volume: 1, year: 2018 - rina-imarest mjbscmonohull and semi-swath by means of prediction....
TRANSCRIPT
A Publication of
Volume: 1, Year: 2018
*Corresponding author: [email protected] 1
Journal of
Marine Science, Engineering & Technology Webpage: https://jmset.rina-imarest-mjbsc.org
JMSET 2018, Vol 1
HULL RESISTANCE AND SEAKEEPING OF MONOHULL AND SEMI-
SWATH
Yahya, M.A.F.
1 and Maimun, A.1,2*
1Department of Aeronautics, Automotive and Ocean Engineering,
School of Mechanical Engineering,
Faculty of Engineering,
Universiti Teknologi Malaysia,
81310 Skudai, Johor Bahru.
2Marine Technology Centre,
Universiti Teknologi Malaysia,
81310 Skudai, Johor Bahru.
ABSTRACT
Hull resistance and seakeeping ability of a vessel are usually being used as indicators for their
performances when operating as crew boat. With many development of hull shape has been done
before, every type of hull has its own advantages as well as the drawbacks. In this paper, hull
resistance and motion response of two different vessels, monohull and semi-SWATH has been
compared to determine which type of hull offers good characteristics in resistance and seakeeping
ability. Both types of hull were compared at same mass displacement. Numerical simulation for both
analyses was carried out by using Maxsurf Resistance and Maxsurf Motions software tools. The result
show that semi-SWATH experience low hull resistance compared to monohull. While, for seakeeping
ability, monohull has good hydrodynamic characteristics in heave and pitch motions. For rolling
motion, result showed that semi-SWATH has good hydrodynamic characteristics.
Keywords : Hull resistance, seakeeping, hydrodynamics, monohull, semi-SWATH, Maxsurf.
1.0 INTRODUCTION
In the last decade, many comparative studies on performances and reliability between two or more
types of hull have been conducted. Offshore Support Vessel (OSV) is widely used in meantime to
provide an assist on offshore exploratory and production activities. Crew boat is usually used as main
transportation to transfer workers from onshore to oil rig. A suitable type of hull use as crew boat can
enhance the performance of the vessel especially on rough sea. The standard monohull concept is
more than 30 years old and some deficiencies have been noticed and confirmed [1]. Inventions for
new type of hulls were conducted as alternatives to this problem. Hence, new concept of hull which
refers to multihull was introduced like Small Waterplane Area Twin Hull (SWATH), and Surface
Effect Ships (SES) [2]. Demands on crew boats design required it to have broad capabilities, not only
for low resistance and good hydrodynamic characteristics but also can offer high number of payload
and broad main deck area for supply carried.
The comparison should be taken on the similar displacement or dimension in order to
distinguish each hull’s abilities. Study conducted by [3] on comparison of hull resistance between
monohull and SWATH at same displacement has yield that there is no significant difference in hull
resistance between these two hull concepts. Moreover, comparison study on resistance between
monohull, catamaran, and semi-SWATH has showed that, semi-SWATH has low percentage of
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variation which means the body is most streamline when moving through the water on constant speed
[4]. For seakeeping aspects, researcher [5] study on seakeeping characteristics between monohull and
SWATH at same displacement has result that SWATH offer great potential in terms of superior
seakeeping characteristics. Furthermore, hydrodynamic characteristics such as added mass or added
inertia for semi-SWATH will decrease exponentially when the frequency is larger [6]. Besides, study
carried by [7] has concluded that multihull vessel shows better characteristics in heave and roll
motions.
The objective of this paper is to compare hull resistance and seakeeping ability between
monohull and semi-SWATH by means of prediction. Predictions were done by using simulation
method for both analyses. The prediction of hull resistance and seakeeping of vessel is essential in
early stage of design. Hopefully it will contribute to naval architect to distinguish the ability of each
type of hull.
2.0 METHODOLOGY
2.1 Software tools
For this study, design of 3D surface model, resistance analysis, and motion response analysis were
applied Maxsurf suite. MODELER, RESISTANCE, and MOTONS are three tools from Maxsurf suite
that were used in this study.
2.1.1 Maxsurf MODELER
This software is used to construct 3D surface modelling for monohull and semi-SWATH. The design
will be based on the existed monohull ship which is Borcos Firdaus 11 [8]. Hydrostatic particulars for
complete model presented on Table 1. The main concern is the displacement of both models which
should be similar.
2.1.2 Maxsurf RESISTANCE
It estimates the resistance and power requirements for any Maxsurf design using industry standard
prediction techniques. Slender body method and wave pattern method were applied to determine hull
resistance and the formation of wave around the hull body. Results were presented in graphical form.
It will consist of total resistance and wave pattern generated by the hull. Speed for the analysis was
taken at range between 5-25 kn and set the design speed is 25 kn.
2.1.3 Maxsurf MOTIONS
It is seakeeping analysis program in the Maxsurf software suite. It uses the Maxsurf geometry file to
calculate the response of the vessel to user-defined sea conditions. Linear strip theory analysis method
was choosing to determine vessel’s motion response at zero-speed conditions. The analysis done for
deep water condition and comparison of results was based on Response Amplitude Operator (RAO)
of each vessel in heaving, pitching, and rolling.
3.0 RESULTS AND DISCUSSION
3.1 Resistance Result
Comparison of resistance for both monohull and semi-SWATH were measured on total resistance,
and wave pattern.
3.1.1 Total Resistance
Figure 1 shows the total resistance for both models at their respective speed.
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Figure 1: Comparison total hull resistance
3.1.2 Wave Pattern Generated
Both Figure 2 and Figure 3 show the wave pattern generated by monohull and semi-SWATH at
Froude number 0.4.
