Using Waves And Power Efficient Loggers For Autonomous Profiling to Collect High Quality, High Density Data For Increased Periods Stefan Stimson 1, Dr. Greg Johnson 2, Chris Kontoes 2 1 Metocean Services International Pty Ltd, Hobart AUSTRALIA. Email: stefan@metoceanservices.com 2 RBR Ltd, Ottawa, CANADA. email: info@rbr-global.com Abstract Traditional methods of acquiring profiled metocean data in real-time are limited by the need to provide a significant power source and the need for the profiler to stay on the surface to transmit the data. Power is required to drive the profiler through the water column and timed so that it stays on the surface long enough to allow it to transmit the data. This requires the operator to balance power budgets, ensuring batteries last whilst both driving the profiler and operating the sensors, resulting in reduced measurement periods and/or reduced amount of data acquired. By using the vertical movement of surface waves, the RBR Wirewalker requires no external power to operate. This paper describes how it uses the movement of a surface buoy, to drive the profiler down and then uses the profilers buoyancy to travel back to the surface. The profiler body is easily modified to fit any type of logger into it and by simply changing the flotation, the system is highly configurable to meet the specific program needs from simple temperature and depth profiling at one site to a full suite of water quality profiling at another. With the addition of an inductive modem and cable as the mooring system, and by putting a modem into the surface buoy, this converts the whole system into a near real time profiling system with data being taken to the surface and transmitted to the world at periods that the operator requires, rather than only when the profiler surfaces. This paper will conclude that with no power required to drive the profiler, using a range of power efficient loggers, and reporting via an energy efficient reliable modem, profiling measurement programs can now measure for longer, or measure more often, or measure faster - or a combination of all three. Keywords: monitoring, profiling, self-powering, instrumentation, wirewalker. 1. Introduction Measuring data at a single point in the ocean has traditionally been achieved using moored instruments or bottom frames as in Figure 1. Although a valuable method of data collection, the desire to obtain data throughout the water column lead to multiple recorders being deployed on long moorings. Of course, this required a significant investment in equipment as well as increasing the risk of financial loss should the mooring fail to be recovered. Subsequently, some sensor technologies were able to develop profilers, such as acoustic Doppler current profilers (ADCP s), which allowed current data to be acquired throughout the water column. Mounted in various configurations, ADCP s allowed the user to measure as much of the whole column as possible. However, primary limiting factors affecting these instruments are power required to operate and memory required to store data. As a result, researchers must balance deployment durations with sampling frequency to ensure maximum data collection without running out of either power or storage. Many other technology manufacturers also recognised this and provided increased battery and memory options. Figure 1: Various methods of acquiring data from single point, multi single point, single current profiler and multi current profiler moorings. Of course these profilers only acquire current data and if the researcher requires other parameters such as conductivity, temperature, DO etc., they were back at the original multiple single point mooring design.
Furthermore, subsurface mooring designs, such as those in Figure 1, have many advantages, particularly in terms of avoiding ship collision or theft. However, there is no ability to transmit data in real-time from the surface. This means that not only is the data collected solely when the mooring is serviced, but also all of the recorded data is lost if the mooring is unrecoverable or lost. surrounding the wire with an internal cam that can grip it securely when the relative motion is in one sense, but release it in the other. See Figure 2. In an effort to reduce the risks while maintaining the quality and quantitiy of data desired, researchers began profiling (i.e. lowering) instruments through the water column. With advances in technology and the ability to sample and record more data faster, profiling has become an ever increasingly popular method of metocean data acquisition. A growing number of parameters such as temperature, conductivity, and numerous optical and acoustic sensors are currently being utilized on profiling platforms and the demand from researchers to push these instruments and measurements to the maximum is natural. Much of the profiled data collected has historically been acquired via shipboard operations or by autonomous profiling platforms. However, with