Dynamic Positioning Systems in Maritime Operations
Dynamically Controlled Vessels
These vessels are capable of maintaining position at sea without the use of moorings or anchors. This is achieved by using propulsion units called computer-controlled thrusters. It is common to see several tunnel thrusters side by side, either at the bow or stern. While tunnel thrusters are fixed, azimuth thrusters can rotate 360 degrees. All these vessels still retain their main thrusters and rudders.
Dynamic Positioning: Principles of Operation
Dynamic Positioning (DP) is the method by which a vessel’s position and course are maintained through computer interaction between specific sensors, thrusters, and main propulsion units. This system automatically counters environmental forces. Anemometers and wind sensors, located in two separate locations on the vessel, measure the strength and direction of the wind. These sensors need to be at the top of the vessel and clear of all structures to obtain accurate measurements. The ship’s bearing to true north (as opposed to magnetic north) is displayed. Two vertical reference systems help in measuring the angle of pitch and roll. Of all sensor positions, one must be “online” and the other immediately available should the primary one fail or stop responding.
Position Reference Systems
- Artemis: This system uses microwave signals between a fixed station and a mobile station, both above water. The distance and bearing between two antennas are calculated by transmitting a microwave beam between them, providing an extremely precise position.
- Taut Wire: A taut wire system uses a vertical cable attached to a heavy weight on the seabed. This cable runs from the weight to a sensor head in a jib, where the angle and deviation are measured. The signal is sent to the computer, analyzed, and movement direction signals are sent to the thrusters. The vessel then moves to maintain a constant wire angle. A motion-compensated winch keeps the weight on the seabed and the cable taut, automatically making adjustments when necessary.
- Hydro-Acoustic System: This system uses acoustic signals passed between two or more transponders placed on the seabed and a transducer lowered below the hull.
System Alert
The Alert System consists of three lights: green, yellow, and red. Yellow and red lights are accompanied by audible alarms:
- Green: Normal operating state.
- Yellow: Degraded operation state. When a yellow alert is signaled, the audio component should be able to be canceled by dive control.
- Red: Emergency state. The diving supervisor must instruct divers to return immediately to the device and/or prepare to return to the surface.
Diving from Dynamically Positioned Vessels
Planning and careful attention to detail are necessary to reduce risk. IMCA Guidance, primarily document IMCA D010, provides assistance in planning.
Deep Sea Diving Procedures
A detailed dive plan is created, risk-assessed, and communicated to all divers and personnel involved. All equipment is inspected and mounted. The supervisor then initiates the dive, monitoring every step and stopping the dive if any problems arise.