Introduction: Transmission pipelines are used to transport crude oil and natural gas from their respective gathering systems to refining, processing, or storage facilities. Transmission pipelines also transport refined petroleum products and natural gas to customers, for use or for further distribution. The standard is in line with the Nigerian Oil Pipeline Acts Cap 338 LFN 1990.
General Gas Pipeline Design Considerations
This section covers the design, fabrication, installation, inspection, and testing, of gas transmission and distribution systems. It shall apply to piping systems required to operate with temperature of between 450F (232 deg. Celsius) and minus 20F (-7 deg. Celsius) only.
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Gas Pipeline Design Criteria and Materials Specifications
Applicable standard in Nigeria shall be the ASME B31.8-2018 or its subsequent editions. Other internationally recognised standards approved by NUPRC are applicable. The minimum requirement are as follows:
i. All materials to be used shall generally conform to the specifications in appendix B of the reference Standard ASME B31.8-2018 for structural materials of the line pipe, valves, fittings, flanges, bolting and tubings.
ii. The pipelines shall be generally seamless or of the ERW and DSAW types.
iii. Thermoplastic and thermosetting pipe materials shall also be acceptable provided that they conform to ASTM specifications 02513 and 02517 respectively and shall be inhibited against material degradation effects of ultra-violet rays if used in locations where the pipeline is exposed to sunlight.
iv. Weldability of the ferrous pipe material shall be tested in accordance with the requirements of API standard 1104 and other internationally recognised standards approved by NUPRC.
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v. Adequate provision shall be made for the flexibility of the pipeline while under pressure in the form of anchorage and guide points and for temperature induced stresses by allowing for expansion joint couplings.
Design and Construction of Gas Pipeline
The design and construction of new gas pipelines, the replacement of existing ones and their corresponding corrosion control installation shall conform to the standards and codes specified in ASME B31.8-2018 , National Association of Corrosion Engineers Standards generally referred to as NACE SP0169-2013 and other international recognised standards approved by NUPRC. The follow shall apply at the minimum:
a. The process of manufacture of the specified pipeline system shall generally comply with the various specifications of API 5L or its internationally recognised equivalent.
b. It shall be expected that operator or owner of a licensed pipeline system shall maintain in its custody sufficient manufacturing records and data to substantiate that acceptable industry standard was employed in the production of the pipeline system. Such records include:
- Manufacturing records such as incoming material inspection reports, welding procedure specification / procedure qualification records, in-factory mechanical and chemical test records
- NDTs results, calibration data or certification of test equipment, approved ITPs and marking schemes, material traceability, dimensional checks.
- Report of workmanship/defects examinations carried out during manufacturing processes etc.
c. Long distance gas transmission pipelines shall be made of steel, the design and construction of which shall be governed by the population density indices specified in ASME B31.8-2018 and the corresponding design factors. Also, the specified construction types of pipelines in the proximity of main roads and railroads and the mode of their crossing shall be complied with.
d. The minimum depth of burial shall be as specified in Section 3.3.1 but where these minimum depths could not be achieved, the pipeline at these points shall be encased, bridged, or specially reinforced to withstand any anticipated external load.
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e. There shall be a minimum clearance of 0.5 meter between the pipeline and any other underground structure not connected with it.
f. It shall be mandatory for all buried pipelines to be protected against corrosion where any pipeline is to transport corrosive or toxic gas, the following shall be ensured:
- Design parameter shall be such that the gas pressure at any time in the pipeline shall not result in a hoop stress greater than 60 percent of the specified minimum yield strength of the pipeline material based on the nominal wall thickness.
- Block valves and check valves shall be so located as to prevent the escape of the corrosive or toxic gas into the atmosphere in the event of a pipeline failure.
g. The design, installation and maintenance of the associated corrosion control system of the pipeline shall be in line with section 3.5, in conformance with to the provisions of NACE SP0169-2013 and other internationally recognised standards approved by NUPRC.
