The OBS software, also known as AGA software, has been upgraded by INTECSEA Over the last few years. This release is the first issue of the upgraded software to the public after the latest issue of Version 3.0. A summary of the main changes to the software include:
- General:
- Complete rewrite on the graphical user interface
- Storing projects models in a database format for easy archiving and handling
- Changing the hard-coded parameters to user defined parameters
- Addition of plotting features
- Addition of the absolute lateral static stability code check module following DNV-RP-F109 (2011)
- Level 1:
- Addition of the logarithmic boundary layer formulation
- Addition of the marine growth specification option
- Level 2:
- Addition of the logarithmic boundary layer formulation
- Addition of the marine growth specification option
- Addition of the JONSWAP spectrum
- Addition of the Verley and Lund for Clay soils
- Addition of the Verley and Sotberg Sand soils
- Resolving the reported bug i.e. excessive embedment prediction in current dominated conditions
- Addition of parametric run functionality for pipe wall thickness and water depth
- Level 3:
- Addition of the logarithmic boundary layer formulation
- Addition of the JONSWAP spectrum
- Addition of running multiple wave seeds per run
- Addition of plotting and reporting per wave seed for multiple seeds runs
- Absolute Stability Method:
- This is a new module added based on the requests from users. The module conducts absolute stability check following methodology presented in the DNV-RP-F109 version 2011 [2].
The new graphical user interface was designed with the intent to maintain the overall layout and workflow of the previous version for the additional comfort of the active users to get familiarized with the new features as they begin to use the new software.
A significant series of verification tests were undertaken for the new additions especially for new developments of Level 2. A summary of finding was published in [18].
In production of this manual several previously generated reports for PRCI, including the works done by Kellogg Brown & Root (KBR) Inc. have been used. The usage includes direct use of texts and pictures that are still applicable to the upgraded version of the software. A list of reference reports is presented in the reference section of this manual.
The main components of the software include:
Level 1
Level 1 is the simplest of the three levels and is only used to get a feel of what has been done historically. The problem is considered to be static and the dynamic effects are not allowed. As a result, the pipeline is required to remain stationary on the seabed and the stability design criterion is that “no movement” occurs.
The results of the analysis give a required submerged weight or coating thickness in order for the no movement criterion to be met. Note that this method uses a single wave with Morison’s force equation, and may allow some movement if unconservative coefficients or a wave smaller than the maximum wave is used. It is anticipated that Level 1 analysis will be performed under the following circumstances.
- To obtain a simple, quick, reasonable estimate of historical concrete coating requirements
- To check Level 2 or Level 3 analysis against a more conservative yardstick, used in earlier designs.
Level 2
The Level 2 design tool is used to determine weight coating requirements based on the latest pipeline hydrodynamic and soil force formulations. It provides the designer a simplified method for estimating pipe embedment and the resulting soil resistance. This program should be used for all preliminary designs, and resulting concrete thicknesses should be adequate for most final designs.
Level 3
In a Level 3 analysis, a two-dimensional dynamic pipeline model is used to calculate stresses and deflections in the line during storm conditions. Input parameters are specified over a grid taking account of lengthwise variations along the pipeline route. A time history of the on-bottom current velocities and wave kinematics is simulated by the program to model the pipeline behavior in irregular seas. The pipeline model considers the impact of end restraints such as risers and anchor points by modeling them as a series of springs at the ends of the pipeline.
It is anticipated that this type of analysis will be used to check and/or calibrate designs produced using a Level 2 analysis, particularly where the Level 2 assumptions do not accurately represent the actual situation. The special features may be the ends restraints, other structural restraints, cases where the axial tension could develop, or if excessive pipe movements are allowed.
Absolute Stability Method
The Absolute Stability Method (ASM) module performs a static stability analysis as per guidelines in DNV-RP-F109 [2]. The module incorporates various elements such as:
- Application of safety class information considering geographic location, operating / temporary conditions, and seabed type
- ‘Extreme horizontal’ and ‘extreme vertical’ forces applied together on pipeline
- Soil friction and passive resistance model
Among other informational outputs, this analysis calculates the lateral and horizontal utilization factors (UFs) for pipeline absolute stability.
