Weak link for a riser system

A weak link (17, 43) for a riser system comprising a pin (2, 25) and a box (1, 24), bolts (11, 34) for releasably connecting the pin (2, 25) and the box (1, 24), the bolts being designed to break at a predefined tension. The link further comprising a pressure balancing mechanism for balancing axial forces acting on the bolts (11, 34) due to end cap effect. The weak link also comprises a strong mode mechanism and a dampening mechanism.

FIELD OF THE INVENTION

The present invention relates in general to a safety joint for a riser system, commonly known as weak link in hydrocarbon exploration terminology. A riser can be disconnected by such link in the event of any unforeseen emergency circumstances such as extreme weather, power failure on the vessel, failure of anchoring or positioning system and so on.

Particularly, the present invention relates to a weak link for a riser system, which has a pressure balancing mechanism for balancing any end cap effect on the release bolts of the weak link. This pressure balancing mechanism preferably operates in conjunction with a damping mechanism, for ensuring that separation is along such weak link takes place in a controlled manner, limiting and dispersing the extreme forces following release.

The present invention also relates to a weak link for a riser system which has a strong mode mechanism for increasing the gripping force between its two releasably joined component portions as and when required.

More particularly, the present invention relates to a weak link for a riser system according to the preamble of the independent claim1.

TECHNICAL BACKGROUND OF THE INVENTION

It is known that during completion and workover operations within the area of subsea operation, weak links are utilized.

The function of the weak link is to provide a given and controlled method of ultimate separation of the riser, if all other known methods have failed, and the operator is in a worst case mode. Such a mode may arise due to extreme weather, failure of anchoring or positioning system, black-out (power failure) on the vessel, or by other unforeseen means.

In such a worst case scenario it is vital that the vessel is able to passively disconnect from the wellhead and infrastructure on the sea bed, in order to remove the vessel from the conduit to the reservoir and to avoid uncontrolled breaking of the riser and subsequent possible blow-out. This is also required to ensure safety to the personnel onboard. Such a passive disconnection is achieved by use of a weak link.

The failure of the weak link (i.e. the disconnection caused by a weak link in its normal mode of operation) can be attributed to several prime failure modes, and in these can be further related to the operational window and physical position of the vessel.

A heave compensation system for a riser system compensates for variations in the vessel's vertical position in relation to the seabed and inherent upward pull, provided by the vessel. It ensures that buckling/tear-off of the riser system is avoided. If this heave compensation system fails, a failure mode known as ‘compensator lock up’ takes place. This then results in application of tension or compression on the riser system, due to changing vertical position of the vessel, caused by wave motion.

Such failure causes buckling or over-pull and unless the over-pull is limited by a weak link then the operator runs the risk of damage to the subsea systems, including the wellhead, and ultimately risks substantial environmental pollution due to leakage of hydrocarbons, and in worst case a blow-out. The weak-link thus has to fail in a mode whereby the vessel is “on station” (in the correct position) but has a compensator that has locked up, resulting in the vessel applying its own heave (vertical motion due to wave patterns) directly to the riser. Ideally a weak link should protect for such a case.

If the system used to maintain on-station position, either via anchoring or dynamical positioning using thrusters, should fail then a situation known as drift-off or drive-off will occur. This results in the vessel rapidly leaving the green (safe) operation window and entering the yellow (unsafe) and red (danger) zones. These are determined by actual vessel position, relative to a nominal purely vertical riser system.

In case of drift-off, the weak link should be able to fail ultimately, with a permanent break and separation of upper and lower riser sections. The most important aspect is to completely and immediately passively disconnect the vessel from the subsea infrastructure, and hence avoid any damage to the wellhead and/or vessel and personnel.

Conventional weak links are constructed most often by the use of two flanged sections of riser that are bolted together at the flanges using tension bolts, whereby the bolts are designed to fail at a given load.

The riser itself may be in a depressurized state (atmospheric pressure) during the course of operation, or it may be filled with oil and/or gas at pressure. Due to the end-cap effect of the riser system, the pressure present in the riser will exert a tension force in the riser equal to the pressure multiplied by the cross sectional area of the pressurized medium. This tension force acts at every cross section of the riser, hence also acts at the tension bolts. Due to varying pressure (from atmospheric during initial installation) and through to full bore pressure, the tension bolts will be subjected to varying pre-tensions in the riser.

This results in the weak link being susceptible to failure at varying mechanical tensions (T fail=T bolts−T end cap). Given the constant value of the bolt tension failure load, and the variation of pressure, the operator will be depending on a weak link with varying and uncontrollable tension limits. This in practice reduces and affects the safe mode of operation.

Hence the tension load (end cap) due to variations of bore pressure has to be balanced; so called “pressure balance” whilst not compromising the normal operation of disconnection/opening of the weak link.

US patent publication number 2011/0127041A1 attempts to teach such pressure balance by providing a riser weak link having an upper housing and a lower housing which are releasably attached by studs. The studs are designed to break at predefined load. There is also a pressure application device which provides a coupling force on the upper housing to counter balance the separation force applied by well pressure. This ensures that the only separation force acting on the top portion of a riser system attached to the upper housing, is the tension applied by the surface vessel.

