SACSTM Wave Spectral Fatigue Analysis Seminar Notes: Topic A. A.
File St Structure an and La Layout
Over Overvi view ew -
Seasta Seas tate te info inform rmat atio ion n is is specified separately from the model file. This will eliminate the need for having multiple model files throughout the course of the analysis.
Topic B. Foundation Super Element Over Overvi view ew -
The The P PSI SI prog progra ram m is is u use sed d to to create a super element to represent the foundation. This super element will replace pile stubs, dummy piles or springs required in past analyses.
Step 1.
Determine which load cases are to be used to determine the foundation stiffness. Add these load cases to a Seastate input file if necessary.
Use either a wave in the range of common waves in the fatigue environment or an effective wave height and period based on center of damage of scatter diagram. Be sure that these load cases create axial load in addition to lateral load in the piles.
Step 2.
Specify analysis options in the Seastate input file including load case options.
Don’t forget to include the dummy load cases added for the foundation.
Step 3.
Add foundation super element input to the PSI input file using the PILSUP line or input under the Foundation options in the Executive run file Wizard.
If dummy load cases are used for SE, may want to exclude them form pile capacity and code check.
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Notes:
Topic C.
Dynamic Characteristics
Overview -
Dynpac is used to create mode shape and mass files required for subsequent dynamic analyses.
Step 1.
Specify retained or master degrees of freedom in the model if they are not already done.
PSI ignores fixities designated with ‘2'.
Step 2.
Create Seastate input file used for hydrodynamic modeling only.
Use FILE ‘J’ option so that only loads in model are considered. Note: Seastate option is overridden by Executive.
Step 3.
Specify load cases to be converted to mass in the Seastate input file.
Use an operational equipment deck mass when converting equipment operational and/or live loads to weights.
Step 4.
Create Dynpac input file.
Use ‘SA-Z’ option if user-defined loads are to be converted to mass.
Step 5.
Create runfile.
If importing a super element, specify import option in Analysis options tab or in model file.
Step 6.
Check mode shapes and periods.
If water depth is greater than 400 feet or period is greater than 3 seconds, dynamic analysis is required.
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Notes:
Topic D.
Wave Steepness Calibration (Optional)
Overview -
The transfer function is usually developed for a constant wave steepness designated in the metocean data. If wave steepness is not available, response functions statistics can be used to determine the appropriate steepness.
The appropriate steepness is usually given. The following describes one method available to determine the appropriate steepness if none is specified.
Step 1.
Create Seastate input file containing the reference wave stepped through the structure.
Use a most probable maximum reference wave height of 1.86*H c and period of T c /1.81 where H c and T c are the center of damage wave.
Step 2.
Determine base shear range from reference wave.
Range = maximum base shear - minimum base shear.
Step 3.
Create Seastate input for transfer functions using various wave steepnesses and grouping waves near the natural period of the structure.
Use the GNTRF feature to define the waves. Group waves such that periods corresponding to Tn, Tn±c, Tn±2c and Tn±3c are used where ‘c’ represents percent of critical damping. If wave spreading is used, be sure to use the same waves for each direction.
Step 4.
Create W ave Response input file specifying a wave spectra defined by the center of damage wave and referencing the appropriate wave numbers of the transfer functions.
Use one WSPEC line for each wave steepness and reference the waves representing the transfer function of the desired steepness. Be sure to use the center of damage wave for most probable maximum wave.
Step 6.
Execute Wave Response and use the Response Function Statistics report to calculate the most probable maximum base shear range for each response function.
Multiply the RMS shear by 3.7 to get the most probable maximum shear value.
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Topic E.
Transfer Function Plots
Overview -
Plot global base shear and over-turning moment transfer functions for dynamic analysis. These plots are used to determine if a sufficient number of waves are used and if the waves are group around critical frequencies.
Notes: The steps outlined should be performed for each wave direction. If wave spreading is to be considered, additional wave directions may be required so that transfer function data is available for a minimum of 22.5 degree increments.
Wave response is used to generate plots for dynamic analysis while Seastate is used for static analysis. Create the Seastate input file used to generate the waves grouping waves near the natural period of the structure. Create wave data for each direction to be considered.
