Underwater Technology 36.3 by SUT

Vol. 36 32 No. No. 332 2019 2014 Vol.

UNDERWATER TECHNOLOGY

ISSN 1756 0543

A Personal View... Seeing the sea

Philomène Verlaan

A novel initiative on vertical-axis underwater turbine suitable for low underwater current velocities

Jasper Ahamefula Agbakwuru and Umar Ukkasha Ibrahim

Signal transmission through seawater for MHz frequencies and medium distances (0–30 m) using ionic current waves

Emeritus Prof J Lucas

Book Review Introducing Sea-Level Change

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UNDERWATER TECHNOLOGY Editor Dr MDJ Sayer Scottish Association for Marine Science Assistant Editor E Azzopardi SUT Editorial Board Chairman Dr MDJ Sayer Scottish Association for Marine Science Gavin Anthony, GAVINS Ltd Dr MA Atamanand, National Institute of Ocean Technology, India LJ Ayling, Maris International Ltd Commander Nicholas Rodgers FRMetS RN (Rtd) Prof Ying Chen, Zhejiang University Jonathan Colby, Verdant Power Neil Douglas, Viper Innovations Ltd, Prof Fathi H. Ghorbel, Rice University G Griffi ths MBE, Autonomous Analytics Prof C Kuo FRSE, Emeritus Strathclyde University Dr WD Loth, WD Loth & Co Ltd Craig McLean, National Ocean and Atmospheric Administration Dr S Merry, Focus Offshore Ltd Prof Zenon Medina-Cetina, Texas A&M University Prof António M. Pascoal, Institute for Systems and Robotics, Lisbon Dr Alexander Phillips, National Oceanography Centre, Southampton Prof WG Price FRS FEng, Emeritus Southampton University Dr R Rayner, Sonardyne International Ltd Roland Rogers CSCi, CMarS, FIMarEST, FSUT Dr Ron Lewis, Memorial University of Newfoundland Prof R Sutton, Emeritus Plymouth University Dr R Venkatesan, National Institute of Ocean Technology, India Prof Zoran Vukić, University of Zagreb Prof P Wadhams, University of Cambridge Cover Image (top): zoonar.com/syrist Cover Image (bottom): Steve Crowther Cover design: Quarto Design/ kate@quartodesign.com

Society for Underwater Technology Underwater Technology is the peer-reviewed international journal of the Society for Underwater Technology (SUT). SUT is a multidisciplinary learned society that brings together individuals and organisations with a common interest in underwater technology, ocean science and offshore engineering. It was founded in 1966 and has members in more than 40 countries worldwide, incIuding engineers, scientists, other professionals and students working in these areas. The Society has branches in Aberdeen, London and South of England, and Newcastle in the UK, Perth and Melbourne in Australia, Rio de Janeiro in Brazil, Beijing in China, Kuala Lumpur in Malaysia, Bergen in Norway and Houston in the USA. SUT provides its members with a forum for communication through technical publications, events, branches and specialist interest groups. It also provides registration of specialist subsea engineers, student sponsorship through an Educational Support Fund and careers information. For further information please visit www.sut.org or contact: Society for Underwater Technology 2 John Street, London WC1N 2ES e info@sut.org

Scope and submissions The objectives of Underwater Technology are to inform and acquaint members of the Society for Underwater Technology with current views and new developments in the broad areas of underwater technology, ocean science and offshore engineering. SUT’s interests and the scope of Underwater Technology are interdisciplinary, covering technological aspects and applications of topics including: diving technology and physiology, environmental forces, geology/geotechnics, marine pollution, marine renewable energies, marine resources, oceanography, salvage and decommissioning, subsea systems, underwater robotics, underwater science and underwater vehicle technologies. Underwater Technology carries personal views, technical papers, technical briefings and book reviews. We invite papers and articles covering all aspects of underwater technology. Original papers on new technology, its development and applications, or covering new applications for existing technology, are particularly welcome. All papers submitted for publication are peer reviewed through the Editorial Advisory Board. Submissions should adhere to the journal’s style and layout – please see the Guidelines for Authors available at www.sut.org.uk/journal/default.htm or email elaine.azzopardi@sut.org for further information. While the journal is not ISI rated, SUT will not be charging authors for submissions.

in more than 40 countries worldwide, including over 190 Corporate Members of the Society.

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Open Access Underwater Technology is available as Open Access. PDF versions of all published papers from Underwater Technology may be accessed via ingentaconnect at www. ingentaconnect.com/content/sut/unwt. All issues from Volume 20 (1995) onwards are available as Open Access. The Society for Underwater Technology also encourages Underwater Technology authors to make their papers available online on their personal and/or institutional websites for Open Access. Through this arrangement, the Society supports the Open Access policy not only in the UK (the Research Councils UK (RCUK) policy) but also the drive towards Open Access in other countries.

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Underwater Technology is also available in electronic format via ingentaconnect as Open Access. To subscribe to the print version of the journal or for more information please email Elaine Azzopardi at elaine.azzopardi@sut.org

Publication and circulation Underwater Technology is published in March, July and November, in four issues per volume. The journal has a circulation of 2,400 copies to SUT members and subscribers

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A Personal View...

doi:10.3723/ut.36.041 Underwater Technology, Vol. 36, No. 3, pp. 41–42, 2019

Seeing the sea How humans see the sea is crucial to the future of life on this planet, which we persist in calling “earth” although nearly threequarters of it is “ocean”, as pointed out by, amongst others, Arthur C. Clarke. ‘Sea blindness’ is a concept usually associated with the public’s lack of knowledge of the shipping world and the dependence of trade on the sea. However, sea blindness also afflicts humans (with the exception of a few fastdisappearing traditional coastal communities) on just about every marine metric, including that of the critical importance of the sea to our well-being. An example of traditional coastal communities seeing the sea is Pacific Islanders thinking of themselves as “Oceanians”. They speak of their world as ‘our sea of islands’ in the striking imagery of Dr Epeli Hau’ofa, who considered himself to be Tongan, Fijian and Oceanian. Oceanians see the sea as a medium of connection between each other, and not of separation. The sea joins the Pacific Islands and their inhabitants; it does not isolate them. Voyaging between islands, using traditional navigation techniques, requires envisioning the islands as moving. The island voyagers in their vessels are the fixed point. Seeing is not simple – it takes different forms. Seeing as a means of learning is not straightforward, because valuable sources of learning may well be in plain sight but invisible to both untutored and over-tutored eyes. Identifying and removing those blinkers are essential to seeing fully. The anthropocentric blinker

(i.e. where it is asked, ‘How is this good for me?’) especially impedes seeing to learn. However, we need to beware of the tyranny of the visual. The deceptive opacity of the sea makes it particularly vulnerable to the ‘out of sight, out of mind’ syndrome, and bedevils not only our attempts to understand it, but also to protect and preserve it. The great Pacific Island navigators do not only use visual cues, such as stars, clouds, water colour and birds, to orient themselves on a journey between islands. Crucial to their portfolio of navigation skills is their ability to disentangle complex wave and current patterns to discern the effects of waves reflected off real, but invisible, islands. They do so by using their entire body as the sensor – usually by lying flat out and face down on the bottom of the vessel – with their eyes closed. Training for this requires immersing themselves well beyond the reef in the deep sea, learning to feel its movements and to differentiate between swell, tide, local currents and deflections from other islands (Brower, 1983). This is physical oceanography seen without eyes and learned through the body. It adds an additional level of meaning to the concept of ‘physical’ in physical oceanography. The tyranny of the visual has other unhelpful aspects when it comes to seeing the sea. For example, we cannot see the adverse effects of noise pollution, acidification or warming on the sea itself. Would we care as much about the effect of warming on corals if they did not lose their vibrant colours because of it, even though we know they are dying?

Dr Philomène Verlaan is an oceanographer specialized in the biogeochemistry and ecology of deep-sea ferro-manganese nodules and crusts (Ph.D., Imperial College London). She has extensive sea-going experience and has participated in 24 oceanographic research cruises and nine submersible dives so far. She is also an attorney-at-law specialized in international law of the sea (J.D., Florida State University; Member of the Florida Bar). She has authored over 50 refereed publications, and is a Visiting Colleague at the Department of Oceanography, University of Hawai’i and Trustee of the Advisory Committee on Protection of the Sea.

Even when we use other sensory technical media to learn about the sea, such as acoustics (e.g. echosounders) and (geo)chemistry (e.g. conductivity‐temperature‐ depth (CTD) instruments), the data obtained must be translated into images in order to be useful. Those data are also used as the basis for models of the sea and its living and non-living processes. Dr Joseph Reid, the eminent physical oceanographer specialising in ocean currents, won the Albatross Award (i.e. oceanography’s Nobel Prize) for ‘his outrageous insistence that ocean circulation models should bear some resemblance to reality.’

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Philomène Verlaan. Seeing the sea

Modelling requires filters and judgments chosen and implemented by humans. What essential information about the sea do we miss with these filters and judgments – in other words, information we can only obtain by looking directly, with our own eyes, into the sea? I emphasise ‘directly’ because when we try to see the sea, we do so ever more remotely from the sea itself, i.e. through cameraequipped devices. The number of human-occupied research submersibles, never large, has plummeted. Just as moving formerly human interactions online to be mediated by machines is probably unhealthy in terms of how humanity is seeing itself, engaging ever more remotely with the sea in order to gather information about it is unlikely to help us to see it most constructively. Nor, to recall the concept of exchange introduced earlier as an intrinsic component of learning through seeing, is it likely to enable the sea to engage with us in response. It is that engagement that seeing for learning should foster. However, it is not easy to have an exchange with the sea. Dr John Craven, a dedicated and creative marine engineer who

never ceased his search for new ways to see the sea once summarised his lifetime’s experience of working in and with the sea as: ‘If you bring something new to the sea, the sea will bring something new to you’ (Craven, 2001). Our removal from direct experience of the sea risks denying ourselves the chance to receive something new from the sea. It also results in the diminution, if not whole-scale removal, of our empathy, and hence, in our alienation from physical realities. I serve on the PhD committee of a student at the University of Hawai’i who is looking at the anthropology of deep-sea mining. The student’s field work includes voyaging on deep-sea mineral vessels to ascertain how the technicians, scientists and crew see the sea and the resource. One finding from this work describes how the use of remote sensing equipment led to descriptions of the sea between the ship and seabed as ‘liquid rock’ (Harris, 2016). We are a deeply visual species. We hunger for direct experience. I have not yet met anyone who considers watching a video of

Venice to be the equivalent of actually going there. Despite my overall caveat about visual tyranny, let us use this predilection for the directly experienced to our and the sea’s advantage: to promote exchange; to leave options open for something new to be brought to us by the sea; for us to at least see something new. This means, inter alia, putting scientists back in research submersibles and to (re)invest in the long-term, direct observational presence by real humans in the real sea.