However, the prior art acknowledged in the preceding paragraph, has a major draw back. On release of the studs at predefined tension load, the upper housing and lower housing are likely to separate with a sudden snap or jerk. Such recoiling of upper housing and lower housing and the corresponding riser portions attached to each, leave potentialities of damage to sub-sea infrastructure and equipment and to personnel on the surface, wide open.

Apart from the disadvantage in the preceding paragraph, the prior art does not teach specifically and explicitly the adaptability of the weak link to effectively function when the riser system is in operation in subsea condition (i.e. weak link operating in weak mode) and also when the riser system is lowered and retrieved; i.e. weak link operating in strong mode when the gripping force between the two principal releasably connected components of the weak link, need to be strengthened.

Accordingly, there is a long felt need for a weak link for riser systems which has a pressure balancing mechanism which can effectively work with a damping mechanism, so that the upper riser portion and the lower riser portion on disconnection by release of the connection tool such as studs or release bolts, are separated in a controlled manner, limiting substantially any sort of recoiling.

There is also a need for a weak link for riser systems which has a simple mechanism for effectively functioning under varying conditions, when the riser system is in operation in subsea condition and also when the riser system is lowered and retrieved. It is common to use a riser as a lowering means for a valve tree (XMT), by attaching the XMT below the emergency disconnect package (EDP) & lower riser package (LRP) at the lower end of the riser. This is a very heavy assembly, and the inclusion of a conventional weak link poses potentially disastrous overloading risks, particularly in poor weather. Alternatively, the operational window is very narrow.

The present invention meets the above mentioned needs and other associated needs by providing a weak link for riser systems having a pressure balancing mechanism which can effectively function in association with a damping mechanism for controlled and smooth separation of the two main releasably joined components of the weak link, each having portions of risers, connected at lower end and top end respectively. The weak link according to the present invention can also effectively function in both weak mode and strong mode as explained before, in a very simple manner.

OBJECTS OF THE INVENTION

It is one of the principal objects of the present invention to provide a weak link for a riser system which has a pressure balancing mechanism for balancing the end cap effect, which pressure balancing mechanism effectively functions with a damping mechanism for controlled separation of a top portion of a riser system, from its bottom part.

It is another object of the present invention to provide a weak link for a riser system which is equipped to effectively function under varying conditions, when the riser system is in operation in subsea condition and also when the riser system is lowered and retrieved.

It is a further object of the present invention to provide a weak link for a riser system which has a simple construction and works on a simple principle for achieving the objects as mentioned above.

All through the specification including the claims, the words “box”, “pin”, “weak link”, “riser system”, “damping”, “anti recoil”, “weak mode”, “strong mode”, “safety joint” are to be interpreted in the broadest sense of the respective terms and includes all similar items in the field known by other terms, as may be clear to persons skilled in the art. Restriction or limitation, if any, referred to in the specification, is solely by way of example and for explaining the present invention. Further, it is hereby clarified that the term “riser system” should be construed in its broadest sense as applicable in subsea operations.

SUMMARY OF THE INVENTION

According to a primary aspect of the present invention a weak link for a riser system is provided, comprising a first member and a second member, a connection means for releasably connecting said first member and said second member, said connection means being designed to break at a predefined tension wherein a pressure balancing mechanism is provided, for balancing axial forces acting on said connection means due to end cap effect of said riser system. This will substantially cancel out end cap effects and provide greater predictability for the break tension of the connection means, e.g. bolts.

In a preferred embodiment the first member is a pin and said second member is a box, said pin and box being releasably interconnected by release bolts. This provides a simple construction based on per se known principles of a telescopic joint.

In a further preferred embodiment the pressure balancing mechanism has a first pressure balance piston for transferring pressure load to said box and a second pressure balance piston for transferring pressure load to said pin, both pistons being located in an annulus between the pin and the box, said annulus being in pressure communication with a bore of the pin, said pressure loads acting in opposite directions on the pistons. This provides a reliable means for ensuring that the pressure in the balancing mechanism is substantially corresponding to the pressure in the riser bore.

In a further preferred embodiment a radially moveable load transfer segment is located in connection with the first pressure balance piston for transferring the load from the first pressure balance piston to the box and the second pressure balance piston connected to the pin, preferably by a threaded connection, for transferring the load from the second pressure balance piston to the pin. This will ensure a reliable load transfer from the pressure balancing mechanism to the pin and box.

In a further preferred embodiment it comprises a stinger that is fixed at a first end to the box and has a second end extending into the bore of the pin, said stinger providing a narrow annulus with the pin, which in turn provides communication between the bore of the pin and an annulus between the pin and the box. This ensures that the riser bore maintains its integrity as long as possible as the weak link strokes and that a seal between the pin (2) and the box (1) is maintained during the separation stroke.

