Pipe hauling and stringing

 

Once the pipeline’s path has been cleared sufficiently to allow construction equipment to gain access, sections of pipes are laid out along the right-of-way, ready for welding, a process called ‘stringing’. The pipe sections are commonly 12m or 24m long. The 24m long sections are made at the yard by welding together two 12m standard section; this is called double-joining. The joints made at the yard may be coated at the yard or left as such for subsequent coating with the line joints. Pipes are specific to their destination and the hauling contractor uses the pipeline layout drawings to string the right pipes at the right place. Indeed, certain areas have different

requirements for coating material, pipe thickness and even pipe material and diameter.

Pre-coated pipe joints are hauled to the right-of-way on stringing trucks. The pipe are

unloaded from the trucks with a side-boom tractor or the crane mounted on the truck and

placed end-to-end alongside the ditch so they are accessible to construction personnel. 

Pipe stringing may occur concurrently with trenching. In some cases, pipe stringing may

occur ahead of the trenching, but it normally follows the trenching crew (to avoid damages to

the pipe by the heavy earth moving equipment). In cases where the trench is to remain open for extended periods (over night or longer) it is sometimes barricaded to reduce any safety hazard. 

Where water and road crossings are to be accomplished, the appropriate pipes are stockpiled

on one or both sides of the crossing so they are to the construction crews that would follow. 

Depending on access and terrain, the trucks off-load the pipe and then turn around and return

to the pipe lay-down area. In cases where there is a narrow construction corridor, the trucks 

have to make a continuous loop by driving a significant distance up the corridor, then off-load

the pipe and follow the corridor a significant distance to exit the corridor. 

 

Bending the pipe

 A pipeline must cross over hills and curve around special places. A hydraulic pipe-bending

machine bends individual joints of pipe to the desired angle at locations where there are

significant changes in the natural ground contours or where the pipeline route changes

direction. The pipes to be bent, the location of the centre of the bend on the pipe joint and the

angle are defined by the surveyor after stringing. Bending data are marked on the pipe joints.

 The bending is limited to making many small bends along the length of a pipe joint until the

desired bend angle is obtained. Doing so, the pipe retains its strength and remains circular

where it is bent because of the characteristics of steel and the bending techniques used.

 The bending technique should be qualified before production (maximum angular deviation for

each stroke, distance between strokes, distance from the ends of the pipe, for each

diameter/thickness, material).

 

Welding 

After the stringing and bending are complete, the pipe sections are aligned, welded together,

and placed on temporary supports along the edge of the trench, waiting for NDT and lowering

in. This step is repeated a number of times until multiple pipe joints are joined to form a pipeline

section which will be lowered in in one shot. Such sections may be as long as 2 km or more in

clear areas. Automatic welding machines (external and internal) are used where possible and some hand welding also takes place, generally for repairs and special points. As for mass production on an assembly line, the various welding phases are performed by specialized crews, having each their specific equipment. The purpose of this organization is to produce as many welds per day as possible (expensive operation). The equipment and staffing of the welding spread depends on the diameter, thickness and welding procedure. A typical set-up looks like the following: the pipe gang follows the pipe stringing and bending crews. The pipe gang uses one or two side booms to set the pipe up on wooden frames (skids) to support the pipe off the ground and line up the pipe with the contour of the trench. Pipe ends are aligned and clamped in place,

using an internal pneumatic clamp or an external mechanical one. Once the pipe is fixed and proper alignment has been achieved, two welders perform the first pass (root pass). Immediately after the first pass, the next group of welders applies the second bead or hot pass. The hot passers can number anywhere from 2 to 4 welders with their helpers. The completion of the root pass (1st pass) and the hot pass (2^nd pass) is the primary function of the pipe gang. Production from the pipe gang is expected to be 500m to 2000m per day, depending on weather and terrain. The firing crew is the largest crew, and consists of 8 to 12 welders using welding machines mounted on pickup trucks. The welding crews follow and place the remainder of the weld material into the joint, including the final “cap” weld. When completed there are a total of four or five layers of welds on the pipe joint. All welders that work on a joint have unique identifying codes. The codes are marked on the area adjacent to the pipe so complete records of the welding are maintained. 

 

Joint Coating 

 

Line pipe, normally mill-coated or yard-coated prior to stringing, requires a coating at the

welded joints and at areas damaged by handling operations. After the welds have been X-rayed, the coating crew cleans the exposed steel at the joint between the pipes and applies a protective coating to it. The coating is commonly a heat-shrinkable polyethylene wrapped around the pipe. Heat is applied to the coating material to shrink it around the joint and form a tight, impervious

covering on the joint. After the coatings have been placed, an inspection crew checks the entire pipeline for nicks and abrasions in the coating with a high-voltage testing device, the holiday detector. The High Voltage Detection Technique locates flaws in insulating coatings on conductive substrates. A power supply within the instrument generates a high DC Voltage which is supplied to a suitable probe. An earth voltage return lead from the instrument is connected to

an uncoated area of the metal of the test substrate. As the probe is passed over the coated surface, a flaw is indicated by a spark at the contact point with an audible alarm in the detector and a visual alarm in the probe handle. The probe can have the shape of a brush or a spring that surrounds the pipe and is rolled all along the line.

 

Lowering in the pipe

 

Once the pipe is welded, bent and coated it can be lowered into the previously dug trench.

This is done with specialized construction equipment acting in tandem to lift the pipe

relatively uniformly and lower it into the trench. Care is taken to avoid damaging the pipe and

its exterior coating. The pipe is lowered off the skids into the ditch by a team of several side-boom tractors. All rocks are removed from the bottom of the ditch prior to the lowering operation. In any areas of rocky terrain, sand-filled bags would be placed in the bottom of the ditch to protect the pipe and coating from damage from rocks. After the pipe is laid in the trench, the wooden skids are picked up and moved ahead to reuse.