References Brower K. (1983). A song for Satawal. New York: Harper & Row, 218 pp. Craven JP. (2002). The silent war: the Cold War battle beneath the sea. New York: Simon & Schuster, 304 pp. Harris L. (2016). Sustainable seabed mining: corporate geoscientists’ visions of seabed mining in the Solomon Islands. Master’s thesis. The University of Hawai‘i at Maˉnoa, Honolulu, USA. Hau‘ofa E. (1993). Our sea of islands. In: Hau‘ofa E, Naidu V and Waddell E. (eds.) A new Oceania: rediscovering our sea of islands. Suva, Fiji: The University of the South Pacific School of Social and Economic Development, in association with Beake House, 2–17.

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doi:10.3723/ut.36.043 Underwater Technology, Vol. 36, No. 3, pp. 43–51, 2019

Technical Paper

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A novel initiative on vertical-axis underwater turbine suitable for low underwater current velocities Jasper Ahamefula Agbakwuru1 and Umar Ukkasha Ibrahim2 1 Center for Maritime and Offshore Studies, Federal University of Petroleum Resources, Effurun, PMB 1221, Delta State, Nigeria 2 Department of Petroleum Engineering, Federal University of Petroleum Resources, Effurun, PMB 1221, Delta State, Nigeria Received 26 March 2019; Accepted 24 August 2019

Abstract The present paper discusses efforts made to reinvent the use of the vertical-axis turbine for use in locations of low underwater current velocities. The present work targets the low flow current of the sub-Saharan ocean system, which has an underwater current record of around 0.3 m/s and a sea state that is mild, benign and with little or no local storms. The present initiative is achieved through a combination of ducting techniques to increase velocity of flow, and the utilisation of a large surface contact area exposed to flowing water per unit of time. Torque estimations are made using three methods: first principle, SolidWorks computational fluid dynamics (CFD) software and physical measurement. The lowest power coefficient for the tested model is computed from SolidWorks CFD software as 0.70. Existing stateof-the-art underwater current power technologies are reviewed and the present initiative described. A future for ocean water current technology in sub-Saharan Africa is also proposed. Keywords: underwater current, marine, water velocity, hydrokinetic, turbine, sub-Saharan, vertical-axis, horizontalaxis, ducting, floating turbine

1. Introduction The regular motion of a body of water in a given direction is referred to as water current. In ocean situations where the surface is stochastically varying as a result of surface waves, the regular motion in a certain direction is found below the free water surface and is referred to as the underwater current. The underwater current from oceans represents a huge energy potential (Lloyd-Evans, 2005; Asseff and Mahfuz, 2009; Rahman et al., 2014; Uihlein * Contact author. Email address: agbakwuru.jasper@fupre.edu.n

and Magagna, 2016; Chen et al., 2018). In the present work, a device that converts hydrokinetic flow of the current or underwater current into mechanical energy for operating an electrical alternator to produce electricity is referred to as underwater current power turbine (UCPT). Over 71% of the world surface is covered by oceans, rivers and seas. It is noted that the use of energy in 0.1% of the ocean can supply the whole energy demand of the earth (US Patent No. 4,219,303, 1980). The minimum physical flow velocity required for viable operation of UCPT is 1.0 m/s(Chen et al., 2018). Based on the measurements by Shell during the West Africa Swell Project (WASP) in 2004 across West African offshore locations, the Atlantic within sub-Saharan Africa has a flow speed less than or equal to 0.3 m/s (Olagnon et al., 2004). This is relatively low for any existing technology to produce meaningful power, and negates the use of UCPT in subSaharan Africa. It has been well noted that the ocean offshore West Africa is benign (Akinsanya et al., 2017). Literature and observed data have shown that unlike various other places of the world, offshore conditions of West Africa are mild, not stormy and are characterised by swells of long wavelengths and long periods of time (Akinsanyaet al., 2017). Available data from the Shell WASP report of 2004 indicates this mild nature and predictability of the ocean off West Africa (Olagnon et al., 2004). UCPT technology is dominated by the horizontal axis turbine. The presented UCPT is a verticalaxis turbine with hinged blades that closes gates against water flow to produce torque on a given side, and opens up over the gates to avoid torque on the opposite side (Fig 1); this differs from existing 43

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Agbakwuru and Ibrahim. A novel initiative on vertical-axis underwater turbine suitable for low underwater current velocities

Active side

Passive side

Water current direction

(a) Start position at 0° shaft rotation

Active side Passive side

Water current direction

(b) New position at 120° shaft rotation

Active side Passive side

Water current direction

Active side Passive side

Water current direction

(d) End position at 360° shaft rotation

Fig 1: A simpliﬁed operating principle of model vertical-axis UCPT

state-of-the-art UCPT. The distinct features of the vertical-axis turbine can be harnessed for use in places where flow speed is below the required 1.0 m/s. Additionally, challenges such as cavitation are reduced in the present technology initiative. The new ideology presents minimal danger to marine life, and its use is economical as the mechanical bearings for all rotating parts are above water (including the alternator). This present work is a research and development effort in sub-Saharan Africa to develop a system of extraction of energy from water, especially for the coastal/riverine communities and localities where access to the natural grid is either not available, or is economically unavailable. The vertical-axis turbine used in a recent study for low-flow velocity was a Darrius turbine in North America (Runge et al., 2018). Runge et al. (2018) state that flow speed must be at least 0.5 m/s for use of the vertical-axis turbine. There is presently no work undertaken in the development of the verticalaxis turbine in low water velocity conditions, less than 0.5 m/s, and therefore this is the basis of the present research. The present paper reviews existing technologies, and an initiative to develop a classical system that could be used for harnessing underwater current energy at low speed is discussed.

2. Existing underwater power turbine technologies Underwater current power turbines can be grouped into three main types, depending on waterflow direction relative to turbine motion axis: horizontal-axis turbine, vertical-axis turbine (or cross-flow turbine) and oscillatory hydrofoil. The vertical-axis turbine (or cross-flow turbine) uses water flow perpendicular to the turbine. The horizontal-axis turbine uses

water flow parallel to the rotation axis. Hydrofoil motion is oscillatory. In all types of underwater power turbine technologies, venturi can be ducted to the blades to improve power production. Ducted systems can increase hydrokinetic power to 40% compared to non-ducted systems (US Patent No. 7,279,803,2007; Chen et al., 2018), and can also reduce vibrations. A venturi duct can be used on both the horizontal- and vertical-axis turbines (Rahman et al., 2014). Some early patents on the technology are well discussed in Bosley (2007); Rahman et al. (2014); US Patent No. 4,219,303 (1980); US Patent No. 4,306,157 (1981); US Patent No. 4,313,059 (1982); US Patent No. 8,324,978 (1994). The patented techniques were based on concepts of wheels and nozzles, manipulation of blade geometry, slidable blades, drag force and floatable hydroelectric turbines installed in water (Rahman et al., 2014). From 2000, early projects and prototypes of ocean current turbines were developed and deployed. These are well discussed by Thake (2005), Van Zwieten et al. (2006), Douglas et al. (2008), Johnson and Pride (2010), Bumsuk et al. (2012), Imran and Badshah (2012), Van Zwieten et al. (2013), Rahman et al. (2014). Most advanced projects and prototypes use horizontal-axis turbines. Examples include the Seaflow project by Bristol-based Marine Current Turbines (MCT) off the north coast of Devon, England, targeting 5 knots (2.5 m/s) water flow. Stringray is a project similar to Seaflow targeting water flow of 2m/s, and the SeaGenis project uses power generation of 1.2 MW connected to the national grid with water speed around 10 mph (5 m/s). These example systems are either of monopile or multi-pile installation. In 2012, efforts were made to submerge a kite-form horizontal-axis turbine in

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Underwater Technology Vol. 36, No. 3, 2019

the Kuroshio current with flow speed of 1 m/s –5 m/s (Rahman et al., 2014). Rahman et al. (2014) state that at least 50% of current ocean turbines are of the horizontal-axis type. This can be seen as a result of the fact that most offshore wind turbine technology is horizontal axis, and the development in ocean current technologies transferred existing wind techniques to marine current systems. In comparing the horizontal-axis and vertical-axis technology of ocean current power turbines, Aly and El-Hawary (2011) and Chen et al. (2018) established that while the former has a wellknown technology, the latter does not. Discussion by Chen et al. (2018) of current technologies, projects and state-of-the-art work in UCPT notes the horizontal-axis turbine as more technologically and economically suitable compared to other types. Although the vertical-axis turbine is less developed compared to the horizontal type, it is beneficial in extracting marine energy independent of direction of water flow (Chen et al., 2018). Types of verticalaxis turbine have been discussed in Chen et al. (2018): Savonius (drag type) and Darrieus (lift type); Darrieus can also be divided into egg-beater type and H type. Most of these designs have already been used in the wind power industry successfully; ‘the vertical axis turbine can directly transfer the mechanical torque without the complicated transmission systems or an underwater nacelle. Furthermore, it can be much more easily applied at any speciﬁc site than the horizontal axis turbine, since vertical axis turbine has more freedom to change its height and the radius’ (Chen et al., 2018). Rourke et al. (2010) and Uihlein and Magagna (2016) identified some problems with the technologies, and Chen et al. (2018) summarised the drawbacks as the need for larger area for installation. If cavitation occurs, the whole blade is affected instead of just the tip of the horizontal-axis turbine; there are also high torque ﬂuctuations with every rotation and no self-starting capabilities. In the present work, effort has been made to review the benefits of the vertical-axis turbine for application in low-velocity water flow less than 0.3 m/s. The present study engages with engineering interests in underwater current energies in the rivers and mild ocean of sub-Saharan Africa. It is noted that while Scandinavian countries and the United Kingdom have already succeeded in benefitting from ocean currents to produce electricity for some small communities and sparsely inhabited islands, their systems are in shallower water and with faster currents. There is a need for SubSaharan Africa to develop technology suited to its own conditions.

