USCG issued a policy letter providing guidance to Officers in Charge, Marine Inspection (OCMI), the Towing Vessel National Center of Expertise (TVNCOE), and Third Party Organizations (TPO), on the use of doubler plates for repairs involving towing vessels constructed of steel.
or the purposes of this policy, USCG explains that a “doubler plate” or “doubler” means a steel plate, compatible with a vessel’s existing steel plate, which is lap welded to a damaged part of that vessel’s structure to regain its original local strength or to restore watertight integrity. The doubler plate should possess physical characteristics equal to or better than that of the original plate.
A doubler plate repair can lead to increased stress concentrated in the area of the repair. In such cases, instead of relieving the stress, the repair can worsen the original problem and lead to a situation of greater risk to the vessel. In addition, a patchwork of doubler plates obstructs the ability to assess the true condition of the hull and may compromise the original strength of the steel.
In fact, the Towing Safety Advisory Committee (TSAC) published a report on 2014, titled “Recommendations Regarding Steel Repair of Inspected Towing Vessels on Inland Service.” The report partially concurs with the recommendation for the continued use of doublers as a permanent repair on steel hulled towing vessels. These guidelines state doublers should be used only as temporary repairs with the exception of very limited applications.
However, the US Coast Guard maintains that permanent doublers are “highly discouraged” and there are very few instances where a doubler is an acceptable repair.
The Coast Guard recognizes that the use doublers in the towing vessel industry has been a common practice. However, as the NTSB identified, poorly installed or fitted doubler plates may lead to the loss of watertight integrity of a vessel and subsequent sinking. As towing vessels are in drydock, the Coast Guard will evaluate the adequacy of existing doublers to assess if further policy is needed to address the removal of existing doublers
Considering the above, USCG recommends OCMIs, TVNCOE, and TPOs to use the following when repairs involving doubler plates are used on steel hull towing vessels:
- New doubler plates should not be permitted for permanent repairs except for allowances in reference (b). Examples where doublers may be approved as permanent repairs include: wastage doublers used in non-strength areas to restore watertight integrity; local reinforcement at overboard discharges or seachests; keel cooler doublers that do not extend beyond the keel cooler; wear plates above propellers; and on sound plating in areas highly susceptible to corrosion as a means for protection.
- New Doubler Plates as Temporary Repairs: After evaluation by the OCMI or an appropriately qualified TPO surveyor, doubler plates may be authorized for temporary repairs in limited circumstances. This evaluation may be limited to the effectiveness of the doubler at restoring watertight integrity and a visual inspection, if possible, to assess the doubler fit up and condition. Doubler plates used for temporary repairs should be removed as soon as possible, but no later than at the next drydock, unless the OCMI or TPO requires permanent repairs to be completed earlier.
- New Doubler Plates as Permanent Repairs: An MI or an appropriately qualified TPO surveyor should visually inspect all new doubler plate repairs for workmanship and verify the doubler plate is free of sharp edges, burrs, carburized flame cut material, and weld spatter. If an onsite inspection of the doubler plate repair is not possible, then the MI or appropriately qualified TPO surveyor may accept documentation suitable to ascertain workmanship and finish such as photographs, non-destructive testing reports or other objective evidence suitable to the US Coast Guard. The doubler plate should be sand or shot blasted to remove rust and scale prior to this inspection. After obtaining MI or TPO approval, the doubler plate repair must be given a complete coat of primer and topcoat based on the environment and compatibility with coatings on the existing structure. For a
new doubler plate repair to be deemed permanent, at a minimum, a certified nondestructive testing Level II inspector should inspect the welds involved in the installation of the doubler plate and provide a report to the TPO or the cognizant OCMI.
- Existing Doubler Plate Repairs: Currently, existing doublers may remain on steel hulled towing vessels. However, OCMIs and TPOs may require existing doublers to be removed if the fit up and installation is not acceptable or creates an unsafe condition. CG-CVC will coordinate with the Traveling Marine Inspectors and specific units to collect data on the adequacy of existing double plates for potential future policy development.