GB2202021A - Safety joint for pipe lines and flow lines - Google Patents

Abstract

A safety joint is provided for use in, for example, offshore oil and gas pipe line systems which includes male and female pieces which are arranged to part under a predetermined axial load. The male piece (1) includes a number of pressure-balance units each of which comprises a piston (5) movable with a radially outward component to urge a release member (9) into engagement with an inwardly extending flange (33) on the female piece (2). The piston (5) and release member (9) are arranged such that an outward force e.g. fluid pressure in the pipeline on the piston (5) tends to force a bearing surface on the release member (9) into engagement with a bearing surface (37) of the flange (33). The arrangement is such that a tensile load applied to the safety joint over a predetermined amount causes the pressure applied by the flange (33) on the release member (9) to fracture a shear pin (7) to allow inward movement of the piston (5) and hence separation of the male and female pieces (1 and 2). For certain applications where pipeline pressure is not suitable e.g. multiple hose bundles, the piston may be driven by mechanical means (Figs. 2 and 3). The release member may take the form of rollers carried by the tension rod (10) (Fig. 7) or the piston (5) (Fig. 10). The shear pin (7) may hold the tension rod (10) (Figs. 8,9,10) or form part of the swivel linkage (11) (Fig. 13). The tension rod (10) may be replaced by a compression rod (Fig. 12). The movement of the piston (5) may have an axial component (Fig. 10). <IMAGE>

Description

SAFETY JOINT FOR PIPE LINES AND FLOW LINES Field of the Invention The invention relates to improved safety joints for pipe lines and flow lines of the type used primarily in undersea pipe lines.

Subsea pipe lines and flow lines are used to convey fluids and gases, typically mineral oil and natural gas from one loc ation to another along the sea bed.Such pipe lines can extend between production platforms and gathering centres, between production platforms and subsea manifolds, or between such subsea manifolds. Recent trends to exploit marginal fields utilising subsea completion structures has meant a requirement for more and longer pipe line networks. Unfortunately these subsea pipe lines are vulnerable because there is a great danger of the pipe lines being snagged by trawling boards or mooring anchors for example or becoming the victims of mud slides.

Because the pipe lines carry the fluids and gases often at a very high pressure (for instance up to 5,000 p.s.i./ 3.447 x 107 a) and also because the pipe line has to be able to accommodate some degree of internal and external corrosion, the pipe line wall has to be relatively thick. This means that the pipe line has a high axial strength which in some cases is not desirable.

At the interface between a pipe line and a platform structure, for example, that particular part of the fabrication has only to support the pipe line and manifold weight plus a reasonable safety factor. Any further additions to the support frame would carry weight penalties. To support the tensile capabil ity of the average pipe line any structural enhancement would have to be quite excessive and, in many cases, totally imprac tical. The load which would be transferred onto a subsea structure by a snagged pipe line prior to pipe line failure would invariably be well in excess of the design strength of the structure support mechanism. The subsequent repair costs, however large, might in insignificant in comparison with the resultant catastrophe of this load being transferred throughout a platform subsea jacket.

Therefore, what is required is to install along the pipeline load limiting connectors which are otherwise known as safety joints and these are connected at the interface between the pipe line and the subsea structure. These joints are arranged such that they part under predetermined axial load so that the pipe line itself will break rather than causing damage to the platform structure or to the subsea manifolds.

Review of the Prior Art Such safety joints have to be able to withstand high internal pressures so that they will retain all the parent pipe line mechanical properties, apart from the tensile ability. A safety joint consists of two primary pieces, each piece defining a length of fluid passageway to co-operate with the pipe line and the two pieces abutting together and being coupled together such that they will separate under a tensile load above a pre-determined amount. The internal high pressure within the fluid passageway of the two pieces will have an affect on the abutting edges of the pieces known as the end cap effect. The effect of this pressure is to urge the two pieces away from each other and out of engagement with each other.In order to overcome this, a safety joint has to be designed to be pressure-balanced so that maximum fluid pressure within the pipe line can be contained without causing separation of the two pieces of the safety joint but still allow tensile force to cause separation of the safety joint.

A method which is often used in a safety joint to cause this pressure balancing is to arrange a piston-like member which is arranged substantially parallel to the axis of the pipe line and being in fluid connection with the inside of the pipe line, arranged such that the pressure in the pipe line causes movement of the piston in a direction away from the abutting edges of the two pieces, in which one of the pieces has a projection which passes over and beyond the other piece onto which the piston bears, such that pressure on the pipeline causes the piston to exert pressure on the projection to pull the two pieces into contact with each other, thus neutralising the end cap effect.

Because of the structure of the two pieces of the safety joint, there has to be an arrangement which collapses when the tensile strength overcomes a predetermined amount, to allow the two pieces of the safety joint to come apart. It is often a complicated arrangement usually initiated by a shear or tension pin failure and incorporating a system of split rings and different movable members.