Force (F ) and power (P) are defined as: F = ρAν 2 P = αρAν 3,

(1) (2)

where F is force; ρ = water density; A = surface area of turbine plate normal to the water flow; ν = water current flow velocity; P is power; and α = power coefficient. Ducting technology was first proposed by Rousseau (2010) for low-velocity flow conditions. The ducting process (as discussed by Ponniah and Mahmood, 2004; Chen et al., 2018; US Patent No. 7,279,803, 2007) can increase the natural velocity of flow. The present initiative is based on ducting techniques, and increases the value of the surface area of fluid contact for use in a vertical-axis turbine.

3. Description of the present vertical-axis UCPT model initiative The model vertical-axis UCPT is a passive-to-activebladed, vertical-axis turbine for extraction of electricity from flowing water bodies. It has a ducted arrangement that introduces a throat towards the active turbine blades within the system. During the 360° revolution of the vertical shaft power hub (Fig 1), reference to the current flow direction is divided into 180°of passive blade power, and 180° of active blade power. On the passive side of the turbine’s shaft, the turbine blade opens to allow flow. On the active side of the shaft, the turbine blade closes, and the result is extraction of the full water hydraulic power. The operating method of the model UCPT is shown in Fig 1. The three turbine blades are hinged at the top of the blade arms. The blades are designed to be larger in surface area than the rectangular frames of the blade arms so that they only flip on one side of the frames. The blade against the current is forced to open or close against the flow depending on whether the blade is on the active or passive side. This produces a resultant force that turns the shaft (power hub) in a clockwise direction, as shown in Fig 1. A simplified version of the model UCPT with turbine blades fixed to a rotating power hub is shown in Fig 2. The rotating power hub is connected to a gearbox that multiplies the low revolution of the power hub to produce electricity from an installed permanent magnet alternator. A floating platform houses the alternator and the gearbox installed at the topside. For improved stability, the entire platform arrangement can be further floated using floaters installed on the extension arms, as shown in Fig 3. The amount of power produced by the model UCPT is dependent on variables such as: maximum

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Agbakwuru and Ibrahim. A novel initiative on vertical-axis underwater turbine suitable for low underwater current velocities

Cover Alternator

Gear box Floating platform

Ducting plate

Power hub

Ducting channel 3 or more blades

Fig 2: The key components of model vertical-axis UCPT

Alternator with gearbox

Floating platform

The throat to active blades

Floaters Fig 3: The ﬂoated model vertical-axis UCPT model showing throat to blades (other parts are submerged)

angular opening of the turbine blades; rate of closingup of the turbine blade openings on rotation from passive to active side; mass from which the material of the blade is made; degree of ducting or throating; exposed area of the blade; and water density and water flow rate. Fig 3 shows the floating system as installed on-site at the River Ethiope in Abraka, Delta State of Nigeria; River Ethiope is the deepest river in Nigeria. Fig 4 shows the initial fabrication of the UCPT in the workshop. As the alternator is an alternating current (AC) system, production of electricity by turning the turbine blade can be achieved from any direction. Fig 4 also shows the technique for inclusion of the floaters.

4. Calculation of power output from the model UCPT The rotation of the shaft as shown in Fig 1 is dependent on the difference in torque generated at the active and passive blade arms. The model UCPT included a throat to optimize the flow velocity; a ‘V’-shaped gate was installed towards the active blade that throated at the ratio of 2:1. The passive section of the turbine is nearly completely shielded from the incoming water current, as shown in Fig 5. The following parameters are then computed: • turbine maximum surface area exposed to the current, A m2

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Underwater Technology Vol. 36, No. 3, 2019

where Vf is the linear velocity of the frame at radius; w is the angular velocity; and R is the frame at radius. R is measured from the centre of the turbine’s blade to the centre of the rotating shaft. The following continuity equation is then applied: SuVu = SdVd ,

Fig 4: The model vertical-axis UCPT at workshop demonstration

• turbine frame maximum surface area exposed to the current, Af m2 • water density, ρkg/m3 • maximum upstream ducting section, Su m2 • minimum downstream ducted section, Sdm2 • upstream water flow velocity, Vu • downstream water flow velocity through the throat of the duct (that powers the turbine), Vd The incoming current at the passive side of the turbine is disregarded as it is shielded by the model. The angular velocity of the turbine is calculated by the following equation: w = 2π ( N ),

(3)

where w is the angular velocity and N is the number of revolutions per second. The linear velocity of the frame at radius (half of the turbine’s arm length) is given by: Vf = wR ,

(4)

The throating ‘V’ shape to the power blade (a)

(5)

where Su is the maximum upstream ducting section in m2; Vu is the upstream water flow velocity; Sd is the minimum downstream ducted section in m2; and Vd is the downstream water flow velocity through the throat of the duct. Equation 5 guarantees that, provided that Su > Sd, then Vd > Vu ; therefore, equation 5 characterises the basic principle of ducting. Maximum theoretical force produced at the active blade is calculated as: Fa(max) = ρAVd2,

(6)

where Fa(max) is the maximum theoretical force; ρ is the water density in kg/m3; A is the turbine maximum surface area exposed to the current in m2; and Vd is the downstream water flow velocity through the throat of the duct. Maximum theoretical force against the returning blade’s frame (assuming the returning blade is fully shielded from the water current) is calculated as: Fp(max) = ρAfVf2,

(7)

where Fp(max) is the maximum theoretical force against the returning blade’s frame; ρ is the water density in kg/m3; Af is the turbine frame maximum

The shielding plate over the passive blade (b)

Fig 5: (a) Throating ‘V’ shape to the power blade; and (b) the shielding plate over the passive blade

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Agbakwuru and Ibrahim. A novel initiative on vertical-axis underwater turbine suitable for low underwater current velocities

surface area exposed to the current in m2; and Vf is the linear velocity of the frame at radius. Maximum effective rotating force on the shaft is calculated as: (8)

where FE(max) is the maximum effective rotating force on the shaft; ρ is the water density in kg/m3; A is the turbine maximum surface area exposed to the current in m2; Vd is the downstream water flow velocity through the throat of the duct; and Vf is the linear velocity of the frame at radius. The site at the River Ethiope has an estimated velocity (Vu) of 0.3 m/s. The investigated prototype of the UCPT has the following parameters: A = 0.9 m2 + 0.2 m2 = 1.1 m2 Af = 0.2 m2 Su = 2 m (wide) × 1 m (depth) =2 m2 Sd = 1 m (wide) × 1 m (depth) =1m2 N = 7 rev/min, R=0.55 m, w = 2π ( rad/sec, Vf = 0.73 × 0.55 = 0.40 m/s

20000

15000

10000

5000

0 0

-5000

Iterations [ ]

Fig 6: CFD torque plot for three-blade model vertical-axis UCPT

6000

)

5000

= 0.73

The downstream water flow velocity is calculated from equation 5 as follows: • 2 × 0.3 m/s = 1 × Vd • Vd = 0.6 m/s

4000 Torque (Y) [N° m]

• • • • •

25000

Torque (Y) [N° m]

FE(max) = ρA(Vd2 – Vf2),

30000

3000

2000

1000

The following calculations are then made: 0

The maximum theoretical force is calculated from equation 6 as: Fa(max) = 396 N The maximum theoretical force against the returning blade’s frame is calculated from equation 7 as: Fp(max) = 51 N The maximum effective rotating force on the shaft is calculated from equation 7 as: FE(max) = 344 N The maximum radius of the arm, r, is calculated as: r = 1.1 m The maximum theoretical shaft power, Pp(max), is calculated as: Pp(max) = 2πNFE(max)r = 285 W

-1000

100

120

Iterations [ ]

Fig 7: CFD torque plot for six-blade model vertical-axis UCPT

model vertical-axis UCPT. The software requires initial water flow velocity into the UCPT (0.3 m/s), and number of rotations of the blades around the power hub per minute (5 rev/min). The CFD obtains the mean output torque by iterating the computational process until a convergence is made. Only converging values are taken into account. The CFD software was used on a model verticalaxial UCPT with three blades, and another with six blades. The plots and derived results are shown in Figs 6 and 7, and Tables 1 and 2.

The maximum torque, FE(max)r is calculated as: FE(max)r = 378 nm

The mean torque, F E(max) ( 2r ), is calculated as: FE(max) ( 2r ) = 189 nm

4.1. SolidWorks computational ﬂuid dynamics application on the vertical-axis UCPT Efforts were made to use standard SolidWorks computational fluid dynamics (CFD) software on the

4.2. Physical measurements In absence of a torque meter to measure the shaft torque of the model UCPT, a spring balance was connected rigidly to the outermost end (1.1 m) of the rectangular frames of the turbine’s blade arms. A maximum load of 15 kg was recorded on the balance to fully retard the rotation of the blade. The obtained torque is calculated as: 15 kg × 9.81 m/s2 × 1.1 m = 161 nm.

