Andrews E
Andrews E
Andrews E
Abstract
Historically the offshore ocean wave-monitoring field has been dominated by surface following wave buoys
utilising accelerometers. However, recent advancements in technology have introduced alternate global
positioning system (GPS) sensors. To monitor wave conditions effectively, it is important to have a
comprehensive understanding of available wave monitoring devices. A three-month field trial was undertaken
at the Gold Coast, Queensland, deploying nine wave monitoring devices for comparison to Datawell’s DWR-
MkIII, which is considered the industry standard. Devices used in this study include four GPS buoys, one
accelerometer buoy, two pressure transducers and two prototype devices. For each device, wave parameters
were either examined, as the on-board calculated parameters, or by using manufacturer provided post-
processing software. In addition, parameters were recalculated from the raw displacements to reduce
variability caused by different calculation methods.
The comparison shows excellent agreement for significant wave height (Hs) and maximum wave height
(Hmax) across all commercially available devices in both the on-board and recalculated parameters. Peak
period (Tp) returned the weakest agreement across all devices compared to the DWR-MkIII, most likely due
to the presence of bi-modal sea-states, differing spectral calculation methods and GPS signal loss. Two GPS
buoys developed by Spoondrift Technologies Inc. sustained damage, with one resulting in device failure after
two months. The SCRIPPS GPS prototype device compared well to the DWR-MKIII across all parameters;
however, suffered antenna damage resulting in loss of satellite connection. The prototype smartphone app
had issues with low-frequency interference and the pressure transducers displayed poor agreement in wave
period, potentially due to considerably shorter recording period and fouling.
The recalculated parameters from the GPS equipment demonstrated that appropriate filtering can help
overcome GPS loss issues, indicating that smaller GPS based devices may be an appropriate alternative in
future wave monitoring applications. However, further consideration of their robustness during extreme events
may be warranted.
is a long-term wave monitoring site for the Coastal personnel at CIU, founded on over 40 years of wave
Impacts Unit (CIU), Department of Environment and monitoring experience, and primarily consisted of
Science, and has been active since 21 March 1987 changes to the anchoring method, and have been
[11]. Nine devices were deployed for comparison utilised in numerus deployments with no negative
with Datawells DWR-MkIII (Table 1), including influence on the buoy. Both the DWR-MkIII and the
seven directional wave measuring devices TriAxys buoy incorporate a rubber cord which, due
consisting of five buoys and two prototype devices. to the added elasticity enables greater flexibility and
Two non-directional pressure transducers were also range of motion when following the orbital motion of
deployed. The estimated water depth at the ocean water particles [7] [9].
deployment site is ~16 m, and the Datawell DWR-
MkIII was located at 27° 57.876’ S, 153° 26.500’ E. 3. Analysis Techniques
Historical wave climate data indicates that the All commercially available instruments provide an
average significant wave height (Hs) is 1.12 m, with automatic wave summary output, either calculated
a dominant east to east-southeast wave direction on the device or by post processing software. As
[11]. The monthly average significant wave height these automatic outputs are what typical users will
(Hs) varies throughout the year, peaking at 1.37 m be relying on for wave measurements it is important
in March to 0.94 m in September, with a modal to examine how they compare to the DWR-MkIII.
height of about 0.9 m. The top five maximum Parameters compared were Hs, Hmax, Tp, and
recorded wave heights (Hmax) range from 10.6 to average period (Tz). Whilst each device provides
12.0 metres. Peak wave periods (Tp) typically range most of these parameters, there are some
from 3 to 15 seconds, with a unimodal period of variations in calculation methods. These
about 10 seconds. Occasional bimodal sea states discrepancies introduce undesired variability and
can be apparent, particularly in cyclone season. are a key limitation in making direct comparisons
between each device. As such, in order to rule out
Table 1 Equipment specifications
differences in calculation methods, non-directional
Device Measurement Record Data parameters have also been recalculated from
Principle frequency
Interval devices that also provide raw displacements,
(min) (Hz) enabling a more consistent comparison of each
Commercially Available Wave Buoys devices ability to measure the motion of waves.