Summary of the Invention According to this invention there is provided a safety joint comprising a first male piece and a second female piece, arranged in abutting relationship to define a fluid passageway, each piece incorporating means to couple the piece to a pipe line, the male piece incorporating at least one pressurebalance unit comprising a piston movable in a direction having a component substantially radial with respect to the axis of the pipeline; each pressure balance unit including a release member having a release bearing surface; the female piece including an extension sleeve which extends beyond the abutting region of the male and female piece and over the male piece, the extension sleeve having an inwardly projecting flange which has a bearing surface substantially parallel to and facing towards the abutting edges of the male and female piece; the release bearing surface of each pressure balance unit being arranged to bear against the bearing surface of the flange of the female piece; each piston and release member being arranged such that an outward force on the piston tends to force the release bearing surface into engagement with the bearing surface of the extension sleeve; each pressure balance unit including at least one shear pin arranged to hold the release bearing surface against the bearing surface of the extension sleeve and a tension pin positioned such that it can be used to apply pre-tension to the shear pin to pre-tension the safety joint such that the load applied via the piston on the release member is transferred to the female piece tending to force the abutting edges together, the safety joint being arranged such that a tensile load applied to the safety joint over a predetermined amount causes the pressure on the release member to be sufficient to break the shear pin to allow the piston of the pressure balance unit to move inwards within the housing and allow the release bearing surface to move out of engagement with the bearing surface of the female flange to allow the extension sleeve to ride over the release member to allow separation of the male and female pieces.

The piston may move in a direction perpendicular to the axis of the pipeline. If however the piston moves in a direction which has a component parallel to the pipeline, then it is possible to arrange the safety joint such that the release member is mounted on and moves with the piston. In this way the release bearing surface can be made to bear against the bearing surface of the flange of the extension sleeve. In this case it is preferred that the release member includes a roller bearing, the surface of which provides the release bearing surface which helps to overcome any frictional forces.

However, it is preferred that the piston is arranged to move substantially perpendicularly to the pipeline and in this case means are required to transfer a force acting radially with respect to the pipeline to a force acting parallel to the pipeline. Preferably this is achieved by providing a release member which swivels about an axis perpendicular to the direction of movement of the piston member and perpendicular to the axis of the pipeline. This allows the change in direction of forces to be effected efficiently.

The shear pin may typically be arranged to co-operate with the tension rod at the region where the extension sleeve contacts the male piece. Alternatively, the shear pin may be arranged to extend through the piston housing and piston member to hold the piston in position with respect to the housing.

Thus, in this way a safety joint is produced in which the piston, which is acted upon by the pressure within the pipeline, is not parallel to the pipeline, and in which the release member which is used to transfer the force applied by the pressure is also used to allow separation of the pieces when required.

This greatly simplifies the structure of the safety joint.

In a preferred embodiment of the invention there is provided a safety joint comprising a first male piece and a second female piece, arranged in abutting relationship to define a fluid passageway, each piece incorporating means to couple the piece to a pipeline, the male piece incorporating at least one pressure balance unit consisting of a housing mounted on the male piece and a movable piston member movable within the housing in a direction substantially radial with respect to the pipe line, a shear pin extending through the housing and the piston member to hold the piston in position with respect to the housing; and, for each pressure-balance unit, a release swivel member capable of rotating about an axis perpendicular to the direction of movement of the piston member and perpendicular to the axis of the pipeline, the release swivel member being rotatably mounted with respect to a tension rod which couples the release swivel member to a flange extending outwardly of the male piece, the female piece including an extension sleeve which extends beyond the abutting region of the male and female piace and over the male piece, the extension sleeve having an inwardly projecting flange which has a bearing surface substantially parallel to and facing towards the abutting edges of the male and female piece, the release swivel member being arranged to bear against the bearing surface of the flange of the female piece and against the piston of the male piece; the pressure-balance unit being arranged such that pressure within the pipeline causes a force on the piston in a direction radially outwards of the pipeline, the tension rod being tensioned to pre-tension the safety joint to preserve abutment integrity, such that the load applied by the piston on the release swivel member is transferred to the female piece tending to force the abutting edges together to counter opposing separating forces, the safety joint being arranged such that a tensile load applied to the safety joint over a predetermined amount causes the pressure on the release swivel member to be sufficient to break the shear pin to allow the piston of the pressurebalance unit to move intwards within the housing and allow the release swivel-member to rotate out of engagement with the bearing surface of the flange of the extension sleeve to allow the extension sleeve to ride over the release swivel member to allow separation of the male and female pieces.

The male and female pieces can be made from a low corrosion alloy or a plastics material but are preferably made from a low alloy carbon steel. The seal at the abutting edges of the male and female pieces is preferably an all metal seal of the type used to seal hub flanges sometimes referred to ds AX-type seal rings or one offering a similar performance. Such seal ing rings are of metal construction and designed to deform elastic ally to seal where sealing lips contact the flanges at the interface.

The seal may be manufactured from carbon steel or other materials such as nickel alloy or plastic to offer enhanced corrosion resist ance.

The design of the male and female pieces has to be such that the structural strength of the joint is preserved as a whole so that It has equivalent or better strength characteristics than the parent pipeline but the joint weight must be kept to a minimum, to aid handling, consistent with good performance.

,The joint must have smooth lines to eliminate any likelihood of snagging during separation. There must be a low friction mechanism and the fluid flow paths must be short and large bore to keep the reaction speeds high.

Preferably the safety joint includes more than one pressure balance unit, equally disposed around the circumference of the male piece. There can be twelve or more pressure balance units but preferably the number of pressure-balance units is between three and eight. The total effective area of the pressure-balance units needs to be as close as possible to the effective area of the joint bore at the seal ring.

Preferably the pressure-balance unit is made from components manufactured from materials exhibiting low corrosion properties such as stainless steel, nickel alloy or plastics materials.

In most cases it is preferred that the male piece includes a fluid passageway from the inside of the pipe to a housing so that the pressure acts directly on the piston. This is not the case where the pipe line is the sort used for housing control bundles and an alternative embodiment of the pressure balance unit will be described below.

