Data Conversion Program for Stress Analysis in Pipeline Design
The Requirement of Local Coordinate Data for Pipeline Design
Pipeline routing is different from normal pipe way arrangement in which the piping configuration is arranged regularly in the northward or eastward direction. Pipeline routing will produce a large amount of a thermal expansion over long distances, and a pipeline turning angle at an expansion loop is not at a right angle in order to smooth the flow of pipeline contents. As a result, the piping stress analysis shall provide pipe support loadings and displacements in local coordinate data in the axial and lateral directions of a pipeline for support load design and shoe length selection.
In general, a piping stress analysis program delivers the analysis result in mixed data between global coordinate data and local coordinate data. Usually, it is a complex and tedious process to use an Excel spread sheet method to convert global coordinate data into local coordinate data. The Data Conversion Program reads the stress analysis output file and automatically transforms the global coordinate data into useful local coordinate data.
Bentley’s AutoPIPE Program
The output file of AutoPIPE contains a Support Force table; in the LOCAL column listed directions can be interpreted as follows:
Back or forward - Axial load;
Left or right – Lateral load;
Up or down – Vertical load.
For a vertical support, there will be vertical loads for weight. The friction forces and movements will be presented in global coordinate values. In order to obtain all data in terms of local coordinates, the vertical support can be input as a guide with a gap. The gap shall be large enough to allow a free movement of the pipe support. The disadvantage of using this method will be the confusion of which one is actual guide and which one is simulated guide in the computer output.
The Data Conversion Program will remove this confusion by transferring those values from global coordinates to local coordinates.
Intergraph’s Caesar II Program
The output file of Caesar II contains a local restraints table which presents local support forces, and the displacements at support locations are presented in a global coordinate data. Consequently, the global displacement data shall be transformed to local coordinate data by using Data Conversion Program.
Stress Analysis in Pipeline Design
CSA Z662 Standard covers oil and gas pipeline systems that convey liquids and gases.
For straight steel pipe, the design pressure for a given design wall thickness shall be determined by the following design formula:
P = (2St/D) x F x L x J x T
P = design pressure, Mpa
S = specified minimum yield strength, Mpa
t = design wall thickness, mm
D = outside diameter of pipe, mm
F = design factor, 0.8
L = location factor
J = joint factor
T = temperature factor
Annex I - Oilfield Steam Distribution Pipeline
There is limitation that CSA Z662 Standard does not apply to piping with metal temperatures above 230 degree C. In this case, the Annex I - Alternate Provisions shall apply the methodology of ASME B31.3, Chapter IX: High Pressure Piping. The basic allowable stress is listed materials from ASME B31.3, Paragraph K302.3.2 (b), and is not more than the lower of two-thirds of the specified minimum yield strength at room temperature and two-thirds of the yield strength at design temperature for materials listed or not listed in the ASME Boiler and Pressure Code, Section II, Part D, Properties.
AutoPIPE does not support Chapter IX High Pressure Piping. User can simulate using regular B31.3 Chapter II by entering the material as non-standard and set the hot and cold allowable as per Chapter IX. A new material library shall create to add the material properties.
Sour Service Pipeline
For pipeline design to convey gas with a content of more than 10 moles of hydrogen sulphide gas per kilomole of natural gas, the design stress levels may not be greater than
(a) 60% SMYS(specified minimum yield strength) for all underground piping, and
(b) 50% SMYS for all above ground piping.
Piping used in sour service can be susceptible to different cracking mechanisms caused by hydrogen sulphide, and due consideration shall therefore be given in design to material selection and heat treatment.
Design of Pipe-supporting Element
Where welded attachments are required for pipelines designed to operate at hoop stress levels of more than 50% of the specified minimum yield strength of the pipe, such attachments shall be welded to a separate cylindrical member that completely encircles the pipe and such an encircling member shall be welded to the pipe by continuous circumferential welds.
Output Report of Stress Analysis Result
The stress analysis output file includes batch report of Support and Model Listing for AutoPIPE, in which the coordinate data listing table will be used to obtain pipeline routing angles. The pipeline stress analysis may comprise a number of design conditions and result in different data, but only the maximum absolute value will be selected for the design as shown in Result tab. The Data Conversion Program lists pipe support design information as shown in the following:
Stress analysis program node number;
Pipe support tag number;
Axial support movement in mm;
Lateral support movement in mm;
Axial anchor force in newton;
Lateral guide force in newton;
Vertical pipe weight in newton.
The advantages of using the Data Conversion Program are simple, fast and accurate.