Answer MEP Question 44

Q. Describe the hull inspection that you would carry out as the senior engineer of the ship in dry dock stating what defects you may find and the repairs that may be necessary with respect to; A. Shell plating; B. Forward end of the ship; C. Aft end of the ship; D. Opening in shell plating; E. Rudder; F. Propeller and stem bush;

Hull Inspection

- Immediately after docking the ship's hull shall be inspected by ship senior officers for the following; 

- Extent of marine growth and fouling of the hull. 

- Extent of any possible damage and corrosion. 

- Distortion of the underwater hull shall be observed. If a dent is discovered, an inspection of the inside of the ship to determine internal damage should be carried out. Serious dents must be repaired; minor dents are left. All repairs and any dents left should be logged. 

- Bilge keel is inspected for any distortion; bilge keels should be faired and their attachment to the hull checked. 

- All cathodic protection systems should be checked for intactness of attachment and worn sacrificial anodes should be renewed if necessary. 

- Forefoot at the after end in way of the stern frame is checked clearly, especially in case of a twin screwed ship around propeller 'A' brackets, for corrosion and cracks. 

A. Shell plating 

- immediately after docking, the ship's hull shall be inspected by ship senior officers for the following; 

- Extent of marine growth and fouling of the hull. 

- Extent of any possible damage and corrosion. 

- Distortion of the underwater hull shall be observed. If a dent is discovered, an inspection of the inside of the ship to determine internal damage should be carried out. Serious dents must be repaired; minor dents are left. All repairs and any dents left should be logged. 

- Bilge keel is inspected for any distortion; bilge keels should be faired and their attachment to the hull checked. 

- All cathodic protection systems should be checked for intactness of attachment and worn sacrificial anodes should be renewed if necessary. 

- Forefoot at the after end in way of the stern frame is checked clearly, especially in case of a twin screwed ship around propeller 'A' brackets, for corrosion and cracks. 

B. Forward end of the ship 

Inspection:  

The structure forward of the collision bulkhead is built to withstand the high dynamic stresses imposed bulbous bow to reduce wave-making resistance to provide increased buoyancy forward. 

After docking the forward shell plating should be hosed with fresh water and brushed down immediately to remove the salt before the seawater dries. 

The plating must be carefully checked for distortion, buckling, roughness, corrosion, the butts and seams should be inspected for cracks. 

The side shell may be slightly damaged due to rubbing against quays. 

If any distortion or dent is observed, an inspection from the inside of the ship is to be made to determine internal damage and if serious should be repaired. 

- Check for any damage to the plate floor, deep floor, panting beams and panting stringers are to be inspected.

- Check for any damage to the flare surface due to falling anchor to be inspected.

- Watertightness of forepeak store forecastle store, forepeak tank to be ensured.

- If fitted with impressed current cathodic protection reference anodes and main anodes are to be inspected and covered during dry dock to protect it against sandblasting and painting.

During repair:

- If any repair is deemed necessary the same is to be communicated to the class and pre-approval is obtained. 

- Cropping and rewilding is done as per the original hull plan and is carried out by approved welders from the yard. 

- Plate thickness, steel grade shall match the original hull material. 

- During welding the internal spaces are to be checked and guarded against any fire. 

After repair:

- Welding is checked for any cracks using optical methods by technicians and the same needs to be witnessed by the chief engineer together with the surveyor. 

- All sacrificial anodes must be checked and replaced if necessary, taking care not to paint over. 

- The plating shall be checked during the flood for any possible leak or wetness. 

C. Aft end of the ship; 

- Inspection of the shell in the aft region is critical, The thickness of the shell plate in this region is less than the midship region. In aged ships in the aft region, wear and tear are significant in this region. 

- Simultaneously the Aft end of the bilge keel is to be checked. Sometimes the crack appears at the aft end. Ensure that the crack (if it exists) does not extend to the bilge strake. Also check for corrosion, because corrosion frequently occurs at the front of the bilge keel. 

- The side shell in the aft area is to be checked. Sometimes the crack of bilge keel propagates to the side shell. 

- Cracks also occur at the local joints of the bilge keel and extend to the side shell. The bilge strake above the bilge keel is often corroded excessively. 

- Aft end inspection is critical as it is subjected to frequent vibrations slamming (impact loading) Presence of different types of materials (for propeller, rudder, hull) may accelerate galvanic corrosion. 

- The Impressed Current Cathodic Protection (ICCP) and sacrificial anode conditions to be checked with the stern frame for cracks, bend, erosion, buckling, etc. 

D. Opening in shell plating;

Shell Opening:

- Plating, fittings & connection In way of shell openings examined to confirm that these are satisfactory condition.

Sea inlets and Discharges & Other openings

- Sea inlets and discharge openings in shell and particularly the shell plating in a way liable to excess corrosion examined to confirm that these arc in satisfactory condition. 

- Confirm that the gauging requirements of shell plating are in way of OBD opening. 

Sea chests & gratings:

(i) Sea chests and their gratings, sea connections and overboard discharge valves and cocks and their fastening to the hull and sea chests examined to confirm that these are in satisfactory condition.

(ii) Confirmation that the corrosion protection system in way of sea chest (Anodes etc.) is in satisfactory condition.