In cases where the piston is in direct fluid communication with the inside of the pipe line the pressure balance housing of the male member comprises a cylinder incorporating a shear pin holder.

The cylinder allows the piston to float within it and has a shear pin holder which allows a shear pin to pass through the piston and the shear pin holder to hold the piston in position with respect to the holder. Preferably a sealing device which may be of an elastomer material is included between the cylinder and the piston.

In an all metal joint such a sealing device is preferably a metal bellows or similar metal seal. This seal may be welded to the shear pin holder and/or to the piston. Whatever system is used, it must be capable of permitting the piston to travel to the bottom of the cylinder with the minimum of restrictive force when required to do so. Because of the favourable arrangement of the pressure balance unit and the location of the shear pin, the seal is, when assembled into the joint in its strongest state, ie., compressed. Further more, . a relatively elevated pressure within the cavity of the cylinder would only serve to enhance the seal integrity by virtue of compressing it even closer together. The Important aspect of the seal design is that it is effective in its minimum compressed state under maximum load and with the shear pin whole, ie, just at the point of shear pin failure with the maximum piston pressure. Also it is advantageous if such a seal is re-usable after joint separation.

The shear pin has to have good performance repeatability and mechanical predictability so that the exact point of failure of the shear pin can be predicted. It must also have low corrosion when used with other materials in the joint environment and low elongation properties to reduce the likelihood of partial shear. The typical material for such a shear pin is phospherbronze or titanium alloy or a plastics material.

It is possible to arrange the release member to be mounted on the end of a tension rod which is arranged with a spherical washer surrounding it to allow the entire tension rod assembly to swivel.

In a further embodiment the release swivel member is arranged so that it may swivel within a section of a projection from the male piece.

Alternatively the release swivel may be in the form of a platelike member which is rotatably mounted to a tension rod which has forks at one end to allow the release swivel to be inserted between the forks, a clevis pin inserted through the members to allow the release swivel member to rotate with respect to the tension rod; or the release swivel may have forks which contain the bearing prong of the tension rod. The flange extending outwardly of the male piece also includes radial slots machined into it to give correct rotational orientation of the release swivel member.

The tension rod is coupled to the male piece by a tension rod assembly which allows pre-load to be introduced into the safety joint. Preferably the tension rods are not the sort of tension rods which have to be rotated to introduce tension since this would introduce unnecessary torsional loads into the tension rod/release swivel-member. Preferably the tension rod is such that it can be tightened by a tension nut which does not rotate the tension rod.

Preferably disc springs are inserted between the outwardly extending flange of the male piece and the tension nut or within the piston assembly to make the tension rod/piston assembly effectively longer and slimmer and increase its apparent slenderness ratio. This has the effect of making the assembly more elastic. This means that if there are any temperature changes or material creep which give minor length variations within the assembly the possibility of losing joint pre-load is significant ly reduced.

The joint tensioning can be effected by means of a hydraulic tensioning device such as those manufactured by Hydratight Limited of Walsall, England, or Pilgrim Systems Limited of West Bromwich England or by utilizing hydraulic cylinders fitted with locking devices Preferably the assembly also includes a torque pin mechanism or a plurality of torque pins to help to ensure that the joint has the ability to resist torsional movement to reduce or eliminate any tendency of the main seal to rotate, thus losing efficiency.

During joint installation a thread on the outer end of the torque pins can allow the pin to serve a double purpose.

This thread can permit a series of installation nuts to be added to render the joint as strong in tension as the parent pipe line. These nuts may be removed after jointing installation by diver or a remotely operated vehicle or, alternatively, the nuts may be manufactured from a sea water corrodable material such as magnesium.

An alternative embodiment of the safety joint is one which was designed specific ally for use with hoses or small-bore pipe assemblies of the type used in control bundles to be protected. Here the pressure balance unit is arranged by incorporating a third piece arranged between the first and second pieces of the safety joint.

The third piece is arranged such that it has an outwardly extending conical flange. The piston of the pressure-balance unit has an inner surface which co-operates with the flange so that pressure acting on the third piece of the safety joint causes movement of the third piece and conical projection to apply force on the piston in a direction radially outwards of the pipe line against the release swivel. This is of special use in small bore pipe bundle assemblies but may also serve to protect a single pipe line.

The safety joint, in accordance with the invention, can be used in oil field and gas system pipe lines, risers and flow lines. It can also be used in tanker loading arms and piped gas and fluid systems in the nuclear power industry.

Brief Description of the Drawings Seven safety joints in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a section through a first safety joint; Figure 2 is a section through a second safety joint; Figure 3 is a section through a third safety joint; Figure 4 is a section along line A-A of Figure 1; Figure 5 is a section along line A-A showing a modification of the first safety joint; Figure 6 is an enlargement of part of Figure 1; Figure 7 is a section through part of a fourth safety joint; Figure 8 is a section through part of a fifth safety joint; Figure 9 is a section through part of a sixth safety joint; Figure 10 is a section through part of a seventh safety joint;; Figure 11 is a section through the piston assembly of an eighth safety joint showing a modification to the first safety joint; Figure 12 is a section through part of a ninth safety joint; and Figure 13 is a section through part of a tenth safety joint.

Description of the Preferred Embodiments A first safety joint shown in Figure 1 comprises a first male piece 1 and a second female piece 2 which are arranged in abutting relationship to define a fluid passageway. Each piece 1, 2 incorporates means 25, 26 in the form of flanges to couple the piece to a pipeline.

