Pipelines provide the most efficient means of transporting fluid from one point to another. Liquid hydrocarbon are mostly conveyed using pipelines. Pipelines can be made of metallic or non-metallic materials; however, liquid hydrocarbons are mostly transported using steel pipelines. Steel pipelines that transport liquid hydrocarbon must have sufficient strength to withstand the internal and external pressure imposed on the pipeline. Other external and internal loads shall also be considered in the design of a liquid hydrocarbon pipeline. The pipeline ability to resist the loads, as detailed in the previous sentence, is a function of the strength, which is also related to the thickness of the pipeline.
This article will focus on calculating pipelines wall thickness and selecting appropriate thickness with a focus on pipelines made of carbon steel materials e.g. API 5L. In addition to performing the wall thickness calculation, inputs to the wall thickness calculation formula will be explained.
Throughout the entire service life of a pipeline, it is operated at a specific pressure to ensure the fluid is transported from the source to its destination. The pressure to propel the liquid from the start point to its destination may be natural, as in the case of crude oil produced from the well or might be generated as in the case of pressure generated by pumps in a pipeline pump station. This implies the pipeline is subjected to considerable pressure; therefore, the pipeline materials selected must have the capacity to withstand the system pressure. Pressure resistance is related to the material strength and its thickness.
The method and procedure to determine the pipeline material thickness might differ depending on the applicable regulation, code or standard utilised. It should be noted that the ultimate governing regulations are the local regulations and standards. Other codes can be utilised provided the local regulatory requirements are met.
There are various regulations, codes and standards governing the transportation of liquids, the most prominent are listed below:
ASME B31.4: Pipeline Transportation Systems for Liquids and Slurries
ISO 13623: Petroleum and Natural Gas Industries — Pipeline Transportation Systems
PD 8010 Code of Practice for Pipelines – Part 1: Steel pipelines on land
PD 8010 Code of Practice for Pipelines – Part 2: Subsea pipelines
CSA Z662: Oil and Gas Pipeline Systems
Amongst the standards and codes listed above, ASME B31.4 is very prominent and widely utilised in Africa, America and the Middle East.
As detailed in section 400 of ASME B31.4, it covers pipeline conveying liquids including but not limited to crude oil, condensate, liquid petroleum products, natural gasoline, natural gas liquids, liquefied petroleum gas, carbon dioxide (supercritical), liquid alcohol, liquid ammonia, produced water, injection water, brine, biofuels and slurries.
This article focus on the determination of steel pipeline wall thickness utilising ASME B31.4; however other codes and standards will be referenced and utilised accordingly
3 Terms related to liquid hydrocarbon pipelines
Before proceeding to perform a wall thickness calculation, there are key terms to be explained as detailed in this section. Definitions in this section are coined from ASME B1.4.
As defined in ASME B31.4. a pipeline is all parts of physical facilities through which liquid moves in transportation, including pipe, valves, fittings, flanges (including bolting and gaskets), regulators, pressure vessels, pulsation dampeners, relief valves, appurtenances attached to the pipe, pump units, metering facilities, pressure regulating stations, pressure limiting stations, pressure relief stations, and fabricated assemblies.
3.2 Pipe Grade
As stated in sections 4.45 and 6.1 of API 5L, the grade of a pipe is a designation of its strength. Table 1—Pipe Grade and Steel Grades and Acceptable Delivery Conditions; shows the available API 5L pipes grade. Except for Grade A and B, the grades of API 5L pipes consist of an alpha or alphanumeric designation that identifies the pipe strength level (Specified Minimum Yield Strength) linked to the chemical composition. The SMYS shown in the numerical portion is in SI units (psi)
Example: pipe grade X42 has a specified minimum yield strength SMYS of 42,100 psi. Note the actual yield strength of a pipe can be higher than the SMYS but shall not be lesser.
Details of other pipe material grades can be found in their respective specifications. Refer to table 2 of ASTM A106/A 106M for details of pipe grades.
3.3 Pipeline Design Life
As per section 400.2 of ASME B31.4, the design life is a period of time used in design calculations. The design life is selected to verify that a replaceable or permanent component is suitable for the anticipated service period. The specified design life does not pertain to the life of the pipeline system because a properly maintained and protected pipeline system can provide liquid transportation service indefinitely.
