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Nowadays, the marine resources especially the development and utilization of offshore oil and gas resources are more and more attention around the world. Currently, offshore oil and gas has become a major source of the world’s oil and gas production growth. According to incomplete statistics, the world has discovered more than 2,000 offshore oil and gas fields, offshore oil and gas reserves account for 30-40% of all oil and gas reserves, and offshore oil and gas production accounts for over 30% of all oil and gas production, and proven rate of about 30 percent, still in the early stages of exploration. In this paper, the relevant application technology of offshore oil and gas drilling will be analyzed and made summarized overview.
2.1 Ocean Drilling
Ocean Drilling include: coastal and marine geology drilling, Marina (offshore) oil and gas drilling, Ocean Drilling scientific investigation.
In order to adapt to different depths, marine drilling facilities can be divided into the following categories:
(1) Artificia island, bridge, honing rack frame—essentially an extension of the land to the shallow water drilling, adapt depth 5-20 m.
(2) Base-mounted platform, adaptation depth 20-30m.
(3) Jack-up drilling rigs (platform) adaptation depth 20-110m
(4) Semi-submersible drilling vessel (the platform above the water 18m), adapt depth 60-200m, the deepest reaches 610m.
(5) Deep-sea drilling ship, adapt depth 300-6000m, is typically less than 610m.
2.2 The requirement of offshore oil and gas exploration for the drilling platform
Compared with onshore drilling, offshore oil and gas drilling need to overcome the impact of the marine environment. Sea condition will always cause drilling ship hull movement, and with changes in wind speed and current speed level exacerbated or diminished. As mush as possible to improve the stability of the hull, to ensure the effective operation of drilling rigs, weakened hull movement is essential.
After the movement of the ship at the sea were analyzed and found the drilling ship work or anchored later, there are six sports: vertical shift, shifting, heave, roll, pitch, roll bow. Any movement of the hull must consider the level that the staff could withstand to ensure the personnel security operations under special circumstances and conduct smooth of drilling.
The hull stability is the ship anti-capsized properties. It is related to the state of motion of the vessel tilted, the stability of the requirement are as follows:
(1) The intact stability —require righting moment must withstand the expected wind speed from any direction.
(2) Destabilize—Any spaces between the main case reasonably submerged vessel from any direction can withstand a certain wind loading.
(3) Dynamic Stability—with the term of wind capsizing moment to express, it is a function of tilt angle.
2.3 The main structure of offshore drilling equipment
Oil and gas drilling methods and technology in offshore oil and gas land drilling are applicable. However, due to the harsh impact of the physical and chemical properties of the marine environment and the natural geography of the sea, many drilling methods and technology is limited. Therefore, the structure of offshore drilling equipment is much more complex, offshore drilling rigs must be used. In addition to the land-based drilling equipment must be the same structure, as well as some of the equipment needed to make appropriate improvements and adjusted according to actual drilling conditions.
2.3.1 Casing head and casing
Underwater casing head and land drilling casing head is very similar, and its role is to:
(1) Must be able to support the drilling BOP stack.
(2) Can be suspended when cementing casing.
(3) In the drilling and exploitation must be able to be sealed between the layers of the casing string.
In order to meet the needs of offshore drilling, casing hanging floating drilling, casing seal, cement head with the land or jack-up rig operations are not the same, such as:
①under the sleeve, the last section of a cannula connected to hang and hang permanently before cementing casing head office, mud hung back through the sleeve grooves return;
②cement plug generally located in the casing head open with a remote control;
③casing seal is installed under the remote control method;
④with special test equipment for telemetry casing seal.
Early casing string has been extended from the seabed casing head to the surface, the traditional cement head injection operations can be carried out. Nowadays, more use of undersea cementing system, the special structure of the system makes use of the cement head in offshore is lighter than onshore, and easy to control.