In an even further preferred embodiment the box comprises an aperture providing communication between the surrounding seawater and a void on the opposite side of the second pressure balance piston from the annulus. This will ensure that the pressure balance mechanism maintains the same pressure conditions when the weak link strokes out.

In an even further preferred embodiment the pin comprises apertures providing communication between the bore of the pin and the annulus. This ensures consistent pressure in the balancing mechanism with the bore of the riser.

In an even further preferred embodiment the box comprises an aperture extending to the surrounding seawater, which aperture is adapted to communicate with at least one of the apertures in the pin when the pin has moved partially out of the box, so as to bleed off pressure within the riser to the surrounding seawater. This will substantially reduce or eliminate the jet effect that would tend to push the riser upward when separation occurs.

In a further preferred embodiment the weak link comprises a damping mechanism for damping any sudden recoiling effect between said first member and said second member during their separation by breaking of said connection means. This substantially reduces the recoiling effect due to separation.

In a preferred embodiment the damping mechanism comprises one or more cylinders and piston arrangements, the damping mechanism being connected to the box by one of the cylinder or the piston arrangement and the other of the cylinder and the piston arrangement being connected to the pin. This will provide an effective dampening mechanism that can be dimensioned according to the requirements independent of the balancing mechanism.

In a further preferred embodiment the dampers are filled with seawater when submerged. This ensures a pollution free system with little complexity.

In a further preferred embodiment the damper has at least one small aperture arranged to slowly expel fluid contained inside the damper through said aperture, for damped separation of said box and said pin. This ensures a controlled dampening with simple and reliable means.

In an alternative embodiment the damper is an integral part of the pressure in balancing mechanism. This provides a compact system.

In a preferred embodiment at least one groove is located on said pin, which groove in the event of separation of said box from said pin, provides space to receive the load transfer segment so as to bring the segment out of engagement with the box, thereby allowing complete separation of the pin and box.

In a further preferred embodiment the weak link comprises a strong mode means adapted to selectively increase the gripping force between said second member and said first member. This reduces substantially or eliminates the risk of accidental or unintended separation when the riser is used for deployment of heavy subsea equipment.

In a preferred embodiment the strong mode means comprises and a strong mode activation dynamic piston operatively coupled to a strong mode locking ring. This provides a reliable means for setting the joint into strong mode.

In an even further embodiment the strong mode means further comprises a first hydraulic fluid pressure conduit that is adapted to deliver hydraulic pressure to a first chamber for displacing the dynamic piston in a first direction and hence displace the locking ring radially into a groove in the box. This provides a simple means for setting the joint into strong mode.

In an even further preferred embodiment the strong mode means further comprises a strong mode static piston situated on the axially opposite side of the locking ring relative to the dynamic piston. This provides a reliable seal for separating the strong mode hydraulically from other parts of the joint.

In a further preferred embodiment the strong mode means further comprises a hydraulic second conduit adapted to deliver hydraulic pressure to a second chamber opposite of the first chamber relative to the dynamic piston for displacing the dynamic piston in a second direction opposite to the first direction, and hence displace the locking ring radially out of the groove in the box. This provides a simple and reliable means to deactivate the strong mode.

DETAILED DESCRIPTION OF THE INVENTION

The following paragraphs describe two preferred embodiments of the present invention which are purely exemplary for the sake of understanding the invention and non-limiting.

In all the figures from1to6a6b,6c,6dand6e, all of which describe one preferred embodiment, like reference numerals represent like features. This is true forFIGS. 7 to 9, 10a,10b,11aand11b, which describe another embodiment. Further, when in the following it is referred to as “top”, “bottom”, “upward”, “downward”, “above” or “below” and similar terms, this is strictly referring to an orientation with reference to the sea bed, where the sea bed is substantially horizontal and below the riser.

It should also be understood that the orientation of the various components may be otherwise than shown in the drawings, without deviating from the principle of the invention.

It is also clarified that the drawings only show the components of the weak link in detail and not the riser system or other components involved in the operation, as those will be understood by persons skilled in the art.

Furthermore, there can be a plurality of the components for weak mode and strong mode operation, which have been described hereinafter. Only one or only two of each have been described hereinafter or shown in the figures, for the sake of ease of understanding only and not for any limitation. Additionally, hereinafter at places the weak link has been referred to as safety joint/joint. All these terminologies indicate the weak link17,43.

FIG. 1is a view of a preferred embodiment indicating the different components of the weak link17. It shows a pin2, which is the internal pressure containing pipe of the joint/weak link and forms a part of the well flow conduit through the joint, having a bore2a. It holds a riser connection at the bottom (i.e. right hand side in the figure) to interface with other riser joints. It is connected to a box1by release bolts11. The box1functions as the outer sleeve of the weak link17and holds a riser connection at the top (left hand side in the figure) to interface with other riser joints.

The box1and the pin2are the two main components of the link and these two are releasably connected by the bolts11. The bolts11are constructed in such a way, as known per se, that those will break if the tension exceeds a predetermined value.

