Deep NINJA: A New Profiling Float for Deep Ocean ... - OnePetro

Proceedings of the Twenty-second (2012) International Offshore and Polar Engineering Conference Rhodes, Greece, June 17–22, 2012 Copyright © 2012 by the International Society of Offshore and Polar Engineers (ISOPE) ISBN 978-1-880653-94–4 (Set); ISSN 1098-6189 (Set)

www.isope.org

Deep NINJA: A New Profiling Float for Deep Ocean Observation Taiyo Kobayashi1, Ken-ichi Amaike1,2, Kazuhiro Watanabe1,2, Tetsuro Ino1,2, Kenichi Asakawa1, Toshio Suga1,3, Takeshi Kawano1, Tadahiro Hyakudome1, and Masami Matsuura1 1: Japan Agency for Marine-Earth Science and Technology. Yokosuka, Kanagawa, Japan 2: Tsurumi Seiki Corporation Ltd. Yokohama, Kanagawa, Japan 3: Tohoku University. Sendai, Miyagi, Japan

surface just after the diving to a greater depth (generally, 2,000 m). At the sea surface, the data are transmitted to stations on land via satellites, and the float again submerges to depth. Powered by internal batteries, the float is designed to continue this cycle for more than four years.

ABSTRACT The importance of deep ocean observations in accurately understanding oceanic effects on anthropogenic climate changes has been gradually recognized. One suitable device for deep ocean observation is a profiling float, and a monitoring network consisting of many deep floats must be built by 2020. We are developing a deep float called Deep NINJA, which can submerge to a depth of 4,000 m. It has a height of 210 cm and a mass of 50 kg. Field tests of Deep NINJA have been carried out in coastal waters, and the first dive to 4,000 m depth is planned for summer 2012.

The float is a sort of ocean vehicle designed for ocean observations. The depth of the float in the ocean is controlled by simply changing the float volume (i.e., buoyancy), because there is no change in the weight of the float during operation. Buoyancy is generated by a buoyancy engine, which inflates and deflates an external bladder (e.g., Davis et al., 1992).

KEY WORDS: Profiling float; deep observations; ocean monitoring. ARGO: A MONITORING NETWORK OF THE UPPER OCEAN BY NUMEROUS FLOATS The ocean plays an important role in the global climate due to its huge heat capacity, and even a very small change in ocean temperature can have a significant effect on climate. Thus, ocean observation is important in order to monitor the status of the ocean and to predict the future climate. The upper layers of the ocean are more closely related to the climate. In order to monitor the upper ocean (at depths shallower than 2,000 m), an international ocean observation program, Argo, was began in the year 2000 (Argo Science Team, 2001; Freeland et al., 2011). In Argo, more than 3,000 profiling floats are operating in the global ocean at every 3 degrees of latitude and longitude (Fig. 1). Japan is a major contributor to Argo, and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), in cooperation with the Japan Meteorological Agency and other governmental agencies and universities, plays an active role in Argo, performing float deployment and data management, for example.

Fig. 1: Distribution of floats under Argo by country. The total number of floats is 3,472 as of the end of October 2011. Courtesy of Argo Information Centre (http://wo.jcommops.org/cgibin/WebObjects/Argo). Ocean observations by floats have several advantages. Such observations can be made under severe conditions, e.g., at high latitudes in winter. As such, we can obtain ocean data without seasonal limitations. After deployment, the float can continue to perform observations for a long period, which provides ocean data in inaccessible regions, such as the Southern Ocean. The higher costperformance of the float observation is another advantage.

Figure 2 shows a schematic diagram of a float operated under Argo. After deployed in the ocean, the float descends to a great depth and drifts at this depth (generally, 1,000 m) for approximately 9 days. The float measures ocean temperature and salinity during ascent to the sea

The Argo network of 3,000 floats was completed in 2007 (Roemmich

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et al., 2009), and, at present, more than 100,000 profiles of temperature and salinity are obtained from the ocean each year. All data become available within 24 hours after float observations, supporting nearly real-time monitoring of the interior of the world’s oceans. The huge amounts of oceanic data provided by Argo have brought about great progress in oceanic and climatological sciences (Freeland et al., 2011).

other components of the global climate. Thus, the deep ocean has been recognized as a major source of uncertainty in predicting global climate in the future.