DWR-MkIII Accelerometer 26.6 1.28 Recalculated statistics are unavailable for the
DWR-G4 GPS 26.6 1.28 Aanderaa and the RBR as they were not set up to
x2 record raw displacement data.
TriAxys Accelerometer 20.0 1.28
Spoondrift GPS 30.0 2.5 Filtering methods were also incorporated into the
Spotter x2 recalculation of wave parameters in order to
Commercially Available Pressure Transducers validate the data and remove errors. This allowed
RBRduet Pressure 8.5 2.5 various errors in records to be identified in the
Aanderaa Pressure 8.5 4 devices where possible. The re-calculation process
Prototype Devices
was adapted from validation procedures developed
SCRIPPS GPS ≈17.0 4
DWSD
by the CIU, aimed at ensuring quality and integrity
of its long-term historical data sets. This process is
WaveApp Accelerometer 1
implemented in four stages: 1) a coarse spike
removal filter is applied in order to remove large
Due to the dominant easterly wave direction spikes exceeding five times the standard deviation;
indicated by historical data, equipment was 2) a Butterworth high pass filter is applied to
deployed parallel to the shore along a contour line frequencies lower than 0.025 Hz, or periods longer
for uniform depth during February 2018, enabling than 40 seconds; 3) wave statistics are recalculated
the instrumentation to capture comparable data using similar methods to those outlined by Datawell
from the same wave train. Devices were deployed [9]; and 4) range and ratio checks that are used to
approximately 150 metres apart in order to prevent identify potential erroneous records for review [5].
mooring system entanglement and the RBR and
Aanderaa pressure transducers were bottom In order to analyse the vast amount of data
mounted together in a weighted triangular frame produced from the nine wave monitoring devices, a
~1 m above the seafloor, with a float as a marker. number of statistical parameters were chosen to
compare the test devices to the DWR-MkIII.
2.1 Mooring Configurations Including Spearman’s correlation coefficient (1),
Manufacturer recommended mooring designs were RMSE, bias (2), and Scatter Index (3).
followed as closely as possible for all wave buoys
∑𝑖(𝑥𝑖−𝑥̅ )(𝑦𝑖−𝑦̅)
[2] [9] [18], however, minor alterations were made to 𝑟= (1)
√∑𝑖(𝑥𝑖−𝑥̅ )2 ∑𝑖(𝑦𝑖−𝑦̅)2
the DWR-MkIII and TriAxys mooring configurations.
Alterations were based on technical advice from
Australasian Coasts & Ports 2019 Conference – Hobart, 10–13 September 2019
Evaluation of Current and Emerging In-situ Ocean Wave Monitoring Technology
Elysia Andrews and Leo Peach
r 0.84 0.72 −0.01 0.32 parameters can be minimised. Due to the presence
DWR-G4 RMSE 0.19 0.44 8.60 0.57 of saw-tooth artefacts in both moored and
BI −0.15 −0.3 −7.91 0.09 unmoored DWR-G4 buoys, it is unlikely that the low-
frequency errors were solely caused by mooring
forces. They are likely due to a combination of high-
energy sea states causing GPS signal corruption,
general loss of the minimum number of required
GPS satellites, and tension placed upon the
mooring configuration, leading to one or both of the
Figure 6 Recalculated parameters (Hs, Hmax, Tp, Tz) for the aforementioned. In order for the DWR-G4 buoys to
WaveApp compared to the DWR-MkIII with a 1:1 line. calculate orbital wave motion via the Doppler shift
principle, a minimum of four visible satellites is
required [8]. The Spotter demonstrated less
frequent saw-tooth patterns in the raw
displacement, possibly due to the Spotter utilising
the Iridium SBD for GPS as opposed to the GPS
satellite system used by Datawell.
