TENSION RING ASSEMBLY

BACKGROUND

a. Field of the Invention

This invention relates to a tension ring assembly adapted to be clamped around a pipe to enable the pipe to be gripped and to enable ancillary devices to be attached to the pipe, for example wires or chains, by which an upwards tension may be applied through the tension ring assembly to the pipe. The tension ring assembly is particularly intended for mounting on a conductor riser pipe which is part of an offshore oil and gas production or exploration facility.

b. Related Art

One particular application is for drilling conductors where ancillary devices, in particular lifting or tensioning lugs, have to be securely attached around the conductor before the conductor is lowered through the rotary table on an offshore rig. Many known tension rings have a relatively large diameter, for example 1500 mm, and cannot pass through the opening in the table with the result that the ring has to be mounted on the pipe below the table.

One prior art tension ring assembly that deals with this problem is disclosed in patent document EP 2143873 A1, which describes a tension ring that is first fitted on a riser pipe, and then lowered through a hole in the table, after which four removable lifting or tensioning lugs are connected around the outer circumference of the tension ring to complete the tension ring assembly. Each of the lifting lugs is attached around the outside of a main body of the tension ring by means of two connection bolts that insert through clearance holes that extend radially inwards through each lifting lug to screw into tapped holes in one of two collar segments that make up an outer collar of the main body. A plurality of slip segments fit between the collar segments and the pipe to make direct contact with the pipe outer surface.

The two outer collar segments are semicircular and are initially secured together by securing bolts that extend horizontally in a substantially circumferential direction. Each bolt passes through aligned clearance holes in opposite end portions of the two collar segments and is then secured by nuts that are tightened until end faces of the collar segments are pressed together.

The inner slip segments are initially retained in an upwards retracted position. The slip collar and slip segments have lifting shackles temporarily screwed into an upper face of the collar segments for the purposes of handling and positioning the collar segments during installation. After the joined collar segments are positioned beneath the rotary table, and the four lifting lugs attached, the slip segments are released, such that they slip downwards on a ramp until they are wedged between the collar segments and the outer surface of the pipe. Each of the slip segments may have an inner surface of steel with a series of parallel ridges and grooves, or may have an inner granular particle gripping surface, as disclosed in patent document WO/2015/173572 A2.

Each of the slip segments has a radially outer face that has an inclined surface which is at 20° to the central axis of the tension ring. Each of the collar halves has a corresponding inner face with an inclined surface at the same angle. This means that as the slip segments are pulled down inside the collar segments as the outer collar is tensioned upwards by wire rope or chains, the ridged surfaces of the slip segments are driven into secure gripping contact with the outer wall surface of the pipe. This arrangement therefore ensures that as upwards tension is increased, the inner tension sleeve grips the outer wall surface of the pipe ever more tightly.

In this process, the outer collar segments become highly tensioned in a circumferential direction, this tension all being borne by the bolts that connect the two collar segments together.

While this arrangement works well after installation, problems arise when it becomes necessary to remove the tension ring from the pipe. To help minimise the outer diameter of the collar segments, the securing bolts holding the collar segments together have ends where bolt heads and nuts are partially recessed in the outer cylindrical profile of the collar segments, instead of being connected through opposed flanges that extend outwardly from the ends of each collar segment. The lifting lugs are positioned at ± 45º around the circumference either side of each of the two joins between the collar segments. The proximity of the securing bolt heads and nuts with the lifting lugs means that the lifting lugs must first be removed in order to provide access for a hydraulic reaction tool to engage with the head and nut at the opposite ends of each securing bolt.

It has been found in practice that the main difficulty comes with removing each lifting lug. The collar segments have an outer diameter of about 1200 mm and a height of about 550 mm. Each lifting lug abuts one of the collar segments over substantially the full height of each collar segment to which it is joined by four connection bolts having a diameter of about 50 mm. Each connection bolt is tightened up to near its torque limit of about 100 kNm. It is difficult to undo such bolts on a conductor riser pipe, which may be moving, particularly as tools for undoing such bolts have to be secured to the pipe in order to provide the required torque.

It is an object of the present invention to provide a more convenient tension ring assembly having a radially compact collar to which at least one lifting or tensioning lug may be removably connected.