Fig. 2: Schematic diagram of float operation in the ocean under Argo. EFFECTS OF THE DEEPER OCEAN AT DEPTHS GREATER THAN 2,000 m ON THE GLOBAL CLIMATE Floats of the present type can be submerged to depths of up to 2,000 m (corresponding to a pressure of 2,000 dbar, or 20 MPa), which means that ocean observations by Argo are limited to 2,000 m in depth. However, since the average depth of the ocean is 3,800 m, only the upper half of the ocean can be examined by Argo using present technologies. Consequently, few ocean observations in the deeper half of the ocean have been conducted. Even now, deeper observations are conducted by research ships, in the same manner before the advent of the floats. In recent years, it was reported that observed variations in the deeper ocean (below 2,000 m) were much larger than expected (e.g., Yashayaev et al., 2003) and that the influences of these variations on the global climate could not be ignored (Levitus et al., 2005). For example, according to comparisons between accurate hydrographic surveys conducted in the 1990s and 2000s, deep and bottom layers in the Pacific warmed significantly (Fukasawa et al., 2004; Kawano et al., 2006). These changes in temperature were on order of 0.001°C, which is smaller than the changes observed in the upper layers. However, this warming significantly affects the global climate because the deeper and bottom layers of the ocean are massive. The heat accumulated in layers deeper than 3,000 m of the entire ocean from the 1990s to the 2000s (15 years) is estimated to be approximately 1.3×1022 J, which is approximately 25 - 30% of that in the layers above a depth of 700 m (Church et al., 2011; Kouketsu et al., 2011; Purkey and Johnson, 2010). The expansion of deep waters due to warming resulted in a rise in sea level: the contribution of the deeper ocean (> 3000 m) is a sea level rise of approximately 0.1 mm yr-1 (e.g., Church et al., 2011), out of the observed sea level rise of 3.1 mm yr-1 (Bindoff et al., 2007) that has occurred since the 1990s.

Fig. 3: Observation systems for the deep ocean adopted at OceanObs’09 (Garzoli et al., 2011). The middle panel shows the system used to monitor temperature and salinity by many deep floats. More floats are required than are shown in the panel. The top and bottom panels represent the monitoring systems of the currents and CO2, respectively. Note that all monitoring networks will fulfill their functions well while repeated hydrographic observations by research ships (Hood et al., 2011) are conducted. OCEANOBS’09: ENHANCEMENT OF OBSERVATIONS FOR THE NEXT DECADE

DEEP

OCEAN

At present, understanding the anthropogenic global change and predicting its future are important issues in the international community. Thus, the importance of deep ocean monitoring has become increasingly recognized from the viewpoints of not only oceanic and climatological sciences but also social security.

Because of the sparseness of ocean observations in deeper layers, the estimated impact of the deeper ocean includes larger errors than the

In the contexts of such scientific and social requirements, international guidelines of observational ocean sciences for the next decade (2010-

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2019) were determined at OceanObs’09 in 2009. The proposal of systematic observations in the deep ocean by autonomous devices was suggested (Garzoli et al., 2011). In the proposal, an observation network involving numerous deep floats should be introduced to monitor temperature and salinity in the deep ocean and to estimate the sea level rise accurately (Fig. 3).

Deep NINJA was designed and built in March 2011 (Fig. 4). The software for operating the float was also designed while considering present floats and user requirements. The validity of the operating software was first confirmed through simulations, and we then carried out field tests using the first prototype in coastal waters during a period from spring to summer in 2011. In August 2011, we carried out a series of comprehensive field tests in Sagami Bay, which is a region of shallow coastal water near Tokyo, by means of R/V Kaiyo of JAMSTEC (Kobayashi, 2011; Fig. 5). Through the series of field tests, we verified the reliability of the fundamental components of the hardware and software under operation in the sea. Based on the results of these field tests, we are currently designing and building a second prototype, which is cheaper and more energy efficient than the first prototype. The second prototype, which is thought to be similar to a future mass-production model, will be built in March 2012. At the same, we are improving the operating software. Thus, Deep NINJA will have better functionality than current floats. The first dive to a depth of 4,000 m is planned for summer 2012. We expect that the mass-production model will be available around 2013.