A pipe is a tube, usually cylindrical, used for conveying fluid or transmitting fluid pressure. The pipes described in this article are steel pipes. These pipes are either seamless or welded.
3.4.1 Seamless Pipes (SMLS Pipe)
Seamless pipes are produced by piercing a billet followed by rolling, drawing, or both. These pipes do not have welded seam. Seamless pipes are usually very popular for diameters smaller than 24”. Larger diameter pipelines are mostly welded pipes.
3.4.2 Welded Pipes
Welded pipes are made by bending and welding metal plates or sheets into cylindrical shape.
The types of welded pipes described in ASME B31.4 section 400.2 are listed below.
Double Submerged Arc Welded Pipe
Electric Flash Welded Pipe
Electric Fusion Welded Pipe
Electric Induction Welded Pipe
Electric Resistance Welded Pipe
Furnace Butt Welded Pipe
Furnace Butt Welded Pipe, Continuous Welded
Furnace Lap Welded Pipe
4 WALL THICKNESS CALCULATION
ASME B31.4 wall thickness calculation is performed per section 403.2 of the code. Below is a description of how to calculate the wall thickness of a pipeline.
4.1 Wall Thickness Calculation Formula
The straight section of pipe shall have a nominal wall thickness satisfying the equation below.
tn = nominal wall thickness satisfying requirements for pressure and allowances.
A = sum of allowances for threading, grooving, corrosion, and erosion.
t = pressure design wall thickness.
The pressure design wall thickness is calculated using the equation below
D = Outside diameter of pipe, in. (mm)
Pi = internal design gage pressure, psi (bar)
S = Applicable allowable stress value, psi (MPa)
The allowable Stress (S) is determined using the formula below
F = Design factor.
E = Weld Joint Factor obtained from Table 403.2.1-1.
Sy = Specified minimum yield strength of the pipe, psi (MPa)
Kindly ensure uniformity is the unit of measurement.
Below is a brief explanation of key inputs to the formula
4.1.1 Internal Design Gage Pressure
The internal design gage pressure is the pressure utilised for the calculation or analysis of the piping component. It should not be less than the maximum steady-state operating pressure at the point being considered, nor less than the static head pressure at that point with the pipeline in a static condition.
4.1.2 Weld joint Factor (E)
The weld joint factor is key in calculating the allowable pipeline stress. The strength of a pipe may be affected by the quality of the weld; therefore, to compensate for the reduction in strength, a factor is incorporated into the allowable stress formula. Kindly note that seamless pipes have no weld; therefore, the weld joint factor is 1; the same applies to some types of welded pipes. Table 403.2.1-1 of ASME B31.4; – 2016 shows weld joint factor (E) values to be used for calculations. The below table is extracted from the reference table. Kindly refer to the table for full details.
4.1.3 Design Factor (F)
The design factor is a safety factor applied to the pipeline’s allowable stress.
As detailed in ASME B31.8, the factor is a function of location class (Class 1, class 2, class 3 and class 4) which defines the population density along the pipeline route. The higher the location class, the lower the design factor, which results in higher wall thickness. For typical design factors used for gas pipelines, refer to table 841.1.6-1 of ASME B31.8.
Unlike the ASME B31.8, the ASME B31.4 code does not define location classes and corresponding design factors; however, the code stipulates that a maximum 0.72 design factor shall be used for wall thickness calculation. Also, the code states that design factors lesser than the 0.72 value may be used. The stated design factor has considered under thickness tolerance and maximum allowable depth of imperfections provided for in the specifications approved by the code.
Note using a lesser design factor will result in thicker pipes which increases the cost of pipe material; therefore, there should be justification for reducing the design factor.
4.1.4 Specified Minimum Yield Strength (SMYS)
Pipe SMYS is the minimum yield strength prescribed by the specification under which the pipe was manufactured. It is expressed in pounds per square inch (psi) or megapascals (MPa)
The pipe manufacturer must guarantee the specified minimum yield strength.
Values of pipes grade SMYS are stated in the specification under which the pipe is manufactured.