Figure 2: Monohull wave pattern
0
50
100
150
200
250
300
350
0 5 10 15 20 25 30
Tota
l Res
ista
nce
, RT
(kN
)
Speed, V (kn)
RT vs V
Total Resistance(kN) monohull Total Resistance(kN) Semi-SWATH
4
Figure 3: Semi-SWATH wave pattern
From the result gained, it clearly shows that at low speed, total resistance for both types of hull has no
significant differences [3]. But, when speed exceeding 15 kn, monohull experience higher resistance
compared to semi-SWATH. At design speed, monohull has excess resistance compare to semi-
SWATH by 13.6%. For the wave pattern generated on Froude number 0.4, monohull produce wake
with large wake angle compare to semi-SWATH’s pattern.
3.2 Resistance Result
For determining seakeeping ability, the comparison between hulls was done by comparing wave
induced forces, hydrodynamics reactions forces, and RAO in heaving, pitching, and rolling motion.
3.2.1 Wave Induced Forces
Wave induced forces are made up by two components, namely pressure field in undisturbed wave and
pressure field from diffraction effects. The first force component is associated partly with the
geometry of the waterline; while the second component is associated with the volume and shape of
the submerged part of the hull.
Figure 4 to 6 show the magnitudes of wave induced forces in heave, pitch and roll mode for
both types of hull. From the graphs, heave and pitch excitation for both hull forms differ in magnitude
at lower frequency and become unity when frequency increase. Contrary for roll motion, the
excitation of semi-SWATH is dominated strongly when compared to monohull.
Figure 4: Heave excitation
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Figure 5: Pitch excitation
Figure 6: Roll excitation
3.2.2 Wave Induced Forces
Forces acting on the hull as a result of motion responses are expressed by an added mass/inertia and a
damping term. Figure 7 to 12 compare the results for heave, pitch, and roll modes. From the curves, it
shown that in heave and pitch motion, the hydrodynamic reaction forces of semi-SWATH is reflected
by relatively small added mass/inertia and wave making damping. While, for roll motion, both added
inertia and damping are substantially greater than observed for monohull.
Figure 7: Heave added mass
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Figure 8: Pitch added inertia
Figure 9: Roll added inertia
Figure 10: Heave damping
Figure 11: Pitch damping
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Figure 12: Roll damping
3.2.3 Response Amplitude Operator (RAO)
Figure 13 to 15 show the RAO of monohull compared with semi-SWATH for heave, pitch and roll
modes in head seas and beam seas. The lightly dampened modes of motion are characterized by sharp
peaks. Graph show that for heave motion, response for semi-SWATH is slightly higher than monohull,
in pitch, semi-SWATH has higher response, but opposite for roll motion where it has low response
compared to monohull.
Figure 13: Heave RAO
Figure 14: Pitch RAO
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Figure 15: Roll RAO
4.0 CONCLUSION
Comparison of both parameters by using prediction simulation has yield that monohull has higher hull
resistance at high speed compared to monohull. For seakeeping ability, monohull has good motions in
heave and pitch, but poor in rolling. These are vice versa for semi-SWATH vessel.
REFERENCES
1. M.Suljic, “Deterrmination of Priorities in Design Lightweight, Fast, Patrol Crafts”, Master
thesis, Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, 1992.
2. Papanikolaou, A. D. “Review of Advanced Marine Vehicles Concepts”, Norwegian Maritime
Technology Forum, November 22-23, 2001.
3. Medaković, J., Dario, B., & Blagojević, B. “A Comparison of Hull Resistances of a Mono-Hull
and A SWATH Craft”, International Journal of Engineering Science and Innovative Technology
(IJESIT) Volume 2, Issue 4, July , 155-162, 2013.
4. Hermie, A. S. “Comparison Study on Ship Resistance Among Monohull, Catamaran and Semi-
SWATH”, UTM Degree Thesis.2009.
5. Dallinga, I. R. “Seakeeping Characteristics of SWATH Vessels”, Maritime Research Institute
Netherlands (MARIN).
6. Salam, A. “Seakeeping Assessment of a Semi-SWATH Design”, UTM, Degree's Thesis.2007.
7. Luhulima, R. B., & Setyawan, D. “Selecting Monohull, Catamaran and Trimaran as Suitable
Passenger Vessels Based on Stability and Seakeeping Criteria”, The 14th International Ship
Stability Workshop (ISSW), (pp. 262-266). Kuala Lumpur. 2014.
8. Research, Clarkson. Malaysian Firms Takes Delivery of Crewboat Built in Singapore. Miri,
Sarawak, 8 September, 2005.
*Corresponding author: [email protected] 9
Journal of
Marine Science, Engineering & Technology Webpage: https://jmset.rina-imarest-mjbsc.org
JMSET 2018, Vol. 1
DEVELOPMENT OF REMOTELY OPERATED VEHICLE
UNDERWATER ROBOT
Aminuddin, M. H., Md Zain, M. Z., Nor, N. S. M., Mastura, A. W.
1 Department of Applied Mechanics,
School of Mechanical Engineering,
Faculty of Engineering,
Universiti Teknologi Malaysia,
81310 Skudai, Johor Bahru.
ABSTRACT
Nowadays, Remotely Operated Vehicle (ROV) robot has been widely used in industry especially in oil
and gas sector. It has been used to do a task inside the sea water environment besides has a capability
to perform a deep sea rescue operation and recover objects from the ocean floor. Development on
the design of the ROV underwater robot is done to increase the performance of the robot in the ocean.
This project will discuss the steps from designing the ROV until its prototype construction. There are
several steps need to be followed in order to produce efficient mechanical structure or design of the
ROV underwater robot. In this project it consist three preliminary designs and one final design. The
final design is produced through evaluation process of three preliminary designs of ROV.