The suggestion is considered to be an extension of the Argo array to the deeper ocean. In Argo, the importance of deep Argo has been recognized and was listed among three important extensions of Argo in the future (Freeland et al., 2011). However, the development of the deep Argo network has not yet begun, because there are no floats that can be implemented in the deeper ocean below 2,000 m. At present, several float makers are interested in the development of a deep float based on the increasing desire for deep ocean monitoring, and a number of float makers have already begun development of a deep float. DEEP NINJA: A DEEP FLOAT DEVELOPED IN JAPAN Japan leads the international community in the development of deep floats. JAMSTEC and Tsurumi Seiki Co. Ltd. (TSK) began a feasibility study to develop a new buoyancy engine for a deep float in 2007. Then, we officially started a project to develop a deep float in 2010. This deep float is referred to as Deep NINJA. Note that NINJA is the name of the Japanese float for Argo, i.e., New profilINg float of JApan, developed by TSK in 2001 (Ando et al., 2003). So far, its development has progressed well.

Fig. 5: The first prototype of Deep NINJA just before deployment on R/V Kaiyo, JAMSTEC. Specifications of Deep NINJA Deep NINJA can observe the global ocean to a depth of 4,000 m (corresponding to a pressure of 40 MPa), which is the upper part of bottom waters. It has a height of 210 cm (including the antenna) and a mass of approximately 50 kg (see Fig. 4). The pressure hull is made of A7075 aluminum-alloy and has a diameter of 20 cm for the cylindrical parts and a diameter of 25 cm for the central bulge. The present prototype has a conductivity-temperature-depth (CTD) sensor (SeaBird Electronics Co.) on top for temperature and salinity measurements only, but it is designed to have sufficient capacity to load additional sensors, for example, dissolved oxygen. The short burst data (SBD) service of Iridium is used to transmit observed data to stations on land in a short time. This also enables two-way communication between float and operator. Thus the float in the sea can change its operation based on commands from the operators. The float locations at the sea surface are accurately fixed by the global positioning system (GPS). The Deep NINJA float is driven by lithium batteries, which provide longer life operation in the sea. The float specifications are summarized in Table 1.

Fig. 4: First prototype of Deep NINJA. (left) External view and (right) internal view. Current Status of Development of Deep NINJA and Plans for the Near Future We have confirmed the reliability of each of the mechanical parts of the float hardware using pressure tanks. In addition, the first prototype of

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Table 1: Specifications of Deep NINJA. Parameter

Value [unit]

Max. depth for operation

4,000 [m]

Dimension

210 (height) × 25 (dia. max.) [cm]

Weight in air

Approx. 50 [kg] A CTD sensor (enough capacity for additional sensors) Iridium SBD, bi-directional

Sensors Data transmission Position fixing Batteries

longer time. Thus, we think that CTD observations by spot sampling at approximately 400 levels within a 4,000 m profile is the standard, which realizes 5×104 Pa (= 5 dbar, ~ 5 m) and 20×104 Pa (= 20 dbar, ~ 20 m) intervals in surface and deep layers, respectively. Of course, CTD observation with mix sampling, in which the continuous (spot) sampling mode is used in shallower (deeper) layers, is also available.