SUMMARY OF THE INVENTION

According to the invention, there is provided a tension ring assembly for securing around a pipe, the assembly comprising a tension ring, at least one lifting lug removably secured to the tension ring so that the tension ring, and a pipe around which the tension ring has been secured, can be upwardly tensioned, the tension ring comprising:

• a plurality of collar segments configured to be assembled into a ring, said assembled ring of collar segments extending around a ring axis and having in a radially outer face of said assembled ring at least one recess to which said lifting lug is secured;
• at least one collar segment connection member for connecting the collar segments together to form said assembled ring when said collar segments are assembled around said pipe; and
• a plurality of slip segments adapted to fit between the collar segments and the pipe, the slip segments having radially inner faces for making direct contact with an outer surface of the pipe, the collar segments and the slip segments when assembled having inclined contacting surfaces such that an inner diameter across said assembled ring, between the inner faces of the slip segments, varies as the slip segments move in a substantially axial direction up and down inside said assembled collar segments;
• characterised in that the tension ring assembly further comprises at least one securing pin for removably securing said lifting lug to the tension ring, and said collar segment has for said recess at least one bore into which a corresponding securing pin may, in use, be removeably inserted, said bore intersecting said recess such that a portion of said inserted pin protrudes inside said recess, and said lifting lug comprises:
• a main body and at least one tongue, said tongue extending away from said main body and being configured to slot in a substantially radial direction into said recess in order to position said main body on said radially outer face, said tongue having a keyway for engaging with said protruding portion of said inserted pin, the configuration of said tongue, recess and bore being such that the keyway comes into alignment with said bore when said tongue is slotted into said recess, so that when said pin is inserted into said bore, the protruding portion of said inserted pin engages with the keyway to secure said lug in position, and so that when the pin is removed from said bore, said protruding portion of said inserted pin disengages from the keyway to free said lug for removal from said collar segment;
• wherein said bore is an axially extending bore that partially intersects said recess and said keyway extends axially to align with said bore where said bore partially intersects said recess, whereby the protruding portion of said inserted pin is a portion in cross-section of said pin.

The tension ring of the invention will normally be used to engage with vertically positioned pipes and, as a result, the plane of the tension ring will in use be generally horizontal. References in this specification to top and bottom, up and down, horizontal and vertical are to be read accordingly, but are not to be taken as limiting the scope of the invention.

In a preferred embodiment of the invention the assembled ring is substantially cylindrical with a corresponding outer circumference, and the, or each, inserted pin lies within this outer circumference.

The means for tensioning adjacent collar segments against one another may comprise at least one screw fixing or bolt and nut fixing, which, when tightened, will pull adjacent collar segments together.

Preferably, the assembled collar has two collar segments, each in the form of an annular arc extending over a semi-circle.

The tension ring may also have a retaining plate associated with each collar segment, and means may be provided in the retaining plates to support the slip segments in their uppermost position when initially locating the tension ring around a pipe. The supporting means can then be operated to release the segments.

The means for tensioning adjacent collar segments against one another to secure the tension ring around the pipe may be used to provide an initial tension of the collar segments prior to full tensioning of the collar segments so that slip segments are held in direct contact with the pipe surface prior to removably mounting the (or each) lifting lug to the tension ring.

The means for tensioning the collar segments may comprise at least one bolt that pull adjacent collar segments together.

The lifting lug may have a main body and a recess in the main body from which the tongue extends. The tongue may then be secured to the main body in this recess.

There may be for the, or each, lifting lug a pair of securing pins and two corresponding protruding portions. The tongue then has having two corresponding keyways.

The two protruding portions may be on opposite inner sides of the recess, in which case, the keyways are also on opposite outer sides of the tongue.

The recess may have a substantially square or rectangular cross-section in a plane perpendicular to an insertion direction of the tongue in the recess.

The pin is preferably cylindrical, in which case, the protruding portion may be substantially half-cylindrical.

In a preferred embodiment of the invention, the keyway has an inverse shape to that of the protruding portion.

The main body of the lifting lug may have a flat inner surface and the tension ring may also have a flat mounting surface against which this flat inner surface abuts, preferably in flush contact, when the main body is positioned on the tension ring.

The collar segments may have substantially circumferentially extending channels in an outer surface of the collar segments. The channels preferably have a straight axis that is parallel with a tangent to the tension at a join between adjacent segments held together by the (or each) connection member. Preferably, the (or each) collar segment connection member is substantially recessed in these channels.

