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After the drilling unit’s anchors have been deployed on the new location the first item lowered is the temporary guide base (the TGB). This serves as a foundation for all the other subsea equipment that will follow, and as an anchorage for the guideline cables on which that equipment will be run down to the sea-bed.
The TGB is a circular, octagonal or square, flat, steel frame of about 100 square feet in area which has compartments in which ballast materials can be placed. The unit alone usually weighs about four tons, but it is heavily weighted with bags of cement, barite or other heavy materials before being lowered to the sea bed on the end of a string of drill pipe. A special tool for releasing the TGB when it is in position is fitted to the drill pipe string, and this connects with a slot in the steel guidebase frame. On the underside of the frame four spikes project to dig into the sea bed and firmly anchor the unit.
Four wires are attached to the edges of the TGB, and when it has been landed these are tensioned up and used for guiding other items of equipment down to their locations above the TGB. There are also two smaller lines for running TV cameras down for monitoring operations from the doghouse on the drill floor. The guide line tensioners are large cylinders, installed in the cellar deck on the rig, that contain pistons which are positioned by air pressure. The amount of tension required on the guide lines is simply set by adjusting the air pressure.
In the centre of the TGB frame is a wide circular aperture with a funnel shape projecting above it into which the bottom of another frame – the permanent guide base – will fit. The drill bit will commence drilling and all subsequent downhole operations will be conducted through this aperture.
3.0 THE PERMANENT GUIDE BASE
The permanent guide base (PGB) is another heavy steel frame, about 3 tons in weight and square in shape, that has a wide central aperture and a tall post on each corner through which the four guidelines run. The PGB serves as a landing seat for the wellhead and as a guide for drilling tools and the blow-out preventer stack which is eventually located above the wellhead. The posts are used to locate the stack, which has arrangements to accept them within its own frame.
The PGB is run down the guide lines to connect with the TGB, and there is a funnel-shaped projection around the aperture on its underside that inserts into the TGB’s funnel-shaped top aperture and ensures an accurate fit.
Beyond about 400 m water depth the guidewire system does not work too well so for deepwater a guideline less system using a capture funnel system is utilised.
4.0 THE WELLHEAD/CASING HANGAR SYSTEM
The wellhead is a large, cylindrical device housing several internal fittings called ‘casing hangars’ that are designed to suspend the required number and sizes of casing and tubing strings that will be used in the well. The wellhead/casing hangar locates through the holes in the two guide bases and fits into the top of the conductor casing after this has been run. It projects above the PGB, and is designed to connect with the BOP stack which is later run above it.
The Subsea Wellhead is the main structural component which supports the loads generated during drilling operations and during production Operations. It is connected (welded) to the inner conductor (18 ¾”) and is locked into the outer conductor, which itself is attached to the guide base.
Over recent years two connection profiles have become the most frequently used. These are proprietary designs known as “Vetco H-4” and the “Cameron Clamp Hub” profiles. Most UK drilling rigs are equipped with a single high pressure stack, though two stack systems were previously in use, to provide a low pressure connection at the 20″ and high pressure for the 13 5/8 inch casing.
The three main functions of a wellhead can be considered as :-
To provide a location for suspension of casing strings.
Each of the casing strings which run up through the well are physically suspended within the wellhead housing. Should the well be used for production, the production tubing is additionally supported and locked in position in the wellhead via the tubing hanger.
To provide Sealing and Pressure Containment
This sealing and containment takes place on two distinct areas, between the well and the environment and to provide isolation from between the casing and downhole structures.
During drilling operations a Blow Out Preventer (BOP) is installed on the wellhead at the base of the Marine Drilling Riser. The BOP is mandatory and used to protect the rig and the environment at the seabed level in case of gas kick from the well, to avoid a Blow Out.
Allows the installation of Flow Control Equipment
Should the well be converted from an appraisal well into a future production (NORMAL CASE) or water injection well, the subsea Xmas Tree can be installed to provide flow control from the well or from the injection line or manifold.
For subsea wells, the wellhead is supported through the 30 inch housing/conductor which is cemented into the first/primary soil formation. The 30 inch housing then supports the internal wellhead housing with the subsequent internal casing strings, each one supported via a casing hanger at the wellhead with a seal mechanism. The 30 inch housing supports the permanent guidebase which forms the main guidance mechanism for equipment being deployed subsea to the wellhead, during drilling and completion operations. Four Guide Posts and guide lines are used down to 400m. For deeper waters guide lines are not practical and funnel down or up systems are deployed.
The BOP stack and subsequently the Xmas Tree are connected to the wellhead by a hydraulically operated connector.
The primary environmental loading on the wellhead occurs during the drilling phase, when a high varying bending moment is transmitted through the riser onto wellhead housing and then to the 30 inch housing/casing/formation through current, wave loads and drilling vessel movement. The wellhead housing is locked into the 30 inch wellhead and loads on the wellhead are transmitted into the 30 inch conductor.
6.0 OVERVIEW OF SUBSEA OPTIONS; SHALLOW AND DEEP WATERS
The challenges to subsea production in the future are considerable, with solutions required to the economical extraction of oil and gas from ever deeper waters in remote locations.
Deep water options are illustrated in Figures 19 A & 19 B.
There are a number of options currently open in the movement of oil and gas from well to shore, and there are some limitations.
Availability of investment capital to achieve a profit from the expectation of sales of gas or oil in a depressed and volatile market.
