The present invention relates to a seal assembly according to the introductory part of claim 1 . In particular it relates to a seal assembly which adapts to variations of dimensions of the tubular elements against which it seals.

Background

In the technical field of subsea wells for production of hydrocarbons, many components exhibit a tubular shape, such as tubing hangers, Xmas tree spools, and well heads. The components are heavy and large, and are designed to bear large mechanical forces, such as from external influence and well pressures. Despite their large sizes, some sections need to accommodate functional components in limited spaces. For instance, when connecting two pressure- containing tubular elements together, one needs to arrange high pressure seals between them in order to provide a pressure barrier for possible well pressures.

This task has existed for a long time in the field of subsea hydrocarbon production and various solutions have been presented. Some seals are ring seals with a U-shaped cross section. It is known to force a wedge element between the legs of the U-shape in order to force the "legs" into sealing contact with the facing sealing surfaces of respective side of an annulus within which the ring seal is arranged. It is also known to have a present pressure aiding the sealing contact, such as with the U-shaped cross section.

Solutions like the ring seal having U-shaped cross section are able to adapt to some variations of the distance between an inner and outer sealing surface. Such variations will arise typically due to varying pressure within the tubular elements, varying temperature, or external forces.

However, if such variations of distance become too large, the seal assemblies of the prior art are not able to maintain their sealing function. An object of the present invention is to provide a sealing assembly that will provide a sealing function over a larger range of such distance variation. The invention

According to a first aspect of the present invention there is provided a seal assembly comprising a seal ring which comprises metal, an inwardly facing first surface of an outer tubular element, and an outwardly facing second surface of an inner tubular element which is coaxially arranged within the outer tubular element. The seal ring is arranged between the first and second surfaces.

Moreover, the seal ring comprises an outer ring seal surface which is adapted to seal against the first surface and an inner ring seal surface which is adapted to seal against the second surface. According to the first aspect of the invention, the first surface or the second surface has an inclined element surface that exhibits an inclination with respect to the axial direction. The seal assembly further comprises an activation arrangement adapted to exert a force on the seal ring, thereby forcing the seal ring against the inclined element surface. One of the outer ring seal surface and the inner ring seal surface, respectively, of the seal ring, exhibits an inclined seal ring surface adapted to seal against the inclined element surface, while the other of the outer ring seal surface and the inner ring seal surface, respectively, exhibits an opposite seal ring surface arranged at the radially opposite side with respect to the inclined seal ring surface. The opposite seal ring surface is a sealing surface. The inclined seal ring surface is adapted to move along the inclined element surface, with sealing contact with the inclined element surface.

The axial direction is the direction parallel to the centre axis of the inner or outer tubular element, such as the centre axis of a Xmas spool, in an embodiment where the Xmas spool constitutes the outer tubular element.

Thus said movement of the inclined seal ring surface along the inclined element surface will expand or decrease the circumference of the seal ring, depending on its direction along the inclined element surface.

The term "tubular element" is meant to include not only elements having cylindrical shape, such as tubes, but also elements exhibiting a coned shape. The outer tubular element will have a bore within it, in order to accommodate the inner tubular element. The inner tubular element may have a bore within it, however it may also be without an internal bore.

The seal ring is preferably a metal seal ring (i.e. made of metal).

The seal ring is arranged between the inclined element surface and the actuation arrangement. Hence the actuation arrangement is pushing the seal ring towards the inclined element surface when the seal assembly is in an activated state. In one embodiment the opposite seal ring surface can be arranged axially closer to the actuation arrangement than what the inclined seal ring surface is.

The force exerted on the seal ring by the activation arrangement is preferably of such direction and/or position on the seal ring that it provides a rotating movement of the seal ring about an axis parallel with the circumference of the seal ring, when the radial distance between the first surface and the second surface increases or decreases. I.e. when the said radial distance increases, the activation arrangement will provide one or both of the following: i) forcing the seal ring against the inclined element surface, thereby moving the seal ring along the inclined element surface to a position where the radial distance between the first and second surface is less;

ii) forcing the opposite seal ring surface radially towards the first surface (of the outer tubular element) or the second surface (of the inner tubular element), respectively. This brings about a rotation of the seal ring cross section (its "enlarged cross section", such as shown in Fig. 3), which will increase the radial distance between the outer ring seal surface and the inner ring seal surface, respectively. In other words, the seal ring adapts to the varying distance between the first and second surfaces by rotation and/or sliding along the inclined element surface. When the radial distance between the first and second surfaces decreases, the opposite movements take place. In some embodiments the seal ring can comprise a shoulder adapted to limit rotation of the seal ring. The shoulder is arranged on the same radial side of the seal ring as the opposite seal ring surface and is axially offset from the opposite seal ring surface. With regard to the axial direction, it is offset towards the inclined seal ring surface, having a distance from the opposite seal ring surface.

