Welding-underwater,

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Welding Profession for Commercial Divers

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Welding-underwater

A general overview of the main types of processes employed is presented in our other page that can be seen by clicking on
Underwater Welding.

In the present page we are going to dig more deeply to clarify certain details.

Some subjects of Welding Underwater were not sufficiently covered in the previous page above.

Anyone wishing to prepare for a Career in Welding Underwater should study very thoroughly theory and practice of this demanding job.

As a Career it is a further specialization for Commercial Diving professionals.

It is essential to absorb all the available information on under water welding presented in our pages.


Visit the NEW Page on Welding Overview, for a thorough
Introduction to Welding.
Visit also the NEW Page on Process-selection, for
Understanding the Selection procedure and
the NEW Page on Process-optimization for
improved productivity.


An Important Book on Welding-underwater

Please be advised that we were authorized by the Author, David J. Keats, of Speciality Welds Ltd., to publish in our Welding Advisers Website,
three previews of his important Book on Underwater Welding titled
"A Welder's Mate".

As it is essential welding reading for anyone involved in building underwater structures and a perfect reference for all welders, we urge our interested readers to read the first excerpt from Chapter 1 by clicking on A Case for Wet Welding.

You will find there the links to other two previews and to the book itself.

Additional information, including the Reference to this important Book on Welding-underwater, can be found in our other page available by clicking on
Underwater Welding.

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Specifications

A well known Specification for Welding-underwater was ANSI/AWS D3.6.
It is now superseded by the more recent Underwater Welding Code with the same number.

The essential information is important to all involved:

  • To the welding engineer, for selecting the recommended Welding-underwater process (wet or dry),
  • To the stress engineer, for verifying the requirements of fitness for purpose,
  • To the design engineer, for specifying details in structural fabrication,
  • To the purchaser, for establishing acceptance requirements,
  • To the contractor, for estimating means and costs of the project.

This Code presents four different Types or Classes for Welding-underwater:

Type A welds are characterized by requirements ensuring a weld, performed by Welding underwater, comparable to a surface weld.
Usually dry (hyperbaric) Welding-underwater meets these requirements.

Type B welds refer to wet Welding-underwater weldments. Requirements are appropriate for less critical applications, despite reduced ductility and increased porosity.

Type C welds have even less requirements than type B welds for applications without load-bearing functions.

Type O welds present requirements of surface dry welds, as well as those of other Codes and Specifications.

ANSI/AWS D3.6M:2010
Underwater Welding Code
Edition: 5th
American Welding Society / 10-Sep-2010 / 144 pages
Click to Order.

Other Welding underwater documents have been developed by different agencies.

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Economic Importance

We may remind that the main drive to the development of Welding-underwater technology is economic, because in spite of the complexity, it permits to save much time and expense when compared with the need of pulling the structure to build or to repair to a dry dock.

Offshore platforms and structures, used for the exploitation of submarine oil fields, are constructions whose continuous availability has important economic consequences.

It is customary to provide a thorough inspection at least once every five years unless unscheduled accidents require immediate intervention.

Damages requiring repair may be due to corrosion or fatigue, or even to impact from massive moving objects.

Physical Conditions influence

Here please find some more insight into the influence of the physical conditions of Deep Water Welding onto the metallurgical microstructures obtained and on the soundness of the welded constructions.

The main factors influencing wet Welding-underwater outcome are the following:

  • the rapid cooling rate provided by the surrounding water quench,
  • the influence of pressure, increasing with depth,
  • the limiting Carbon Equivalent permissible,
  • the concern for Hydrogen cracking,
  • the presence of porosity and its influence on fatigue life.

Electric Arc

The process most commonly used for Welding-underwater is SMAW (Shielded Metal Arc Welding with covered Electrodes of special types) although FCAW (Flux Cored Arc Welding) has been also employed.

Wet Welding-underwater has been carried out as a building or repairing technique at depths of 100 m (330 ft) and down to 200 m (660 ft) under sea level and over.

As already remarked in the previous page (see above), Welding-underwater by the Electric Arc is performed in either the wet environment or in a protective chamber to be constructed around the weld zone and evacuated of water by compressed helium gas, to create the dry conditions.

Although the quality might be higher, the cost of hyperbaric Welding-underwater is much higher than that of wet welding. Assembling and dismantling the additional chamber equipment can be very expensive in a deep-sea environment.

Depth Influence

In both cases pressure increases with depths: this fact brings about unexpected consequences like a decrease of Silicon and Manganese and an increase of Carbon in the weld pool, correlated to an increase of Oxygen.

Also Hydrogen content may increase with depth, with increased danger of cracking. And the arc is less stable with increasing depth, because the high ionization potential of Hydrogen makes it more difficult to sustain the arc.

Hydrogen Cracking

Wet Welding-underwater of higher strength steels is very susceptible to Hydrogen cracking because Hydrogen is more available in presence of water.

To limit the tendency to Hydrogen induced cracking certain provisions may be attempted:

  • designing the weld for minimum stress, using improved fit up,
  • reducing weld deposits or using temper beads to partially relieve the stresses of a previous weld,
  • using special consumables like austenitic stainless or nickel electrodes that can hold a higher concentration of Hydrogen in molten weld pools,
  • introducing carbonates in the flux coatings which, upon decomposing, deliver alternate gases in the welding plasma, thereby reducing Hydrogen content,
  • using the addition to the weld pool of specific elements that getter the Hydrogen by forming hydrides.

