Floating LBL concept for underwater position detection

Determination of underwater coordinates is important in such activities like underwater vehicle operation, installation of underwater structure and so on. The determination methods are using LBL, SSBL or an inertia navigation sensor, which are all very expensive. In some scientific or academic operations, the cost of position determination of an underwater structure or a vehicle is too expensive to realize in the sea experiments and a low cost position detection system should be developed. In this paper, a floating LBL concept is presented. The LBL system consists of at least three floating buoys each of which has a GPS, a pair of ultrasound transmitter and receiver, wireless communication device and a microcomputer board which controls the total procedure. We will introduce and explain about each module of the buoy system, ultrasound transmitter/receiver.


Introduction
Determination of underwater coordinates is important in such activities like underwater vehicle operation, installation of underwater structure and so on.The determination methods are using LBL, SSBL or an inertia navigation sensor, which are all very expensive.In some scientific or academic operations, the cost of position determination of an underwater structure or a vehicle is too expensive to realize in the sea experiments and a low cost position detection system should be developed.One example supposed in our laboratory is position detection of a ROV in the coral reef monitoring operation.In this operation, we need to take underwater pictures of coral reefs to record and monitor the condition of the reefs, where we need underwater coordinates of the picture taken to make a map of the total reef condition in the supposed area.The price of ROVs or underwater cameras is becoming cheaper and it seems ridiculous to use SSBL whose price is almost from five to ten times of ROV itself.Another supposed operation is installation of underwater devices or structures.To put the structure down on the desired position of the seabed, we need to monitor the coordinate of the structure during its installation operation.If the project is related to oil/gas development, this coordinate detection cost is relatively small compared to the total project investment.However, if the project is related to an academic research like settling a seismic sensor down on the seafloor, the cost of position detection must be much cheaper.
In this paper, a floating LBL concept is presented.The LBL system consists of at least three floating buoys P -246 each of which has a GPS, a pair of ultrasound transmitter and receiver, wireless communication device and a microcomputer board which controls the total procedure.We will introduce and explain about each module of the buoy system, ultrasound transmitter/receiver.

Floating LBL system
The floating LBL system consists of at least 3 buoys each of that has a pair of ultrasound receiver and transmitter.A buoy also has a set of Xbee wireless communication device to make a serial communication link with a master computer.
Fig. 1 The concept of floating LBL system A GPS unit is attached on each buoy.The master buoy is the origin of the coordinate and controls the total system.The procedure of the coordinate calculation of underwater structure is as follows.
(1) The master buoy transmits the start signal to slave buoys using Xbee module.At the same time, the master buoy transmits the ultrasound signal to underwater structure.The timer of the master buoy is cleared and set zero.
(2) As soon as a slave buoy receives the start signal, it clears its own clock and set zero.At the same time, the ultrasound receiver of the slave buoy goes waiting mode.
(3) The underwater structure receives the transmitted signal from the master buoy and responds back the ultrasound signal toward water surface where each buoy is floating.
(4) The buoys receive the ultrasound signal from the underwater structure.Each buoy stops the timer and records the GPS (N,E) coordinate at the same time.
(5) If the master buoy receives the returned signal from the underwater structure, it stops its timer and records the time as well as its GPS position values.(6) The master buoy calls to the slave buoy No.1 through Xbee and the slave No.1 calls back its timer count value and GPS coordinate through Xbee communication to the master buoy.(6) The slave buoy No.2 calls back according to the request from the master buoy.(7) The master buoy calculates the distance between the master and the slave respectively using the GPS coordinate values gotten from slaves.
(8) The master calculates the distance between each buoy and the underwater structure using the timer values gotten from slaves and its own.(9) Using the distances mentioned above, the master can calculate the underwater coordinate of the structure.(10) This procedure will be repeated periodically in the given interval time.

An example of underwater structure motion
As an example of the application of the floating LBL system, we considered a hanged underwater structure from a crane on the vessel.The offset angle from the vertical axis is a parameter of the nonlinear pendulum motion of the structure as shown in Fig. 2.
It is assumed that the crane cable is paid out from the initial length l_0 with v m/s speed.The initial length corresponds to the height from the initial submerged point of the structure to the hanging point of the top of the crane arm.
Considering the added mass and fluid resistance force, the equation of motion of the pendulum system is, , where, f is control force, Cm is added mas coefficient, m is the mass of the structure, theta is the angle of the cable, l is the length of the cable, v is the speed of the cable pay out, Cd is the drag coefficient, A is the effective area of the structure.

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The simulation is executed using next parameters.The structure size is 5m wide 5m long 4m height.The mass is assumed 50tf.The water depth is assumed as 2500m.The initial offset angle is 20 deg.The paid out rate of the cable are 2 cases, which is 1m/s and 0.5 m/s.Added mass coefficient is assumed 1.0, whereas drag force coefficient is assumed as 3.0.
The simulation result of the paid out rate is 1.0m/s is shown in Fig. 3.The horizontal displacement which means the fluctuation from the desired installation point is shown in Fig. 4. As Fig. 3 shows, the offset angle is decreasing according to time, which is due to damping effect of fluid resistance force.However, as the crane cable length is increasing, the horizontal offset from the desired point is developing as shown in Fig. 4. I assumed the drag coefficient higher assuming that some damping device is attached to the structure.But from the result shown in Fig. 4, it shows very difficult to reduce the offset vibration amplitude only using passive fluid damping force.So the active control method should be employed to reduce the vibration of the structure.For the active control of the structure, we need to detect the underwater position, so the floating LBL system should be developed.

Ultrasound transmitter/receiver
Ultrasound receiver/transmitter is expensive in general.So we are developing a set of ultrasound receiver/transmitter for this experiments.Fig. 5 shows the sea trial of our ultrasound receiver/transmitter.We succeeded more than 20m distance measurement, however, we found that the directivity of the sensor probe is too narrow so we need to improve it to realize the sea experiments.Through this experiments we realized the delivery of the master and slave buoys on the sea surface is very hard task.So we modified a UAV drone to be floatable and watertight to bring the buoys from the support vessel to the desired sea surface.

Conclusion
In this paper, we presented our floating LBL concept for the detection of the coordinate of the underwater structures.Using Xbee wireless communication and ultrasound ranging system, we can constitute a set of LBL much cheaper than the traditional LBL system that needs very expensive setup for seafloor baseline installation.To put the buoys on the desired surface position from the support vessel, we also modified a UAV drone to deliver the buoy system from the vessel to the sea.

Fig. 2
Fig. 2 Sand captured by the blocks

Fig. 5
Fig.5 Distance detection experiments at sea