There are four basic types of UWW techniques in use today.

15.3.1 Dry Hyperbaric Chamber Process (See Fig. 15.3)

(i) Weldment and welder completely enclosed.

(ii) Weld properties similar to air welds.

(tit) Equipment-bulky, costly, and complex.

(iv) Fit-up time is more.

(v) Two or more support ships and a crane are needed.

(b) Hot-tap welding of pipelines Fig. 15.3 Use of Hyperbaric chambers (Habitat welding)

(c) Making Weld-ball pipeline joint Fig. 15.3 Use of Hyperbaric chambers (Habitat Welding)

15.3.2 Local Chamber Welding (See Figs. 15.4, 15.5 (b) and 15.6)

(i) Weldment in dry environment.

(ii) Weld properties are similar to air welds.

(iii) Equipment is not as bulky and costly.

(iv) Fit-up time is less.

(v) Usually requires a small crane.

Tube to wire Gas switch Wire feed trigger control

"Dry spot” design

feed

(a) Portable dry spot (PDS) welding

(b) Stages in the repair of damaged riser using Local Dry Environment “Hydrobox' Fig. 15.5 Underwater dry welding

Fig. 15.5 Underwater dry welding

15.3.4 Wet Welding

(i) Weldment and welder both exposed to water.

(ii) Weld properties are inferior to air welds.

(iii) Standard air welding equipments can be used.

(iv) No fit-up time or negligible fit-up time.

(v) Process is convenient.

Advantages of Wet-Welding

1. Welders can reach positions inaccessible by other methods.

2. Process is fast.

3. Cost of welding is very low.

4. More freedom of repair design and fit-up.

5. Standard welding equipment could be used.

Disadvantages of Wet-Welding

Due to direct contact of the arc and the molten weld-pool with water, there is a ‘Quench­ing’ effect that increases tensile strength but reduces ductility. The porosity and hardness also increase.