SeaTrack™ Digital Acoustic Pinger Technology *
What is it?
CDI's SeaTrack™ represents nothing short of a revolutionary shift in the state-of-the-art of acoustic pinger pipeline pig and mobile asset tracking. In conjunction world-class acoustic engineers from Canada and the UK, CDI engineers spent three years designing, testing and refining our novel tracking system. SeaTrack yields major improvements to acoustic pingers' reliability, probability of detection, quality of information, user programmability and overall system flexibility.
Put simply, CDI's SeaTrack acoustic pingers don't work the same way other company's pingers do - and that's a good thing. Why?
Before embarking on our own acoustic pinger design CDI sought out the opinions and experiences of a great many owners, operators and general users of existing acoustic pinger equipment. Without fail their loudest and most frequent complaints were with a simple lack of detection confidence and sense of these systems exhibited an almost random behavior when used. Pings wouldn't be detected when it seemed obvious that they should, and pings would sometimes inexplicably come and go during a job.
These discussions informed CDI's decisions on what to design, and it was clear that existing systems were falling short with the clients but were being used due to a lack of better alternatives. Taking this customer feedback to heart, CDI decided to take the time and effort to create a new and novel system which overcame these shortcomings. We threw out the rule book and started with a clean sheet of paper.
With this in mind, CDI's goals with our SeaTrack project were ambitious:
1. To increase the reliability of subsea pig tracking and the confidence level of users when tracking.
2. To have the ability to send digital information from the pinger to the receiver.
3. To be able to provide the speed of the pig or other asset.
4. To be able to provide a target's GPS position and velocity relative to the receiver.
5. Create an acoustic pig tracking system which would allow for continuous real-time tracking of a moving target without the need of a diver or ROV.
6. Reduce client dependence upon expensive resources - namely divers and ROVs.
So how did we solve these problems? Let's discuss them point-by-point.
1. Increasing Reliability and Confidence of Detection: Digital, Multi-Tonal Pings
Rather than emitting a ping consisting of a milliseconds-long burst of a single frequency, SeaTrack pingers emit a longer ping which contains many simultaneous frequencies. We call these pings "Chords", because if you imagine striking a chord on a piano - this is essentially what SeaTrack does within a single ping.
So how does this Multi-Tonal ping increase reliability and probability of detection?
Sound traveling through water is quite complex and bounces around a lot, even in the open ocean. There are multiple thermoclines, the ocean's surface and bottom and potentially structures and vessels in the water around a pipeline all reflecting the energy emitted by a pinger. As it bounces around, the acoustic ping gets broken up into acoustic wavefronts which will arrive at the receiver hydrophone location at different times and out of phase from one another due to the reflections. This phenomenon is called "Multipath Propagation" or simply "Multipathing". Multipathing is a common problem that must be dealth with in transceiver systems; common examples are GPS systems and cellular telephones.
Here's an image to illustrate this point. Here we see a pig in a pipeline emitting pings from an existing single-tone system. (A) and (B) represent thermoclines, (C) are underground structures and (D) is the receiver hydrophone:
Because the acoustic energy from the multiple paths arrive at the hydrophone out of phase, each of the acoustic waves gets added and subtracted from one another. This often results in an overall loss of received signal strength, and in fact it's possible to be quite close to a pinger and not be able to hear it at all. This problem - multipathing - cannot be overcome with a stronger ping or a different frequency. Multipathing represents the biggest problem to reliability and ease of use of pig tracking acoustic pinger systems.
How SeaTrack Combats Multipathing
When the SeaTrack pinger emits a Chordal Ping, each of the constituent frequencies in that ping have slightly different transmission coefficients. As these frequencies are reflected and refracted at the interfaces between mediums with differing transmission velocities, each frequency reflects and refracts slightly differently than its neighboring frequency. Because of this effect (described by Snell’s Law) the SeaTrack receiver hydrophone is guaranteed the arrival of some, if not all, of the Chord’s frequencies. Snell's Law also describes how a prism works - each color of the visible light spectrum refracts through a prism at a slightly different angle, which results in a rainbow of color exiting the prism.