Figure 7 Recalculated parameters (Hs, Hmax, Tp, Tz) for the
WaveApp compared to the DWR-G4 #6 with a 1:1 line. 4.5 Device Longevity
4.4 Raw Heave Displacements Longevity of wave monitoring equipment is a
A comparison of a half-hour segment of raw significant factor to take into consideration when
displacements from the DWR-MkIII and DWR-G4 undertaking wave monitoring due to the harsh
on 19 April demonstrates the extent to which heave ocean environments that devices are subjected to
measurements were influenced by GPS corruption during deployment. There are a number of
(Figure 8). Whilst the DWR-MkIII fluctuates around situations that can result in ocean monitoring
0.0 m in a relatively uniform manner, the DWR-G4 equipment being damaged or destroyed: lightning
has multiple anomalies which severely impacted strikes; boat strikes; and extreme events. Datawell
calculated parameters (Table 6) in both the and TriAxys buoys are made from stainless steel
automatic data outputs and recalculated (DWR-G4 and TriAxys) or cunifer alloy (DWR-MkIII)
parameters. in order to resist potential damage, while Spoondrift
Spotters are made from a marine grade plastic.
Table 6 Automatic output wave parameters for the DWR- Throughout this trial damage was sustained to the
G4 and DWR-MkIII on 19 April 2018 at 08:30
hull of the Spotters which resulted in water intrusion.
Device Hs (m) Hmax (m) T p (m) Tz (m) Additionally, the Spotter’s size is of concern when
DWR-G4 1.61 2.53 40 6.36 deployed for extended periods as mooring forces
DWR-MkIII 1.247 2.22 14.29 5.128 and biofouling may affect the buoyancy of the
device. In warmer climates biofouling is a serious
concern for equipment within the marine
environment. For long-term deployments of both the
Spotter and DWR-G4 extensive biofouling may
severely hamper a buoys ability to follow the orbital
motion of the water, reducing the quality of wave
parameters recorded and affecting the high
frequency response of the buoy [19]. For GPS
buoys an additional concern is that the added
weight of marine growth can cause the buoy to sit
Figure 8 Vertical heave displacement produced by the lower in the water, becoming more susceptible to
DWR-MkIII and DWR-G4 to demonstrate the effect of wave over topping thus causing GPS interference.
GPS loss on raw displacements. Observations are from Additionally the DWR-G4 is limited by a 4–6 week
19 April 2018.
battery life. Due to the submerged nature of the
pressure transducers biofouling was apparent,
This anomaly (known as a saw-tooth pattern that however, no negative effects on wave parameters
introduces erroneous low frequency energy) has records were apparent. Both the DWR-MkIII and
been reported in a number of studies, utilising both TriAxys buoys were subject to barnacle growth, but
moored and drifting DWR-G4 buoys [3] [4]. The they were not notably affected due to the size and
saw-tooth like artefact occurred frequently subsequent buoyancy of the devices. However,
throughout the raw displacements of the DWR-G4s consideration of biofouling for long duration
and infrequently in the Spotters, and is a result of deployments is recommended.
the buoy losing track due to loss in GPS signal.
By applying a high-pass filter to the data, the effect
of these erroneous displacements on wave
Australasian Coasts & Ports 2019 Conference – Hobart, 10–13 September 2019
Evaluation of Current and Emerging In-situ Ocean Wave Monitoring Technology
Elysia Andrews and Leo Peach
5.1 Limitations [3] Björkqvist, J.-V., Pettersson, H., Laakso, L., Kahma, K.
While the authors endeavoured to undertake a K., Jokinen, H., & Kosloff, P. (2016). Removing low-
comprehensive comparison of wave monitoring frequency artefacts from Datawell DWR-G4 wave buoy
equipment there were a number of limitations, measurements. Geoscientific Instrumentation Methods
and Data Systems, 5, 17–25.
primarily, due to spatial differences, measurements
of the exact same wave were not possible. Future [4] Boswood, P. (2017). Monitoring and Modelling
trials would benefit from an on-board inter-sensor Extreme Wave Conditions during Tropical Cyclone
deployment where the sensors of each device are Nathan. In Coast and Ports 2017.
deployed within one hull. Additionally this will
minimise variations caused by alternate mooring [5] Butterworth, S. (1930). On the Theory of Filter
configurations and hull designs, allowing a direct Amplifiers. Experimental Wireless and the Wireless
Engineer, 7, 536–541.
Australasian Coasts & Ports 2019 Conference – Hobart, 10–13 September 2019
Evaluation of Current and Emerging In-situ Ocean Wave Monitoring Technology
Elysia Andrews and Leo Peach
[14] Jeans, G., & Bellamy, I. (2012). Sea Trial of the New
Datawell GPS Directional Waverider.