SUGISAKAMASANORI

Journal of Robotics, Networking and Artificial Life
Vol. 9(4); March (2023), pp. 326–330
ISSN (Online):2352-6386; ISSN (Print):2405-9021
https://alife-robotics.org/jrnal.html

Keisuke Watanabe
Department of Marine Science and Ocean Engineering, Tokai University, 3-20-1 Orido Shimizu-ku, Shizuoka, 424-8610, Japan.

Received 25 November 2021
Accepted 06 November 2022

Deep sea mining

In the near future, deep-sea mineral resource development will be possible to be commercialized.
To minimize the CAPEX of the project, estimation of deposit quality is critically important andexploratory drilling is indispensable for estimating the amount of resources. To reduce the cost of
exploratory drilling, we are going to develop a system of seabed drilling which can move on theseabed by itself without a support vessel on the sea. This exploration platform must be able tomove along with the undulations of the seafloor and have a structure that supports the reaction
force of the drilling. The authors have been studying an eight-legged drilling platform. This paper
introduces our tank test system and some results of tank experiment.

Exploration phase
In-situ drilling.
Sea bed walking robot© 2022 The Author. Published by Sugisaka Masanori at ALife Robotics Corporation Ltd.
This is an open access article distributed under the CC BY-NC 4.0 license(http://creativecommons.org/licenses/by-nc/4.0/).

money is spent, and we believe that exploration cost willneed to be kept much smaller in the development ofseafloor mineral resources than in the development ofoffshore oil fields.
In offshore oil field development, the size of subsea
reservoirs can be estimated by seismological exploration.
Then, an exploration well will be drilled for thepromising reservoir. There are two types of exploration
wells. One is called wildcat wells, and the other is called
appraisal wells. Wildcat wells are drilled to find out ifhydrocarbons are really there beneath the seabed of thetarget area. Once the discovery has been approved,
appraisal wells are drilled to obtain more data about thesize and extent of the reservoir to estimate the feasibility.
On the other hand, in the development of seafloor
mineral resources, it is impossible to see veins by seismic
exploration. Therefore, it is necessary to carry out a large
number of wildcat-like explorations to estimate the size
and range of veins by obtaining actual ore samples. For
exploration drilling, it is possible to use MODU (MobileOffshore Drilling Unit), which is commonly used in

1. Introduction
Acquiring mineral resources such as rare metals from theseabed has long been debated. However, a commercial
seafloor mineral resource development project has not
yet been realized. The reason for this is that although
there is a technical problem that the mining system in thedeep sea has not been established, there is also a bigproblem that the method of deposit exploration which isnecessary for judging the economic feasibility of theproject has not been established.
Currently, there is a track record of drilling to a depthof around 3000 m in the development of technologicallymature offshore oil fields and the ratio of exploration costto the entire project is becoming relatively smaller. For
example, according to a report [1] by NorwegianPetroleum, “In 2020, the overall costs were around NOK
245 billion. Investments made up about 60 per cent of
this, operating costs 25 per cent, and exploration costsabout 10 per cent.”. Nevertheless, a very large amount ofCorresponding author’s E-mail: keisuke@tsc.u-tokai.ac.jp326

offshore oil/gas field development. However, thismethod is not a good idea not only because of its highoperational cost, but also because of the transportation ofore samples from the seabed to the ocean. This is because,in the case of crude oil, the underground pressure causes
the fluid to blow up inside the riser, but the ore sample
requires energy to be sucked up the ore from the seabedto the top facility through inside the riser pipe [2].
Moreover, in this case, the ore sample needs to be madeinto fine particles and mixed with seawater to havefluidity. Therefore, in addition to the drilling device, it isnecessary to install a crusher that finely crushes thedrilled ore and a mixer that mixes the ore with seawaterto enhance fluidity on the seabed or at the riser entrance.
Based on the above background, the author isstudying a method of deploying a large number of in-situ
type subsea drilling equipment and conducting
exploration drilling at low cost without using MODUs[3,4,5,6]. In this paper, a tank experiment of the 8-legged
walking robot currently under consideration is introduced.

As shown in Figure 1, each leg is L1 to L4 and U1 to U4.
Walking movement is realized as follows. While the UD
Fig. 1 Prototype design of the eight-legged robot.
Fig. 2 Deck layout and dimensions.