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Table 1: Summary result for CFD conducted on three-blade model vertical-axis UCPT Goal name

Unit

Value

GG Torque (Y)1

[N*m] 306.0617472

Averaged value

Minimum value

Maximum value

Progress Use in [%] convergence

Delta

227.8272422

158.6932242

306.0617472

100

147.368523

Yes

Iterations [ ]: 78 Analysis interval: 23

Table 2: Summary result for CFD conducted on six-blade model vertical-axis UCPT Goal name

Unit

Value

Averaged value

Minimum value

GG Torque (Y)1

[N*m] 274.5567692 211.140971 163.0516547

Maximum value

Progress Use in (%) convergence

Delta

274.5567692

100

35.36506744

Yes

Iterations [ ]: 104 Analysis interval: 24

5. Results and discussion The model vertical-axis UCPT initial concept was to be built with 16 mm-thick fibre glass materials to reduce its weight. However, owing to costs and other financial constraints, the UCPT model was built using mild steel material. Aluminum anode was installed on the immersed structure to protect the paint coatings against corrosion. Table 3 compares the torque measurements and results from first principle, SolidWorks CFD software and physical measurement. The measured torque is less than the torques measured by the other two computation methods. This is expected, as the first principle and SolidWorks CFD software methods do not consider opposing mechanical frictional forces and other possibilities. The use of the measured torque as a divisor against the values obtained by the other two methods gives the power coefficient, α(discussed in section 2). It is observed that the SolidWorks CFD for the three-blade vertical-axis UCPT gave the lowest power coefficient of 0.70. Even at the minimum computed power coefficient of 0.70, the tested model UCPT is encouraging considering the low-level manufacturing method used. Comparing the SolidWorks CFD application on a three-blade vertical-axis UCPT and six-blade vertical-axis UCPT, it is noted that the former has higher torque than the latter. This is also expected.

There was an observed variation in the velocity flow of the River Ethiope even without seasonal variation. This is presumed to be tidal variations that occur at the mouth of the river to the Atlantic. This effect, including the variations of the velocity with seasons, will be investigated in another study. It is noted that ducting is an essential and relevant design aspect of the model vertical-axis UCPT. The low-flow velocity of 0.3 m/s will be unable to make any useful impact on power output without the duct design. There was a rapid change in momentum (and therefore a rapid increase in velocity) at the exhaust from the active side of the UCPT, as shown in Figs 8 and 9. This effect is documented in earlier experiments conducted by Agbakwuru and Ombor (2016). A circulatory flow of the water around the blades was also observed (Fig 10). This implied that reducing the angle of lift of the passive blades may not have serious consequence. Based on these observations and results, the present study shows that the model UCPT could be applied to in the arrangement of multiple power extraction stations on a single floating vessel. The stations in the forward section of the vessel would exhaust the flow for the stations immediately behind it, and the power arm of the turbine would be alternated between port and starboard of the floating vessel. The arrangement of port and starboard

Table 3: Comparison of actual and theoretical results Method of computation on the model three-blade vertical-axis UCPT

Obtained mean torque

Calculated power coefﬁcient, α

First principle theoretical computation SolidWork CFD application Actual measurement

189 nm 229 nm 161 nm

0.85 0.70 1.0

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Agbakwuru and Ibrahim. A novel initiative on vertical-axis underwater turbine suitable for low underwater current velocities

at full power. This will be managed in further work on this project.

0.932 0.828 0.725 0.621 0.518 0.414 0.311 0.207 0.104 0 Velocity [m/s] Flow trajectories 1

Fig 8: The CFD analytical image of the velocity ﬂow around the active blades

0.932 0.828 0.725 0.621 0.518 0.414 0.311 0.207 0.104 0 Velocity [m/s] Flow trajectories 1

Fig 9: The CFD analytical image of the velocity ﬂow exhausting from the active blade

5.1. Future opportunities to enhance the use of model vertical-axis UCPT The present work proposes the use of the model vertical-axis UCPT in sub-Saharan African rivers and ocean characterises with low-flow velocities. This region benefits from abundant solar energy, and therefore an optimal engineering of the model UCPT will involve extraction of energies from underwater and above water. The extraction of solar energy will imply the use of solar panels over the platform. The present work proposes that ten stations of the model are made, with each station delivering 1000 watts of underwater current power. In total, the UCPT will provide 10 kW. Referencing the model UCPT, the length of such floating system should be around 30 m. It is proposed that 100 watts of solar panels are used over the platform; each panel is 1 m × 1 m surface area. Referencing the model UCPT, the number of panels over the surface platform will be around 200, delivering approximately 20 kW average power by daylight. This hybridised technology of UCPT will deliver 30 kW of power per day – enough to supply energy to 750–1000 homes in Nigeria. Additionally, the model UCPT turbine blades operate at relative rotational speed of seven revolutions per minute. This low speed will rarely permit cavitation, and enables marine animals to move around and maneuver through them without harm.

6. Conclusion 0.932 0.828 0.725 0.621 0.518 0.414 0.311 0.207 0.104 0 Velocity [m/s] Flow trajectories 1

Fig 10: The CFD analytical image of the velocity ﬂow into the active blades

power blades would also contribute to good balance of the floating vessel. Fig 10 shows the in-flow of the low velocity into the ducted active blade. The presented UCPT has an installed 300-watt alternator. The available gear ratio of the gearbox for the model vertical-axis UCPT is 1:18, which is not optimal enough to enable the alternator to produce

Effort has been made to demonstrate that the verticalaxis turbine can be used for power generation from low-velocity water flow. The relevance of the application of ducting and water current shield over blades has also been demonstrated. The low-flow velocity of the Sub-Saharan rivers and ocean system, coupled with mild nature of the ocean waters, prompted the present work. The present authors have considered that the abundance of solar power in sub-Saharan Africa, combined with advanced harvesting technology could provide for hybridised solar and marine power. This could enable energy extraction from the sun and water for the benefit of sub-Saharan African people requiring power for development and industrialisation.

Acknowledgement The present authors are grateful to the Center for Maritime and Offshore Studies (CMOS) of the

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Federal University of Petroleum Resources Effurun (FUPRE), and Shell Nigeria for providing the in situ measurements used in the present study.

References Agbakwuru J and Ombor PG. (2016). Water current velocity intensification by momentum diffusion method. The International Journal of Science and Technoledge 4: 93–99. Akinsanya A, Gudmestad OT and Agbakwuru J. (2017). Swell description for Bonga offshore Nigeria location. Ocean Systems Engineering 7: 345–369. Aly HHH and El-HawaryME. (2011). State of the art for tidal currents electric energy resources. In: Proceedings of the 24th Canadian Conference on Electrical and Computer Engineering (CCECE), 8–11 May, Niagara Falls, Ontario, Canada. Asseff NS and Mahfuz H. (2009). Design and finite element analysis of an ocean current turbine blade. In: Proceedings of IEEE Oceans 2009, 26–29 October, Biloxi, Mississippi, USA. Bosley KR. (2007). US Patent No. 7,279,803. Washington, DC: United States Patent and Trademark Office. Bumsuk K, Bae SY, Kim WJ and Lee SL.(2012). A study on the design assessment of 50kW ocean current turbine using fluid structure interaction analysis. Earth and Environmental Science 15: 2037. Chen H, Tang T, Aït-Ahmed N, El Hachemi Benbouzid M, Machmoum M and El-Hadi Zaïm M. (2018). Attraction, challenge and current status of marine current energy. IEEE Access 6: 12665–12685. Douglas CA, Harrison G and Chick JP. (2008). Life cycle assessment of the Seagen marine current turbine. Journal of Engineering for the Maritime Environment 222: 1–12. Haining ML. (1994).US Patent No. 8,324,978. Washington, DC: United States Patent and Trademark Office. Imran M and Badshah S. (2012). Vibration analysis of an ocean current turbine blade. International Journal of Scientific and Engineering Research 3: 879–883. Johnson JB and Pride DJ. (2010). River, tidal, and ocean current hydrokinetic energy technologies: status and future opportunities in Alaska. Report prepared for Alaska Energy Authority. Fairbanks, Alaska: Alaska Center for Energy and Power, 28 pp. Available at: http://acep.uaf.edu/media/172246/2010_ 11_1_State_of_the_Art_Hydrokinetic_Final.pdf, <last accessed 22 September 2019>. Lloyd-Evans LPM. (2005). A study into the prospects for marine biotechnology development in the United Kingdom: Volume 1, Strategy. Report: FMP Marine Biotechnology Group – 02 – Volume 1. London: Foresight Marine Panel, 86 pp. Available at: https://www.biobridge.co.uk/ClientArea/files/