The collar segments may have a substantially cylindrical outer surface.

In a preferred embodiment of the invention, the bore of the pin extends fully through the collar segment and the pin, when fully seated in the bore, has one end with a head that limits the insertion of the pin, and an opposite end that protrudes from the bore.

Such a head preferably has a threaded hole for receiving a threaded member for pulling the pin out of the bore when the lifting lug is to be removed from the tension ring.

According to the invention, there is also provided a method of securing a tension ring assembly around a pipe, the assembly comprising a tension ring and at least one lifting lug by which the tension ring and a pipe, around which the tension ring has been secured, can be lifted or upwardly tensioned, the method comprising:

• assembling said tension ring by connecting a plurality of collar segments together into an assembled ring of collar segments around the pipe, and providing for each collar segment a plurality of slip segments disposed between the collar segments and the pipe for making direct contact with the pipe surface, the collar segments and the slip segments when assembled having inclined contacting surfaces, such that an inner diameter across said tension ring, between the inner faces of the slip segments, varies as the slip segments move up and down inside said assembled collar segments;
• tensioning adjacent collar segments against one another to secure said tension ring around the pipe;
• providing an apparatus separate from the tension ring assembly, said apparatus having a body and in the body a through hole;
• passing said secured tension ring through said hole and then removably mounting said at least one lifting lug to the tension ring to complete the assembly of the tension ring assembly;

wherein, after said mounting of said at least one lifting lug, the tension ring assembly has an outer diameter that is greater than the diameter of the tension ring prior to mounting of said at least one lifting lug to the tension ring, such that the secured tension ring assembly and pipe may not pass back through said through hole, characterised in that said removable mounting of said at least one lifting lug to the tension ring comprises:

inserting along a substantially radial direction at least one tongue of the lifting lug into a corresponding recess in a radially outer face of the tension ring in order to position a main body of the lifting lug on said radially outer face;

inserting a pin into an axially extending bore in the tension ring, said bore partially intersecting said recess such that a portion in cross-section of said pin protrudes into said recess; and

as said pin is being inserted, engaging said protruding portion of said inserted pin with a keyway in said tongue, the configuration of said tongue, recess and bore being such that the keyway comes into alignment with said bore when said tongue is inserted into said recess so that said when said pin is inserted into said bore, said protruding portion of said inserted pin engages with the keyway to secure said lug in position, and so that when the pin is removed from said bore, said protruding portion of said inserted pin disengages from the keyway to free said lug for removal from said collar segment.

The method may include, after passing the secured tension ring and pipe through the hole, the step of applying additional tension between adjacent collar segments to further secure the tension ring around the pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which:

• Figure 1 is a perspective view of a tension ring for a tension ring assembly according to a preferred embodiment of the invention, having a segmented collar securing by bolts into a full collar or ring assembly that surrounds a plurality of gripping slip segments mounted around a pipe;
• Figures 2 to 5 show sequential steps in the assembly of the tension ring assembly of Figure 1, and in particular how a tension ring is formed in two halves, each having one collar segment and associated slip segments, the collar segments with associated slip segments being joined together around the pipe and inserted through a hole after which lifting lugs are attached to each tension ring half to form the tension ring assembly;
• Figure 6 is a top plan view of one of the tension ring halves of Figure 2;
• Figure 7 is a side plan view of the tension ring half, taken along line VII-VII of Figure 6, showing the inside surfaces of the slip segments and opposite circumferential end surfaces of the collar segment, within each of which are four clearance holes for connecting bolts;
• Figure 8 is a side plan view of the tension ring half, taken along line VIII-VIII of Figure, showing a radially outer surface of the collar segment 6, in which there are four recesses for the clearance holes and two recesses for mounting one of the lifting lugs;
• Figure 9 is a perspective view of one of the tension ring halves of Figure 2, showing how the lifting lug has two tongues and the direction in which these tongues are slotted into the recesses is shown in Figure 8, prior to securing by two securing pins which are inserted in an axial direction into holes in a top surface of the collar segment;
• Figure 10 is a top plan view of the assembled tension ring assembly of Figure 1, having four lifting lugs spaced equidistantly around the circumference of the assembled collar segments;
• Figure 11 is a sectional view of the tension ring assembly, taken along line XI-XI of Figure 10 in a plane through the axis of the assembly and centered on two of the lifting lugs, showing how each of the tongues is held by a corresponding one of the recesses;
• Figure 12 is a sectional view of the tension ring assembly, taken along line XII-XII of Figure 10 in a plane perpendicular to the plane of Figure 11 and centered on the two securing pins;
• Figure 13 is a sectional view of the tension ring assembly, taken along line XIII-XIII of Figure 10 in a plane perpendicular through one set of four connecting bolts;
• Figure 14 is a side view of the assembled tension ring assembly of Figure 1; and
• Figure 15 is a sectional view of the tension ring assembly, taken along line XV-XV of Figure 14 in a plane through two bolts through the lifting lug which connect the two tongues to a main body of the lifting lug.