GPS Lithium, 20 DD cells

OBSERVATIONS IN SEA BY DEEP NINJA Float Operation in Water Deep NINJA can realize most observation patterns required by users. Figure 6 shows a flow chart of its operating software. Its standard operation, we consider now, is as follows. The first profile is observed from the maximum depth (4,000 m) within a couple of days of deployment1. The accuracy of the float observation can be evaluated through comparison with ship-board observation. Later, the float operates repeatedly on the track expressed in red shown in Fig. 7, drifting at a depth of 1,000 m and profiling from a depth of 2,000 m or 4,000 m, depending on the cycle. (Although the case of three cycles is shown in the figure, the standard is five cycles.) The observation interval is 10 days. Another observation pattern is available by simply changing the setting parameters. Figure 7 shows some examples of the available patterns for Deep NINJA operation. The float observes the 4,000 m profile every cycle (green), which is a similar operation to the floats under Argo. In addition, the deep float carries out CTD observations at, for example, 6-hour intervals while drifting for approximately 9 days at a depth of 4,000 m and profiles during the ascent to the sea surface (yellow). Furthermore, the float will also be allowed to operate a sequence focusing on deeper observations, where the float observes CTD profiles between depths of 2,000 m and 4,000 m for a time and then ascends to the sea surface to transmit the data (blue). In this case, the locations at deeper profiles should be interpolated from the previous and following surfacing positions fixed by GPS.

Fig. 6: Flow chart of operating software for Deep NINJA. Function to Avoid Deaths Caused by Accidents Due to Sea Ice Deep floats are expected to be used in high-latitude waters, such as the Antarctic Ocean, where the sea surface is seasonally covered by sea ice. Thus, Deep NINJA has a function to reduce the risk of death due to sea ice by a method similar to that of Klatt et al. (2007). If the deep float predicts that sea ice exists above it, it stops ascending (and observing) and promptly descends. The observed data are then transmitted to stations on land when the float arrives at the sea surface.

Through commands from operators on land, Deep NINJA can change its operation while in the sea. Most of the fundamental parameters of float observation, such as parking depth, profile depth, interval of observation cycle, and frequency of deeper observations, are changeable. In addition, the float can observe a CTD profile from a depth of 4,000 m within a couple of days of receiving commands, only because it takes approximately 24 hours to return from a depth of 4,000 m, by skipping the parking drift at a shallower depth. Therefore, Deep NINJA is able to observe oceanic events that occur after its deployment. The deep float has two options for CTD sampling. One is spot sampling, in which CTD observations are carried out at preset pressures given by a list, and the other is continuous sampling, by which processed temperature and salinity data are obtained at pressure intervals of 2×104 Pa (= 2 dbar, which is equivalent to approximately 2 m). The latter method provides higher-resolution data, but more energy is also consumed because the CTD sensor must be operated continuously for a 1

Fig. 7: Typical operation patterns that will be available for Deep NINJA. Red is the standard, in which the float skips deeper observations every few cycles. Blue is an exceptional pattern, in which deeper profiles are observed repeatedly. Black lines represent the standard pattern operated under Argo.

It is possible to change the first observation 10 days after deployment.

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the float volume is restricted to 88 cm3, and most of the effect is concentrated on the layers shallower than the depth at 3 MPa (approximately 300 m), which reflects the temperature structure at that location. In the Antarctic Ocean, since the temperature of the water does not change significantly, almost no buoyancy generation by the thermal effect is expected.

Function to Avoid Groundings Deep floats have a higher possibility of grounding on sea floor than current floats. Grounding, during which the float cannot escape from shallow waters, causes subsequent groundings. The cost per profile for deep floats will be more expensive than that for current floats for Argo. Thus, we provided Deep NINJA with a function to avoid groundings. The float ascends by, e.g., 50 m above the bottom in order to drift away from the shallow region when it is expected that the float has contacted the sea floor during decent. DESIGN OF THE PRESSURE HULL It is desirable that Deep NINJA can be taken onboard by hand by two persons, which means that its mass should be less than approximately 50 kg. Its lightweight could be realized by a multi-stage-shaped pressure hull (Fig. 4). The multi-stage structure could make the cylindrical parts of the hull shorter, which resulted in enhanced pressure proofing. Therefore, the thickness of the hull could be thinner than that of a long and simple cylindrical hull. The multi-stage design yields other advantages. The cylindrical parts are formed and dressed more easily (and at less cost). In addition, the diameter of the pressure hull can be increased, which, in general, is desirable for an underwater vehicle because the hull capacity of the vehicle is increased for the same weight. However, with respect to the hull of the float, the diameter of the top part of the pressure hull with a CTD sensor must be less than 16.5 cm because of the size of the sensor calibration bathes at the manufacturer. The multi-stage structure can avoid the restriction.