Publications/MARINE%20BIOTECHNOLOGY%20 DEVELOPMENT%20in%20UK%20M%20Lloyd-Evans% 20Jan%202005%20Vol%201.pdf, last accessed <22 September 2019>. Mouton, WJJ and Thompson DF. (1980). US Patent No. 4,219,303. Washington, DC: United States Patent and Trademark Office. Olagnon, M, Prevosto M, Van Iseghem S, Ewans K and Forristall GZ. (2004). WASP – West Africa Swell Project: Final report. Available at https://archimer.ifremer.fr/ doc/00114/22537/, last accessed <22 September 2019>. Ponniah M and Mahmood B. (2004). Feasibility study of harnessing on-shore wave energy at Waipapa, New Zealand: a case study. In: Proceedings of the International Conference on Sustainability Engineering Science, 21–23 February, Auckland, New Zealand. Rahman N, Badshah S, Rafai A and Badshah M. (2014). Literature review of ocean current turbine. International Journal of Scientific and Engineering Research 5: 177–182. Rourke FO, Boyle F and Reynolds A. (2010). Marine current energy devices: Current status and possible future applications in Ireland. Renewable and Sustainable Energy Reviews 14: 1026–1036. Rousseau N. (2010). Oceans of energy: European ocean energy roadmap 2010–2050. Brussels, Belgium: European Ocean Energy Association, 36 pp. Available at: https://www.icoeconference.com/publication/oceans_of_energy_european_ ocean_energy_roadmap_2010_2050/, last accessed <22 September 2019>. Runge S, Stoesser T, Morris E and White M. (2018). Technology readiness of a vertical-axis hydro-kinetic turbine. Journal of Power and Energy Engineering 6: 63–85. Thake J. (2005). Development, installation and testing of a large-scale tidal current turbine. Report T/06/00210/00/ REP, URN 05/1698.London: The Department of Trade and Industry, 60 pp. Available at: https://webarchive. nationalarchives.gov.uk/+/http:/www.berr.gov.uk/ files/file18130.pdf, <last accessed 22 September 2019>. Uihlein A and Magagna D. (2016). Wave and tidal current energy – A review of the current state of research beyond technology. Renewable and Sustainable Energy Reviews 58: 1070–1081. Van Zwieten J, Driscoll FR, Leonessa A and Deane G. (2006). Design of a prototype ocean current turbine – Part I: Mathematical modeling and dynamics simulation. Ocean Engineering 33: 1485–1521. Van Zwieten JH, Vanrietvelde N and Hacker BL. (2013). Numerical simulation of an experimental ocean current turbine. IEEE Journal of Oceanic Engineering 38: 131–143. Wracsaricht LJ. (1981). US Patent No. 4,306,157. Washington, DC: United States Patent and Trademark Office.

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CALL FOR PAPERS Underwater Technology: InternaƟonal Journal of the Society for Underwater Technology The Society for Underwater Technology is calling for papers for its internaƟonal journal, Underwater Technology. The journal publishes peer-reviewed technical papers on all aspects and applicaƟons of underwater technology, including: • • • • • • • • • • • • •

diving technology and physiology environmental forces geology/geotechnics marine polluƟon marine renewable energies marine resources oceanography subsea systems underwater acousƟcs underwater roboƟcs underwater science underwater vehicle technologies salvage and decommissioning

Original papers on new technology, its development and applicaƟons, and papers covering new applicaƟons for exisƟng technology, are parƟcularly welcome. Submissions should adhere to the journal’s guidelines available at www.sut.org/publicaƟons/underwater-technology/guidelines-for-authors/ For more informaƟon or to make a submission, please contact the Assistant Editor, Elaine Azzopardi, at Elaine.Azzopardi@sut.org

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doi:10.3723/ut.36.053 Underwater Technology, Vol. 36, No. 3, pp. 53–61, 2019

Technical Paper

www.sut.org

Signal transmission through seawater for MHz frequencies and medium distances (0–30 m) using ionic current waves Emeritus Prof J Lucas Department of Electrical Engineering and Electronics, The University of Liverpool, Liverpool, L69 3GJ, United Kingdom Received 16 April 2019; Accepted 19 August 2019

Abstract Electromagnetic (EM) signals can only be transmitted through seawater for short distances (<1 m) for frequencies (>1 MHz). Therefore a new technique, the ionic current wave (ICW), has been developed for signal propagation at MHz frequency. This technique uses the conduction current produced in seawater as a result of thermal ionisation releasing H+ and OH – ions. A small voltage (<1.5 V pk) is applied between two metal electrodes submerged in the seawater to avoid ionisation by the input electrical energy. A detailed theoretical analysis of the ICW process has shown that ionic currents can be transmitted at MHz frequency over distances of 10 m with low signal loss per decade. For longer propagation distances of 100 m the theory predicts a signal loss of –20 dB per decade. Propagation experiments have been carried out in Liverpool dock seawater for distances of 2 m–28 m between parallel 0.5 m × 0.3 m electrodes placed vertically in the seawater at a depth of 2 m. Signal frequencies within the range of 1 MHz– 8 MHz have been investigated. In each experiment the received propagated signal power was approximately –67 dBm (well above the dock electrical noise of –140 dBm) and only showed a small power loss over the full range of propagation. The ICW system will be able to measure longer propagation distances in deep seawater conditions suitable for ship and submarine communications. Its performance is comparable to that of sonar systems. Keywords: seawater communications, ionic current, MHz frequency, long range propagation, duplex transmission

1. Introduction It has been shown that it is difficult to propagate electromagnetic (EM) waves through seawater at MHz frequencies (Lucas and Yip, 2007; Yip et al., 2008). Research by Dunbar (1994) has demonstrated that * Contact author. Email address: j.lucas@liv.ac.uk

the attenuation of EM signal power (P ) at a given distance (d) and frequency (f ) is related to the skin depth (Δ), and is given by: P = Po e (–2d/Δ),

(1)

where Po is the initial emitted power and Δ = 250// f . The maximum propagation distance is obtained when the received signal power is equal to the seawater noise level of –170 dB. Analysis of the equation for P0 = 1 W shows that a propagation distance above 100 m is possible, but only for a frequency of 2.4 kHz. When the frequency is 1 MHz, the propagation distance is 4.89 m. Research by Yip (2007) and Goudevenos (2008), within the present author’s University of Liverpool laboratory, endeavoured to increase the propagation distance by investigating new forms of antenna design, such as using parallel plate electrodes enclosed by distilled water, but the results were not successful. Therefore the EM waves research was suspended in 2016. During 2017–2019 the present author has theoretically and experimentally researched a new technique which uses thermal ionic current waves (ICWs). The theory shows that these waves can be propagated at MHz frequency over distances above 100 m, with only a small signal power loss. A further advantage is that the nominal transmitted power for a distance above 100 m is ~ –95 dB, which is well above the seawater noise level of –170 dB. The ICW system extends seawater communications capabilities presently provided by sonar systems only. Although the ICW system is an electrical device it can propagate similar distances to those by sonar, but with lower battery power and attenuation per metre. It can also operate at higher frequencies 53

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Lucas. Signal transmission through seawater for MHz frequencies and medium distances (0–30 m) using ionic current waves

Table 1: Comparison of sonar and ICW systems Mode Freqency Attenuation Intermediate Power (MHz) (dB/m) range (m) loss (dB) Sonar <1.5 ICW >1

0.7 0.03

100 100

70 3

(above 10 MHz), as shown in Table 1. The ICW system will allow underwater communications to move towards the advantages of terrestrial communications by having higher bandwidths and good integration of communications, navigation and sensing functionality, all within a reduced power budget. Defence and security offshore activities require two-way (duplex) transmissions which can be provided by the ICW system. Therefore, the ICW system is useful for such applications as ship to submarine communications, underwater vehicle/robot technology, harbour security systems, online subsea camera imaging and navigation in fog. The present paper describes the theory and results of several propagation dock trials at frequencies between 1 MHz–8 MHz, measured over an horizontal propagation distance of 2 m–28 m within dock seawater of 6 m depth, 30 m width and 100 m length.

2. The far ﬁeld transmission characteristics 2.1. Non-linear seawater conductivity characteristic The seawater current (I ) conducted between two parallel plates has an unexpected nonlinear characteristic, which depends on the voltage between the electrodes. The Voltage(V) -Current(I) characteristic is shown in Fig 1 (Kumari et al., 2016). The diode-like relationship is given by: V = VD + I R ,

(2)

where R is the resistance and VD is the transition voltage. Current Density, J (mA cm-2)

DI water Seawater Seawater after 60 hours

For voltages less than VD (~1.5 V) the conduction current (I ) is small and the seawater electrical conductivity is comparable to that for distilled water (σ = 4 × 10–5 S/m). For V > VD, the current rapidly rises and the seawater conductivity approaches the normal value obtained for seawater (σ = 4 S/m). This non-linear effect occurs as a result of low voltages V < VD where the water is ionised by thermal energy into hydrogen and hydroxide ions. The equilibrium reaction is: H2O + thermal energy = H + + OH –

(3)

The electrical conductivity indicates the number of ion pairs existing in water per m3. The formula for the conductivity of water (σ) for the ion pair concentration per m3 (n), the ion pair mobility (u) and the electronic charge (e) is given by: σ=neu n = σ/(e u)

(4) (5)

The mobility for H+ and OH– is 36.3 10–8 and 20.5 10–8 (units of m2/(s V)), respectively. Hence, triple distilled water at 18°C gives σ = 4.3 10–6 S/m (see Table 2), so that n = 4.3 10–6/(1.6 10–19 56.8 10–8) = 4.73 1019, and therefore n = 4.73 1019 ion pairs per m3. The number of water molecules (N) is given by Avogadro’s number in units of kg mole, hence N = 6.023 1026 for 18 kg. Since 1 kg of water occupies 1 litre (equal to 10–3 m3), then: N = 6,023 1026 (1000/18) N = 3.346 1028 per m3. The degree of ionisation is n/N = 4.73 1019/3.346 1028 = 1.42 10–9. The electrical conductivity of triple distilled water as a function of temperature is given in Table 2 and indicates that an increase in thermal ionisation occurs with increasing temperatures. The surface temperatures for Liverpool dock and Fort William are given in Table 3. When V > VD electrolysis occurs in water; the water decomposes to give hydrogen and oxygen gas as a result of the input of electrical energy: 2H2O + electrical energy = O2 + 2H2

(6)

Table 2: Electrical conductivity of triple distilled water 0 0.0

Temperature (°C) 0.5

1.0

1.5

2.0

Voltage, V (V) Fig 1: The relationship between current density and voltage in seawater and distilled water (DI)

0 18 25 34 50

Electrical conductivity × 10–6 (S/m) 1.5 4.3 6.2 9.5 18.7

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Table 3: The sea temperatures at the testing sites

CATHODE

ANODE

Month

Liverpool sea temperature ( C)

January March May July September November December

7.6 7.4 10.9 15.6 16.1 12.4 9.6

OPEN WATER Equal concentration + – of H and OH ions

(a)

No voltage

OPEN WATER Equal concentration of H+ and OH– ions

Table 4: Electrical conductivity of water at 18 °C Type of water

Electrical conductivity (S/m)

Deionised or triple distilled Single distilled Tap Seawater

5.6 10–6 4 10–5 (5–50) 10–3 4–5

The anode is 2H2O = O2 + 4H+ + 4e–, and V = 1.229. The cathode is 4H+ + 4e– = 2, and V = 0. The minimum necessary voltage to initiate water electrolysis is 1.229 V, but the amount necessary to initiate water electrolysis without drawing heat energy from the surroundings is 1.481 V. For V > 2.7 ionisation of NaCl molecules commences with Na+ and Cl– ions being produced. This increase in ion concentration produces a further increase in the electrical conductivity of the seawater. When electrolysis occurs for higher voltages, this results in the various types of water as summarised in Table 4 at 18 °C and identified by their electrical conductivity value.

