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Ocean Engineering 290 (2023) 116217
Available online 24 November 2023
0029-8018/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Structure design and implementation of a high stability semi-submersible
optical buoy for marine environment observation
Shizhe Chen
a
,
b
,
c
, Jiming Zhang
a
,
c
, Shixuan Liu
a
,
c
,
*
, Bangyi Tao
d
, Yushang Wu
a
,
c
,
Xiaozheng Wan
a
,
c
, Yuzhe Xu
a
,
c
, Miaomiao Song
a
,
c
, Xingkui Yan
a
,
c
, Xianglong Yang
a
,
c
,
Zhuo Lei
a
,
c
a
Institute of Oceanographic Instrumentation, Qilu University of Technology (Shandong Academy of Sciences), Qingdao, 266100, China
b
Laoshan Laboratory, Qingdao, 266237, China
c
Shandong Academy of Sciences, Institute of Oceanographic Instrumentation, Qingdao, 266100, China
d
Second Institute of Oceanography, Ministry of Natural Resource, Hangzhou, 310012, China
ARTICLE INFO
Handling Editor: Prof. A.I. Incecik
Keywords:
Marine satellite calibration
Marine satellite verication
Optical buoys
High stability
Transverse tethering
Semi-submersible
ABSTRACT
Marine optical buoys are recognized as highly effective technical means for obtaining long-term continuous
ocean optical observation data for on-orbit ocean color satellite radiometric calibration and remote sensing
product validation. The high stability of a buoy’s body structure is a crucial factor for marine optical buoys to
operate in complex environments, such as in the presence of wind, waves, and currents. In this study, we propose
a highly stable self-counterweight transverse-tethered semi-submersible buoy structure achieved through buoy
structure design, mooring system design, mechanical analysis and numerical simulation analysis, and transverse-
tethered point analysis. The novel buoy design underwent a marine environment adaptability test, demonstrating
an improved ability to resist wind and waves while also greatly reducing the difculty and cost of design and
deployment. In-situ verication showed that the buoy’s swing angle was better than ±10◦under a 3–4 level sea
state, meeting marine optical buoy requirements and proving the new buoy design suitable for harsh and
complex marine conditions.
1. Introduction
Ocean color remote sensing has greatly developed in the past de-
cades since the proof-of concept “Coastal Zone Color Scanner” (CZCS)
was launched in November 1978 by the National Aeronautics and Space
Administration (NASA; Hovis et al., 1980). Since then, more than twenty
ocean color satellite sensors from several space agencies have been
launched (www.ioccg.org/sensors_ioccg.html). These sensors capture a
virtually daily coverage of the ocean surface continuous global ocean
color data (e.g. chlorophyll concentration, primary production), which
provide signicant benets for research in areas such as global carbon
cycle research biological oceanography, sheries regulation and coastal
zone monitoring and management (Platt and Sathyendranath, 1988).
In the international effort to develop a global, multi-year time series
of consistently calibrated ocean color products using data from a wide
variety of independent satellite sensors, the top-of-atmosphere (TOA)
observations from these satellite sensors have to be properly calibrated.
In particular, the normalized water-leaving radiance, noted L
WN
(λ),
which is the primary quantity derived from the TOA satellite observa-
tions after the atmospheric effects are removed, should a relative com-
bined standard uncertainty of only 5 % as a longstanding goal of the
SeaWiFS and MODIS (Ocean) Science Teams (Clark et al., 1997). To
achieve a such radiometric uncertainty of water-leaving radiance, the
uncertainty dictated by uncertainty requirements in the TOA products,
the at-sensor radiometric uncertainty requirement is 0.5 %. This level of
uncertainty cannot be achieved in the laboratory and transferred
on-orbit through the launch. Consequently, marine optical buoys are
widely recognized as the most effective and indispensable technical
means for obtaining long-term continuous radiometric quantities
observation data, performing on-orbit marine water color satellite ra-
diation correction, and verifying the authenticity of remote sensing
products (Antoine et al., 2008; Brown et al., 2007).