Computational fluid dynamic (CFD) software is used in order to analyse and identify the drag
coefficient of the ROV underwater robot structure. Other than that, other software and calculation is
used to determine the behaviour of the robot inside the water. This thesis also will provide the
overview of the process in designing, constructing and testing the ROV with respect to mechanical
part only. The main material used for this ROV is Aluminium square hollow and Acrylonitrile
butadiene styrene (ABS). Entire joint and holder used in this ROV is custom made in order to
maintain the originality of the ROV design. All the steps are carefully conducted in order to design
and construct an effective structure for underwater robot in term of its drag coefficient and stability
performance.
Keywords : Acrylonitrile butadiene stryrene (ABS), Aluminium hollow bar, Ocean.
1.0 INTRODUCTION
Nowadays, it is common for remotely operated vehicle or ROV are used to extract images from the
sea and solve environmental problems such as removing the waste that can cause water pollution. A
tether is used as signal transmitting between the operator and robot since radio signal cannot be used
to any depth of water greater than 1m. In recent years, ROV become popular due to replacement
human role in work at dangerous underwater condition for a specific task.
The Remotely Operated Vehicles (ROVs) received increasing attention because of its
significant impact in several underwater operations. Examples are in monitoring and maintenance of
off shore structure or pipeline or the exploration of the sea bottom. Skilled human operator is needed
to operate, control and in charge of command vehicle; a failure detection strategy will help in human
decision making.
However, ROV system will not completely replace divers in the near future due to the
weaknesses and lack of the sensory feedback needed to complete a task. But the ROV, in many cases,
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can replace the putting a human in dangerous condition or environment. Other than that, using the
ROV also can simplify the human work. Human only needed to searching and monitoring the ROV
thus require less effort as well as less risk for human when using the ROV [1-7].
2.0 PROJECT METHODOLOGY
There are few steps in order to achieve the objective that is to build the efficient structure of the ROV
underwater robot. The efficient structure means the robot has the ability to move smoothly and also
high stability when it going into the sea water. Other than that, it also must have enough strength to
experience the sea water pressure at the required depth. In this case, material selection is important
aspect to ensure the frame do not deform plastically when it performs in the required depth due to the
high pressure inside the sea water.
Next step is to understand the function of every compulsory part in the ROV underwater robot.
The knowledge on function of every compulsory part will guide to the effective usage on every part.
Besides, studies about previous design is also important because it can avoid from doing the same
mistakes or weaknesses that has be made on previous design. For example, the usage of welding and
rivet as a joining part will make the robot is fixed and cannot be adjustable. Besides that, it can make
the robot became difficult to undergo maintenance process. This mistakes had been applied on robot
ROV RECFRS when it entering the ROV competition.
Research background on ROV is studied in detail after the analysis on design weaknesses and
strength is completed. The purpose for this step is to determine the components that are needed in the
ROV. ROV is used in the water so that there are several equipment needed to make sure our robot
stable when it operates in order to complete the task. This step will show the importance of every part
needed in the ROV mechanical design.
This is the step where all the data or knowledge from previous step is needed to apply it into the
design. This step is called design process. This step required SolidWork® software. This software is
used as a 3D-drawing tool to draw the ROV design. In this stage, team member is required to give any
idea about the design that compatible to do the task given. In design process also require other
software to check the drag force occur to the robot during the movement in the water despite to check
the stability of the robot during movement. The software is called ANSYS 16.0. This software is the
computational fluid dynamic software and used to check the behaviour of our robot in the water
theoretically. In order to produce accurate data, there are several tests that are conducted to get the
correct parameter. The tests are grid independent test, solver test and turbulence model test.
Last step is the construction of the ROV. This step required the skill of construction of team
member. Team work among team member also important to make sure the prototype design
construction follow the Gantt Chart. In this stage also it is critical where problem will appear. All the
technical knowledge and experience is needed to solve the problem during prototype design
construction of ROV.
After the construction process, there are several tests are conducted to make sure the efficiency
of the robot in term f stability and maneuvering system of the robot. The main purpose of conducting
the tests is to ensure some development on ROV underwater robot has been made to achieve the
objective of this study.
3.0 DEVELOPMENT OF ROV
In this section, it will explain the steps before the construction of the ROV underwater robot. There
are a few process involved in this section that are design specification, ROV’s final design and
analysis of the design. The final design produced based on three preliminary designs evaluation.
Entire analysis is used to check the behaviour of the ROV in the water.
3.1 Design specification
There are few important aspects need to be in a ROV. The specifications of the ROV were listed
before proceed to the design process and construction of the ROV. The specification is based on the
design requirement and it is different compared to standard industry’s ROV due to money and time
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constraint to build the ROV. Developments of the ROV also need to be included in the design
specification. The specification of the ROV has been listed below:
1. Maximum operating depth must be 30m
2. The weight of the ROV must be less than 30kg
3. Length of tether must be at more than 30m
4. Electrical tank must be in the robot.
5. The part of the robot must be easily attached and detached
6. Shape of the robot must be hydrodynamic shape
3.2 Final Design concept
Final design is chosen based on evaluation on three preliminary designs. In final design, it can be
divided into three four structures: Frame, holder, electrical tank and manipulator arm. All of the
materials were used without further purification.
3.2.1 Frame Design
Figure 1 shows the frame of the robot. Frame is used as a base to install all the components or parts of
the robot. The body frame of our ROV was built using Aluminium hollow bar. The aluminium hollow
bar was used to make sure our ROV has the ability to float and sink easily. The dimension of
Aluminium hollow bar used in the frame is 30 cm x 40 cm. All the joint and connection of the frame
use the custom made joint that made from 3D print technique. Joint made up from 3D print technique
to make sure our joint easy to re-changeable and replaceable. This because almost failure is occurs at
the joint of the robot.