Use the GNTRF feature to define the waves. Group waves such that periods corresponding to Tn, Tn±c, Tn±2c and Tn±3c are used where ‘c’ represents percent of critical damping.
Step 2.
For dynamic analysis, create Wave Response input file or specify transfer function plot options to create plots only.
Specify wave numbers on the TFLCAS line if an input file is used.
Step 3.
Create Wave Response runfile for dynamic or Seastate runfile for static fatigue.
For static fatigue, select the ‘Transfer Function Plots’ Seastate option in the Executive (if not specified on the LDOPT line).
Step 4.
View plots to be sure that no peaks or valleys have been missed and to ensure that valley does not occur at natural period.
If natural period falls in valley of BS transfer function, may have to adjust operational mass assumption to shift period to a more conservative location. Natural period may be shifted by 5 - 10% by varying operational mass.
Step 5.
Modify wave definitions in the Seastate input file as needed.
Step 1.
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If wave spreading is used, be sure to use the same waves for each direction.
Topic F.
Transfer Function Nominal Stresses
Notes:
Overview -
For dynamic analysis, Wave Response is used to generate equivalent static loads which are solved by the SACS module. Seastate is used to generate static wave forces for static fatigue analysis.
The steps outlined should be performed for each wave direction.
The SACS module creates a solution file containing the nominal stresses required by the Fatigue program for creating transfer functions. If fatigue is to be checked for the pile below the mudline, a PSI analysis must be used to solve the transfer function nominal stresses.
Must specify the ‘PP’ option on the PSIOPT line or in the Executive Foundation options when creating the run file so that a separate solution file for the pile below the mudline will be generated.
Step 1.
For dynamic analysis, create the Wave Response input file used to generate equivalent static loads at the desired wave crest positions or specify Transfer function options in the Executive run file wizard.
If using a Wave Response input file, specify ‘-1' maximum iterations if no iterations are to be performed. Plot features can be left in the input file.
Step 2.
Create Wave Response runfile using the solve for equivalent static loads option for dynamic analysis. For static analysis, create a linear static analysis runfile.
If appropriate, be sure to include the effects of the foundation by using either the Super element import option if a foundation super element file is to be used, or including PSI if a pile soil interaction analysis is to be included. .
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Topic G.
Fatigue Analysis
Overview -
The Fatigue program builds a separate transfer function for each point around the connection for each wave direction.
Step 1.
Create the Fatigue input file containing S-N curve, SCF options, joints to be checked and the fatigue environment to be used to calculate the expected life.
Notes:
Use ‘SEAS’ option so that wave height and period data can be retrieved directly from the Seastate input data. No need to specify number of positions per wave if a load case was created for each crest position. Specify the wave environments for each direction. Use scatter diagram feature to define wave environments if possible. If using wave spreading, may have to create dummy fatigue cases if no environment data is available for intermediate directions.
Step 2.
Create run file for fatigue of structure.
Be sure to specify solution files (and Seastate input files if applicable) in the same order that the wave environments are defined in the Fatigue input file.
Step 3.
Create run file for fatigue of pile below the mudline if applicable.
Be sure to specify pile solution files (and Seastate input files if applicable) in the same order that the wave environments are defined in the Fatigue input file.
Step 4.
Check fatigue lives. Add Extract data to Fatigue input file to extract connections to be reviewed by Interactive Fatigue.
This requires recreating the runfile if original input file does not contain extract daqta.
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File Description
Directory
File Name
Model file Seastate Input file for foundation linearization PSI Input file PSI listing file Foundation Super Element file PSI Plot file
Dynpac Input file Seastate Input for Dynpac Dynpac Listing file Dynpac Mode Shape file Dynpac Mass file
Wave Response Plot Input file Wave Response EQS Input file Seastate Input file defining transfer function periods Transfer Function 000 Degree TF Plot file Transfer Function Listing file Transfer Function Solution file for structure Optional Transfer Function Solution file for piles
Fatigue Input file for structure Optional Fatigue Input file for piles Fatigue Listing file Fatigue Extract file
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