|A RADius 1000 distance-measuring transmitter on Seacor’s John G. McCall. Manufactured by Kongsberg, the RADius system uses radar technology for close-in positioning situations in which GPS signals may not be accurate. (Photo courtesy Gulf Craft)|
Dynamic positioning may be the most important shipboard navigation system since the compass.
Once used only on the largest offshore vessels working in the deepest parts of the Gulf of Mexico, these systems are now standard on smaller OSV vessels. The main reason is that oil companies, contract drillers and other owner/operators of drilling rigs and platforms insist that any vessel coming in close contact with their structures must have DP systems.
DP is especially important for vessels approaching drilling rigs. While drilling, these rigs must stay directly over a precise spot on the seabed. A collision or even a slight bump might invite disaster by moving the drilling vessel off station, causing considerable equipment damage and lost drilling time. Many of these rigs lease for $500,000 a day or more.
In a DP system, a computer coordinates the main engines, bow thrusters, rudders and propellers to keep the vessel in position. Inputs to the system come from wind sensors, gyrocompasses and special devices such as vertical reference units.
DP systems are rated DP0, DP1, DP2 and DP3. The basic difference is system redundancy. DP3 systems have the most redundancy and are reserved primarily for drilling rigs. As a general rule, crew/supply vessels and shelf supply boats use DP1 systems, while deep-water supply vessels 220 feet and longer and construction ships equipped with moonpools and ROVs have DP2 systems.
|The laser head of an MDL Fanbeam target-positioning system aboard the crew boat Joyce McCall. (Photo courtesy Gulf Craft)|
DP has become so important that existing vessels are being retrofitted with DP systems. Many of them can be upgraded at a fraction of the cost of vessel replacement.
DP systems typically use azimuthing thrusters with either variable-speed, fixed-pitch propellers or fixed-speed, controllable-pitch propellers. The installation of a DP system normally would require extensive design work and time-consuming dry-docking.
However, there is another way, according to Thrustmaster of Texas, a manufacturer based in Houston. A packaged system using pre-engineered modules can convert an older OSV or barge to a vessel able to command the day rates that are normally reserved for far newer DP-equipped vessels.
The heart of the Thrustmaster system comprises deck-mounted, over-the-side thrusters with deck-mounted self-contained power units, a DP sensor suite, and all the interconnecting cables and interfaces. Often a vessel is completely renovated while the DP system is installed.
The 218-foot Elkhorn River started out as a 178-foot twin-screw OSV. Besides a 40-foot mid-body extension, the conversion added a Kongsberg-Simrad DP system and two Thrustmaster 500 retractable combination thrusters, one in the bow and one in the stern skeg between the two shaft lines. Each thruster uses a Cummins KTA-19 driving a hydrostatic transmission. The thrusters can be used in the tunnel mode or azimuthing mode, depending on water depth.
The packaged DP systems can be portable and taken from one vessel to another. This is useful especially if the vessel needs DP capability only for a specific project. The packaged DP concept often uses podded thrusters with electric drives.
“We have been building podded thrusters for many years and many of these units have been operating for 10 years or more without any failures,” said Joe Bekker, president of Thrustmaster.
DP systems are especially important in the operation of large deep-water vessels referred to as IMRs, or inspection, maintenance and repair vessels. IMRs are not supply vessels, since all of the liquids on board are for its own use rather than for transfer to rigs or platforms.
The 2007 newbuild Chloe Candies is an example. Designed with a large moonpool, two ROV units, 100-ton knuckle boom crane and 100-ton deep-sea winch, the vessel can handle large lifts through the moonpool or over the side in deep water. The 280-by-59-foot boat has a pair of 1,000-hp bow thrusters and two Steerprop 2,145-hp aft thrusters. The DP2 system is critical for holding position while the deck crane, winch and ROVs are working.
|The bow of the OSV John G. McCall with the tunnels for the vessel’s two bow thrusters, which are essential elements of the DP system. (Photo courtesy Gulf Craft)|
Chloe Candies is contracted to Saipem Americas and features an Innovator ROV. The vessel will work exclusively in the Thunder Horse field, assisting this huge platform to come on line after numerous delays.
Thunder Horse, the largest moored semi-submersible oil and gas platform in the world, is located in 6,300 feet of water in the Gulf of Mexico about 150 miles southeast of New Orleans. The platform cost approximately $1 billion dollars to build and at its peak will produce 200 million cubic feet of gas per day and 250,000 barrels of oil. Thunder Horse was badly damaged by Hurricane Dennis and sustained a direct hit from Hurricane Katrina six weeks later.
New systems to improve the tracking of the relative position of a vessel and the rig or platform it is serving are introduced frequently. The first target positioning system to gain wide acceptance is the Fanbeam from MDL, a British company. The Fanbeam is a laser radar system that uses the latest gallium arsenide laser diode technology suitable for repetitive, high-accuracy positioning and tracking of vessels, rigs, platforms and other targets. The basic principle of laser ranging is to measure the time a pulse of light takes to travel to the target and back again.
The system is primarily used to control or assist automatic DP systems of a vessel next to a platform or other vessel. Fanbeam can be employed as either a stand-alone collision monitoring system or local backup system for position control within a DP system. It is also used to position towed seismic vessel gun-array floats during geophysical survey operations.
The basic system consists of a laser-scanning head typically mounted on the mast of the vessel, an auto-tilting yoke that compensates when large height differences occur between a vessel and a rig or two vessels in different states of ballast. Also in the system is a universal control unit with embedded control software. Over 200 of these systems are in service.
Seacor Marine, Houston, is now using the RADius position reference and tracking system on its new crew/supply vessels such as the crew/supply vessel John G. McCall. The new vessel is equipped with three bow thrusters set in a DP2 configuration.
Built by Kongsberg, RADius uses radar technology for short-range and directional monitoring. The system is designed for use in harsh deep-water environments where GPS signals may be inaccurate and the need for an independent system is critical.
The RADius 1000 (for its range of 1,000 meters) measures reflected radar signals from a number of passive transponders in the area. Each reflected signal is mixed with a unique ID to differentiate it from signals emanating from other targets. Advanced signal processing allows simultaneous and continuous measurements from each transponder.
|Broussard Tide is a 220-foot supply boat whose DP system allows it to work throughout the Gulf. (Photo courtesy Gulf Craft)|
The interrogator unit is mounted on the maneuvering vessel mast and several transponders are attached to remote objects such as rigs or platforms. RADius is designed to fill the need specified by IMO for DP2-class vessels.
In addition to the interrogator and the transponders, the system uses a RADius processing unit, a graphical user interface and serial interfaces on the vessel with a keyboard and video-display unit.
DP systems are also used to automatically set up the course the vessel will follow. Many vessel pilots use the DP system like an autopilot to keep the vessel on course when the vessel is moving from one area to another.
In the future, DP may be called upon to assist in more ship functions, such as positioning of ROVs and operation of winches that position infrastructure on the sea floor. DP will also play a pivotal role in the operations of floating production, storage and offloading vessels, and for the shuttle tankers that will carry oil from them to shore unloading points. FPSOs and their associated shuttle tankers are expected to begin operating in the Gulf of Mexico in the next few years.
DP systems continue to become more accurate and more stable. Once considered a luxury, DP systems have proven their navigational superiority in the harsh conditions of deep-water operations in the Gulf of Mexico.