Figure 1: Final design frame
3.2.2 Customize joint and holder
Figure 2 shows several joint and holder that had been used in the ROV. All the joint and holder were
custom made to maintain the originality of the ROV design. This will make the ROV different
compared to other ROVs. Joint and holder used were made from ABS material. 3D printer was used
to produce all the parts that use ABS material. The most important mechanical properties of ABS are
impact resistance and the toughness. This will make this material suitable to be used as a joint and
holder of the ROV. Other than that, ABS also has a strength, flexibility and machinability that make it
a preferred plastic for ROV application.
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Figure 2: The customize holder at ROV.
3.2.3 Electrical tank
Figure 3 is the container that acts as electrical tank of the ROV underwater robot. All the electric
circuit that cannot be exposed to the water is placed at one aluminum container with the dimension 30
cm x 40 cm. In this container, there were several holes used for the wire to give the signal to motor
and sensor. The electrical tank was closed with Perspex plate. Between Perspex plate and the
container there was rubber to seal the gap between the Perspex plate and the aluminum container to
prevent the water from leaking. All the electronic par including lighting, power, LAN, motor and the
signal cables are installed within a PVC cylinder. Epoxy was used to seal the PVC fitting wires
together.
Figure 3: The electrical tank
3.2.4 Customize joint and holder
Figure 4 shows the manipulator arm design at the ROV. Another feature that ROVs should have is the
manipulators. Manipulators are mechanical arms that are able to perform various jobs underwater.
Because the underwater environment is not suitable and very dangerous to humans, using remotely
manipulated mechanical arms is a natural way to perform subsea work. The main objective behind
creating the ROV is so it would complete the tasks given by interacting with some objects inside the
pool such as pick up and handle various objects that need to be moved while competing. The actuator
chosen for the manipulators was a pneumatic cylinder. Pneumatic cylinder was chosen due to its
advantage in reducing the complexity of the manipulator design, as well as simpler electronic circuit
to be used for controlling it. This manipulator was positioned on the lower front of the ROV in order
to attach a flange, install the cap over the flange and insert the cable connector into the port on the
power and communication hub. By putting some rubbers at the hand grip, the ability to grip can be
improved.
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Figure 4: Manipulator arm
3.3 Design Analysis
After completing the design process, the design was analysed using SolidWork simulation and
Computational Fluid Dynamic software. Other than that, some calculations also need to be conducted
to make sure the design fulfil all design requirements. The analysis was needed to ensure effective and
workable design after construction process and this will prevent from built the design that has not
fulfilled the design requirements. In this section, SolidWork® simulation and Computational Fluid
Dynamic software have been used to analyse the behaviour of the robot inside the water. Calculation
on maximum depth which the robot can withstand also was calculated to make sure the design
requirement had been fulfilled.
3.3.1 Solidwork Simulation
The objective of Solidwork simulation is to determine the behaviour of the frame in the water with
30m-depth. Solidwork Simulation is used to make sure the frame of the robot can withstand the high
pressure in 30m depth in the water. The change the depth will increase the pressure exerted at the
frame. Frame is the critical part in the 30m-depth due to high pressure of water exerted most is at the
frame. Other than that, frame also is the place where all the parts is placed and it must be strong to
stand in the depth and suitable to hold the part. The detail parameter used in this simulation is shown
at table below.
Table 1: Parameter of Solidwork analysis
Material of the parts Aluminium 6061 alloy
Acrylonitrile butadiene styrene (ABS)
Connection Global Contact (-Bonded)
Fixture Fixed on the leg of the robot
External Load Pressure on the frame : 402879 Pa
Force at electrical tank holder : 150N
Gravity with 9.81 m/s^2
Mesh detail
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3.3.2 Solidwork Result Analysis
Figure 5 shows the effect of pressure inside the water of 30m depth. In this figure, it shows that there
is no critical part experience the load inside the sea water. The most critical load exerted at the frame
is 2.134×107 Pa which is lower compared to modulus elasticity of the ABS and aluminium 6061alloy.
The safety factor of ABS due to the load or pressure at 30m depth can be calculated as below:
From the calculation, it shows that the ABS and aluminium alloy material used at the frame is
suitable to use and safe to withstand high pressure inside the sea water. It shows also there is no
plastic deformation of the ABS material when it exerted the pressure at the depth of 30m.
Figure 5: The result analysis on SolidWork® software
3.3.3 Computational Fluid Dynamic Simulation
The objective of this study is to measure the drag forces on underwater robot when changing the
motion speed in the horizontal direction at a constant wave speed (5 knot) at 30m depth.
In this problem, the enclosure was used to limit the observation area of the model. The
enclosure was used to define the inlet of the outlet of the problem. The type of enclosure used in this
problem was a rectangular shape and its inlet and outlet were placed at the front and back of the
model. Inlet speed used to solve the problem is 5 knot with the pressure 402879 Pa. The speed of the
inlet is referring to the ocean wave speed and the pressure is referring the pressure of the sea at the
30m depth. The reference line use is between the inlet and the outlet. Reference line is chosen to show
the reaction of the water flow toward the ROV underwater robot. The model used in this analysis was
a simplified model from the actual model to reduce the computational time for the analysis. The final
parameters were obtained after conducting several tests: Grid Independent test, Solver test and
Turbulence model test.
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Table 2: Parameter for CFD analysis
3.3.4 Computational Fluid Dynamic Result analysis
The Ansys Fluent software is used to find the drag coefficient exerted at the design body of the ROV.
There are two velocities used in this test which are when the robot in stationary and when robot is
move (0.5m/s). The drag coefficient of the ROV underwater robot design is 0.0813.The data shows
that the drag coefficient remains constant although the speed is changing. The drag coefficient is
depending on the shape of the structure of the ROV underwater robot. Next step is to change the drag
coefficient to drag force by using drag equation. Drag equation is a formula used to calculate the force
of drag experienced by design body due to the movement through a fully enclosing fluid. Drag force
was calculated based on drag coefficient produced all the data. In order to validate the result, journal
with title “Verification of CFD analysis method for predicting the drag force and thrust power of an
underwater disk robot” that have been done by Tae-Hwan Joung et al. [3]. In this journal, it shows
that when speed increasing, the drag forces exerted at the ROV also will increase. It is similar with the
data produced using CFD analysis. Table 3 shows the drag force produced when the robot in
stationary and in motion.
