Inertial FOG and acoustic aiding references for Dynamic Positioning applications


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By Yann Casamajou technical. product line manager Ixblue

With DP-PHINS, iXBlue has expanded the functionality of its industry-leading PHINS inertial navigation system (INS).

The new DP-PHINS is designed to interface with any third-party acoustic positioning equipment to provide INS-enhanced acoustic data input to marine dynamic positioning (DP) systems. Additionally DP-PHINS can also take data from a range of other sensors, some not normally associated with DP, such as Doppler velocity logs (DVL), for use in maintaining vessel position.

Using DP-PHINS with INS produces positioning data that is smoother, more accurate and is updated at a higher rate. Consequently, station-keeping performance is significantly improved, vessels use less fuel, and wear and tear on the DP system components is reduced.

The system has been fully qualified at sea with industry leaders operating in West Africa O&G development field.

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  • An inertial navigation system is a set of electronics and sensors that very accurately measure acceleration and rotation. Due to the accuracy of these measurements it is possible to calculate both the gravity vector and the rotation of the earth. This information can be used to calculate the orientation of the device with respect to North and is the basis of how a fiber optic gyro compass works. If the sensors are accurate enough, we can mathematically integrate the acceleration and rotation data to get velocity and direction. If we know the starting point we can use the velocity and direction data combined with the initial starting point to calculate subsequent points.
    IXBlue proposes a wide range of INS, in various housings for different applications.
  • PHINS is fast becoming the standard instrument for high accuracy ROV navigation , here we see a PHINS 6000, the standard unit for ROV use.
    Inside we can see the three cans holding the fibre coils with the accelerometers mounted in the centre of each coil.
    Between the coils is the electronics stack including optical sources and components, interface cards and main signal processing card.
    A Kalman filter is implemented to fuse the inertial measurements with external aiding data.
  • Of course, no sensor is perfect and errors will always crepe in to an inertial solution. Bias and Scale factor errors accumulate over time leading to a drift in the calculated position over time. The longer we try to navigate on inertial only data the further the navigation results deviate from reality.
    The majority of the errors in an inertial calculate are due to gyroscope errors, in general the higher quality the gyroscopes the better will be the navigation accuracy.
    Therefore we find that a good INS requires good gyroscopes. IXBLUE manufactures fibre optic gyroscopes, we control the whole process, from the manufacture of the fibre itself and the optical components that are essential for the measurement of the optical parameters.
    IXBLUE manufacture a range of Fibre optic gyroscopes, the FOG90 used in our ROVINS and OCTANS products, the FOG 120 used in PHINS and Hydrins, the FOG180 used in our military product MARINS. And the FOG210 used in our space products STARINS.
    A FOG 90 based INS typically has a free inertial drift rate of 6m in two minutes.
    The PHINS with it’s FOG120 has a free inertial drift rate of roughly half that 3.2m in two minutes, (that's 0.6Nmi per hour).
    Whereas the Marins is an order of magnitude better than that at 1Nmi in 24 hours.
  • As described earlier, no inertial sensor is perfect, errors will accumulate over time. The kalman filter implemented within IXBLUE inertial products is able to take external aiding data and fuse that data with the inertial measurements in order to calculate the current position.
    A large buffer within the Kalman filter allows the use of old data within the position calculation so long as the age of the data is known.
    Typically in the offshore environment, the INS will be initialised on deck with GPS, then when the ROV is deployed aiding will switch over to USBL or LBL, and as the vehicle gets closer to the seabed, Doppler Velocity Log becomes available.
    The PHINS contains a very accurate real time clock, if necessary the PHINS can operate entirely on it’s own clock, but normally a GPS time signal (ZDA) is used to tie the PHINS time to GPS time. This allows the age of time stamped data to be properly taken in to account in the Kalman filter.
    PHINS is a fully self contained system, all calculations are done within the subsea unit with no surface equipment required. PHINS is as well suited to AUV operation as it is to ROV operations and no link to the surface is required.
  • This example concerns a PHINS, or a GAPS, which also improves GPS position accuracy using its internal INS.
  • Note: PME = position Measuring Equipment
  • Sparse Array LBL: fuse asynchronously into PHINS the measured distance to transponders to compute LBL position
    Flexible : can virtually adapt to any system including non-iXBlue ones
    Expandable : additional functionalities become easily implementable in the computation cabinet…. See the list of potential future sensors to be added
  • Tests passed on SeisRanger (SS7 vessel) in CLOV field.
  • PHINS performances:
    Heading 0.02deg x
    Pitch/Roll 0.01deg
    DVL aided position drift < 0.1% x travelled distance
  • Inertial FOG and acoustic aiding references for Dynamic Positioning applications