Fig. 8: Observed profiles of (black) temperature, (red) salinity, and (green) in situ density in the western tropical Pacific (4.976°N 147.058°E, observed on 1 March 2006, indicated by solid lines) and the Antarctic Ocean (59.837°S, 139.841°E, observed on 19 November 2001, indicated by dashed lines). Note that 1 MPa of pressure corresponds to approximately 100 m of depth.

Buoyancy generation due to thermal and pressure effects A float in water is compressed by pressure and its body is also diminished by low temperature in the deep ocean. The effects of pressure and temperature provide additional buoyancy to the deep float, which helps it ascend from deep layers to the sea surface. Here, we estimated these effects with oceanic profiles observed in the western tropical Pacific and the Antarctic Ocean (Fig. 8). The western tropical Pacific (Antarctic Ocean) is the region in which the water covering the sea surface is the least dense (densest) and warmest (coldest). The coefficient of thermal expansion of the Deep NINJA body is considered to be same as that of A7075 aluminum alloy, i.e., 6.48×10-5 [K-1] (= 3 × 2.16×10-5, at a temperature of from -60 to 20°C). The compressibility of the float body was estimated to be 2.1×10-10 [Pa-1] (= 2.1×10-6 [dbar-1]) by a similar method to that reported by Izawa et al. (2001) for an APEX type float, and the compressibility is about half that of (pure) water (4.5×10-10 [Pa-1]). The compressibility of the APEX float, which can submerge to a depth of 2,000 m, was 2.23×10-10 [Pa-1] (= 2.23×10-6 [dbar-1]) (Izawa et al., 2001). The compressibility of Deep NINJA is almost the same as that of the APEX float, which may indicate that the pressure hull of the deep float is fairly thin, probably because of the multi-stage structure. The increases in volume of Deep NINJA, which are assumed to be 5.0×104 cm3 (50 liters) in the calculation, due to the pressure and thermal effects are shown in Fig. 9 for the western tropical Pacific and Antarctic Ocean. The pressure effect is the same anywhere in the ocean, and the value of the deep float increases linearly by 420 cm3 (0.84% of the entire body) during ascent from depth at 40 MPa (approximately 4,000 m) to the sea surface. On the other hand, the thermal expansion is a minor effect. Even in the western tropical Pacific, the increment of

Fig. 9: Change in volume of Deep NINJA due to the effects of (red) pressure and (blue) temperature in (solid lines) the western tropical Pacific and (dashed lines) the Antarctic Ocean. The float volume is assumed to be 5.0×104 cm3 at 40 MPa.

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ENERGY BUDGET ESTIMATION

depths shallower than 5 MPa (approximately 500 m), the structure of in situ density varies primarily due to variations of the temperature structure. In the western tropical Pacific, the float requires more buoyancy around the depth at 1 to 3 MPa than in the Antarctic Ocean in order to ascend through the sharp pycnocline (i.e., thermocline). The total volume expansion by the deep float engine is estimated to be 707 cm3 in the western tropical Pacific and 522 cm3 in the Antarctic Ocean when the float statically ascends from the depth at 40 MPa to the sea surface.

