Insert plate or doubler: choosing the permanent repair

#Two ways to deal with bad plating

A doubler is a plate welded over the damaged area. An insert replaces it: the old plate is cut out and a new one is welded into the opening. The doubler is quicker; the insert restores the structure as it was designed.

#Why class expects inserts

A doubler leaves the wasted plate in place, and the gap between the two plates traps water and corrosion. It also changes the way loads flow through the structure. That is why class societies generally accept doublers on primary structure only as temporary repairs, for a period agreed with the surveyor, and expect wasted or cracked plating to be renewed. IACS Recommendation 47, the shipbuilding and repair quality standard, describes how both should be made.

#An insert done right

• Size and shape. The opening reaches sound plate all round and has radiused corners — square corners concentrate stress.
• Position of the seams. New butts and seams are placed clear of existing welds and of the stiffeners behind, so welds do not cross or crowd each other.
• Material. The grade and thickness of the original, or as required by the renewal, with mill certificates.
• Fit-up. Bevels prepared, an even root gap, the plate in line with the surrounding structure.
• Welding sequence. Welds laid in an order that controls shrinkage, so the panel stays fair.
• Testing. Visual inspection of every new weld and NDT of butts and seams as agreed.

#When a doubler still makes sense

To keep a ship trading until a planned docking, a doubler with a clear expiry date can be the sensible choice — provided class agrees, the extent is known from measurements and the permanent repair is already in the plan.

#The point

Renewal takes longer than a patch. But it closes the item, and the ship does not come back to the same plate at the next survey.