Growing awareness of DP

The dynamic positioning (DP) market is diverse and systems are being fitted to a wider range of vessels from small survey vessels to large cruise ships.

DP automatically controls a ship?s heading and station keeping (either a fixed or moving target) while minimising thruster activity and has considerably expanded the options for ship owners and operators. This is particularly relevant where vessels remain at sea for long periods of time, often in deep water and in areas where laying anchors is either not an option due to the nature of the seabed, or impractical because of timescales.

Examples include drill ships, shuttle tankers, FPSOs, cable ships and pipe layers, offshore support and platform supply vessels, anchor handlers, diving and semi-submersible support vessels. Increasingly however, DP is becoming more common in cruise ships, enabling them to keep station in locations where anchoring could result in environmental damage to coral reefs.

Forefront

One of the companies designing and developing DP equipment is the French-based Alstom which has been at the forefront of DP technology for offshore vessels for more than 30 years. Before then, control of thrusters was very much a manual process, using only manual controls or the vessel?s joystick to keep station; a procedure which challenged the most experienced of captains particularly in changing weather conditions. Alstom?s involvement in vessel control began with the development of the first UK-built and owned DP system for the coring and surveying vessel Wimpey Sealab.

A ?stay-still? solution was needed for the vessel, which was to be used for surveying coal deposits off the northeast coast of England. The then GEC spent three years developing its first Simplex (stand alone) DP control system, which was purpose-designed for Wimpey Sealab. GEC followed this in the mid 1970s with a Duplex system for Uncle John, a semi-submersible and diver support vessel. This order marked a significant technical challenge beyond the Wimpey Sealab installation as the thruster configuration was more complex and required a redundant console to control the vessel.

Development continued through the rest of the 1970s and 1980s with each system being application driven and designed as a one-off to meet the clients? specific needs. In the 1980s, Alstom developed the DPS 800 series, a series which helped to establish the company?s position as a leader in DP technology. Its key features were retained and further developed as the main attributes for its successor, the 900 series.

Flagship

Alstom?s flagship family of DP control systems is currently the ?A? series which is suitable for approval by Lloyds, DNV, ABS, BV, GL and other classification society standards. The company has recently modified its ‘A’ series range that provides not only DP but also ?vessel management? (power management, bilge/ballast, alarms, auxiliaries control and monitoring, etc). The enhancements include larger screens with higher resolution. Software features include enhanced power management load shed facilities, extensive data logging, analysis and enhanced remote diagnostics. Another advancement is the inclusion of ‘on line’ documentation.

Other advances which have aided the on-going development of DP, and hence more accuracy in vessel positioning, have included developments in measuring equipment.

The introduction of Differential GPS, some 10 years ago, revolutionised the whole process. The accuracy of global positioning systems, the satellite based passive ranging navigation system which provides latitude, longitude and altitude data for anywhere in the world, has been greatly enhanced with DGPS with improved standard positioning service (SPS) accuracy for non-military users from 1 to 5 metres.

Most vessels are now fitted with DGPS as standard as it is a cost-effective, widely available solution and, more importantly, makes dynamic positioning a viable proposition for many more vessels.

Laser-based

Alstom further enhanced this new technology with the development and launch in 2001 of CyScan, a laser-based positioning and tracking system. This high precision system provides another means of independent measurement when standing by other vessels and has advantages over DGPS which can be affected by interference from the sun?s radiation and other factors, knocking it out for short periods of time. CyScan even has the ability to identify false targets, such as people wearing reflective clothing.