Tracking an ROV in shallow waters or at high elevation has always been a challenging operation with typically poor results due to acoustic interrogations and transponder responses bouncing off the seafloor or underwater structures. Ed Ceurstemont, Sonardyne’s Team Leader for Subsea Product Development, describes the challenges of acoustic positioning in shallow water environments and how the recent dramatic improvements to Sonardyne’s Ultra-Short BaseLine technology is overcoming them.
The positioning of an ROV is typically achieved using USBL techniques where the position is calculated by measuring the range and bearing from a vessel-mounted transceiver to an acoustic transponder fitted to the target.
Using Sonardyne USBL technology, system accuracy is better than 0.1% of slant range. This accuracy, however, not only depends on the assumption that the two way acoustic signal (interrogation and response) is able to follow an unobstructed path but also the system calibration, signal to noise ratio, acoustic aperture and the environmental monitoring of the sound velocity profile. As soon as the path is no longer clear, the system receives acoustic responses from the wrong directions and ranges. In shallow water environments, this multipath is caused by multiple reflections off the ocean’s surface and seafloor.
Depending on the geometry between the ROV and the USBL, the arrival of multipath signals overlapping with the direct signal can affect the ability of a USBL system to identify the direction the signal arrived from. Additionally, shallow water creates a highly dynamic sound velocity profile which constantly changes the direct path angles and sometimes even obscures the direct signal altogether due to refraction. However, Sonardyne’s Wideband 2 digital signals, can help users overcome multipath issues as the technology improves the detection by using complex signals to discriminate between direct and indirect paths, improving timing precision.
With multipath signals, resolving the depth of the beacon can also be difficult.
“In some situations, the environment is so challenging that depth cannot be resolved, we can identify the direction of the ROV from the vessel but to calculate the elevation, we need to use depth aided responses.”
To resolve depth issues, Sonardyne’s transponders are fitted with highly precise internal pressure sensors to give an accurate measurement of depth which is communicated to the USBL.
Noise, the unknown variable
Another challenge associated with shallow water, high elevation tracking – and one that Sonardyne’s customer Global Marine Systems experienced on a recent cable trenching project – is that of noise.
A leading provider of engineering and underwater services, Global Marine was tasked with the trenching and subsea cable installation for a wind farm in depths of approximately 15 metres, using Sonardyne’s Ranger 2 USBL system to position the ROV carrying out the trenching work. At these depths, noise from the vessel bounces off the seabed and reverberates. Noise varies greatly from one vessel to another so although the transceiver can measure it, it is difficult to determine an accurate level.
This noise is picked up by the transceiver, degrading the true signal from the ROV. To address the issues, Global Marine contacted Sonardyne’s Survey Support Group (SSG). After reviewing the data and location, the SSG recommended using the ROV-mounted transponder in Responder mode, removing the need to send an interrogation acoustic signal.
“Noise is one of two primary factors which make it difficult to predict the performance of an acoustic tracking system; the more noise there is at the receiver, the less accurate the acoustic positioning will be. Using Responder mode instead means that electrical signals are sent to the ROV via its Tether Management System (TMS) and the only acoustic signal in the water is the response from the ROV’s transponder, reducing the likelihood of signal masking,”
Tracking by (acoustic) sight
The second primary factor affecting positioning is the line of sight between the transponder and the acoustic receivers on the transceiver. “Line of sight is a simple but important issue to consider and it is applicable to anyone undertaking USBL operations,” notes Ed. “An acoustic receiver has to ‘see’ the signal it is to detect and variations in topography and the water column will impact this line. Careful consideration must also be given to the location of the transponder on the ROV. In Global Marine’s case, any tilt or pitch on the ROV beyond just 3.3 degrees would cause the transponder to be masked by the vehicle itself. To rectify this, we suggested the transponder be moved from the front of the ROV to the rear.”
A similar issue is that of transceiver directionality. “Sonardyne’s 6G transceivers work in any water depth in the world, from ultra-deep to ultra-shallow, the settings just need optimising to compensate for the varying cone of operation,” comments Ed. “Our transceivers track at 90° elevation but to do so accurately, lower frequency channels should be used. Higher frequencies would reduce the cone of operation but improve the precision on bore sight.”
Supporting Global Marine
Throughout Sonardyne’s history, the company has always had a hands on approach to being involved with its customers’ operations; the customer support team being the first port of call for users needing help.
“As well as advising Global Marine from our UK headquarters, engineers visited the vessel to optimise their Ranger 2 for shallow water,” recalls Ed. “Once we had accounted for all the environmental challenges and run shallow water tracking tests and a trial trenching operation, our system was chosen for use on the project over the tenderspecified technology from another vendor. Performance was proven in just 14.5 metres of water with only 2.9 metres vertical distance between the transceiver and transponder. We also verified the system by tracking over 170 metres horizontal distance in 35 metres of water and over 70 metres in 14.5 metres. Most notably, in shallow water operations on a vessel of this size, the line of sight problem encountered highlights the importance of understanding the physics and geometry of the situation in order to maximise performance. This is something that Sonardyne can always help with.”