Welding repair review
TWI first conducted a weld repair methods review, taking into account the critical environment, the (difficult) access, materials involved, weld repair method and equipment. The following methods were considered: structural weld overlay (SWOL), excavated groove weld, external overplating, internal overplating, or an inserted plate. The review concluded that the best repair option for both cracks was an inserted plate. An excavated groove weld was identified as the second best option.
First repair
The inserted plate method was selected for the first repair, of the through-thickness crack. As this method required removing a section of the leg wall at a water depth of 15m, a cofferdam (fig 1) would need to be prepared and made watertight prior to the damaged plate being cut out and removed, and the repair area then dried for more than 12 hours. The plate with the crack would then be completely removed and a new one welded into place. The weld repair had to be carried out from inside the leg so a method was devised of positioning the root pass ceramic backing tiles prior to fitting the insert plate. The ceramic tiles can be seen in fig 1 as the cofferdam is retrieved to the platform following successful completion of the repair.
However, it has been determined that the repair would be the more efficient and reliable with an insert plate. The removal of the defective area and the addition of a new plate would avoid structural change to the original design. This kind of repair provides an equivalent service life to the original fabrication.
To ensure a fully efficient repair and procedure, a full-scale simulation of the repair process was first carried out in controlled environment at the premises of one of the company’s subcontractors.
Finally, when the repair was carried out offshore, a TWI engineer stayed onsite to ensure the welding procedure was correctly followed at all stages. The welding operation was carried out without problems and the weld was fully inspected using ultrasonic and magnetic particle techniques with no recordable defects being found.
During the welding operation a very close watch was kept on the cofferdam as failure of this component could have caused a catastrophic flood into the leg.
Second Repair – procedure development
The second crack did not extend fully through the wall thickness. This meant that excavating the crack without breaching the wall was a possibility, and simpler than another cofferdam repair. However, the underwater location of the repair presented complications. The seawater on the opposite side of the repair site prevented any preheat, and also would lead to a rapid cooling rate of the weld, creating a hard and brittle heat-affected zone (HAZ) prone to further crack initiation and propagation.