Table 3: Result of CFD analysis
Design condition Drag force (N)
During robot in Stationary ( 0 m/s ) 23.29
During robot move ( 0.5 m/s) 33.24
3.3.5 Buoyancy of the robot analysis
The objective is to identify the mass need to be placed at the robot so that the robot can submerge in
the water. One of the important aspects is the buoyancy of the ROV underwater robot. Since it needs
to float and sink at the water at the same time, the buoyancy is needed to make sure the robot can float
using a minimum amount of thrust. Too heavy will make the robot cannot float on the surface while if
it too light, it will make the robot difficult to be submerged in the water. So calculation is needed to
make sure the robot was not too heavy and not too light so that the robot can float and submerged
easily in the water. In the ROV, air is trapped in the electrical tank that gives the ability for robot to
float. Calculation is needed to make sure the robot can submerge by counter back effect of air in the
tank by adding the mass on the robot [6-7].
Grid size 0.015m
Pressure-velocity coupling SIMPLE
Pressure Second order upwind
Momentum Second order upwind
Turbulence model K-Epsilon
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∑
⁄ (Sea water density)
⁄
The weight need to be placed at the robot including the frame weight must be more than 8.21
kg for robot to submerge in the water. Too much buoyancy force exerted in the robot also will cause
difficulty for robot to submerge.
4.0 DISCUSSION
This is the section to discuss all the result taken before and after the construction of the robot.
Every decision is made to make sure the robot can perform during operation under the water. The
underwater test for performance evaluation for the ROV is conducted in UTM marine laboratory’s
towing tank. The test is conducted to ensure the robot has high stability to perform the task and can
move smoothly inside the water.
4.1 Construction of ROV Underwater Robot
There are many criteria need to be considered during choosing the suitable material for ROV
underwater robot. The consideration taken in chooses the materials are the corrosion resistance,
weight, strength to high pressure inside the water and oxidation resistance. So that, some analysis of
the material is taken and Aluminium 6061 alloy hollow bar and acrylonitrile butadiene styrene (ABS)
is chosen as a main material for ROV underwater robot. These materials are chosen due to the
strength of the material that can withstand with the high pressure besides has the good corrosion
resistance. Hollow bar is chosen as the shape for aluminium alloy due to the light weight of the
material and easy for machining process. The pneumatic system was used in controlling the vertical
movement of the robot. In fact, pneumatic system more reliable compared to the actuator system. In
Malaysia, it’s hard to find the suitable motor or thruster for ROV underwater robot which a powerful
thruster is needed to control the ROV’s movement. Combination of actuator system and pneumatic
system was used in the ROV to control the vertical movement of the robot. These combination make
the robot more unique compared to other industrial ROVs. Figure 6 shows the robot after construction
process.
4.2 Submerge depth of the ROV underwater robot
ROV is able to submerge into 30 metre based on analysis conducted by using SolidWork® software.
Practically, the ROV had been tested in 5 metre below bottom of swimming pool successfully without
any leaking problem and able to perform perfectly. Other than that, the electrical tank in the ROV can
withstand the pressure with the depth of 30 metre under the water. The tank was made up from
stainless steel and it has high tensile strength while for the lid was made from thick Perspex that can
withstand high pressure in water. Tank is the most critical part due to its function to place all the
microcontroller and electronic parts that are sensitive to the water. Practically, the tank had been
tested in 5m-depth of water without any leaking as a result of the test.
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Figure 6: Final ROV Design
4.3 Stability of the ROV Underwater robot
The ballast tank is inserted in the ROV underwater robot to be functioned as a stabilizer of the robot.
It is used to make sure the robot to be in correct orientation besides to prevent the robot from inverted
during operation of a task inside the water. The ROV frame initially show positive buoyancy of the
ROV means that the ROV unable to submerge in the water but this problem has been solved by
placing weightage to make sure the ROV can submerge easily. The weightage is used to make sure
robot can submerge and float easily. The ROV become stable and able to submerge, float and
successfully perform forward and reverse motion. The stability because of the design has symmetry in
axis that makes the robot become more stable. Figure 7 shows the ROV stability while moving in the
water.
Figure 7: ROV stability in water
5.0 CONCLUSION
Design and prototype construction of an underwater robot with manipulator arm according to the
design specification that has been made require careful analysis during the design and fabrication
phase. Entire decision is selectively made because it may affect the performance result of underwater
robot as well as delay the process. Simulation has been conducted to measure the efficiency of the
design performance of underwater robot in the sea water. Computational fluid dynamic software was
used to calculate the drag coefficient of the design. For a result, drag coefficient for the design is
0.0813 is similar to the half streamlined body and it also shows that the shape of the design is
hydrodynamic shape. Besides, SolidWork® simulation also was conducted to make sure the frame
has the ability to withstand the pressure at 30m depth. The efficient design had been made based on
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several simulations and design performance analysis after the prototype’s design developement
process.
REFERENCES
1. Muhammad Zuhdi bin Mohd Zin. Design and construction of remotely operated underwater
vehicle with manipulator arm. Undergraduate Project Report. Universiti Teknologi Malaysia.
2014
2. Tae-Hwan Joung, Hyeung-Sik Choi, Sang-Ki Jung, Karl Sammut and Fangpo He. Verification
of CFD analysis methods for predicting the drag force and thrust power of an underwater disk
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Form Design of Underwater Vehicle Applying CFD (Computational Fluid Dynamics).
