IEC TS 62600-40:2019

IEC TS 62600-40 ®
Edition 1.0 2019-06
TECHNICAL
SPECIFICATION
colour
inside
Marine energy – Wave, tidal and other water current converters –
Part 40: Acoustic characterization of marine energy converters

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IEC TS 62600-40 ®
Edition 1.0 2019-06
TECHNICAL
SPECIFICATION
colour
inside
Marine energy – Wave, tidal and other water current converters –

Part 40: Acoustic characterization of marine energy converters

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.140 ISBN 978-2-8322-6926-8

– 2 – IEC TS 62600-40:2019 © IEC 2019
CONTENTS
FOREWORD . 5
INTRODUCTION . 7
1 Scope . 8
2 Normative references . 8
3 Terms and definitions . 9
4 Symbols and abbreviated terms . 11
5 Outline of method . 11
6 Instrumentation . 12
6.1 Sound measurement system . 12
6.1.1 General . 12
6.1.2 Frequency range . 13
6.1.3 Sensitivity and dynamic range . 13
6.1.4 Hydrophone directionality . 13
6.1.5 Data acquisition and playback system . 13
6.1.6 Calibration . 13
6.2 Deployment platforms for sound measurement systems . 14
6.2.1 Fixed platforms . 14
6.2.2 Drifting platforms . 14
6.2.3 Flow-noise and self-noise minimization . 16
6.3 Contextual measurements . 16
6.3.1 General . 16
6.3.2 Winds . 16
6.3.3 Waves . 17
6.3.4 Currents . 17
6.3.5 Sound speed profiles . 18
6.3.6 Marine energy converter output . 18
7 Measurements and measurement procedures . 19
7.1 Metocean conditions . 19
7.1.1 General . 19
7.1.2 Winds . 19
7.1.3 Waves . 19
7.1.4 Currents . 19
7.2 Potential sources of acoustic masking . 19
7.2.1 General . 19
7.2.2 Vessel noise . 20
7.2.3 Biological noise sources . 20
7.2.4 Precipitation . 20
7.2.5 Air traffic . 20
7.2.6 Physical sources. 20
7.3 Underwater sound from marine energy converters . 20
7.3.1 Levels of characterization . 20
7.3.2 Measurement frequency range. 20
7.4 Wave energy converters . 21
7.4.1 WEC characteristics . 21
7.4.2 Sound measurement system deployment . 22
7.4.3 Temporal resolution . 22

7.4.4 Spatial resolution . 22
7.4.5 Sound speed profiles . 23
7.5 Current energy converters . 23
7.5.1 CEC characteristics . 23
7.5.2 Sound measurement system deployment . 23
7.5.3 Temporal resolution . 24
7.5.4 Spatial resolution . 24
7.5.5 Sound speed profiles . 25
7.6 Ocean thermal energy converters . 25
7.6.1 Overview . 25
7.6.2 Sound measurement system deployment . 26
7.6.3 Temporal resolution . 26
7.6.4 Spatial resolution . 26
7.6.5 Sound speed profiles . 27
8 Data analysis procedures . 27
8.1 General . 27
8.2 Metocean conditions . 27
8.2.1 Winds . 27
8.2.2 Waves . 28
8.2.3 Currents . 28
8.3 Sound speed profiles . 28
8.4 Marine energy converter output. 28
8.4.1 General . 28
8.4.2 Wave energy converters . 28
8.4.3 Current energy converters . 28
8.4.4 Ocean thermal energy converters . 28
8.5 Underwater sound . 29
8.5.1 General . 29
8.5.2 Acoustic signal processing. 29
8.5.3 Geo-referencing of drifting acoustic measurements . 30
8.5.4 Global sample acceptance criteria . 30
8.5.5 Considerations specific to wave energy converters . 31
8.5.6 Considerations specific to current energy converters . 32
8.5.7 Considerations specific to ocean thermal energy converters . 32
8.5.8 Aggregate statistics . 33
9 Information to be reported . 33
9.1 Sound measurement system . 33
9.2 Marine energy conversion system . 34
9.3 Measurement site . 34
9.4 Contextual measurements . 34
9.5 Specific reporting for wave energy converters . 35
9.5.1 Level A . 35
9.5.2 Level B . 37
9.6 Specific reporting for current energy converters . 37
9.6.1 Level A . 37
9.6.2 Level B . 37
9.7 Specific reporting for ocean thermal energy converters . 37
9.7.1 Level A . 37
9.7.2 Level B . 38