2. The Concept of Eight-legged Walking Robotlegs U1 to U4 land and support their own weight, the LDlegs L1 to L4 are lifted and the LD is slid. Also, bylanding these legs, it supports the reaction force ofdrilling the seabed of the drill unit. The crawler-type
drilling machine being developed by JOGMEC isdifficult to climb up and down the slope of the seabed.
On the other hand, since this 8-legged robot canchange the length of each leg, it can move even in acomplicated seabed topography. That is, it is possible tomove along the unevenness of the seafloor topography
and to go up and down the slope. If the seabed is soft
ground, it is necessary to devise a mud mat so that eachleg is not trapped in the ground.

As a method of moving the device for in-situ drillingon the seabed, a traditional navigation type underwater
vehicle can be considered. However, in the drilling robot,
it is necessary to increase its own weight enough to
support the reaction force generated by the drill into theseabed. When moving by a thruster, it is necessary togenerate a buoyancy that cancels this own weight, and abuoyancy variable mechanism will be required. As thereaction force will be at least several hundred kN, it is noteasy to realize a variable buoyancy mechanism to yieldsuch force at deep water. Therefore, a walking platformthat can move while supporting its own weight is suitable.
There have been many researches related to walking
robots design and control as a mobile platform in therough terrain on land [7,8,9]. However, conventionally,
crawler-type robots have been the main method ofmoving on the seabed [10,11,12] and there has not beenmuch research on walking-type robots.
Figure 1 shows the prototype design of the eightlegged robot. Figure 2 shows the deck layout anddimensions. The robot consists of two decks that canslide in orthogonal XY directions. These are called Upper
Deck (UD) and Lower Deck (LD), respectively. Eachdeck has four legs, each of which can slide up and down.

327

The drilling unit is attached to the Upper Deck. The
details of the drilling unit need to be further examined in
the future, but the important functions required of thisrobot, including the drill unit, are as follows.
(1) Being able to collect the drilled ores so that they are
not scattered in the sea.
(2) Automatically connect the drill pipes vertically to dig
deeper.
(3) Equipped with drilling and moving power(4) Being able to accurately detect the position on theseabed
In order to realize these functions, it is necessary to build
experimental system and accumulate knowledge throughexperiments.

direction. Vertical movement of each leg was performed

3. Tank Experiment System
Y

In order to study the walking algorithm of the 8-leg
drilling platform, an experimental device as shown in
Figure 3 was manufactured. The motor was stored in awatertight container and an underwater walking
experiment was conducted. An attitude (roll, pitch)
sensor is attached to each deck, and the length of each leg
is adjusted so that the deck is horizontal. In addition, atouch sensor is attached to the tip of each leg to know thatit has touched the seabed.
The purpose of this experiment is to confirm that therobot can walk even if obstacles are placed at the bottomof the tank and that the robot can climb up and down aslope of about 30 degrees.
As shown in Figure 4, a slope was installed in the tankand several blocks were placed at the bottom of the tankto achieve the unevenness.
In this experiment, the coordinates of the robot in thetank were calculated from the land using a 3D image
processing system. In the actual system on the seafloor, apositioning system such as LBL should be installed on
the seafloor. The coordinate system is shown in Figure 4.
The control algorithm is basically sequence control.
First, from the starting point, the robot moves a given
distance in the X or Z direction toward the target point.
When climbing a slope, it automatically climbs the slope
because the leg stops when it senses the slope as it lowers
the leg that was lifted to move forward. The same is true
when descending a slope. Currently, it is not possible tocontrol both the X and Z directions at the same time, andthe robot will always move forward in either the X or ZZ

Xby servo motors with a feedback function.
Fig. 3 Tank experiment robot schematic viewFig. 4 Basin experimental setup with a slope

When the amount of stage movement in the X
direction is set to ΔX and the possible height of one leg
is ΔH, the theoretical tilt angle θ, which allows climbing
up and down, is given as tanθ=ΔH/ΔX.
In this experiment, the robot is not equipped with asensor to detect the angle of inclination of the slope infront of it.
Therefore, each leg was raised to ΔH, the maximum
height of the movable part, and moved a predetermined
distance in the X direction. This method would raise thelegs to an unnecessary height when the tilt angle is small,which is inefficient in terms of both time and energy.
Therefore, it was found that in the actual system, theslope angle of the slope in front of the robot should be328

measured and the legs should be raised or lowered by theminimum necessary height.
A touch sensor attached to the tip of the leg is very
effective. When the touch sensor detects the seabed, thedescent of the legs is automatically stopped. Despite thesimplicity of the mechanism, the touch sensor and theattitude sensor on the deck enable the robot to keep thedeck level even on uneven seafloors, and the robot canascend and descend slopes very easily.
4.

