Underwater Welding

AMIT K. SONCHHATRA (11BME081)ATHAR KOTHAWALA (11BME082)

KARAN C. PRAJAPATI (11BME083)YUDHISTHIR RAMNANI(11BME084)RAJ ANUPAM (11BME085)

Knowledge Corridor, Raisan, Gandhinagar-382007,Gujarat

INTRODUCTION

•  The fact that electric arc could operate wasknown for over a 100 years. The first everunderwater welding was carried out by BritishAdmiralty - Dockyard for sealing leaking shiprivets below the water line.

•  Underwater welding is an important tool forunderwater fabrication works.

•  In 1946, special waterproof electrodes weredeveloped in Holland by ‘Van der Willingen’.

CONTD.

•  In recent years the number of offshorestructures including oil drilling rigs, pipelines,platforms are being installed significantly.•  Some of these structures will experience failuresof its elements during normal usage and duringunpredicted occurrences like storms, collisions.Any repair method will require the use ofunderwater welding.

CONTD.•  Underwater welding is the process of welding at

elevated pressures, normally underwater. Underwater weldingcan either take place wet in the water itself or dry inside aspecially constructed positive pressure enclosure and hence adry environment. It is predominantly referred to as"hyperbaric welding" when used in a dry environment, and"underwater welding" when in a wet environment

•  The applications of underwater welding are diverse—it is oeenused to repair ships, offshore oil platforms,

and pipelines. Steel is the most common material welded

CONTD.•  Dry hyperbaric welding is used in preference to wet

underwater welding when high quality welds are requiredbecause of the increased control over condiVons which can beexerted, such as through applicaVon of prior and postweld heat treatments•  This improved environmental control leads directly to

improved process performance and a generally much higherquality weld than a comparative wet weld. Thus, when a veryhigh quality weld is required, dry hyperbaric welding isnormally utilized

CONTD.•  Research into using dry hyperbaric welding at depths of up to 1,000 metres (3,300 e) is ongoing. In general, assuring the integrity of underwater welds can be difficult (but is possible using various non-destructive testing applications), especially for wet underwater welds, because defects are difficult to detect if the defects are beneath the surface of the weld.

Classification1.  Dry2.  wet

DRY WELDING•  Dry hyperbaric welding involves the weld beingperformed at the prevailing pressure in achamber filled with a gas mixture sealed aroundthe structure being welded.•  Most welding processes SMAW, FCAW, GTAW,GMAW, PAW could be operated at hyperbaricpressures, but all suffer as the pressureincreases. Gas tungsten arc welding is mostcommonly used.

CONTD.

•  The degradation is associated with physicalchanges of the arc behavior as the gas flow

regime around the arc changes and the arc rootscontract and become more mobile.

•  Of note is a dramatic increase in arc voltagewhich is associated with the increase in

pressure. Overall a degradation in capability andefficiency results as the pressure increases.

•  Although, a large number of techniques areavailable for welding in atmosphere, many ofthese techniques can not be applied in offshoreand marine application where presence of wateris of major concern.•  In this regard, it is relevant to note that, a greatmajority of offshore repairing and surfacing workis carried out at a relatively shallow depth, in theregion intermittently covered by the waterknown as the splash zone.

CONTD.

•  Though, numerically most ship repair and welding jobsare carried out at a shallow depth, most technologicallychallenging task lies in the repairing at a deeper waterlevel, especially, in pipelines and occurrence/creation ofsudden defects leading to a catastrophic accidentalfailure.

•  The advantages of underwater welding are ofeconomical nature, because underwater-welding for

marine maintenance and repair jobs bypasses the needto pull the structure out of the sea and saves valuabletime and dry docking costs.

•  Special control techniques have been appliedwhich have allowed welding down to 2500m

simulated water depth in the laboratory, but dryhyperbaric welding has thus far been limitedoperationally to less than 400m water depth bythe physiological capability of divers to operatethe welding equipment at high pressures andpractical considerations concerning constructionof an automated pressure / welding chamber atdepth

ADVANTAGES

1)  Welder/Diver Safety - Welding is performed in a chamber,immune to ocean currents and marine animals. The warm,dry habitat is well illuminated and has its ownenvironmental control system (ECS).

2)  Good Quality Welds - This method has ability to producewelds of quality comparable to open air welds becausewater is no longer present to quench the weld and H2 levelis much lower than wet welds.

3)  Surface Monitoring - Joint preparation, pipe alignment, NDTinspection, etc. are monitored visually.

4)  Non-Destructive Testing (NDT) - NDT is also facilitated bythe dry habitat environment.

DISADVANTAGES

1)  The habitat welding requires large quantities of complexequipment and much support equipment on the surface.The chamber is extremely complex.2)  Cost of habitat welding is extremely high and increases with

depth. Work depth has an effect on habitat welding.3)  At greater depths, the arc constricts and corresponding

higher voltages are required.4)  The process is costly - a $ 80000 charge for a single weld

job. One cannot use the same chamber for another job, if itis a different one.

WET WELDING•  Simply means that job is performed directly in the water•  It involves using special rod and is similar to the process inordinary air welding

PRINCIPLE OF OPERATION•  The process of underwater wet welding takes in the followingmanner:

•  The work to be welded is connected to one side of an electric circuit,and a metal electrode to the other side.