Requirements for Gas Pipeline Inspection and Testing
i. Inspection of pipeline construction materials and its appurtenances, welding, ditching, stringing and the general installation shall follow the procedure outlined in chapter IV of the ASME B31.8-2018 and other internationally recognised standards approved by NUPRC.
ii. All gas pipelines laid in farmlands, virgin areas and sparingly populated areas shall be tested with water up to a minimum pressure of 125 percent of their maximum operating pressure or with air up to 110 percent of their maximum operating pressures. The test result shall be lodged with the nearest office of the Department.
iii. Pipelines laid in densely populated areas shall be tested ONLY with water up to a minimum of 140 percent of their design maximum operating pressures and the test result shall be lodged with the nearest NUPRC office.
iv. Leak test shall however be performed on the pipeline system with the use of pressurized air or gas between 100 PSI to 20 percent of the minimum specified yield of the material for pipelines that has been designed to withstand 20 percent or more of the specified minimum yield strength.
v. Pipeline to be operated below 100 PSI shall be subjected to a leak test at maximum system pressure.
vi. Any gas pipeline that will not be put into use for more than six months after construction shall be filled with inert gas or nitrogen and if it is to be put into use after one year of completion shall be pressure tested and certified by an official of Department of Petroleum Resources before being put into use.
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vii. Gas pipeline systems shall be well purged with water, air or inert gas before undertaking any repairs and the environment of the repair site shall be constantly monitored with gas detecting device to ensure adequate safety.
Gas Pipelines Made with Materials Other Than Ductile Steel
Cast Iron Materials
Cast iron Materials shall be used for gas pipelines under specified application to and with special approval by the Department. Such applications shall be supported with the compelling reasons for using Cast Iron Material. However, the design considerations shall strictly follow the specifications in paragraph 842 of ASME B31.8-2018 and other internationally recognised standards approved by NUPRC.
Plastic Materials
The use of thermoplastic and thermosetting plastic materials of the grades specified in ASTMD 2513 and ASTMD 2517 shall be generally allowed for laying service lines only.
i. The design pressure for plastic gas piping system shall be governed by the formula given in reference standard ASME B31.8-2018 where the value of the plastic design factor shall not be less than 0.32 for any case.
ii. Plastic pipe materials shall not be used in any service where the maximum and minimum operating temperatures shall be higher or lower than 100F and 20oF respectively
iii. Plastic pipe materials to be used shall be inhibited against the effect of Ultraviolet rays which renders such materials brittle when exposed to sun light
iv. Plastic pipes or tubing shall not be threaded at joints but shall be joined by the solvent cement method, adhesive method, and heat-fusion or by the means of compression coupling or flanging whichever conforms to the manufacturer’s specifications.
v. Extreme caution shall be taken in laying plastic pipelines to avoid any damages to the material. All buried plastic pipelines of ½ inch nominal diameter and above shall have a minimum wall thickness of 0.1 inch while those below ½ inch nominal diameter shall have a wall thickness not less than 0.06 inch.
vi. Plastic pipes shall be buried in undisturbed or well-compacted soil while no mitre bends shall be permitted at any portion of the pipeline.
vii. Plastic pipes shall be tested at pressure not less than 150 percent of their maximum operating pressure or 50 PSI whichever is greater and shall not on any occasion be subjected to a pressure more than 300 percent of their maximum operating pressure.
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viii. Certificate of conformity to required specifications shall amongst other items provide information on the following parameters for plastic gas transmission system:
Compound
- Conventional density
- Volatile content
- Water content
- Carbon Black dispersion
- Resistance to gas condensate
- Resistance to Rapid crack propagation (critical pressure, Pc) (e>/= 15mm)
- Resistance to slow crack growth (dn: 110mm or 125mm- SDR 11)
Tensile strength for butt fusion (dn:110mm or 125mm- SDR 11)
Pipeline
- Hydrostatic strength (20 deg centigrade, 100h, 80 deg c, 165h, 80 deg c,1000h)
- Elongation at break
- Resistance to slow crack growth e</= 5mm (Cone test)
- Resistance to slow crack growth e> 5mm (Notch test)
- Resistance to rapid crack propagation (critical pressure, Pc)
- Oxidation induction time (Thermal stability)
- Melt mass flowrate (MFR)
- Longitudinal reversion
Valves
- Resistance to weathering
- Leak-tightness of seat and packing
- Actuation mechanism resistance
- Resistance to bending between supports
- Thermal cycling resistance dn>63mm
- Leak-tightness under bending with thermal cycling, dn</= 63mm
- Leak-tightness under tensile loading
- Leak-tightness under and after bending applied to the operating mechanism
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