Planning Horizontal Directional Drilling for Pipeline Construction

items will increase the final bore diameter and will dictate the minimum radius of
curvature.
8.4.3 Coating
As mentioned in Section 4.3, one operational risk that should be addressed is
external corrosion due to damaged pipeline coatings. Protective coatings can often
be damaged during pull through by the forces involved, and by contact with soils,
rocks, and other debris present in the bore hole. The consequences of coating
damage are multiplied by the nature of the HDD method. A pipeline installed by
HDD will not be readily accessible to make future pipeline or coating repairs.
External pipe coatings for HDDs must be carefully selected to minimize the risks.
Given the potential for coating damage it may be necessary to select a different
coating system for the HDD section(s) of a pipeline.
Coatings used for HDD drag sections must be flexible and have sufficient
abrasion resistance to limit damage. The economic and environmental
consequences of a future failure are significant. Cathodic protection (CP)
compatible coatings will allow protective current to reach the pipe regardless of
any damage to the coating. Certain coatings such as single or double layer
extruded polyethylene, or polyethylene tape, can shield cathodic protection
current when damaged. Such coatings should be avoided on HDD projects.
Often the most suitable coatings for an HDD project are Fusion Bond Epoxy
(FBE) or similar liquid coatings. An additional layer of pipe coating should be
applied for abrasive protection. This layer is often referred to as the abrasion
resistant layer or the sacrificial layer. The exact type of coating should be selected
based on a number of factors including the amount of abrasion expected.
The selection of field applied joint coatings also requires careful attention. It is
recommended that the joint coatings be liquid epoxies with similar properties,
especially abrasion resistance, as the main plant applied coating. Since the joint
coatings are field applied, proper application methods, qualified
workers/applicators, and qualified coating inspectors are recommended.
In addition to inspecting the coatings during the actual application process, a
careful visual inspection should be made of the first few pipe joints at the exit
location. Often these leading joints are believed to receive the most damage. If
these joints are in good condition it is likely that the remaining coating is in
similar, or even better condition.
Another method of inspection is an in situ electrical method to determine the
coating resistance. A competent cathodic protection technician can complete this
work. This type of inspection can provide a relative understanding of the coatings
efficiency. The field CP measurements must be done prior to completing any tiein
welds to the rest of the overland pipeline. Therefore, the timing of this work
must be carefully coordinated.
September 2004 Planning Horizontal Directional Drilling for Pipeline Construction Page 8-4