References
- Verley, R. L. P. and Lund, K. M. A Soil Resistance Model for Pipelines Placed on Clay Soils, OMAE – Volume 5, 1995
- DNV-RP-F109, On-Bottom Stability Design of Submarine Pipelines, Det Norske Veritas, Nov-2011
- Ochi, M. K. Hubble, E.N. Six-parameter wave spectra, Proceedings 15th International Conference on Coastal Engineering, Honolulu, 1976
- DNV-OS-F101, Submarine Pipeline Systems, Det Norske Veritas, Oct-2013
- On-Bottom Stability in Deep Waters Note, Det Norske Veritas
- Catalog No. L51522e, PR-170-185, Hydrodynamic Forces on Pipelines: Model Tests, Prepared for PRCI by the Danish Hydraulic Institute, Dec-1986
- PR-170-716, Hydrodynamic Forces Acting on a Pipeline in an Open Trench, Prepared for PRCI by the Danish Hydraulic Institute, Jan-1988
- Ochi, M. K. and Hubble, E.N. Six-Parameter Wave Spectra, Proceedings 15th International Conference on Coastal Engineering, Honolulu, 1976
- Mardia, K. V., Statistics of Directional Data. London: Academic Press, 1972
- Borgman L.E., Directional Wave Spectra from Wave Sensors. In: Earle M.D., Malahoff A. (eds) Ocean Wave Climate. Marine Science, vol 8. Springer, Boston, MA, 1979
- Mardia, K.V., Some properties of clasical multi-dimesional scaling,Communications in Statistics – Theory and Methods, 1978
- PR-169-186
- PR-175-420
- Catalog No. L51570e, Contract PR-194-719, Energy Based Pipe-Soil Interaction Models, PRCI, June 1988
- PR-162-157
- PRCI Project PR-178-04405, Submarine Pipeline On-Bottom Stability Volume 1: Analysis and Design Guidelines, Kellogg Brown & Root, Inc., Dec-2008
- PRCI Project PR-178-04405, Submarine Pipeline On-Bottom Stability Volume 2: Analysis and Design Guidelines, Kellogg Brown & Root, Inc., Dec-2008
- Abdolmaleki, K. and Gregory, C. J., Performance of Pipe-Soil Interaction Models in a Quasi-Dynamic Approach to Pipeline Stability Analysis, OMAE-77988, 2018, Spain
- Jacobsen, V., Bryndum, M.B., and Bonde, C., “Fluid Loads on Pipelines: Sheltered or Sliding,” Proc. 21st Offshore Technology Conference, Paper No. OTC 6056, Houston, 1989
FAQ
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Can OBS accurately model for HDPE Pipe?
Except for the Level 3, the software can be used for stability assessment of HDPE pipes.
The level 3 uses a beam-element type of finite element approach that takes the stiffness of the pipe into account. However, the software has only been applied to steel pipes and no validation has been conducted on using Level 3 for analysis of flexible pipes.
Therefore, it is not recommended to use Level 3 at this stage. Check Out
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What is the reference height of current velocity in Level 3?
The reference level for current input in Level 3 is the elevation of the top of the pipe, including any coatings, and assuming no embedment.
The theory used to convert specified currents to the elevation of the top of pipe is at the discretion of the user. Check Out
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Does OBS have a water depth limitation?
In the PRCI OBS V3.0, the software could not receive water depths less than 6m.
In the PRCI OBS V4.0, this restriction has been removed and users can input any water depth. However, the applied theory has not been revised is still based on linear wave theory.
Therefore, it is the users’ responsibility to ensure that ratio between the inputted wave data at the given water depth does not exhibit very nonlinear / breaking wave scenario. Check Out
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What is lateral factor of safety in Level 2?
“Lateral factor of safety” means Horizontal Safety Factor at Theta.
The vertical factor of safety is computed but ignored. It is not significant if the pipe moves up and down slightly so long as it does not move horizontally as determined by the horizontal safety factor calculation.
Pipeline stability for Level 2 analysis requires that at the end of 4 hr storm buildup the horizontal safety factor at theta for U(1/100) must be 1 or greater and that at the end of the additional 3 hr storm the horizontal safety factor at theta for U(1/1000) must be 1 or greater. Check Out
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What is the meaning of Factor of Safety in Level 3?
This is the momentary ratio of hydrodynamic force and lateral resistance at each time history. Check Out
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In Level 3, can factor of safety be used as a Pass/Fail criteria like Level 2?
Currently, no way has been found to use the factor of safety as calculated in Level 3 as a pass/fail criterion like Level 2. Check Out
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When does hydrodynamic ramping force apply for Level 3?
Consult section 5.8.4 from the version 4.0 User’s Manual:
Ramping force begins at t=0. To simulate slowly built-up dynamic forces use a “Build-up sea-state ramp” of 12 or less. Check Out
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What is the general guidance on usage of Level 3?
Preliminary stability analyses should be completed using Level 2 or the ASM module, so as to have a good starting point for a Level 3 stability analysis.
- For each pipe outer diameter and weight, sufficient runs need to be made with different “seeds”. Random seeds are numbers used as internal input to generate random irregular sea states, with two different seed values resulting in two different sea states. Performing the analysis with a sufficient number of storm simulation seeds provides a reliable indication of the range of results which may be obtained for the specified wave spectrum parameters. Check Out
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Can I use OBS Level 2 or 3 to assess stability of power cables of wind turbines at crossings areas?
No.
From the embedment point of view, it is realistic to assume that the cable has zero embedment, but the limitation of OBS is that it cannot capture the influence of free span zones on the cable’s lateral stability. Check Out
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In Level 2, if the horizontal factors are shown as >1 but the vertical are <1, is the pipeline considered stable?
The software produces two vertical safety factors. One is corresponding to the minimum horizontal safety factor at wave angle “Theta” and the other one is the independently minimum vertical safety factor.
The lateral soil resistance includes a passive component, which is solely dependent on the pipe embedment.
In scenarios where lift force momentarily gets larger than the Submerged weight, the vertical safety factor < 1 and the pipe might lose contact with seabed, however, the embedment related component of the passive soil can remain nonzero. Check Out
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PRCI OBS V3.0 Illegal Operation! Message
If you receive an Illegal Operation! message when switching between Levels, this could provide a workaround for you. Check Out
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OBS V4.0 Error Contacting Registration Service Message
If you are trying to activate OBS Version 4.0 and receive an error message, this quick guide should help us gauge where the root of the issue is and how best to resolve this for you. Check Out