Optical buoys monitor various parameters, including water irradi-
ance, multilayer underwater irradiance, and underwater radiation
* Corresponding author.
E-mail address: lsx@sdioi.com (S. Liu).
Contents lists available at ScienceDirect
Ocean Engineering
journal homepage: www.elsevier.com/locate/oceaneng
https://doi.org/10.1016/j.oceaneng.2023.116217
Received 17 May 2023; Received in revised form 25 September 2023; Accepted 28 October 2023
Ocean Engineering 290 (2023) 116217
2
brightness. Compared to conventional buoys, optical buoys have unique
requirements, stabilizing the instruments in the water column and
avoiding shading them. As to high stability, which means that the buoy’s
swing angle should be within ±10◦in a 3–4 level sea state (Clark et al.,
2003; Antoine et al., 2008). This is because considering the angular
structure of the upward light eld (e.g., Morel and Gentili, 1993), a 10◦
tilt represents an uncertainty of less than 3% in the measurement of the
upwelling nadir radiance. During the three years of deployment between
September 2003 and September 2006, the behavior of BOUSSOLE buoy
showed that 85% of the buoy tilts are within 10◦. Additionally, the
buoy’s body should not create a cover effect on underwater multi-layer
optical observation. In summary, the successful operation of a marine
optical buoy depends on the high stability of its body structure, making
it an essential consideration in its design.
In recent years, the technology of ocean buoy observation has
developed rapidly, and some new buoy structures are constantly
emerging (Zhang et al., 2022; Amaechi et al., 2021; Doyle and Aggidis,
2019). In addition, there have been new technological advancements in
buoy structure design, anchor system design analysis, and other aspects
(Ja’e et al., 2022; Amaechi et al., 2022; Neisi et al., 2022; Yu et al.,
2020). However, due to the unique nature of optical buoys, the most
representative optical buoys in the world are the MOBY in the United
States (Clark et al., 1997, 2003; Voss et al., 2015; Feinholz et al., 2017)
and BOUSSOLE in Europe (Antoine et al., 2006, 2008), as shown in
Fig. 1. MOBY utilizes a sub-mother form, where a small optical obser-
vation buoy is connected to a large buoy by a cable. Although this
method has high safety standards, the small buoy is located on the water
surface, making it is difcult to avoid the inuence of wind and waves,
and in some positions, it may obstruct underwater optical observations.
On the other hand, the BOUSSOLE buoy uses a semi-submersible cy-
lindrical buoy, which has better stability than that of the sub-mother
form adopted by MOBY. Moreover, it provides better performance by
reducing the inuence of the shadow of the sub-mother surface buoy
during underwater observations (Lee et al., 2010). However, BOUSSOLE
uses a taut anchor system and is mainly deployed in deep-sea areas with
very low currents(usually <10 cm/s) (Antoine et al., 2008). To minimize
the inuence of external sea conditions, both types of buoys were
deployed in oligotrophic open waters with excellent water quality, at-
mospheric conditions, and minimal dynamic changes (Voss et al., 2015).
In China, optical buoy research and sea trials similar to the MOBY
sub-mother buoy design have been conducted (Cao et al., 2003; Yang
et al., 2009). Although improvements have been made to its
saddle-tethered structure, better stability and resistance are required for
the buoy to endure the environmental damage caused by the complex
Fig. 1. Comparison of several typical optical buoys locally and abroad.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
3
sea environment and harsh sea conditions found in China (Cao et al.,
2010).
