Most pipeline welding involves girth welding from external side only, because the diameters are too small to permit welding from the inside. The commonly used joint design is shown in Fig. 11.9. It is well suited for the stovepipe technique described below. In special cases, the angle of bevel is increased from 30° to 37.5°.

Internal backing rings are avoided as far as possible, because they not only cause turbu­lence in the flow of material, but also make it difficult to use devices for internal pipe cleaning. Moreover, the stovepipe technique enables the welder to deposit sound weld-metal at the root through the entire 360° in 5G position. If welders cannot guarantee complete root fusion and freedom from internal protrusions (icicles), the use of backing rings is indicated.

11.7.1 Stovepipe Technique

Stovepipe welding is the term used when a number of pipes are laid and welded together in G5 position one after another to form a continuous line, and welding is carried out vertically downwards, and not by the conventional vertical upwards method which is time consuming and expensive.

In this technique, welding starts at the 12 o’clock position on the pipe, and progresses vertically down until the 6 o’clock position is reached. On completion of one half of the pipe, the opposite side is welded in the same manner, thus producing an endless root run known in the field as a stringer bead.

The second run, known as the hot pass, is then put into the joint. Its name comes from the fact that a high current is used to deposit the run, so as to burn out any defects that may be present from the stringer bead. With the exception of the final run, all subsequent runs after the hot pass are termed filler beads. Their purpose is to bring the weld deposit to just below the level of the pipe surface. The number of filler beads required will depend largely on the pipe-wall thickness and the preparation.

There are times, however, when it is necessary to deposit a filler bead all round the pipe periphery, especially as the weld nears completion. In most cases only the areas between 2 to 4 and 10 to 8 o’clock on the joint (see Fig. 11.10) will require additional weld-metal. These concave areas are rectified by the quick deposition of a weld run called a stripper bead, which brings the concave areas flush with the remaining weld-metal elsewhere in the joint. To finish the pipe weld the final run is made, which is appropriately called the capping bead.

The joint preparation and fit-up is as shown in Fig. 11.9. Welding is done with AWS E6010 and E7010 class electrodes. These are chosen because the small volume of stiff, thin slag coating deposited on the weld bead, together with the forceful arc, facilitates rapid changes of electrode angle during vertical-down welding on fixed pipes.

To compensate for the thin slag coverage, extra protection from the atmosphere is pro­vided by a gaseous shield of carbon monoxide and hydrogen evolved from the cellulosic coating during welding.

For stovepipe welding, the maximum current specified by the producer for the size of electrode is increased by approximately 10%. DC supply with electrode positive (positive polarity), is often recommended. There may be occasions, however, where scale on the pipe causes surface porosity. In such cases, changing the electrode polarity from positive to negative tends to reduce this problem.

For deposition of the stinger bead (root run), once the arc has been established, the cup of the electrode must be literally pushed into the root of the joint. No weave of the electrode is necessary, only a light drag action as welding proceeds, to ensure that the arc is allowed to burn inside the pipe. An electrode angle of 60° in the direction of travel to the pipe tangent (see Fig. 11.11) must be held throughout.

This practice produces a very small root run, which allows for a controlled penetration bead. If one or more burn-throughs (windows) occur during the laying of the stringer bead, they can be quickly rectified by the remelting process of the second run.

Immediately following the stringer bead and while it is still warm, the hot pass is put down with an electrode angle held at 60° to the pipe tangent. A short arc must be held with a light drag, together with a forward and backward movement of the electrode (see Fig. 11.12), in order to fuse out any undercut and/or wagon tracks, caused by the stringer bead. In addition to remelting the portions containing windows, the higher current used for this run prevents the formation of slag lines at the toes of the stringer bead.

For the filler bead deposition, it is necessary to alter the electrode angle from 60° to 90° to the pipe tangent. However, on reaching the 4 o’clock (8 o’clock on side 2 of the pipe) the electrode angle is increased from 90° and reaches 130° at the 6 o’clock position of the pipe (see Fig. 11.13).

From the 12 o’clock down to 4 o’clock (8 on side 2), a normal arc length with a rapid weave across the weld face is required, pausing memontarily at the toes, from 4 o’clock (8 o’clock) down to the 6 o’clock position, the electrode manipulation is changed from a weave to a lifting or vertical movement of the arc away from the deposit on to the weld pool. By adopting this technique on the filler beads, flat weld faces with the absence of undercut are produced.