The male piece 1 incorporates at least one pressure balance unit consisting of a housing 3 mounted on the male piece.

Here the housing consists of a cylinder 3 and shear pin holder 6 welded to cylinder 3. A piston 5 is movable within the housing in a direction radial of the pipeline. A shear pin 7 extends through the housing 6 and the piston 5.

The male and female pieces 1 and 2 are axially slidably arranged in relation to each other to effect a fluid passage for a pipe system which will contain pressure when clamped.

around a seal ring 24. The pieces would normally be manufac tured from low alloy carbon steel but could alternatively be made from a low corrosion alloy or plastics material.

Interconnecting flanges 25 and 26 permit the joint to be connected to ' the pipeline. These interconnecting flanges may be of various types (typically of the ANSI variety) which might include one which swivels to assist alignment. The interconnect ing flanges may be added to the joint by welding or other means or may be machined to form an integral part of the male and/or female flanges.

The seal ring 24 shown is typical of the variety used to seal Hub flanges (sometimes referred to as AX type). It is of metal construction and designed to deform elastically to seal where sealing-lips contact the pieces at interface 29.

Sealing preload pressure in the order of 20,000 p.s.i (1.379 x 108 Pa) is required at this interface which is derived from the axial make-up force of the two pieces: the sealing efficiency is normally reinforced by increased internal pressure. The seal may be manufactured from carbon steel or other, more exotic, material (such as nickel alloy) or plastic to offer enhanced corrosion resistance. Other styles of seal may be alternatively used without detracting from the effectiveness of the joint.

Critical interfaces within the pieces may be weld-clad with a low-corrosion alloy to minimise the effects of galvanic or chemical corrosion that might occur if the parts were manufac tured from carbon steel. These interfaces would include (but not be restricted to) the main sealing faces and the Pressure Balance unit connection port.

The whole joint would normally be filled in its joint cavity 30 with a corrosion-inhibiting fluid or grease with all seawater exposed surfaces being treated with paint or other protective medium.

The design of the safety joint takes into consideration a number of factors. The structural strength of the joint as a whole must be preserved, ie. it must have equivalent or better strength characteristics than the parent pipeline. Also joint weight must be kept to a minimum consistent with good performance. Smooth joint lines must be maintained to eliminate any likelihood of snagging during separation. Also of importance are: Ease of assembly; keeping material and manufacturing costs to a minimum; reduction of interfaces requiring sealing; and low friction mechanism. It is also desirable to maintain short, large-bore fluid flow paths to keep reaction speeds high.

The pressure balance unit is a critical component which provides a force to counterbalance the effects of pressure tending to separate the joint at the sliding interface (sometimes referred to as Bendcap effect"). The units would be equally disposed around the circumference of the male piece. A minimum of two would be used with three to eight being the more likely number but their could be as many as twelve or more.

The total effective area of the pressure balance units needs to be as close as possible to the effective area of the joint at the seal ring 24. Minor adjustments may be made to the balance of areas by adjusting the geometry of a release swivel 9 to give a mechanical advantage in one direction or other.

Positioning of the pressure balancing unit should be optimised to give a short flow path for the balancing fluid with as large a flow port cross section as practical in order to ensure maximum response speed along with minimum likelihood of blockages occurring through waxing, etc.

It is likely that the components comprising the pressure balance unit would all be manufactured from a material exhibiting low corrosion properties such as stainless steel, nickel alloy or a plastics material The pressure balance unit incorporates cylinder 3 which supports piston assembly 5 and provides an interconnection to the pipeline bore via the male piece 1. In this "all metal" joint configuration this part is fixed to the male flange utilizing a conical seal type threaded adaptor 31 to give metal to metal sealing along with an easy means of assembling and disassembling the unit. For added security, however, the periphery of seam 32 may be welded.

The piston 5 supports one side of the seal, carries the pressure force due to the fluid pressure on its innermost face, reacts with the mechanical balancing loop for the joint assembly via release swivel 9 and provides one half of the shear pin mechanism/support.

The piston "floats" (within the boundaries permitted by the preload and shearing forces). On its outermost face it sees the combined force due to tensile separation loads/preload, joint separation pressure and joint cavity 30 pressure. The innermost face is exposed to the pipeline pressure to provide the balancing force.

During the operational life of the joint prior to separation the piston moves very little in its cylinder and may be consid ered to be virtually static. '' In this "all metal" joint the seal between the pistons and cylinder 3 is a metal bellows. The seal may or may not be welded to the shear-pin holder 6 and the piston 5. Whatever system is used it must be capable of permitting the piston 5 to travel to the bottom of the cylinder 3 - with the minimum of restrictive force - when required to do so. 7 Because of the favourable arrangement of the pressure balance unit and the location of the shear-pin the seal is, when assembled into the joint, in its strongest state (i.e. compressed).

Furthermore, a relatively-elevated pressure. within the cavity of the cylinder would only serve to enhance the seal integrity by - virtue of compressing it even closer together. However, for the purposes of this design, the important aspect of the seal design is that it is effective in its minimum compressed state, under maximum load and with the shear-ptn whole (i.e. just at the point of shear-pin failure with maximum piston pressure). A desirable feature of the seal is that it is reusable after a joint separation; however this is not a compulsory feature of the unit especially where, in order to achieve seal reusability, joint performance might have to be sacrificed. Once joint separation is underway this seal need not be functional.

