Code Checking - DNV Postprocessing
The DNV Postprocessing capability allows you to obtain local buckling unity checks at both load controlled and displacement controlled regions along the pipeline. All included checks are in accordance with the offshore standard, ‘DNV-OS-F101, October 2013’.
The DNV Postprocessing section on the Code Checking tab of the Analysis component is where you define the various inputs required for local buckling checks, such as load/resistance factors, section properties, etc.
Four buttons are present in the DNV Postprocessing section, Load Data, Section Properties, Resistance Factors and General Data. Each of these buttons opens an individual input dialog and these dialogs are discussed in turn over the following four sub sections. Note that default values are provided for the majority of inputs in the DNV 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.
As shown in the figure below, there are six inputs on the Load Data dialog. The first of these inputs is the number of the Functional Stage for the analysis. Once the relevant stage number is specified all remaining installation stages are assumed to be environmental in nature. Note that only local buckling checks are performed on environmental stages. However, these checks will be directly dependent on the output from the functional installation stage. The specified functional stage must be a static installation stage. You can omit a dedicated functional stage from DNV Postprocessing and instead functional loads are taken from the start of each environmental stage.
The next four inputs on the dialog correspond to functional and environmental load effect factors for two different load combinations, ‘a’ and ‘b’. The sixth and final input is a condition load effect factor. These various effect factors are used to scale functional and environmental loads during the calculation of design loads (moments, effective tensions and compressive strains) for the two load combinations.
Load Data Dialog
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 DNV Postprocessing. It also enables you to define a list of DNV 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 section; however it is worth mentioning here that the corresponding inputs fall under three categories:
1.‘Engineering Characteristics’, such as specified minimum yield strength, specified minimum tensile strength, Young’s modulus, etc.
2.‘Code Factors’, such as the fabrication factor, girth weld factor, material strength factor, etc.
3.‘Load Scale Factors’, which are used to modify the magnitude of both functional and environmental loads prior to the computation of design loads.
Section Properties Dialog
Three inputs are available on Resistance Factors dialog, which correspond to the material resistance factor, the safety class resistance factor and the strain resistance factor. These factors are applied directly during the calculation of local buckling unity checks.
Resistance Factors Dialog
The General Data input dialog, as shown in the figure below, is used to define general parameters that influence DNV Postprocessing. The first of these parameters is the Transience Period and it refers to the duration at the start of the analysis (for dynamic stages) that you wish to exclude from DNV 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.
The second and third inputs on the dialog correspond to drop down lists that allow you to specify whether the overbend and sagbend regions of the pipeline are displacement controlled or load controlled. The significance of these inputs, and indeed all DNV inputs, will become more apparent in the proceeding section, which outlines the overall operation of DNV Postprocessing.
General Data Dialog
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 ‘DNV-OS-F101’:
•Plastic Moment Capacity
•Plastic Effective Axial Force Capacity
•Flow Stress Parameter
•Characteristic Collapse Pressure
•Characteristic Bending Strain Resistance
2.Extraction of Functional Loads
For all elements/nodes in the pipe section sets, the relevant loads are extracted from the functional stage. In the case of elements/nodes in a load controlled region (typically the sagbend), these extracted loads correspond to effective axial force (tension) and resultant bending moment. While for elements/nodes in a displacement controlled area (usually the overbend) there is a single load extracted and it is the maximum compressive strain. Note that maximum compressive strain is essentially the Von Mises strain at the point on the circumference of the pipe where the largest negative (compressive) bending strain occurs. Also note that all extracted functional loads are multiplied by the relevant scale factors defined in the Section Properties dialog.
3.Computation of Local Buckling Unity Checks
Looping over all environmental stages, local buckling unity checks are performed at each element/node in the selected pipe sections, as follows:
a.For each time step, after the specified transience period for dynamic analyses, external/internal pressures and relevant load terms are extracted. As in the case of the functional stage, the type of extracted loads depend on whether the element/node in question is in a load or displacement controlled region. Also, as before these loads are scaled by the associated scale factors defined under Section Properties.
b.The extracted load values for the time step are converted into environmental loads by subtracting the related functional values. If by chance the resulting environmental numbers are opposite in sign to their functional counterparts, then they are discarded and assumed to be zero (conservative assumption).
c.The environmental and functional loads are combined with the various load effect factors from the Load Data dialiog to obtain a set of design loads for the time step (for both load combinations, ‘a’ and ‘b’).
d.Local buckling unity checks are then calculated for the time step using the design loads from above in conjunction with resistance factors, extracted pressure terms and engineering parameters (from Step 1). Note that the exact equations used for these unity checks depend on the location of the element/node. For elements/nodes in a load controlled region, equation 5.28b from ‘DNV-OS-F101’ is used, whereas for a displacement controlled region, equation 5.31 is used.
e.The local buckling unity checks from above are compared with maximum values from previous time steps and if they are greater in magnitude then the maximum values are updated accordingly.
4.Creation of DNV 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 DNV 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 contains the maximum local buckling checks at each element/node, as well as the time of occurrence and the load/pressure values that went into the checks. Note that for the functional stage no checks are performed and so only the functional load values are displayed for this stage.
•‘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 DNV 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 DNV 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.