Studies have shown that the BOUSSOLE buoy is more stable than
other optical buoys. However, it has some limitations. Firstly, the design
of the taut anchor system is difcult (Brewin et al., 2016; Liu et al., 2018,
2019), as it requires accurate measurement of water depth, tidal dif-
ferences, seabed conditions, and other parameters, especially for sea
areas of several kilometers in depth. Accurately conguring the anchor,
particularly in terms of controlling the cable expansion ratio, is also
challenging for kilometer-level anchor systems. Secondly, deploying the
buoy is difcult and requires a large engineering vessel with high
tonnage lifting capacity. This makes construction at sea expensive and
generally complicated. Additionally, the taut anchor system
semi-submersible buoy has some drawbacks. The buoyancy cylinder is
located at the bottom of the buoy, resulting in a large current resistance
area and a small distance between the buoyancy and mooring point. As a
result, it is challenging to provide signicant recovery torque, and it can
easily experience substantial inclination under external interference,
making it not suitable for use in sea areas with complex environments
and harsh conditions.
As the marine satellite industry continues to develop, designing op-
tical verication and calibration buoys with high stability and strong
environmental adaptability is becoming increasingly urgent. Therefore,
building on previous research, this paper proposes a self-counterweight
laterally tethered semi-submersible buoy design. This design not only
improved the buoy’s ability to resist wind and waves and its overall
stability, but also greatly reduced the difculty and cost associated with
designing and deploying the buoy. Sea tests have demonstrated that this
design meets the requirements for the optical buoy’s attitude swing
angle, providing a new technical solution for optical verication and
calibration in vast and complex sea areas.
2. System integration
The primary objective of optical buoys is to achieve reliable obser-
vations of optical parameters, including irradiance (in air), multi-layer
underwater irradiance, and underwater radiance. Compared to con-
ventional buoys, optical buoys require high stability (i.e., the swing
angle of buoys should be within ±10◦in 3~4 level sea state) and should
not have any cover effect from the buoy’s body on underwater multi-
layer optical observation.
As shown in Fig. 2, the overall structure of the optical buoy includes a
water irradiance sensor, underwater multilayer irradiance, and radiance
sensors. The data acquisition processor collects the sensor data and
transfers it to the shore station through a communication network. The
upper end of the buoy is equipped with an attitude sensor that monitors
the buoy’s real time attitude. The solar power supply system provides a
long-term renewable energy supply to meet the long-term work needs of
buoys at sea.
In terms of the optical buoy structure, this paper proposes a trans-
verse moored anchor scheme with a counterweight to address the
shortcomings of existing optical buoys. As shown in Fig. 3, a certain
length and mass counterweight was added to the lower end of the semi-
submersible buoy to form a stable tumbler structure, which helps the
buoy maintain a stable, upward position. This study changed the
traditional method of mooring the bottom of the buoy by designing the
tether point at the position of the horizontal force balance point on the
side of the buoy, in order to ensure that the buoy can maintain a stable
upward posture under the action of wind and waves. The mooring sys-
tem adopts horizontal mooring instead of taut mooring, which greatly
reduces the difculty of buoy deployment and maintenance. Therefore,
ordinary ships and conventional buoy deployment schemes can be used,
providing the advantages of simplicity and reliability. The force of the
buoy in the water was simulated and analyzed.
Fig. 2. Overall block diagram of the optical buoy observation system.
Fig. 3. Overall diagram of a transverse tethering scheme with counterweights.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
4
3. The design and optimization of the buoy structure
Fig. 4 shows the proposed distribution design scheme for the buoy-
ancy cabin to address the issue of increased current resistance caused by
the existing buoy cabin located at the root of the buoy. The buoyancy
chamber is distributed into three layers in the middle of the buoy’s body,
with the counterweight at the root of the buoy’s body, the center of
gravity below, and the oating center at the top, forming an effective
tumbler structure that ensures the buoy’s vertical stability in the water.
It also provides sufcient recovery moment to allow the upper part of
the buoy to automatically return to the vertical state when tilted by wind
and waves. Moreover, the distributed structure and downstream setting
of the buoyancy cylinder signicantly reduce the current resistance area
and the current’s impact on the buoy.
In Fig. 5, the buoy is also equipped with brackets to serve as a
platform for the installation of observation sensors. On the water surface
part of the buoy’s bracket, four 30-W solar panels were installed to
charge the underwater lithium battery, which provides power to the
spectrometer, data collector, anchor lamp, GPS, and other system
equipment. This design enables the buoy to operate reliably and safely
for an extended period at sea.
