The invention relates to a safety joint device for a pipe, and in particular for a riser extending between a vessel and a subsea installation, which joint comprises two telescoping parts that are interconnected by means which are designed to break at a preset axial load.

Operations in seabed wells usually take place by establishing a closed column that connects the well to a vessel on the surface and which will provide safe access to the well. A column of this kind is usually termed a riser or riser system and comprises not only the pipe itself but also a number of other devices which in addition to the actual pipe are necessary for safe access to the well. All operations in the well are carried out through the riser as it forms a barrier between well fluids and the surrounding seawater. The operations are carried out in a "live" well, i.e., the well is open all the way up to the vessel and contains well fluids which have a pressure corresponding to the formation pressure. The riser must therefore be dimensioned to withstand high well pressures. On the other hand, an uncontrolled blow-out can cause the riser to be filled with gas from the well, which will result in the pressure within the riser falling to almost zero.

The riser system normally comprises a lower riser package (LRP) having a plurality of valves for shutting down the well, and which therefore in terms of function corresponds to a blowout preventer (BOP). An emergency quick disconnect package (EQDP) and a stress joint are also provided. At the upper end of the riser, i.e., in the vessel, a surface BOP is usually provided. In addition, the riser may be equipped with flexible joints, buoyancy elements and optional other devices for operations in a seabed well.

When operations are to be carried out in wells located at substantial ocean depths, a vessel is used that is held in the correct position by means of propellers and/or thrusters. Such vessels are called dynamically positioned (DP) vessels. These vessels are highly dependent on all systems functioning satisfactorily and usual practice requires them to be equipped with several systems as safeguards against the vessel drifting out of its position.

During operations from a dynamically positioned vessel, situations could arise in which it becomes necessary to quietly leave the position above the well. This may be controlled, such as when a warning of deteriorating weather conditions makes it necessary to evacuate the position, or uncontrolled, such as in the event that some of the systems fail and the vessel begins to drift out of its position. A situation of this kind may also arise in the event of sudden bad weather, but in particular in situations in which the vessel's systems are incapable of holding the vessel in the correct position over the well. The consequences of such a situation may be that the heave compensation system ceases to be effective, or that the riser is bent at an unacceptable angle which causes loads above the design load of the riser.

The vessel's heave compensation system, which ensures that the riser system remains vertically still relative to the seabed, whilst the vessel heaves because of waves and weather, may also in some cases lock and then exert high tensile forces on the riser and cause a danger of the loss of the well barrier(s).

Such situations may cause breaks in the riser. In such situations, it is vital that the break is controlled, i.e., that it comes at a point where the well barrier remains intact.

Breaks in the riser can cause damage to the vessel and be a danger to personnel in addition to causing environmental damage, i.e., discharge of hydrocarbons, hydraulic fluid or the like. This may happen because of the energy in the extended riser and the contents of the riser. A complicating factor will be present if the riser has an internal pressure of an instabilised fluid or a mixture of gas and fluid. The fluid which then flows out of the lower end of the riser will cause an upwardly directed force which seeks to press the riser up into the rig against the heave compensation, thereby rendering the situation more unstable. In its most extreme consequence, the riser can be pressed upwards with such force that the equipment in the vessel is damaged and even sustains a shipwreck. A situation of this kind could also result in the loss of human life.

It is previously known to equip pipes with a safety joint that breaks if the pipe is subjected to a tension above a predetermined value. These joints comprise shear pins that break in the event of tension in the pipe. However, in some applications, such as risers connected between a floating platform and the seabed, a greater problem may be that the riser could be subjected to large bending moments if the vessel drifts out of its position.

The invention seeks to remedy this problem by equipping the riser with a device that causes the pipe to break at a predetermined point when it is subjected to a bending moment of a preset value.

This is achieved in that the joint comprises means for translation of bending moment into axial load.

In what follows the invention will be described in more detail with reference to the attached drawings, wherein:

Fig. 1 shows a section through a joint according to the invention; and

Figs. 2-5 show the joint at different stages of bending. Fig. 1 shows a joint 10. This joint is made as a separate part with coupling ends 12, 14 for connection to a pipe. It can be positioned wherever it is expedient, but it is advantageously positioned immediately above the joint for emergency disconnection. Optionally, the joint can be a part of the emergency disconnect unit. In the embodiment shown in the drawing, the joint is used in a dual bore riser and thus has a through-going main duct 16 and a side duct 18, 18' which are connected to each other by a flexible hose 19.

