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Tech Tidbits, vol. 9 - Ultrasonic Wind Speed Sensors
Recent work by WEST Engineering Services on newbuild rigs have identified anomalies during the use of certain ultrasonic wind speed sensors in drilling rig DP (Dynamic Positioning) systems. On three separate rigs, rain and/or lightning rendered them inoperative for short periods of time (up to 2 minutes). Because these sensors are utilized in the feed forward (predictive) calculations for setting thruster parameters, this loss of wind velocity and direction data can be expected to result in increased deviation from the vessel’s desired position. Until the full consequences are identified and/or this situation is mitigated, it is recommended that at least one of the rig’s three ultrasonic wind sensors be replaced with a mechanical wind sensor.
Description of Main Issue
DP rigs maintain position by changing thruster output and/or direction. Wind and current are monitored and changes in their speed and direction are input to a rig specific model to calculate the changes required of the thrusters to remain in position. During periods of rain or lightning, one or more of the three ultrasonic wind sensors located on at least three rigs have been observed to lose signal input to the DP system. These redundant devices are inputs to the DP control system that uses software to determine defective readings and the actual wind conditions. At times, all three sensors were inoperative.
The greater the wind changes, the more significant the required change in thruster operation to result in minimal position movement. Because the most significant wind changes can be expected during serious weather conditions, these sensors can fail when most needed, unnecessarily causing excursions. Technical reasons for loss of wind signals are currently unclear. These sensors have been installed as the standard sensors in certain DP packages starting at some time in 2006. The manufacturer has reported that the newest version of software/firmware has reduced this behavior for conditions of wind and snow. Ways to determine the software/firmware version currently installed on rigs, as well as the effectiveness of this fix, are being investigated but have not been determined at this time.
Other Issues
During a rain storm, one rig noticed that the wind speed readings on the three ultrasonic wind sensors varied as much as 20 mps. This indicates that either the sensors are not calibrated, or the scaling on the DP system is not correct. Each DP system includes a factor in the software that corrects measured wind speed to a predefined height above the water line. This adjusted height provides a better estimate of the wind’s effect on the effective sail area of the vessel. It was observed that the factor used on one semisubmersible resulted in corrected wind speed at 10 meters above sea level; the sail area on a semi at drilling draft is expected to be around 30 meters. This is an optional function in the software. The default setting when enabled is 10 meters, which is about right for a typical OSV/PSV vessel but not necessarily right for a drilling rig. The numerical value can be adjusted by the attending software engineer during sea trials to achieve the best performance based on factors such as vessel type. Placement of the sensors is critical. IMCA guidelines for Dynamic Positioning vessels recommend that at least one sensor be in a separate location than the other two when three sensors are used. On at least one rig, all 3 wind sensors were located at the top of the drilling derrick. For more information or technical questions, please contact WEST Engineering, west@westengineer.com, or phone at WEST Engineering.