In Fig. 6, to prevent rotation of the buoy’s body caused by changes in
current, water sails were installed on the buoy to increase its down-
stream capacity and improve its stability. In order to enhance the buoy’s
marine adaptability and prevent it from sinking due to biological
attachment or accidental entanglement of foreign objects such as drift
nets, auxiliary oats were added above the water surface. If the buoy
unexpectedly sinks, the auxiliary oats can provide reserve buoyancy to
prevent it from sinking completely. Beneath the waterline of the buoy is
an adjustable buoyancy chamber, with two water pipes located directly
above the water surface. The position of the waterline of the buoy can be
ne-tuned by lling or pumping water into the adjustable buoyancy
cabin to adjust its buoyancy.
4. Design and optimization of the buoy mooring
4.1. The design of mooring system
In Fig. 7, the buoy is moored using a horizontal transverse tethering
method with the mooring point set at the horizontal force balance point.
This design ensures that the buoy maintains a stable and vertical posture
even under the inuence of the current, overcoming the inherent
problem of traditional taut anchor system that are easily tilted by large
tidal differences and strong currents. In taut anchor systems, the
mooring point is located at the root of the buoy. Unlike taut anchor
systems, which require large engineering vessels, this scheme can be
deployed using conventional vessels, thus reducing operating costs and
overcoming the difculties associated with taut anchor systems.
The mooring of a buoy primarily consists of horizontally moored
cables, anchor mooring oats, cables, anchor chains, and anchors. The
horizontally tethered cable has a diameter of 30 mm and a length of
approximately 30 m, which provides horizontal tension for the buoy’s
body, avoiding the vertical tension of the conventional anchor tether,
and ensuring that the buoy does not descend owing to the large anchor
tension typical of harsh environments. The anchor oat is made of EVA
(Ethylene Vinyl Acetate Copolymer) foam with a diameter of 1 m and
oats above the water surface, providing a vertical recovery force for the
anchor system to offset its vertical tension. The lower cable and anchor
chain jointly provide anchor mooring tension. The cable has a diameter
of approximately 30 mm and a length of approximately 15 m, while the
anchor chain is a 24-mm diameter geared anchor chain with a length of
27.5 m. The anchor is a combination of naval and gravity anchors,
providing grip on the entire anchor train.
4.2. Mechanical analysis of the transverse mooring system
The forces acting on buoys under real sea conditions include wind
resistance, current resistance, wave force, and mooring force. The hor-
izontal mooring force provided by the transverse anchor system is
balanced with the horizontal component forces of the wind, waves, and
current, and the inclination moment of the buoy generated by the hor-
izontal external force is balanced with the recovery moment of the buoy
after tilting.
In Fig. 8, due to the truss structure, the structural feature scale is
much smaller than the wavelength, and the structure is xed relative to
the wave, which can be simplied to a cylindrical structure. The wave
Fig. 4. Diagram of the self-counterweight semi-submersible buoy.
Fig. 5. Diagram of the solar power and equipment at the top of the buoy.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
5
force can be calculated according to the Morison equation, while the
wind resistance and current resistance are estimated according to the
empirical formula.
According to the Morison equation, the horizontal wave force acting
at any height z of the cylindrical buoy and column height dz is expressed
as follows:
dFwave =1
2CD
ρ
Du2
xdz +CM
ρπ
D2
4
˙uxdz (1)
where CD is the drag force coefcient, CM is the additional mass coef-
cient,
ρ
is the uid density, D is the diameter of the cylinder, ux is the
horizontal velocity of the wave water quality point, and ˙
ux is the
horizontal acceleration of the wave water quality point.
The horizontal wave force acting on the buoy can be integrated with
the volume of the buoy as:
Fwave =∫d+a
0
1
2CD
ρ
Du2
xdz +∫d+a
0
CM
ρπ
D2
4
˙uxdz (2)
where d is the total length of the cylindrical buoy and a is the amplitude
of the incident wave.
