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INTRODUCTION .............................................................................................................. 1
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POSITIONING OF DRILLSHIPS ..................................................................................... 3 2.1
Spread Mooring System .............................................................................................. 3
2.2
Turret Mooring Systems.............................................................................................. 3
2.2.1
External Turret Mooring System ......................................................................... 4
2.2.2
Internal Turret Mooring System .......................................................................... 5
2.2.3
Disconnectable Internal Turret Mooring Systems ............................................... 5
2.3
Dynamic Positioning System ...................................................................................... 6
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MOONPOOL ..................................................................................................................... 8
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DRILLING EQUIPMENTS ............................................................................................... 9
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4.1
Prime movers: ............................................................................................................. 9
4.2
Hoisting equipment: .................................................................................................... 9
4.3
Rotating equipment: .................................................................................................. 10
4.4
Circulating equipment ............................................................................................... 10
Heave Compensation ........................................................................................................ 11 5.1
Passive Heave Compensator: .................................................................................... 11
5.2
Active Heave Compensator:...................................................................................... 12
REFERENCES ................................................................................................................. 13
LIST OF FIGURES Figure 2-1: General Layout of Mooring Lines .......................................................................... 3 Figure 2-2: Working of Dynamic Positioning System. ............................................................. 7 Figure 2-3: Thruster ................................................................................................................... 7 Figure 3-1: Moonpool ................................................................................................................ 8 Figure 4-1: Schematic Representation of a Drillship. ................................................................ 9 Figure 4-2: Rotary Drilling Equipments .................................................................................. 10 Figure 5-1: Schematic Representation of Heave Compensating Equipments ......................... 11
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INTRODUCTION
The drillship is an adaptation for a standard seagoing ship of mono-hull form with the addition of a substructure with a moon pool and/or cantilevers from which the drilling operations may be carried out. These vessels are also equipped with some additional means of positioning the unit over the drill centre so that the vessel will maintain a close relationship with the bore hole in the seabed. These vessels may be held in position by either a mooring system or a dynamic positioning system. Drillships are often used to drill in very deepwater, which can often be quite turbulent. Drillships are exactly as they sound: ships designed to carry out drilling operations. These boats are specially designed to carry drilling platforms out to deep-sea locations. A typical drillship will have, in addition to all of the equipment normally found on a large ocean ship, a drilling platform and derrick located on the middle of its deck. In addition, drillships contain a hole (or 'moonpool'), extending right through the ship down through the hull, which allow for the drill string to extend through the boat, down into the water. Drillships are generally preferred for deepwater drilling in remote locations with moderate weather environments because of their mobility and large load carrying capability. Several marine drilling contractors operate drill ships as well as Semis and Jack-ups. Because of its conventional ship shaped, hull the drill ship is more prone to movement in a seaway than the semi-submersible, and is therefore subject to longer periods of downtime due to wind and wave action. For this reason drill ships are more usually (but not always) found working in the smoother waters of the world, while semi-subs can drill in the most hostile environments. This disadvantage is partially offset by the drill ship's ability to move from one location to the next rapidly and under its own power, with considerable economic advantage. For oil and gas offshore Exploration and Production (E&P) operations in waters deeper than 300 meters, floating platforms such as Drillships and Semi- Submersible Platforms are used. These vessels must be kept stationary at a desired location (reference) to accomplish their offshore E&P tasks. Therefore, the platform must have means of producing forces and moments to counterbalance environmental forces (wind, currents and waves) in order to keep it at the desired location. In the most common case, the platform is equipped with anchor lines. A mooring system usually has 8 to 12 anchor lines for each platform. However, in water depths deeper than 1000 meters, a mooring system becomes uneconomic or impracticable. This problem was overcome with the development of the Dynamic Positioning System (DPS).. There is always a lag of time between start of exploration drilling and first production in a 1
new area. This has to do with the time to make commercial finds but also, when it comes to the increasing water depths, it has to do with the qualification of new deepwater completion and production technologies. When a field moves into the production phase there is another time lag before well maintenance and intervention is needed. Some oil companies have made three categories of operations on this basis and the service industry has answered by proposing suitable vessels for the three different segments.
