Wells – Submerged well – Means removably connected to permanent well structure
Reexamination Certificate
1999-02-03
2001-08-21
Bagnell, David (Department: 3673)
Wells
Submerged well
Means removably connected to permanent well structure
C166S350000, C166S367000, C405S168100, C405S168300, C405S170000, C405S224200, C405S224300
Reexamination Certificate
active
06276456
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a riser system for accessing and servicing sub-sea oil or gas wells. The riser system may be used for production or to access an existing well to carry out intervention operations. Such access is required, for example, to take further measurements of the reservoir by introducing logging devices, for servicing or installation of electric submersible pumps to enhance production rates, replacing down-hole safety valves, cleaning out debris, zonal isolation re-perforating and for other reasons.
BACKGROUND OF THE INVENTION
Typically for a sub-sea production well, the original drilling platform will have been removed and the well head will have to be accessed by means of a suitable surface vessel. In order that the required operations can be carried out to the well, it is necessary that the movement of the vessel, which is floating on the surface of the sea, be compensated, to ensure positional consistency with respect to the well itself, which is fixed on the sea bed. This is conventionally provided by means of a heave compensation system on the vessel itself which is extremely cumbersome and expensive.
GB-A-2297337 is an example of a riser system which overcomes this problem and provides compensation for the heave and swell of the sea by a continuous coiled tubing riser which extends from the surface vessel to the well and which adopts a controlled ‘S’ profile which accommodates the movements of the surface vessel without these movements affecting the lower regions of the riser at the well head end. The disadvantage of this system is that the whole riser has to be made out of special grade materials in order to be sufficiently flexible, and this is expensive, in particular for wells which are located on a sea bed more than several hundred or several thousand feet below the sea level.
Other existing systems which involve fixed or semi-fixed platforms are expensive to install and maintain.
OBJECT OF THE INVENTION
It is an objective of the invention to provide a riser system which can be rapidly deployed to provide a conduit between the well head and a floating vessel, and which can be disconnected safely in the event of an emergency and without incurring damage to the system.
SUMMARY OF THE INVENTION
According to the invention there is provided an offshore oil or gas well riser system which forms a connection between the well head and a surface vessel, the riser system comprising a first riser section which extends substantially vertically and is maintained vertically under tension, and a second riser section which is flexible and is arranged above the first riser and flexes to accommodate movements in the surface vessel, the second riser section including curvature limiting means to limit the extent of curvature of the second section.
Preferably, the curvature limitation is provided by a variable stiffness along the length of the second riser. Thus starting from the vessel it will be relatively stiff so that the bending of the riser in the region of the vessel is limited and the stiffness decreases with the distance from the vessel along the second riser reaching a minimum stiffness approximately midway along the riser, increasing in stiffness again towards its junction with the first riser.
Preferably emergency disconnect means are provided between the lower riser section and the upper flexible section.
Preferably, tension may be induced in the lower riser by a tensile tie to the vessel, wherein a constant load tensioning means is provided to maintain constant tension. The lower riser will preferably be a coiled tubing riser which is lowered down from a coiled tubing reel.
The upper flexible section of the riser extends for a depth which is sufficient to accommodate the expected heave on the surface vessel. This will vary according to the sea or ocean conditions but is preferably between 30 and 150 meters.
The lower section preferably has a control valve at its upper end which may be closed when it is required to access the inside of the riser at the surface vessel. Such access will be required to lower different tools and instruments into the well and the upper end of the riser needs to opened for this purpose which exposes the well fluids to the surface vessel. Because this is potentially dangerous the well fluids are isolated by closing the control valve. The well fluids in the riser above the control valve can either be bled away after the valve has been closed or alternatively pressed back down the riser by an inert fluid before the control valve is closed. Preferably the control valve is a double ball valve.
The position of the vessel may be adjusted so that a smooth, continuous and sufficiently shallow curve is provided in the flexible upper section of the riser permitting equipment to be transmitted inside between the surface vessel and the well.
Preferably the second riser comprises a plurality of discrete collars which are connected together and surround the outer diameter of the second riser. The collars are connected together by connection means which permit a limited degree of relative axial movement which controls the radius of curvature of the second riser section.
Preferably the curvature control means of the second riser section has two states, a first state in which the second riser section is permitted to bend sufficiently to be wound on a reel and a second state in which the second riser section is permitted to bend only to a lesser extent.
Preferably activation means are provided on the vessel to convert the second riser section from the first state to the second state as the riser is unreeled from a reel.
There is also provided by the invention a method of installation of the well riser system which comprises the steps of;
a. positioning a vessel approximately over a well head in which well intervention operations are required to be carried out;
b. lowering a first lower riser section towards the well head;
c. connecting the second flexible upper riser section to the upper end of the first lower riser section to form a continuous riser;
d. connecting the first lower riser to a support line at its upper end and lowering it further to the well head and connecting it thereto preferably with the assistance of a remote vehicle and/or guide lines; and
e. activating curvature limiting means on the second riser section from a first state in which the second section is able to be wound on the reel to a second state in which the curvature is limited to a greater extent.
Alternatively, the curvature of the second flexible riser section is maintained at a radius of curvature which is sufficiently large to accommodate the passage of long tools therethrough and at the same time is permitted to flex to accommodate the movement of the vessel.
In the preferred embodiment of the invention a method and apparatus is provided which provides positional consistency between the well head and the vessel and permits intervention into the well from a surface vessel with pumps, measuring equipment, drilling equipment and other tools without the need for an expensive heave compensation system on the vessel. The apparatus and method according to the invention also ensures that there is no damage caused to the well head by bending moments applied by movement of the riser connecting it to the surface vessel.
The preferred embodiment of the invention accommodates itself between the well head and the moving boat by the flexing of the second riser section and it is also maintained rigidly above the well head which prevents any bending of the riser in this region which could damage the well head and the connection thereto.
REFERENCES:
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patent: 4225270 (1980-09-01), Dareing
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patent: 4647255 (1987-03-01), Pow
patent: 4704050 (1987-11-01), Wallace
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patent: 4735267 (
Bagnell David
Dubno Herbert
Lee Jong-Suk
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