Underwater Welding & Inspection: A presentation-Part 1 of 2

Underwater Welding & Inspection: A short Presentation Part 1 of 2 [With PDF]

What is Underwater Welding?

Underwater welding, as the name properly hints, is the welding inside the water. This is mainly used to repair offshore pipelines and structures, ships, submarines, and nuclear reactors. Joining two metal pieces underwater requires a lot of safety considerations. The purpose of this presentation is for general knowledge only on how “underwater welding” is carried out. Safety is also emphasized here as we are dealing with two types of activities, Diving, and Welding. One must remember that underwater welding is a different world, and so special precautions are adhered to for maximum safety of the welder/diver.

Introduction of Underwater Welding

Underwater welding began during World War 1 when the British Navy used it to make temporary repairs on ships. Those repairs consisted of welding around leaking rivets of ship hulls. Underwater welding was also restricted to salvage operations and emergency repair works only. In addition, it was limited to depths below the surface of not over 30 ft (10 meters).

At first, underwater welding was just applied to weld a patch until a more thorough repair could be performed. But as soon as more experience was gained, ambitious individuals and companies joined forces to improve results and to establish achievable specifications.

Use of Underwater Welding

While underwater welding has been used for new construction & installation of offshore structures, subsea pipelines, & harbor facilities, it is most often used for maintenance and repair applications. These include repair of damage caused by corrosion, fatigue, and accidents of offshore structures such as oil platforms, repair & replacement of damaged subsea pipeline sections, repairing holes in ship’s hulls or collision damage to harbor facilities.

Judging from the photographs shown below (Fig. 1), it is obvious that these structures have to be repaired. And one of the tools of repair is underwater welding.

Damaged Offshore Structure
Fig. 1: Damaged Offshore Structure

The Welder

  • Welder – is a certified welder who is also a commercial diver, capable of performing tasks associated with commercial subsea work, weld setup and preparation, and who has the ability to weld in accordance with the AWS D3.6, Specification for Underwater Welding (wet or dry), and other related activities.
  • Welder qualifications required for a given assignment vary from project to project. Most diving contractors would like their welder/diver to be “a jack of all trades”. This means that the welder/diver must know how to do underwater cutting, fitting and rigging, inspection and nondestructive testing, and underwater photography.

Classification of Underwater Welding

Underwater welding is classified into two categories.

  • Welding in-the-wet environment = this was used primarily for emergency repairs and salvage operations in shallow waters due to poor quality welds.
  • Welding in-the-dry environment = this technique produces high-quality welds that meet X-ray and code requirements.

Welding in-the-wet environment

As the name implies, underwater wet welding is done in an environment where the base metal and the arc are surrounded entirely by water. The electrode types used conform to AWS E6013 classification. These electrodes are waterproofed by wrapping them with waterproof tape or by dipping it into special sodium silicate mixes and allowing them to dry. The power source is a direct current machine rated at 300 or 400 amperes.

The power of the arc generates a bubble of a mixture of gasses that lets metal melting and joining occur more or less normal as shown in Fig. 2.

Welding in the wet and dry environment
Fig. 2: Welding in the wet and dry environment

Welding in-the-dry environment

Welding in-the-dry environment is again divided into two:

  • Hyperbaric welding, in which a chamber is sealed around the structure to be welded and is filled with breathable gas at the prevailing pressure.
  • Cofferdam welding, which is carried out in the dry, in air, where a rigid steel structure to house the welders is sealed against the side of the structure to be welded, and is open to the atmosphere.

Hyperbaric welding is done with the use of a welding chamber or habitat. This method provides high-quality weld joints that meet radiography and code requirements. The chamber is sealed into a structure or pipeline and filled with a breathable mixture of helium and oxygen (90-95% helium and 5-10% oxygen).

Cofferdam welding is also a type of dry welding where a rigid steel structure to house the welders is sealed against the side of the structure to be welded and is open to the atmosphere. It is normally used for harbor works or ship repair

Photographs in Fig. 3 are examples of Cofferdam Welding

Cofferdam Welding
Fig. 3: Cofferdam Welding

Principle of Operation

The figure (Fig. 4) below right shows the general arrangement for underwater welding. Underwater welding should always be a direct current machine grounded to the ship. The welding circuit includes a knife switch that is operated on the surface by an assistant upon the signal of the welder/diver. The knife switch cuts off the welding current and is design this way for safety reasons. The electrode holder utilizes a twist type head for gripping the electrode. The work lead is attach within 3 ft. from the point of welding and is perfectly insulated to avoid leaks. The welding circuit should be direct current electrode negative. Refer Part 2 of this article for more details…..

General Arrangement for underwater welding
Fig. 4: General Arrangement for underwater welding

Few more Resources for you..

Underwater Welding & Inspection: A short Presentation Part 2 of 2
General requirements for Field Welding
Piping Design and Layout

Reference

https://www.sciencedirect.com/topics/engineering/underwater-welding

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