As part of a demonstration project for the Innovations for Navigation Projects Research Program, new positioning technology for river construction has been successfully tested. Under the work unit titled "Positioning and Placement of Large Underwater Structures" new positioning software was used to monitor the procedures and results of placing a large steel protection cell at Lock and Dam 24 in Clarksville, MO. Funding for this work unit was provided by the Implementation and Demonstration work unit.

The purpose of the protection cell is to protect the downstream guide wall from collision by barge traffic. Using the differential GPS technique to obtain position data, the actual location can be compared with the desired location in real time. GPS technology can assist the construction crew in placing the structure more efficiently and accurately. This report gives a brief overview of the methods and results of the demonstration project.

The cell was designed to be placed over 3 casings which were positioned by standard geodetic methods using a total station surveying instrument. The casings were set by rotary drilling approximately 10 feet into the river bottom. One of these casings is shown in Figure 1. Our purpose in the first phase of the project was to monitor the positioning of the casings and to verify that position by demonstrating the use of DGPS technology and high accuracy positioning software in placing large structures.

To monitor the position we installed a small diameter GPS antenna on the drill string centered over the drill bit which was approximately 50 feet above the drill floor (Figure 2). We also installed a second GPS antenna on the drilling platform so accurate heading information could be calculated.

Cables were attached at the antennas and connected to the geodetic receivers on the deck of the drilling barge. One radio antenna was connected to the two receivers to provide the real time kinematic (RTK) corrections. These corrections are computed to give more accurate positioning. Since a local coordinate system was being used by the survey construction crew, we used a coordinate transformation program to convert from WGS84 to the local system. We interfaced a monitor to the computer and receiver and used software developed by Trimble Navigation, Inc. to acquire and process the position data. This software, Target: Structures, consists of algorithms to compute the coordinates of the antenna and to give a graphic display of the location of the antenna. The actual position is compared to the desired position and is shown in real time. The construction personnel can see where the structure or in this case the drill string is and maneuver to the desired location.

In addition, an inclinometer was attached to the derrick for the installation of the second casing to give real time data on the derrick orientation. This data was also displayed on the monitor and gives a real time display of the antenna position.

Once the casings were positioned, they were filled with concrete and allowed to harden. The following week, the steel cell was scheduled to be positioned. The cell measures approximately 35 feet in diameter and 45 feet in height. Inside the cell are 5 structural triangular frames oriented horizontally with openings to fit around the casings. This was required to secure the cell to the casings. To position the cell over the casings we mounted two GPS antennas with receivers to the side of the cell. We also installed a radio transmitter at each antenna site to transmit the position to the shore station. This equipment was mounted in steel boxes at the axis of two of the members of the triangular frames (Figure 3).

Knowing the distance from the center of cell and the angle between the triangular arm members of the frames (120) we calculated the position of the antennas and entered those values into the software. We also entered the designed coordinates of the three casings. This provided a display of the desired final position of the steel cell (Figure 4).

Because the same software used for the casing placement was to be used for the cell placement, no further geodetic information needed to be entered. The cell was lowered using two cranes with a bridal attached in four places on the cell (Figure 5). During the placement of the cell, transmission was lost from one of the antenna and had to be replaced. This emphasizes the need for redundancy in critical electronic systems. The real time position of the cell was recorded and transmitted to the computer on shore and displayed on the monitor. The graphics showed the position of the cell relative to where it was supposed to be positioned.

Results

The results of the placement for casings 6 and 8 are shown in Table 1.

Casing 6 was the first casing to be installed and prior to the inclinometer being installed on the drill derrick. The errors shown here are larger than would be expected with RTK and are due in part to the GPS antenna mount not being directly above the center of the Kelly bar as well as the tilt of the drill string. With more precise positioning of the antenna on the derrick and an absolutely vertical derrick, better results are expected.

Casing 8 was positioned after the inclinometer had been installed but without the unit having been calibrated to zero. The difference in the position in the northing direction was confirmed by the surveyor on site using the total station and electronic distance measuring instrument (EDM).

The results of the final cell placement are shown in the Table 2:

Conclusions

Although the results are good, they do not reflect the true accuracy of the Target: Structure system. There are several reasons for this. The installation of the GPS antennas on both the drill derrick and the cell and the subsequent measurements to determine their location are critical. If there are errors in the location of the antennas relative to the points on the structure being positioned then all subsequent measurements will be incorrect. Adding the inclinometer improved the results from the casing measurement. This could have been improved even further if it had been calibrated. This would have required that the drillers raise the derrick to vertical position while the values were read and recorded. Calibration needs to occur only once during the project. In addition, the cell placement results could have been improved if the antenna mounts had been place directly on to the cell rather than a box on the side of the cell where measurements may have been inaccurate. According to the technicians at Trimble, experience on other projects of this type has shown that including GPS into the positioning plans from the start will significantly improve the accuracy.

Despite the results shown in the above table, the demonstration project was a success. The concept of using GPS to locate and position a large structure from a remote location was proved. The location of the cell as it was positioned was observed and recorded unobtrusively from a remote site. Since the positioning data was being recorded digitally via radio link, it could easily have been displayed on the barges to the crane operators.