Dynamic Positioning from A to Z (Part II)

I. History

It all started in the 1960s when the offshore drilling moved to deeper waters and Jack-up barges could not be used anymore and anchoring became less economical. The first vessel to use dynamic positioning was “CUSS 1”, made of an old war surplus YF barge at 3400 LT displacement. The ship had 4 propellers and on its first operation, in La Jolla, California, managed to keep its position above the well at a depth of 948 meters. After this first well, the vessel did other 5 drillings at a depth of 3560 meters, managing to maintain its position within a radius of 180 meters. The vessel’s position was kept manually and it was determined by radar ranging to buoys and sonar ranging from subsea beacons. “Eureka” was the first true DP vessel, built from scratch by Shell at the Orange shipyard in Texas. The vessel was equipped with port and starboard propellers which could be rotated 360 degrees in order to move the ship in any direction. The ship had also a positioning device tied to the ocean floor, made of a thin wire that ran up through an on-board tiltmeter. By measuring the angle of the wire, this mechanical device managed to calculate the position of the ship relative to the wellhead. The operators used a joystick to engage the forward and aft thrusters, but later on this joystick it was changed with electro-mechanical devices which used vacuum tubes and performed much better. All this was possible thanks to Howard Shatto, a Shell engineer who came up with the whole idea of Dynamic Positioning. Although the “Eureka” revolutionized the offshore industry, it was mainly used for drilling core samples. Ten years later, in 1971, the first purpose built dynamically positioned drill ship was released. “SEDCO 445” was equipped with a Honeywell Automatic Station Keeping System developed by Howard Shatto and it was ready for exploration wells. In the year 1980 there were about 65 DP capable vessels operating in the offshore industry.  By 1985, due to the integration of the Satellite technology being used as reference, the number of DP operated vessels increased to over 150. What really modernized and caused the growth of worldwide DP fleet was the GPS development. In 1990, due to the continued use of the DP vessels, the Dynamic Positioning Vessel Owners Association (DPVOA) was formed. This association began to collect incident data and developed guidelines based on that information. Working with the IMO they formed the Maritime Safety Committee Circular 645 and established International guidelines for all the vessels equipped with DP built after the 1st of July1984. IMCA (International Marine Contractors Association), the main DP regulatory body was created in 1995 by the merging of the DPVOA with the International Association of Diving Contractors.  Since its appearance and up until present the IMCA has released over 50 guidance documents based on the information gathered over the years.

While the first DP vessels had analogue controllers and lacked redundancy, up until now, a lot of progress was made. The DP system is no loner used only in the oil industry to maintain a fixed position, but also on other various types of ships and operations such as cable lay, survey, pipe laying, supplying etc.

II. Pros and Cons of the Dynamic Positioning System

The advantages of using a DP vessel are:

• The vessel is funny self-propelled
• No tugs are ever required
• Great maneuverability of the vessel
• Rapid response to weather changes and in the requirements of the operation
• The system is very versatile
• Can work in any water depth
• More economical
• Can complete short tasks more quickly
• Does not damage the seabed hardware with mooring lines and anchors
• Doesn’t exist the risk of cross-mooring with other vessels or fixed platforms
• Can change location very quickly

The disadvantages of using a DP vessel are:

• It can fail to keep position if the equipment has a failure
• Higher day rates for the personnel on board
• Higher fuel consumption
• The thrusters can hurt divers and ROVs
• In strong tides, shallow waters or extreme weather can lose position
• Position control and equipment rely on a human operator
• More personnel needed to operate and maintain equipment
• Only specially trained seafarers can operate the system and the courses are very expensive