– 4 – IEC TS 62600-40:2019 © IEC 2019
Annex A (informative)  Sound within and around arrays . 39
Annex B (informative) Additional measurements for source level estimation . 40
Annex C (informative) Approaches to minimizing flow-noise . 41
C.1 General . 41
C.2 Methods specific to measurements in waves . 41
C.3 Methods specific to measurements in currents . 41
C.4 General methods . 42
C.5 Methods to mitigate flow-noise distortion . 42
Annex D (informative) Approaches to minimizing self-noise . 43
Bibliography . 44

Figure 1 – Hydrophone orientation relative to WEC . 23
Figure 2 – Zones for drifting CEC measurements . 25
Figure 3 – Zones for drifting OTEC measurements . 27
Figure 4 – Example grid of median MEC sound pressure levels as a function of
significant wave height and energy period . 35
Shaded region denotes interquartile range. . 36
Figure 5 – Example of median mean-square sound pressure spectral density level
variations as a function of sea state for a single spatial position . 36
Figure 6 – Example of median decidecade sound pressure level variations as a
function of sea state for a single spatial position . 36

Table 1 – Summary of measurement procedures . 12
Table 2 – Minimum measurement grades for metocean observations by category of
marine energy converter . 16

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MARINE ENERGY – WAVE, TIDAL AND
OTHER WATER CURRENT CONVERTERS –

Part 40: Acoustic characterization of marine energy converters

FOREWORD
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• the subject is still under technical development or where, for any other reason, there is the
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Technical specifications are subject to review within three years of publication to decide
whether they can be transformed into International Standards.
IEC TS 62600-40, which is a Technical Specification, has been prepared by IEC technical
committee 114: Marine energy – Wave, tidal and other water current converters.

– 6 – IEC TS 62600-40:2019 © IEC 2019
The text of this Technical Specifications based on the following documents:
Draft TS Report on voting
114/297/DTS 114/307/RVDTS
Full information on the voting for the approval of this technical specification can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
A list of all parts in the IEC 62600 series, published under the general title Marine energy –
Wave, tidal and other water current converters, can be found on the IEC website.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to
the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates that it
correct understanding of its
contains colours which are considered to be useful for the
contents. Users should therefore print this document using a colour printer.

INTRODUCTION
The purpose of this document is to provide uniform methodologies that will ensure
consistency and accuracy in the measurement and analysis of acoustical emissions from
marine energy converters. These systems include wave, current (tidal, ocean, and river), and
ocean thermal energy conversion. The document provides guidance on the measurement,
analysis, and reporting of acoustic emissions from marine energy converters and has been
prepared with the anticipation that it would be applied by:
• Marine energy converter manufacturers striving to meet well-defined acoustic emission
performance requirements and/or a possible declaration system;
• Purchasers of marine energy converters to specify such performance requirements;
• Operators of marine energy converters who may be required to verify that stated, or
required, acoustic performance specifications are met for new or refurbished units;
• Operators of marine energy test sites, who may be required to assess conformity with
consented acoustic levels at their sites;
• Marine energy converter planners or regulators who must be able to accurately and fairly
define acoustical emission characteristics of marine energy converters in response to
environmental regulations or permit requirements for new or modified installations.
The methods and reporting requirements in this document ensure that continuing
development and operation of marine energy converters is carried out in an atmosphere of
consistent and accurate communication relative to environmental concerns.