JAMSTEC (Japan Agency for Marine-Earth Science and Technology) Yokosuka; 2010
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*Corresponding author: [email protected] 19
Journal of
Marine Science, Engineering & Technology Webpage: https://jmset.rina-imarest-mjbsc.org
JMSET 2018, Vol 1
PREDICTION ON THE INFLUENCE OF DIAMETER TO DRAUGHT
RATIO TO THE WAVE FREQUENCY MOTION OF CYLINDRICAL
FPSO
Edesson, M.
1 and Siow, C. L.1,2*
1Department of Aeronautics, Automotive and Ocean Engineering,
School of Mechanical Engineering,
Faculty of Engineering,
Universiti Teknologi Malaysia,
81310 Skudai, Johor Bahru.
2Marine Technology Centre,
Universiti Teknologi Malaysia,
81310 Skudai, Johor Bahru.
ABSTRACT
The cylindrical Floating, Production, Storage and Offloading (FPSO) has become the game changer
in the offshore industry. The focus of this research is to design cylindrical FPSO which suitable for
Malaysia seawater and to study the relationship between draught to diameter ratio towards the
dynamic motion of the cylindrical FPSO. Total five cylindrical models were sketched by using
AutoCAD with varies of design parameters and were exported into the Ansys AQWA to simulate its
motion behavior in wave which conducted in head seas condition based on the wave condition in
Malaysia. The results of Response Amplitude Operator (RAOs) are only focused on the heave and
pitch motion. Further analysis was conducted to obtain the motion response spectrum analytically in
which the Pierson-Moskowitz wave spectrum is selected to represent the wave condition in Malaysia.
The response spectrum obtained by mapping the Pierson-Moskowitz wave spectrum with the RAOs
from Ansys AQWA. The cylindrical FPSO with the lowest motion response amplitude was selected as
the most suitable model for Malaysia wave condition.
Keywords : Cylindrical FPSO; RAO.
1.0 INTRODUCTION
In offshore industry, the study on the floating structures becomes the crucial subject matters especially
the hydrodynamic interaction effect on the motion of the floating structures is concern. In many cases,
development of offshore gas exploration industry, particularly are shifted into deep water. Global
demand of oil and gas energy is rising with time. According to Energy Information Administration
(EIA), Malaysia is a significant net exporter of oil and the second largest exporter of LNG in the
world behind Qatar. In other word, Malaysia maintain the credibility as one of the leading oil, gas and
& energy sector and remains one of South East Asia’s most dynamic owner oil & gas reserves [3].
The development of the cylindrical FPSO begin in 2001 by the Sevan Marine, company from
the Norway which began the study and develop the cylindrically-shaped FPSO designs The
cylindrical Floating, Production, Storage and Offloading (FPSO) was selected for this study.
Accoding to Almeida, The advantage of cylindrical FPSO in the development of the offshore industry
such that the large capacities of oil storage tank, the capability to adapt on the harsh environment and
20
convenience for maintenance and repair made the cylindrical FPSO the good application in the
offshore industry [1].
The wave condition of the seawater in Malaysia is one of the obstructions due to effect of the
sea wave loading, the proability of occurences of wave heights and wave periods which unable to
predict and hinders to assess this research. The study on wave and wind condition in Malaysia by
Chiang et al. shows that the wave condition and the direction of wave in Malaysia is influenced by the
monsoon wind from the northeast and also from the southwest. Furthermore, the wave and wind data
collected are derived from the marine surface observation, oilrigs and lighthouse respectively [2]. The
study implemented by Mirzaei et al. stated that the wave energy is higher at the East Malaysia (Sabah
and Sarawak) approximately 0.5 Wh/m2 and the wave data are third generation numerical wave
model of the NOAA WAVEWATCH III [9].
The heave and roll response amplitude operator (RAO) for the cylindrical FPSO is excellent
based on data from sea trial. According to Perunovic et al., the heave RAO of cylindrical FPSO is
smaller than the conventional ship shaped FPSO either in head or beam sea condition [11]. The heave
motion of the will change based on the waterplane area of the cylindrical FPSO. The pitch RAO will
affect by center of gravity (COG) of the FPSO. According to Wang et al., the cylindrical floating
bodies enable to generate large pitch motion response which ultimately alters the center of gravity and
it will affect the volume displacement respectively [13]. The motions characteristics of the cylindrical
FPSO are favourable regardless of the environment which enables it to withstand not only the harsh
environment, but also even in the harshest hurricane conditions respectively.
Motions and hydrodynamic coefficient of floating structures can be estimated using several
simulation methods. The basic hydrodynamic used in this study in predicting the motions of floating
structures is diffraction potential theory. According to Kvittem et al., the diffraction potential theory
can predict the hydrodynamic behaviour of large floating structure accurately [6]. This is because the
effect of wave diffraction is significant when the incident wave interacts with large floating structure.
When the motion of floating structure is dominant by the mass term or dominant by restoring force
term, the motion of the floating structure estimated by the diffraction potential theory is close to the
experiment result. However, the viscous effect is ignored by the diffraction potential theory, causing
the motion predicted by the theory at damping dominant region to become over-estimated
significantly [12].
Based on the available literatures, the weakness of the diffraction potential theory as
mentioned was also reported by Loken [7]. Loken found that the diffraction potential theory would
over predict the motion response of floating structure in damping dominant region due to under
prediction of radiation damping by the theory. Besides, Lu et al. reported similar finding in their
research when comparing the potential theory and viscous theory [8]. According to Lu et al., the
numerical results indicated that the viscous fluid model performs well in predicting the violent free
surface oscillation at the fluid resonance and leads to good predictions compared with the
experimental observations. However, the computational efforts are considerably excessive. They
found that the viscous theory mostly under-predicts the wave force in the calculation while the
potential theory over predicts the motion when the viscous effect is ignored in the approach of
potential theory [8].