The adverse effects of hydrostatic pressure on sensitive subsea equipment.
The limiting effects of hydrostatic pressure on the means of maintenance and intervention with subsea equipment.
Mechanical limitations on the type of ‘host’.
The limiting factors of distance between well and host.
Surface climatic conditions affecting the host annual storm patterns, ice, etc.
Subsea production equipment used to be much more expensive than land-based equipment, adapted for use on offshore platforms. This was by way of specification, manufacturing quality, complexity etc., as well as the requirements for specialist tools with which to run the equipment. Nowadays costs of subsea developments are very competitive to platform based systems. This is due to more than 15 years of operational experience, fabrication simplification, and competition coupled with the use of more simple functional specifications.
An additional feature of subsea production systems is their designed ability to operate in deepwater (diver less) areas. The development of deepwater systems, the installation and operational support systems using ROVs has been a major set in recent years. In earlier years such diver less systems were more expensive that diver assisted systems, but the advances now make then very similar in cost. In fact diver less operations using ROVs can be more cost effective than the use of very costly divers (requiring all the life support systems and complex DSV).
The purpose of subsea wellhead is
To support BOP and seal with well casing during drilling
â€¢ To support and seal the subsea production tree
â€¢ To support and seal the well casing
â€¢ To support and seal the production tubing hanger
The three main functions of a wellhead can be considered as;
i)To provide a location for suspension of casing strings;
Each of the casing strings which run up through the well are physically suspended within the wellhead housing. Should the well be used for production or injection, the tubing is additionally supported and locked in position in the wellhead via the tubing hanger (T.H.) when conventional dual bore trees are used.
(ii)To provide Sealing and Pressure Containment;
This sealing and containment takes place on two distinct areas, between the well and the environment and to provide isolation from between the casing and the down hole completion structures.
During drilling operations a Blow Out Preventor (BOP) is installed on the wellhead at the base of the Marine Drilling Riser. The BOP is mandatory and used to protect the rig and the environment at the seabed level in case of gas kick from the well, to avoid a Blow Out and the riser is mandatory to return the drilling fluids, etcâ€¦ to the rig. Refer to additional figures in Lecture L 5 B.
(iii)Allows the installation of Flow Control Equipment;
Should the well be converted from an appraisal well into a future production (NORMAL CASE) or water injection well (or gas injection well), the subsea Xmas Tree can be installed to provide production flow control from the well or into the well from the injection line or manifold.
In these notes, the emphasis is on the subsea wells drilled from a semi submersible or a drill ship. In the southern North Sea and in water depths generally up to 100 m, jack-up rigs are also used. These provide a fixed platform from which to drill the wells and the well can be directly tied back into the rig. During drilling operations, a high pressure riser system connects the well with the jack-up and no subsea B O P system is necessary.
For subsea wells, the wellhead is supported through the 30″ housing / conductor which is cemented into the first / primary soil formation. The 30″ housing then supports the internal wellhead housing with the subsequent internal casing strings. Each one is supported via a casing hanger at the wellhead and with a seal mechanism. The 30″ housing supports the Permanent Guide Base which forms the main guidance mechanism for equipment being deployed subsea to the wellhead, during drilling, completion operations and work over operations. Four Guide Posts and guide lines are used down to 500m .For deeper waters, guide lines are not practical and other guidance systems are in use with exploration drilling now reaching -2800m. Refer to section 6.
The BOP stack and subsequently the Xmas Tree are connected to the wellhead by an hydraulic operated collet connector. Over recent years two connection profiles have become the most frequently used. These are proprietary designs known as “Vetco H-4” and the “Cameron Clamp Hub” profiles. Most UK drilling rigs are equipped with a single high pressure stack, though two stack systems were previously in use, to provide a low pressure connection at the 20″ and high pressure for the 13 5/8 ” casings. A single connection of the BOP stack to the wellhead is the generally accepted method.
The primary environmental loading on the wellhead occurs during the drilling phase, when a high varying bending moment is transmitted through the riser onto wellhead housing and then to the 30″ housing/casing/formation through current, wave loads and drilling vessel movements. It is now common policy to run the two top joints of 1 ½” wall thickness joints of 30″ to provide the resistance to the anticipated loads, when previously, only 1″ casing was considered. The wellhead housing is locked into the 30″ conductor and loads on the wellhead are transmitted into it.
In addition to the drilling loads the wellhead may be subjected to impact and snagging loads associated with construction activities and accidental incidents such as fishing trawl board impacts and pull over forces. These loads are transmitted through the production tree into the wellhead connector and through into the 30″ conductor.
SUBSEA XMAS TREES FUNCTIONS;
The Xmas Tree is the primary flow control system for fluid flow from or into the well. The flows which can be considered include the following :-
(a) Production, – produced fluids from the oil/gas/condensate bearing formation pass-up through the well tubing and the Xmas Tree.
(b) Injection, – treated, inhibited seawater is pumped from the host facility into the well to provide pressure support for the reservoir.
Gas injection/disposal – excess gas is pumped into an adjacent reservoir for temporary storage or in limited cases, pressure maintenance.
Oil production with Gas lift, – gas is pumped from the host facility, through the annulus to subsurface injection points in the tubing. The inclusion of gas into the produced fluid reduces the column density to allow the reservoir pressure to flow the well. It is a flow boosting technique but with a low efficiency.
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