Prototype development of tethered underwater robot for underwater vessel anchor release

Received Mar 4, 2020 Revised Mar 31, 2020 Accepted Apr 29, 2020 Tethered underwater robot (TUR) for underwater vessel anchor release is presented. In off-shore oil and gas enviromnment, there has been series of reported cases on stuck vessel anchors after mooring operations and divers are sent to release these anchors for the vessels to be in motion. The use of divers to perform such function is very risky because of human limitation and some divers have been reported dead on the process due to high pressure underwater or being attacked by underwater wide animals. This has caused very serious panic to the vessel owners and hence, this work is aimed to develop TUR that would be used by the vessel operators instead of divers to release the stuck anchor without loss. The underwater robot system comprises of three basic sections namely graphical user control interface (GUCI) that would be installed in the operator’s laptop, the WiFi LAN router for network connection, and TUR system hardware and software. Each of these sections was strictly designed. Various high-level programming languages were employed to design the GUCI and code the interface buttons, robot controller program codes etc. The implementation carried out and the prototype system tested in the University of Port Harcourt’s swimming pool of 6m depth for validation. The robot performed extremely good in swimming and release of constructed anchor underwater.


INTRODUCTION
Undersea robotics has shown an increasing interest in the last 50 years for oceanic cartography, sea exploration and underwater oil extraction that has led to the creation of an underwater vehicle to be controlled from distance [1][2][3][4]. Recently, underwater robots have been used for various tasks such as underwater data collection, underwater surveillance, underwater structure inspection, pipe handling in drilling operations, pipe inspection, in-pipe inspection robots (lPIRs), ship hull inspection, ocean exploration, maintenance of underwater equipment etc. [5][6][7][8][9]. Remotely operated underwater vehicles shortened as ROVs are tethered and manned underwater vehicle used to perform some certain functions underwater. The main benefits of using underwater robotic vehicles could be removing divers from the dangers of the undersea space and reduction in cost of exploration of deep seas.
In oil and gas offshore and marine environments in Nigeria, vessels transporting crudes are always anchored during crudes offloading period at the sea port. There has been series of reported cases of trapped anchors used to tension vessels during loading/offloading or mooring operation in the sea. The vessel anchor 197 might get trapped after operation and, for as long as the anchor is hooked, the vessel cannot move. Personnel on duty often find it difficult to remove the trapped anchor after mooring operation or after waiting for its offloading turn. The sea divers would typically be sent to release the anchor, depending on the depth of the water which sometimes may be very difficult to release. When the situation seems so difficult, the vessel anchor would be cut-off which is a huge loss to the vessel owners. To prevent any further delay of the trapped vessel or loss to its anchor as observed, a remotely operated underwater robot prototype that would release the vessel anchor is developed.
The following researched works were reviewed as stated: -In 2015, Vedachalam et al [10] developed a 500 m depth rated remotely operated vehicle (Prove 500) for carrying out scientific research in shallow waters and in challenging polar regions. The vehicle with dimensions of 0.96 m × 0.61 m × 0.63 m and weighing 175 kg in air is designed for a speed of 3 knots at an electric power input of 5 kW. The vehicle which is powered by 300 V DC through the 500 m length of a neutrally buoyant electro-optic umbilical communicates with the surface console through the redundant fiber optic cores of the umbilical. The developed vehicle is tested for its hydrodynamic stability, low temperature performance in the in-house test facilities and for navigation at the Idukki Lake in Kerala, where the vehicle is navigated at a depth of 106 m at 2 knots speed with the navigation system's position error of less than 5 % in the dead reckoning mode. The vehicle is being equipped with accessories for carrying out research in polar regions. -In [11], studies showed the development of an underwater ROV with fuzzy logic motion control for a shallow water environment i.e. up to 10 m depth. The ROV was developed with the associated electronics for motion and power control. The control electronics are mounted inside the ROV main body and communicated with via a tethered cable running from the surface which also carries the required power. The ROV also has a camera for obtaining video and a set of LED lights for illumination. The main controlling unit of the electronics is a Raspberry pi microcomputer which also operates the video. Test trials of the ROV underwater were conducted in a laboratory water tank to a depth of about 1.5 m. Very satisfactory operation was achieved. Some drawbacks and possible improvements were identified during these tests and addressed in the second phase with the introduction of fuzzy logic for motion control. -According to [1], Ahmad et al developed remotely operated vehicle (D20-ROV) for anode ship hull inspection in 2017. This D20-ROV has three thrusters to control the maneuvering forward, reverse, left, right, raise and submerged. The vehicle with dimensions of 0.5 m × 0.46 m × 0.22 m and weighing 15 kg in the air is design for a speed around 3knots and it is powered by 16VDC on-board battery. The umbilical transmits and receives the data through the 20-meter length by universal serial bus (USB) cable to communicate with the operator at the control room. A real-time streaming camera and command from a surface room were able to do a visual inspection of the vessels. -A full-fledged robot (HYDROBOT) for underwater surveillance and survey was developed in 2018 [12].
Underwater surveillance is a new emerging technology as a promising field of research in recent years. The potential applications include marine investigation, ocean exploration etc. The tasks to be carried out by the robot would be detecting and mapping submerged wrecks, rocks and obstructions that could hinder the navigation systems such as in commercial and recreational vessels. HYDROBOT is made up of polyvinyl chloride pipes and is balanced by the principle of center of gravity. This structure is capable of rotating in 360 degrees as well as changing the depth according to the user. The underwater video footage is taken by camera for the identification of underwater life. Sensors such as the accelerometer, hall effect sensor, and temperature sensor were used to make it more efficient for research and surveillance.

