ensure that the slag is thoroughly removed from the weld.
Stage 4:- Frames affected are now connected to insert plate by staggered welding which the normal procedure is for welding frame to shipside or by full welding if that is the style followed in that area. Finally repairs are tested by hose testing if it is a hold or hydro testing if it is part of ballast tank or wing tank.
Assume the purifier room is adjoining the ship side and ship side plating in way requires renewal. The ship is in dry dock when this repair has to be carried out and completed.
Precautions: The following checks to be carried out prior to commencing the repairs The room should be checked for hot work and suitable permit obtained from ship yard The oil pipelines within the room should be dismantled so that necessary hot work can be safely carried out.
Tools and Rigging: – Apart from the regular welding tool, electrodes transformer and gas cutting torch extra rigging and gear required are.
(a) Sufficient pad eyes and lifting hooks
(b) Wedge brackets.
The above two items are fabricated on spot with extra steel available
(c) Chain blocks
(d) Slings
(e) Staging to be erected as required
Stage 1:- The portion to be renewal by an insert plate of similar original thickness and same quality, is identified by U.S gauging (if wasted) or marked out by surveyor (if damaged). The identification is roughly marked by white paint
Stage 2:- The wasted or damaged portion of plating is cut by oxy-acetylene flame in bits (only if frames are not required to be renewal). The excess metal of plating sticking on frames is gouged out. The corners are properly rounded off and finally the edges are bevelled by grinding the edges with the portable disc grinder. The bevelling angle is 30 dge to 400 deg
Stage 3:- A suitable strake conforming to the material specification and scantling is selected by the workshop from a steel stockyard. If the material specifications are in accordance with national standards organization, Such material is acceptable to the classification but they have to undergo a UTS, % elongation and impact test, provided the chemical composition of the material is broadly in accordance with the original steel. If so a test portion is marked out of the strake and duly stamped for identification of test pieces. The original specification is got from the shell expansion plan.
This strake is then used for cutting out the insert plate, edges are bevelled by grinding to the corresponding angles as made on the dressed cut on ship side sufficient penetration allowance is given on the edge for proper welding. The corners are suitably rounded to the corresponding radius of curvature.
The insert is lifted up by chain blocks rigged at proper support. Supports are made by pad eyes with lifting plate hooks. The insert is positioned in place, supported by wedge brackets welded on Hull along the cut edges. This insert is adjusted to ensure that the welding gap or penetration allowance is uniform all along the edges. Welding can proceed from outside and inside after correct setting of the penetration gap. The welding is done in two or three continuous runs with proper wire brushing and chipping after each run to ensure that the slag is thoroughly removed from the weld.
Stage 4:- Frames affected are now connected to insert plate by staggered welding which the normal procedure is for welding frame to shipside or by full welding if that is the style followed in that area. Finally repairs are tested by hose testing if it is a hold or hydro testing if it is part of ballast tank or wing tank.
B. Common welding defects that can occur there:
Under-cutting
: A groove is formed in the parent metal along the sides of the weld
bead. Groove reduces the thickness of the plate and thus the area along
the bead, which in turn weakens the weld. Causes are as follows :
1. Wrong manipulation and inclination of electrode and excessive weaving.
2. Too large electrode diameter
3. Higher currents.
4. Longer arc.
5. Faster arc travel speeds.
6. Magnetic arc blow.
7 Rusty and scaly job surfaces.
Overlapping : It is just reverse to undercutting. An overlap occurs when the molten metal from electrode flows over the parent metal surface, and remains there without getting properly fused and united with the same. Overlapping may occur due to the following :
1. Lower arc current.
2. Slower arc travel speed.
3. Longer arcs.
4. Improper joint geometry (i.e. root gap).
Poor Fusion : Sometimes the molten metal deposited by the electrodes does not fuse properly with the cold base metal and the two do not unite properly and completely. The fusion may be lacking at the root, sides or between two runs in a multi-run weld. Causes are:
1. Lower arc current.
2. Faster arc travel speed.
3. Improper weaving technique.
4. Presence of oxides, rusts, scale and other impurities.
5. Incorrect joint preparation.
6. Incorrect electrode manipulation
Slag inclusion : It may be in the form of slag or any other foreign material, which does not get a chance to float on the surface of the solidifying weld metal and thus get entrapped within the same. Inclusions lower the strength of the joint and make it weaker. Causes are: 1. Too high or too low arc current.
2. Long arcs.
3. Too large electrode diameter
4. Insufficient chipping and cleaning of previous passes in multi-pass welding.
Incomplete penetration : It is the distance of fusion from base plate top surface to the maximum extent of weld nugget. Reasons are :
1. Improper joint preparation ( U-Joint is simpler to have good penetration than J butt joint).
2. Too large root face.
3. Root gap too small.
4. Too small bevel angle.
5. Less arc current.
6. Faster arc travel speed.
7. Too large electrode diameter
8. Longer arc length.
9. Incorrect polarity when welding with direct current.
10. Wrongly held electrode.
1. Wrong manipulation and inclination of electrode and excessive weaving.
2. Too large electrode diameter
3. Higher currents.
4. Longer arc.
5. Faster arc travel speeds.
6. Magnetic arc blow.
7 Rusty and scaly job surfaces.
Overlapping : It is just reverse to undercutting. An overlap occurs when the molten metal from electrode flows over the parent metal surface, and remains there without getting properly fused and united with the same. Overlapping may occur due to the following :
1. Lower arc current.
2. Slower arc travel speed.
3. Longer arcs.
4. Improper joint geometry (i.e. root gap).
Poor Fusion : Sometimes the molten metal deposited by the electrodes does not fuse properly with the cold base metal and the two do not unite properly and completely. The fusion may be lacking at the root, sides or between two runs in a multi-run weld. Causes are:
1. Lower arc current.
2. Faster arc travel speed.
3. Improper weaving technique.
4. Presence of oxides, rusts, scale and other impurities.
5. Incorrect joint preparation.
6. Incorrect electrode manipulation
Slag inclusion : It may be in the form of slag or any other foreign material, which does not get a chance to float on the surface of the solidifying weld metal and thus get entrapped within the same. Inclusions lower the strength of the joint and make it weaker. Causes are: 1. Too high or too low arc current.
2. Long arcs.
3. Too large electrode diameter
4. Insufficient chipping and cleaning of previous passes in multi-pass welding.
Incomplete penetration : It is the distance of fusion from base plate top surface to the maximum extent of weld nugget. Reasons are :
1. Improper joint preparation ( U-Joint is simpler to have good penetration than J butt joint).
2. Too large root face.
3. Root gap too small.
4. Too small bevel angle.
5. Less arc current.
6. Faster arc travel speed.
7. Too large electrode diameter
8. Longer arc length.
9. Incorrect polarity when welding with direct current.
10. Wrongly held electrode.
C. Tests may be carried out before returning the rudder to service:
1. Visual inspection
2. NDT by ultrasonic
3. pressure test by filling the rudder with water to a height of 2.45m above it.
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