– 8 – IEC TS 62600-40:2019 © IEC 2019
MARINE ENERGY – WAVE, TIDAL AND
OTHER WATER CURRENT CONVERTERS –

Part 40: Acoustic characterization of marine energy converters

1 Scope
This part of IEC 62600 provides uniform methodologies to consistently characterize the sound
produced by the operation of marine energy converters that generate electricity, including
wave, current, and ocean thermal energy conversion. This document does not include the
characterization of sound associated with installation, maintenance, or decommissioning of
these converters, nor does it establish thresholds for determining environmental impacts.
Characterization refers to received levels of sound at particular ranges, depths, and
orientations to a marine energy converter. Informative Annex B provides guidance on
additional measurements that would be necessary to estimate source levels.
The scope of this document encompasses methods and instrumentation to characterize sound
near marine energy converters, as well as the presentation of this information for use by
regulatory agencies, industry, and researchers. Guidance is given for instrumentation
calibration, deployment methods around specific types of marine energy converters, analysis
procedures, and reporting requirements.
This document is applicable to characterization of sound from individual converters and
arrays. This document primarily describes measurement procedures for individual converters,
with extension to arrays discussed in informative Annex A.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their
content constitutes requirements of this document. For dated references, only the edition
cited applies. For undated references, the latest edition of the referenced document (including
any amendments) applies.
IEC 60565, Underwater acoustics – Hydrophones – Calibration in the frequency range 0,01
Hz to 1 MHz
IEC 61108-4, Maritime navigation and radiocommunication equipment and systems – Global
navigation satellite systems (GNSS) – Part 4: Shipborne DGPS and DGLONASS maritime
radio beacon receiver equipment – Performance requirements, methods of testing and
required test results
IEC 61400-12-1, Wind energy generation systems – Part 12-1: Power performance
measurements of electricity producing wind turbines
IEC TS 62600-1, Marine energy – Wave, tidal and other water current converters – Part 1:
Terminology
IEC TS 62600-20, Marine energy – Wave, tidal and other water current converters – Part 20:
Design and analysis of an Ocean Thermal Energy Conversion (OTEC) plant – General
guidance
IEC TS 62600-100, Marine energy – Wave, tidal and other water current converters – Part
100: Electricity producing wave energy converters – Power performance assessment

IEC TS 62600-200, Marine energy – Wave, tidal and other water current converters – Part
200: Electricity producing tidal energy converters – Power performance assessment
ISO 17208-1, Underwater acoustics – Quantities and procedures for description and
measurement of underwater sound from ships – Part 1: Requirements for precision
measurements in deep water used for comparison purposes
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC TS 62600-1 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following
addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
acoustic self-noise
sound at a receiver caused by the deployment, operation, or recovery of the receiver, and its
associated platform
[SOURCE: ISO 18405:2016, 2.1.1.5]
3.2
ambient noise
all sound, except acoustic self-noise and sound associated with a specified signal
[SOURCE: ISO 18405:2016, 2.1.1.6, modified – Notes not relevant to this document have
been removed.]
3.3
flow-noise
non-acoustic pressure fluctuations measured by a pressure-sensitive instrument
Note 1 to entry: This is sometimes referred to as “hydrodynamic noise”.
Note 2 to entry: Moving water can also excite structures, causing them to radiate acoustic pressure, but this is
categorized as acoustic self-noise.
3.4
mean-square sound pressure spectral density
distribution as a function of frequency of the mean-square sound pressure per unit bandwidth
of a sound having a continuous spectrum
Note 1 to entry: Mean-square sound pressure spectral density is expressed in units of pascal squared per hertz
(Pa /Hz).
Note 2 to entry: For operational purposes, mean-square sound pressure spectral density is estimated as the
mean-square sound pressure in a finite frequency band divided by the frequency bandwidth. The averaging time
and frequency band shall be specified.
[SOURCE: ISO 18405: 2016, 2.1.3.13, modified – Notes 2, 4, and 5, which are not relevant to
this document, have been removed, as has the preferred formula.]
3.5
mean-square sound pressure spectral density level
ten times the logarithm to the base 10 of the ratio of the mean-square sound pressure
spectral density to the specified reference value, in decibels