CDI’s SeaTrack deck box software is designed to be tolerant of the loss of some of the constituent frequencies of a Chordal ping, and by default SeaTrack can identify and validate a Chordal Ping with up to 20% of its frequencies missing entirely, either lost to multipathing or destroyed by noise in the environment (i.e. a nearby ship or sonar system). In particularly noisy environments, the fault tolerance number can be adjusted upward, loosening the system’s fault tolerance from its default of 20%.
2. Transmitting Digital Information: Chordal Semaphores
One of CDI’s goals with the design of the SeaTrack system was the transmission of digital data from inside the pipeline to the deck of a ship. SeaTrack’s robust Chordal pings also allow for this through the construction of what CDI has termed “Chordal semaphores”.
The SeaTrack system (pinger and deck box pair) has a database of 40 unique pings constructed of 40 individual frequency patterns. These patterns are used to signal conditions inside the pipeline to topside receivers. This system is similar to the way in which acoustic modems work.
The most basic use of Chordal semaphores is to provide unique and unambiguous pipeline pig identification. Each SeaTrack pinger which is attached to a pipeline pig is assigned its own device ID. This device ID is contained and transmitted within each ping of the system. Therefore, when the deck box receives a ping from a SeaTrack pinger, it’s actually a coded semaphore indicating the device ID of the pig to which it is attached. Pigs quite literally travel down the pipeline repeatedly broadcasting their own unique identification semaphore.
Because of this, distinguishing between multiple pigs in a pipeline or multiple assets in a field is a basic and inherent function of the system.
In addition simple asset ID, the system has the capability of stringing together Chordal semaphores into decodable sequences. These sequences can represent small amounts of digital data. This information can be sent by the inline pinger and received shipside by the deck box with the same fault tolerance discussed previously.
Information Types Supported by Chordal Semaphore Sequencing
The information transmitted within Chordal semaphore sequences may be arbitrary, meaning that the pinger itself may contain analog sensors, such as pressure sensors, shock and vibration accelerometers, etc., and the changing analog values may be transmitted in near real-time to surface vessels.
In fact SeaTrack’s Chordal semaphores may comprise simple TRUE/FALSE state change information, such as the condition change of a pipeline pig's gauge plate.
To better accommodate the demands of product customization, SeaTrack may also ultimately be used by clients to transmit arbitrary digital information from their own computer data systems through the use of an RS232 serial input. In other words, an intelligent inspection tool or seabed asset could use the system as a simple, fault tolerant acoustic modem.
Data rates are low, on the order of eight bits per second. However, for in situ pipeline monitoring and signaling this is typically sufficient. For high-speed acoustic transmission through water alone, many solutions exist.
3. Speed Measurement: Doppler Shift
One of the goals of the SeaTrack project was to provide a pig’s speed from shipside.
Because the individual frequencies of each of the Chords are known precisely, Doppler Shift measurements may be performed by the deck box software. These Doppler Shift calculations result in a speed representing the velocity of the pig relative to the measurement platform - in our case a surfacegoing vessel.
In the simplest example, that in which a stationary vessel is positioned directly over a pipeline, the pig’s approach speed and retreat speed can be known quite accurately, as shown below.
Below, we see a screen capture from SeaTrack’s deck box software displaying the target asset’s speed. The proprietary algorithms which CDI uses to perform these Doppler measurements have resolution of 1/3 of a knot, or about 0.5 meters per second. This image shows that the system has detected pinger Chord 2, that the pig’s measured Doppler velocity is 2.4 knots, and that the signal to noise ratio of the ping is 9.8dB.
There are many possible uses for knowing a pig or other asset's speed from the vessel. The most basic use of Doppler speed information is to know quickly whether in fact a pig is moving at all, or is stationary, perhaps stuck. Arrival into a subsea pig receiver may also be detected, as the pig’s speed will, of course, drop to zero. Conversely a pig launch could be monitored and confirmed by observing a pig's speed change from zero to non-zero.
Because of the Chordal construction of SeaTrack’s pings, Doppler measurements are considerably more accurate than those that could be made with pings of a single frequency. Given that a single frequency ping provides only one frequency data point for comparisons, the signal to noise level at the receiver will need to be quite high to achieve an acceptable level of accuracy. Increasing the signal to noise level at the receiver for single frequency systems involves increasing pure output power beyond these systems’ already high levels, but even with higher output powers, the problem of multipathing persists.
