DP MAINTENANCE INTRODUCTION TO SENSORS
Sensors Wind Sensors PMEs Workstations Gyrocompasses Network Switchbox Network Switchbox Controller Cabinets Dual Ethernet Motion Sensors (VRU) Field Stations Thrusters UPSs
Gyrocompasses Measurement of vessel s heading Location - typically on bridge Interface to DP system: Serial (RS 422) SG Brown Litton Anschutz
Anemometers Measurement of wind speed and direction Location - ideally at the top of the mast not on top of derrick Ultrasonic sensors subject to weather induced errors and icing Interface to DP System: Speed and Direction - Serial (RS 422) Young Gill Observator
Vertical Reference Units Measurement of pitch, roll and heave Location - ideally on the vessel s centre of roll and pitch Interface to DP system: Serial (RS 422) Offsets need to be correct otherwise errors induced into DP TSS Datawell
Sensors - Rejection Criteria Gyrocompasses Is the input signal OK? How close is the measurement compared to the other compasses? Vertical Reference Units Is the input signal OK? How close is the measurement compared to the other VRUs? Is the vessel pitching and rolling? Anemometers Is the input signal OK? How close are the measurements compared to the other anemometers? Is the wind strong enough?
PMEs Wind Sensors PMEs Workstations Gyrocompasses Network Switchbox Network Switchbox Controller Cabinets Dual Ethernet Motion Sensors (VRU) Field Stations Thrusters UPSs
Taut Wire Measurement of vessel s position relative to the weight. Suppliers: Converteam Simrad DPS Accuracy < 1m Advantages: High accuracy in <150m water depth Good reliability Quick and easy to set up Disadvantages Short range only Susceptible to strong tides Possibility of weight dragging Only up to 300m water depth
Taut Wire Guide Arm with Inclinometers in F/A Axes Vessel offset contant Constant Tension Winch Correction Required For vessel motion Constant VCG Vessel Variable Water Depth Manual Input by Operator or automatic calculation from line length at Mooring 60 deg Cone Angle Maximum Working Depressor Weight Sea Bed
Taut Wire
Hydro-Acoustics Measurement of vessel s position relative to the transponder Suppliers: Sonardyne Simrad Nautronix Applications: Vessel positioning ROV positioning Survey work Drilling operations Accuracy: 1-5m Update rates: 1s to 8s, dependent upon water depth
Hydro-Acoustics Range Measurement d = v t Speed of Sound in Water: 1483 m/s
Hydro-Acoustics Short Base-Line (SBL)
Hydro-Acoustics Super Short Base-Line (SSBL or USBL) Phase Not Time Lower Position Discrimination More economic installation. Operates Multiple Transponders 10 khz around 3.5 cm wavelength 35 khz around 1 cm wavelength
Hydro-Acoustics Long Base Line (LBL) Intelligent Transponders Deepwater Accuracy No VRU. Difficult to deploy minimum three transponders
Hydro-Acoustics SBL USBL LBL Simple Operation - Expensive Ship Installation Accurate to beyond 1000 M Requires real time MRU Complex Operation - Less Expensive Ship Installation Accurate to 500 M Sensitive to Noise Requires real time MRU Complex Operation - Less Expensive Ship Installation Difficult deployment 3 transponders + min separation Accurate to beyond 1500 M Time Delay Less Sensitive to Noise No need for MRU in deepwater
Hydro-Acoustics SBL - Short Base Line USBL - Ultra Short Base Line LBL - Long Base Line LSBL - Long Short Base Line LUSBL - Long Ultra Short Base Line
Hydro-Acoustics Mud Sources of Interference Propeller Noise and Wash Reflections Fish Marine Growth Water Ingress Hydrophone offsets need to be correct to avoid errors being introduced Sudden with visual confirmation Intermittent obs ship s head Shallow water Xpdr power Theft Gradual loss of sensitivity over time, poor response after layup
Global Positioning System (GPS) 31 satellites + 2 in-orbit spares 6 orbital planes 55 degree orbital inclination Altitude 20200km 12-hour orbit GPS Accuracy 5m to 10m
Differential GPS (DGPS) DGPS Accuracy <1m to 5m Suppliers Thales Sercel Seatex Trimble Dedicated local corrections Coastal beacons Global corrections via satellite
DGPS - Typical Hardware Configurations
DGPS - Inmarsat Coverage
DGPS - Spotbeam Coverage
Correction Signals Radio Band Inmarsat Spotbeam UHF HF LF Range World-wide World-wide 40 km 600 km 700 km
Artemis Measurement of vessel s position relative to the fixed station Suppliers - Reson Range 10-30000m Range/bearing accuracy 0.1-1m / 0.02 degrees Advantages: short to medium range high accuracy at short range Disadvantages: line-of-sight fixed-end requires set-up reflections interference from radar
Fanbeam Measurement of vessel s position relative to the reflector Suppliers: MDL Range (DP use): up to 200m Accuracy: 1m Advantages: easily installed low cost in comparison passive target Disadvantages: dirty lens line of sight false targets low sun
Laser Reflectors Tube Targets and Prism Cluster Arrays
CyScan Similar to Fanbeam, but more advanced. Measurement of vessel s position relative to the reflector(s). Suppliers: Converteam/Guidance Navigation Range (DP use): up to 400m Accuracy: 1m Advantages: easily installed low cost in comparison passive target Disadvantages: dirty lens line of sight
CyScan Use Continuously rotating fanned laser head measures targets bearing, range, width, etc.
CyScan Use Auto-tracking of target ship s speed and position Auto-tracking of target ship s heading Target ship
PME Comparison