The actuation arrangement can be adapted to be axially compressed between a lock ring of the internal tubular element and the seal ring. It may also be compressed between other parts of either the inner tubular element or the outer tubular element, depending on to which tubular element it is attached.

According to a second aspect of the present invention, there is provided a seal ring of metal adapted to be arranged in the annulus between an inner and outer tubular element and to seal against these. According to the invention the seal ring comprises

- an inclined seal ring surface adapted to seal against an inclined element surface of the inner or outer tubular element;

- an opposite seal ring surface adapted to seal against the other of the inner or outer tubular element;

- an actuation surface, adapted to be engaged by an actuation arrangement exerting a force onto the actuation surface, the force having at least an axial component of an axial component and a radial component;

wherein the inclined seal ring surface is axially offset with respect to the opposite seal ring surface.

In an embodiment of the second aspect of the present invention, the point of engagement between the inclined seal ring surface and said inclined element surface is axially offset with respect to the point of engagement between the opposite seal ring surface and a first surface or second surface of the outer or inner tubular element, whichever it seals against.

Moreover, said point of engagement between the opposite seal ring surface and a first surface or second surface of the outer or inner tubular element, against whichever it seals, can be closer to the actuation surface in a radial direction than what the point of engagennent between the inclined seal ring surface and said inclined element surface is.

Furthermore, the said point of engagement between the opposite seal ring surface and a first surface or second surface of the outer or inner tubular element, against whichever it seals, is preferably closer to the actuation surface in an axial direction than what the point of engagement between the inclined seal ring surface and said inclined element surface is. According to a third aspect of the present invention, it is provided a seal assembly comprising a seal ring arranged in an annulus between an inner and outer tubular element, wherein the seal ring comprises an inner seal surface facing the inner tubular element and an outer seal surface facing the outer tubular element. The inner seal surface and the outer seal surface of the seal ring are arranged at different axial positions, whereby their radial mutual distance changes by rotation of the seal ring about an axis extending parallel to the circumference of the seal ring.

In a preferred embodiment of the third aspect of the invention, the seal ring further comprises an inclined seal ring surface which is adapted to slide on a facing inclined element surface of the inner or outer tubular element. When sliding on such an inclined element surface the diameter and the circumference of the seal ring changes. Moreover, tension is brought about in the seal ring, resulting in a contact force between the inclined element surface and the seal ring.

It should be appreciated that, in the general description above as well as the detailed description of example embodiments below, when rotation of the seal ring is described, it is not referred to a rotation of the entire seal ring about its axial centre axis. Neither is meant a rotation that would alter the direction of the plane along which the seal ring extends. Rather, it is referred to a rotation of the shape of its enlarged cross section, i.e. a cross section through a portion of the ring. The rotation can be construed as a twisting of the seal ring about an axis extending parallel to the circumference of the seal ring. This axis is thus of a concentric shape, as is the seal ring.

Example of embodiment

While the invention has been described in general terms above, a more detailed and non-limiting example of embodiment will be described in the following with reference to the drawings, in which

Fig. 1 is a cross section view of a subsea internal tree cap (ITC), illustrating one possible application of the seal assembly according to the present invention;

Fig. 2 is an enlarged cross section view of a part of the ITC of Fig. 1 , however its lock ring in a locked position;

Fig. 3 is an enlarged cross section view through a seal ring which is part of the seal assembly;

Fig. 4 is a cross section view through the entire seal ring;

Fig. 5 is a principle cross section view through a part of the seal assembly

according to the invention, showing parts of the inner and outer tubular elements, the seal ring, and the actuation arrangement;

Fig. 6 is a cross section view corresponding to Fig. 5, however showing the seal assembly in an activated mode;

Fig. 7 is a cross section view corresponding to Fig. 6, however with an

increased distance between the inner and outer tubular elements;

