Survey Sensors Hydrofest Ross Leitch Project Surveyor

Transcription

1 Survey Sensors Hydrofest 2014 Ross Leitch Project Surveyor

2 Satellite Positioning Only provides position of antenna Acoustic Positioning Only provides position of transponder relative to transceiver How do we connect the two? Offsets Sensors Offsets Provide a relative physical distance between all sensors/systems and a Central Reference Point Sensors Measure movement relative to the Central Reference Point

3 VESSELS & VEHICLES

4 Construction Vessels

5 ROV Remote Operated Vehicle

6 ROTV Remote Operated Towed Vehicle

7 AUV Autonomous Underwater Vehicle

8 SURVEY SENSORS

9 Survey Sensors Positioning Systems Surface Positioning Acoustic Positioning Attitude Systems Gyro Compass Motion Reference Unit SVP Sound Velocity Probe Inertial Navigation Systems Doppler Velocity Log Depth Sensor Cross-Profilers, Singlebeam & Multibeam Echo sounders Pipetracker Side Scan Sonar

10 Gyro Compass Gyros always output True North but we generally present Grid North Determine heading/bearing relative to True North Different types available Mechanical older style, spinning parts Ring Laser (RLG) or Fibre Optic (FOG) uses light and mirrors (no magic though) GPS calculated between two antennas not subsea suitable! Accuracy is Typically 0.1+ sec lat (secant latitude) The gyro corrects the offsets between all the different sensors so if it s wrong, so are the positions!

11 MRU Motion Reference Unit Measures the motion of Vessels & ROVs in 3 axes Pitch Roll Heave Many modern Gyros have MRU functionality built in Essential to correct for accurate subsea positioning

12 Heading error Pitch or Roll error GPS ROV USBL ROV

13 Sound Velocity Sound Velocity is required for the majority of subsea sensors SV probe measures: Sound Velocity, Pressure, Temperature & Conductivity. SV probe calculates: Salinity, Density & Sound Velocity. SV profile required to: predict ray trace compute ranges from TOF Distance = Speed x Time Time Time of Flight Speed Sound Velocity Profile Calculate Distance North Sea = 1490m/s, Greenland = 1455m/s 2 way TOF 3.273s NS = m, Gr = m

14 Sound Velocity Salinity Density VOS Temp

15 Inertial Navigation Systems Not a positioning system on it s own Aid to position Increases position update & relative accuracy Provides low cost/high accuracy positioning Vessel INS & ROV INS Absolute accuracy is limited to host positioning system

16 Doppler Velocity Log Relative measuring device smoothes ROV position Based on Doppler effect apparent change in frequency due to motion 4 beams reflected from seabed

17 Depth Sensors Depth Sensor combined with altimeter Quartz Crystal vibrating under pressure Pressure output (PSI, Bar etc) Conversion to depth uses Gravity (changes depending on latitude) Density of local seawater Atmospheric pressure Accuracy - typically 0.01% FSR (Full Scale Range) e.g. 0.07m for 700m sensor

18 Bathymetry Sounding originates from Old English meaning the sea or to swim and does not imply the use of sound pulses Historically a sounding line (leadline) would be used which provided Depth Seabed Information The combination of accurate positioning and accurate depth allows the generation of bathymetry charts and seabed terrain models Acoustic systems allow data to be collected quickly and efficiently Echosounders Singlebeam Multibeam

19 Bathymetry Singlebeam Echosounders Generation of acoustic pulse from source Pulse time of flight timed to calculate range to target (typically the seabed) Most vessels, small and large, have an echosounder

20 Cross profilers / Multibeam Echosounders Hull Mounted & ROV Mounted Scanning the surface of the seabed to determine depth points relative to ROV/vessel High frequency sonar MBES Swathe ~ 120 beams What s the difference between scanning sonar and MBES? Mechanically moving single beam vs. simultaneous multiple beams Data density Price!

21 Cross profilers / Multibeam Echosounders

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23 Cross profilers / Multibeam Echosounders

24 Bathymetry Multibeam Echosounder (MBE)

25 TSS Pipetracker Highly sophisticated metal detector! Active Pulse Induction system (440) Passive tone system (350) Target Scaling (Calibration)

26 Side Scan Sonar Imaging the surface of the seabed to identify objects, pipelines etc. High frequency sonar Sometimes towed What s the difference between SSS and MBE? SSS looks sideways, perpendicular to direction of ROV travel Very wide data coverage for SSS SSS data gap under ROV No depth data determined from SSS Data shows intensity of reflection rather than depth Free span Rock dump

27 Seabed Imaging Sidescan Sonar

28 Geotechnical Survey (Brief overview) Additional sensors Sub-bottom information can also be gathered using additional sensors Sub-bottom profiler Fish, ROV or AUV mounted Can identify layers within the seabed Magnetometer Used to identify features with a magnetic return such as submerged metal structures or rock formations with a magnetic signature

29 Survey Navigation Packages Three main survey suites EIVA / NaviPac Quincy Starfix.Nav,.Seis,.NG All survey software does the following Interfaces to survey sensors Logs data Defines objects or vehicles for sensors to be referenced to Takes WGS84 (GPS) positions and converts to local ellipsoid and grid Displays objects on a screen including background graphics and vehicles Problem - As data density and data collection speed increases so does the need for fast update rates and accurate time stamping of data Prime example is multibeam echosounder and AUV data.

30 Summary To compute an accurate position for a ROV or object we need: Satellite Positioning Acoustic Positioning Offsets Sensors Any Questions?