External Coating Rehabilitation

The main reasons for pipeline breakdown are

  • corrosion (both internal and external),
  • mechanical damage.
 
 

another example of damaged coating

 

There are several approaches to external coating rehabilitation:

Option A

  1. a section of the pipeline is decommissioned,
  2. excavated,
  3. cut out,
  4. lifted out onto the side of the ditch,
  5. the coating is replaced
  6. and lastly the repaired section is tied in.

Option B

  1. a section of the pipeline is decommissioned,
  2. the ditch is excavated,
  3. the repair work is done inside the ditch without cutting out the segment.

In this scenario, the work is done using highly productive, but heavy and large equipment (such as described
in the Earth Moving section
).

 

Option C

  • Repair work is done inside the ditch, on an operating line, with reduced pressure.

All units are designed for light weight, minimal underpipe clearance and easy pipe mounting.

A mobile, relatively lightweight system for performing repairs on live pipelines without causing damage to the pipe surface is available.

 
 

rehabilitation to Option C

 
 

The quality of coating rehabilitation is largely determined
by the quality of cleaning and pipe surface preparation.

Mechanical cleaning methods should be avoided as much as possible due to the risk of damaging the pipe as well as the likelihood
of causing sparks.

Where climatic conditions allow, the most effective and safe solution for removing practically all coating types is hydrocleaning
— cleaning with a high pressure stream of water. Thousands of kilometers of pipeline in various countries have been rehabilitated using hydrocleaning.

 

rehabilitated external coating

 

Currently, the most promising and economical approach for in situ rehabilitation is bell hole technology, which consists of the following steps:

  1. excavate a small (approximately one shift’s worth of work) section of the main, and set it on supports,
  2. clean the pipe surface of damaged or old coating,
  3. perform visual inspection of the cleaned surface,
  4. prepare the cleaned surface for application of new coating,
  5. If necessary, repair the damaged areas, for example with Wrap Master (Permawrap and Weldwrap) sleeves,
  6. Apply new corrosion protection coating (for example cold applied tapes by Polyken, or Acothane TU
    — a two–component polyurethane coating by Metrotect),
  7. Backfill the repaired section of the pipeline.

Thus the equipment needed for coating repair can be subdivided into three categories:

  • Equipment for removal of old coating (HydroKleaner)
  • Equipment for pipe surface preparation (sand or short blast systems)
  • Equipment for application of new coating (prime and tape or liquid airless spray machine)
 
 

rehabilitation spread

 

The equipment systems used for this method of rehabilitation have a clam shell design (machines open up from one side) which allows quick assembly and disassembly without cutting the pipe.

 

After digging up the pipe section and placing it on supports, the HydroKleaner is run along it.

The line travel HydroKleaner is the modern solution for removing coal tar enamel, asphalt or polyethylene tape from pipelines.
High–pressure water jets leave the pipe surface, longitudinal seams and girth welds clean and ready for inspection.

Deeply pitted areas are free of corrosion deposits and easily visible. Hydrocleaning is also the only practical way to remove water soluble salts and other contaminants in the field.

To support the HydroKleaner, a vehicle with a platform containing a high pressure pump, a diesel drive and a water tank is deployed on the right of way. The support platform is connected to the working unit by high pressure hoses which may be as long as 30 meters.

Water is supplied by a water tank truck, and replenished by nurse truck. Vacuum truck or portable containers are used to collect
the spent water and removed coating material.

 
 

removing of old coating with HydroKleaner

 

After the HydroKleaner has passed over the pipe surface, a visual inspection of the cleaned pipe takes place. Depending on the rehabilitation spread and the type of defective coating to be removed and replaced, the pipe can be recoated either after HydroKleaning or after an additional pass with shot or sand blasting by an abrasive cleaning unit.

 
 

The abrasive cleaning unit (sand or shot) removes the remaining traces of active primer and prepares the surface profile of the pipe prior to application of new coating.

The blasted pipe takes on a surface profile which aids in bonding
of the adhesive to be used in the new coating application.

If the replacement coating is to be polyethylene tape blasting, while not obligatory, is highly desirable.

If the replacement coating is a plural component liquid system, then blasting is absolutely essential.

 

sandblasting machine to prepare pipe surface

 

When applying coating to a pipe that is in operation, it is sometimes necessary to pre–dry the pipe, since condensation can collect between the time it was sandblasted and the time coating is to be applied.
A hot–air machine is used for drying the pipe. The time interval between hot air treatment and coating application must be minimized.

 

After visually checking the quality of the pipe surface and repairing any defects on the bare pipe, the pipe is coated

When two–component polyurethane coating system (Metrotect)
is applied, heated coating material components are metered
to the correct ratio, blended in static mixing tubes, mounted above the oscillating rings, and then sprayed onto the pipe surface.

Coating thickness is governed by spray pressure, travel
and oscillation speed.

 
 

application of two–component coating
onto cleaned surface

 

When cold applied tape systems (Polyken) are used, in a single pass — as in case two–component coating — the unit applies primer and 2–layer polyethylene to the cleaned and prepared pipe surface.

 

cold tape applied to cleaned surface

 

The efficiency of such systems is about 250–300 meters per shift.

An important quality of these systems is that they have been developed in accordance with environmental safety standards.