Code Checking - API Postprocessing
The API Postprocessing capability allows you to obtain local buckling due to combined bending and external pressure unity checks along the pipeline. There is also a check for collapse due to external pressure and pipe diameter over thickness ratio. All included checks are in accordance with the API Standard: ‘API Recommended Practice 1111 – 5th Ed., Sept 2015’
The API Postprocessing section available on the Code Checking tab of the Analysis component is where you define the various inputs required for local buckling checks such as bending safety/collapse factors, section properties etc.
Two buttons are present in the API Postprocessing section, Section Properties and General Data. Each of these buttons opens an individual input dialog and these dialogs are discussed in turn over the following two sub sections. Note that default values are provided for the majority of inputs in the API Postprocessing section and these defaults reflect typical values for installation analysis. However caution is also urged in the use of the defaults and it is recommended that the user fully understands their significance before accepting them.
The Section Properties dialog, as shown in the figure below, allows you to select which pipe sections from the related model are to be included in the API Postprocessing. It also enables you to define a list of API specific properties for each of the selected pipe sections. Note that there is an option to select all pipe sections from the model in which case you can define a single list of properties for all sections. The properties list itself is discussed in detail in the Data Inputs article; however it is worth mentioning here that the corresponding inputs fall under three categories:
1.‘Engineering Characteristics’, such as specified minimum yield strength and Young’s modulus.
2.‘Code Factors’, such as the collapse/collapse-reduction factors and ovality.
3.‘Load Scale Factors’, such as bending safety factors for installation or operation.
Section Properties Dialog
The General Data input dialog, as shown in the figure below, is used to define the Transience Period that influences API Postprocessing. This refers to the duration at the start of the analysis (for dynamic stages) that you wish to exclude from API computations (as it may contain spurious output from dynamic transient effects). This input is expressed as a percentage of the total duration for the installation stage.
General Data Dialog
The operation of API Postprocessing is best described as a series of steps:
1.Calculation of Engineering Parameters for Selected Pipe Sections
For every element in each selected pipe section set, the following parameters are calculated using the input section properties in conjunction with relevant equations from ‘API RP 1111’:
•Pipe Collapse Pressure
•Buckling Strain under Pure Bending
•Collapse Reduction Factor
•Combined Pressure & Bending Collapse Factor
2.Computation of Buckling Unity Checks
Looping over all included installation analysis stages, and using equation (13) of section 184.108.40.206 in ‘API-RP-1111’, a buckling unity check due to combined bending and external pressure is calculated at each node in the selected pipe section sets. This calculation is based on the maximum predicted bending strains and pressures at the nodes in conjunction with engineering parameters and load/scale factors taken from the user inputs. Note that for static stages the calculation is done at the last step in the analysis whereas for dynamic stages the calculation is done at each time step and then the maximum/governing check across the entire analysis duration is selected for the each node.
3.Creation of API Report File
Once the computation process is completed the relevant data can be output to the report file. The content and format of this file is self-explanatory, but nonetheless it is worth noting the main sections:
•‘Input Data Echo’, which summarises all the inputs made in the API Postprocessing section of the Code Checking tab.
•‘Element Parameters’, where the calculated parameters from Step 1 are displayed for each element in all selected pipe section sets.
•‘Stage Results’, which contain the maximum combined bending and external pressure buckling unity checks at each element/node, as well as the time of occurrence and the bending strain/pressure values that went into the checks.
• ‘Summary Results’, where the maximum unity checks across all elements/nodes are indicated per stage. There is also a flag to highlight whether the checks in question exceed the allowable value of 1.
Note that unlike other PipeLay output files, the API report file is in CSV (Comma Separated Values) format and so it is viewed using Windows™ Excel. When you launch this file using the Open API Report button on the Results tab of the Analysis component, it is recommended that you zoom out to 70% and resize the columns to fit the data (click on the ‘Select All’ button on the top left corner of the sheet and then double click on the first column partition). This will then allow you to fully view the data in the file.