A stinger10functions as the inner sleeve of the weak link. It is connected to the box1preferably by threaded connections (not shown in detail) and fits with a small but distinct clearance at the inside of the pin2to ensure pressure balance and fluid containment throughout the stroke. The stinger also forms a part of the well flow conduit through the joint, having a bore10a.

FIG. 1also shows the upper pressure balance piston(s)4and the lower pressure balance piston(s)3. The pressure balance pistons are situated at a mutual distance, which is at least the same as one bore diameter of the bore10a. The distance may be greater than this to allow for more time to close safety valves to prevent spill of hydrocarbons or drill mud to the environment. Pressure balance load segments5or load transfer segments5are located below the lower pressure balance piston(s)3. These will be explained in detail below.

The features which enable the strong mode activation of the weak link are located at the lower portion of the box1. Those comprise a dynamic strong mode piston9above which is situated a strong mode locking sleeve6and a split ring7above this. Above the split ring7is located a strong mode static piston12. The functioning of these components is explained later.

FIG. 1also shows bore seals8which seal the bore for pressure balancing and dampers13that extend along the outer periphery of the weak link17.

FIG. 2is a cross-sectional view showing the components inFIG. 1more elaborately. This figure also shows the handle15, which may be used by an ROV for the purpose of gripping the weak link. Also shown are grooves5a, located in the box1, with which the pressure balance load segments5remain locked into under normal operation. The figure also shows a lower small aperture1aand an upper small aperture1b. Also shown is a hydraulic hot stab14, which may be accessed by an ROV to exert hydraulic pressure in a hydraulic line23to deactivate the strong mode.

FIG. 3is a view of an enlarged portion of the lower part of the weak link17which is shown inFIG. 2. Other than the features shown in the previous figures, it also shows a groove23bin the lower part of the box1inside which the split ring7can be received. There is also shown a hole1aextending through the wall of the box1. The function of this will be explained later.

FIG. 4is an enlarged cross-sectional view of the weak link similar to the one shown inFIG. 2. In addition to the features shown in the previous figures, it also shows slits10bat the lower part of the stinger10, openings18in the middle and upper part of the pin2and the annulus19between the pin2and the box1.

In the same area as the upper openings18is formed a circumferential recess40.

FIG. 5is a more elaborate cross-sectional view of the lower portion of the weak link similar toFIG. 3, but it shows the strong mode operation of the weak link, which is explained later. This figure also shows a chamber23aimmediately beneath the dynamic piston9and also hydraulic lines23and22through which hydraulic pressure can be applied.

FIG. 6ashows four dampers13, which are located on the outer periphery of the safety joint17. The dampers13are cylindrical tubes with piston arrangement16. The cylinders of the dampers13are secured to the box1while the piston rod16of the dampers extends inside the cylinders to their top end. Longitudinal grooves21are present on the piston rods16, for release of seawater as will be explained later.

TheFIGS. 6b, 6c, 6dand 6eshow the various stages of separation of the weak link17when the box1separates from the pin2.

Having described the basic structures of the weak link17, first the functioning of the pressure balancing mechanism of the weak link will be explained with reference toFIGS. 1 and 4in particular. As the riser is subjected to internal pressure when connected, the safety joint17contains a pressure balancing mechanism that makes sure that this pressure is not exerting any axial force on the bolts11due to the end-cap effect, and thereby reducing the operational window of the safety joint17.

Referring back toFIG. 1again, the box1functions as the outer sleeve of the joint and holds a connection at the top to interface with other riser joints. The pin2may be attached directly to a valve tree or an LRP or may have a riser portion beneath.

Referring again toFIG. 4, the stinger10is connected to the box1preferably by a threaded connection (not shown in detail) and is received with a small clearance along the inside of the pin2to ensure pressure balance and fluid containment throughout the stroke. The small clearance provides a narrow annulus (not shown in detail) between the stinger10and pin2, and through this annulus pressure is communicated between the pin bore2aand an annulus19between the pin2and the box1. Apertures18in the pin2provide communication between the narrow annulus and the annulus19.

The lower pressure balance piston3and the upper pressure balance piston4function to feed back the pressure separation force (due to the end-cap effect) to the box1and pin2respectively.

As indicated by arrows inFIG. 4, the pressure inside the riser20is transferred from the well flow bore through slits10bat the lower part of the stinger10, through the small annulus between the stinger10and the pin2and from there through openings18in the middle and upper part of the pin2into the annulus19between the pin2and the box1. At the two ends of this annular enclosure19are the two opposed circular pistons3,4. The pistons3,4transfer the resultant force to the box1and the pin2respectively, in opposite directions, as shown, thereby cancelling the effects the internal pressure is exerting on the bolts11. InFIG. 4, the light grey shaded portions in general indicate fluid path.

Actually, during operation of the well the medium being transferred (gas/oil and so on) is at high pressure, which gives rise to the end cap effect and results in adding tensile force to the riser segments. Since this pressure varies with time and also with the length of the riser, the force acting on the failure bolts11of the weak link17cannot be exactly ascertained at any point of time. To get around this problem, the pressurized medium being transferred is allowed into the chamber19(also shown inFIG. 4) between pressure balancing pistons3,4, so that the pressure acts in both directions. The exposed area of balancing pistons3,4being equal to the cross section of the production bore; the opposing forces cancel each other out.