RESEARCH METHOD
The research method adopted is top-down approach which splits the entire TUR system into hardware, software and mechanical systems ( Figure 1). The software system includes graphical user control interface (GUCI) and robot control program codes which are written using various high-level programming languages in order to achieve the desired result. Mechanical system is made up of PVC materials that are used to form system casing.   Figure 3. All are integrated to ensure effective distribution of power to every unit of the system.  ) is used to establish network within the ship environment and enable the user laptop to communicate with the robot system underwater during the anchor release operation. The router operates at a frequency of 2.4GHz a. Algorithms and flow chart for TUR system The algorithm for connecting GUCI to WiFi Router and controlling TUR is shown below and the flow chart is shown in Figure 12.

Start 2. Connect GUI to WiFi Router 3.
Input battery level reading from TUR 4.
Display TUR battery level on screen 5.
Input Camera stream from TUR 6.
Display video on screen 7.
Input IMU reading from TUR 8.
Display TUR orientation on screen 9. Listen for signals from GUCI buttons 10.
Is control character received? 11.
No: go to step 9 12.
Yes: Send control signal to TUR 13.
No: go to step 9 15. Yes

TUR graphical user control interface (GUCI)
This GUCI interface is designed in Microsoft Visual Studio using Extensible Application Markup Language (XAML) and programmed with C# which would run in the operator's computer system. The GUCI would be used by the operator to control and assist the TUR to swim to the target (trapped anchor location) underwater and releases it. The operator's computer system must have a Wi-Fi facility for easy connection to the wireless access point (WAP) router linking the TUR. There are five sections in the GUCI that the operator would use for TUR monitoring, controlling and assistance during its operation underwater. These are TUR orientation, TUR status, TUR obstacle detection, display panel, and TUR locomotion control as shown in Figure 13. a. Algorithm for float mode The algorithm that activates the float switch sensor is stated below and the flow chart to accomplish it is as shown in Figure 14.
Is float switch above water? 4.
No: go to step 2 5.
Yes: go to step 6 6. Return . Algorithm for manual mode The algorithm that controls the TUR inside the water through the graphical user control interface at the operator's laptop, the flow chart to accomplish it is as shown in Figure 15.  No: is '↓' character received? 10.
Yes: call move down subroutine and turn ON light 11.
Yes: call float subroutine and turn ON light 13.
Yes: call move forward subroutine and turn ON light 16.
Yes: call move backward subroutine and turn ON light 18.
Yes: call turn subroutine and turn ON light 20.
No: go to step 2 22.
Yes: is light ON? 23.
Yes: Turn off light 24.
No: go back to step 2 27.
No: go back to step 2 30.
Yes: go to step 31 31. Return

TUR equation of motion & influential forces
TUR influential forces are TUR hydrodynamics (kinematics & kinetics) and TUR hydrostatics (gravitational & buoyancy). Other forces considered are drag force, propulsion force, added mass force, environmental forces (wind, sea current, waves etc.) as stated in [13,14]. a. Drag force Drag force is the resistance force caused by the motion of a body through a fluid. The drag force along x, y, z is shown in (1), (2) and (3). (1) depends on the shape of the TUR and the reynolds number. b. Derivation of the equations of TUR motion Since TUR moves underwater in six degree of freedom (6-DOF), then six independent coordinates are necessary to determine the position and orientation. Translational motion and rotational motion components shown in Table 1 are considered to derive the necessary equations of motion as listed in [15]. The translational motions are derived using Euler-Newton's first law (5).
Therefore, substituting , ⃗ , ⃗ ⃗⃗ and ⃗⃗⃗⃗⃗ in (19) and resolving into vector yields torques acting on x, y and z axes as shown in (20), (21) and (22). Roll motion-Torque acting on x-axis: Pitch motion-Torque acting on y-axis: Yaw motion-Torque acting on z-axis: -TUR hydrostatics (gravitational + buoyancy forces) Restoring forces due to Archimedes principles (gravitational and buoyancy) constitute the fundamentals of hydrostatics: The submerged weight (Newton) of the body and buoyancy force (Newton) written were picked from Fossen [16] and stated as in (24) For translational motion about x, y, z axes; , and are shown in (31), (32), and (33). The restoring force along x-axis: The restoring force along y-axis: The restoring force along z-axis: For rotational motion about x, y, z axes, torque is given as: