Dynamic Positioning

Friday, August 12, 2011

The R/V Thomas G. Thompson and TE SubCom Dependable----the two ships now working at sea on behalf of the OOI regional cabled network--are each equpped with dynamic positioning (DP) systems. DP allows a vessel to automatically maintain position. Scott McMullen, the Oregon Fishermen's Cable Committee representative on board the Dependable during cable installation has contributed the entry below about that vessel's dynamic positioning system.

The Dependable is equipped with DP2, which stands for Dynamic Positioning System class 2. This control system directs the ships thrusters, which have the ability to swivel in all directions and vary the amount of thrust. (See blog entry for July 18, 2011.) It uses a variety of inputs to sense the ship’s position, and then provides precise control to the thrusters to direct the ship’s movement. The system can be used to hold the ship in the same position, move along a line at a given speed or follow our remotely operated vehicle (ROV) at a set offset amount. DP2 indicates that the Dynamic Positioning system is fully redundant, having two of all critical components.

Position Inputs
A variety of DP system sensors provide the ship’s position. In relatively shallow water (less than 50 m) a “taut wire” input can be used. This uses a weight lowered to the seafloor. Sensors at the pulley on the davit arm inform the DP system if the ship starts to drift from its position. When the ship needs to hold station near a fixed object, such as an offshore oil rig, then a “fan beam” laser can be used to measure the distance to reflectors mounted on the rig to precisely control the ship’s position as it holds station. Temporary beacons can also be installed on the seafloor for precision work in a localized area. For our cable-laying operations, the Differential Global Positioning System (DGPS) is used to fix the ship’s position with meter accuracy. The DP system also uses other sensors to calculate navigation solutions, such as water depth, wind speed and wind direction.

Functions
The DP system can be used in a number of ways: 1) as a traditional ship’s autopilot, steering the ship on a given course during transit, 2) to hold the ship in a precise position, or 3) in a “marked move” mode where the ship moves to a specific point. DP can direct the ship over a series of points along the route to lay the cable as designed. The accuracy is so great that offsets between the ship’s GPS antennas and the stern roller (where cable is lowered into the water) are accounted for. Another function of the DP system is the “follow target mode.” The Dependable uses this function to follow the ROV at a specified offset, as the ROV tracks the cable on the seabed. Beacons on the ROV transmit signals to a transducer lowered from a pole through the ship’s hull. The beacons provide a range and bearing from the ship to the ROV, and the DP system then calculates the ROV’s position.

Heading Vs Course
In DP mode, the ship is able to move in any direction or hold station with her bow pointing in any direction. The ship can head into the current or wind to reduce forces on the hull. During cable-laying operations, the ship’s heading and her course over the ground can vary. DP allows the vessel to continue to work in weather and sea conditions that would otherwise halt operations.