09-04-2011, 10:48 AM
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INTRODUCTION
Dynamic positioning (DP) is a rapidly maturing technology, having been born of necessity as a result of the increasing demands of the rapidly expanding oil and gas exploration industry in the 1960s and early 1970s. Even now, when there exist over 1,000 DP-capable vessels, the majority of them are operationally related to the exploration or exploitation of oil and gas reserves.
The demands of the offshore oil and gas industry have brought about a whole new set of requirements. Further to this, the more recent moves into deeper waters and harsh-environment locations, together with the requirement to consider more environmental-friendly methods, has brought about the great development in the area of Dynamic Positioning techniques and technology.
1.1 HISTORY
The first vessel to fulfil the accepted definition of DP was the "Eureka", of 1961, designed and engineered by Howard Shatto. This vessel was fitted with an analogue control system of very basic type, interfaced with a taut wire reference. Equipped with steerable thrusters fore and aft in addition to her main propulsion, this vessel was of about 450 tons displacement and length 130 feet.
By the late 1970s, DP had become a well established technique. In 1980 the number of DP capable vessels totalled about 65, while by 1985 the number had increased to about 150. Currently (2002) it stands at over 1,000 and is still expanding. It is interesting to note the diversity of vessel types and functions using DP, and the way that, during the past twenty years, this has encompassed many functions unrelated to the offshore oil and gas industries
1.2 APPLICATIONS
• coring
• exploration drilling (core sampling)
• production drilling
• diver support
• pipelay (rigid and flexible pipe)
• cable lay and repair
• multi-role
• accommodation or 'flotel' services
• hydrographic survey
• pre- or post-operational survey
• wreck survey, salvage and removal
• dredging • rockdumping (pipeline protection)
• subsea installation
• lifting (topsides and subsea)
• well stimulation and workover
• platform supply
• shuttle tanker offtake
• Floating production (with or without storage)
• heavy lift cargo transport
• passenger cruises
• mine countermeasures
• oceanographical research
• seabed mining
DP is also used in
• rocket launch platform positioning
• repair/maintenance support to military vessels
• ship-to-ship transfer and
• manoeuvring conventional vessels
CHAPTER 2
BASIC PRINCIPLES OF D
Dynamic Positioning can be described as an integration of a number of shipboard systems to obtain the ability of accurate manoeuvrability. DP can be defined as:
“A system which automatically controls a vessel's position and heading exclusively by means of active thrust.”
A convenient way of visualizing the inter-relation of the various elements of a DP system is to divide the system into six parts, as the following sketch shows.
Sketch 1 - Schematic Diagram of a DP system
The prime function of a DP system is to allow a vessel to maintain position and heading. A variety of further sub-functions may be available, such as track-follow, or weathervane modes, but the control of position and heading is fundamental.
2.1 DEGREES OF FREEDOM
Any vessel (or other object) has six freedoms of movement; three rotations and three translations. In a vessel they can be illustrated as roll, pitch, yaw, surge, sway and heave.
Sketch 2 - The Six Freedoms of Movement
Dynamic positioning is concerned with the automatic control of surge, sway and yaw. Surge and sway, of course, comprise the position of the vessel, while yaw is defined by the vessel heading. Both of these are controlled about desired or "setpoint" values input by the operator, i.e. position setpoint, and heading setpoint. Position and heading must be measured in order to obtain the error from the required value. Position is measured by one or more of a range of position references, while heading information is provided from one or more gyrocompasses. The difference between the setpoint and the feedback is the error or offset, and the DP system operates to minimise these errors.
The vessel must be able to control position and heading within acceptable limits in the face of a variety of external forces. If these forces are measured directly, the control computers can apply immediate compensation. A good example of this is compensation for wind forces, where a continuous measurement is available from windsensors.. Sensors connected to the cable tensioners, and the fire monitors allow direct feedback of these "external" forces to the DP control system and allow compensation to be ordered from the thruster before an excursion develops.
In addition to maintaining station and heading, DP may be used to achieve automatic change of position or heading, or both. The DP operator (DPO) may choose a new position using the control console facilities. The DPO may also choose the speed at which he wants the vessel to move. Similarly, the operator may input a new heading. The vessel will rotate to the new heading at the selected rate-of-turn, while maintaining station. Automatic changes of position and heading simultaneously are possible.
CHAPTER 3
DP MODEL
Every vessel is subjected to forces from wind, waves and tidal movements as well as forces generated from the propulsion system and other external elements (fire monitors, pipelay tension, etc). The response to these forces is vessel movement, resulting in changes of position and heading. These are measured by the position reference systems and gyro compasses. The DP control system calculates the offsets between the measured values of position and heading, and the required (or setpoint) values, and calculates the forces that the thrusters must generate in order to reduce the errors to zero. In addition the DP control system calculates the wind force acting upon the vessel, and the thrust required to counteract it based on the model of the vessel held in the computer.
Modeling and filtering enable a 'dead reckoning' or 'DR' mode (often called 'memory') to operate if all position references are lost. The vessel will continue to maintain position automatically, although the position-keeping will deteriorate with the increasing length of time since the last position data received. In practical terms, this means that the DPO does not need to immediately select "manual" control upon the loss of all position reference.
The difference between the thrust calculated from the model and the wind speed and direction is the force taken as the current. The current force or 'sea force' is therefore a summation of all the unknown forces and errors in the DP model and displayed in the model as the speed and direction of the current.
