Double Jointing Yards

In many cases, when pipeline construction infrastructure allows, it is feasible to make use of double and triple joining of pipe joints.

Pipe welding can be done:
— internally and externally,
— externally only.

The entire process consists of the following stages:
1. Pipe end preparation — if required (see Pipe Handling Equipment),
2. Joint assembly on line–up clamp,
3. Actual welding on a conveyor which is equipped with rollers to turn the tack–welded pipe in the weld zone.

The primary welding method employed at double jointing yards is submerged arc welding.

For internal and external double jointing:

— first, an external floating head that tracks the pipe contour and maintains a constant gap between the electrode and the joint welds the root pass,

welding of the root pass
using an external floating head

     

floating welding head

— then, welding is performed by an internal welder located on a boom which passes inside the pipe.

internal pipe welding

     

welding head mounted on boom

The boom is equipped with a video camera for precise positioning of the weld head in the centre of the pipe,
and an extraction system to remove air from the welded pipe, along with weld fumes and spent flux. This system allows improved visual control over the welding process and, more importantly, significantly reduces the likelihood of undesirable impurities and gases entering the weld metal.

For thick–walled pipe, additional fill passes may be performed by external floating welding heads.

Each phase of the welding operation is carried out using the submerged arc process.

For double jointing pipe sections externally only:

In this case the root and hot passes of the weld are carried out using one of a number of well–known manual or semiautomatic welding methods. Fill passes, as in the first case, are welded using an external floating head with submerged arc.

In most yards the root pass is done manually using stick electrodes.

However, the process of welding with stick electrodes turns out
to be the weakest link, since it does not offer the same productivity and quality that are expected from double jointing at a fixed site.

root pass stick welding

The STT process holds undisputable advantages over manual welding in terms of quality, repeatability
of results, productivity, and, finally, in terms of expense. The volume of metal deposited in a single pass equals the root and hot pass combined using stick electrodes.

The STT process applied similarly to over the ditch welding both for semi–automatic and automatic welding is here again the inevitable replacement of stick electrode welding.