– 10 – IEC TS 62600-40:2019 © IEC 2019
Note 1 to entry: Mean-square sound pressure spectral density level is the level of the power quantity mean-
square sound pressure spectral density.
Note 2 to entry: Mean-square sound pressure spectral density level is expressed in decibels.
Note 3 to entry: In underwater acoustics, the reference value of mean-square sound pressure spectral density
is 1 µPa /Hz.
[SOURCE: ISO 18405:2016, 2.2.1.10, modified – Symbolic notation and formulas from the
definition and Notes 1 and 3 have been removed and Note 4 has been removed because it is
not relevant to this document.]
3.6
marine energy converter sound pressure level
ten times the logarithm to the base 10 of the ratio of the integrated mean-square sound
pressure spectral density over the measurement frequency range to the specified reference
value, in decibels
Note 1 to entry: Marine energy converter sound pressure level is expressed in decibels.
Note 2 to entry: The reference value is 1 µPa – see: mean-square sound pressure spectral density level.
3.7
decidecade sound pressure level
ten times the logarithm to the base 10 of the ratio of the integrated mean-square sound
pressure spectral density over a specified decidecade frequency band to the specified
reference value, in decibels
0,1
Note 1 to entry: The frequency ratio corresponding to a decidecade (1 ddec) is 10 . One decidecade (0,1dec) is
equal to 0,1 log (10).
Note 2 to entry: Decidecade bands are defined in IEC 61260-1:2014 as “one-third octave bands”.
Note 3 to entry: Decidecade sound pressure level is expressed in decibels (dB).
Note 4 to entry: The reference value is 1 µPa – see: mean-square sound pressure spectral density level.
3.8
sound pressure
difference between instantaneous total pressure and pressure that would exist in the absence
of sound waves
Note 1 to entry: Sound pressure is expressed in pascals (Pa).
[SOURCE: ISO 18405:2016, 2.1.2.1, modified – Notes 2, 3, 4, and 5, which are not relevant to
this document, have been removed, the preferred formula has been removed, and it has been
made explicit that this is an instantaneous quantity.]
3.9
self-noise
fluctuations in signal caused by the combination of acoustic self-noise and non-acoustic
self-noise
Note 1 to entry: An example of acoustic self-noise is hydrodynamic excitation of the sound measurement system
that generates propagating sound.
Note 2 to entry: An example of non-acoustic self-noise is electrical noise internal to the sound measurement
system electronics.
[SOURCE: ISO 18405:2016, 2.6.1.6, modified – Note 1, which is not relevant to this
document, has been removed and notes have been added with examples of acoustic self-
noise and non-acoustic self-noise.]

4 Symbols and abbreviated terms
H significant wave height
m0
T energy period
e
L mean-square sound pressure spectral density level
p, f
L decidecade sound pressure level
p,ddec
L marine energy converter sound pressure level
p,MEC
V mean-square voltage spectral density
f
p mean-square sound pressure spectral density
f
CEC  current energy converter
MEC  marine energy converter
OTEC  ocean thermal energy converter
WEC  wave energy converter
AEP annual energy production
5 Outline of method
Measurements of underwater sound around marine energy converters (MECs) are undertaken
to characterize MEC sound. This document focuses on measurements of underwater sound
utilizing hydrophones, which are sensitive to fluctuations in acoustic and non-acoustic
pressure. Deployment mechanisms to minimize measurement contamination by non-acoustic
pressure fluctuations are given for each class of MEC, specifically current energy converters
(CECs), wave energy converters (WECs), and Ocean Thermal Energy Conversion (OTEC)
systems. These measurements are complemented by contextual observations of metocean
conditions, sound speed profiles, and MEC operation (e.g., rated capacity, annual energy
production (AEP)). The acoustic frequencies of interest for these measurements coincide with
those of aquatic species, extending from a few Hz to over one hundred kHz. Two levels of
characterization are described. Level A characterization describes the temporal and spatial
characteristics of MEC sound. Level B characterization describes the characteristics of MEC
sound at reduced temporal and spatial detail. Both levels utilize the same instrumentation,
analysis, and reporting. “Level B” characterization is included within the document to provide
a mechanism for reduced-effort reconnaissance surveys to be compliant with the
specification. For example, a Level B characterization might be sufficient for a prototype
demonstration of a MEC, whereas as Level A characterization might be more suitable for a
commercial deployment. For both levels of characterization, analysis requires identifying, from
all collected acoustic samples, a sufficient valid set at specified marine energy converter
operational conditions and spatial positions. Measurement procedures for the three classes of
MEC are summarized in Table 1.