The elongate pressure balancing chamber19ensures that the safety joint17is able to keep the pressure balance throughout the separation stroke of the joint, as will be explained more detailed below.

Weak mode operation of the weak link17is required when the riser system is deployed and is functioning normally. To be precise, at this stage, the tension on the release bolts11is below the predefined level and the box1and the pin2remain connected. This is explained particularly with reference toFIGS. 2 and 3. When the riser is connected to the infrastructure on the seabed, the safety joint17is set to weak mode as shown inFIG. 2. At this stage, the bolts11connecting the box1and the pin2are limiting how much force the safety joint can transfer. This way, the maximum permitted force on the riser is known, and if the riser is subjected to a force beyond this calculated maximum, the bolts11will fail.

The load transfer segments5are engaged with the grooves5ain the box1. This ensures that the lower pressure balance piston3can not move further down.

Consequently, the pressure within the annulus19acting on the lower piston3, will be transferred to the load transfer segments5and from these to the box1. The pressure on the pistons3,4will therefore act to push the box1and the pin2towards one another. This force pushing the box and pin towards one another will be substantially equal to the force acting to push the pin and box away from one another due to the end cap effect. This is due to the fact that the area of the pistons3,4is substantially equal to the cross sectional area of the bore of the riser and that the pressure in the bore of the riser is communicated to the annulus19, so that the pressure in the annulus19is substantially equal to the pressure in in the bore. Consequently, the only force acting on the bolts11is due to the tension in the riser.

Further, as it would be clear from the enlargedFIG. 3, which is a view of an enlarged portion of the lower part of the weak link17, inFIG. 2, the lower pressure balance piston3can not move upwards. This is due to the presence of a ledge3aon the piston that abuts a shoulder3bin the box1.

FIG. 3further shows the approximated load path (shaded in dark grey) when the weak link is running in weak mode. Here the dynamic strong mode piston9and the sleeve6are in their lower most position, and the locking ring7is disengaged away from the groove23b.

FIG. 4, described hereinbefore, while explaining pressure balancing, is also a view showing weak mode operation. During weak mode operation there may occur a situation with a sudden need to separate the top portion of the riser system from the lower portion due to increase of tension in the riser and hence a force acting on the release bolts11crossing a certain threshold limit. In such a situation the box1and the pin2will separate, in order to prevent uncontrolled damage on the riser.

In order for the separation process to take place in a dampened manner, to substantially reduce any sudden jerk due to elastic energy in the riser system, is further explained with reference toFIG. 6a. As shown inFIG. 6adampers13, in the form of cylindrical tubes, are located on the outside of safety joint17to absorb the sudden jerk on failure of bolts11for enhanced safety. These are ideally cylindrical tubes with piston arrangement16as best shown inFIG. 6a. These are automatically filled with seawater when submerged. The cylinders of the dampers13are secured to the box1while the piston rods16of the dampers extend inside the cylinders to the top end of these. TheFIG. 6ashows a stage, when separation has not started and the weak link17is functioning normally.

To understand how separation of the box1and pin2takes place, in the event of the tension in the riser reaching and crossing a predefined limit, reference is now in again made toFIG. 1. Here it is clarified to avoid any confusion, that during the separation, the pin2remains stationery in attachment with some equipment/riser system on the sea bed and the box1moves axially upwards relative to the pin2. The joint may however, be configured the other way around, so that the pin moves upward while the box is stationary.

In the event of failure of bolts11, the lower pressure balance piston3moves axially with the box1when the box1is commencing the separation from the pin2, since the load transfer segments5are locked in the groove5ain the box1.

The top portion of the upper pressure balance piston4has preferably threads (not shown in detail) that positively attaches the piston4to the upper end of the pin2. Consequently, the upper piston4will move downward relative to the box1(i.e. the box moves upward while the piston is stationary). As this creates a void on the upper side of the upper piston4, an small aperture1bis provided through the wall of the box, through which seawater can flow to fill the void.

Referring toFIG. 6b, which is a cross sectional view of a portion of the joint, is shown a partial separation of the box1and the pin2. At this stage a set of apertures18in the pin have reached the aperture1ain the box1. The pressure within the bore10aof the joint is bled off into the sea water through these apertures. Due to the upper and lower pistons3and4moving closer to one another, the fluid in the annulus19awill be pushed through the apertures18and into the bore of the riser until the lowermost apertures18reach the lower aperture1a. Also during the bleed-off through the lower aperture1athe annulus19, and the pressure therein acting on the pistons3,4, will provide pressure balance to cancel out the end cap effect. Also during the separation, seawater is continuously flowing into the void19aabove the upper piston4.

FIG. 6cis a front view of the weak link, which corresponds to the view inFIG. 6b. As, it would be clear, the box1has moved up relative to the pin2.

