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Weld procedure developed for underwater repair

Background

TWI recently developed a new weld procedure that overcomes difficulties associated with carrying out underwater weld repairs, for a Member company in the oil and gas sector, operating an FPV (floating production vessel) situated midway between Aberdeen and Stavanger.

The company found two fatigue cracks in one of the legs of the vessel. The cracks, situated inside the leg, 15m below sea level and immediately above the tank top of the pontoon, required permanent repair. One of the cracks was through-thickness (17mm wall thickness) and 109mm long; the other only partially penetrated through the wall thickness. The repair was to be completed within a time period of six months.

The company first consulted TWI to provide expert advice on the different repair possibilities. TWI’s involvement then extended to developing a new repair welding procedure specific to the particular circumstances, and assisting onsite during the repair to oversee the correct application of the welding procedure.

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.

Figure 1: Cofferdam used for the insert plate
Figure 1: Cofferdam used for the insert plate

To overcome these problems, TWI developed a new method of dry repair by insulating the seawater side of the plating during weld repair with neoprene rubber pad (fig 2). This significantly retarded the cooling rate of the weld, avoiding or substantially reducing the risk of hydrogen cracking in the HAZ.

Tests were carried out in TWI’s welding laboratory using a piece of neoprene adhesively bonded to one side of a 25mm S355J2 plate, which was then partially submerged in cold water to provide a quench effect. Preheat of 50⁰C was applied by oxy-propane on the ‘dry’ side, with the neoprene providing insulation from the water. TWI conducted non-destructive and mechanical tests to measure the repair effectiveness. All test assemblies were subjected to MPI (magnetic particle inspection) following a 24-hour delay period. Then bend test samples from all plates were bent to 180° over a 4t former, and hardness surveys were carried out to include unaffected parent metal, HAZ at cap and base of groove, and weld metal. The results showed a peak hardness of 290HV: significantly lower than without an insulated plate and safely below the maximum value of 380HV allowed by the specification. The results achieved with an insulating pad were identical to those achieved for welding in air using identical preheats.

Onsite support

TWI provided onsite support on the FPV throughout the repairs, which were performed successfully. Assistance available included assessing the criticality of a repair, helping identify the potential difficulties to take into account when determining a welding procedure, and ensuring correct practice onsite during the execution of the repair.

For more information please email contactus@twi.co.uk.

Figure 2: Weld procedure development for the second repair
Figure 2: Weld procedure development for the second repair

For more information please email:


contactus@twi.co.uk