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Presentation Pieter Toor gave during the Hydrographic Society Benelux workshop about the z-axis positioning with Veripos.

Presentation Pieter Toor gave during the Hydrographic Society Benelux workshop about the z-axis positioning with Veripos.

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  • 1. Real-time Tidal Observation using High-Accuracy GPS Positioning Pieter Toor Research & Development Manager VERIPOS Hydrographic Society Benelux, Dordrecht 7 th May 2008
  • 2. Company Information
      • A product line within Subsea 7
      • Established in 1989
      • Providing precise navigation & positioning services
      • Global business
      • Specialising in marine operations in offshore oil & gas sector
  • 3. Corporate Structure Subsea 7 Inc. Construction / IRM VERIPOS i-Tech Drill Rig Support Oslo S.E.
  • 4. VERIPOS Background
    • 2008 – VERIPOS +/- 75 people
  • 5. Overview
    • Advances in Positioning Technologies
    • Overview of Tides
    • Using High-Accuracy GPS to Determine Tide
    • Veripos Implementation
    • Benefits and Applications
    • Future
  • 6. Advances in Positioning Technologies
    • Positioning is essential technology for offshore operations
      • Satellite Navigation technology can now be considered a mature and accepted technology
    • Augmentation data from service providers allows a higher level of accuracy & precision over standalone GPS
      • Services offered range in accuracy from the metre level to the decimetre level
    • Satellite navigation will remain a key technology for offshore operations
  • 7. Advances in Positioning Technologies
    • Positioning techniques can now deliver position solutions at the decimetre level
      • Future enhancements will see new satellite constellations and modernisation of the current systems
      • Will see developments with new position solutions
        • Different levels of accuracy
        • Different combinations of satellite signals
      • High-Accuracy positioning systems allow use in new applications
        • One area is the determination of Tidal information
  • 8. What is Tide?
    • Tide is the periodic rise and fall of the sea level under the gravitational pull of the moon and sun
    • Tides cause periodical variations in the ‘equilibrium’ sea level called Mean Sea Surface
  • 9. Tidal Trend in High-accuracy Positions
    • Veripos Ultra DGPS (PPP) system offers 10-20cm vertical accuracy
      •  High correlation between tides and DGPS heights after removing antenna motion relative to waterline
    Ultra PPP Height and Predicted Tide
  • 10. GPS to Vertical Reference - Offsets
    • Reduce GPS antenna height via a few basic steps:
    • reduce WGS84 height of antenna reference point to vessel’s CRP
      • inputs: antenna offsets, pitch & roll and heading, heave
    • reduce vessel CRP to waterline
      • inputs: draft, squat
    • correct for additional local effects to finally derive the waterline in WGS84 (optional)
      • e.g. currents, atmospheric pressure
    Geoid: T1 = h – H + D – N Chart datum: T2 = h – H + D – L Ellipsoid: T3 = h – H + D
  • 11. Veripos Tides Implementation
    • Reduce the effect of heave, pitch and roll by averaging the Ultra high-accuracy heights
      • ensure that the position used is an accurate Ultra position (not Standard DGPS or converging) using QC tag to Error Ellipse/SD threshold of less than 0.1m
    • After 39hrs de-tide the heights by using the Doodson filter
    • Leave actual tidal reduction to client, i.e. antenna offsets, draft & squat
  • 12. Doodson Filter
    • The Doodson X0 filter is a simple filter designed to damp out the main tidal frequencies
    • It takes hourly values, 19 values either side of the central one
    • A weighted average is taken with the following weights (1010010110201102112 0 2112011020110100101)/30
    • ‘ Ultra Tide ’ estimate only available after initial 39hrs of observation
    • Secondary instantaneous tide value, ‘ Geoid Tide ’, is based on the deviation from the EGM96 geoid model
  • 13. Ultra Tide vs. Geoid Tide 39 hrs 0 hrs Doodson X0 Ultra Tide