4. Pig Location: Doppler Inversion
Since, through Doppler measurements, velocity of the pig relative to the measurement platform can be known, it is also possible to move the vessel relative to a stuck or stationary pig and locate that pig using Doppler information created by the vessel’s movement. CDI refers to this technique as Doppler Inversion.
With Doppler Inversion, a towable hydrophone is deployed behind a work vessel and towed. As the vessel is moving relative to the pinger, Doppler shift information is created. If the vessel is underway at 5 knots and the measured Doppler is 3 knots, then it is known that the heading to the pinger is not correct.
SeaTrack’s deck box is GPS equipped and, therefore, constantly knows its own velocity on the surface of the water. By comparing the known GPS velocity and the calculated Doppler speed, SeaTrack measures and displays the angular difference between the two.
There are two possible solutions for this angular difference, one to port and one to starboard. If the Doppler measurements are positive (i.e. the frequency of the Chordal pings has been upshifted), SeaTrack knows and displays that we are moving toward the target. If the Doppler measurements are negative (the ping frequencies are downshifted) SeaTrack displays that we are moving away from the target.
By maneuvering slowly in a circle while within range of SeaTrack, the captain matches the speed of the vessel as closely as possible to the speed reported by SeaTrack's Doppler Speed. When the speeds closely match, a bearing to the stationary pinger has been ascertained, as depicted below.
If one achieves this bearing and continues along the course, eventually the target will be overrun. As the vessel approaches the target, the Doppler will begin to downshift, with the rate of downshift dependent upon the water’s depth.
SeaTrack’s deck box software monitors this Doppler downshift, and when the measured Doppler inverts, or switches from positive “approaching” Doppler to negative “retreating” Doppler, SeaTrack drops a GPS waypoint on the screen showing the estimated location of the stationary pig. An illustration of the Doppler Inversion technique is shown below.
This works to ascertain the stationary location of a pig or other asset. Below is a screen shot of CDI's deck box software showing a Doppler Inversion with the vessle traveling at 5 knots. With the speed in knots listed along the righthand margin, it's easy to see the Doppler shift of a stationary pinger from 5 knots to -5 knots. The location of the pinger and asset are at the point where the line crosses through zero - near the center of the image.
Dots represent individual pings.
5. Continuous Real-Time Tracking: Cooperative Target Motion Analysis
Continuous real-time tracking of a moving pig through a pipeline from the ocean's surface has been a major part of the development of SeaTrack. This has been accomplished through the use of a technique which CDI has dubbed "Cooperative Target Motion Analysis".
"Plain" Target Motion Analysis (TMA) is a complex but well understood acoustic technique (often military) which allows, for instance, a submarine to passively track a moving war ship across the surface of the water at some distance.
CDI's proprietary Cooperative Target Motion Analysis (CTMA) takes "Plain" TMA and simplifies it for use by a layperson. It is able to do this because, unlike a military submarine application, CDI's SeaTrack deck box knows beforehand what frequencies it will be listening for. Using these known frequencies, SeaTrack can discard erroneous acoustic information and arrive at a triangulated point on the surface of the ocean below which the pinger lies.
CTMA is an advanced technique and requires a towable hydrophone system and a moving vessel, however, unlike Doppler Inversion discussed above, CTMA can give yield the GPS coordinates of a pinger (or other subsea asset) without having to know anything of the geometry of the pipeline below the surface.
For more information on CTMA, please contact CDI.
CDI's primary objective with SeaTrack is to increase the reliability and usefulness of acoustic pig tracking while also reducing reliance upon expensive client-provided resources - namely diver and ROV vessels. The time of work vessels is a resource which should be focused onto tasks that only they can perform. Robust acoustic pipeline pig tracking should be viable from lesser boats - and it is.
SeaTrack can locate and record the GPS coordinates of a stuck or stationary pipeline pig from shipside, with relatively inexpensive hardware, and without ever incurring the risk and expense of deploying an ROV or diver into the water. Small amounts of digital information such as the state of a gauge plate can be transmitted as well as short strings of a user's binary data.
This is a relatively brief introduction to the techniques and capabilities of CDI's SeaTrack system. However, it's easy to see that these features culminate in an acoustic pinger tracking system unlike anything on the market today. CDI is proud of the accomplishments of our staff in the development of the SeaTrack system and we look forward to discussing your job requirements.