The shear pin holder 6 supports one side of the seal 4 and provides the means of enclosing the assembly via a threaded interface which is preferably welded to the cylinder 3 or sealed by other suitable means. Along with the piston, it provides the shear-pin support and shear mechanism. A short thread in one side of the holder ensures that the pin does not fall out during assembly.

The shear pin 7 must be made of a material which offers good performance repeatability and mechanical predictability, low corrosion when used with other materials in joint environment, and low elongation properties to reduce likelihood of partial shear.

Such a material might be phosphor bronze or titanium alloy or a plastics material.

For each pressure balancing unit there is provided a release swivel member 9. The release swivel member is rotatably mounted to a tension rod 10 via a clevis pin 11. The release swivel member is arranged to be rotatable about an axis perpendicular both to the radial and axial direction of the pipeline. The release swivel member 9 provides means of transferring the direction of the various forces to planes normal to the reaction faces. In the embodiment shown the pressure balance unit forces are acting in a plane generally at right angles to the plane of the forces in the female piece (which is parallel to the joint axis). However these forces may be disposed at some other relative angle in which case the swivel would still be capable of transferring the direction of forces as required.

The female piece 2 includes an annular extension sleeve which extends about male piece 1. The annular extension sleeve of 2 includes an inwardly extending flange 33 having a bearing surface 37 facing towards and substantially parallel to the seal 24 at the abutting edges of pieces 1 and 2.

The release swivel member 9 is arranged to bear against bearing surface 37 and against face 36 of the piston 5.

During breakaway the swivel forms the major component of the release mechanism. It transfers the axial component of breakaway tension from face 37 onto the shear-pin holder/ pressure balance unit at face 36. On failure of the shearpins the swivel compresses the piston thus swinging out of the way of the swivel clearance diameter 33 consequently permitting the female piece to pass over the release swivel and joint separation to take effect. Forces due to friction are low.

The ratio of the effective length of the "arms" of the swivel allows for some manoeuvring of the relative effective areas of the main bore seal ring 24 and the pressure balance unit.

This feature may also be used to advantage in adjusting the geometry of the joint as a whole for optimum size and perform ance.

Location of the release swivels (which are approximately rectangular in section) in correct rotational orientation for operation is provided by means of radial slots 34 machined into the flange extending from male piece 1.

A tension rod assembly comprising tension rod 10, clevis pin 11, disc spring assembly 12 and tension nut 13 provide the means of "pulling together" the joint via the release swivel 9, engaging the main seal 24 and effecting the correct level of preload.

On assembly the tension rods are pushed in towards the joint cav ity 30 to permit the diameter 33 of the female flange to pass over the release swivels 9 (temporary adhesion or small bias springs in the clevis may be required to orientate the swivels to their innermost position). The tension rods are then gradually tensioned whilst ensuring that the swivels are correctly orientated into their slots 34.

Continued tensioning will engage the main seal ring and create joint abutment at face 35. The joint will become pretensioned with a force which will ultimately manifest itself in the shear-pin. This pretension will be of a value determined by various parameters such as accuracy of breakaway force, joint tolerance build-ups and the ability of the main metal seal to tolerate movement. It would typically be within the region of between 60% and 90% of breakaway load.

The disc springs 12 may be used in the tensioning mechanism to make the tension rod effectively "longer" and "slimmer", i.e.

increase its apparent slenderness ratio. This has the effect of making the rod assembly more elastic and consequently more forgiving of minor length variations as might be caused by temperature changes or material creep. It reduces the possibil ity of losing joint preload for any of these reasons. An alternative configuration of this device might advantageously be repositioned in the piston assembly as shown in Figure 11.

Joint tensioning would normally be effected by means of an hydraulic tensioning device such as those units manufactured by Hydratight Limited of Walsall, or Pilgrim Systems Limited of West Bromwich. This would ensure joint tensioning without the likelihood of introducing stresses and strains into the mechan ism via torsional effects from the tension rod: it also permits more even make-up by virtue of a plurality or all of the tension rods being made up at the same time.

Once joint tensioning has been completed a protection cap 14 is placed over the tension nut 13 and 0-ring 15 seals the cap 14 into position to prevent corrosion of the tension rod assembly.

A torque pin 16 or plurality of torque pins 16 will help to ensure that the joint has an ability to resist torsional movement - primarily to reduce or eliminate any tendency of the main seal to rotate and thus lose sealing efficiency. The joint has an inbuilt ability to resist a certain amount of this movement by virtue of anti-rotation frictional forces built up during joint assembly. It is also possible that other means - external to the joint - may. be employed to prevent this eventuality.

The torque pins 16 are made to be a good fit in the female flange by virtue of a thread and reamed hole - the latter mating with a finely machined section on the torque pin itself.

Beyond this finely-machined section on the torque pin is a machined section 17 with a spherical profile or conical profile. After assembly these pins - which had previously been fully threaded home into their recess - may be unthreaded by means of socket 19 to permit the spherical section or conical section to engage into the rim of a mating hole, a counterbore or a conical section (of different frustrum angle from the torque pin cone if used in the male flange member). Gland seal 18 helps the seal of the torque pin 16. This is shown in more detail in Figure 6.

The force of this fit must be significantly low so as to have little effect upon the preload force or taken into consideration when formulating the makeup forces such that its effect will not impair the accuracy of the breakaway force. (The latter may be satisfactorily achieved by not implementing the final, say, 10% of joint tensioning until the torque pins have been tightened). On joint separation the pins will disengage without imparting any significant frictional effects because of the relative angular contact.