For the stripper beads, a medium to long arc is required to spread the weld deposit. A slight weave of the electrode may be found beneficial, depending on the current setting and width and depth of the bead required. The angle of the electrode is held at 90° to the pipe tangent, irrespective of the position on the pipe periphery.

Finally the capping bead completes the joint, using a medium to long arc length, with a rapid side-to-side movement of the electrode tip. The angle is maintained at 90° to the pipe tangent except from 4 to 6 and 8 to 6 o’clock positions when the electrode angle is increased to 130°.

For these sections, the electrode should be manipulated to produce a lifting and flicking action. To achieve best results, the capping bead should be restricted to the width and depth of ~19*1.6 mm. Weld beads wider than this are somewhat difficult to control.

The electrode size for various passes depends on wall thickness. For depositing the stringer bead, for example, 3.25 mm diameter electrode is used for wall thickness below 6.3 mm, and 4 mm diameter for larger thicknesses. For first and second filler passes, 4 mm diameter electrode is commonly preferred. For third filler, stripper and cover passes, 4 or 5 mm diameter electrodes are used depending on wall thickness.

It is difficult even for a normally well-experienced welder to use stovepipe technique successfully, unless he is given special training with suitable electrodes on actual pipe joints. Experience has shown that only about 20% of the otherwise skilled welders are capable of mastering the stovepipe technique.

The adoption of stovepipe technique in pipeline construction demands a well-planned disposal of the crew, in order to ensure that welding operations take place rapidly along the line. The pipes are first lined up by the line-up crew with the help of an internal line-up clamp. A good joint fit-up is the necessary condition for a flawless, well penetrated stringer bead, and it is the responsibility of the line-up crew to ensure it. Two welders then complete the stringer bead (first pass). The line-up men and these welders then move on to the next joint, while a second group of welders deposit the hot pass (second pass). They then shift to the next joint, while the third group of welders completely fill the joint. The third group, called firing line, includes a larger number of welders, since more welding is involved in completing the joint. The stringer welders and the hot pass welders work in groups of two or four.

Stovepipe technique is not possible with rutile type (E6013 class) electrodes, because the relatively large volume and high fluidity of the slag render vertical downward welding difficult with these electrodes, good joints can be made by welding vertically upwards. But the technique is slow and results in lower productivity.

11.7.2 LH Electrodes

In recent years, increasing use is made of high-yield steels for pipeline, for example, the SL x 60 and SL x 52 steels. These steels are more prone to hydrogen-induced cracking in the HAZ than the conventional mild steel. Hence the pipe ends need to be preheated when E6010 - E7010 electrodes are used. When this is done, the stringer pass and the hot pass have to be made with an increased speed of 230 — 300 mm/min. This increases the strain on the welder.

Special LH electrodes have been developed for welding SL x 52 and SL x 60 steels using the stovepipe technique, without the need for preheating. With these electrodes, the root gap is increased to 2.5 mm to accommodate the heavier coating and the welding speed is kept as low as 150 mm/min. The disadvantage of reduced speed is more than made up by the thickness of the root pass, which is twice that deposited with E6010 type. The deposition efficiency of the LH electrode being 20% higher than the E6010 type, the joint can be completed with fewer layers and in shorter arc time.

11.7.3 MIG/CO2 Process

The inherent advantageous features of this process could make it preferable to MMA welding, but there are several difficulties. The normal spray transfer technique which is capable of giving high deposition rates would give rise to burn-through and considerable spatter when CO2 is used for shielding. The dip transfer technique using argon/CO2 mixture for shielding is better suited for 360° welding, but the shallow penetration of this process can lead to incom­plete fusion. Moreover, the upkeep of the equipment at site demands the services of properly trained mechanics and a regular supply of spares.

For the welding of pipes large enough to accomodate a MIG/CO2 welding head inside, fully automatic equipment has been developed. A typical piece of equipment consists of four welding heads, mounted at 90° spacing, for internal welding and two welding heads for exter­nal welding. The two top internal welding heads proceed simultaneously from the top of the pipe downward to make the weld. The two opposite internal heads then counter rotate to complete the joint. The external welding units are light and portable, and they are used in conjunction with a tracking band, which is attached around the pipe at a fixed distance from the weld. The two units operate simultaneously on each side of the joint, proceeding from the top of the pipe downward. It is also possible to use the external units simultaneously with the internal units.

