Dynamic Positioning System
Dynamic Positioning (DP) is a computer-controlled system that allows vessels to hold position and heading in a location that is otherwise too deep to launch anchors. DP is achieved by integrating the control units of all thrusters onboard the vessel into a single computerised system. The system, commonly known as the DP system, uses a carefully constructed algorithm to provide a continuous and responsive thrust vectoring on all three degrees of freedom of the vessel (yaw, surge, and sway) to keep the vessel in the same position and at the same heading. Without the DP system, offshore activities such as personnel transfer, subsea pipe-laying, and offshore construction will be risky if not impossible.
DP started to be applied to offshore oil industry assets on a large scale in the early 1980s, particularly with the exploration of the North Sea and deeper water oil and gas fields worldwide. With drilling moving into ever deeper waters, Jack-up barges could not be used any more, and anchoring in deep water was not economical. The initial DP system development had begun in the USA with drilling ships in the late 1960s and early 1970s.
The DP system is made up of 4 main sensors – Gyrocompass, Anemometer, Vertical Reference System (VRS) and Position Reference System (PRS). The gyro measures the heading of the vessel; the anemometer measures the wind speed and wind direction; the VRS measures the pitch, roll and heave of the vessel and the PRS determines the position of the vessel. There are 2 types of PRS – Relative PRS and Absolute PRS. The Relative PRS consist of CyScan, SpotTrack, Fanbeam, RadaScan, RADius, Light Tautwire and HiPAP, while the Absolute PRS consist of DGNSS.
DP Equipment Class 1 (DP-1)
A vessel classed under the IMO DP Equipment Class DP-1 or its equivalent is equipped with a DP system that may fail after a single point failure. The vessel has adequate thrusters and sensors in place for a DP system to monitor and keep the vessel in position and at the same heading for prolong period of time. However, the vessel lacks redundancy and as such a loss of any DP-critical component may render the DP system inoperable.
DP Equipment Class 2 (DP-2)
A vessel classed under the IMO DP Equipment Class DP-2 or its equivalent is equipped with a DP system that shall not fail after a single point failure. This can be achieved by having all DP-critical components duplicated and/or providing them with multiple power supplies. On top of that, the vessel's power system, fuel oil system, lubrication oil system, cooling system, compressed air system, and the HVAC system have to be redundant as well.
DP Equipment Class 3 (DP-3)
A vessel classed under the IMO DP Equipment Class DP-3 or its equivalent is equipped with a DP system that is as redundant as what is defined in DP equipment Class 2. In addition to having DP-critical equipment duplicated, these redundancies and their essential complementary systems shall also be separated by an A-60 class fire shielding and kept in their respective watertight compartment such that no flooding or fire in a single compartment shall render the DP system is operational.
The DP system is advantageous:
Operational at any water depth
Instantaneous and reactive response to weather changes.
Excellent manoeuvrability – relocation is a matter of a few keystrokes.
Increased ease of repositioning – no need for complicated structural setups.
Ease of use – reduces the number of training time required to reach acceptable competence.
Anchor deployment is not needed – eliminate the risk of damaging underwater installations and natural marine formations.