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On-Bottom Stability

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SIMPLIFIED ANALYSIS. COMPREHENSIVE DESIGN. COMPLEX MODELING

On-Bottom Stability Software (OBS) features include:

  • Calculation of kinematics from 3-D irregular waves
  • Hydrodynamic force models
  • Models for partial burial
  • Non-linear, time dependent interaction between pipeline and soil
  • Non-linear pipeline material properties (for boundary elements)

These features allow for detailed analysis of specific pipeline sections subjected to specified environmental conditions. Additionally, the new release consists of three levels with Level 1 being used for quick simplified analysis, Level 2 for comprehensive detailed design and modeling and Level 3 for advanced and very complex subsea pipeline design modeling.

Overview of PRCI OBS V 4.2

The new release of the PRCI OBS Software Tool is the first public issue of the upgraded application since the original 3.0 version. Upgrades include modernization, bug-fixes and the addition of new functionality to the software.

The new interface was designed to maintain the overall layout and workflow of the previous version so existing users can quickly and easily become familiarized with the new features, which include:

  • Database structural enhancements for storing and accessing project models
  • Increased flexibility with an increase in user defined parameters
  • Plotting features added
  • Addition of the absolute lateral static stability code check module following DNV-RPF109 (2011)
Level One – Simplified Analysis
  • The addition of the logarithmic boundary layer formulation.
  • The addition of the marine growth specification option.
Level Two – Comprehensive Design
  • The addition of the JONSWAP spectrum.
  • The addition of the Verley and Lund for clay soils.
  • The addition of the Verley and Sotberg for sand soils.
  • The addition of parametric-run functionality for pipe wall thickness and water depth.
  • Resolving a reported bug (i.e. excessive embedment prediction in current dominated conditions).
Level Three – Complex Modeling
  • The addition of running multiple wave seeds per run.
  • The addition of plotting and reporting per wave seed for multiple seed runs.
Absolute Stability Method
  • This new module was added based on feedback from users. The module conducts absolute stability check following methodology presented in the DNV-RP-F109 version 2011 [2].

Getting Started

References

  1. Verley, R. L. P. and Lund, K. M. A Soil Resistance Model for Pipelines Placed on Clay Soils, OMAE – Volume 5, 1995
  2. DNV-RP-F109, On-Bottom Stability Design of Submarine Pipelines, Det Norske Veritas, Nov-2011
  3. Ochi, M. K. Hubble, E.N. Six-parameter wave spectra, Proceedings 15th International Conference on Coastal Engineering, Honolulu, 1976
  4. DNV-OS-F101, Submarine Pipeline Systems, Det Norske Veritas, Oct-2013
  5. On-Bottom Stability in Deep Waters Note, Det Norske Veritas
  6. Catalog No. L51522e, PR-170-185, Hydrodynamic Forces on Pipelines: Model Tests, Prepared for PRCI by the Danish Hydraulic Institute, Dec-1986
  7. PR-170-716, Hydrodynamic Forces Acting on a Pipeline in an Open Trench, Prepared for PRCI by the Danish Hydraulic Institute, Jan-1988
  8. Ochi, M. K. and Hubble, E.N. Six-Parameter Wave Spectra, Proceedings 15th International Conference on Coastal Engineering, Honolulu, 1976
  9. Mardia, K. V., Statistics of Directional Data. London: Academic Press, 1972
  10. 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
  11. Mardia, K.V., Some properties of clasical multi-dimesional scaling,Communications in Statistics – Theory and Methods, 1978
  12. PR-169-186
  13. PR-175-420
  14. Catalog No. L51570e, Contract PR-194-719, Energy Based Pipe-Soil Interaction Models, PRCI, June 1988
  15. PR-162-157
  16. PRCI Project PR-178-04405, Submarine Pipeline On-Bottom Stability Volume 1: Analysis and Design Guidelines, Kellogg Brown & Root, Inc., Dec-2008
  17. PRCI Project PR-178-04405, Submarine Pipeline On-Bottom Stability Volume 2: Analysis and Design Guidelines, Kellogg Brown & Root, Inc., Dec-2008
  18. 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
  19. 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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

  • 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
  • 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

  • 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

  • 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

  • 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

Updated on October 13, 2023
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