One of the most important issues for a float is its energy budget, because the energy budget of a float is critically related to the “theoretical” lifetime of the float (Kobayashi et al., 2009)2. For a float, in general, most of the energy is consumed by the buoyancy engine for the control of vertical movement (Davis et al., 2001). Thus, we briefly explain the performance of the engine of Deep NINJA and then examine the energy budget. Performance of the Buoyancy Engine for Deep NINJA The buoyancy engine of Deep NINJA, which was newly developed by JAMSTEC and TSK in 2008, has a hybrid mechanism of a singlestroke piston (Davis et al., 2001) and a hydraulic pump (Davis et al., 1992). This engine is suitable for a small vehicle operating in the deep ocean and is described in detail by Kobayashi et al. (2010). The current prototypes have the engine optimally adjusted to the high pressure of 40 MPa. Figure 10 shows a summary of the engine performance for ejecting 50 cm3 of hydraulic oil to the external bladder under pressures of from 0 to 42 MPa. A constant 14 V direct current (DC) was supplied to the driving motor from an external device. In order to eject 50 cm3 of hydraulic oil, the engine required a greater electric current (from 0.44 A at 0 MPa to 2.06 A at 42 MPa) in order to drive its motor as the pressure on the external bladder increases. Furthermore, it took longer to complete the action (from 132 s at 0 MPa to 154 s at 42 MPa). Both parameters changed approximately linearly with the external pressure and their respective regressions are shown in Fig. 10. The energy consumed at the buoyancy engine is, for example, 813 J (14 V × 0.44 A × 132 s) at 0 MPa and 4,215 J (14 V × 1.93 A × 156 s) at 40 MPa. The engine was designed to be supplied with 7 V DC from internal batteries when the engine works under low load for energy savings. In this case, a current of 0.27 A was supplied to the driving motor and a total of 267 s was required to complete a one-way action at 0 MPa. Thus, the engine consumes 504 J (7 V × 0.27 A × 267 s) to eject or take in a volume of 50 cm3 of hydraulic oil.

Fig. 10: Results of a laboratory test to examine the buoyancy engine for Deep NINJA. Solid circles represent the electric current on the driving motor, and red triangles represent the duration for one-way action of the piston upon pushing out 50 cm3 oil against external pressure. Regression lines are obtained linearly. A constant 14 V DC was supplied to the driving motor from an external device. The energy consumption required to ascend the float by 1 MPa is also estimated. When the float ascends statically, for example, from the depth at 40 MPa to 39 MPa, approximately 1.06 kJ (= 11.2 cm3 × 4,729 J/50 cm3) of energy is consumed. Thus, Fig. 11b shows the function of the energy consumption at pressure. Therefore, Deep NINJA consumes an energy of 35.85 kJ (30.14 kJ) in the western tropical Pacific (Antarctic Ocean) to statically ascend from the depth at 40 MPa to the sea surface.

The engine must pull back its piston to replenish the hydraulic oil supply (by means of the supplied 7 V DC) after ejecting the oil (14 V DC). Thus, the engine consumes 4,729 J per 50 cm3 of oil ejected at 40 MPa, considering an energy consumption of 10 J for the two actions of the three-way valve. The regressions shown in Fig. 10 enable us to calculate the energy consumption per 50 cm3 of oil ejection at any pressure, which is shown in green in Fig. 11b. Energy Budget

Deep NINJA controls its upward velocity in water to be faster than 0.08 m s-1 during ascent. This requires additional buoyancy (25 cm3 in the calculation) at the deepest depth in order to generate the velocity, which consumes an additional energy of 2.38 kJ. In this situation, the float always receives a positive buoyancy of at least 0.245 N (= 25 g × 9.8 m s-2) during ascent in water. At the sea surface, the float lifts the Iridium/GPS antenna for position fixing and data transfer. Preliminary experiments clarified that both functions worked sufficiently by lifting up with additional buoyancy corresponding to a volume increase of 250 cm3 from the neutral density with the sea surface water. The volume increase at the deepest depth (25 cm3) is set aside for the additional buoyancy at the sea surface, and the volume of the float is required to be increased by 225 cm3 at the sea surface. The duration of sea surface drifting of the float should be as short as possible in order to avoid problems such as sensor contamination due to biofouling, shellfish adhesion on float body, drifting ashore, and unintentional pickup. Thus, Deep NINJA is designed to use 14 V DC for the volume expansion, which requires approximately 7.34 kJ of energy.

