Ensuring safety and reliability in critical operations
Dynamic Positioning (DP) systems are essential for maintaining a vessel’s position and heading automatically, using its own propellers and thrusters. They are crucial in a wide range of offshore operations, from drilling and construction to cable laying and survey work. The accuracy, reliability, and continuity of position information are paramount for the safe and effective operation of these systems.
In many DP operations, even small errors in position can have significant consequences. Some tasks, such as precision subsea installations or close-proximity operations near platforms, require relative accuracy better than 3 meters. This level of precision ensures that equipment is placed correctly and that collisions are avoided. Furthermore, DP control systems need position data at a high rate, typically once per second (1 Hz), to react quickly to disturbances like wind, waves, and currents. This rapid data acquisition allows the system to make continuous adjustments, maintaining the vessel’s position with high accuracy.
Reliability: A Matter of Life and Property
The reliability of position information is not just a matter of operational efficiency; it’s a matter of safety. In operations where life and property are at risk, such as diving support or heavy lifting near offshore structures, incorrect position data can lead to catastrophic failures. A sudden drift due to inaccurate information could result in collisions, equipment damage, or even loss of life. Therefore, DP systems must be designed to provide continuous and reliable position data, even in challenging environmental conditions.
DP vessels use a variety of Position Reference Systems (PRSs) to determine their position. Here are five main types:
Hydroacoustic Position Reference (HPR): Uses underwater acoustic transponders or beacons placed on the seabed or on subsea structures. The vessel transmits acoustic signals, and the time it takes for the signals to return is used to calculate the distance to the transponders.
Taut Wire: A physical wire is deployed to the seabed with a known weight attached. The angle and length of the wire are measured to determine the vessel’s position relative to the anchor point.
DGPS (Differential Global Positioning System): Uses satellite signals corrected by reference stations on land or at sea to provide more accurate position information than standard GPS.
Laser-based systems (Fanbeam and CyScan): These systems use lasers to measure the distance and bearing to reflectors placed on nearby structures. Fanbeam typically uses a rotating laser, while CyScan uses a laser scanner to create a 3D image of the surrounding environment.
Artemis: A microwave-based system that measures the distance and bearing to a transponder on a fixed location, typically a platform or another vessel.
Data Fusion and Weighted Averages
Modern DP control systems don’t rely on a single PRS. Instead, they pool or combine data from two or more PRSs to improve accuracy and reliability. These systems often use weighted averages, where each PRS’s contribution is weighted based on its estimated variance or uncertainty. For example, a PRS with a lower variance (i.e., higher accuracy) will have a greater influence on the final position estimate. This approach ensures that the most reliable data is given the most weight, while less reliable data is given less weight, minimizing the impact of individual sensor errors.
Kalman Filters for Enhanced Station Keeping
Many modern DP systems incorporate Kalman filters, which are sophisticated algorithms that estimate the vessel’s position and velocity based on multiple sensor inputs and a mathematical model of the vessel’s dynamics. Kalman filters can effectively filter out noise and errors in the sensor data, providing a more accurate and stable position estimate. This leads to improved station keeping performance, especially in challenging sea conditions.
DP Redundancy and Sensor Voting
For operations requiring DP redundancy (Class 2 or 3), it is necessary to use at least three different PRSs. This allows the DP system to detect and isolate faulty sensors through a process called “two-out-of-three voting.” In this scheme, the system compares the position data from all three PRSs. If one PRS deviates significantly from the other two, it is identified as a rogue sensor and its data is discarded. This ensures that the DP system continues to operate safely and reliably, even in the event of a sensor failure.
It is also crucial to choose different types of PRSs to avoid common-mode failures. For instance, relying solely on three DGPS systems would leave the vessel vulnerable to a GPS outage. By using a combination of HPR, DGPS, and laser-based systems, the risk of a single point of failure affecting all PRSs is significantly reduced.
Summary
In conclusion, accurate, reliable, and continuous position information is fundamental to the safe and effective operation of Dynamic Positioning systems. From achieving sub-3-meter precision to safeguarding against catastrophic failures, the role of PRSs cannot be overstated. By utilizing a combination of different technologies, advanced data fusion techniques, and robust redundancy measures, DP systems can maintain station keeping performance and ensure the safety of personnel and assets in even the most demanding offshore environments.