DETAILED DESCRIPTION

Figure 1 shows in phantom outline a conductor pipe 10, used for example for an oil or gas production installation, which has a cylindrical outer surface 11 that is surrounded by a tension ring indicated generally at 12 to which are joined four lugs 30 for applying an upwards attention to form a tension ring assembly 1. In this specification, such upward tensioning lugs are referred to as "lifting lugs". Figures 2 to 5 show the stages of assembling the tension ring assembly to the outer surface 11 of the conductor pipe 10 where the conductor passes through an aperture 4 in rotary table 6 (the table is shown only schematically).

The tension ring 12 comprises a collar 8 made up of two arc-shaped halves 7, 9, each of which extends substantially around a half circle and which, when assembled into a full collar, also referred to as a "ring assembly", define an axis 2 of the tension ring 12. Each collar half 7, 9 has a corresponding collar segment 14, 16 and inside each segment is provided a plurality of wedge-shaped slip segments 20. The slip segments are held inside the ring assembly and provide an inner cylindrical gripping face 21 of the tension ring 12. In this example, the gripping faces are provided by grit-face segments 22 affixed to a main body 23 of each slip segment 20. Such grit face segments are described in more detail in WO/2015/173572 A2. It would however, alternatively be possible for the gripping faces to be provided by a ribbed steel surface of each slip segment, as described in EP 2143873 A1. The particular type of gripping faces to be used is not a central aspect of this invention.

With reference now also to Figures 6 to 8, the collar segments 14, 16 and the slip segments 20 when assembled present to each other inclined contacting surfaces 24, 25, such that an inner diameter across the assembled ring, between the inner faces 21 of the slip segments 20, varies as the slip segments move in a substantially axial direction up and down inside the assembled collar segments. As can be seen from Figure 7, in this example the inclined contacting surfaces 24, 25 have a saw-tooth profile. An advantage of the saw-tooth profile as opposed to a continuous inclined ramp, such as that disclosed in EP 2143873 A1, is that this allows for a reduction in the radial thickness of the tension ring 12. It will be seen that as the slip segments 20 drop down inside the collar segments 14, 16 under gravity, the presence of the inclined surfaces 24, 25 means that the grit-face segments 22 of the slip segments 20 come into gripping contact with the outer wall surface 11 of the pipe 10.

The tension ring 12 has a substantially cylindrical outer surface 26. Between the outer surface 26 and inner gripping face 21 the tension ring 12 has substantially planar opposite upper and lower ends 28, 29. An arc-shaped upper retainer plate 18 is fitted on each of the collar halves 14, 16 at the upper end 28 of the tension ring 12, and an arc-shaped lower retainer plate 19 is fitted on each of the collar halves 14, 16 at the lower end 29 of the tension ring 12. Together, the retainer plates 18, 19 constrain the axial movement of the wedge slip segments 20.

One of the top retainer plates 18 is omitted from Figure 1 to show threaded bores 32 in a top surface 33 of one of the collar segments 16. When the tension ring 12 is first fitted around the pipe 10, the slip segments 20 are held in an uppermost position by retention bolts 31 each of which extends axially to engage in one of the threaded bores 32 in the top surface 33 of the slip segment. Only when the retention bolts 31 are loosened are the slip segments 20 free to drop under their own weight. Each of the retention bolts passes through a radially elongate clearance bore (not shown) in the top retainer plate 18 and has a nut 35 in contact with the upper retainer plate 18. When the nut 35 is turned by hand, with the bolt securely screwed into the bore 32 in the slip segment 20 so as not to rotate, the bolt is loosened allowing the slip segment 20 to drop in a controlled manner.

