More than once we have been asked why we don’t put a GPS on our ROV’s in order to position them. Were it only that simple! Unfortunately, RF energy at the frequency used in GPS positioning systems only travels a couple of inches through water. Instead, we have to rely on a number of other alternatives available to us. We will briefly discuss them here.
Pipeline Tunnel Positioning: Penetration Distance
This is the simplest means of positioning available to us and provides an accurate means of determining position when performing pipeline and tunnel penetrations where we are only moving significantly in one direction, down the pipe or tunnel. This system works by running the ROV’s umbilical cable through a series of spring loaded idler wheels attached to an optical encoder. The encoder is attached to a computer which keeps track of the number of “counts” coming from the encoder. The count is then scaled for feet or meters as required. We write our control software in-house so we are able to configure the system to meet job-by-job requirements. One feature that we often customize for our clients is the ability to show not only the ROV’s position in a pipeline as a penetration distance but also as a station position. By using a station position the inspection engineer reviewing the recorded video and sonar data is more easily able to correlate the position to their construction drawings. We find that this can save a lot of time and confusion. If you think this service would aid your project, please don’t hesitate to ask us about implementing the option.
Open Water Positioning: Ultra Short Baseline Acoustic Positioning (USBL)
USBL is a means of providing a position of the ROV using acoustic positioning. It works by using a Differential Global Positioning System (DGPS) derived position and an accurate heading, pitch, roll and heave sensor to position a transceiver carried on a vessel on the surface. The transceiver is equipped with a phased transducer array capable of resolving both range and bearing to a transponder or responder carried on the ROV. Since the position and attitude of the transceiver on the vessel is known, the absolute position of the ROV is able to be derived by combining that position with the relative range, depth and bearing from the USBL system. This makes for the most simple and efficient means of providing a position in open water conditions. Download the USBL Fact Sheet here.
What’s the difference between a Transponder and a Responder?
The difference between a transponder and a responder is generally more about the way the USBL device is used on a mobile underwater object (ROV for instance) than the hardware itself. A USBL transponder is a device that receives an acoustic signal from the surface transceiver, waits a predetermined time (milliseconds) and then sends out a reply pulse. The surface transceiver receives this pulse and calculates the range to the transponder by halving the time and using the speed of sound through water (approx 1500m/s). This system has the advantage that it is simple to deploy as it doesn’t require any independent wiring and can often run on internal batteries. The downside is that as the sound path has to travel both from the surface transceiver down to the transponder and then back from the transponder to the transceiver, inaccuracies are introduced to the calculation due to variations in speed of sound in water, thermoclines etc.
The Responder is a similar device except that instead of taking the trigger signal as an acoustic signal from the surface transceiver, it receives the trigger as an electronic pulse sent down the ROV’s umbilical. This pulse then causes the responder to send out an acoustic pulse to surface where it is received by the transceiver. Though this method requires that the ROV be configured by a competent electronic technician to allow the signal to travel from copper on the surface to fiber optics in the umbilical and then back to copper in the ROV, the advantage is that the responder system doesn’t have to travel twice through the water column but rather just once. This results in us providing a significantly tighter position to the client.
When working in deep water we generally place a transponder on our aluminum deployment garage and a responder on the ROV itself. This way, we are able to keep a general track of the deployment cage while achieving a tight position on the ROV itself. An example of this method was demonstrated in this recent NOAA/NURC project.
Restricted Access Positioning: Doppler aided Inertial Positioning.
Originally designed for navigating nuclear powered submarines and space craft, the IMU is essentially an array of very sensitive accelerometers (based around a fiber optic gyroscope) sensing movement in all 6 degrees of freedom. The device measures the rate of change of velocity in the fwd/rev, up,down, port,stbd planes as well as the corresponding rotational axis of the sensor. These accelerations are then integrated two times (the first integration giving velocity and the second integration giving displacement). This process is done many times a second to give the device the ability to not only dead-reckoning its position in space relative to a known start point but also to know its attitude to a very precise degree with respect to North and vertical.
When fitted to a specially designed skid on the Falcon DR and mated with Doppler sonar (to null out any drift errors when the unit is stationary) and a profiling scanning sonar, the system allows us to geo-reference any solid underwater object. This means we are able to model the interior of a tunnel or pipeline or perform very high resolution hydrographic surveys in locations otherwise inaccessible to conventional navigation devices (USBL)
It is a very impressive technology. While CDL initially intended it for use in performing tunnel investigations we (SeaView Systems and SeaVison Marine Services) have been pioneering its application for performing other restricted access surveys.
Please don’t hesitate to contact us for further information about this technology, how we deploy it, the available deliverables and how they may be used to support your application.