Patent Application
20170356265
The invention relates to a device for reducing the strain on a wellhead casing from a bending moment generated by a horizontal load component from a well element arranged over a wellhead.
As a rule, installing elements on a wellhead, in particular a blowout preventer (BOP), at the top of a wellhead casing which extends down through unconsolidated masses in the sea floor, usually with an upper wellhead-casing portion surrounded by and fixed to a conductor casing, involves a risk of fatiguing the wellhead casing, by the wellhead being subjected to lateral forces so that the wellhead casing is being bent. The lateral load may arise in consequence of drift of a riser extending through the water masses from the wellhead upwards to a surface installation. When a blowout preventer weighs 250-500 tonnes and has a vertical extent of up to 14-16 metres and a horizontal extent of 5-6 metres, such a bending strain will increase in that the load that is resting on the wellhead casing will have its center of gravity displaced away from the original, vertical center axis of the wellhead. The problem is described among other things by Dahl Lien: “Methods to Improve Subsea Wellhead Fatigue Life”, a project assignment at the Faculty for engineering science and technology, the Institute for petroleum technology and applied geophysics, NTNU, Trondheim, Norway, 2009. The situation may lead to deformation of the wellhead casing and, at worst, fatigue and rupturing. The problems intensify as the safety requirements are being increased, for example illustrated by the fact that while pressure barriers were earlier dimensioned to withstand 5000 psi, the requirements have gradually increased to 15000 psi, and associated valves have gone from 4 to 6 levels. The use of deep-water rigs with heavy subsurface safety equipment at moderate water depths has further intensified the problems. It has been recorded that the wellhead has been subjected to strains of up to 90% of the critical limit of the wellhead as regards fatigue.
From the prior art describing solutions to the problem of fatiguing the wellhead casing which forms the foundation for wellhead elements, the inventor's own suction foundation (Conductor Anchor Node=CAN) may be mentioned, disclosed in NO U.S. Pat. No. 313,340, included in its entirety herein by reference, in principle providing a larger contact surface between the upper part of the conductor casing and the surrounding seabed mass, the diameter of the suction foundation typically being approximately 6 metres, whereas the diameter of the conductor casing is in the range of 0.75-0.90 m (30-36 inches).
It is also known (Dahl Lien 2009, see above) to use moorings extending at outward and downward angles from an upper portion of a wellhead installation to the seabed where the moorings are secured to anchors.
From NO U.S. Pat. No. 305,179, a suction anchor enclosing an upper portion of a conductor casing and parts of a wellhead is known. To the wellhead, a frame is connected, arranged to carry a swivel device for the horizontal connection of a riser et cetera, the frame resting on separate suction anchors placed at a distance from the former suction anchor.
From the applicant's own NO U.S. Pat. No. 331,978 (and the corresponding WO publication 2011162616 A1), a stabilizing device for a wellhead with the upper portion of a wellhead casing projecting up above a seabed is known, in which a wellhead valve which projects up from the upper portion of the wellhead casing is completely or partially supported on the suction foundation by several supporting elements being arranged between the wellhead valve and the suction foundation.
US2006162933A1 discloses a system and a method of establishing a subsea exploration and production system, in which a well casing, projecting up from a seabed where a well is to be stablished, is provided with a buoyancy body arranged at a distance above the seabed. The buoyancy body is stabilized by means of adjustable stabilizing elements, which are anchored to the seabed at a distance from the well casing.
US2010/0212916 A1 is disclosing a stabilizer for a wellhead, comprising: a ground engaging support structure having lateral dimensions suitable for laterally stabilizing the wellhead; wellhead stabilizer elements disposed within the ground engaging support structure, the wellhead stabilizer elements having wellhead abutting faces spaced to laterally cage the wellhead to restrict lateral movement of the wellhead while permitting the wellhead to move in a vertical direction. The wellhead may include various wellhead components, including for example casing bowls, spools, blowout preventers, and other suitable components. The portion of wellhead that is laterally caged need not be circular in cross-section, but may be a suitable geometry.
To try to meet the constantly increasing challenges when it comes to avoiding fatigue fracturing of the wellhead, the dimension of the wellhead casing has gradually been increased, the diameter having increased from 30 inches to 36 inches and further to 42 inches, with a wall thickness that has increased from 1 inch all the way up to 2 inches.
In the further description, the term “wellhead valve” covers both a blowout preventer (BOP) alone and also a combination of a blowout preventer and other valve types (for example production valves), and other valve types or combinations of valve types alone, said wellhead valve being arranged on a wellhead on an end portion of a wellhead casing projecting above a seabed.
The invention has for its object to remedy or reduce at least one of the drawbacks of the prior art or at least provide a useful alternative to the prior art.
The object is achieved through the features, which are specified in the description below and in the claims that follow.
The invention provides a method and a device for reducing the risk of fatigue in a wellhead without increasing the pipe dimension, that is to say the pipe-wall thickness, the pipe diameter or the material quality, of the wellhead casing projecting up above the seabed and forming the wellhead, and without intervening in valves and so on mounted on the wellhead. The invention involves having a supporting frame, which, at a distance from the well center, is supported on a foundation that rests on a seabed, rigidly connected to the wellhead casing to absorb a substantial portion of a bending moment applied to the wellhead casing by a horizontal load component. Calculations show that the bending stresses on the wellhead casing can be reduced considerably by the supporting frame absorbing a substantial part of the load caused by horizontal load components affecting the wellhead. Such horizontal load components may, for example, be caused by a connected riser being bent out sideways, for example because of sea currents. Studies have shown that bending stresses on the wellhead casing can be reduced to a range of 5-25% of the total torque by the supporting frame relieving the wellhead casing. The material stresses in the wellhead casing will thereby be reduced correspondingly and, with a view to fatigue, the lifetime of the wellhead casing will increase. With a conservatively estimated effect by which the load on the wellhead casing is reduced to 10%, the supporting frame taking 90% of the load, the stresses in the wellhead casing will be reduced to 10%, which results in an increase in the estimated lifetime of the wellhead casing by 1000 times seen in relation to fatigue.