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2 2.1
POSITIONING OF DRILLSHIPS
Spread Mooring System
Spread mooring systems are multi-point mooring systems that moor vessels to the seabed using multiple mooring lines. The mooring lines can be directly attached to the hull of the vessel as well as indirectly using buoys on the sea surface. When they are directly attached to the hull, structural modifications are required to provide strong points for the mooring lines. Spread mooring systems with buoys are called Conventional Buoy Mooring (CBM) systems and do not require any modifications to the vessel hull structures.
Figure 2-1: General Layout of Mooring Lines 2.2
Turret Mooring Systems
The turret system performs four main functions 1. Maintaining the vessel on station through mooring. 2. Allowing weathervaning or rotation of the vessel to adjust to climate conditions. 3. Fluid transfer from the risers to the process plant. 4. Providing transfer of electrical, hydraulic and other control signals. The turret mooring system consists of a turret assembly that is integrated into a vessel and permanently fixed to the seabed by means of a mooring system. The turret system 3
contains a bearing system that allows the vessel to rotate around the fixed geostatic part of the turret, which is attached to the mooring system. The turret mooring system can also be combined with a fluid transfer system that enables connection of (subsea) pipelines to the vessel. The fluid
transfer system includes
risers between the pipeline end manifold (PLEM) at the seabed and the geostatic part of the turret. In the turret a swivel provides the fluid transfer path between the geostatic part and the free weathervaning vessel that rotates around the turret. The turret system is fully passive and does not require active vessel heading control or active rotation systems in the turret or swivels. The turret system can be located externally or internally with respect to the vessel hull structure. 2.2.1 External Turret Mooring System The external turret mooring system includes a turret system that is located at the extreme end of an outrigger structure attached to the bow of the vessel. This turret contains one largediameter 3-race roller main bearing that transfers both the vertical and the horizontal loads from the riser and mooring system to the outrigger structure. In addition, it transfers the own weight of the turret and the turntables above the main bearing to the outrigger. The turntables include the process facilities for commingling fluids from the risers using manifolds, as well as pigging facilities and/or chemical injection facilities to assure the flow in the risers. The swivel stack is located at the highest deck of these turntables and transfers fluid and electrical power to the turret access structure located on the outrigger. This access structure also provides support access to the different turntable decks. The external turret including (a part of) the outrigger can be installed as one integrated structure on the vessel bow, minimising the scope of work for the integration of the turret into the vessel. Furthermore this integration work can be executed while the vessel is afloat. Typically the cantilever type outrigger structure is integrated above the main deck of the vessel. However, depending on the departure angle of the mooring lines and on the size of the outrigger structure, part of the bow might need to be removed to avoid clashes between the vessel and the mooring lines. Optionally, risers are available that allow for fast disconnection from the turret. This disconnectable turret uses a buoy to keep the risers afloat when disconnected. Typically the 4
disconnectable option is required when the vessel operates on dynamically positioned (DP) station keeping. Disconnection within minutes is required to avoid damage to the risers during failures in the DP system causing drift-off or drive-off conditions. 2.2.2 Internal Turret Mooring System The internal turret mooring system includes a turret system that is integrated into the hull structure at the bow of the vessel. The internal turret is a slender structure that is connected to the vessel structure via a large-diameter 3-race roller main bearing at the top and via a largediameter sliding bearing at the bottom. The slender turret shaft ensures that the horizontal loads from the risers and mooring lines are transferred via the sliding bearing to the vessel, while the remaining forces from the turret and the vertical loads from the risers and mooring lines are transferred via the 3-race roller bearing. The supports of the bearings are integrated into the turret casing, which is part of the modified vessel hull structure. Typically the turret casing is a cylindrical structure that is integrated between the bottom and the deck of the vessel and transfers the turret loads via the vessel web frames to the vessel hull structures. The turntable decks are positioned on top of the 3-race roller bearing. The turntables include the process facilities for commingling fluids from the risers using manifolds, as well as pigging facilities and/or chemical injection facilities to assure the flow in the risers. In addition, the turntables provide space for subsea control facilities, to enable safe operation of the subsea manifolds and well heads. The highest deck level of the turntables provides space for the swivel stack, which transfers fluid and electrical power to the turret access structure that is located on the vessel main deck. This access structure also provides support access to the different turntable decks. 