380ft Towing Tank Carriage Control System - STOP Analysis.pdf

MEMORANDUM

SUBJECT: Investigation of 380 Carriage Stop procedures

The Hydro-lab staff conducted a series of tests in response to the towing carriage position over-run incident that occurred on 3/2/26. During this incident, the carriage operator pressed the manual “stop” button with the intent of aborting a run early. Instead of stopping early, the carriage decelerated too slowly and overran its stop position, contacting the arrestor cable.

The investigation tests were performed to identify the root cause of the incident and for use in guiding any procedural changes for future carriage operations.

Description of test procedure:

The Hydro-lab staff began with low speed (5 ft/sec) steady speed runs to establish a procedure for triggering the various stops. Once comfortable with the stop procedures, multiple runs were performed at 20 ft/sec, which is sufficiently “high speed”, but allows for significant margin in the stop position to account for potential position over-run.

Additional considerations were made to minimize the risk to equipment on the towing carriage that could be damaged due to high deceleration rates. The staff removed any loose components and strapped down what was necessary to remain on the carriage.

Description of Stop Conditions in order of Safest to least Safe:

Normal Stop – The carriage control system uses internal logic to bring the carriage to a controlled stop based on input parameters (stop position, steady speed, deceleration rate, jerk rate). There is no operator injection for this condition.

Set 0 ft/sec – An alternative method of safely stopping the carriage was identified by the operators;

changing the carriage speed to 0 ft/sec while it is moving produces identical deceleration profiles to when the carriage decelerates normally. This is useful when needing to stop the carriage early, as it is less jarring to carriage riders. This is not useful for high speed runs, since it requires several seconds for the operator to input the speed; at high speed runs, the carriage operator will not have sufficient time to react.

Emergency Stop at Speed – The operator presses the Emergency stop at constant speed. This eliminates the “jerk” which is meant to soften the ramp into and out of deceleration. This is considered safe since the operator triggered the action and will have radioed any carriage riders to alert them. The carriage stops in a shorter distance than the normal stop due to lack of jerk. The carriage will not over-run its stop position.

Emergency Stop during Deceleration – The operator presses the Emergency Stop just after the carriage has begun its normal deceleration. Although it would be better to allow the carriage to decelerate to a normal stop, this condition is also safe as the carriage will not over-run its stop position.

Position Trip at Speed – The carriages position reading is interrupted and the carriage triggers an

Emergency Stop. This is slightly less safe than an Emergency Stop since it is un-planned and the carriage operator will not have radioed to alert carriage riders. The carriage will not over-run its stop position.

Position Trip during Deceleration – Hydro-lab staff was unable to trigger this condition. The carriage performed a normal stop when attempting to interrupt the position signal. This condition is not presented in figures 1 and 2.

Stop button at Speed – The operator presses the stop button while the carriage is at constant speed.

The carriage has an undesired two stage deceleration; initially decelerating at -2 ft/sec^2, then a more aggressive -11ft/sec^2. The carriage takes a significantly longer distance to stop and MAY over-run the stop position.

Stop button during Deceleration – The operator presses the stop button just after the carriage has begun its normal stop deceleration. The carriage has an undesired two stage deceleration; initially decelerating at -2 ft/sec^2, then a more aggressive -11ft/sec^2. The carriage takes a significantly longer distance to stop and WILL over-run the stop position.

Of note, carriage coasting was not observed under any of the tested conditions. Powered braking is maintained for all conditions, with the intensity of the deceleration and jerk varying depending on the condition.

Summary of findings:

The stop conditions can be broken into three categories; normal stop, emergency stop, and stop button.

Due to the risk for position over-run, use of the stop button is considered unsafe.

The stop button was originally intended as a controlled means of aborting a run. A material defect exists in the control logic. Hydro-lab staff speculates that this logic error was introduced during a service call when the vendor was troubleshooting carriage position errors. Since the carriage control system is planned for total replacement, the hydro-lab staff recommends a procedural change where use of the stop button is prohibited.

Figure 1 – Distance to stop from 20 ft/sec

Figure 2 – Stop Comparison from 20 ft/sec

Table 1 - Test Matrix

Settings:

Accel Rate 10

Accel Jerk 5

Decel Rate 10

Decel Jerk 5

Data File Speed Entered Stop Location Actual Stop Location Peak Accel (Trendline) fpss Peak Decel (Trendline)fpss Stop Type Notes

1 thru 5 Push Pull, noisy accelerometer

6 5 100 100.24 5 4 Normal Actual Stop is from Velocity Integration

7 20 100 100.37 10 9 Normal

8 32 100 100.33 10 12 Normal

9 5 100 102.87 5 3 Stop at Speed Tripped out after stop, wouldn't clear.

10 20 100 91.13 9 17 Stop at Speed 2 stage decel, slow then faster. Tripped, then cleared. May have hit stop after normal decel started

11 20 At Speed 183.92 9 12 Stop at Speed Stopped as soon as speed reached, 2 stage decel. Tripped with ESB error.

12 5 At Speed 286.12 5 11 E Stop at Speed Very fast decel. Tripped, then cleared.

13 20 At Speed 218.51 10 13 E Stop at Speed Very fast decel. Tripped, then cleared.

14 20 During Decel 110.55 10 12 E Stop during decel definite "Knuckle" in decel.

15 20 At Speed 194.81 10 10 0 fps at speed consistent decel

16 20 During Decel 123.58 10 10 0 fps during decel consistent decel

17 20 At Speed 117.33 10 13 covered Trimble Sharp decel.

18 20 During Decel error 8 10 covered Trimble Covered late; normal stop. Speed at rest is 0.53fps? Bad rezero?

19 20 During Decel 100.35 10 10 covered Trimble consistent decel

*Note: Decel plots for stops at speed should be overlaid manually; stops will vary due to human timing