During joint installation a thread on the outer end of the torque pins will allow the pin to serve a double purpose.

This thread - if substantially designed - will permit a series of installation nuts 20 to be added to render the joint as strong in tension as the parent pipeline. These nuts may be removed after joint installation by diver or ROV (remotely operated vehicle) to rarmw the joint ready for use; alternatively the nuts may be manufactured from a seawater corrodable material such as magnesium.

Two pressure relief valves 8a and 8b (only one of which is shown in Figure 1) - one acting in one direction and one acting in the opposite direction - provide a means of equalizing the pressure within the joint cavity 30 to that in the surround ing seawater. It also provides a means of relieving a pressure build up from, say, an inadvertant but small pressure leak from the pipeline through the joint main seal or a pressure balance unit.

The valve seats (which would preferably be of a metal to metal seal type) would be spring biased to close until a predetermined pressure differential is reached - typically 20 p.s.i ) (1.379 x 105 Pa). Material of manufacture would probably be chosen for good anti-corrosion properties in seawater.

The joint cavity 30 is filled with grease or fluid via grease plug 27. A further O-ring 28 further seals the safety joint.

The integrity of the safety joint main seal may be tested using test port 21, and test seal assembly 23. Test plug 22 seals this port off.

Thus a simple, reliable and effective safety joint is produced.

It is compact and low weight with components which are easy to manufacture and to assemble into the joint. The joint is resistant to corrosion from both seawater and the fluid within the pipeline. It is compatible with subsea and topside systems for rigid and flexible pipeline systems. The system works with both positive and negative pressure differentials.

There is a minimum number of components and therefore mechan ical interfaces. There is low fricton breakaway mechanism and a fast reaction speed on pressure balancing mechanism.

Applications are not restricted to oilfield and gas system pipelines and flowlines. The concept also lends itself to other applications including tanker loading arms and piped gas and fluid systems in the nuclear industry.

The primary application of the safety joint is to protect attachment structures or similar equipment from overtensions.

As a secondary consideration, prevention of spillage from the pipeline would also be an important feature. The normal method of implementing this - should it be required - is to add a downstream non-return valve and an upstream pilot activated valve to the respective end flanges of the safety joint.

In the second and third safety joints many of the features of the joint are common with the first safety joint and will not be described. The difference lies in the pressure-balancing units.

In the second embodiment of the invention the pressure balance assembly may be simplified in some respects by adding a sliding sleeve member 38 along with sealing devices, O-rings 39 and 40. (The sealing device would probably be of elastomer material as shown or may alternatively be of metal construction such as that type supplied by the company "Advanced Products (Seals and Gaskets) Limited" of Newcastle upon Tyne, England).

This particular embodiment of the invention would permit hoses of the type used in control bundles to be protected.

On assembly, abutment would take place at interface 41 which would carry the majority of the make-up preload. The innermost face of piston 5, in this case, is reacted upon by the frustrum of a conical section 42 which forms an integral part of member 38. This reaction force is initially derived from disc spring assembly 43 and is determined by the geometry of the assembly when made up. This initial reaction force is ideally of a magnitude which represents an axial thrust representative of the pressure due to maximum anticipated sea depth: this will have benefits of offering compensation when the internal pressure is zero at maximum sea depth. Furthermore, this reaction force is arranged such that it will be enhanced in proportion to any pressure (or sum of separate pressures) inside the pipeline thus supplying the pressure balancing force for the whole assembly.

Pressure inside a small bore pipe or hose axially - sliding connection 49 or plurality of connections within the joint bore would cause a reaction force in member 38 causing the member 38 to enhance the reaction force already existing at interface 42. This in turn would offer compensation for the "endcap effect" so as to put the joint into balance and make the primary breakaway force that as determined by the shearpins 7 and disc springs 43. Flexibility in the small bore pipe is effected by the addition of a flexible sliding or bellows connection 45.

On breakaway the shear-pins will be fractured thus initially forcing the member 38 to compress springs 43 and so permit the pistons 5 to travel inwards and swivels 9 to rotate to their separation orientation. During this cycle the joint will part at Interface 41 thus allowing member 38 to reverse its direction of travel and thus not become effected by excessive force from springs 43. Axial movement on small bore pipes due to movement of member 38 is contained by flexible joints 45.

Restraining pins 46 ensure, by means of location in slots or counterbores 47, that on joint separation the member 38 stays connected to male assembly 1 and does not impart excessive stress onto small bore pipes.

Figure 3 shows a third embodiment of the invention which is essentially a variation upon that shown in Figure 2. The difference between the two lies in the location of the abutment area which, in this embodiment, is located at the internal interface between the male and female pieces - in fact the couplings 49. The benefits of doing this are that accurate control is exercised over the dimensional relationships in that area which, when under preload, remains fairly static, thus permitting the use of metal to metal seals (48) on individual small bore pipes. Seals (48) on these pipes may be of the hub flange AX type as used on the main pipe joint in Figure 1 or of other types such as the "Conoseal" as manufactured by Aeroquip Limited of Redditch, England.

Make up procedure is similar to that of Figure 2 except that the making up of the tension studs will tend to pull interface 35 into abutment via piston/sliding member 38 path. The disc springs and flexible couplings must be selected such that enough compression is still available after preload has been applied to allow relative axial movement between male piece 1 and sliding member 38 at the point of shear-pin separation - the relative geometry is more critical than in the previous embodiment if a clean sequence of separation is to be assured.