BOP is designed for closed the wellhead under the pressure state. In order to maintain continuous control of the well, so that the formation fluid in the wellbore can be rotated out, require the use of several types of blowout preventer. At the same time in order to spare, the same type of blowout preventer should have two or more sets of numbers. Assembled several BOP together called BOP together.
The design process and technology of BOP and the experience of use BOP on land, are borrowed for floating drilling, but when the blowout preventer placed on the seabed, some improvements are needed for BOP, control systems, drilling technology, the use of methods aspects. The main changes are:
①BOP size increased;
②structural design must consider the seabed hydrostatic pressure;
③Since the delivery line is longer, the pressure drop increases, large blowout preventer corresponding require more liquid than onshore to manipulate BOP;
④To prevent the return line pressure drop, exhaust hydraulic fluid to go to sea;
⑤hydraulic fluid can contaminate water, with corrosion resistance, low viscosity, good lubrication, and can be mixed with highly mineralized water features;
⑥the change of BOP stack layout;
⑦long choke line pressure drop affect the process changes of well control.
2.3.3 Riser component
The riser is the transport links channel between drilling ship and submarine casing head. Riser components include: riser short; ball joints; sliding short; shunt; transition pipeline; buoyancy wheel.On the offshore oil and gas drilling platforms require a lot of additional equipment to compensate for the movement of the hull to ensure normal operation of the riser. The meaning of motion compensation is compensation of floating drilling rigs heave motion, the drilling platform relative to the sea bed to keep from affecting their movement, ie floating drilling process, the ship affected by the ups and downs role, while always keeping the drill pressure constant. Motion compensation modes: passive systems; active and semi-active systems; buffer short section.
2.3.4 Underwater equipment
The consisting of the offshore drilling equipment from the bottom to the top are as follows: drilling template (temporary drilling template)—TV guide tope and acoustic wave devices attached; permanently guide frame—permanent guide rope, casing head (group),hydraulic connector, BOP(group)—four double plate BOP, a ring-shaped hydraulic BOP, control box, riser system guide arm and television cameras attached; hydraulic connector—ball joints, clamps, transition line, lower ball joints, riser, telescopic riser, riser tensioning rope, shunt device and stretch shortened festival.
The main function of the underwater equipment is:
(1) Constitute an isolated sea channel from the wellhead to the rig. In order to put variety of drilling tools, return and export the drilling fluid.
(2) Through the BOP, well control-blooie pipe to control the subsea wellhead and well pressure.
(3) By ball joints, sliding short section, skew and telescopic of tensioning system to accommodate the heaving and sway of the drilling ship.
(4) Between the wellhead equipment and BOP group, BOP group and marine riser system, hydraulic connector for a drilling vessel with a quick disconnect with riser system in an emergency situation.
2.4 The technical features of offshore oil and gas exploration
At sea,due to the influence of physical and chemical properties of harsh natural environment and seawater, is difficult to use ground geological survey, a number of technical methods are limited, various methods of exploration need change to conduct in the survey vessel and drilling platform and the result is impact by the water depth and nature. It is to adapt to these factors weaken its adverse impact, offshore oil and gas drilling technique form some characteristics different from the land:
(1) Drilling platform has long life, high reliability, mainly reflected in the high strength requirements, fatigue life requirements, high construction technology requirements and high production management demanding.
(2) In order to med different sea area, different depths, different orientations and diversified operations, the drilling platform is variety, the variety drilling equipment and methods are also more complex.
(3) Offshore drilling exploratory wells in the layout must be very careful to ensure that maximize drilling efficiency, take the whole, taking quasi firsthand.
(4) Due to the offshore oil and gas engineering plant produced very serious marine accident, so the requirement of its safety performanceis greatly improved, especially for the design and implementation requirements, fire and environmental design, such as more stringent HSE.
In general, offshore oil and gas drilling technology are generally rendering as common technology, new technology and high technology absorption fusion each other to form a more technical structure.
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