Background And History of Dynamic positioning

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Offshore Support Vessel Toisa Perseus with, in the background, the fifth-generation deepwater drillship Discoverer Enterprise, over the Thunder Horse Oil Field. Both are equipped with DP systems.

@drillingnet Allen Killian

Dynamic positioning is a technique of automatically maintaining the position of a floating, unanchored vessel within a specified tolerance by using generated thrust vectors to counter the forces of wind, wave, and current forces tending to move the vessel away from the desired location. Design evolution and improvements in reliability allow station keeping by dynamic positioning for extended periods. Increases in available power and advances in sophistication of control equipment allow station keeping to be maintained in increasingly severe levels of wind and wave intensity.

Position is usually defined in terms of percent of water depth. Percent of water depth is the horizontal positional error divided by the water depth multiplied by 100, Position error, expressed in percent of water depth, is preferred because it defines the position and it is also related to the stress level in the riser or drill pipe. Generally, accuracies of the positioning system itself are about 1 percent and positioning to an accuracy of 1 percent or less is possible only in calm water and wind.

Five percent represents a common maximum permissible error with respect to permissible stress levels in the tubulars running from the ship to the ocean floor. At 5 percent of water depth, the angle off verticle of the drill pipe would be 3 degrees, a very small angle. At 10 percent of water depth, most drill strings would become bent or damaged. With significant wave action and vessel motion due to wave, the string may be lost.

Increasing water depth makes the task of dynamic positioning much easier because the same percent of water depth permits more movement in deeper water. For instance, given a 5 percent accuracy requirement a nearly impossible accuracy requirement of 5 feet is set up for 100 feet of water. Similarly, with the same 5 percent requirement applied to 1,000 feet of water an off-the-hole allowance of 50 feet is given, a much more reasonable tolerance. For 10,000 feet of water, the allowable radius of surface movement is a generous 500 feet.

This desirable feature of deeper water, however, is partially offset by some of the difficulties found in position determination in deep water.

Mohoie Tests

The first dynamic positioning involved a feasibility study and test for the now defunct Project Mohoie in March and April, 1961. In tests, the barge Cuss I was maintained on location by 4 harbormaster units controlled essentially by manual control from visual interpretation of position reference displays. The 4 harbormaster units were attached to the 4 corners of the vessel. Position was indicated by radar reflecting surface buoys and sonar signals from a ring of subsurface buoys anchored at a relatively shallow depth in the very deep 12,000 feet of water off La ¡olla, California and Guadeloupe Island, Mexico.

The Mohole tests were successful in that they did show that dynamic positioning could be effective in recovering drilled cores from such deep water depths. Difficulties encountered in both position determination and automatic operation of the positioning equipment (thrusters) pointed out the need and direction for further improvement in these areas. The great water depth of the tests, 12,000 feet, in most respects was an advantage. It allowed an operating radius of 600 feet at the 5 percent of water depth limit.

Core Boat Eureka

Figure 1-97 shows the first fully successful dynamically positioned vessel, the M/V Eureka, developed by Shell Oil Company. This small 36 ft. x 136 ft. core boat was originally designed to be manually operated, but early demonstration of the difficulty of manually coordinating the two 200 horsepower steera-ble thrusters resulted in a fully automatic, nonredundant, closed loop control system. The Eureka first began operating in May 1961. Separate analog controllers were used for each of the three degrees of freedom of motion—surge, sway, and yaw. Standard startup procedures were used for optimizing rate, proportional band, and reset settings for each of the three controllers.

The Eureka was a successful vessel, having drilled core holes in as little as 30 feet and as deep as 4,500 feet of water. Drilling has been performed with winds up to 40 mph and 20 foot swells. As many as 14 core boles have been drilled in one day.

Caidrilt I

The Caldrill I was the second dynamically positioned vessel. Figures 1-98 and 1-99 show the vessel at dock and also drilling while dynamically positioned. The Caldrill 1,176 ft. long and 33 ft. in beam, was powered by four fully steerable 300 horsepower

Fig. 1-97 Core boat Eureka
  1. 1 -99 Caldrill I drilling with dynamic positioning. (Courtesy ofCaldrill)
  2. 1 -99 Caldrill I drilling with dynamic positioning. (Courtesy ofCaldrill)
  3. The vessel was equipped to drill 6,000 feet with 4V2 inch drill pipe. The Caldrill I has operated in 25 foot swells off Nova Scotia and has maintained position in the face of wind gusts up to 60 mph and ocean currents of 3 knots. The vessel is still in service and has operated in the oceans of the world.

For reliability for live well operations, the Caldrill I has four 300 horsepower thrusters with two separate dynamic positioning systems. The thrusters on both the Caldrill I and the Eureka are open propeller type and retracting. Two taut line systems 011 the Caldrill I provide position sensing.

Other Ships

The French vessel La Terebel made its debut in December 1964. This vessel was 85 meters long, 12 meters in beam (278 ft, by 39 ft.) with two 300 horsepower azimuthing thrusters. The thrusters were directly driven by diesel engines.

In 1967, the Mission Capistrano, under U.S. Navy contract to Hudson Laboratory, was used for anti-submarine warfare work. The Mission Capistrano had two 1,000 horsepower azimuthing thrusters. The position computation system was made by General Motors AC Division.

Challenger

In August 1968 the Glomar Challenger appeared, 400 ft. long by 65 ft. beam, displacing 10,500 long tons (Figure 1-100). The Challenger had two main screws with 2,250 horsepower each and was powered by three GE 752, 750 horsepower DC traction motors. Four tunnel thrusters were each powered by 750 horsepower traction motors to provide 3,000 horsepower for thwart-ships thrust. The Challenger has used two acoustic positioning systems—a phase comparison system, and a time of arrival system, Position computation was provided by a General Motors AC division system.

Saipem II, Pelican, Sedco 445

In 1971 and 1972, three major drillships having an obviously similar mission but showing distinct differences in design

Fig. 1-100 Glomar Challenger. (Courtesy of Globe! Marine)

philosophy were brought into existence. These ships were the Sedco 445, the Pelican, and the Saipem II dynamically positioned ships (Figures 1-101,1-102, and 1-103). It is interesting to note in Figure 1-104 that each ship has about 14,000 tons displacement, each is between 400 and 500 ft. long, and each ship is about 70 ft. beam. Cruising speed on all three vessels is about 14 knots. However, the similarities end here. Saipem II uses 4 Voith-Schneider omni-directional thrusters, the Pelican uses five 1,500 horsepower fixed tunnel thrusters and 2 main screws all with controllable pitch (CP), and the Sedco 445 uses nine 800 horsepower fixed thrusters with kort nozzles and a reversing, variable rpm fixed propeller drive on the thrusters and 2 main screws.

Specific references to design characteristics and operating experience of these vessels may be found in OTC (Offshore Technology Conference) papers beginning with 1970.

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Posted on October 28, 2011, in Drilling, Dynamic positioning and tagged , , , , , , . Bookmark the permalink. Comments Off on Background And History of Dynamic positioning.

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