2.2.3 Disconnectable Internal Turret Mooring Systems Optionally, mooring and riser systems are available that can be disconnected from the turret. The disconnectable turret uses a buoy that is locked into a receptacle located at the lower end of the turret. The buoy provides support to the risers and mooring lines when disconnected. The buoyancy of the buoy matches the weight of the risers and mooring lines to ensure equilibrium at the required submerged depth. In ice-covered Arctic waters, the disconnection might be required while the mooring lines are highly tensioned in one direction. This asymmetrical mooring load can cause jamming of the buoy when it is disconnected. 5
The Arctic Turret offers a two-step disconnection mechanism to avoid this problem. In the first step the buoy is lowered below the hull while the mooring lines remain attached to the turret; in the second step the mooring lines are released to allow disconnection. The mooring lines are also connected to the buoy using jumper lines, to ensure that the mooring lines remain attached to the buoy in submerged position. External turrets are best suited for mild to medium environments, and hold a moderate number of risers. Internal turrets provide a better option for harsher environments and allow for the inclusion of a greater amount of risers. Disconnectable turrets are used in extreme environments, such as locations prevalent to typhoons, hurricanes or severe ice conditions. 2.3
Dynamic Positioning System
Drillships use what is known as 'dynamic positioning' systems. Drillships are equipped with electric motors on the underside of the ship’s hull, capable of propelling the ship in any direction. These motors are integrated into the ships computer system, which uses satellite positioning technology, in conjunction with sensors located on the drilling template, to ensure that the ship is directly above the drill site at all times. Dynamic positioning is a computer controlled system to automatically maintain a vessel's position and heading by using her own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyro compasses, provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position The DPS controls platform displacements in the three horizontal degrees of freedom: surge, sway and yaw. The DPS is composed of a controller, a sensor system, a thruster system, and a power system. The sensor system feeds to the controller (computer) with information about the platform positioning and environmental parameters – winds, currents and waves. The controller commands the action of thrusters, installed on the bottom of the platform hull, that generate the forces and moments needed to counteract the environmental forces and thus keep the platform at the reference. The controller keeps the platform within a tolerance radius of about 2% to 6% of the water depth, depending on the surface equipment and the operation to be executed. Furthermore, a DPS can also assist a moored platform as, for example, changing its head to minimize the environmental loads.
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Figure 2-2: Working of Dynamic Positioning System.
Figure 2-3: Thruster
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3
MOONPOOL
A moonpool is a large wall-sided hole in the bottom of a ship through which, for instance, equipment can be lowered into the sea or through which pipes (riser, cables or drills) are going. When not in operation the drillship moonpool is secured by airtight plugs. The bottom of moonpool in case of drillships is below the waterlevel. To prepare for drilling operation, the airtight plugs of moonpool are opened slowly and the water starts flooding inside moonpool.
The column of water inside the moonpool can, however, be excited at its own natural frequency resulting in large vertical motions. Water motions inside moonpool tend to be more violent when moonpool size increases. Internal sloshing can also occur, resulting in transverse breaking waves that are added to the vertical motions. Water motions in the moonpool can be excited through different mechanisms, in waves or in calm water with forward speed of the vessel. This dynamic magnification can cause slamming on diving bells or ROVs that are launched, green water over the edge of the moonpool which can be dangerous for the crew, or can increase drastically the resistance of the vessel in transit conditions. This can also lead structural damages due to slamming of equipments installed inside moonpool. To reduce the water motion moonpool damping devices are used.
