by Evert van den Berg (2nd Officer, DPO)
To be able to drill in the seabed hundreds of meters below the sea surface Fugro Synergy needs to stay in the same position on the surface, sometimes for weeks at a time. As anchors would be impractical in the water depths we work in Fugro Synergy is equipped with a Dynamic Positioning system (DP for short), meaning she is kept in position using her propellers.
The DP system receives input from a number of sensors (DGPS and hydro-acoustic beacons for position, gyro compasses for heading and several wind meters) and then continuously calculates the trust settings (both force and direction) required on the propellers to generate the exact forces needed to counter the outside forces and stay in position. In the generally quiet waters of the Gulf of Corinth we are often able to keep the ship within about 20cm of the set position, though we may move over a meter in the strong squalls that sometimes blow through the Rion strait. On one occasion we recorded a 75kn (140km/u) gust while holding position on the DP.
An interesting piece of history: While DP systems are quite common today in the offshore oil/ gas and wind energy sectors as well as on cruise ships, the first predecessor of the technology was actually developed for a ship used by an early predecessor of the IODP. They used converted barge called the CUSS1 during project Mohole in 1961 that attempted to drill through the earth’s crust where it was thinnest: on the bottom of the ocean. As the water depths were much too deep for anchors the barge was kept over the borehole using 4 thrusters operated manually in combination with a crude sonar system to indicate the position which kept them within around 180m of the site, just enough not to bend the pipe too much. 60 years of improvement in computers, positioning systems, electric drive converters and ship design have helped a lot in making our jobs easier!
The DP system is controlled from the bridge, and while the computer is doing the hard work of constantly correcting the settings of the propellers it is monitored by the one of the duty Dynamic Positioning Operators (DPO). The DPO has to make sure that all parameters stay within limits and also anticipate changes in the weather and other operating conditions. In case of failures the DPO can switch between the multiple systems that provide redundancy, or can take over manually as required; different control modes are available in case of failures from fully automatic, via joystick to control to full manual. As on most ships the DPOs are also the officers in charge of the navigation of the vessel and our normal DP watches consist of a senior and a junior deck officer who are on the bridge together, taking one hour turns at the DP system controls. The chief officer and first officers are changing shifts at midnight and mid-day, while the two 2nd officers are changing at 0600 and 1800 to make sure there is enough overlap between shifts.
Also on the bridge is the survey station, here the surveyor determines the exact position of the borehole. First the position of the ship is fixed using GNSS receivers that can use signals from both GPS and GLONASS satellites which are then corrected using Fugro’s Starfix system to an accuracy of <10cm. The exact position of the Seabed Frame (and drill pipe leading through it) is then fixed using the USBL (ultra-short baseline) hydro-acoustic systems integrated in Synergy’s hull and a transponder mounted on the frame.