A 334 meter long containership with a gross tonnage of 91,649 t, equipped with an open loop scrubber system, experienced a cavitated/corroded hull section with substantial material loss at the penetration of the super duplex scrubber overboard discharge pipe and the carbon steel hull, approx. 3,900 mm above BL.

Previous Repair

The owner had the damage previously repaired through a local company by welding a doubler plate underwater over the compromised area, without the use of class approved procedures, welders, unknown material for the doubler plate as well as unknown underwater wet welding consumables. After the classification society rejected the repair, the owner turned to SGS for help. SGSs welding department developed a repair procedure, which was submitted and sequentially approved by class for a temporary repair to bridge the time until the next scheduled dry dock of the vessel, at which time the permanent repair will be performed.

Hull Damage

The hull material iwo the penetration was identified on the shell expansion drawing as 16 mm thick higher tensile steel grade AH32. The overboard discharge pipe was identified as a DN 300 SCH. 80 fabricated from 2505 super duplex steel, with specific levels of Cr, Ni, Mo, and N.

Subsea Global Solutions Attendance

Attending the vessel, SGS divers performed an initial inspection dive to confirm the locationand condition in way of the area of concern.

Repair

Following the inspection dive, SGS divers removed the previously installed doubler plate including marine growth, corrosion protection and other foreign material from the repair area, followed by cleaning the cavitated/corroded hull section to bare metal. Once the compromised area was cleaned, it was filled with a two-part underwater epoxy, followed by placing a 12 mm thick higher tensile steel plate grade AD/DH36 over the treated area and welded to the hull and to the OD of the overboard discharge pipe between the 12 o’clock and 5 o’clock position.

The weld metal between the ferritic hull and the ferritic doubler plate was deposited by welding stringer beads with a HYDROWELD FS electrode, a ferritic underwater wet welding electrode. The weld metal between the ferritic doubler plate and the duplex pipe section was deposited by welding stringer beads with a Ni based underwater wet welding electrode.

All welds were executed as multiple layer fillet welds.

Exhaust Gas Cleaning Systems (EGCS)

Scrubbers, or exhaust gas cleaning systems (EGCS), are becoming a more frequently used technology for ships to achieve compliance with the 2020 IMO sulphur cap. A scrubber sprays seawater or chemically treated freshwater into exhaust gases. Sulfur dioxide reacts with the alkaline water, and in open-loop systems the resulting wash water is discharged through overboard discharge pipes.

For overboard discharge pipes in ship scrubber systems, materials are chosen to withstand corrosive wash water, acidity, and seawater exposure. Commonly used materials include:

• Duplex stainless steels – very common due to high corrosion resistance and strength
• Super duplex stainless steels
• High-alloy austenitic stainless steels (254 SMO)
• Stainless steels (300 series)

The above listed stainless steels offer superior corrosion resistance and strength, required for the discharge of the exhaust gas cleaning systems. Since the hulls of most commercial vessels are made from carbon steel, the afore mentioned stainless steels in combination with the carbon steel hull and the exposure to an electrolyte (sea water) may create galvanic corrosion if the corrosion protection at the interface between the dissimilar metals is compromised.

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Corrosion Protection

After welding and NDE was completed, an underwater two-component epoxy coating corrosion protection was applied over the areas where the corrosion protection had to be removed or burned off during welding, to assist in the protection and reduction of metal wastage in those areas due to immersion in salt water by providing permanent anticorrosive protection.

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Underwater Welding with Subsea Global Solutions

Subsea Global Solutions provides specialized underwater welding and structural installation services for offshore vessels, drilling units, and marine assets, including hull-mounted piping, structural reinforcements, and subsea support systems—all performed without the need for drydocking. Our certified commercial dive teams perform underwater wetwelding, fabrication, inspection, and corrosion protection in accordance with approved procedures and class requirements, helping operators maintain critical systems while reducing downtime. Learn more about our underwater welding and structural installation capabilities here.