•  These two parts of the circuit are brought together, and thenseparated slightly. The electric current jumps the gap and causes asustained spark (arc), which melts the bare metal, forming a weldpool.

•  At the same time, the top of electrode melts, and metal droplets areprojected into the weld pool. During this operation, the flux coveringthe electrode melts to provide a shielding gas, which is used tostabilize the arc column and shield the transfer metal.

•  The arc burns in a cavity formed inside the flux covering, which isdesigned to burn slower than the metal barrel of the electrode.

WET WELDING

PARAMETERS•  Power supply : DC 300 OR 400 AMP

•  Polarity : straight polarity

EFFECT OF WET ENVIROMENT

•  WATER = HYDROGEN + OXYGEN•  Dissolve in weld pool

•  Solubility decreases and then comes out = porosity•  Oxygen as solid , liquid inclusions or gases

•  Hydrogen combines with oxygen forming vapor.

V-groove wet weld deposited at 100 m depth (a) and its radiographic image

Formation of Hydrogen

• Once the welding circuit is completed by striking an arc, the heat of the arc is sufficient to vaporise the surrounding water. Hence, the arc is surrounded by a vapor shield. However, reaction with the molten metal influences the composition of this vapor and it comprises of about 70% hydrogen, 25% carbon dioxide and 5% carbon monoxide.

• The risk of hydrogen induced cracking increases as welding depth increases in water (sea/ocean).

• D4301, D4313 and D4327 are electrode classification in JIS (Japanese Welding Society).

• Actually these electrodes are used in arc welding• For underwater welding, special type of ilmenite

coating is applied around the electrodes• Picture Courtesy: Kobelco Welding and Hyundai

Welding Co. Ltd.

ADVANTAGES

1)  The versatility and low cost of wet welding makes thismethod highly desirable.

2)  Other benefits include the speed. With which the operationis carried out.

3)  It is less costly compared to dry welding.4)  The welder can reach portions of offshore structures that

could not be welded using other methods.5)  No enclosures are needed and no time is lost in building.

Readily available standard welding machine and equipmentsare used. The equipment needed for mobilization of a wetwelded job is minimal.

DISADVANTAGES•  Rapid quenching decreases impact strength and ductility.•  Hydrogen embrittlement.

•  Poor visibility in water.•  Higher energy density of hydrogen, higher efficiency.

REQUIREMENTS•  Power supply requirements - 400 amp or larger. DC

generators, motor generators and rectifiers are acceptablepower supplies

•  Power converters.•  Welding Generator, Pre-Setup

•  Polarity.•  Diesel Driven Welding Generator Amperage and Voltage settings.

•  Gas Manifolds

Figure showing schematic diagram forunderwater welding or cutting

DANGERS AND DIFFICULTIES

•  Hydrogen and oxygen are dissociated from thewater and will travel separately as bubbles.

•  Oxygen cuung is about 60 percent efficient

•  Above river beds, especially in mud, becausetrapped methane gas in the properconcentrations can explode.

CONTD.

•  There is a risk to the welder/diver of electricshock.•  There is a risk that defects may remainundetected

•  The other main area of risk is to the life or healthof the welder/diver from nitrogen introducedinto the blood steam during exposure to air atincreased pressure

SAFETY MEASURES

•  Start cuung at the highest point and work downward

•  By withdrawing the electrode every few seconds to allowwater to enter the cut

•  Gases may be vented to the surface with a vent tube(flexible hose) secured in place from the high pointwhere gases would collect to a position above thewaterline.•  Precautions include achieving adequate electrical

insulation of the welding equipment

•  Areas and voids must be vented or made inert.

DEVELOPMENTS

•  Wet welding has been used as an underwater weldingtechnique for a long time and is still being used.•  With recent acceleration in the construction of offshorestructures underwater welding has assumed increasedimportance.•  This has led to the development of alternative welding

methods like friction welding, explosive welding, and studwelding. Sufficient literature is not available of theseprocesses.

SCOPE•  Wet welding is still being used for underwater repairs, but thequality of wet welds is poor and are prone to hydrogen cracking.•  Dry Hyperbaric welds are better in quality than wet welds. Presenttrend is towards automation. THOR - 1 (TIG Hyperbaric OrbitalRobot) is developed where diver performs pipe fixing, installs thetrace and orbital head on the pipe and the rest process isautomated.•  Developments of driverless Hyperbaric welding system is an evengreater challenge calling for annexed developments like pipepreparation and aligning, automatic electrode and wire reelchanging functions, using a robot arm installed. This is in testingstage in deep waters.•  Explosive and friction welding are also to be tested in deep waters.

REFERENCES1)  D. J Keats, Manual on Wet Welding.

2)  Annon, Recent advances in dry underwater pipeline welding,Welding Engineer, 1974.

3)  Lythall, Gibson, Dry Hyperbaric underwater welding, WeldingInsVtute.

4)  W.Lucas, InternaVonal conference on computer technology inwelding.

5)  Stepath M. D, Underwater welding and cuung yields slowly toresearch, Welding Engineer, April 1973.

6)  Silva, Hazlel, Underwater welding with iron - powder electrodes,Welding Journal, 1971.

7) wikipedia.org

8) Amit Mukund Joshi, Underwater Welding, JRF, IIT-Bombay,