FIG. 6dis a cross-sectional view of a portion of the joint. It is a view of a stage when separation of the box1relative to the pin2is almost complete. It would be clear from this figure as also fromFIG. 6e, which is a front view of the weak link corresponding to the view inFIG. 6dthat the box1has moved further up, as compared to what is shown in the views inFIGS. 6band 6cand is almost separated from the pin2. It also shows that the lower pressure balance piston3has moved upwards, together with the box1, relative to the upper balance piston4and has in fact met the upper pressure balance piston4. The void19above the upper piston4is now filled with seawater.

At this point, a groove40in the upper part of the pin2has reached juxtaposition with the load transfer segments5, and allow these to move inwards and out of the grooves5ain the box1. The locking segments are preferably spring biased inwardly to facilitate this action. The disengagement from the groove5aallows the load transfer segments5and the lower balance piston to move downwards with the pin2and out of the box1. Thus, the box1now moves axially away from the pin and is fully separated from the pin2. The dampers, which have also reached their end of the stroke are separated by the piston rods dislodging from the cylinders. This separation may be facilitated through a mechanism of segments in the cylinder13that is allowed to expand when the piston at the upper end of the piston rod16reaches a certain position.

At the end of the separation, which stage is not shown, the pin2remains at the bottom with the upper pressure balance piston4, the lower pressure balance piston3and the load transfer segments5. The box1completely separates and the dampers13having the pistons inside release completely from the piston rods16, which are connected to the pin.

The dampers thus ensure controlled and smooth separation of the box1relative to the pin2and the risers/equipments attached therewith.

Now the strong mode action of the weak link will be explained, referring toFIG. 5. The strong mode is required to strengthen the joint between the box1and the pin2when the riser is lowered to the sea bed with heavy equipment, e.g. a EDP/LRP/XT assembly, hanging from its lower end. This is also required when the riser assembly is retrieved.

The strong mode ensures greater gripping force between the box1and the pin2by reducing the load on the release bolts11. This strong mode is inactive when the weak link is in normal operation and is subject to well pressure. The strong mode has to remain inactive also, during separation the box1and the pin2along the release bolts11.

Strong mode is particularly required to ensure that the bolts11do not fail during lowering and retrieval operations, when substantial tension acts on the release bolts11of the weak link. This tension may be much greater than the pre-defined tension at which the release bolts11are designed to break.

During strong mode operation hydraulic fluid is forced through the valve22(best shown inFIG. 5) into a chamber23aat the lower part of the box1. This forces the strong mode dynamic piston9upwards. The piston pushes the strong mode locking sleeve6, which subsequently forces the split locking ring7into a groove23b(best shown inFIGS. 4 and 5) in the lower part of the box1. The ring7is pressed radially outward by the strong mode locking sleeve6, so that it enters the groove23band sits in it. The locking sleeve may be designed to keep the locking ring7in the groove23bwithout having to apply a continuous hydraulic pressure in the chamber23a.

Through the locking ring7, the pin1and the box2are locked together and hence the bolts11are partially relieved, and the overall tensile capacity of the safety joint is increased.

The strong mode is deactivated by releasing the hydraulic pressure through a strong mode deactivating hydraulic line23, which pushes the sleeve6and the dynamic piston9back downwards so that the locking ring7can move inwards again to disengage out of the groove23bat the lower part of the box1. When strong mode is deactivated the cavity between the static piston12and the dynamic piston9contains hydraulic pressure, to ensure that the split ring cannot engage with the groove23b.

The static piston12is fixedly attached to the pin2by threads. This ensures that the static piston12remains static with the pin2.

FIG. 7is a view indicating the different components of a further embodiment of the weak link43of the riser system. The weak link43has a pin25and a box24. The pin25has a portion of a riser system (not shown) connected at its bottom (left hand side inFIGS. 7 and 8), while the box24has a portion of a riser system (not shown) connected at its top (right hand side inFIGS. 7 and 8). The pin25and box24are releasably connected to each other by release bolts34. These release bolts34are designed to break at a predetermined load. The riser system connected to the pin25is either connected to further risers or to other equipments/infrastructure on the sea bed.

There is an upper pressure balance piston27for transferring load on the pin25and a lower pressure balance piston26for transferring pressure load to the box24. A pressure balance load ring29is located above the upper pressure balance piston27for transferring load from the upper pressure balance piston27onto the pin25. A pressure balance load segment28is located below a lower pressure balance piston26for transferring pressure load from the lower pressure balance piston26onto the box24.

A strong mode load segment30is located above a strong mode activation ring32, which is in contact with both the pin25and the box24. The upper part of the pin25forms an anti-recoil piston rod with an upper anti-recoil piston36at the end. The upper part of the box24forms an anti-recoil cylinder39with a lower anti-recoil piston41at its lower end.

An anti-recoil load ring37is located above the upper anti-recoil piston36, while an anti-recoil support segment42is located below the lower anti-recoil piston41for supporting these.

A stinger33is arranged within the upper part of the pin25to contain pressure in the joint43. Bore seals31are provided at the end of the stinger33to prevent leakage.

FIG. 7also shows the location of the anti-recoil support segment groove40, the stinger retainer38.