increasing desire for real-time data, broadcasting the data from these autonomous profilers becomes more and more complex. Traditional methods of acquiring and telemetering real-time profiled data is limited by the need to provide a significant power source to run the data logger, buoy controller, transmitter, as well as the need for the profiling platform itself (or a part of the platform). The power required to mechanically propel the platform through the water column must also be used to keep the platform on the surface and well timed to allow the full data transmission to be sent. This requires the researcher to not only carefully balance buoyancy for minimal power consumption, but also power budgets for both the platform as well as the instruments on board. Many of these limitations can be overcome through the use of a wave powered profiling platform, the Wirewalker (WW), and the use of power efficient data loggers and a unique telemetry system from RBR Ltd. This combination allows for high-quality, high-density real-time data to be collected for extended periods of time from an autonomous profiler. 2. The Wirewalker Profiler The Wirewalker (WW) was originally designed at the Scripps Institute of Oceanography and in concept is actually quite simple.[1] The basic system consists of a buoy, mooring wire, and weight suspended above the bottom, plus a profiling body Figure 2: Main Components of the Wirewalker The motive force of the WW is provided by surface waves, which drives the vehicle and sensor package down the wire suspended from the surface buoy. At the limit of the profiling range, the vehicle is mechanically decoupled from the wire, and it rises smoothly toward the surface under its own buoyancy. Because no on board energy is required for profiling, extended deployments are possible. The design utilizes low drag and large entrained mass in order to exploit wave motion and continually profile to an arbitrary depth [2]. The system is currently manufactured by Del Mar Oceanographic (www.delmarocean.com) and is also available through RBR Ltd and the RBR worldwide agent network. The WW is constructed around an elongated central box that provides good strength and stiffness for its weight. The rectifying cam is installed at the centre of the box, and guide rollers for the wire are fixed at each end. The box, cam, and roller assemblies can be opened quickly, so that the deployment wire can be laid in and then contained. Blocks of floatation foam can be added to the central box to render the basic WW slightly positively buoyant. See Figure 3.
Figure 3: (a) Schematic of the WW, showing the cam (black), centre core (yellow), flotation (white), and external crash guards (transparent). (b) The full sized profiler with a CTD / battery case attached. The crash guards are removed here. (c) The WW with the core open and the cam disassembled. (d) The original drag based WW of Rainville and Pinkel (2001). (e) Schematic of a 0.3m WW. Normally, the one-way cam (Figure 4) is engaged to draw the profiler downward with each wave movement of the surface buoy, by clamping on when the orbital motion is downward and letting go when the cable returns upward. Upon hitting a bottom stop the cam is disengaged, and the profiler, being slightly positively buoyant, floats up smoothly to the surface, where the upper stop reengages the cam. Figure 4: Schematic of the Wirewalker cam during (a) ascent and (b) descent It is critical that the sensor payload be firmly attached to the central core of the platform. For this cause, instrument specific clamps are attached to rails that are an integral part of the WW central core. This modular mounting approach allows instruments, wiring, and batteries to be approximately ballasted in the laboratory, prior to being attached to the WW frame. Access to the instruments and wiring is unobstructed. To protect the payload, split-cylinder crash guards are affixed to the outside of the instrument mounts (Fig. 3a, 5 and 7). These lock the mounts into a rigid structure and enable the WW to be dragged over the rail of a small boat or launched from the A-frame from a larger vessel. Figure 5: Profiling body reaching the surface buoy Clearly, a significant requirement for high performance is a cam that is reliable, does not slip when it is supposed to be clamped on yet is practically frictionless in release mode, and does not damage the wire; however, this is not the subject of this paper but can be reviewed in greater detail in many of the references listed here. In practice, current design of the cams have been successful in gathering tens of thousands of profiles in actual deployments. Initial deployments were limited most often by the battery requirements of the carried instruments operating continuously. Typical profiling speeds achieved are of order 0.5 m/s, on both the downward (erratic) and upward (smooth) profiling directions. Figure 6: Example of typical profile noting 'jagged' downward profile and smooth return to surface.