The invention is defined by the independent claim. The dependent claims define advantageous embodiments of the invention.
The invention relates, more specifically, to a device for reducing the strain on a wellhead casing from a bending moment generated by a horizontal load component from a well element arranged over a wellhead, characterized by a supporting frame being connected to an upper portion of the wellhead casing and projecting outwards from the center axis of the wellhead casing and being provided with abutments resting in a supporting manner on a base at a radial distance from the wellhead casing, the supporting frame being arranged to absorb a portion of said bending moment.
The supporting frame may include a well-casing extension adapted for connection to the wellhead casing. The advantage of this is that the wellhead casing can thereby be protected from bending stresses from drilling operations during the establishing of the well, as, in this phase, the bending moment from a blowout valve and other elements temporarily installed over the wellhead subject only the supporting frame and the well-casing extension to strain, and this is removed after the drilling operations have been carried out, and the well casing is possibly provided with a new supporting frame connected directly to the wellhead casing.
The ratio of the maximum bending moment absorbed by the supporting frame to the bending moment applied to the wellhead casing may be at least 1:2, alternatively at least 3:4, alternatively at least 9:10.
The connection between the supporting frame and the wellhead casing, possibly between the supporting frame and the well-casing extension may be formed as a zero-clearance connection. An advantage of this is that any bending moment applied will, in the main, be absorbed immediately by the supporting frame.
The supporting frame may include a coupling formed as a sleeve enclosing a portion of the wellhead casing or the well-casing extension, by a press fit. The sleeve may have been shrunk around a portion of the wellhead casing or the well-casing extension. An advantage of this is that the connection can be machined with moderate requirements of tolerance, and the shrinking may be provided by heat development during the welding-together of the sleeve and the projecting elements of the supporting frame.
The base may be a seabed or a wellhead foundation. The advantage of this is that the supporting frame may be placed on the type of base that is the most suitable in each situation.
In what follows, an example of preferred embodiments is described, which is visualized in the accompanying drawings, in which:
Reference is first made to
The wellhead casing 12 is shown here as a casing 122 extending up through a so-called conductor casing 121 which bounds the well 1 in a manner known per se towards the unconsolidated masses in the upper part of the base 4. An upper portion 12a of the wellhead casing 12 is rising from an upper portion 121a of the conductor casing 121.
Connected to the upper portion 12a of the wellhead casing 12, there is a supporting frame 6 which projects radially outwards from the wellhead casing 12 and is provided with several abutments 61 resting in a supporting manner on a base 13 shown schematically here as an element which is partially embedded in the seabed 41. The base 13 may be any wellhead foundation, for example a suction foundation or a well frame which provides a sufficiently large degree of stability and ability to absorb a load Lv which is transmitted through the supporting frame 6.
The wellhead casing 12 and the supporting frame 6 are connected to each other in a way that makes it possible for the supporting frame 6 to absorb a bending moment Mf as a reaction to the horizontal load component Lh from the wellhead element 2 subjecting the wellhead casing 12 to said bending moment Mw. A coupling 62 may be arranged in such a way that the wellhead casing 12 is allowed a certain deflection before hitting the supporting frame 6 and the further load being substantially absorbed by the supporting frame 6. The design of the coupling 62 and the dimensioning of the supporting frame 6 can thereby be used to control how great a load the wellhead casing 12 may be subjected to. Calculations carried out by the applicant and other instances have shown that the supporting frame 6 may absorb 75 to 95% of the strain caused by said horizontal load component Lh.
To ensure a greatest possible relief of the wellhead casing 12, the coupling 62 is advantageously formed as a sleeve 621 surrounding a portion of the wellhead casing 12 without radial clearance. This is advantageously achieved by shrinking the sleeve 621.
The supporting frame 6 according to
When the supporting frame 6 is mounted on the wellhead 11 and the wellhead 11 is subjected to a bending moment Mw generated by a horizontal load component Ln from above-lying elements 2, 3, the supporting frame 6 is subjected to a vertical load Lv which is transmitted to the seabed 41 at a distance from the center axis of the wellhead casing 12 through the abutment of the supporting frame 6 against the base. Depending on the amount of play the coupling 62 between the supporting frame 6 and the wellhead casing 12 allows and how great a bending stiffness the wellhead casing 12 and the supporting frame 6 exhibit, the portion of the applied bending moment Mw absorbed by the supporting frame, that is to say Mf/Mw, Mf being the bending moment absorbed by the supporting frame 6, will vary. Calculations show that it is quite possible to dimension the supporting frame 6 to enable absorption of at least 9/10 of the bending moment Mw applied.
It should be noted that all the above-mentioned embodiments illustrate the invention, but do not limit it, and persons skilled in the art may construct many alternative embodiments without departing from the scope of the dependent claims. In the claims, reference numbers in brackets should not be regarded as restrictive. The use of the verb “to comprise” and its different forms does not exclude the presence of elements or steps that are not mentioned in the claims. The indefinite article “a” or “an” before an element does not exclude the presence of several such elements.
The fact that some features are stated in mutually different dependent claims does not indicate that a combination of these features cannot be used with advantage.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20141427 | Nov 2014 | NO | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/NO2015/050222 | 11/25/2015 | WO | 00 |