Next, we estimate the buoyancy that the engine of Deep NINJA must generate in order to ascend 1 MPa (approximately 100 m) from depth. As shown in Fig. 8, the vertical difference of in situ density between depths by 1 MPa at depths greater than 10 MPa is approximately 0.45 to 0.5 kg m-3 uniformly throughout the world ocean. Thus, the float having dimensions of 5.0×104 cm3 must increase its volume by 22 to 25 cm3 in order to ascend statically by 1 MPa, i.e., to be balanced by the water at a depth shallower by 1 MPa than the present position. Due to the thermal and pressure effects, Deep NINJA increases its volume by 0 to 1.5 cm3 and 10.5 cm3, respectively (see Fig. 9). Therefore, the volume of around 11 to 14 cm3 must be increased by the buoyancy engine to ascend by 1 MPa in the deep layers. This suggests that large compressibility of float body is very important to save energy. At 2

The “actual (or average)” lifetime of a float is also related to the frequency (or the possibility) of occurrences of fatal accidents during its operation (Kobayashi et al., 2009).

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The energy consumption by other components is smaller than that by the buoyancy engine. For CTD observation (32 mA × 14 V) during ascent, approximately 2.69 kJ will be consumed by 400 level measurements, assuming 15 seconds of sensor operation per measurement3. Pressure should be monitored (5 mA × 14 V) during ascent (approximately 40,000 s, by an average upward velocity of 0.1 m s-1) and energy of 2.80 kJ is consumed. At the sea surface the float uses GPS for position fixing (50 mA × 7 V). If this requires approximately 5 minutes, then 0.11 kJ per cycle is consumed. In addition, the Iridium system (130 mA × 14 V) is used for 10 minutes. Thus, 1.09 kJ of energy will be consumed for data transmission per cycle. The energy consumption of the other components, i.e., operating float central processing units (CPUs), background current on electric boards during a cycle of 10 days, is estimated to be 4.81 kJ per cycle. Therefore, the total energy consumption per cycle is estimated to be approximately 79.96 kJ (70.47 kJ) in the western tropical Pacific (Antarctic Ocean), and about 85% of the energy is consumed by the buoyancy engine. The results are summarized in Table 2, in which the estimation for 2,000 m profiling is also shown.

Next, we consider the energy consumption during its descent. At the sea surface, 8.97 kJ is consumed by the float in order to put down the antenna and to submerge in water by deflection of the volume of 275 cm3. In the subsurface, the deep float decreases its volume by a total of 682 cm3 (496 cm3) under pressure to arrive at the depth at 40 MPa in the western tropical Pacific (Antarctic Ocean). Here, the buoyancy engine is allowed to work slowly by supply of 7 V DC electricity, and the pressure under which the engine works is assumed to be 0 MPa. Thus, the engine consumes 13.92 kJ (= 682 cm3 × 1,020 J/50 cm3) (10.14 kJ). Therefore, Deep NINJA is estimated to consume 68.46 kJ (58.97 kJ) per cycle by its vertical movement between the depth at 40 MPa and the sea surface in the tropical Pacific (Antarctic Ocean).

The present prototypes were designed to hold 20 DD lithium batteries, for a total of 7,344 kJ (= 20 × 30 Ah × 3.4 V) of energy. Thus, the deep float is expected to be able to carry out approximately 91 (104) CTD observations from the depth of 4,000 m during its theoretical lifetime in the western tropical Pacific (Antarctic Ocean). When the float carries out the observation of the 4,000 m profile every five cycles, which is the standard operation we consider, the expected number of observation cycles is extended to 127 (153), including 25 (30) deep profiles. Thus, for the standard operation, Deep NINJA can continue to perform observations for more than 3 years with a 10-day interval (like Argo) and deep profiles up to a depth of 4,000 m are obtained every 50 days during the lifetime of the float. The above values are estimated under numerous assumptions, and the expected number of observation cycles in the study will be changeable. Therefore, accurate estimation of its energy budget will be performed again in the future. Table 2: Energy budget estimation for Deep NINJA. Unit is kJ per cycle. See text for the details of assumptions for the estimation. The values outside (inside) the parenthesis are the values for the western tropical Pacific (Antarctic Ocean). Phase of float operation Energy consumption [kJ / cycle] 4,000 m 2,000 m profile case profile case Ascending in water 35.85 (30.14) 17.60 (11.84) Acceleration at max. depth