2.2. Electrical resistance of thermally ionised water It can be assumed that seawater between the electrodes is divided up into a number of imaginary parallel cells equidistant apart, as shown in Fig 2. The ions of the ionised water, H+ and OH–, lie in the cells. When a voltage is applied between the electrodes the positive ions move towards the cathode and the negative ions move towards the anode, where they are then discharged to generate the electric current. A change in ion concentration will take place in the vicinity of the cathode and anode (referred to as the cathode and anode compartments). However, the ion concentration in the body of the water, represented in Fig 2 by the central box, remains unchanged. At the start of the sequence (Fig 2a), the number of positive and negative ions in the compartments will be the same with, e.g. eight cells of ions. It can now be assumed that the sinewave voltage is applied and that the anion moves faster than the cation. From section 2.1, vA/vB = 1.77; however, for representation in Fig 2b, vA/vB = 2 has been assumed. When the voltage reaches its peak it will be assumed that the positive ions will have moved four spaces while the negative ions will have moved two spaces,

V=0

(b)

Peak

Positive

V = V pk

Voltage

V pk R

V A (c)

Voltage distribution

Fig 2: Ionic movement between the electrodes for an applied voltage; the curve in Fig 2c shows the electric ﬁeld strength

as shown in Fig 2b. At this positive voltage (V ) four positive ions and two negative ions will have been discharged at the electrodes. Within the cathode compartment there will be four positive ions, and within the anode compartment two negative ions. When starting from zero voltage and increasing this value, the ions will move in opposite directions with velocities vA and vB. The compartments shown in Fig 2b will have different sizes, xA and xB , and different voltage drops, VA and VB, as illustrated in Fig 2c. The ions in the cathode compartment are all H+ ions and those in the anode compartment are all OH– ions, as shown in Fig 2b. The value for x is: x = e E T 2/8M, where M is the atomic weight. Thus: xA2 = e VA T 2/8MA xB2 = e VB T 2/8MB , so that: xA/xB = square root (VA MB/VB MA)

(7)

The velocity (vA) is given by: vA = e EA T/4MA vA xA = e VA T/4MA 55

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Lucas. Signal transmission through seawater for MHz frequencies and medium distances (0–30 m) using ionic current waves

Thus:

Anode electrode

vB xB = e VB T/4MB ,

Cathode electrode

and therefore:

E S y

(vA/vB) × (xA/xB) = (VA MB/VB MA)

Substituting from equation 6 gives: (vA/vB) = square root (VA MB/VB MA)

V0 b Image plate

(8)

Since the velocity ratio (vA/vB) = 2 and the mass (M) is equal to the atomic weight with MB/MA = OH –/H + = 17/1, equation 8 gives: VA/VB = 4/17 = 0.235

-Q

(9)

From equation 7 the ratio xA/xB = square root (17 VA/VB) is given, and therefore:

Fig 3: The electrode arrangement of the cathode and anode for a gap separation of x

to be a circular plate with a radius b and the cathode electrode is assumed to be a point source. The distance between the electrodes is x. The electric field (E ) at the plate is given by: E = 2 Q cos(φ)/(4πε s2) = Q/4πε x 2x/(x2 + y2)3/2

xA/xB = square root (4/17 × 17) = 2

The current (I ) received by the plate is: Hence, the cathode compartment is twice as large as the anode compartment (as shown in Figs 2b and 2c). The inter electrode voltage across the body of water will have a value of Vr so that: V = VA + VB + Vr

(10)

Since VA and VB are generated by isolated ions, they will produce substantial values; the body of water, which contains neutralised ions, produces a much smaller voltage drop. As the current passes through all three compartments, the resistance (R) is given by: R = (VA + VB + Vr)/I R = RA + RB + Rr

(11) (12)

From equation 14 (section 2.3), Rr = d/(0.3 σ). The transmitted power (P) recorded by the Anritsu spectrum analyser with 50 Ω input resistance is P(0) = 50 V 2/(RA + RB)2 and P(d) = 50 V 2/ (RA + RB + R r) 2, with: P(d)/P(0) = (RA + RB)2/(RA + RB + Rr)2 (13) The assumption that vA/vB = 2 rather than vA/vB = 1.77 will not change the value of V = VA + VB , which retains the same magnitude.

2.3. Electrical resistance (Rr) between electrodes in open seawater For equal-sized cathode and anode plate electrodes the current (I ) can be calculated by using the method illustrated in Fig 3. The anode electrode is assumed

∫

σ E 2πr dr b

) x π ∫ xr / 3 x 2 + y 2 dr

Q /(

⎛ Vo x 4 x 2 ⎜⎜ x1 − x1 ⎝

(

x x

)⎞⎟⎟⎟⎠

If x >> b, then: I = σ Vo 2π b 2/x The inter plate resistance is: Rr = V/I = x/(2πσb2) = x/(2σA)

(14)

If the circular plate has the same area (A) as the rectangular plate used in the experimental system (0.15 m2), then Rr = x/(0.3 σ). When the open-water resistance (Rr) dominates the compartment resistances RA and RB, then the transmitted power measured by the Anritsu spectrum analyser is P = 50 I 2, and P = 50 V 2/Rr2. When the resistance (Rr) between the electrodes dominates, then for a fixed output voltage the output power is proportional to 1/x 2. This means that the power reduction is –20 dB per decade. This relationship between the power reduction and the propagation distance allows substantial far-field propagation distances to be attained, as expressed by: 10 log P(d) = 10 log P(0) – 20 log (d),

(15)

where d is the propagation distance.

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2.4. Simplex and duplex operation All the electronics used were battery operated: the signal generator had +5 V and –5 V Li-ion rechargeable batteries, and the B~K oscillator and the Anritsu had +12 V lead acid rechargeable batteries. For the simplex system, the transmitted path is via the electrical resistance (R) between the electrodes, and the return path is via the earths at the

Transmitter

Receiver

50Ω

Dock water

50Ω

Anritsu spectrum analyser

+Q, -Q, +Q, etc

decaying charge sequence in water -Q, +Q, -Q, +Q, etc +Q, -Q, +Q, -Q, etc

Local charge neutralisation occurs near the earth wire

(a) SIMPLEX 1

SIMPLEX 2

ANRITSU SPECTRUM ANALYSER

I1 f1

WATER TIGHT RECTANGULAR PLASTIC BOX IP68

pickup

BK PRECISION I2 OSCILLATOR f2 MHz 3.5V pk-pk, 50Ω f2, V2, I2

ZONE 2 RECEIVER PLATE 1

f1 MHz

and jointly TRANSMITTER PLATE 2

f2 MHz d

TRANSMITTER PLATE 1 f1 MHz and jointly RECEIVER PLATE 2

ZONE 1

f2 MHz

V1 , I1

I2 Pickup f1

ANRITSU SPECTRUM SIGNAL GENERATOR f1 MHz 5V pk-pk, 50Ω

LEAK PROOF CYLINDICAL PLASTIC BOX

ANALYSER

electrodes, as shown in Fig 4a. The forward current loop is from A to B, and the return loop is from C to D in the seawater. This process is aided by the oscillator charge (Q ) that constantly alternates from +Q to –Q at the oscillator frequency ( f ). The local charge in the water at the earths also follows an alternating sequence of +Q , –Q , +Q , –Q.... and hence creates local charge neutralisation at both the transmitter and receiver earths (see Fig 4a). A confirmatory dock test using a short-circuit wire from C to D gave the same result. The system was also tested in the laboratory by using a 135 kΩ resistance to represent the resistance (R) between the electrodes, placing the earth connections at C and D into small jars containing seawater and separated by 2 m. The same result was obtained as with the short-circuit wire. The duplex system is illustrated in Figs 4a and 4b. Each oscillator has a transmitter and receiver with seawater providing the closed loop. This is therefore only a simplex transmission system. To generate a duplex transmission system, an identical setup in reverse is required. This also requires another oscillator (B~K) and Anritsu spectrum analyser. The duplex system can, however, jointly use the two electrodes. To avoid signal cross-talk the frequency of the oscillator ( f1) was fixed, while the frequency of the B~K ( f2) was changeable so that each signal could be separately measured by the Anritsu spectrum analyser. In the present paper only measurements for simplex communications were recorded in the Liverpool Dock (sections 3 and 4), but in the laboratory a demonstration of duplex communications has been made.