The joint comprises a lower housing 20 comprising a portion having an enlarged diameter of the inner passage 16, for receiving a pipe end piece 22. Screwed onto the housing 20 at its upper end is a cup-shaped flange 24 which is outwardly flared in the upward direction and has an annular end face 26. Sealing elements 23 and 25 are disposed between the housing 20 and the pipe end piece 22. A floating piston 30 is arranged in a recess 28. The volume above the piston 30 communicates via a duct 31 with the passage 16, and the volume below the piston communicates via a duct (not shown) with the surroundings. Thus, the piston functions as a tension equaliser which renders the separation force between the pipe end piece 22 and the housing 20 independent of varying pressure within the passage 16. The pipe end piece 22 is secured in the housing 20 by means of two sets of shear pins 32, 34. These may be pins evenly distributed around the circumference or may be formed as a ring. The shear pins are designed to break when they are subjected to a predetermined force.

The pipe end piece 22 has an upper outwardly flared portion 36 with lower 37 and upper 38 faces. These faces are advantageously spherical faces.

The joint further comprises an upper housing 40 which has a lower part 42 that is a portion of enlarged outer diameter. A cavity 44 is provided in the projection 42 for receiving the outwardly flared portion 36 of the pipe end piece 22. The cavity is defined by an inner cylindrical part 46 with an end face 47 that is designed to rest against the end face 38, and an outer cylindrical part 48 in which a plurality of bores 49, 50 (only two are shown) are provided for receiving bolts 51, 52. A flange 54 is screwed to the end of the part 42 by means of the bolts 51, 52. The outer diameter of the flange 54 is approximately equal to the outer diameter of the cup-shaped flange 24. The inside of the flange is made having a upward facing spherical face 56. A rubber sleeve 60 is arranged so that it lies between the faces 56 and 37. The rubber sleeve functions as a bending limiter in order to reduce the loads on the riser during normal use and.also is a pressure barrier between passage 16 and the surroundings, which is well known. The outer side of the flange is made having a downward facing circular face 55. One or more locking means 70 may be provided to stiffen the joint that is made up of 40, 22 and 60 and to hold the joint together in those cases that neither bendability nor having the safety joint activated is desirable. This may be during the installation and pulling of the riser system, and during the handling of the safety joint and its opposite components.

When the riser is subjected to a tension in excess of a predetermined value, the shear pins 32 will shear so that the upper part begins to separate from the lower part. This separation will continue until the piston 30 meets the upper wall of recess 28, whereupon shear pins 34 shear with subsequent complete separation. The upper shear pins 32 are dimensioned not to shear until the tension in the riser has exceeded an allowed value, but before damage is done to the well control elements or other parts of the riser system. Shear pins 34 are dimensioned to withstand tensile forces as a result of the separation force the pressure in passage 16 exerts, which will be much lower than the capacity of the shear pins 32. Thus, it can be ensured that the riser breaks at a predetermined point.

When the riser is subjected to a bending moment in excess of the permissible bending moment that is taken up by the flexible joint, the upper part of the riser will be at an angle relative to the lower part, as shown in Fig. 2. The flange face 55 will thus rest against the face 26. Continued bending will cause the flange 54 to act as a torque arm which provides upward directed force against the rubber sleeve 60. Thus, the pipe end 22 will be forced axially upwards so that a tension arises in the pipe end piece 22. If the deflection is great enough, it will cause the shear pins 32 to break, Fig. 3. The pipe end piece 22 will then take the piston 30 and shear pins 34 with it until the piston 30 meets its end stop and shear pins 34 shear (Fig. 4). The upper part 40 of the joint will thus separate from the lower part 20 and can be pulled up, as shown in Fig. 5.

The riser is designed, under normal operations, to be able to deflect (bend) within certain limits. This deflection is taken up by the flexible joint that is provided by the rubber sleeve 60. The riser must be regarded as a rigid column and because of the vessel's motions in waves and the wind, the riser will always be subjected to a certain deflection which may result in a danger of fatigue fractures in the pipe, and which renders a flexible joint necessary. The vessel's dynamic positioning system will normally ensure that the vessel is held in a position within the allowed limits that can be taken up by the flexible joint. However, the positioning system may fail and the vessel may move out of its safe position. Such a situation may occur unexpectedly and develop very rapidly so that there is no time to activate the normal procedures for emergency disconnection, i.e., disconnection with the aid of EQDP. The invention will cause the riser to be broken at a predetermined point, which avoids damage to other equipment or to the well. The remaining part of the joint can be brought up to the surface later and the joint can be made ready for continued use.

Although the invention has been described with reference to a riser for offshore use? it is clear that the invention may be used in any application where a pipe is subjected to large bending moments and where it is desirable that the pipe should break at a predetermined point.