– 12 – IEC TS 62600-40:2019 © IEC 2019
Table 1 – Summary of measurement procedures

WEC (7.4) CEC (7.5) OTEC (7.6)

Level A Level B Level A Level B Level A Level B
Acoustic measurements
Default low- 10 Hz (7.3.2)
frequency limit
Default high- 100 kHz (7.3.2)
frequency limit
Measurement Fixed Fixed or Fixed or Drifting Drifting
platform Drifting
Sea states Any sea Currents Currents • Rated Rated
corresponding state that corresponding corresponding capacity
capacity
to 50 % of AEP results in at to 0 %, 25 %, to 100 % of
Temporal extent
• Idle power
or 6 months of least 75 % 50 %, 75 %, rated capacity
measurements of rated and 100 % of
capacity rated capacity
• Two in-line One in-line Four zones Upstream or Four zones Any of the
with downstream bracketing four zones
• Upstream
zone plant in bracketing
dominant
ordinate plant in
wave
Spatial extent • Downstream
directions ordinate
direction
directions
• Port
• One offset
• Starboard
Required data for 20 valid acoustic 10 valid acoustic measurement 20 valid acoustic
each combination measurement sequences sequences (8.5.6) measurement sequences
of temporal and (8.5.5) (8.5.7)
spatial extent
Contextual measurements
Sound speed 6.3.5 6.3.5 6.3.5
profile
Current 6.3.4 (Grade 2) 6.3.4 (Grade 1) 6.3.4 (Grade 2)

Wave 6.3.3 (Grade 1) 6.3.3 (Grade 2) 6.3.3 (Grade 2)

Wind 6.3.2 (Grade 2) 6.3.2 (Grade 2) 6.3.2 (Grade 2)

MEC 6.3.6 6.3.6 6.3.6
Valid sequences are those sequences that do not include obvious contamination by flow-noise, self-noise,
anthropogenic sources, or biological sources.

6 Instrumentation
6.1 Sound measurement system
6.1.1 General
The term “sound measurement system” refers to the complete instrumentation system
consisting of the sensitive elements (also known as the “hydrophone”), signal conditioning
stages (including gain and filters), acquisition (analog to digital conversion), communication,
transmission, and data storage. This term applies to systems where these components are
closely integrated as a single package by a manufacturer, as well as discrete components
from one or more manufacturers assembled into a system.
NOTE The signal is defined here as the time-varying voltage.