Over the profiling cycle, the greatest impact that the WW encounters is when colliding with the bottom stop. Here the velocity difference between the surface and local depth is the greatest. Transition shocks at both the lower and upper limits of the profiling range are cushioned by mounting the stops at the head of a ~1-m section of rubber hose that surrounds the deployment wire. conductivity, temperature, pressure, turbidity, various fluorometers, ph, dissolved oxygen, PAR, and more. The greatest wear on the wire occurs at the top of the cycle, where the cam is reengaged. The actual force on the wire is not large relative to that at greater depth, but the impact occurs repeatedly at the identical spot on the wire. The cam works equally well on 3mm Spectra line, however longterm wear tests on the Spectra have yet to be conducted and real-time telemetry is then restricted [2]. The profiling body is designed to be positively buoyant and can be easily adaptable for housing different sensors and in multiple configurations, as well as being streamlined to avoid drag / knock down. This configurability allows the user to use the same system at multiple locations with different instrumentation giving flexibility based on the required monitoring. In order to broadcast data in real-time, the measurements are made from a data logger on the WW platform, sent through the mooring wire to the surface buoy data controller, and broadcast via a wireless telemetry modem (e.g. VHF, GSM, Iridium, etc.). Traditionally, profiling bodies must stay at the surface for sufficient time to allow the data to be transferred to the data controller or directly transmitted by the modem (depending on the configuration). As well as precise timing, this requires significant power to hold the profiler at the location as well as transfer and transmit the data. The RBR telemetry system helps overcome this limitation. 2.1 Power efficient data loggers As described above, initial projects with the WW provided continuous profiles but deployment endurance was limited on the order of several weeks, primarily due to the need to provide constant power to the profiling instruments. Naturally, low powered equipment lends itself well to increasing the deployment possibilities. With this in mind, RBR Ltd. has been consistently improving power consumption and efficiency of their loggers. A RBRconcerto (up to 5 channels) or RBRmaestro (up to 13 Channels) data logger may now be integrated with the WW platform. The data loggers have been designed in such a way to allow for complete customization of the parameters users request. Power is generally dictated based on the sensors chosen for integration, which often include Figure 7: Cutaway of a WW profiling body with RBR CTD and Fermata battery pack With the addition of a directional dependant sampling scheme, the logger can become even more power effacing by changing sampling rates based on the direction of the profile. Traditionally, acceptable CTD data is collected on the down cast in order to sample undisturbed waters; however, the unique dynamics of the WW allow the smooth free floating upcast to be the direction of choice. Hence, the sensors are facing upward as in figure 5 and 7. The reduced sampling rate of the ratcheting downcast, allows for diagnostic data and monitoring of the WW dynamics, while the data rich upcasts can be collected in high resolution. 2.2 Wirewalker payload power supply With waves powering the mechanics of the WW, any battery power supplied can be utilized solely to operate the sensors and telemetry components. A standard RBRconcerto CTD measuring continuously at 6 Hz will last approx. 16 days. Extending the standard battery housing (and memory) will extend this to 30 days, still at 6Hz continuous sampling.
2.3 Inductive mooring line modem Further expanding the capability of the WW, a RBR manufactured inductive mooring line modem (MLM) is utilized to transfer the data from the data logger to the surface data controller. The MLM consists of 3 major components; a subsurface modem mounted on the WW (SSM), a head end modem (HEM) which is attached to the data controller at the surface, and the insulated wire rope utilized by the WW cam. The RBR MLM operates at 4800 baud, which has proven sufficient for transmitting up to 6Hz data from a 10-parameter data logger. Figure 8: Comparison of standard CTD and extended housing CTD However, as more parameters are added, this is still somewhat limiting and therefore the RBRfermata was designed. This Polyoxymethylene (POM) battery canister is rated to 750m and supplies 1KWh at nominal 12V (welded alkaline D-cells). This is sufficient to increase endurance of standard RBR loggers by up to 40 times (of the standard logger). The SSM is mounted internal to the WW, which allows the ferrite to constantly encompasses the wire rope while remaining free to move along the wire. This design enables the data to be transmitted from any position along the profile, preventing the need to carefully budget the timing of broadcasts, and allows the profiler to remain subsurface conducting uninterrupted autonomous profiling for gapless data collection. In order to broadcast data in real-time, the measurements are made from a data logger on the WW platform, sent through the mooring wire to the surface buoy data controller, and broadcast via a wireless telemetry modem. 