According to the empirical formula, the wind and current resistances
acting on the buoy’s body are expressed as follows:
Fwind =1
2CD
ρ
Av2(3)
where CD denotes the drag force coefcient,
ρ
is the uid density, A is
the wind/current projection area of the buoy, and v is the wind/current
velocity.
In Fig. 9, based on the above theoretical formulas, the HydroStar
boundary element analysis software developed by Bureau Veritas Ser-
vices (France) is used to establish a surface element model of the buoy
and divide the surface mesh. The forces of wind, wave, and current on
the surface of the buoy are obtained through integration of each mesh.
Considering the worst working conditions of wind, waves, and cur-
rents in the same direction, after software analysis and calculation, the
combined horizontal force F
e
of the three was approximately 4800 N,
and the resultant action point was located He(3.1 m) above the lowest
point of the buoy’s body.
Due to processing errors, structural simplication, and equipment
layout, the transverse anchor connection point may deviate from the
theoretical calculation result point by approximately 10%, and the dis-
tance between the two is approximately of 0.31 m. The tilting moment
M
e
generated by the wind, waves, and current is:
Me=Fe•S(4)
Fig. 6. Optimal design of the buoy structure.
Fig. 7. Diagram of the horizontal anchorage structure of the semi-submersible buoy.
Fig. 8. Diagram of the force model of the cylindrical buoy.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
6
The recovery torque of the semi-submersible buoy is primarily pro-
vided by the tumbler structure. The three-dimensional model of the
buoy showed that the total displacement G of the buoy was 1384 kg, and
the distance between the center of gravity and the center of gravity L
BG
was 1.678 m. When the buoy’s inclination angle is θ, its recovery
moment M
f
is:
Mf=G•L•g•sin θ (5)
In the three-level sea state, when the semi-submersible buoy is in tilt
equilibrium, the inclination of the semi-submersible buoy is: Me=Mf.
θ=arcsin (Fe•S
G•L•g)=3.74◦(6)
From the above analysis and calculation, it can be concluded that the
design of the transverse anchorage semi-submersible buoy meets the
requirements for the inclination angle of the buoy’s body to be less than
10◦under a three-level sea state.
4.3. Numerical simulation analysis of semi-submersible buoy
Although traditional potential ow theory and the Morison equation
can quickly estimate the force and inclination of a buoy, they cannot
accurately simulate a oating body in a real ow eld. With advance-
ments in modern computer performance, it is now possible to simulate
the viscous ow eld near the buoy using the uid simulation software
Fine/Marine, and consider the coupling effect between the anchor moor
and buoy.
In this study, a full-scale model of a semi-submersible buoy was used
as the object of calculation to establish the calculation domain for uid-
structure interaction. In Fig. 10, the calculation domain was a cuboid
and was divided into four parts: the background domain, overlapping
mesh domain, wave elimination domain, and wave generation domain.
The size of the background domain was 100 m ×100 m ×75 m with a
water depth of 50 m. The size of the overlapping grid domain was 7 m ×
4 m ×20 m, and the length of the wave-suppression area was twice the
wavelength. The wave generation area is located 1.5 times of the
wavelength in front of the buoy, and the source function wave gener-
ating method is used for wave making. The buoy model was placed at
the center of the calculation domain.
In Fig. 11, to achieve accurate simulation results and minimize the
computational workload, a ne mesh was created around the buoy, with
the surface mesh size not exceeding 0.1 m, and the maximum mesh size
in the overlapping grid domain was 0.3 m. In order to precisely capture
the uctuations of the free liquid level in the calculation domain, the
free liquid level was encrypted throughout the entire domain. The total
number of computing domain grids was 15 million, of which 10 million
were overlapped grid domains and 5 million were background domain
meshes.
Using k-
ω
SST (Shear Stress Transport) turbulence model set up a
two-phase ow in the calculation domain, with air in the upper part and
seawater in the lower part. The VOF (Volume of Fluid) method was used
to accurately capture the free liquid surface in the calculation domain.
