Hydrofest 2016 - The Hydrographic Society UK

Hydrofest 2016
The Hydrographic Society in Scotland
HYDROFEST 2016
Survey Sensors
20/04/2015
Danny Wake
Group Support Surveyor
i-Tech Services
The Hydrographic Society in Scotland
HYDROFEST 2016
What do we need sensors for?
• For pure hydrographic surveying:
– Depth measurements
– Hazard identification
– Seabed composition
– Tides & currents
– Water column information
• For construction survey
– Identifying suitable seabed (composition, topography, stability)
– Positioning infrastructure within tolerance
– Installing infrastructure at the correct attitude and elevation
– Assessing integrity during and after installation
HYDROFEST 2016
What is a suitable survey platform?
HYDROFEST 2016
Other end of the scale
HYDROFEST 2016
Other end of the scale
HYDROFEST 2016
Other end of the scale
HYDROFEST 2016
Other end of the scale
HYDROFEST 2016
Other end of the scale
HYDROFEST 2016
Other end of the scale
HYDROFEST 2016
Typical platforms for Subsea Sensors
• Vessels of all shapes & sizes
• ROVs
• ROTV/towed sensors
• UUVs / AUVs (AIV)
• Subsea structures
• Surface structures
• ASVs
• Highly dynamic platforms
• All require a number of
integrated survey sensors
• Let’s focus on ROVs
HYDROFEST 2016
Typical platforms for Subsea Sensors
HYDROFEST 2016
Typical platforms for Subsea Sensors
02/05/2016
HYDROFEST 2016
Subsea Sensors
• ROV Position - USBL
HYDROFEST 2016
Subsea Sensors
• ROV Depth / Altitude
• Essentially this can be done in two ways:
– Depth from USBL (Z component)
– Depth (pressure) sensor
Depth
USBL
Altitude
• Altitude from small HF ES
HYDROFEST 2016
Subsea Sensors
• Cameras
• A variety of cameras may be used
• For pipeline survey typically there are three : one centre and two booms
HYDROFEST 2016
Subsea Sensors
• Vehicle Heading
• Can be derived crudely from position track
• Not accurate enough for normal survey operations
• Heading sensor (gyrocompass) provides vehicle heading to e.g. 0.1° seclat
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Subsea Sensors
• Vehicle Attitude
• i.e. pitch and roll
• Varying options depending on accuracy requirements
• Systems capable of 0.01° accuracy
• iXBlue Octans is a multiple output sensor
HYDROFEST 2016
But what if there is zero visibility?
• Obstacle avoidance sonar (OAS)
• May be mechanical scanning type
or beam-forming
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Let’s survey a pipeline. What else do I need?
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Subsea Sensors
• Echosounders
• Scanning profilers
– Low cost option
– Lower costs supporting sensors
• Multibeam echosounders
– Higher cost
– Higher performance
– Requires higher accuracy supporting sensors
• Positioning
• Attitude
• Normally in dual head configuration
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Multibeam / DHSS pipeline profiling
• How do we survey a pipeline?
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Multibeam pipeline profiling
courtesy
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Sound Velocity Sensors
• SV Probe
– Measures time of flight of an
acoustic pulse over a known
baseline length
– Very precise; required for MBE
and LBL metrology
– Does not provide
environmental information
• CTD Probe
– Measures conductivity,
temperature and density
– Derives sound velocity using
standard formulae
• Chen & Millero WD <1000m
• Del Grosso WD>1000m
HYDROFEST 2016
Sound Velocity
• What else is sound velocity
important for?
• It is also needed to apply the full
water column sound velocity
profile to the USBL system to
correct for refraction effects in
positioning
• Sound bends through layers in the
water column
• The sound velocity profile is used
to model the path of sound taken
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Survey Sensors
• What if the product we are trying to
survey is buried?
• Pipetracker
• Essentially a big metal detector – 3 coils
• Uses pulse induction techniques to induce
a voltage in ferrous targets
• Calculates range to target both vertically
and horizontally from centre coil
• Requires knowledge of target size and
composition (diameter and material)
• Best results using “target scaling”
• Requires altimeter in order to calculate
depth of cover (DOC)
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Pipetracker
• Pipetracker
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Example Data – pipeline in and out of burial
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Survey Sensors
• Subsea Laser
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Subsea Laser
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Sidescan Sonar
• Seafloor search system (area based)
• 12kHz to 1200kHz
• Operating depths typically 5-200 metres
• Can be used for seabed composition
• Comprises:
– Towed body, or “fish”
– Armored tow cable
– Processing electronics
– Display
Seabed Composition
HYDROFEST 2016
Survey Sensors
• Sub bottom profiler
• Sub seabed composition
HYDROFEST 2016
Subsea Positioning – Revisited
• Inertial Navigation Systems (INS)
• Subsea acoustic positioning has relatively low precision and update rates
• USBL, 1-5 second update rate
• INS aided by USBL, DVL and bathy:
– Increased precision
– Faster update rates
• Accuracy is still depended on USBL
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Subsea Positioning – Revisited
Rough guideline depths for use
Surface
USBL only
500m
USBL aided INS
1200m
USBL aided INS / LBL
2000m
LBL
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Summary: hardware integration
ROV Sensors
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HPR TXP/RXP
Gyro
Attitude
Bathy/Altimeter
OAS
Pipetracker
DHSS / MBE
HYDROFEST 2016
Summary: software integration
HYDROFEST 2016
Survey Sensors
20/04/2015
Danny Wake
Group Support Surveyor
i-Tech Services
The Hydrographic Society in Scotland
Hydrofest 2016
The Hydrographic Society in Scotland