Drill rigs, for obvious reasons, must maintain a fixed position above the ocean floor. As rigs moved off the continental shelf into water too deep for jack-up rigs or where conventional mooring or anchoring systems were not feasible, computer controlled DP systems became the only practical method for keeping the drilling rig on station.
The spread of DP systems from rigs and drill ships to support vessels came at the insistence of the oil companies. They did not want supply and crew vessels striking their expensive rigs, something that often happened in advanced sea states or during lengthy transfers of drilling fluids.
Now DP systems are becoming commonplace on the newest generation of crew/supply vessels and supply boats because they increase safety and operating efficiency, according to boat owners and operators.
“DP systems are one of those technological advances like cell phones,” said Wes Bordelon, president of Bordelon Marine, Lockport, La. “I don’t know how we got along without them. DP systems are indispensable for much of what we do. Holding position is critical in drilling operations, diving and especially work-over situations where we might be transferring fluids or coiled tubing directly into a well.”
DP also has significant safety implications. “If we are transferring liquids to a rig or platform over several hours, DP can hold position automatically. While a captain’s eyes and judgment are invaluable, fatigue can set in over a long period of time, making DP an extra layer of safety in boat operations,” Bordelon said.
Recent advances in the electronics of DP systems have made them more compact, more user friendly and certainly more affordable. “Today, DP systems have spread from 280-foot supply boats down to crew/supply and utility boats, often in the 150-foot range,” Bordelon said. “Our DP system also works very well with chartplotters, AIS and other cutting-edge electronic navigational aids.”
DP is helping the industry move to a practically all-computerized wheelhouse, said Karl Baier, president of Baier Radio, Belle Chasse, La., a leading designer, supplier and installer of DP systems. “Chances are if a customer wants a DP system, they also order a touch-screen automatic monitoring and alarm system, since they tie together so well. Another important trend is high-definition LCD monitors,” Baier said.
Not all DP systems are the same. Depending on the features needed and system redundancy required, DP systems are typically rated DP-0, DP-1, DP-2 or DP-3 by the American Bureau of Shipping. A DP-0 rated system would be considered “bare bones” with just the basic components to hold a vessel on station.
Most operators have opted for a DP-1 system that offers dual positioning referencing systems; two wind sensors, two gyrocompasses, a vertical positioning system and an uninterruptible power supply. This redundancy gives the operator the ability to hold station in case of the failure of one of the components.
What DP-1 does not do is offer two complete systems. DP-2 systems, by contrast, would typically have two bow thrusters, two vertical reference units and dual communication systems between the bridge and the engine room. DP-2 systems are used on larger supply vessels and special-purpose vessels such as well stimulation vessels that inject chemicals directly into wells or vessels that support ROV operations.
DP-3 systems are seldom found on vessels, but are used on jack-up rigs or drill ships where critical positioning holding capabilities are needed.
While DP-2 systems are typically used on larger vessels, some of the newer crew/supply boats are incorporating DP-2 into their design. For example, a new 190-foot-by-35-foot crew/supply vessel for Seacor Marine of Houston is set for a December 2006 delivery from Gulf Craft, Patterson, La. This vessel will carry liquid mud as well as the typical liquid cargoes â€” fuel and water.
“Because liquid mud is considered environmentally hazardous cargo, this has driven the design to include a DP-2 rated system so that all propulsion and maneuvering components are redundant,” said Joe McCall, project manager for Seacor.
The new vessel has five 1,800-hp Cummins main propulsion engines driving five 54-inch-by-54-inch propellers and three rudders, assuring needed redundancy. “The rudders are located behind the two outboard propellers and the center propeller, so if either outboard engine fails, there are still two ruddered propellers for maneuvering,” said Scotty Tibbs II, one of the three Tibbs brothers who run the yard.
For even more redundancy, the crew/supply boat has three bow thrusters â€” two 200-hp tunnel designs and a 400-hp, 360Â° azimuthing thruster. Three thrusters give this vessel significant operational flexibility and maneuverability.
Two Cummins engines drive hydraulic pumps to power the bow thrusters. One of the engines drives the 400-hp thruster and another Cummins engine powers the other two.
Also tied into the DP-2 system on this new vessel is controllable-speed propeller (CSP) technology that allows the props to turn more slowly to save fuel while on station operating in the DP-2 mode.
“The design challenge here is to reduce the speed of the props for fuel savings yet keep less than 50 percent load on the props required by DP-2 operation,” said Grant Pecoraro, head of the Gulf Craft design team on this vessel. “A computer supplied as a part of the CSP system calculates the thrust on each prop so operator input is minimized,” Pecoraro added.
This new vessel will sacrifice passenger carrying capacity to haul liquid mud and other liquids plus deck cargo. “We will have 36 passenger seats on this vessel. It is designed to carry specialized products at 20 knots with a maximum load,” McCall said.
As prevalent as DP systems are in offshore vessels, other vessel markets also rely on DP systems. Oceanographic research vessels depend on DP systems to hold position for many research tasks. New vessels such as Hugh R. Sharp, a research vessel owned by the University of Delaware and based at the university’s Marine Sciences Center in Lewes, Del., typically have a DP-1 system installed.
“We use our Kongsberg DP system every time we get underway,” explained Bill Byam, captain of Hugh R. Sharp. “Our DP system is also an autopilot, and most of the ship’s steering when we are transiting is done in autopilot. We also use the auto track feature of DP that allows us to enter specific waypoints from a survey grid, and the ship will track down that line.”
Byam also said he uses DP for station keeping when deploying sampling equipment such as water-sampling bottles, light meters, coring, net tows and other research equipment.
Most vessel operators have come to the conclusion that DP systems are a necessary system for safe and efficient operation of their ships. “I can’t conceive of building any more supply vessels without a DP system,” said Bordelon.