2.5. Temperature with depth In a simple temperature–depth ocean water profile for low-to-middle latitudes, the temperature decreases with increasing depth. The vertical propagation power of the ICW system from Zone 1 to Zone 2 for a fixed separation distance (d) decreases as the system is lowered into the water as a result of the temperature decrease. At a depth of 4500 m the temperature is 4 °C, and the electrical resistance decreases from the sea surface resistance value at a temperature of 18 °C. If Rr > (RA + RB), then from Table 2: Rr (4°C)/Rr (18°C) = σ18/σ4 = 4.5/2.1 = 2.14

DUPLEX

(b) Fig 4: (a) The simplex propagation system using dual earths; and (b) the experimental two-zone system operating at different frequencies (f1 and f2 MHz) to produce the duplex system

Therefore, the received power has only a small additional loss when compared with the same initial power transmitted horizontally near the sea surface over a similar distance (d). The additional loss is equal to: (1/2.14)2 = 0.218 = –6.62 dB for a change of temperature: 18 – 4 = 14 °C.

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Lucas. Signal transmission through seawater for MHz frequencies and medium distances (0–30 m) using ionic current waves

3. The experimental system The experimental system is illustrated in Figs 4a and 4b. The system is able to transmit from Zone 1 to Zone 2 with a frequency of f1, and simultaneously from Zone 2 to Zone 1 with a frequency of f2. Each zone has a metal planar electrode that can operate in both transmit and receive modes simultaneouly. The transmitter and receiver electrodes are stainless steel metal plates 500 cmm × 300 cmm × 1 mm each supported by a polyethylene board 1000 mm × 500 mm × 17 mm. The electrode operating in Zone 1 is referred to as the transmitter 1 plate and the receiver 2 plate, while the electrode operating in Zone 2 is referred to as the transmitter 2 plate and the receiver 1 plate. Each zone has a low power signal generator capable of operating at frequencies of 1 MHz–10 MHz. The rf output voltages are between 3.5 V–5 V peak to peak. The electronic systems are kept in water-tight enclosures, and the earth is connected to the seawater. Zone 2 contains the receiver plate 1 and the water-tight plastic box (IP68) with an eight-way Subcon connector to the Anritsu spectrum analyser. An eight-way Subcon connector is used to connect the IP68 box electronics to the receiver plate 2. The IP68 box is leak-free for long-term placement in seawater at depths of up to 2 m. Zone 1 has the transmitter plate 1 connected by an eight-way Subcon connector to the polycarbonate Prevco cylindrical container, which is capable of operating in very deep water. This contains the 1 MHz–10 MHz crystal oscillator, EL2045 operational amplifier and two sets of four rechargeable AA batteries. The maximum current that can be generated using the EL2045 operational amplifier is 75 mA. The crystal oscillator generates the f1 frequency. The Zone 2 system is shown in Fig 5. The plate is supported vertically in the seawater by buoys with a plate depth greater than 2 m. Fig 6 shows the Zone 2 system floating in the dock water. The zone systems are manoeuvred in the dock water using flexible drainage rods of 9 m length that can place the zones parallel and up to 6 m perpendicular from the dock pontoon. The Zone 1 system is shown in Fig 7 with the transmitter plate 1. It is also used as the receiver plate 2 for a frequency f2 return signal; the return current can be measured by an Anritsu spectrum analyser when fitted. The Zone 1 receiver plate 2 is energised by a BK precision oscillator capable of transmitting up to 10 MHz (in steps of 1 kHz) frequency. It is housed in the IP68 plastic box of Zone 2. The two signal generators are activated at the same time. A small frequency difference exists between f1 and f2 so that they can be clearly identified by the

Fig 5: The Zone 2 system

Fig 6: The Zone 2 system operating in seawater and supported by buoys

Anritsu spectrum analyser. The following frequencies have been used for f1: 1 MHz, 2 MHz, 4 MHz, 4.9 MHz and 8 MHz, and the corresponding values used for f2 were: 1.3 MHz, 1.7 MHz, 4.3 MHz, 5.1 MHz

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Underwater Technology Vol. 36, No. 3, 2019

4. Experimental far ﬁeld transmission results 4.1. Results Measurements have been carried out in Liverpool dock. Transmitted power results have been measured for transmission from Zone 1 to Zone 2 at a frequency of f1 MHz for distances of 2 m–28 m. At receiver plate 1 the received f1 signal was measured, as well as the transmitted signal for frequency f2 MHz from Zone 2 to Zone 1. Five sets of results have been recorded for the frequencies listed in Table 5. The results of power (P) versus propagation distance (d) for three frequencies: 2 MHz, 4 MHz and 8 MHz, are given in Figs 8, 9 and 10, and show that P has an initial value of –67 dBm, which is nearly constant for the propagation distance of 2 m–28 m. -50 -55

1.7MHz (f2) -2dBm

-60

Noise: -82dBm

P(dBm) -65

Outward Return

-70 -75

Fig 7: The Zone 1 system

-80 2

and 8.3 MHz. This difference in frequency allows the f1 and f2 signals to be clearly identified without interference from cross-talk. The current (I ) flowing between the plates is measured by an Anritsu spectrum analyser with an input impedance of 50 Ω. The transmitted power (P) signal is expressed in dB units and is given by: P = I 50 2

(16)

Propoga on distance (m)

Fig 8: The propagated signal power (P(d)) from Zone 1 to Zone 2 as a function of zone separation distance (d) for a frequency of 2 MHz (f1)

-50

4.3MHz(f2)

-55

-6dBm Noise: -82dBm

-60 P(dBm) -65

The current (I ) is expressed in amps units and is given by:

Outward Return

-70 -75

P / 50

(17)

-80 2

The seawater resistance (R) between Zone 1 and Zone 2 is expressed in ohm units and is given by: R = V/I

(18)

Propagation Distance (m)

Fig 9: The propagated signal power (P(d)) from Zone 1 to Zone 2 as a function of zone separation distance (d) for a frequency of 4 MHz (f1)

Table 5: Results summary Propagation distance (m); starting distance is 2 m 20 28 20 20 26

Zone 1 frequency (f1 MHz); ampliﬁer output voltage 5V pk-pk 1 2 4 4.92 8

Zone 2 frequency (f2 MHz); Ampliﬁer output voltage 3.5V pk-pk 1.3 1.7 4.3 5.1 8.3

Power (dBm) received by Zone 2 at f1 MHz with propagation distance increasing/decreasing –67/–67 –67/–64 –67/–64 –68 –64/–64

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Lucas. Signal transmission through seawater for MHz frequencies and medium distances (0–30 m) using ionic current waves

-50

-55

8.3MHz(f2) -1dBm

-60

Noise: -82dBm

P(dBm) -65

Outward Return

-70

-75

From equation 14 (section 2.3), the water conductivity is given by σ = d/(0.3 Rr) = 30/(0.3 × 1.525 105) = 6.54 10–4 S/m. From Table 3, σ is in the range between single distilled water (of 1.4 10–4 S/m) and tap water (with a value varying from 50 10–4 to 500 10–4 S/m). This result agrees with the value expected for σ in seawater when the amplifier output voltage is less than 1.5 V as illustrated in Fig 1.

-80 2

Propoga on distance (m)

5. Conclusion, summary and exploitation Fig 10: The propagated signal power (P(d)) from Zone1 to Zone 2 as a function of zone separation distance (d) for a frequency of 8 MHz (f1)

A detailed explanation of the propagation technique has been given in section 2.2 of the present paper. As shown in Fig 2, the propagation occurs between a cathode compartment and an anode compartment. The cathode compartment extends a distance (xA) from the electrode; it has a resistance (RA) and a voltage drop of VA. Likewise, the anode compartment extends a distance (xB) from the electrode, and has a resistance (RB) and a voltage drop of VB. For a propagation distance (d) the seawater has a resistance (Rr) and a voltage (Vr), and the voltage between the electrodes (V) is given by: V = VA + VB + Vr VA + VB >> Vr The propagation power (P(d)) is given by equation 13 (section 2.2) as P(d) = 50 Vo2/(RA + RB + Rr)2, with Vo = 2.5 V pk. The current (I) is given by equation 17, so that at d = 2 m and P(2) = –67 dBm:

5.1. Conclusion The dock results shows that an rf signal can be transmitted through seawater by using thermallygenerated ionic currents. The theory is outlined in sections 2.2 and 2.3 of the present paper. There exists an electrical resistance (R) between the transmit Zone 1 and the received Zone 2, which have a voltage (V) operating at a frequency (f1) MHz. The current is given by I = V/R. The experimental system also allows a current to be propagated in the opposite direction from Zone 2 to Zone 1 with the same magnitude but at a different frequency (f2). The current is measured using an Anritsu spectrum analyser that is able to separate the signals f1 and f2 when they are transmitted at the same time. The applied voltage needs to be less than 2.5 V pk so that only thermally-produced ions (H+ and OH–) are present in the water. No dissociation of seawater occurs by keeping the voltage low, and thus keeping the dissociation current negligible. The experimental conditions obtained from the Liverpool dock tests are summarised in Table 5. The deduced results are:

I 2 = P(2)/50 = 10–9.7/50 = 4.00 10–12, so that I = 2.00 10 amps. The Zone 1 to Zone 2 resistance is given by equation 18, so that R = V/I = 2.5/2 106 = 1.25 106 Ω. From equation 11, for d = 2 m, R = RA + RB + R r. Hence, RA + RB = 1.25 106 Ω, since for short propagation distances the dock water inter-zone resistance is Rr << RA + RB. From Fig 8, for f = 2 MHz, it is estimated that there is only a 1 dBm drop in power at d = 30m. Therefore, from equation 13: -6

P(30)/P(2) = (RA + RB)2/(RA + RB + Rr)2 = 10–0.1 = 0.794, with 1 + Rr/(RA + RB) = 1.122 and Rr = 0.122 × 1.25 106 Ω = 1.525 105 Ω. Hence, R = (1.250 + 0.153) 106 = 1.403 106 Ω, and I = 2.5/(1.403 106) = 1.78 10–6 amps.

V = 5V pk-pk I = 2.00 10–6 amps (d = 2) R = RA + RB = 1.25 106 Ω (d = 2) I = 1.87 106 amps (d = 30 m) R = RA + RB + Rr R r/d = 5.08 103 Ω/m R r = 1.525 105 Ω (d = 30m) Therefore, R = (1.25 + 0.153) 106 = 1.403 106 Ω (d = 30 m). Hence, σ = 6.54 10–4 S/m for d = 2 m–30 m.