but it is a fluctuation of 40 mm for a movement of 1000
mm in the X direction. Although it will be necessary toSlope

Floor unevenness

Experimental Results

Figures 5 through 8 show the results of the tankexperiment. Figure 5 shows the Y coordinate, i.e., theamount of movement of the height of the deck section.
The vertical and horizontal axes of the figure represent
the amount of movement in the Y direction and time,respectively. The unevenness of the graph from the start
to about 240 seconds reflects the unevenness placed on
the bottom of the tank. From about 240 to 260 seconds,
the graph shows the movement up the slope. In thisexperiment, the robot was only able to climb a distanceof about 100 mm due to the size limitation of the tank,
but we have confirmed that the same robot can climb alonger distance and a 45-degree slope in land-based
experiments.
Figure 6 shows the time series of X coordinate of thedeck during the experiment of climbing a slope. It started
from around -2350 mm and moved around 1000 mm.
Based on the results of this experiment, the forward speed
is about 2.8 mm/sec. Assuming that this model is 1/100
scale, the speed of the actual machine is 28 mm/sec,
calculated from Froude's law of similarity. This speed
observed here will be too slow in practical use. The
forward speed was physically determined by the control
speed of the servomotor employed in this study. The
forward speed also depends on the distance the legs areraised. We found that it is necessary to consider how toincrease the forward speed while taking these points into
account in the future.
Figure 7 shows the time series of Z coordinate of thedeck. It started from around -1365mm and moved around
40mm, which means the robot moved almost only X
direction along the base on the tank floor shown in theFigure 4. In this experiment, the movement wasprogrammed only in the X direction and not in the Z
direction. Therefore, the fluctuation observed here isconsidered to be caused by the unevenness of the bottom,

Fig. 5 Time series of Y coordinate of the experimentFig. 6 Time series of X coordinate of the experimentFig. 7 Time series of Z coordinate of the experiment

Robot
direction

Fig. 8 Robot trajectory in the XZ plane329

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implement feedback control in the future, it wasconfirmed that the robot has, in principle, higher straightline performance than a navigational robot.
Figure 8 shows the XZ plane path result. Considering
the fact that the movement over the unevenness of thebottom and the slope, this fluctuation is considered to bevery small, and the walking method presented by thissystem is considered to be effective in the developmentof seafloor mineral resources.
5. Conclusion
In this paper, we introduced an in-situ type submarine
drilling walking robot and conducted an underwater
walking experiment in a tank. We confirmed that theplatform can be moved to the target point with a simple
algorithm even if the seafloor is uneven or sloped. It turns
out that the designed robot can climb sloping terrain witha very simple algorithm. In addition, we found that thestraight-line performance of this system is much better
than navigation type robots. As a result of this tankexperiment, some future problems were also found. For
example, in designing the shape of the platform, it isimportant to survey the seafloor topography of theplanned exploration site in advance. This is because themaximum slope angle that the robot can climb isdetermined by the ratio of the height difference between
the front and rear legs to the length of the platform deck.
Since it raises and lowers its legs while searching for thehardness of the seabed, its forward moving speed is veryslow. In addition, we need to implement a rotating
mechanism to change direction on the ocean floor. Theseissues need to be resolved in the future.
As a whole, it was confirmed through experimentsthat this concept is suitable for drilling for seabedresource development because it can support the reaction
force during drilling and various devices can be mounted
on the deck.

Author Introduction

Acknowledgements

Dr. Keisuke Watanabe
He is a Professor of School of Marine
Science and Technology, TokaiUniversity in Japan. He received hisB.E., M.E. and ph.D. degrees from theDept. of Naval Architecture andOcean Engineering, The University ofTokyo, Japan in 1991,1993 and 1996.
His research interest is autonomous
marine vehicles application.

This work was supported by JSPS KAKENHI Grant
Number 18K04585.
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