Fig. 8 is a cross section view corresponding to Fig. 7, however showing a

situation where a well pressure exerts a force onto the seal ring;

Fig. 9 is a principle view of the embodiment shown in the previous drawings; Fig. 10 is a principle view of another embodiment of the seal assembly according to the present invention;

Fig. 1 1 is an enlarged cross section view through an alternative embodiment of the seal ring;

Fig. 12 is an enlarged cross section view through an alternative embodiment of the seal ring;

Fig. 13 is an enlarged cross section view through an alternative embodiment of the seal ring; Fig. 14 is an enlarged cross section view through an alternative embodiment of the seal ring;

Fig. 15 is an enlarged cross section view through an alternative embodiment of the seal ring;

Fig. 16 is a principle cross section view through yet an alternative embodiment in a non-activated state, wherein the seal ring is split into a seal ring part and a seal stop ring;

Fig. 17 is a cross section view according to Fig. 16, however in an activated

state;

Fig. 18 is a cross section view according to Fig. 17, however in a state where well pressure exerts a force onto the seal ring part;

Fig. 19 is an enlarged cross section view through the seal ring part of the

embodiment shown in Fig. 16 to Fig. 18;

Fig. 20 is a cross section view through the entire seal ring part of the seal ring part shown in Fig. 19;

Fig. 21 is an enlarged cross section view through an alternative seal ring and inclined element surface; and

Fig. 22 is an enlarged cross section view through another alternative seal ring and inclined element surface.

Fig. 1 illustrates an internal tree cap (ITC) 100 which is adapted to be installed within the spool of a subsea Xmas tree (not shown in Fig. 1 ). The ITC 100 is provided with a seal assembly 1 according to the invention, which will be described further below. In order to lock the ITC 100 to internal locking profiles of the XT spool, the ITC 100 comprises a lock ring 103 which is actuated in the radially outward direction by means of an actuation sleeve 105. This method of locking an internal tubular element to the inner bore of an outer element is well known to the skilled person in the art. It should be understood that the ITC 100 and the XT spool are only examples of inner and outer tubular elements between which the seal assembly 1 according to the invention may seal. It is noted, however, that the seal assembly 1 according to the invention is particularly useful in the field of subsea wells, which involves a plurality of tubular metal

components between which reliable sealing is needed. Fig. 2 is a segment of the left portion of the cross section of Fig. 1 , however with the lock ring 103 in the locked position (radially outward position). In Fig. 2 an actuation arrangement 2, here in the form of a spring assembly, and a seal ring 3 are indicated, which are both part of the seal assembly 1 according to the present invention. The actuation arrangement 2 provides a biasing force on the seal ring 3 and is able to do so over an axial distance. That is, the seal ring 3 is adapted to move a certain axial distance and the actuation arrangement 2 will follow the axial movement of the seal ring 3 and exhibit a biasing force through this entire distance. It should also be appreciated by the person skilled in the art that the actuation arrangement 2 can be of another type than the one shown in the example embodiment.

The seal ring 3 extends about an inner tubular element which in this embodiment is the actuation sleeve 105.

Above the seal ring 3 is arranged a wedge seal 31 . The wedge seal 31 is arranged about and surrounds an inclined wedge seal surface 33 of the inner tubular element, which in this embodiment is the actuation sleeve 105 of the ITC 100. As will be described further below, the seal ring 3 is adapted to move in a substantial axially direction in the annulus 19. As the wedge seal 31 lands on the top of the seal ring 3, the wedge seal 31 is adapted to move with the seal ring 3.

Fig. 3 is an enlarged cross section through the seal ring 3, corresponding to the right portion of the cross section through the entire seal ring 3 in Fig. 4.

Fig. 5 is an enlarged cross section showing the seal ring 3 arranged in the annulus 19 between an inner tubular element 105 and an outer tubular element 107. In this embodiment inner tubular element is the actuation sleeve of the ITC 100 and the outer tubular element 107 is a spool of a Xmas tree. The outer tubular element 107 has an inwardly facing first surface 7. The inner tubular element 105 has an outwardly facing second surface 5.

In this embodiment the second surface 5 is provided with an inclined element surface 9. Thus the inclined element surface 9 is arranged on the outwardly facing surface of the inner tubular element. The seal ring 3 comprises an inclined seal ring surface 1 1 that abuts and faces the inclined element surface 9. The inclined seal ring surface 1 1 and the inclined element surface 9 form together a sealing engagement.

