Saturday, 22 February 2014

Dynamic Positioning System - Part One

Unlike olden ages when ships were only used for cargo trades and passenger transport purposes, the present era has a lot more to do with ships. Ships are now not only used to transport cargo, but operations like mid-ocean research activities, underwater pipeline laying, dredging, sea bed drilling have also found themselves on the priority list. The offshore industry has also evolved rapidly from the primitive jacket platforms to the present day SPAR Platforms, Tension Leg Platforms, Semi-submersible platforms, Floating Production Storage and Offloading Vessels (FPSO), etc. Among these, certain category of ships and offshore structures are required to operate at conditions in which their movements and stability in maintaining positions at even the roughest seas must be controlled to the highest precision. 

You must have seen a helicopter hovering over an area, without gliding either way from its position. Well then, how is that done by ships? Dynamic Positioning System- A technology that was developed in the 1960s, which is designed to assist a vessel or an offshore structure to either maintain perfect station keeping or precisely follow a predefined path at sea; even at the roughest sea conditions where wave heights may rise to 21 meters. 

The dynamic positioning is a computer controlled system that uses a combined operation of underwater tunnel thrusters, propellors and rudders to assist the structure in maintaining its position or track. The entire system is controlled by a combined action of sensors, GPS, and mechatronic systems onboard. The range of operations carried out by vessels equipped with DP is wide today. Some of them are:
  1. Gravity Coring.
  2. Drilling (Oil Exploration)
  3. Oil Production Drilling
  4. Diving Support
  5. Pipe and cable laying operations
  6. Flo-tel Services
  7. Hydro-graphic survey purposes
  8. Survey of wrecks at sea
  9. Dredging
  10. Underwater rock dumping
  11. Heavy lifting operations
  12. Platform supply
  13. Shuttle tanker offtake operations
  14. Oceanographic research activities
  15. Underwater mining practices
The only reason why DP systems are used is their efficiency and redundancy. Efficiency in the sense, DP systems are precise to the orders of few meters (50 meters of precision at open sea is equivalent to a precision in micrometers in brain surgery). The redundancy of DP systems is sufficiently reliable; that is, if one computer control system fails, there are three more backups to keep the operation running without a single glitch.  

In this article various applications of DP systems have been explained with appropriate examples to ensure the realization of the necessity of dynamic position in the modern day operations at sea. The original article is divided into parts. Each part would deal with a couple of DP System applications in different ships or offshore structures.

  • Dynamic Positioning in Cruise Ships:
Ever wondered how does the captain of a cruise ship manage to dock the monster vessel with centimeters of precision in spite of the space constraints at the port? That's correct. Dynamic Positioning is also used in cruise ships for counteracting the challenges faced during berthing at shallow draft ports. Not only that, it improves the flexibility of the ship and avoids the requirement of anchoring in areas where the sea bed is sensitive. Note in the video below, how the Queen Mary 2 berths precisely at ports using its DP System.

  • Dynamic Positioning in Pipe-laying Ships:
Subsea pipelines and cables are layed for transfer of gas, oil, and communication lines across oceans. With today's cutting edge ship design it is evident that ships are well built to carry out the laying of these pipelines and cables on the sea bed. These operations are highly risky with big money at stake. Any damage to the pipeline or the vessel itself during the process can jeopardize the entire operation. To understand why such a vessel would require a DP system, it is important to first know how pipelines are laid on the sea bed. 

Subsea pipe laying operation
Pipe segments are welded in a weld assembly line within the pipe laying vessel at then passed on to the sea bed through the aft of the ship. The pipe emerging out of the assembly line is supported at the aft by a steel structure called the stinger. In spite of being made of stainless steel, the pipe is nothing but a strand of thread when lowered into the ocean. Thus, it forms a curved shape right from the end of the stinger to the touchdown point, where it touches the sea bed. As the ship moves forward, the pipe is laid on the sea bed. Due to a pre-designed map of the pipelines, the ship is required to follow the track exactly without any room for error. If the tension in the entire line is lost, the pipe would buckle at the touchdown point and the entire operation would shut down. Precise calculations are done to predict all the parameters before the operation is commenced. 

One could wonder what would possibly have happened if a strong wave hit the ship at the time of operation. Or, what is the ship failed to follow the predefined track along which the pipe was to be laid? Once the pipe is laid, it is almost impossible to lift it back up. The saviour here is again Dynamic Positioning System onboard the vessel which keeps the ship constrained against any sway or yaw motions, maintaining a predefined surge (forward movement of the ship is called surging motion). The DP system also controls the movements of the ship according to the tension in the emerging in the pipeline, resulting in overall dynamic control over the entire process. 

You can read the second part of the series here. Note in the following video how the world's largest pipe laying vessel MV Solitare uses its Dynamic Positioning to lay pipes in the worst sea conditions. LSD

Article By: Soumya Chakraborty

Author's Note: This article is the first part on Dynamic Positioning Systems. Prime focus has been laid on basic understanding of DP systems and two applications of the same have been depicted with videos and illustrations. I have written this article based on my experience and technical knowledge on the same as a Naval Architect. The videos and the figure donot belong to LSD, and full credit for the same goes to their respective owners. The second part of the article would deal with how dynamic positioning is used in Drillships. If you have any queries or doubts, write to me

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