Page 93 - Pipeline Rules of Thumb Handbook

80 Pipeline Rules of Thumb Handbook

d. External pressure from static head in the drilled hole Joint Coating—The coating application of the weld area
and/or overburden pressures must be considered. It is the most critical field operation to maintain a smooth
is recommended that the static head resulting from abrasion-resistant pipe string. It is recommended that the
the maximum envisaged drilling fluid density should girth weld be coated with FBE powder utilizing the induc-
be used with a factor of safety of 1.5 to provide con- tion heating coil and powder application machine to a
servative estimations of resulting hoop and longitu- minimum dry film thickness of 25 mils. As an alternate, two
dinal stresses. component catalyzed liquid epoxy may be applied to the girth
3. Post-installation weld area to a minimum dry film thickness of 25 mils using a
a. The longitudinal curvature stresses calculated for paint brush or roller. Tape should never be used for joint
stage 2.c. above are used again here. coating on the pullback portion of a directional crossing.
b. External pressure stresses from 2.d. apply. Coating Repair—It is recommended that small coating
c. Hoop and longitudinal stresses resulting from the damaged areas be repaired with a polymeric melt stick patch-
final hydrostatic test are calculated. ing material. Holidays larger than 1in. in diameter should be
4. In-service repaired utilizing the two component catalyzed liquid epoxy
a. Curvature—see 2.c. applied with a paint brush or roller. Tapes should never be
b. external pressure—see 2.d. used for repair of coating damaged areas on the pullback
portion of a directional crossing.
The maximum working pressure of the pipeline is used Abrasion-Resistant Overcoat—As an extra abrasion-
in calculating longitudinal and hoop stresses that will be resistant barrier for crossings that may encounter stones,
imposed during service. boulders or solid rock it is recommended the FBE coated
Allowable Stresses—Having determined the individual pipeline be overcoated with an epoxy-based polymer con-
and combined stresses at each stage of construction and those crete. The material should be applied at a mill or with a
for the in-service condition, they must be compared with portable yard coating machine to a minimum thickness of
allowable limits. 40mils. Girth weld and coating damaged areas should be field
coated using an epoxy-based polymer concrete compatible
1. ASME B31.8—1992, Table A842.22 provides the fol- with the overcoat material. The patch material should be
lowing limits: applied so the material tapers uniformly and feathers into the
— Maximum allowable longitudinal stress: 80% SMYS. original coating. Stability of the pipeline in drilled crossings
— Maximum allowable hoop stress: 72% SMYS. is not normally a concern so a Portland cement type concrete
— Maximum allowable combined stress: 90% SMYS. coating is not recommended.
(Where SMYS is the Specified Minimum Yield Strength
of the pipe material.)
2. Regulatory bodies may impose additional limits to those Drilling slurry—containment, recycling
specified above—owner companies should identify any and disposal
such further constraints and ensure the adequacy of the
design. The directional crossing process requires the use of large
volumes of slurry that provide the following functions:

Pipeline coating 1. Hydraulic cutting with a jet.
2. Provide energy to the drill motor.
Coatings are applied to provide a corrosion barrier and an 3. Lubricate the cutting head.
abrasion barrier. Directional crossings generally encounter 4. Transport drill cuttings to the surface.
varying materials and often can be exposed to extra abrasion 5. Stabilize the hole against collapse.
during the pullback. An outer abrasion-resistant overcoat is 6. Guard against loss of slurry into surrounding formations.
often warranted. To facilitate the pullback of the pipeline the
coating should bond well to the pipe to resist soil stresses and Slurry Composition—The slurries most commonly used
have a smooth, hard surface to reduce friction and maintain are bentonite based. Bentonite is a naturally occurring
the corrosion barrier. As in any pipeline construction, the rec- Wyoming clay known for its hydrophilic characteristics. Often
ommended external coating system should be compatible polymer extenders are also added to enhance certain charac-
with any specifications for the field joint coating or any inter- teristics. Material Safety Data Sheets (MSDS) are readily
nal coating. available from suppliers and can be presented to regula-
The recommended pipe coating is mill applied fusion tory/disposal authorities.
bonded epoxy (FBE). The recommended minimum thickness Containment—The slurry is pumped downhole and cir-
is 20 mils. culates back to the surface and collected in “return pits.”