Porosity

Another considerable concern in wet Welding-underwater is weld metal pore formation which increases with pressure (depth).

Weld metal porosity due to Hydrogen absorption, in the case of wet welding, can be minimized by altering welding parameters as follows:

  • using a low current with direct current electrode positive (DCEP) (but this polarity produces electrolysis that destroys metals in the electrode holder),
  • a high current with direct current electrode negative (DCEN),
  • a short arc, and
  • a fast travel speed.

Alternating current is not used in wet Welding-underwater.

CE and Quench Rate

For wet repair welding, the Carbon Equivalent of the base metal (steel) should be limited to max. 0.40 %, when calculated using the following accepted formula:

CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Ni+Cu)/15

But the rapid cooling rate provided by the surrounding water quench must be considered. Whereas in normal dry conditions the time for cooling from
800 to 500 oC is generally between 8 and 16 s (second), under water the same cooling interval is covered in 1 to 6 s, depending on joint thickness and heat input.

Obviously this rapid quench rate in wet Welding-underwater, produces significant amounts of heat affected zone (HAZ) martensite in carbon steels and has measurable consequences on microstructure and properties of welds.

Hardness of 400 Vickers measured under a load of 10 kg can be found in the weld when the Carbon Equivalent is about 0.40%.

This hardness may be considered medium to high, and quite worrying as an indication of limited ductility.

The major changes in microstructure and in mechanical properties occur in the first 50 m (165 ft) of depth. At greater depths, the weld metal composition and microstructure remain fairly constant.

If necessary, compositional modifications of consumables for wet Welding-underwater must be studied to provide for suitable properties.

Friction Welding Developments

The application of friction welding as a repair procedure to be conducted underwater was attempted in the last decades, especially in England by
The Welding Institute, (www.twi.co.uk) with progressive involvement and success.

One of the most important advantages of friction welding is the absence of liquid phase, with the complete elimination of the influence of gases (Hydrogen and Nitrogen) and the fact that the metal composition is not altered, with consequent uniformity of results independent of depth.

Therefore a procedure demonstrated at a certain depth, needs not be re-qualified at another one. This fact would represent a considerable advantage for friction welding, as for all other Welding-underwater processes re-qualification is a requirement.

A process was developed with the intent of providing a method of repair for linear cracks. A sort of stitch welding was employed by which subsequent friction welds of tapered studs (short conical rods) were performed in precise holes to bridge the gap between the two sides of a crack.

The continuity was assured by a certain overlap of the rods along the crack. And the integrity was demonstrated by destructive bending tests.

Future Automatic Equipment

Special Welding equipment and procedures were and are developed to perform this kind of repairs. Most of the functions of the welding head were designed to be performed automatically.

This is due to the known fact that at a depth of more than 400 meters under sea level, the ability of human divers-welders to concentrate in performing demanding tasks deteriorates rapidly.

Deep water is that at more than 400 m, and ultra-deep water at about 1500 m.

The trend of these developments is for increasing the independent autonomy of systems in order to dispose of the requirement for manual operation of divers.

The programmed developments of automatic systems will enable to perform pipe laying down and Welding-underwater even at extreme depths still not heard of, without manual intervention.

Two relevant notes were published in Issue 69
of Practical Welding Letter for May 2009.

An article on automatic Underwater Flash Butt Welding of Pipes in section (2)
and a short note on Underwater Friction Stud Welding in section (11).
Click on PWL#069 to see both.

Download the following:
U.S. Navy Underwater Cutting & Welding Manual (200 pages)
http://www.hnsa.org/doc/pdf/cut_weld.pdf

An Article on Developments in Underwater wet Welding was published In Section 11 in Issue 29 of Practical Welding Letter for January 2006. To read the article click on PWL#029.

An Article on Filler Metals for Underwater Welding (2) was published in Issue 48 of Practical Welding Letter for August 2007.
Click on PWL#048 for reading it.

An Article on Electrodes for Underwater Welding was published (4) in Issue 61 of Practical Welding Letter for September 2008. In the same issue there is also a note (11) on Underwater Welders Pay.
Click on PWL#061 to read them.

An Article on Underwater Inspection and Welding was published (3) in Issue 116 of Practical Welding Letter for April 2013.
Click on PWL#116 to see it.

An Article on Underwater Laser Cutting for Nuclear Decommissioning was published (7) in Issue 138 of Practical Welding Letter for February 2015.
Click on PWL#138 to see it.

An Article on THERMAL SPRAY ALUMINUM COATINGS FOR SPLASH ZONE STRUCTURES was introduced in a publication (iTSSe) reported (8) in Issue 148 of Practical Welding Letter for December 2015.
Click on PWL#148.

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Watch the following Video on
Diver Repairing Corrosion Damage.

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Hardness Testing
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It is always important to know the hardness of any material before welding.

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Welding-underwater is making rapid progress towards automatic implementation, thanks also to friction welding...