2.38

1.46

Lift up at sea surface

7.34

7.34

Lift down at sea surface Descending in water CTD observation P monitoring Fig. 11: (a) Volume increase and (b) energy requirement for Deep NINJA to ascend 1 MPa (approximately 100 m) statically. (a) The total volume increase required is expressed in red, and the compensations by the thermal and pressure effects are expressed in blue and yellow, respectively. Thus, the volume increase that the buoyancy engine must generate is shown in black. (b) The energy required by the engine to eject a volume of 50 cm3 of hydraulic oil is shown in green. Solid and dashed lines represent the cases in the western tropical Pacific and the Antarctic Ocean, respectively.

3

8.97

8.97

13.92 (10.14) 2.69 (400 levels) 2.80

9.09 (5.30) 2.02 (300 levels) 1.40

Position fixing by GPS

0.11

0.11

Data transfer by Iridium

1.09

1.09

Others

4.81

4.81

Total

79.96 (70.47)

53.89 (44.34)

In the case of the continuous sampling mode, the CTD sensor consumes 17.92 kJ (= 32 mA × 14 V × 40,000 s) to profile from the deepest depth.

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CONCLUSIONS The importance of deep ocean observations has been gradually recognized to understand oceanic effects on anthropogenic climate changes accurately. One suitable device for deep observation is a deep float, and a monitoring network comprising many deep floats is required to be built by 2020. JAMSTEC and TSK are developing a deep float, Deep NINJA. The deep float is available for observations in deep layers of up to 4,000 m everywhere in the world ocean. Its height is 210 cm (including the antenna) and its weight is approximately 50 kg due to a multi-stage-shaped pressure hull. Thus, Deep NINJA can be easily handled by two persons. A CTD sensor is attached to the top of the prototype; but the float is designed to have sufficient capacity for additional sensors. Locations are fixed accurately by GPS while drifting on the sea surface. The float is designed to transmit data to and to receive commands from stations on land bi-directionally by the Iridium SBD service. The deep float can operate most patterns for observation requested by users. The patterns are changeable by changing parameters before deployment and by sending commands from operators after deployment. Finally, the deep float, which will be used in the polar region, has a function to avoid problems due to sea ice. Deep NINJA is driven by lithium batteries, which provide a longer lifetime for operation in the sea. A trial energy budget estimation revealed that the float requires around 70-80 (45-55) kJ for a 4000 (2000) m profile observation and that about 85% of energy will be consumed at the buoyancy engine to control the float depth. Under many of the assumptions, the estimation yielded a theoretical lifetime of Deep NINJA of more than 3 years for an observation interval of 10 days, in which the profiles of 4,000 m are obtained every 50 days. Since the spring of 2011 field tests of Deep NINJA were carried out in coastal waters to verify the reliability of its fundamental components. At present, we are assembling the second prototype, which will be similar to a future mass-production model. The first dive to a depth of 4,000 m is planned for the summer of 2012, and the mass-production models will be made available around 2013.

ACKNOWLEDGEMENTS The development of Deep NINJA was financially supported by the Accelerating Program for Advance of Practical Utilization of JAMSTEC. REFERENCES Ando, K, et al. (2003). “Results of field experiments and laboratory test of domestic profiling floats (NINJA),” Rep. Japan Mar. Sci. Tech. Center, Vol. 48, pp 55-65. (in Japanese with English abstract) Argo Science Team (2001). “Argo: the global array of profiling floats,” Observing the Oceans in the 21st Century, Eds. Koblinsky, CJ, and Smith, NR, Godae Project Office, Bureau of Meteorology, Melbourne, Australia, pp 248–258. Bindoff, NL, et al. (2007). “Observations: Oceanic Climate Change and Sea Level,” Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Eds. Solomon, S, et al., Cambridge University Press, Cambridge, UK. and New York, NY,

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