Furthermore, the structural strength of the piston/shearpin assembly needs to be capable of supporting the full static loading condition as well as the balancing forces.

Figures 4 and 5 show alternative arrangements of the release swivel member 9 and tension rod 10. As can be seen here the safety joint includes six pressure balance units arranged equidistantly around the circumference of the male piece.

As in Figure 4 the release swivel member 9 comprises a plate with the tension rod 10 including a forked region exhibiting two spaced apart prongs between which swivel member 9 rotates Alternatively, as shown in Figure 5 the release swivel member 9 may include two spaced apart prongs with the tension rod 10 fitting between them.

The advantage of the second and third safety joints over the first is that the second and third joints can accommodate a plurality of hoses whereas the first can only accommodate a single pipeline.

Figures 7 to 13 show fourth, fifth, sixth, seventh, eighth, ninth and tenth safety joints which are all modifications of the first safety joint. In each example only the features which are different from those in the first safety joint are illustrated in the drawings and described.

In the fourth safety joint, as shown in Figure 7, the release swivel 9 is replaced by rollers 51 which are mounted on the end of the tension rod which has a spherical washer 50 mounted about it to render the effective pivot arm longer by moving the pivot point backwards. Clearly greater clearance would be required around the tension stud to facilitate an orientation of the assembly that would permit release. However, all other features are as shown in the first safety joint.

In the fifth to seventh safety joint the shear pin/tension assembly is separated from the piston unit. The shear pin 7, instead of lying between the shear pin holder 6 and the piston 5 is moved to a position where it is embedded within the extension sleeve of the female piece. The tension rod 10 is also moved such that the shear pin 7 passes through the tension rod 10 so that the tension rod 10 can be used to pre-tension the shear pin 7. The rest of the tension rod assembly is as it is arranged in the first embodiment. However, clearly the tension rod 10 does not provide the means for mounting the release swivel.

This offers the advantage of removing the accumulative pre tension and pressure compensation forces off the tension rod and swivel pin assembly by separating them. The new tension rod 10 may have its elongation properties more accurately predetermined over the whole tension range through to joint separation. Similarly the swivel member 9 may be made from a section' of low elongation performance to enhance its properties.

The bellows 4 provides pressure compensation as in previous embodiments and also forms along with the tension/shear pins the escape mechanism for the joint.

In the fifth safety joint shown in Figure 8 the release swivel member 9 includes a spherical section part 53 which locates in a trapezoidal groove 54 in the outwardly extending projection from male piece 1. A mating slot 55 in a piston cap 52 locates the swivel member 9 on assembly and acts as a retainer Here the release swivel member 9 rotates with respect to the tension rod 10 but is not mounted upon it.

In the sixth safety joint (Figure 9) the shear pin/tension rod assembly is as in the fifth safety joint but the release swivel member 9 is mounted with respect to the male piece 1 via a spherical pin 56 which passes through a bore 57 in the swivel member 9. This serves to retain the swivel member 9 in its required position.

In the seventh safety joint (Figure 10) the shear pin/tension rod assembly is as described in the fifth and sixth embodiments but here the housing and piston are arranged such that the piston may move in a direction which is non perpendicular to the axis of the pipeline and has a component in a direction parallel to the pipeline. This means that it is possible to arrange the piston such that it applies force in the direction of the release member onto the abutting surface 37 of the extension sleeve. The piston 5 has mounted upon it a roller bearing 58 which helps to overcome friction between the angle of contact of face 37. The roller bearing 58 provides a curved release bearing surface. This somewhat simplified arrangement eradicates the need for a swivel at the expense of increased side forces on the bellows assembly 4 and greater hoop stresses in the female flange.

In the eighth safety joint (Figure 11) the mechanism to increase flexibility of the tension rod/swivel/piston assembly linkage has been transferred from the tension rod nut shown in Figure 1 to the piston assembly. It comprises a disc spring 12 which acts on a holder 59 for the shear pin 7 to urge the holder 59 into a position in which the heads of bolts 60 engage the bases of corresponding bores formed in the holder 59. This helps to preserve preload at the abutment face by reducing the axial elongation of the tension rod due to the axial component of increased rod tension from end cap pressure effects. The product of flexibility is transferred to the radial force plane which may be readily compensated by the balancing piston. A slight amount of over-balancing may be employed to compensate for any remaining axial elongation.

In the ninth safety joint (Figure 12) the tension rod assembly is replaced with a compression rod 61 and compression screw 62. The shear-pin 7 is repositioned to the interface between the compression rod 61 and the release swivel 9 and forms part of the swivel linkage.

In the tenth safety joint (Figure 13) the shear-pin 7 is positioned at the interface between the tension rod 10 and release swivel 9 and forms part of the swivel linkage.

In all the safety joints the bellows unit 4 would more likely be manufactured to have its natural "set" in a compressed state, thus biassing it to an engaged position on assembly even before the joint is internally pressurized. Benefits are also offered when the design assembled position is that with the bellows almost fully compressed to render it more effective as a pressure vessel. This would also reduce lamination wall thickness and permit a high level of bellows flexibility, thus not inhibiting separation by imposing high breakaway. loads upon the joint.

The above principle of the fourth, fifth, sixth, seventh, eighth, ninth and tenth safety joints can be applied to the bundle protecting variants shown in Figures 2 and 3. In the cases shown in Figures 8, 9, 10, 11, 12 and 13, the pistons 5 of the bundles joints would be free running in their bores (possibly being held in position by light duty location shear pins for assembly purposes).