The application of software Ansys AQWA was proven as the reliable commerical software to
study the hydrodynamic interaction of floating structure or marine structure. The comparative study
had been conducted by Nallayarasu and Prasad using experimental and Ansys AQWA to predict the
motion response of semi-submersible and TLP [10]. In their simulation they included the viscous
damping coefficient in the calculation to improve the numerical prediction in damping dominant
region. Also, Ansys AQWA also used by Yus-Farid to simulate the effect of main deck diameter to
the motion response of the designed X-round shape floating structure [15].
2.0 METHODOLOGY AND MATHEMATICAL MODEL
Since the scope of the research is focusing on the Malaysia seawater, the case study decides to select
Kanowit, Bintulu as the operation location for the cylindrical FPSO design in this research. The
location is selected because there is high in oil and gas activity, in which Kanowit is the location of
the PFLNG vessels. In short, Kanowit contain high content of natural gas basin. Secondly, as shown
21
in Figure 1, since the location 11 is near to the onshore, it will less time take to shuttle tanker ship to
reach the location of the cylindrical FPSO. Table 1 shows the wave condition of the location 11. Table
2 shows the particular of the cylindrical FPSO modelling for this study.
Table 1: Wave condition of the location 11 (Bintulu) [14]
Case study
location
Wave height
(m)
Wave period
(s)
Wave energy
(MWh/w)
Water depth
(m)
11 (Bintulu) 0.5 – 1.0 5-6 30.92 200
Figure 1: Case study location [2]
Table 2: Boundary modelling for cylindrical FPSO
Symbol Model
Diameter (m) 60-120
Depth (m) 30-50
Draught (m) 15-32
Free board (m) 15-18
D/T ratio 2.0-2.4
The variables are considered in this research to study the effect on the motions response of
cylindrical FPSO. In this study, the dependent variables are the motions response of cylindrical FPSO,
in which the RAOs only emphasize in heave motion response and pitch motion response because the
study only focus in head sea condition. The manipulation variable for this research is the diameter to
draught ratio (D/T). The constant variable that fixed in this research is the displacement of the
designed cylindrical FPSO.
In this study, RAO of cylindrical FPSO had been estimated by using commercial software,
Ansys AQWA. The RAO data from the Ansys AQWA were used to conduct a comparative study
between the RAOs of different cylindrical FPSO. Ansys AQWA estimated the RAOs of cylindrical
FPSO in frequency domain technique and this software were developed based diffraction potential
theory. In the wave frequency motion, the motion of model oscillate follow the frequency of wave.
The diffraction potential equation is normally written in Eular form in respective to the flow direction
as shown in equation (1) [5].
( ) [ ( ) ]
(1)
Where g is gravity acceleration, is the incident wave amplitude, and is the circular
frequency. The total of the diffraction potential ( ) is the summation of the incidents-wave potential
22
( ) and the scattering wave potential ( ). While radiating potential, is related to the wave
generate by floating structure in each direction of motions.
In addition, the equation (1) also fulfill the boundary condition as follow:
i. Laplace equation
and for 0 ≤ z ≤ h (2)
ii. Free surface boundary condition
and
at z = 0 (3)
iii. Sea bed boundary condition
and
at z = (4)
iv. Radiating condition
The condition stated that the diffraction potential and the radiating potential would
disappear when the distance from the floating structures is of great distance.
√ and
√ should be ~0 if r = (5)
v. Kinematic sea-surface condition
The condition is applied to submerge surface area of the floating structure in its mean
position.
and
(6)
Where is the normal vector of the hull in direction j and it is positive into the fluid
As mentioned, the RAO of each cylindrical FPSO is calculated using Ansys AQWA. The
cylindrical FPSOs are first modeling in AutoCAD and then imported to Ansys AQWA. The sample of
the FPSO designed is shown in Figure 2 and Figure 3
Figure 2: Cylindrical FPSO modelling in AutoCAD
23
(a) (b)
Figure 3: Sample of Cylindrical FPSO in Ansys AQWA
To estimate the wave frequency motion of the FPSO, Pierson-Moskowitz (PM) spectrum is
used to represent the ocean environment in the targeted location. PM spectrum selected in this
research because the research is purposed to study the motion of the FPSO in fully developed sea
condition. The equation of the PM spectrum is shown in equation (7)
( )
[
] (7)
Where ( ) is the wave spectrum in the function of wave speed, is significant wave height,
is wave speed, is the significant wave period.
To estimate the wave frequency motion of these FPSO, the response spectrum can be calculate
using the wave spectrum data and RAO of the floating structure. The motion response spectrum is
calculated using equation (8) and the significant motion response is calculated using equation (9) [4]
( ) ( ) ( ) (8)
(9)
Where is the motion response spectrum; is encounter wave frequency, since the FPSO is
not moving with any speed, then it is assume that ; is the response amplitude operator in
specific motion direction; is the significant response amplitude; is the moment for the area
under the graph of response spectrum.
3.0 RESULTS AND DISCUSSION
The heave RAO simulated by Ansys AQWA for each modelled FPSO is shown in Figure 4.
The FPSO with diameter of 95 meter shows the highest peak heave RAO of 3.112 at period of 14
seconds. From the simulation, it is found that the peak heave RAO simulate from Ansys AQWA is
very large because of underestimation of damping effect from Ansys AQWA. This is because the
Ansys AQWA does not consider the effect of viscous damping [12]. Based on the simulation result
from Ansys AQWA, it can observe that the diameter of the FPSO play the important role to determine
the peak heave response. By changing the diameter of the FPSO, it will result in the changes in the
waterplane area. From the theoretical point of view, the change of waterplane area would affect the
restoring force coefficient and the added mass coefficient. Therefore, changing on waterplane area
would lead to the shift of heave natural frequency.