    1. 1. PHINS for DP Applications Yann CASAMAJOU Europort 2013
    2. 2. Agenda  Introduction to INS  DP-PHINS : bringing INS benefits to DP  Augmented USBL Performances  LUSBL Performances  Robustness to Outages  Conclusion 2
    3. 3. 3 What's in an INS?  What is an Inertial Navigation System (INS)? An instrument (electronic + sensors) which is using its initial state (position) and internal motion sensors (gyroscopes + accelerometers) to measure and calculate its subsequent positions in space with high accuracy, stability and update rate PHINS PHINS 6000 ROVINS
    4. 4. 4 What's in an INS? 3 “FOG” gyrometers monitor rotation and speed in X, Y & Z axis  3 accelerometers measure acceleration (>> speed >> motion) in 3 axis  Powerful electronic / firmware package PHINS “knows” in real time its motion in space . Firmware (Kalman filter) calculates its position in real time + heading, pitch, roll, heave, etc…  All integrated small, lean, powerful! (PHINS 6000 example)
    5. 5. INS Introduction  What makes an INS good.. or not?    Gross figures:    Internal sensors (gyroscopes & accelerometers) are never perfect, bias and scale factors accumulate over time Navigation is mostly about Gyroscopes Accelerometers errors are not heavily involved in the position drift – 4m – Schuller period Gyroscope are heavily involved in the drift – 400 m Conclusions    A good INS requires good gyro’s IXBLUE manufactures FOG (Fiber Optic Gyroscope) and controls the whole process A range of FOG’s (FOG90, FOG120, FOG180…) for a range of INS 5
    6. 6. INS Introduction  The best sensors are still not perfect, accumulating small errors vs. time makes the system drift on the long term  External sensors (aiding) are required to bound drift within acceptable limits. PHINS & ROVINS includes interfaces for most common external sensors       GPS DVL (Doppler Velocity Log) Pressure sensor Acoustic positioning references (USBL, LBL) …and all IXBLUE products! IXBLUE INS are fully integrated Inertial Positioning solutions designed for ease of installation & operation, flexible enough to fit most requirements, with no specialist engineer to install / operate. 6
    7. 7. Benefits of data fusion: Robustness to signal losses GPS positioning with masking 7
    8. 8. Benefits of data fusion: outages GPS + INS positioning with masking 8
    9. 9. 9 Benefits of data fusion: accuracy Data Fusion    Use various and different technologies to measure the same parameter Blend (fuse all this data (Kalman filter) in order to correlate it and obtain a better result A simple example GPS + INS (PHINS or GAPS typical use case) 3 position accuracy (m)  2,5 PHINS pure inertial drift (0.0002 x t^2 m) 2 Averaging of DGPS data (3 / sqrt(t) m) 1,5 PHINS+DGPS 1 0,5 0 0 20 40 60 time t (s) 80 100
    10. 10. Benefits of data fusion: accuracy (USBL case)  Acoustic positioning can be poor, low update rate, or out of range. INS + USBL combination / data fusion provides continuous high quality positioning: Survey @2500m depth: INS accuracy is much better than USBL’s one (noise rejection) 10
    11. 11. Agenda  Introduction to INS  DP-PHINS : bringing INS benefits to DP  Augmented USBL Performances  LUSBL Performances  Robustness to Outages  Conclusion 11
    12. 12. DP-PHINS, Why raising expectations?  Augmented GNSS is an accurate and generally reliable positioning solution.  For DP in deep water, the main issue is the lack of other performing positioning systems.  What to do in water that is too deep for useful acoustics?   After a certain depth acoustics become too noisy, too deep for taut wire, no other structures for relative based systems. Too many vessels relying on single PME – GNSS  DP PHINS is the simplest way to raise positioning performances on a vessel 12