The safety joint described above, particularly that shown in Figure 1, has a number of important advantages. These include the basic simplicity of the design as a result of which the joint is relatively easy and inexpensive to manufacture, assemble and inspect. The design is not too tolerance critical and there are very few potential failure points.

The bellows arrangement used in the embodiment of Figure 1 provides the maximum strength in the compressed state, i.e. at the highest pressure. It has a low spring rate due to the use of relatively thin wall section components. The modular design of each piston and cylinder unit facilitates testing and calibration. The safety joint can be assembled as two separate sub-assemblies, i.e. the male and female pieces. This ensures rapid assembly of the joint proper from two fully inspected sub-assemblies and this of advantage for underwater or otherwise remote quick assembly and release of the joint. The design of the release swivel member allows for adjustment of the piston stroke and bore.

Claims (13)

CLAIMS:

1. A safety joint comprising a first male piece and a second female piece, arranged in abutting relationship to define a fluid passageway, each piece incorporating means to couple the piece to a pipeline, the male piece incorporating at least one pressure balance unit comprising a piston movable in a direction having a component substantially radial with respect to the axis of the pipeline; each pressure balance unit including a release member having a release bearing surface; the female piece including an extension sleeve which extends beyond the abutting region of the male and female piece and over the male piece, the extension sleeve having an inwardly projecting flange which has a bearing surface substantially parallel to and facing towards the abutting edges of the male and female piece; the release bearing surface of each pressure balance unit being arranged to bear against the bearing surface of the flange of the female piece; each piston and release member being arranged such that an outward force on the piston tends to force the release bearing surface into engagement with the bearing surface of the extension sleeve; each pressure balance unit including at least one shear pin arranged to hold the release bearing surface against the bearing surface of the extension sleeve and a tension pin positioned such that it can be used to apply pretension to the shear pin to pre-tension the safety joint such that the load applied via the piston on the release member is transferred to the female piece tending to force the abutting edges together, the safety joint being arranged such that a tension load applied to the safety joint over a predetermined amount causes the pressure on the release member to be sufficient to break the shear pin to allow the piston of the pressure balance unit to move inwards within the housing and allow the release bearing surface to move out of engagement with the bearing surface of the female flange to allow the extension sleeve to ride over the release member to allow separation of the male and female pieces.

2. A safety joint according to claim 1, in which the piston is arranged to move in a direction perpendicular to the axis of the pipeline.

3.- A safety joint according to claim 1, in which the piston is arranged to move in a direction which has a component parallel to the pipeline, the release member including a roller bearing the surface of which provides the release bearing surface.

4. A safety joint according to claim 2, in which the release member is arranged to swivel about an axis perpendicular to the direction of movement of the piston and perpendicular to the axis of the pipeline.

5. A safety joint according to claim 3, in which the shear pin is arranged to co-operate with the tension rod at a region where the extension sleeve contacts the male piece.

6. A safety joint according to claim 2, in which the shear pin is arranged to extend through the piston housing and piston member to hold the piston in position with respect to the housing.

7. A safety joint comprising a first male piece and a second female piece, arranged in abutting relationship to define a fluid passageway, each piece incorporating means to couple the piece to a pipeline, the male piece incorporating at least one pressure balance unit consisting of a housing mounted on the male piece and a movable piston member movable within the housing in a direction substantially radial with respect to the pipeline, a shear pin extending through the housing and the piston member to hold the piston in position with respect to the housing; and, for each pressure balance unit, a release swivel member capable of rotating about an axis perpendicular to the direction of movement of the piston member and perpendicular to the axis of the pipeline, the release swivel member being rotatably mounted with respect to a tension rod which couples the release swivel member to a flange extending outwardly of the male piece, the female piece including an extension sleeve which extends beyond the abutting region of the male and female piece and over the male piece, the extension sleeve having an inwardly projecting flange which has a bearing surface substantially parallel to and facing towards the abutting edges of the male and female piece, the release swivel member being arranged to bear against the bearing surface of the flange of the female piece and against the piston of the male piece; the pressure balance unit being arranged such that pressure within the pipeline causes a force on the piston in a direction radially outwards of the pipeline, the tension rod being tensioned to pre-tension the safety joint such that the safety joint is in balance, such that the load applied by the piston on the release swivel member is transferred to the female piece to force the abutting edges together, the safety joint being arranged such that a tensile load applied to the safety joint over a predetermined amount causes the pressure on the release swivel member to be sufficient to break the shear pin to allow the piston of the pressure balance unit to move inwards within the housing and allow the release swivel member to rotate out of engagement with the bearing surface of the flange of the extension piece to allow the extension sleeve to ride over the release swivel member to allow separation of the male and female pieces.

8. A safety joint according to claim 7, in which the housing of each pressure balance unit comprises a cylinder incorporating a shear pin holder.

9. A safety joint according to claim 8, in which a sealing device acts between the cylinder and the piston.

10. A safety joint according to claim 9, in which the sealing device is a metal bellows which surrounds the piston and is contained within the housing.

11. A safety joint according to claim 7, in which the release swivel member is mounted on the end of a tension rod which is -arranged with a spherical washer surrounding it to allow the entire tension rod assembly to swivel.

12. A safety joint according to claim 7, in which the release swivel member is in the form of a plate-like member which is rotatably mounted to a tension rod which is coupled to the male piece by a tension rod assembly which allows a pre-load to be introduced into the safety joint.

13. A pipeline safety joint substantially as hereinbefore described with reference to and as shown in any one or more of the accompanying drawings.