    • Ultra Tide can only be calculated after 39 hours
      • Unbiased but long initialisation
    • Geoid Tide can be calculated instantaneously by using EGM96 Geoid Model as an approximate of mean sea level
      • Instantaneous but biased
    EGM96 Geoid Tide
  • 14. Ultra Tide vs. Geoid Tide 39 HOURS 1010010110201102112 0 2112011020110100101/30 Geoid Tide Ultra Tide = Avg. Ultra Height – Doodson X0 output Geoid Tide = Avg. Ultra Height – EGM96 (Earth Gravitational Model 1996) Tide Value Availability Ultra Tide Start of Ultra positioning 39 hours
  • 15. Application Level
    • Tidal estimation is available in the Verify-QC positioning & QC software application
    $UltraTide,20070827,12:20:00,989,600,600,5708.8614,N,00057.7000,E,20.76,0.06,1.17,18.98,22.45,20.62,0.14,-0.36,21.13,46.43,EGM96*08 $UltraTide,20070827,12:30:00,990,600,600,5708.9052,N,00057.7425,E,20.80,0.06,0.97,19.28,22.21,20.62,0.18,-0.33,21.13,46.43,EGM96*06 $UltraTide,20070827,12:40:00,991,600,600,5708.9503,N,00057.7887,E,20.83,0.06,1.02,19.47,22.18,20.62,0.22,-0.29,21.13,46.43,EGM96*0A $UltraTide,20070827,12:50:00,992,600,600,5708.9828,N,00057.8221,E,20.83,0.07,1.06,19.26,22.27,20.62,0.21,-0.29,21.13,46.43,EGM96*08
    • Output average height and associated quality information to ASCII file
  • 16. Validation Against Predicted Tides
    • Comparison of Veripos Tides against predicted tide (SHM159a v2.00) in Singapore
    • C-O showed Standard Deviation of 11cm and a Mean of 1cm
  • 17. Validation Against Tide Gauge
    • Comparison of Veripos Tide observation against tide gauge close to work site
      • Conducted over a three day period in United Kingdom
      • Comparison of both the Geoid Tide and Ultra Tide at 10 minute intervals
      • C-O for the Ultra Tide showed a Standard Deviation of 10 cm and a Mean of 5 cm
      • C-O for the Geoid Tide (EGM96) showed a Standard Deviation of 8 cm and a Mean of 12 cm
  • 18. Comparison Against Tide Gauge
  • 19. Comparison Against Tide Gauge
  • 20. Comparison Against Tide Gauge
  • 21. Validation on Subsea Vessel
    • Comparison of Veripos Tide observation against tide gauge close to work site
      • Subsea 7 ‘ Kommandor Subsea ’, in Aberdeen
      • Comparison of both the Geoid Tide and Ultra Tide at 10 minute intervals
      • C-O for the Ultra Tide showed a Standard Deviation of 13 cm and a Mean of 2 cm
      • C-O for the Geoid Tide (EGM96) showed a Standard Deviation of 12 cm and a Mean of 50 cm
  • 22. Validation on Subsea Vessel Results highlight biases in EGM96 Geoid Model (accurate to +/- 1m)
  • 23. Performance & Verification
    • Veripos tide solution has ability to observe tide at different locations with different sea state conditions
    • Figure below is tide observations recorded during transit from Scotland to Cyprus
  • 24. Applications & Benefits
    • Real-time observation of tidal variations using high-accuracy GPS offers many advantages over conventional methods
      • Cost and time savings if deployment of tide gauges not required
      • Correct bathymetric data in real-time removing need for post-processing
      • Better representation of tides compared to model based predictions since it includes tidal variations at a much higher frequency
      • Used for tidal current prediction especially if tidal range is very high
        • Current at minimum during peak or trough of tide and strongest current during the tide rise or fall
        • Useful when conducting critical operations
  • 25. Future Activities
    • Further improvement with the inclusion of pitch and roll data
    • During vessel operations (loading, unloading, crane movement, etc.) will affect the vessel draft which can impact tidal observations
      • Investigate how these values can be incorporated
    • Any improvement in positioning accuracy will immediately benefit the tide observation process
      • Future enhancements to Satellite Navigation
    • Improved MSS / Geoid models will aid in linking GPS height (ellipsoidal) to MSS
      • New MSS anticipated with greater resolution than EGM96 but more importantly higher accuracy
  • 26. Positioning for Success Thank you for your attention ? QUESTIONS