    13. 13. 13 DP-PHINS What is it? Missing link between PHINS and USBL PME Goals of the system : Improve USBL performances to make it usable for DP during GPS scintillation Use PHINS fusion algorithms to enhance raw USBL positioning DP-PHINS cabinet extends PHINS capabilities to : use raw acoustic detections from any USBL for positioning provide positioning telegram to DP Desk Context: Offshore works and installation jobs Deep sea (>1000m) Necessity to pursue operations without any reliable GPS Severe daily scintillation phenomena (Africa, Brazil...)
    14. 14. 14 DP-PHINS What is it? DP-PHINS USBL system Time stamped beacon positions (in vessel reference frame) Enhanced Position ... Time Signals DP desk DGPS GNSS Acoustic Positions Time and Position For Initialisation Only Processed Positions
    15. 15. DP- PHINS, Features  PHINS Natural Features:     DP-PHINS cabinet additional improvements:       Native fusion of a wide range of sensors (GPS, DVL...) Strong noise rejection Sparse-array capability Relative to Global co-ordinate transforms Unlimited number of beacon Sparse array LBL Flexible Expandable Future Sensors     Taut Wire Fan Beam Radascan Etc. Etc. 15
    16. 16. Agenda  Introduction to INS  DP-PHINS : bringing INS benefits to DP  Augmented USBL Performances  LUSBL Performances  Robustness to Outages  Conclusion 16
    17. 17. 17 DP- PHINS Performances: Augmented USBL, case 1 Nov2012: 1300m depth operations Single vessel, 2 cases: DP with manoeuvring operations 1x standard omnidirectional transducer, tonal codes DP with static vessel position 1x directional transducer  improved USBL accuracy Test CASE USBL SD DP-PHINS SD Accuracy Accuracy Gain (1DRMS,DGPS ref) (1DRMS,DGPS ref) (%slant range) Dynamic, omni 2.34m 0.71m x3.3 0.055% Static, directional 1.22m 0.46m x2.65 0.035% DP-PHINS raises USBL PME to submetric performances on performing vessels USBL raw data PHINS + USBL, SD live PHINS + DGPS
    18. 18. 18 DP- PHINS Performances: Augmented USBL, case 2 Less performing USBL Kongsberg CAT, 1360m depth Standard tonal transponders DP performances in dynamic test USBL vs GPS accuracy PHINS vs GPS accuracy Accuracy GAIN (GPS ref) % Slant range max 86,79m 7,94m x10,93 0.57% 1DRMS 6,21m 2,64m x2,35 0.19% Error Type DP-PHINS raises poor DP-USBL to acceptable levels of performances in most fields
    19. 19. Agenda  Introduction to INS  DP-PHINS : bringing INS benefits to DP  Augmented USBL Performances  LUSBL Performances  Robustness to Outages  Conclusion 19
    20. 20. 20 DP-PHINS, LUSBL performances  In May this year iXBlue commissioned DP-PHINS onboard Subsea7 vessel Simar Esperanca.  Augmented-USBL brings x2 to x3 improvement over raw USBL, operating in over 1,350m on acoustics  LUSBL makes it even better     Most of the USBL error is on the angle measurements Range measurements are always consistent Extract ranges out of USBL data string and generate Pseudo LBL data Simply add a beacon... And take the full benefit of DP-PHINS ! DP-PHINS operating in 1364m (4475ft)
    21. 21. DP-PHINS, LUSBL performances Measured ranges extracted computed from HiPAP USBL Data  Slant range standard deviation on CW signals: <30cm 1DRMS  B14 2 beacons used during this test B14, 312m north B15, 285m south Vessel B15  LUSBL achievements in such a situation: Optimum precision on latitude No improvement on longitude 21
    22. 22. 22 DP-PHINS, LUSBL performances Performance improvement on latitude: 1DRMS Standard deviation on latitude At 1360m depth, LUSBL is 15.9x better than basic USBL 5.1x better than augmented-USBL ... With same beacons, same vessel, pole and USBL transceiver !