The disposition of the various components of the weak link43as described hereinbefore with reference toFIG. 7are further elaborated in the enlarged view inFIG. 8, which show only the lower part of the joint43.

FIG. 8also clearly shows a strong mode load segment30located above the strong mode activation ring32and that this sits in a groove46in the box24. The ring32is in contact with the pin25and the box24.FIG. 8also shows a groove28ain the lower part of the box24, with which the pressure balance load segment54remains in engagement, when the box24and the pin25are connected.

FIG. 9is a view showing the pressure balancing, when the joint43is under weak mode operation. It shows the pressure balancing chamber45between the box24and the pin25and the openings50through which pressure in the riser44is conveyed to the chambers45.

FIG. 10ais a view showing the weak mode operation whileFIG. 10bis a view showing the strong mode operation.FIG. 10ashows the openings50through which pressure is conveyed from the riser to the pressure balancing chamber45best shown inFIG. 9. It also shows the box24, the pin25and the release bolts34.FIG. 10bshows these features and additionally shows the static strong mode piston30above the strong mode activation piston32and the strong mode activation port47.

FIGS. 11aand 11bare views showing the damping mechanism for dampened separation of the box24and the pin25. There is an annular chamber39between the pin25and the box24. This chamber39is filled with sea water, when submerged, through holes48(shown inFIG. 11b) at the lower part of the box24. The shaded portion in theFIG. 11bindicates water in the cavity39.

The pressure balancing mechanism during weak mode operation will now be explained with reference also toFIGS. 7, 8 and 9.

When the riser44is fully deployed the weak link is in weak mode, where the bolts34connecting the pin25and box24are limiting how much force the weak link43can support. This way the maximum permitted force on the riser44is known, and if the riser is subjected to force beyond this calculated maximum the bolts34will fail.

A force acts on the riser system due to end cap effect which needs to be first of all balanced by the pressure balancing mechanism of the weak link43, in order to cancel out the axial force exerted on the release bolts34due to pressure in the riser.

As particularly shown inFIG. 9, the pressure inside the riser44is transferred through openings26into a chamber45between the pin25and the box24where it exerts force on two opposed circular pistons (a lower pressure balance piston51and an upper pressure balance piston52, as best shown inFIGS. 7 and 8) each of which transfers the resulting force respectively to the box24and the pin25in opposite directions. Thus the effects of the internal pressure on the bolts34, is cancelled out.

A pressure balance load ring54is located above the upper pressure balance piston52for transferring the pressure load from the upper balance piston52to the pin25. Similarly, a pressure balance load segment54is located below the lower pressure balance piston51for transferring pressure load from the lower pressure balance piston51to the box24. These segments54are so constructed that they may move radially and allow the box24to release, once the bolts34break. When the box24and the pin25remain connected, the segments54are in engagement with the grooves28ain the box24by a holding ring28that has a chamfered surface, as best shown inFIG. 9.

When, the threshold limit of the release bolts34is crossed, the box24and the pin25start separating, the holding ring28will move downward with the pin and the segments54are allowed to move radially inward to disengage from the box.

Towards the end of the stroke a groove40will reach anti-recoil support segment42allow this to move radially into the groove40in the pin and hence out of engagement with the box24, thus allowing the pin25to pass all the way out of the box24and separation to occur.

Without any form of dampening, this separation would be like a rubber band breaking, and the resulting recoil has potential chances of damaging the subsea infrastructure as well as equipment and personnel on the surface.

To avoid any recoil during separation of the box and the pin, the weak link43is designed with a built-in recoil prevention system to minimize any recoil. This mechanism will now be explained with reference toFIGS. 11aand11b.

The recoil prevention mechanism works by providing a chamber39between the upper parts of the pin25and the box24, this chamber39is filled with sea water through holes48(shown inFIG. 11b) at the lower part of the box24. When the weak link separates, the water is forced slowly out again through the holes48.

The holes48are so sized that the water is restricted in its flow, thus providing an effective damping to the separation movement. This causes the separation process, or the stroke, to be limited to an acceptable speed, thereby limiting the impact of the released energy.

FIG. 11ais a view of a stage when the box24and the pin25have not started to separate and seawater (shaded portion) is present in the chamber39.

FIG. 11bis a stage when the separation has begun by breaking of the bolts34. The box24has moved upwards relative to the stationery pin and water is partially expelled so that it is now present only in the cavity portion39a.

Thus the pressure balancing mechanism of the weak link works in association with the anti-recoil mechanism of the weak link. However, as opposed to the in embodiment ofFIGS. 1-6, the pressure balance feature will not work after the bolts34have been broken. This means that the damper mechanism deals with both the end cap effect and the riser tension forces, namely, the axial force acting on the release bolts due to end cap effect of the riser is cancelled out and that the pin and the box separate in a controlled and smooth manner, without any sudden snap when the release bolts reach the threshold value as preset.