5.2. Summary The offshore industry requires two-way transmissions over long distances. An estimate of the transmitted power has been made as follows: As d increases P(d) will decrease as given in equation 13: P(d )/P(0) = (RA + RB)2/(RA + RB + Rr)2

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Underwater Technology Vol. 36, No. 3, 2019

Table 6: Propagation power attenuation with distance Distance (m)

Power (dBm)

I (10–6 A)

Vr volts

(VA + VB) volts

Rr ohms

RA + RB ohms

0 10 100 1000* Dock noise

–67 –67 –70 –81 –140

2.0 2.0 1.41 0.40

0 0.102 0.719 2.03

2.50 2.40 1.78 0.47

0 5.1 104 5.1 105 5.1 106

1.25 106 1.20 106 1.26 106 1.18 106

*estimated

Therefore, when d = 100 m: P(0)/P(100) = (1 + 5.08 103 × 100/1.25 106)2 = 1.4022 = 1.966 = 2.93 dB, and thus the power (P(100)) decreases by –2.93 dB from the power at d = 0. Therefore when d = 1000 m: P(0)/P(1000) = (1 + 5.08 ×103 ×103/1.25 106)2 = 5.022 =25.2 = 14.0 dB, and thus the power (P(1000)) decreases by –14.0 dB from the power emitted at d = 0. A summary of the measured and estimated transmitted powers is given in Table 6, showing that when d = 1000 m, Vr increases to 2.03 V and Rr = 5.1 106 Ω. The value of RA + RB decreases to 1.18 106 Ω. When Vr is 2.5 V, then the power attenuation with increasing distance is –20 dB per decade.

5.3. Exploitation The ICW system is a relatively low-level technology system with low cost and short construction time. If successful for long-distance and deepwater trials, then the ICW system can be considered to be compatible with the sonar systems, but able to operate at much higher frequencies. Customer uptake is expected once the ICW performance is proven. To become commercially viable the ICW system requires a line-of-sight setup of the electrodes with rotary movement of the planar electrodes to optimise the output signal. This action may be automatically

achieved by using fixed spherical shell electrodes, with the electronics placed internally. The ICW system can be set up as a duplex communications system by fitting a signal generator in both electrodes having different but close frequencies. Large bit rates can be achieved by operating above 10 MHz frequencies.

Acknowledgement The present author wishes to acknowledge the assistance of Mr B Lucas with tests carried out in Liverpool dock.

References Dunbar RM. (1994). A surface contour electromagnetic wave antenna for short range sub-sea communications. In: Proceedings of Sixth International Conference on Electronic Engineering in Oceanography, 19–21 July, Cambridge, UK. Goudevenos A. (2008). Through water electromagntic communications. PhD thesis. The University of Liverpool, Liverpool, UK. Kumari S, White RT, Kumar B and Spurgeon JM. (2016). Solar hydrogen production from seawater vapour electrolysis. Energy & Environmental Science 9: 1725–1733. Lucas J and Yip CK. (2007). A determination of the propagation of electromagnetic waves through seawater. Underwater Technology 27: 1–9. Yip CK. (2007). Underwater communications using electromagnetic waves. PhD thesis. The University of Liverpool, Liverpool, UK. Yip CK, Goudevenos A and Lucas J. (2008). Antenna design for the propagation of EM waves in seawater. Underwater Technology 28: 11–20.

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Society for Underwater Technology International multidisciplinary learned society This non-aligned membership-based organisation seeks to further the dissemination of knowledge and lessons learned in the underwater environment through networking, events and publications

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12/6/2019 5:23:56 PM

Introducing Sea-Level Change Alastair Dawson Published by Dunedin Academic Press

Paperback edition, 2018 ISBN 978-1-780-46087-1 101 pages Sea-level change is a topic of great interest that increasingly features in university geography courses and even in the A-level syllabus at UK schools. Various physical geography and coastal texts on the market today contain some background on sea-level change, sometimes as a separate chapter (e.g. Masselink and Gehrels, 2014), but entire textbooks on sea-level changes are few and far between. The most useful books that have been published recently are arguably Murray-Wallace and Woodroffe (2014) on Quaternary sea levels, and Pugh and Woodworth (2014) on more recent sea levels. Now Dunedin Publishers have entered the market with a colourful book that is competitively priced (£14.99) and more introductory than the aforementioned books. It is part of a series of ‘Introducing …’ topics, all related to the earth and ocean sciences. The author is an expert in coastal geomorphology and sedimentology who has worked on Quaternary sea-level changes since the early 1980s, mainly in Scotland. The book is quite short in length (84 pages plus a glossary), but is beautifully illustrated. Many interesting topics

appear in the book, and at first glance one might be tempted to put it on the reading list for a coastal course at undergraduate level. Unfortunately, closer scrutiny reveals some shortcomings. First of all, the book lacks a clear structure, making it challenging for lecturers to tailor it to their course content. It jumps from topic to topic, from concepts to field evidence, and from examples of sea-level changes in the past to some (not so current) debates. It also jumps between timescales, from contemporary processes to long-term millennial processes, creating confusion for the novice who often struggles with the concept of timescales. The level of the book is variable, and not all material is easy to follow for a non-specialist. It is therefore not quite clear who the intended target audience is. I was expecting the book to have been written for first year geography undergraduates, but not all material covered is of equal importance or of interest to this group. Scottish examples feature frequently – not surprisingly, given the provenance of the author – so perhaps the book is intended for the UK market in the first instance. Some errors occur in the book. For example, the section on using foraminifera as sea-level indicators in salt marshes states that they are only used in limestone areas. The accompanying illustration shows foraminifera from very deep water. (Agglutinated foraminifera used in saltmarsh sea-level reconstructions have siliceous tests – they are not calcareous – and bedrock type doesn’t matter.) This type of error makes one wonder if the book was adequately reviewed.

www.sut.org

Book Review

doi:10.3723/ut.36.063 Underwater Technology, Vol. 36, No. 3, pp. 63–64, 2019

The book lacks a true global perspective and is written from a mainly Northern Hemisphere, and often Scottish, viewpoint, sometimes leading to omissions or errors. For example, the Younger Dryas, and not the Antarctic Cold Reversal, is discussed is relation to ice build-up in Antarctica during the late glacial period. The name of the sea-level scientist Anny Cazenave is consistently misspelled. The definition of ‘steric’ (i.e. density-related) sea-level change is mistakenly said to include sea-level changes generated by changes in ocean circulation and air pressure. Other definitions presented in the book were popularised in the 1970s by Nils-Axel Mörner but have since been abandoned (e.g. tectono-eustasy, geoidal eustasy), giving the book a distinct ‘oldschool’ flavour.1 Dawson credits Nils-Axel Mörner – in sea-level science considered to be a highly controversial individual – with making us aware that ‘there is no such thing as a global sea-level curve’, only to show on the next page a global sea-level curve by the same Nils-Axel Mörner. Given Mörner’s well-publicised crusades against the Intergovernmental Panel on Climate Change (IPCC) it comes as no surprise that, towards the end of the book, the IPCC is placed in a very misleading light. The author appears to suggest that the IPCC keeps changing its mind about rates of sea-level rise and melt contributions by ice sheets. In reality, the ice sheets are changing, not the IPCC, which only reports on the science.

1 For proper terminology, students of sealevel change should read the excellent paper by Jonathan Gregory and co-authors (Gregory et al., 2019).

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Alastair Dawson. Introducing Sea-Level Change

From time to time throughout the book, the author gets grumpy with the main-stream media (e.g. ‘One despairs what is written in the media’), and oddly blames the media for popularising the view of a uniform global sea-level rise. Although we can blame some portions of the media for its dubious reporting of climate change issues, I am not aware that debates on regional sea-level variability have featured in the press. Dawson also states that some misunderstandings about sea-level changes have entered the academic debate, which is undeniably true, but the author doesn’t help himself by ignoring current 21st century sea-level topics, such as new studies on interglacial and Pliocene sea levels and the recently proposed marine ice-cliff instability hypothesis. Current research appears to be substituted by discussions that were

mainstream some decades ago. The lack of ‘state-of-the-art’ material shines through in the list of recommended reading that includes books from 1967, 1983 and 1987. Perhaps most disturbingly, one of the recommended web links leads to a climate change denial website (www. wattsupwiththat.com). Would I recommend this book? Probably not, despite the attractive price tag. I’d wait for an adequately reviewed and well edited second edition in which the shortcomings are hopefully resolved. That said, this first edition is safe in the hands of a specialist who can guide the student, correct errors and provide additional explanations where required. For now, Pugh and Woodworth (2014), and MurrayWallace and Woodroffe (2014) remain my recommendations for text books on sea-level changes.

References Gregory JM, Griffies SM, Hughes CW, Lowe JA, Church JA, Fukimori I, Gomez N, Kopp RE, Landere F, Le Cozannet G, Ponte RM, Stammer D, Tamisiea ME and van de Wal RSW. (2019). Concepts and terminology for sea level: mean, variability and change, both local and global. Surveys in Geophysics https:// doi.org/10.1007/s10712-019-09525-z. Masselink G and Gehrels R. (2014). Coastal environments and global change. Oxford: Wiley, 438 pp. Murray-Wallace CV and Woodroffe CD. (2014). Quaternary sea-level changes: a global perspective. Cambridge: Cambridge University Press, 504 pp. Pugh D and Woodworth P. (2014). Sealevel science: understanding tides, surges, tsunamis and mean sea-level changes. Cambridge: Cambridge University Press, 525 pp.

(Reviewed by Professor Roland Gehrels, Department of Environment and Geography, University of York)

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11/19/2019 4:03:50 PM

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