24
Figure 4: Heave RAO of cylindrical FPSOs with different D/T ratio
As shown in Figure 5, the pitch RAO of the FPSO models is plotted with respect to the wave
period. The FPSO model with the highest peak of the pitch RAO response is the model with diameter
of 107 m which is 86.74 deg/m at the period of 27 seconds. From the theoretical point of view, the
pitch RAO strongly depend center of gravity of the FPSO. This is because the center of gravity of
FPSO would influence the moment arm which lead to the change of restoring moment coefficient for
pitch motion. Large peak RAO for both the FPSO with diameter 107m and 120m is due to ignore of
the viscous damping in the simulation using Ansys AQWA.
Figure 5: Pitch RAO of cylindrical FPSOs with different D/T ratio
The Pierson-Moskowitz spectra shown in Figure 6 are plotted based on the wave condition at
Bintulu (refer Table 1). The wave spectrum used to calculate the response spectrum of the FPSOs in
heave and pitch direction using equation 8. The heave and pitch response spectrum of each models of
FPSO are shown in Figure 7 and Figure 8 respectively.
-0.2
0.3
0.8
1.3
1.8
2.3
2.8
3.3
0 5 10 15 20 25 30 35 40
RA
O
Period (s)
HEAVE
D = 130 m D = 120 m D = 107 m D = 100 m D = 95 m
0
10
20
30
40
50
60
70
80
90
0 5 10 15 20 25 30 35 40
RA
O (
de
g/m
)
Period (s)
PITCH
D = 130 m D = 120 m D = 107 m D = 100 m D = 95 m
25
Figure 6: Pierson-Moskowitz wave spectra for wave condition at the Kanowit, Bintulu.
Figure 7: Response spectrums of 5 models for heave motion
Figure 8: Response spectrums of 5 models for pitch motion
-0.02
0
0.02
0.04
0.06
0.08
0.1
0 0.5 1 1.5 2 2.5
Spec
tral
(m2
.s/r
ad)
Wave frequency (rad/s)
PIERSON MOSKOWITZ WAVE SPECTRA
0
0.0000005
0.000001
0.0000015
0.000002
0.0000025
0.000003
0.0000035
0.000004
0.0000045
0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5
SR(W
e)
W (rad/s)
RESPONSE SPECTRUM (HEAVE)
D = 130 m D = 120 m D = 107 m
D = 100 m D = 95 m
0
0.0001
0.0002
0.0003
0.0004
0.0005
0.0006
0.0007
0.0008
0 0.5 1 1.5 2
SR(W
e)
W(rad/s)
RESPONSE SPECTRUM (PITCH)
D = 130 m D = 120 m D = 107 m D = 100 m D = 95 m
26
The area under the graph is response moment (MR) of the cylindrical FPSO when it operates in the
wave condition present in Table 1. The response moment also represents the energy of the motion.
Based on Figure 7, the response spectrum for heave motion for each model is differing to each other.
The highest response for heave motion is the model with diameter 100 meter. From Table 3, the
response moment for FPSO with diameter 100 meter is 1.34862e-05 m2s. Similarly from the Table 3,
the lowest response for the heave motion is the model with diameter of 95 meter which the response
moment is 2.30695e-07 m2s.
Based on Figure 8, the pattern of response spectrum for pitch motion of each model differ each
other. The highest response for pitch motion is the FPSO with diameter of 100 meter. As presented in
Table 3, the pitch response moment is 0.002607 m2s. The lowest response for the pitch motion is the
FPSO with diameter of 107 meter which is 1.673e-06 m2s. The response spectrum for the FPSO with
diameter 107 meter is almost near to zero in any wave period. From the Figure 8, it is obtained that
only the FPSO with diameter 100 meter have higher pitch response spectrum compare to other FPSO.
This is because the pitch natural period for the FPSO with diameter 100 meter is closed to the wave
significant period for this selected location. Therefore, resonance effect is observed in the response
spectrum for this FPSO.
Table 3: Response spectrum of 5 models for pitch and heave motions
Model diameter Motions Area under the graph (MR) M(1/3)
130 meter Pitch 4.33882E-06 0.0083 deg
Heave 2.73543E-06 0.0066 m
120 meter Pitch 7.36858E-06 0.0108 deg
Heave 4.60332E-07 0.0027 m
107 meter Pitch 1.67312E-06 0.005 deg
Heave 3.08915E-07 0.002 m
100 meter Pitch 0.002607354 0.2042 deg
Heave 1.34862E-05 0.0146 m
95 meter Pitch 8.19835E-05 0.0362 deg
Heave 2.30695E-07 0.001 m
From Table 3, the diameter model of 107 meter shows the lowest pitch significant response
within 5 models. However, in heave motion, the model with diameter 95 meter shows the lowest
response for the heave motion, but the FPSO with diameter of 107 meter only a big larger than it.
Hence, the FPSO with diameter 107 meter is chosen and assume suited for the Kanowit, Bintulu.
4.0 CONCLUSION
This research aims to study the diameter to draught ratio for the cylindrical FPSO suitable to operate
in Malaysia seawater. The numerical simulation was conducted using Ansys AQWA commercial
software. The RAO of the FPSO with different draught (T) to diameter (D) ratio is estimate by Ansys
AQWA before it is used to calculate the FPSO response spectrum. Then, the mapping of all of the 5
models with respect to the heave motion and pitch motion are done to select the suitable model which
enables to shows the lowest wave frequency motion. From the comparison made, the FPSO with
diameter of 107 meter is the suitable for the wave condition in Bintulu. From the study, the natural
frequency play a big role in determines the motion amplitude of the FPSO in wave environment.
Therefore, the draught to diameter ratio need to be proper select since it can be directly influence the
natural frequency in heave and pitch motion. By shift the natural frequency of the motion far away
from significant wave frequency, the resonance phenomena can be avoided and the motion amplitude
can be lower.
27
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