(iii) Confirm that the gauging of sea chest plating is carried out.

Pre - Inspection 

* Inspection of large seawater inlet chest and valves are done from the dock bottom. All underwater valves (injections and discharges) should be examined. 

* After removing the grating of the sea chest in way of the engine room or the pump room, climb onto the staging and inspect the internal parts of the sea chest. The sea chest forms a discontinuity with the rest of the bottom shell; therefore, cracks might appear at fillet welds of girders and floor plates. 

* Valve grids and ship's boxes should be examined from the dock bottom to ensure the grids are clear and secure, and then from the engine room, etc. to see the attachments to the hull are sound, the valve seats tight (hammer test) and the valve lids and seats free of scores and pits. 

* Always check the valve spindles are free to turn in their bridges. Always each valve is overhauled by shore labour before it is boxed up. It is good practice to list all underwater valves before entering the dry dock and then ticks off each one overhauled, in this way none are missed. The ship side valves and cocks are examined, glands repacked and greased. 

* All internals grids arc examined for corrosion and freed from any blockages. 

During the repair:

- If severe wastage has occurred the grid may be built up with welding, ensure the surface is prepared well and the corroded portion is completely sliced. 

- Ensure proper welding rod is used and the welding fabrication work is satisfactory. 

- Inspect during welding minimum of 2 runs are made. 

- Ensure the shell boxes are wire brushed and painted with an anti-fouling composition.

- Any sacrificial anodes must be checked and replaced if necessary, taking care not to paint over the surface. 

- While flooding all sea chest and underwater valves are to be inspected for leakages till the vessel is fully afloat.

 

E. Rudder; 

- Checking that the rudder is watertight. When a vessel is in the dock the plug can be removed to see if it has leaked. Air and water testing can be done to check the rudder. For a water test, the rudder must be removed and laid horizontally and then filled. It is then subjected to a pressure head of 2.45 m. If this is done with the rudder in the position it may overpressure large rudder an air test would have a pressure equivalent to this pressure head. 

- Checking the general condition of the rudder with respect to corrosion and damage. The general condition of the paintwork and cathodic protection and thickness may be taken. 

- Checking the condition of the pintles. Wear down and ovality of the pintle bearings are checked. - The rudder stock attachment is checked. The palm bolts or hydraulic nuts are inspected to ensure that they are installed correctly. 

- The leading edge of the rudder, which is made in one piece, should be examined for smoothness as eddy making at the leading edge produces rapid corrosion. 

F. Propeller and stem bush; 

Inspection:  

- Thorough inspection of the propeller is essential as propeller and stern bush inspection is one of the main objectives of any dry-docking 

- The propeller should be examined for erosion (cavitation), cracks and bent blades. 

- Pitting may occur near the tips on the driving face and on the whole of the foreside due to cavitation. 

- Propeller blades are sometimes damaged by floating debris that is drawn into the propeller stream. Such damage must be repaired as it reduces the propeller efficiency, while the performance is improved by polishing the blade surface. 

- If a built propeller is fitted, it is necessary to ensure that the blades are tight and the pitch should be checked at the same time. 

- For keyless propeller slip between the tapered shaft and propeller boss to be examined and forward one-third of shaft cone is to be examined by magnetic particle crack detection method - Stern gland should be checked for any deterioration. 

- For keyed propeller, key ways to be checked for any surface crack by the magnetic particle detection 

- The wear down of the tail shaft should be measured by inserting a wedge between the shaft and the packing. If this wear down exceeds about 8mm the bearing material should be renewed. 10 min being regarded as an absolute maximum. 

- There should be little or no wear down in an oil-lubricated stern tube. The wear down in this type is usually measured by means of a special gauge as the sealing ring does not allow the insertion of wear down the wedge. 

-Stern bearing wear down to be measured by poker gauge for the oil-lubricated stern bearing condition of oil glands and shaft liner to be examined. In case of the previous record of oil leakage outside or seawater leakage inside oil sealing ring gaiter spring or worn seal ring to be replaced. 

- Grooving on the shaft liner to be checked and measured. 

Rudder Repair:

- After all the above inspections, the repairs are to be carried out according to the extent of any damage. 

- If pits due to corrosion are found they shall be polished off (very small pits (less than 1 mm) can be ground out and polished. Deeper pits can be repaired by welding, grinding and polishing if not possible temporary repair can be carried out using resin fillers. 

- Minor edge cracks are more serious, especially at the leading edge. If the cracks are less than 10mm in length, they can be ground out and the edge 'faired'. Larger cracks may need to be repaired by drilling a small hole at the root of the crack., gouging out. Welding, grinding and polishing (not in the inner third of the blade). 

- The stern gland should be carefully repacked and the propeller nut examined for movement. Such movement usually results in cracking of the cement filling which is readily seen. 

After Repair:

- Groove on shaft liner if noticed due to which leakage has occurred position of seal ring has to be shifted by a distance ring. 

- After complete assembly and during flooding close examination is required for any leakage. 

- Crankshaft deflection and thrust hearing clearance to be checked to ensure alignment has not been disturbed. 

- The efficiency and safety of the ship depend to a great extent on the care taken in carrying out such an inspection.