INTRODUCTION
Dynamic positioning (DP) is a rapidly maturing technology, having been born of necessity as a result of the increasing demands of the rapidly expanding oil and gas exploration industry in the 1960s and early 1970s. Even now, when there exist over 1,000 DP-capable vessels, the majority of them are operationally related to the exploration or exploitation of oil and gas reserves.
The demands of the offshore oil and gas industry have brought about a whole new set of requirements. Further to this, the more recent moves into deeper waters and harsh-environment locations, together with the requirement to consider more environmental-friendly methods, has brought about the great development in the area of Dynamic Positioning techniques and technology.
1.1 HISTORY
The first vessel to fulfil the accepted definition of DP was the "Eureka", of 1961, designed and engineered by Howard Shatto. This vessel was fitted with an analogue control system of very basic type, interfaced with a taut wire reference. Equipped with steerable thrusters fore and aft in addition to her main propulsion, this vessel was of about 450 tons displacement and length 130 feet.
By the late 1970s, DP had become a well established technique. In 1980 the number of DP capable vessels totalled about 65, while by 1985 the number had increased to about 150. Currently (2002) it stands at over 1,000 and is still expanding. It is interesting to note the diversity of vessel types and functions using DP, and the way that, during the past twenty years, this has encompassed many functions unrelated to the offshore oil and gas industries
1.2 APPLICATIONS
• coring
• exploration drilling (core sampling)
• production drilling
• diver support
• pipelay (rigid and flexible pipe)
• cable lay and repair
• multi-role
• accommodation or 'flotel' services
• hydrographic survey
• pre- or post-operational survey
• wreck survey, salvage and removal
• dredging • rockdumping (pipeline protection)
• subsea installation
• lifting (topsides and subsea)
• well stimulation and workover
• platform supply
• shuttle tanker offtake
• Floating production (with or without storage)
• heavy lift cargo transport
• passenger cruises
• mine countermeasures
• oceanographical research
• seabed mining
DP is also used in
• rocket launch platform positioning
• repair/maintenance support to military vessels
• ship-to-ship transfer and
• manoeuvring conventional vessels
CHAPTER 2
BASIC PRINCIPLES OF D
Dynamic Positioning can be described as an integration of a number of shipboard systems to obtain the ability of accurate manoeuvrability. DP can be defined as:
“A system which automatically controls a vessel's position and heading exclusively by means of active thrust.”
A convenient way of visualizing the inter-relation of the various elements of a DP system is to divide the system into six parts, as the following sketch shows.
Sketch 1 - Schematic Diagram of a DP system
The prime function of a DP system is to allow a vessel to maintain position and heading. A variety of further sub-functions may be available, such as track-follow, or weathervane modes, but the control of position and heading is fundamental.
2.1 DEGREES OF FREEDOM
Any vessel (or other object) has six freedoms of movement; three rotations and three translations. In a vessel they can be illustrated as roll, pitch, yaw, surge, sway and heave.
Sketch 2 - The Six Freedoms of Movement
Dynamic positioning is concerned with the automatic control of surge, sway and yaw. Surge and sway, of course, comprise the position of the vessel, while yaw is defined by the vessel heading. Both of these are controlled about desired or "setpoint" values input by the operator, i.e. position setpoint, and heading setpoint. Position and heading must be measured in order to obtain the error from the required value. Position is measured by one or more of a range of position references, while heading information is provided from one or more gyrocompasses. The difference between the setpoint and the feedback is the error or offset, and the DP system operates to minimise these errors.
The vessel must be able to control position and heading within acceptable limits in the face of a variety of external forces. If these forces are measured directly, the control computers can apply immediate compensation. A good example of this is compensation for wind forces, where a continuous measurement is available from windsensors.. Sensors connected to the cable tensioners, and the fire monitors allow direct feedback of these "external" forces to the DP control system and allow compensation to be ordered from the thruster before an excursion develops.
In addition to maintaining station and heading, DP may be used to achieve automatic change of position or heading, or both. The DP operator (DPO) may choose a new position using the control console facilities. The DPO may also choose the speed at which he wants the vessel to move. Similarly, the operator may input a new heading. The vessel will rotate to the new heading at the selected rate-of-turn, while maintaining station. Automatic changes of position and heading simultaneously are possible.
CHAPTER 3
DP MODEL
Every vessel is subjected to forces from wind, waves and tidal movements as well as forces generated from the propulsion system and other external elements (fire monitors, pipelay tension, etc). The response to these forces is vessel movement, resulting in changes of position and heading. These are measured by the position reference systems and gyro compasses. The DP control system calculates the offsets between the measured values of position and heading, and the required (or setpoint) values, and calculates the forces that the thrusters must generate in order to reduce the errors to zero. In addition the DP control system calculates the wind force acting upon the vessel, and the thrust required to counteract it based on the model of the vessel held in the computer.
Modeling and filtering enable a 'dead reckoning' or 'DR' mode (often called 'memory') to operate if all position references are lost. The vessel will continue to maintain position automatically, although the position-keeping will deteriorate with the increasing length of time since the last position data received. In practical terms, this means that the DPO does not need to immediately select "manual" control upon the loss of all position reference.
The difference between the thrust calculated from the model and the wind speed and direction is the force taken as the current. The current force or 'sea force' is therefore a summation of all the unknown forces and errors in the DP model and displayed in the model as the speed and direction of the current.