Introduction
The Pipeline Toolbox is home to many tools and calculators. The PLTB User’s Guide presents information, guidelines and procedures for use during design, construction, operations and integrity tasks for field or office applications.
Analysis and design of buried pipelines are usually based on classical theories and semi-empirical methods. The confidence in these approaches is very limited and use of them could lead to a very conservative design, which could cause major increases in costs. One of such methods is the American Petroleum Institute API recommended practice 1102 to design pipelines which are to be located under highways and railroad crossings.
The Wheel and Track Load Analysis calculations were designed to evaluate the overburden and vehicle loads on buried pipe with a Single Layer System (soil only) or a Double Layer Systems (timbers, pavement and soil). Spangler computed the load transmitted to the pipe due to surface loading using Boussinesq theory for a surface point load or distributed load based on numerical integration.
Gas Piping Technology Guide (GPTC) is a method that is proven and acceptable that can be used in the cases when the crossing conditions for design are out of the scope and the limitations of API RP 1102 and PCPISCES. Examples include depth of cover and 48” diameter pipes.
The CEPA Wheel Load, Track Load, and Grid Load calculations were designed to model the effects of different surface loading scenarios for buried pipelines that allows a variety of vehicle types to be analyzed more accurately because it accounts for the distribution of load from the vehicle, wheel contact, axle weight, etc.. The calculations offered here were developed by report released by Canadian Energy Pipeline Association (CEPA) in October 2009 for “Development of a Pipeline Surface Loading Screening Process and Assessment of Surface Load Dispersing Methods” that was performed by Keifner & Associates, Inc.
Module/Application
- API 1102 Pipeline Crossing Highway
- Use Case: Design road crossings using trenchless methods
- Limitations: Applicable to pipes between with depth of cover 3-10 feet
- Differentiator: Accounts for fatigue due to cyclic loading
- API 1102 Pipeline Crossing Railroad
- Use Case: Design road and railroad crossings using trenchless methods
- Limitations: Applicable to pipes between with depth of cover 3-10 feet
- Differentiator: Accounts for fatigue due to cyclic loading
- Design of Uncased Pipeline Crossings
- Use Case: design of Uncased Pipeline Crossings where conditions for design are out of the scope and limitations of API 1102 & PC Pisces.
- Special Considerations:
- Total calculated combined stress should not exceed 100% of SMYS (Safety Factor of 1).
- GPTC Guide sets impact factor at 1.5 for non‐ridged piping.
- Recommend for railroad crossings an impact factor of 1.75.
- Track Load Analysis
- Use Case: Calculate the overburden and track loads on buried pipe with a Single Layer System (soil only).
- Limitations:
- Does not account for fatigue due to cyclic loading
- Soil load on pipe is assume that backfill soil slides down without friction, pipe is supported by itself and does not assist in the support
- Differentiator: Accounts for Single Layer System (soil only) or Double Layer Systems (timbers/pavement and soil)
- Wheel Load Analysis
- Use Case: Calculate the overburden and vehicle loads on buried pipe
- Limitations:
- Does not account for fatigue due to cyclic loading
- Soil load on pipe is assumed that backfill soil slides down without friction, pipe is supported by itself and does not assist in the support
- Differentiator: Accounts for Single Layer System (soil only) or Double Layer Systems (timbers/pavement and soil)
- CEPA
Reference
- Battelle Petroleum Technology Report on “Evaluation of Buried Pipe Encroachments” which considered the theoretical work done by M.G. Spangler on overburden and vehicle loads on buried pipe.
- “Technical Summary and Database for Guidelines for Pipelines Crossing Beneath Railroads and Highways” (GRI-91/0285, Final Report)
- Cornell/GRI Guidelines are given in “Guidelines for Pipelines Crossing Beneath Highways” (Stewart, et al., 1991b) and “Guidelines for Pipelines Crossing Beneath Highways”.
- ASME B31.8 “Gas Transmission and Distribution Systems” “Evaluation of Buried Pipe Encroachments”, BATTELLE, Petroleum Technology Center, 1983
- API RP 1102 – Steel Pipelines Crossing Railroads and Highways
FAQ
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What are crossings – Live Load?
Many questions are asked about API 1102 regarding wheel load cyclic stresses on highways and railroads.
Live highway load, w, the load due to the wheel load at the highway surface. The load from only one wheel set needs to be considered. An axle is considered to have two-wheel sets.
Live rail load, w, pounds per sq inch, is the load applied at the surface of the crossing. It is assumed that the load is evenly distributed over an area that is 8’x 20’.The recommended default is 80,000 pounds (80 Kips) per axle. Check Out
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Validation checks in place for API 1102 – Pipeline Crossing Railroad?
Below are the list of validation checks we have incorporated in API 1102 – Pipeline Crossing Railroad (Gas and Liquid). Check Out
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Validation checks in place for API 1102 – Pipeline Crossing Highway?
Below are the list of validation checks we have incorporated in API 1102 – Pipeline Crossing Highway (Gas and Liquid). Check Out
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Track Load – Interpolation Method (for Influence Coefficient)?