Figure 3-1: Moonpool
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DRILLING EQUIPMENTS
Figure 4-1: Schematic Representation of a Drillship. Rotary drilling uses a sharp, rotating drill bit to dig down through the Earth's crust. The spinning of the drill bit allows for penetration of even the hardest rock. Main group of components include 4.1
Prime movers:
The prime movers in a rotary drilling rig are those pieces of equipment that provide the power to the entire rig. The energy from these prime movers is used to power the rotary equipment, the hoisting equipment, and the circulating equipment. 4.2
Hoisting equipment:
The hoisting equipment on a rotary rig consists of the tools used to raise and lower whatever other equipment may go into or come out of the well. The most visible part of the hoisting equipment is the derrick, the tall tower-like structure that extends vertically from the well hole. The hoisting system is made up of the drawworks, derrick, crown block, traveling 9
block, hook and wire rope. If a drill bit needs to be changed, either due to tear or a change in the subsurface rock, the whole string of pipe must be raised to the surface. The hoisting equipment is used to raise all of this equipment to the surface so that the drill bit may be replaced. 4.3
Rotating equipment:
The rotating equipment consists of components that actually serve to rotate the drill bit. The drill bit is located at the bottom end of the drillstring, and is responsible for actually making contact with the subsurface layers, and drilling through them 4.4
Circulating equipment
The circulating system consists of drilling fluid, which is circulated down through the well hole. The most common liquid drilling fluid, known as 'mud', may contain clay, chemicals, weighting materials, water and oil. The circulating system consists of a starting point, the mud pit, where the drilling fluid ingredients are stored. Mixing takes place at the mud mixing hopper, from which the fluid is forced through pumps up to the swivel and down all the way through the drill pipe, emerging through the drill bit itself. From there, the drilling fluid circulates through the bit, picking up debris and drill cuttings, to be circulated back up the well, travelling between the drill string and the walls of the well (also called the 'annular space'). Once reaching the surface, the drilling fluid is filtered to recover the reusable fluid.
Figure 4-2: Rotary Drilling Equipments 10
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HEAVE COMPENSATION
It is well known that ship type drilling units are very susceptible to wave action and will tend to move in a direct relationship with the sea state encountered. Since the vessel is connected to the seabed by a riser and the drill string is in contact with the bottom of the bore hole, motions of the vessel with respect to the seabed are extremely important to be able to maintain the drilling posture. Special equipments known as heave compensators are used for this purpose. There two heave compensating devices- Active heave compensators, passive heave compensators
Figure 5-1: Schematic Representation of Heave Compensating Equipments 5.1
Passive Heave Compensator:
The Passive Heave Compensator (PHC) is a reactive device. Using a large air cushion, the PHC attempts to isolate the drill string from the ship heave and has to overcome the friction 11
of seals each time the ship heaves up or down. When the ship heave is <3 m, the response time to overcome the seal friction is slow, resulting in a PHC efficiency <40%. The resulting drillstring motion is relative to the sea. floor is ~ 1m. On the other hand, when the ship heave is >4 m, the response time is faster and the PHC efficiency is ~85%. The resulting drill string motion is ~ 0.66m. 5.2
Active Heave Compensator:
The Active Heave Compensator (AHC) is a hydraulic power assist device to overcome the PHC seal friction and the drill string guide horn friction, which act in the opposite direction of the ship motion. By monitoring AHC rod position and the ship motion via a ship mounted motion reference unit, the hydraulic forces of the AHC counteract the PHC seal friction. Thus, the efficiency of the AHC is better than 95% for any ship heave condition, or about 4 in. at the rig floor for any heave. Prior to operation, the AHC is preloaded (biased) with the required weight on bit (WOB) using the PHC before the bit is landed on the bottom.
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REFERENCES
1. S K Chakraborty, “Handbook of Offshore Engineering Vol 1”. 2. Gang MA,“Analysis of Mooring Systems on a Drillship” 3. Lloyd's MIU Handbook of Maritime Security 4. Ocean Drilling Program “Active Heave Compensation”
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