On an opposite end of the cross section through the seal ring 3, as shown in Fig. 3 and Fig. 5, the seal ring 3 exhibits an opposite seal surface 13. The opposite seal ring surface 13 is adapted to form a sealing engagement with the first surface 1 of the outer tubular element 107.

Below the seal ring 3 is arranged an actuation arrangement 2 which will now be described. The purpose of the actuation arrangement 2 is to provide a biasing force on the seal ring 3. If the radial distance D (Fig. 5) between the first surface 7 and the second surface 5 (of the outer and inner tubular element 107, 105, respectively) changes, the seal ring 3 also needs to change its position and/or orientation. The biasing force of the actuation arrangement 2 provides for this change.

In this embodiment, the actuation arrangement 2 has a centre ring 201 with an upper inclined face 207 and a lower inclined face 209. Abutting the upper and lower inclined faces 207, 209 are an upper ring 203 and a lower ring 205, respectively. The upper and lower ring 203, 205 are adapted to slide on the upper and lower inclined faces 207, 209. Referring again to Fig. 1 and Fig. 2, when the actuation sleeve 105 is moved downwards, the actuation arrangement 2 is axially compressed between the lock ring 103 and the seal ring 3. During this compression, the centre ring 201 is elastically compressed as it is forced radially inwards by the upper and lower rings 203, 205 sliding on the upper and lower inclined faces 207, 209. Contrary to this, the upper and lower rings 203, 205 are elastically expanded. As a result, the axial extension of the actuation arrangement 2 is reduced. However, the actuation arrangement 2 exerts a significant force onto the seal ring 3, pushing it upwards towards the inclined element surface 9. Fig. 6 shows the actuation arrangement 2 in a compressed state. It should be appreciated by the person skilled in the art, that the type of actuation arrangement is not essential for the function of the seal ring 3. Thus other arrangements that are able to bias the seal ring 3 through the required axial varying distance will be applicable. However, the actuation arrangement 2 shown in this embodiment has proven to fulfill this technical task in a satisfactory manner.

In addition to bias the inclined seal ring surface 1 1 towards the inclined element surface 9, the actuation arrangement 2 also bias the opposite seal ring surface 13 against the first surface 7 (i.e. the inwardly facing surface of the outer tubular element 107 in this embodiment). An upper inclined face 21 1 of the upper ring 203 abuts a facing actuation surface 15 of the seal ring 3. Due to the inclination of the upper inclined face 21 1 as well as the position of engagement between the upper inclined face 21 1 and the actuation surface 15 of the seal ring 3, this abutment provides a rotating movement of the seal ring 3, in addition to an axial movement of the seal ring 3. This will be described further below.

In the following, various situations will be described with reference to Fig. 5 to Fig. 8.

Fig. 5 shows the seal assembly 1 according to the present invention in a non- activated state. The seal ring 3 rests freely between the inclined element surface 9 above it and the actuation arrangement 2 below it. The actuation element 2 is in a non-compressed mode. Thus, there is a free space within the centre ring 201 so that it may move radially inwards. Correspondingly, there is space on the radial outside of the upper and lower ring 203, 205 so that they may move radially outwards. Fig. 6 shows a situation where the seal assembly 1 has been actuated. The actuation arrangement 2 has been axially compressed so that it exerts a force onto the seal ring 3. The centre ring 201 has moved radially into contact with the second surface 5 of the inner tubular element 105. The upper and lower ring 203, 205 have moved slightly radially outwards. Due to the force exerted onto the activation surface 15 of the seal ring 3 by the actuation arrangement 2, the seal ring is in sealing contact with the inclined element surface 9 as well as the first surface 7. Fig. 7 shows a situation similar to that shown in Fig. 6. However, in Fig. 7 the radial distance D (Fig. 5) has increased. This may be due to internal pressure, thermal expansion or removal of mechanical forces that previously was exerted onto the external face of the XT spool 107. This may also be due to reduced diameter of the inner tubular element 105. Due to this larger annulus 19, the seal ring 3 may expand radially and move along the inclined element surface 9, and it may also rotate as the actuation arrangement 2 exerts a force from it from below ("below" with respect to the drawing of Fig. 7). In the drawings shown it rotates in an anti-clockwise direction, as indicated with the curved arrow in Fig. 7. As a result, the opposite sealing surface 13 of the seal ring moves after the first surface 7 and ensures continuing sealing.

