Cracks in deck plating in way of deck penetrations of oil tankers has been an increasing problem and is typical on vessels with high strength steel in deck and most typically found on Buoy loading Oil Tankers trading in the North Sea. The damaged areas (as shown in this post) were limited to cracks in the upper deck plating in way of the pipe penetrations resulting in small leaks, but could easily have resulted in more serious damage to the deck structure and the safety of the ship if left unattended over time.
The probable cause of some of the cracks could have been a rough edge cut with an oxy-acetylene torch, which has resulted in the initiation of a crack. For other cracks welding had been carried out near the edge of the hole in the deck plating. Cracks have also developed from pipe penetrations where a doubler has been used instead of insert plates.
Example 1 – Inert Gas Pipe
An inert gas pipe with a collar plate was welded from the top only and the opening in the deck was found to have been cut with an oxy-acetylene torch resulting in a very rough edge.
The rough cut edge caused a crack to initiate and propagate quickly. The edge should be cut properly and the edge ground smooth all round. An insert plate would be a better solution instead of a collar plate.
The best design for a pipe that does not actually need to protrude through the deck, particularly, if access below deck is difficult, is the “improved design” shown in the following sketch below.
It should be pointed out that a collar plate should always be welded from both sides. In the engine room transverse bulkheads etc. the collar method is completely acceptable provided both sides are welded, but on the main deck in the cargo area it is not recommended.
Example 2 – Scupper Pipe
A scupper pipe was fitted in the main deck and it was found likely that the welding had been near the edge of the hole. A rough cut hole is also a possible reason for the crack, although on the picture it looks good.
Again the hole should be cut properly and ground smooth. Welding on or near the edge is not recommended. The distance from plate edge to the toe of the weld is recommended to be above 10–15 mm.
Example 3 – Pipe Penetration for Heating Coils
Pipe penetration for heating coils is shown in the picture below where a doubler or collar plate was used on top of deck with the pipes passing through the doubler. To add to the problems the cut out in deck was oriented in the transverse direction.
On the following picture an insert plate repair is shown. This is always better than a doubler welded on top. A doubler should only be used in engine room and fore and aft structure where longitudinal stresses are not a problem.
If the hole was cut in the longitudinal direction the risk of cracks would be reduced, since the effective loss of deck area is less.
When cutting holes in the deck plating on oil tankers the following should be adhered to:
- The edge should be cut properly and the edge should be ground smooth all round.
- Welding is not recommended to be closer to the edge than 10–15mm.
- An insert plate repair is always better than collardoubler welded on top. If doublers are used, particular attention should be paid to the workmanship and design, unless used in areas where longitudinal stresses are low.
- A collar plate should always be welded from both sides.
- The risk of cracks would be reduced if elongated holes in deck was cut in the longitudinal direction, since the effective loss of deck area is less.
This paper by DNV has been prepared with specific focus on deck penetrations on oil tankers but will be relevant also to other type vessels where such penetrations are to be made.
Source: DNV
Officer of the Watch 
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