Appropriate boundary conditions were set on the outer surface of the
Fig. 9. Diagram of the mechanical analysis of the semi-submersible buoy.
Fig. 10. Schematic diagram of the calculation domain.
Fig. 11. Schematic diagram of the calculation domain meshing.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
7
calculation domain and combined with numerical wave technology to
simulate wind, waves, and current conditions under three-level sea
states. Based on the aforementioned transverse anchorage mechanical
analysis results, a mooring point was established 3.1 m above the lowest
point of the buoy to limit the horizontal displacement of the buoy while
allowing rotational displacement. This simulation accounted for longi-
tudinal movement of the buoy in wind, waves, and current, the load
change of the mooring force, and ensured that the buoy always remained
in a reasonable calculation domain. To reduce the number of simulation
calculations, wind, current load, and wave load were simulated as two
separate working conditions, and the simulation results were as follows
in Fig. 12:
Based on the above simulation results, it was observed that the
longitudinal angle of the buoy under the combined effect of wind, cur-
rent, and wave loads, was approximately 5◦, which is consistent with the
theoretical analysis results. The wave load caused a signicant change in
the mooring force, with a maximum value of approximately 2800 N.
However, the mooring force gradually stabilized over time under the
inuence of wind and current load, reaching an average value of
approximately 1850 N. The combined force between the two is in close
agreement with the theoretically calculated value.
Overall, the results indicate that the mechanical analysis of the
transverse anchorage system nearly matches the numerical simulation
analysis results of the semi-submersible buoy. The swing angle of the
buoy’s body under the three-level sea state was less than 10◦, which
demonstrates that the semi-submersible buoy design and the anchor
system design proposed in this study meet the requirements for optical
buoys in complex sea states.
5. Design of the data collection and control module
The function of the buoy data collection and control module is to
achieve high-speed collection, control, and transmission of ocean optical
parameters and buoy attitude. In Fig. 13, the data acquisition controller
collects ocean optical parameters (mainly including data from above
water irradiance sensors and underwater multi-layer irradiance and
radiance observation sensors) through data logger(strox), as well as
buoy attitude data. After encoding the observation data, it is transmitted
back to the shore station through the communication network.
The buoy data acquisition controller is selected as Raspberry Pi Zero
& expansion board SIM868. The radiance radiometers and irradiance
radiometers are selected as HYPEROCR hyperspectral radiometer from
Satlantic company(Canada). The attitude sensor is selected as TCM2.5
Fig. 12. The simulation results.
Fig. 13. Concept design of the data collection and control.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
8
from PNI company (United States).
6. Buoy development and offshore deployment
This buoy has a long cylindrical structure that differs from conven-
tional disc-type buoys. The deployment of such a buoy is also a problem
that must be resolved. The deployment plan is as follows. Step1:
Considering that the buoy’s buoyancy is mainly concentrated in the
lower part of the structure, the upper buoyancy is very small. Therefore,
before deployment, a oat was added to the upper part of the buoy to
provide sufcient buoyancy. This ensures that after the buoy is sus-
pended in the water, it can be balanced up and down, lying at in the
water, without ipping over. Step2: To deploy the buoy, it was rst
placed horizontally on the water, and a self-counterweight was fastened
to the side of the boat. The buoy was then towed to the deployment site
with the ship, while gradually releasing the self-counterweight to ach-
ieve an upright position in the water. Step3: Once the buoy was stable,
the horizontal anchor was securely fastened to complete the deployment
process.
After successful development of the buoy prototype and laboratory
testing, the system was installed and adjusted at Qingdao Wharf, as
shown in Fig. 14.