It should be noted that in the situations shown in Fig. 6 and Fig. 7 (also Fig. 8), the circumference of the seal ring 3 has been increased by forcing it to move along the inclined element surface 9. Hence, tension in the seal ring 3 will ensure an appropriate contact force between the inclined seal ring surface 1 1 and the facing inclined element surface 9, which will provide a sealing between these surfaces.

In Fig. 8 the situation corresponds in many ways to the one in Fig. 7. However, a significant pressure, such as a well pressure, exerts an additional force onto the lower side of the seal ring 3. This additional force makes the seal ring 3 slide an additional distance along the inclined element surface 9. Since the opposite seal ring surface 13 abuts the first surface 7, the seal ring 3 now rotates in the other direction, i.e. the clockwise direction. In order to limit this rotation, an upper shoulder 17 of the seal ring 3 abuts the first surface 7 at an upper portion of the seal ring 3. The upper shoulder 17 may also exhibit a surface adapted for sealing against the first surface 7. Hence it may fulfill the task of being a rotation limiter while also being a secondary seal surface. If the radial distance D of the annulus now increases, the seal ring 3 will be forced downwards by the inclined element surface 9, thereby compressing the actuation arrangement 2. Also, if the pressure is relieved the seal ring 3 will also move downwards along the inclined element surface 9.

Thus, the seal assembly 1 according to the invention comprises a seal ring 3 arranged between an inner tubular element and an outer tubular element which will continue to exhibit sealing performance both during varying pressure conditions, as well as during varying sizes of the inner and outer tubular elements.

It is again referred to Fig. 3, which shows an enlarged cross section view through a seal ring 3 of a seal assembly 1 according to the invention. The inclined seal ring surface 1 1 , which is adapted to abut the inclined element surface 9, exhibits a curved shape. This curved shape makes the seal ring 3 suitable for rotation, as described above, while still maintaining sealing performance. The curved shape also reduces the risk of damage to the facing seal surfaces, such as by scratching, denting and deformation. As appears from Fig. 3, the opposite seal ring surface 13 also exhibits a curved shape. This is advantageous since the radial distance D varies, as described above. Thus, the seal ring 3 is adapted to seal in various rotational positions, with both the inclined seal ring surface 1 1 and the opposite seal ring surface 13. Fig. 9 and Fig. 10 are principle drawings to show how the principle of the seal assembly 1 according to the present invention will also if the first surface 7 comprises the inclined element surface 9, in lieu of the second surface 5 as shown in Fig. 1 to Fig. 8. Fig. 9 corresponds to the embodiment shown with reference to Fig. 1 to Fig. 8, in which the inclined element surface 9 is on the inner tubular element 105, that is on the second surface 5, which faces radially outwards. Fig. 10 shows an embodiment where the inclined element surface 9 is arranged on the inwardly facing first surface 7 the outer tubular element. The cross section of the seal ring 3 is mirrored so that the inclined seal ring surface 1 1 faces the inclined element surface 9. Moreover, when the actuation arrangement 2, here only indicated with an arrow, exerts force on the seal ring 3, the circumference of the seal ring 3 is reduced elastically. This is in contrast to the embodiment shown in Fig. 1 to Fig. 8, wherein the circumference was increased. Due to the reduction of circumference, the seal ring 3 will exert a force onto the inwardly facing inclined element surface 9, to make a sealing performance. The opposite seal ring surface 13 will, on the other hand, be forced radially against the second surface 5 of the inner tubular element.