We recently issued an enhancement for Track load calculation by providing an option to use the Interpolation Method to calculate the Influence Coefficient (Ic). The old method calculated the Influence Coefficient using a roundoff method for ‘m-Influence Factor’ and ‘n-Influence Factor’ to get the Influence Coefficient from the table. Check Out
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API 1102 Uncased Crossing 10 Foot Limit Understanding?
Uncased carrier pipe is subjected to internal loading from pressure and external loading from earth loading (dead load) and live (cyclic) loading from highway or railroad traffic. Other loading due to special or temporary conditions should be evaluated on the specific situation in the field (See Knowledge Base Article for Temporary Conditions). Check Out
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Surface Load Mitigation Measures – Best Practice?
Method
Reduce the operating pressure of the pipeline.
Advantages
Provides a direct reduction of the hoop stress due to internal pressure. This reduction allows for additional circumferential stress due to equipment loads.
Disadvantages
– Reduces the beneficial effect of internal pressure on the pipe circumferential bending stresses due to fill and traffic loads.
– Could reduce the overall capacity of the pipeline and therefore should not be considered as a long term fix.
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Difference between “Operating Weight” in Track Load Analysis and “concentrated surface load” in Wheel Load?
The term “Operating Weight” used in the track load analysis is a straight forward analysis as it takes the total weight of the equipment into consideration. We multiply the total weight by 0.5 to reflect the load on each track (see below the equation).
“Concentrated surface load” in the Wheel load calculation requires a more detailed understanding of the vehicle that is crossing. Below schematic will help better understand the requirement for “Concentrated surface load” entry in Wheel Load analysis.
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Determining Trench Width for Crossings?
The basic analysis developed by M.G. Spangler includes frictional forces between the trench wall and the backfill. This permits the weight of the overburden to be partially carried by the surrounding soil and reduces the total soil load on the pipe. The equations require the following information to determine internal friction. Check Out
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Combined Stress for Wheel Load and Track Load?
The question comes up from time to time how to address local stresses for wheel and track load calculations. ASME B31.8 833.9 Local Stresses states that the maximum allowable sum of circumferential stress due to internal pressure (Barlow Design Formula) and circumferential through-wall bending stress caused by surface vehicle loads or other local loads is 0.9ST, where S is the specified minimum yield strength, psi (MPA) per para. 841.1 (a), and T is the temperature derating factor per para. 841.1.8. Check Out
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What Are Timber Mats in Crossing?
Timber mats are used for temporary access roads, work pads, staging areas, and to stabilize the ground beneath heavy equipment, dissipating heavy loads and providing equipment stability.
For short-term water crossings over narrow spans timber mats (also referred to as bridge mats) or crane mats may be an option.
For projects that require moving heavy equipment safely across pipelines, timber mats can be used to create air bridges which will support the weight of heavy equipment while protecting the pipelines below. Air bridges can also be used to cross culverts, ditches, or sensitive ground that needs to be bridged. Check Out
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Redistributing Wheel and Track Loads (Pavement and Other Materials)?
This knowledge base article is a takeoff from “Pipeline Toolbox (PLTB) and Vehicles over Buried Pipelines – Maximum Allowable Stress”. These predictions through Spangler’s work, Battelle and others who contributed to validation makes PLTB one of most used programs in the industry. It includes temporary wheel and track load crossings that considers a uniformly distributed load at the surface while calculating the load on the pipe. This article will focus on concrete, asphalt and timber mats as well as other materials such as steel plates and composites. The other materials are not part of the Pipeline Toolbox which will be discussed. Check Out
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Pipeline Crossings – External Loads and Vehicles over Buried Pipelines “Maximum Allowable Combined Stress”?
This article was created due to the number of inquiries regarding the Maximum Allowable Combined Stress for Class 3 Design Areas. The research by M. G. Spangler, Battelle and industry allowed this information to be a standard practice today with Wheel and Track Load analysis today. Check Out
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Applicability of Class Location for Liquid Lines? What is the design factor used for Liquid Pipeline Operation?
Design factor specification for Liquid Pipelines are governed by ASME B31.4 Table 402.3.1(a)
Excerpts from B31.4: Check Out
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Loading Requirements for API 1102 E-80 and E-90 Railroad Crossings?
API 1102 for railroad crossing program is based on the design methodology that has been used in analyzing existing uncased pipelines and designing new uncased pipelines that cross beneath railroads. This API design methodology relates to the train engine (E-80) which is the heaviest load: Check Out
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What are Cased Crossings?
The pipeline industry begins to experience casing problems such as carrier pipe leaks/failures under road crossings.
- Atmospheric corrosion due to condensation (particularly at elevated temperatures and cold soils)
- Metal to metal shorts
- Electrolytic coupling or contact
Can the casing pipe protect the carrier pipe from external loadings? Check Out
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Understanding Impact Factors in Pipeline Design (CEPA)
What is an Impact Factor?
An impact factor is a multiplier used in pipeline design to account for the additional stress caused by moving vehicles above the pipeline, compared to static loads. Think of it as a “safety buffer” that accounts for the dynamic nature of traffic. Check Out