After installing the buoys on the pier, they were launched and
deployed. Prior to the launch, the buoy’s body was not connected to the
counterweight, and the underwater instrument rack was in a folded
state. Since most of the buoyancy of the buoy’s body is concentrated in
the lower part, a suitable number of oats were temporarily bundled in
the upper part to provide additional buoyancy and maintain overall
horizontal oating for easy towing. The status of the buoy after it was
hoisted and launched is shown in Fig. 15. The anchor, counterweight,
and sensors were hoisted onto the deployment vessel, and the buoys
were towed to the Wheat Island sea area.
After arriving at the deployment area, the sensors were installed and
the buoy’s body was connected to the counterweight. The counter-
weight was then slowly released using an onboard winch until the buoy
was in a vertical state and the temporary oats were removed.
Subsequently, the anchor bolts were deployed separately in accordance
with the conventional buoy placement operation processes. Finally, the
semi-submersible was connected to an anchor motor. On October 25,
2020, it was successfully deployed off the coast of Qingdao, as shown in
Fig. 16. A large 10-m buoy was deployed nearby for long-term contin-
uous observation of the marine environment, which could provide data
support for the performance study of semi-submersible buoys, as illus-
trated in Fig. 17.
Fig. 14. Self-counterweight optical buoys were installed at the pier.
Fig. 15. Self-counterweight optical buoys being deployed with the ship.
Fig. 16. Self-counterweight optical buoy deployment at sea.
Fig. 17. Self-counterweight optical buoy offshore deployment position map.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
9
7. Analysis of the results from the buoy experiment
After the successful deployment of the buoy prototype, it operated
effectively for nearly seven months, from October 2020 to May 2021.
This is the rst report on the long-term continuous operation of optical
verication and calibration buoys in China’s complex and harsh offshore
waters. The signicant wave height (Hs) and maximum wave height
values reached 2.1 m and 3.3 m, respectively. The maximum wind speed
reached 22.6 m/s, and the maximum current speed reached 110 cm/s.
These results demonstrate that the buoy has good survivability at sea.
The upper part of the buoy was equipped with an attitude measurement
sensor (TCM2.5, developed by the Premier Sensor Technology Company
in the United States), which measured the buoy’s attitude in real-time at
a frequency of 1 Hz.
For the statistical analysis of the buoy’s offshore observation data,
the signicant wave height was divided into four situations: Case I
(signicant wave height 0.1–0.5 m), Case II (signicant wave height
0.6–1.0 m), Case III (signicant wave height 1.1–1.5 m), and Case IV
(signicant wave height 1.6–2.1 m). Typical data were selected for
analysis in each situation, including a buoy attitude data curve and
corresponding wind and wave current data curves. Table 1 lists exam-
ples of buoy’s swing angles under different sea states, and Figs. 18–25
show the corresponding buoy’s swing angles and sea state curves.
Observations of the data show the buoy’s swing angle was within
±5◦under a three-level sea state (signicant wave height ≤1.5 m), while
it stayed within ±10◦under high wind and wave conditions. These re-
sults generally demonstrate that the buoy’s stability meets the design
requirements for optical buoys.
Case I: Signicant wave height 0.1–0.5m.
Case II: Signicant wave height 0.6–1.0m.
Case III: Signicant wave height is 1.1–1.5m.
Case IV: Signicant wave height 1.6–2.1m.
Based on the above observations, it can be concluded that the buoy’s
pitch inclination remained within ±5◦, and the roll inclination remained
within −5◦–15◦even under harsh sea conditions. This suggests that the
pitch inclination of the buoy’s pitch was mostly symmetrical, while the
roll inclination had a systematic error of 5◦. This xed tilt of 5◦in the roll
inclination may have resulted from the engineering process error in
positioning the horizontal mooring point, which should be minimized in
subsequent tests.
8. Conclusions
Taking into account the limitation of current optical satellite veri-
cation and calibration buoys and the demands of complex marine en-
vironments, this study proposes an innovative transverse-tethered semi-
Table 1
The buoy’s swing angle under different sea states.