2.4 Data controller and telemetry The RBRcervello data controller is mounted in the WW surface buoy. It controls and retrieves the data inductively from an RBRconcerto or RBRmaestro mounted on the subsurface WW. Real-time data can then be transmitted via Iridium or GSM modem, which is immediately available on a private website. Data is also archived on the RBRcervello for redundancy, and is accessible via a USB stick upon buoy recovery. Figure 9: RBRfermata 1KWh external battery pack The RBRfermata includes 4 MCBH connectors for powering more than one instrument at a time and the form factor allows it to be tucked away nicely behind the WW protective guard (see figure 8). Therefore, a combination of powering the profiling mechanism by the waves, employing clever management of the measurement regime and installing an external battery, it is possible to extend the measurement program significantly using the basic CTD example above of 6Hz continuous sampling, this can potentially increase endurance from ~16 days to well over 600 days. Adding additional parameters will vary endurance accordingly. Figure 10: RBRcervello data controller
3. Operational Experience To date, WW s have been deployed world-wide, from lakes to the open ocean. The system has been proven in both large waves as well as small wind waves generated by only 3 knots of wind. Here we will briefly introduce two experiences, which confirms that the WW is an ideal power efficient platform for both moored and free drifting deployments. 3.1 The ASIRI Bay of Bengal Deployment A WW equipped with a RBRconcerto CTD and integrated DO optode (JFE Rinko), fluorometers (Turner Cyclops 7 for ChlA and DCOM), and turbidity (Seapoint) was deployed in the central region of a poorly structured cyclonic mesoscale eddy in the Bay of Bengal (BoB). All sensors were sampling at 6Hz. Additional microstructure and velocity measurements were also measured concurrently, yet autonomously. Further details and a full scientific report of this deployment can be reviewed by reading Lucas et al. 2016 [3] 3.2 La Jolla Canyon Deployment Just offshore and south of the Scripps Pier is the La Jolla Canyon. Here, a moored WW, equipped with a RBRconcerto CTD with integrated DO (JFE RinkoIII), chla, backscatter and phycoeryhrin sensors (WETLabs ECO Puck), has been deployed nearly continuously for >150days in 2016-2017. This WW has been transmitting data in real time via the RBR inductive mooring line modem and a GSM modem. The real-time data is made public and can be viewed at: http://data.rbrglobal.com/scripps/instrument/865798 Over a two-week span, the eddy became increasingly organized, and correspondingly, the mesoscale velocities accelerated. The WW was allowed to free drift for 13.5 days within the eddy and profiled from about 2m to 105m depth. The total dataset comprised of 2414 profiles (Figure 11). Figure 11: Figure taken from Lucas et al 2016 showing Bay of Bengal data collected. Figure 13: Sample data from website showing profile to 100m Figure 12: 3D representation of the Bay of Bengal drifting WW data set. Figure 14: Sample data timeseries with temperature on the top row and chlorphyl on the bottom. Left hand column shows data on the order of a few days, while the right hand column presents just a few hours.
4. Summary In summary, the Wirewalker system, built by Del Mar Oceanographic and adapted by RBR Ltd, can be used in either a moored or drifting scenario. The WW design allows for all on board power to be utilized solely to collect and transmit data. This in turn allows the user possibilities of greater deployment endurance, faster sampling capabilities, more frequent telemetry, or some combination of the above. Tandon, A. Mahadevan, M.M. Omand, M. Freilich, D. Sengupta, M. Ravichandran, and A. Le Boyer. 2016. Adrift upon a salinity-stratified sea: A view of upperocean processes in the Bay of Bengal during the southwest monsoon. Oceanography 29(2):134 145, http://dx.doi.org/10.5670/oceanog.2016.46. The system has been proven in both large period seas as well as small wind waves generated by only 3 knots of wind. Since the platform is able to continuously profile without interruption due to transmission time at the surface, users are able to also collect data without interruption, greatly reducing the burden on the programmer to carefully balance buoyancy, power budgets, and transmission times. Significant savings in terms of vessel costs are made using a WW given a vessel is not required to continually profile at one location or drift with an area of interest such as ocean eddy during the period of investigation. The high-speed communications via the telemetry system allow constant data streaming to anywhere in the world. Not only does the user get the benefit of real-time data and capturing events as they happen, but he/she also gets peace of mind knowing where their assets are at all times, and that the components are functioning. Future developments are planned with the system, including looking at using the wave energy to not only power the downward movement of the profiler but to also generate power for the onboard electronics to further increase the flexibility and duration of this system. 5. Acknowledgements The authors would like to thank Andrew Lucas, Tyler Hughen, and Rob Pinkel from Del Mar Oceanographic for their continued development and operation of the Wirewalkers and RBR integrations. 6. References [1] Rainville, L., and R. Pinkel, 2001: Wirewalker: An autonomous wave-powered vertical profiler. J. Atmos. Oceanic Technol., 18, 1048 1051. [2] Pinkel, R., M. A. Goldin, J. A. Smith, O. M. Sun, A. A. Aja, M. N. Bui, and T. Hughen, 2011: The wirewalker: A vertically pro- filing instrument carrier powered by ocean waves. J. Atmos. Oceanic Technol., 28, 426 435. [3] Lucas, A.J., J.D. Nash, R. Pinkel, J.A. MacKinnon, A.