6.1.2 Frequency range
The default frequency range is 10 Hz – 100 kHz. This frequency band is selected to cover
most of the frequencies that may be audible to aquatic life, with the recognition that this may
be adjusted to satisfy case-specific regulatory requirements.
Conditions under which it is allowable to increase the low-frequency limit and change the
high-frequency limit in the absence of case-specific regulatory requirements are described
in 7.3.2.
The frequency range applies to all components of the sound measurement system (i.e., from
hydrophone to recording device).
6.1.3 Sensitivity and dynamic range
The sound measurement system should have sufficient sensitivity and dynamic range to
measure ambient noise exceeding typical sea-state 1 conditions up to maximum levels
received by the MEC without saturating. Combined receiving voltage sensitivity and amplifier
gains should be between -165 dB to -145 dB re 1 V/μPa for 16-bit systems. Combined
sensitivities between -180 dB to -145 dB re 1 V/μPa should be acceptable for higher bit
depths.
The non-acoustic self-noise mean-square sound pressure spectral density level should be
less than 55 dB re 1 μPa /Hz for frequencies between 100 Hz-1 000 Hz.
To establish the non-acoustic self-noise, an air-side test of the sound measurement system
should be conducted in a relatively quiet environment where the hydrophone is isolated from
vibrations (e.g., wrapped in foam in a laboratory setting). The mean-square sound pressure
spectral level may be calculated as for in-water acoustic measurements (8.5.2).
6.1.4 Hydrophone directionality
The hydrophone response should be omnidirectional in the horizontal plane to ±5 dB over the
specified frequency range. There is not a similar requirement for vertical directionality. The
sound measurement system should be assembled in such a way as to minimize additional
horizontal directionality and minimize the impacts of vertical directionality.
6.1.5 Data acquisition and playback system
The data acquisition, recording, processing, and display system should be capable of
accurately acquiring, recording, processing, and displaying data from the hydrophone(s) in
accordance with ISO 17208-1.
The analog-to-digital conversion system should be at least 16-bit.
NOTE A higher bit depth will allow a more sensitive hydrophone to be used without saturating at maximum
received levels around a MEC.
Such systems may comprise tape recorders, computer-based data acquisition systems, or
hardware-specific devices (such as spectrum analyzers) or combinations of such. The data
acquisition system should have an appropriate sampling rate following Nyquist requirements
and appropriate dynamic range for either analog or digital systems.
6.1.6 Calibration
During acoustic measurements of marine energy converters, the complete sound
measurement system should be calibrated immediately before and after the measurement
session at one or more frequencies using an acoustical calibrator on the hydrophone in
accordance with IEC 60565. The calibrator should, itself, be calibrated at least every 24
months.
– 14 – IEC TS 62600-40:2019 © IEC 2019
Because hydrophones are a critical component of the measurement system, hydrophone
calibration values should be provided by a calibration laboratory or by the manufacturer, and
not merely be indicative or nominal values indicated by the manufacturer’s design
specification. The hydrophone calibration should be over the specified frequency range and
conducted in accordance with IEC 60565 with the exception that the calibration should be
updated every 24 months (rather than 12 months). The calibration should also be updated if
the acoustical calibrator indicates a change in sensitivity of more than 5 dB.
NOTE IEC 60565 allows for multiple methods of calibration, including in-air methods for lower frequencies.
If it is impractical to test the entire sound measurement system (e.g., data acquisition system
powered by a local grid that cannot be recreated in a laboratory setting), it is acceptable to
provide justification and clear reasoning why the entire sound measurement system was not
tested, along with an assessment of the implication for measurement accuracy.
If the data acquisition system is not integrated with the hydrophone, then proper operation
should be verified in a laboratory setting at least every 24 months.
6.2 Deployment platforms for sound measurement systems
6.2.1 Fixed platforms
6.2.1.1 General
“Fixed” deployment platforms generally constrain the movement of the acoustic measurement
system in response to metocean forcing, such that their average spatial position does not
change in time.
The advantage of fixed platforms is their persistence at a specific location over long periods
of time, which is a particularly useful attribute for acoustic measurements around wave energy
converters.
The disadvantage of fixed platforms is that it is possible for relative velocity to develop
between the hydrophone and surrounding water (e.g., wave orbital velocities in water that is
shallow relative to the wavelength), which will produce flow-noise contamination.
It is allowable to introduce compliant elements that decouple the hydrophone element from
platform vibration, including vibration transmitted through the seabed. It is also allowable to
introduce compliant elements that allow the hydrophone to move in response to wave orbital
motion. If compliance is introduced between the hydrophone and the fixed anchor point, then
the maximum vertical and horizontal displacement relative to a neutral position shall be
estimated and reported.
6.2.1.2 Establishing platform location
If the fixed platform is deployed by a surface vessel, its location on the seabed should be
confirmed by an IEC 61108-4 compliant Global Navigation Satellite System (GNSS) co-
located with the deployment system and the line angle should minimized during deployment.
If the fixed platform is deployed by divers, its position on the seabed should be estimated with
an accuracy of at least ±10 m.
NOTE Because the minimum separation distance between a fixed platform and a MEC is 100 m (7.4.4 and 7.5.4),
a positioning error of 10 m would be expected to contribute to a propagation loss uncertainty on the order of 1 dB.
6.2.2 Drifting platforms
6.2.2.1 General
“Drifting” deployment platforms are generally unconstrained in response to external forcing,
including metocean forcing, buoyancy engines, and thrusters, such that their average spatial

position changes in time. Drifting platforms includes systems with surface expressions
(e.g., spar buoys) and fully submerged platforms (e.g., floats, autonomous underwater
vehicles).
The advantage of d
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