    23. 23. Agenda  Introduction to INS  DP-PHINS : bringing INS benefits to DP  Augmented USBL Performances  LUSBL Performances  Robustness to Outages  Conclusion 23
    24. 24. DP-PHINS: what if the positioning source fails ?  INS Station keeping on GPS 24
    25. 25. DP-PHINS: what if the positioning source fails ?  1mn outage 25
    26. 26. DP-PHINS: what if the positioning source fails ?  2mn outage 26
    27. 27. DP-PHINS: what if the positioning source fails ?  3mn outage 27
    28. 28. DP-PHINS: what if the positioning source fails ?  4mn outage 28
    29. 29. DP-PHINS: what if the positioning source fails ?  5mn outage 29
    30. 30. DP-PHINS: what if the positioning source fails ?  6mn outage 30
    31. 31. DP-PHINS: what if the positioning source fails ?  7mn outage 31
    32. 32. DP-PHINS: what if the positioning source fails ?  8mn outage 32
    33. 33. DP-PHINS: what if the positioning source fails ?  9mn outage 33
    34. 34. 34 DP-PHINS: what if the positioning source fails ?  10mn outage  11m in 10 minutes
    35. 35. 35 DP-PHINS: what if the positioning source fails ?  20mn outage  38m in 20 minutes
    36. 36. DP-PHINS: what if the positioning source fails ?  36 INS will drift quickly if the acoustics fail.    3m in 2 min, 20m in 5 min. 0.6Nmi in an hour  When using INS with a single aiding sensor, a single failure will take out your INS also.  INS is most effective with multiple aiding sensors.  INS SHOULD be used with multiple aiding sensors
    37. 37. 37 DP-PHINS: what if the positioning source fails ? INS free inertial specification is based on time. The more time, the greater the drift.  Adding a DVL to DP-PHINS is an option to contain this drift  DVL aided INS specification is based on distance travelled. If you don’t move the error can’t grow as much.  Error, % DVL Update travelled Drift speed Rate distance m/h 1S 0.03% 0.18 2S 0.13% 0.82 3S 0.26% 1.67 4S 0.27% 1.77 6S 0.32% 2.08 8s 0.30% 1.98 PHINS-DVL performance is still good at low update rate  compatible with deep water  Problem: DVL is only available in water depths up to around 1,000m 
    38. 38. Agenda  Introduction to INS  DP-PHINS : bringing INS benefits to DP  Augmented USBL Performances  LUSBL Performances  Robustness to Outages  Conclusion 38
    39. 39. Conclusion: DP-PHINS as a PME  INS is a proven technology on Land, Under water, in Space, why not in DP? INS has a long track record, Modern FOG based systems bring extreme robustness and reliability.  INS can produce heading and attitude data as well as positioning.  INS Should not be aided just by GPS for DP applications.  DP-PHINS can make your acoustics as good as high accuracy GPS.  The biggest benefits can be obtained by combining aiding sensors.  With the addition of DVL, PHINS can even be considered a stand alone PME for a significant period of time.  39
    40. 40. 40 Conclusion: Why moving to DP-PHINS ? Intrinsic improvement of USBL due to PHINS IMU performances  Augmented-USBL: x2 to x3 better than USBL with single USBL beacon  Augmented-LUSBL: up to 16x times better with additional beacons   Sequential use of beacons for LUSBL ➯ battery savings of field transponders Extended acceptable water depth for DP ➯ extended DP class  Continued operation in case GPS outage ➯ recurring financial gain  Easy refit of vessels with existing USBL ➯ unchanged USBL, pole… DP-PHINS can be installed anywhere on board  Fuel saving thanks to DP-PHINS output position smoothness  Positioning system open to any additional sensor (DVL, deep water CVL…) 
    41. 41. 41 Thanks for your attention