The strong mode operation is explained further with reference toFIGS. 10aand 10b.FIG. 10ashows the approximate load path (shaded portion) when the weak link43is set in weak mode andFIG. 10bshows the approximate load path (shaded portion), when the weak link43is set in strong mode. By comparingFIGS. 10aand 10bit can be seen that inFIG. 10athe strong mode activation piston32is in its lowermost position, whereas inFIG. 10bthe piston32has been displaced to its uppermost position, and hence the strong mode has been activated.

As shown inFIG. 10b, to activate the strong mode, hydraulic pressure is applied through a port47located near the lower part of the box24and below the piston32, whereby the piston32is pushed upward and in turn pushes the load segment30axially into a groove46(best shown inFIG. 8) in the box24. Hence, the gripping force between the pin25and the box24is increased. This strong mode application of hydraulic force, thus substantially increases the strength of the weak link43by ensuring greater contact between the pin25and box24. Naturally, in that event, the load on the release bolts34is also reduced.

FIG. 12ashows a longitudinal section through a damper13. The damper comprises a cylinder100that is sealed at a first end101by a first open ended end cap102. At the opposite second end103, is a second end cap103. Within the cylinder100is a piston104. A piston rod105is attached at a first end106to the piston104and the piston rod extends out through the second end cap103to a second end107that is outside the cylinder100. InFIG. 12a, the damper13is in the fully retracted position, i.e. the piston104is next to the first end cap102.

A longitudinal groove108is formed along the piston rod105. This groove is stepped in depth as follows (from the outer end to the piston104): an outer deeper groove portion108a, an intermediate shallower groove108band an inner shallowest groove108c. In addition there is a short deeper groove108dclosest to the piston104.

FIG. 12bshows the damper13in a partial longitudinal section 90° to the section inFIG. 12a.

FIG. 12cshow a detailed longitudinal cross section of the inner end101of the damper13in the same view as inFIG. 12a. It shows the piston104and the end cap12. The piston is equipped with a connecting piece109, a one way valve110and a fill channel111, to fill the cylinder100with hydraulic fluid. The channel111is in communication with the innermost groove108d.

Distal of the piston104is a gripping mechanism112that forms a connection between the piston104and the piston rod105. The gripping mechanism comprise a plurality of dogs120that couple the piston104and the piston rod105by intermeshing grooves113and projections114. The dogs120have a distal end119that extends obliquely outward from the piston rod105to the inner wall of the cylinder100. Thereby a conical cavity121is formed between the rod105and the distal end of the dogs120.

FIG. 12dshows a detailed view of the outer end of the damper13in the same section as inFIG. 12a. The piston rod105has a bore115that puts the outermost groove108ain communication with the surroundings through an opening116in the distal end of the piston rod105. In addition the rod has a circumferential groove117that also puts the interior of the cylinder100in communication with the surroundings.

The end cap103has an inwardly ring shaped projection118, the function of which will be explained later.

When the force of acting to separate the box1and pin2of the weak link, as described hereinbefore, and the release bolts11break the piston rod105will be pulled outward from the cylinder100. The rod105will pull the piston104along with it.FIG. 13ashows a cross section similar toFIG. 12a, but where the piston104and piston rod105have been pulled somewhat outward. Due to the pulling force, hydraulic fluid within the cylinder will be forced out of the cylinder through the bore115in the rod105. Initially, fluid will also be forced out via the circumferential groove117. As the first part108aof the groove108travels through the end cap103, fluid will also flow along this groove to the outside. The first groove part108ais relatively deep and thus the travelling speed of the piston rod105will be relatively high as long as the first groove part108aallows flow through the end cap103.

FIGS. 13a, canddshows the situation when the inner end of the groove part108areaches the end cap103.

When the whole of the groove part108ahas penetrated through the end cap103, the fluid will flow along the second and shallower groove part108b. This will reduce the travelling speed of the piston rod105.

When the whole of the second groove part108bhas penetrated through the end cap103, the third and shallowest groove part108chas been reached. This will slow down the travelling speed of the piston rod even more.

Only when the shallowest groove part108chas penetrated the end cap103, the last and deeper groove part108dis reached. Then the traveling speed of the piston rod is increased again to ensure separation of the piston rod105and the piston104, as will be explained below.

Referring toFIGS. 14aandb: when the piston104reaches the outer end of the cylinder100, the dogs120will meet the ring shaped projection118. The ring shaped projection118will enter into the conical cavity121and force the distal end119of the dogs120outward. The end cap103has a larger inner diameter than the cylinder100, allowing the distal ends119of the dogs to move outward and thereby release their grip in the piston rod105.FIG. 14cshows the piston rod105being completely separated from the piston104.

The release of hydraulic fluid from the cylinder100can be done in a controlled manner by adapting the width and depth of the groove108and the channel117to achieve the desired separation rate.

The present invention has been described with reference to preferred embodiments and some drawings for the sake of understanding only and it should be clear to persons skilled in the art that the present invention includes all legitimate modifications within the ambit of what has been described hereinbefore and claimed in the appended claims. It would be readily understood that the pin can be attached to the upper part of the riser and the box to the lower part or seabed equipment without deviating from the invention.