Refer to tables 6 and 7 of API Specification 5L, table 2 of ASTM A106 etc. for values and SMYS requirements
4.1.5 Pressure Design Wall Thickness (t)
The pressure design wall thickness is the calculated wall thickness satisfying the internal design gage pressure, excluding allowances for corrosion, erosion threading etc.
4.1.6 Pipe Wall Thickness Allowance
Depending on the design consideration, several allowances should be added to the calculated pressure design wall thickness to cover up for the reduced wall thickness.
Typical examples of allowances include
Allowance for pipe corrosion
Allowance for pipe erosion
Allowances for threading, grooving etc.
Threaded pipe wall thickness is reduced due to the tread depth at the threaded ends. An allowance shall be added to the calculated pressure design wall thickness to cover the loss of thickness.
Corrosion and erosion allowance is the most significant allowance in pipeline wall thickness calculation. The estimated corrosion rate is multiplied by the pipeline’s design life and added to the calculated pressure design wall thickness. Corrosion rate can be estimated using software such as hydrocor. Also, NORSOK M-506 presents a recommended practice for the calculation of corrosion rates in hydrocarbon production and process systems where the corrosive agent is CO2
4.1.7 Pipe Nominal Wall Thickness (tn)
This is the wall thickness listed in applicable pipe specifications or dimensional standards. Listed wall thicknesses are subjected to tolerances usually specified in the applicable standard; however, the design factor specified in ASME B31.4 has considered the specified under thickness and imperfection tolerances.
The nominal thickness is the selected standard thickness listed in ASME B36.10M. The thickness should be greater than or equal to the sum of all allowances as earlier defined and pressure design wall thickness.
Note the pipeline may be subjected to other conditions during construction and operation. These external loads may exceed the internal pressure, therefore the selected pipe wall thickness shall provide the adequate strength required to prevent pipeline collapse taking into consideration mechanical properties, variation in pipe wall thickness permitted by material specification, bending stress, external loads, and out of roundedness. In addition detailed pipe stress analysis shall be performed for the entire pipeline. Refer to section 402 of ASME B31.4 for details of stress calculations.
4.1.8 D/t Ratio Check
After selecting the wall thickness the D/t ratio should be checked. Section 403.2.5 of the code stipulates that the D/t ratio should not be greater than 100. Pipes having D/t ratio greater than 100 are susceptible to flattening, ovality, buckling and denting. Pipe having D/t ratio greater than 100 may require additional protection during construction and installation.
This aim of this example is to perform a wall thickness calculation for a pipeline transporting crude oil. Estimate the pressure thickness required for a pipeline conveying crude oil from a pump station to a tankfarm. Also select an appropriate commercially available thickness. See below pipeline parameters.
Pipeline Nominal Pipe Size: 16
Pipeline Material: API 5L Grade X52
Pipe Type: Seamless pipes
Design Pressure = 102 Barg
Maximum Design Temperature = 38 °C
Internal Corrosion Allowance, (CA) = 3.0 mm
Since the product is crude oil, ASME B31.4 will be utilized for the wall thickness calculation.
SMYS for API 5L Gr. X52 = 360 MPa = 52200 psi (refer to API 5L, Table 6 and 7)
Design Factor = 0.72 (Refer to section 403.2.1 of ASME B31.4)
Pipe Type = Seamless.
Weld Joint Factor (E) = 1.00 (Table 403.2.1-1 of ASME B31.4, for API 5L material;
The only allowance given is the corrosion allowance and will only be considered.
Calculating the Allowable Stress (S)
Substituting the applicable data into equation (3) above
Calculating Pressure Design Wall Thickness
Using equation 2
The only applicable allowance is the corrosion allowance which should be added to the pressure design thickness.
Obtaining Nominal Wall Thickness
As stated in equation (1)
Any available thickness listed in ASME B36.10 equal or greater than 10.996mm can be selected.
This implies the thickness required to satisfactory hold the pressure of 102 Barg is 7.996mm, while the nominal thickness as selected from ASME B36.10 is 12.7mm. As previously explained, the wall thickness shall be used to further perform stress calculation as per section 402.
API 5L: Specification for Line Pipe
ASME B31.4: Pipeline Transportation Systems for Liquids and Slurries
ASME B31.8: Gas Transmission and Distribution Piping Systems
ASTM A 106/A 106M: Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service