Fig. 1 1 to Fig. 15 show enlarged cross sections through different alternative embodiments of a seal ring 3 which could be part of other embodiments of the seal assembly according to the invention. All the illustrated seal rings 3 have an inclined seal ring surface 1 1 adapted to seal against the inclined element surface 9 (cf. Fig. 5). Common to them all is also that they have an actuation surface 15 adapted to be engaged by an actuation arrangement 2 and an opposite seal ring surface 13 adapted to seal against the first or second surface 7, 5 against which the inclined seal ring surface 1 1 does not seal. Furthermore, the axial position of the inclined seal ring surface 1 1 is offset with respect to the axial position of the opposite seal ring surface 13. Or at least the point of engagement between the inclined seal ring surface 1 1 and an abutting inclined element surface 9 is axially offset with respect to the point of engagement between the opposite seal ring surface 13 and an abutting first surface 7 or second surface 5. This axial offset brings about the desired varying radial distance between the inclined seal ring surface 1 1 and the opposite seal ring surface 13 when the seal ring 3 rotates (about its enlarged cross section as shown in Fig. 3 and Fig. 1 1 to Fig. 15). Common to all the seal ring 3 embodiments shown in Fig. 1 1 to Fig. 15 (as well as the one shown in Fig. 3) is that the actuation surface 15 exhibits an inclination and faces in a partial radial direction which is opposite of the radial direction which the opposite seal ring surface 13 faces. This inclination of the actuation surface 15 contributes in bringing about the rotational movement of the seal ring 3 when adopting to varying distances of the annulus 19.

One should also note that the point of engagement between the actuation surface 15 of the seal ring 15 and the actuation arrangement 2 is radially offset towards the opposite seal ring surface 13 with respect to an axial centre line between the radially outermost parts of the seal ring 3 (with respect to the enlarged cross sections shown in Fig. 3 and Fig. 1 1 to Fig. 15). This feature also contributes in bringing about said rotational movement.

As shown with the embodiments illustrated in Fig. 12 and Fig. 14, the part of the seal ring 3 that exhibits the opposite seal ring surface 13 and the actuation surface 15 may have a slim shape (i.e. this part of the cross section of the seal ring 3) that will allow for some bending. Thus, for such embodiments the seal ring 3 will adapt to the varying dimensions of the annulus 19, as described above, by movement along the inclined element surface 9, rotation, as well as some degree of bending in the seal ring 3. This bending may allow for an even further variation of annulus distance without loosing the sealing capability of the seal ring 3. In one possible embodiment of the seal ring 3, as shown in Fig. 13, the seal ring 3 is not provided with the upper shoulder 17 which is arranged in the other embodiments to limit the amount of rotation.

It should be noted that the shown cross sections of various embodiments of the seal ring 3, as shown in Fig. 3 and Fig. 1 1 to Fig. 15, may be employed both on a radially inwardly and outwardly facing inclined element surface 9, such as described with reference to Fig. 9 and Fig. 10.

Fig. 16 is a principle cross section view through yet an alternative embodiment of a seal assembly V according to the invention. In Fig. 16 the assembly is shown in a non-activated state. In this embodiment the seal ring 3 is split into a seal ring part 3a and a seal stop ring 3b. The function of this seal assembly V is

substantially corresponding to the function of the seal assembly 1 described above. The seal ring part 3a exhibits an inclined seal ring surface 1 1 , an opposite seal ring surface 13, and an actuation surface 15. The seal stop ring 3b is arranged directly axially adjacent and abutting the seal ring part 3a. The seal stop ring 3b serves to limit the axial movement of the seal ring part 3a. Without the seal stop ring 3b, the seal ring part 3a could also be inclined to rotate in the "wrong" direction, as the inclined seal ring surface 1 1 could be allowed to slide an excessive distance along the inclined element surface 9. Thus, one of the radially facing sides of the seal stop ring 3b is comparable to the upper shoulder 17 shown in the embodiments above. Fig. 17 shows the same cross section as Fig. 16, however with the seal assembly 1 ' in an activated state. Fig. 18 is a cross section view according to Fig. 17, however in a state where well pressure exerts a force onto the seal ring part 3a.

Fig. 19 is an enlarged cross section view through the seal ring part 3a of the embodiment shown in Fig. 16 to Fig. 18. Fig. 20 is a cross section view through the entire seal ring part 3a of the seal ring part shown in Fig. 19.

Fig. 21 shows an alternative embodiment of the invention. Here the inclined seal ring surface 1 1 is not curved. Rather, its cross section shows a straight line, while the abutting inclined element surface 9 is curved in order to make it possible for the seal ring 3 cross section to rotate with respect to the inclined element surface 9.

Fig. 22 is yet an alternative, somewhat similar to the one shown in Fig. 21 . Here both the inclined element surface 9 and the inclined seal ring surface 1 1 exhibit a curved cross section. The curving of each surface may then be less.

In all the embodiments described above, both the seal ring 3, and the inner and outer tubular elements are preferably of metal, such as in a subsea well application.