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.
Backfilling the trench
Before testing the pipeline, the ditch is backfilled. Sometimes the excavated soil is used to fill
the trench and sometimes other selected backfill is used. The trench is backfilled using a
backfilling; no foreign materials are permitted in the trench.
Backfilling requires special attention in the following areas:
- in steep slopes (where the soil is generally rocky), the backfill material used to prepare
the pipeline bed and which is in contact with the pipe may be easily washed away.
Indeed, the trench is working like a drain and over the years the backfill will tend to
slip down the hill. To avoid this, special arrangements are made to stabilize the backfill in place (sand bag walls for instance) and to evacuate the water under control (drain pipe, diverters).
- when crossing underground structures or in any case when the minimum cover cannot
be achieved, special measures should be taken to provide the same level of protection:
backfilling with concrete, concrete slabs, etc… More than in normal cases, the backfill around the pipe should be properly compacted to provide the necessary strength to the line against static and dynamic pressure from loads crossing the trench.
- The above also applies when for some reasons the normal cover could not be achieved
due to construction problems: collapsing of the trench during the lowering in operation or river crossings (when the crossing is in open trench, the backfill of this trench shall be stabilized against erosion from the river).
Testing
After backfilling, the pipeline is hydrostatically tested following applicable regulations.
Actually, pipeline testing is more than just leak detection and resistance checking. Pipelines
are tested even when there is no applicable regulation. Indeed, the purposes of this “testing” operation are multiple:
- remove all debris from the inside;
- verify that there is no obstruction or diameter reduction;
- make sure the pipeline will not come out of the ground under pressure (Bourdon
effect);
- make sure the additional weight of the test water can be supported by the trench;
- check for leaks with maximum accuracy,
- check for resistance, verify the strength of the pipe and the weld,
- empty totally the test water (even this little tiny pocket under this river crossing)
- clean and dry (mainly for gas pipelines) the internal surface of the pipeline before the
client introduces valuable product into it,
- close the line at both ends to avoid any introduction of foreign materials and water.
- avoid corrosion;
- avoid polluting the environment with the huge quantities of rusty test water;