No Date Marine environment Pitch (◦) Roll (◦)
Hs (m) Cmax (cm/s) Wmax (m/s) Min. Max. Range Min. Max. Range
1 2020-11-22 0.3 80 15 −2 4 6 0 8 8
2 2020-11-12 0.8 105 13 −5 5 10 0 10 10
3 2021-5-10 1.7 75 19 −5 5 10 −5 15 20
4 2020-11-18 2.1 110 12 −10 10 20 0 20 20
(Hs is the signicant wave height, Cmax is the maximum current speed, and Wmax is the maximum wind speed).
Fig. 18. buoy’s 24-h tilt attitude data on November 22, 2020.
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Ocean Engineering 290 (2023) 116217
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submersible optical buoy scheme with a counterweight that overcomes
the difculties associated with the design, processing, and deployment
of traditional taut semi-submersible buoys. This can be accomplished
using ordinary boats and conventional buoy placement techniques,
which are simple, well-established, and reliable. The buoyancy cylinder
of the buoy is dispersed and downstream-oriented to achieve high sta-
bility, reducing the difculty of placement and maintenance.
Additionally, the buoy system retention point is located at the force
balance point of the buoy, overcoming the issues caused by the tradi-
tional taut semi-submersible buoyancy cylinder being located at the root
of the buoy, which is heavily impacted by currents. Furthermore, a
water sail was installed on the buoy to adjust its orientation in real-time
to achieve a stable posture during operation. Through simulation anal-
ysis, the prototype was developed. Sea tests were carried out for seven
Fig. 19. 24-h sea situation in the buoy’s deployment area on November 22, 2020.
Fig. 20. buoy’s 24-h tilt attitude data on November 12, 2020.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
11
consecutive months, and optical parameters and buoy’s attitude pa-
rameters were achieved, including irradiance (in air), multi-layer un-
derwater irradiance and radiance, and pitch and roll of buoy, etc. The
results showed that the buoy’s swing angle was better than ±10◦under
the 3–4 level sea state, effectively reducing the impact of the wind and
waves, as well as the difculty of deployment.
At the same time, it was also noted that due to the limitations of the
buoy structure, solar panels cannot be installed too much, posing sig-
nicant challenges for long-term high-power supply. New renewable
energy supply technology is the important focus of future research.
In summary, the innovative approach presented in this study was
shown to improve the stability of the buoy and reduce the impact of
Fig. 21. 24-h sea situation in the buoy’s deployment area on November 12, 2020.
Fig. 22. buoy’s 24-h tilt attitude data on May 10, 2021.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
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target occlusion on observation, accumulating valuable experience in
buoy deployment and maintenance and providing a new scheme and
equipment for the calibration of marine satellites in harsh and complex
sea areas.
Funding
This work is supported by the National Natural Science Foundation
of China (41976179), National Key Research and Development Program
(2022YFC3104201, 2018YFC0213103).
CRediT authorship contribution statement
Shizhe Chen: Writing – original draft, Validation, Methodology,
Investigation, Conceptualization, Data curation. Jiming Zhang: Meth-
odology, Investigation, Formal analysis, Software. Shixuan Liu:
Fig. 23. 24-h sea situation in the buoy’s deployment area on May 10, 2021.
Fig. 24. buoy’s 24-h tilt attitude data on November 18, 2020.
S. Chen et al.
Ocean Engineering 290 (2023) 116217
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Methodology, Writing – review & editing, Funding acquisition, Super-
vision, Project administration. Bangyi Tao: Methodology, Validation,
Formal analysis. Yushang Wu: Visualization, Investigation, Formal
analysis, Software. Xiaozheng Wan: Methodology, Investigation,
Formal analysis, Software. Yuzhe Xu: Visualization, Investigation,
Formal analysis, Software. Miaomiao Song: Visualization, Investiga-
tion, Formal analysis, Software. Xingkui Yan: Resources, Data curation,
Investigation. Xianglong Yang: Resources, Data curation, Investigation.
Zhuo Lei: Resources, Data curation, Investigation.
Declaration of competing interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
We would like to especially thank all teams involved in the research
and development for their help.
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