For the internal weld which is made first, a small V-groove is provided. For external welding, a V-groove with 20° included angle is adequate to ensure complete fusion. This means reduced weld-metal required to complete the joint. The welding wire is of 0.8 mm diameter and the shielding gas is 70% argon - 25% CO2. This argon-rich shielding reduces spatter to the minimum.

The system may also incorporate a pipe-end preparation machine, which is used ahead of the welding operation. The internal welding machine may be combined with a line-up clamp.

Such systems have been used with success for various onshore and offshore construc­tion projects in the U. S.A., Canada and England.

11.7.4 Flux-cored Process

A typically system utilising this incorporates an end preparation machine and makes all the weld passes from the outside. It uses two welding heads, mounted 180° apart, for the root pass and four welding heads, spaced at 90°, for the subsequent passes. The root pass is deposited over a copper back-up attached to a specially designed internal line-up clamp. All welding proceeds from the top to the bottom. The flux-cored welding wire is of 2 mm diameter. No external gas shielding is used, which is a welcome feature for site welding. The joint consists of 58° included angle, 1.6 mm root and 2.5 mm root face.

11.7.5 Underwater Pipelines

Pipelines for underwater service are laid in marshy land, shallow waters or in considerable water depths. MMA process is commonly used for welding. The welders work at stations located on barges. The pipe laying starts from the land or shore and proceeds towards deeper waters. As many as five welding stations may operate on several barges, followed by two radiographic stations and a coating station. Coating is meant for corrosion protection. Large diameter pipes are preferably concrete coated to provide corrosion resistance as well as negative buoyancy. In offshore construction, the completed pipe sections is lowered gradually by means of a semi - buoyant stringer, which holds the pipe until it has neared the sea bed. After laying, the pipe is buried in the sea bottom.

11.7.6 Inspection and Testing

For important pipeline construction, the welding procedure as well as the welders must be qualified. The necessary guidance is obtained from any of the following or equivalent standards:

(a) API Standard 1104, Standard for Welding Pipelines and Related Facilities

(b) ASME Boiler and Pressure Vessel Code, Section IX

(c) ANSI B 31.8, Code for Gas Transmission and Distribution Piping

In the qualification test a sample pipe is welded in accordance with the procedure adopted and coupons are removed by gas cutting; they are then subjected to various tests such as tensile, nick break, root and face bend tests. If these tests meet the code requirements the welder or procedure is taken as qualified.

Inspection is carried out both during and after welding. During welding, the points to be checked are: (i) edge cleanliness, edge preparation and joint fit-up; (ii) physical condition of the electrodes; (iii) functioning of the power source and current setting; (iv) soundness and penetration of the stringer bead; (v) soundness and quality of hot passes; and (vi) interpass cleaning. After welding, the joints are subjected to visual and radiographic inspection. The latter is carried out with X-rays or gamma-rays. Special radiographic equipment has been designed for large diameter pipelines, which enables the X-ray or gamma-ray source to be propelled through the pipeline on a battery driven or engine-driven crawler unit. The unit is provided with a mechanical or radiological device to locate and stop at a welded joint. Film belts are wrapped around the joint circumference to radiograph the entire joint in one expo­sure. The unit is programmed for speed, exposure time and other radiography parameters before insertion into the pipeline. Such an equipment can travel several kilometres through a pipeline, thus enabling the contractor to proceed continuously with welding without waiting for radiographic inspection to catch up with him.

For small diameter pipe, radiography has to be done from outside. In this case, the source is placed on one side and the film 180° opposite. At least three exposures are necessary to cover the entire joint, and increased exposure time per exposure is required. Hence external radiography is more time-consuming than internal radiography. Other NDT methods are rarely used. Ultrasonics, for example, cannot perform reliably because of the irregularities of the manual-arc welded stringer bead and cover pass.

Sometimes the completed pipeline needs to be pressure-tested prior to being placed in service. The common practice is to test it hydrostatically with water to stress levels equal to the actual yield point of the base metal.