As shown in Figures 2 to 5, the tension ring 12 is first assembled to the conductor pipe 11 above the rotary table 6, and then the lifting lugs 30 are assembled to the tension ring below the rotary table to form the tension ring assembly 1. The collar segments 14, 16 are secured to one another in a first stage by first positioning each collar half 7, 9 on opposite sides of the conductor pipe 10. To facilitate handling of each collar half the upper end 28 of each half is provided with eyes 34 to which rope or chain from lifting gear (not illustrated) may be connected.

The initial alignment of the collar segments 14, 16 with respect to each other is set by the engagement of a pair of dowels 41 in each circumferential end face 42 of one of the collar segments 16 that insert into a pair of bores 43 in each circumferential end face 44 of the other collar segment 14.

Once positioned around the pipe 11, four connecting bolts 36 are inserted in a generally circumferential direction through four aligned straight bores 37 in each of the collar segments 14, 16. Each bolt is then secured with a nut 38 held in place in the other collar half to provide an initial connection between the collar halves 14, 16. The bolts and nuts therefore provide connection members for connecting the collar segments together to form the tension ring. At this stage the slip segments 20 are held in their uppermost position, away from the pipe wall surface 11, by the retaining bolts 31.

The bolts and nuts therefore provide means for tensioning adjacent collar segments against one another to secure the tensioning ring around the pipe. The diameter of the tension ring 12 in this example is 1167 mm. The securing bolts 36 may protrude radially by a small amount, for example by about an additional 23 mm as illustrated. However, the bolts may alternatively be fully recessed. The hole 4 in a rotary table typically has a diameter of about 1200 mm, and so the tension ring may therefore be lowered downwards through the hole 4 in the rotary table. After connection of the lugs 30, the diameter of the tension ring assembly is 1665 mm.

Usually, the tension ring 12 is lowered together with the conductor pipe 10, and so the tension ring is first tensioned loosely against the pipe. It would, however, alternatively be possible to lower the just tension ring through the hole and then tension the two collar halves 7, 9 against the pipe, rather than lowering the pipe and tension ring together.

When the two collar halves 7, 9 are to be tensioned, the axial retention bolts 31 are loosened, as described above, so that the slip segments 20 drop under gravity in an axial direction and move radially inwards into contact with the pipe surface 11. The connecting bolts 36 and nuts 38 are then tightened to provide an initial frictional grip between the collar halves 14, 16 and the outer surface 11 of the pipe 10.

During this initial tightening, the contacting inclined surfaces 24, 25 slide along each other, causing a corresponding radial inwards movement of each of the grit-face segments 22, which then make direct contact with and press into the outer surface 11 of the pipe 10. The position of the tension ring 12 on the pipe 10 is then fixed.

In this example, the tension ring assembly 1 is designed for connection to a conductor pipe 10 having a nominal diameter of 30 inches ± 1 % (762 ± 6 mm). The collar segments are therefore adjustable over a diameter range of 15 mm, and this equates to an axial adjustment range 90 of 40 mm.

Once fully tensioned, a gap 45 remains between the opposed circumferential end faces 42, 44 of the collar segments 16, 14, so that all the tension maintained by the connecting bolts 36 and nuts 38 is transmitted to the pipe outer wall 11. The nominal width of this gap is 14 mm, which is sufficient to accommodate the expected variation in conductor pipe diameter.

As shown in Figure 5, once the tension ring 12 has passed through the hole 4, the lifting lugs 30 are fitted, as will be described in more detail below, to form the tension ring assembly 1, and in this assembled configuration, the weight of the conductor pipe 10 can be taken by attaching suitable lifting equipment to the lifting lugs 30, for example by shackles 39 bolted to the lugs. It will then be noted that when the weight of the pipe is taken through the lifting lugs, the engagement between the segments and the pipe wall will tend to drive the slip segments down the inclined faces 25 of each collar half 14, 16 to increase the grip of the slip segments 20 on the pipe wall 11. In this way an installed conductor pipe can be upwardly tensioned.

It will be apparent from Figures 2 to 5 that the tension ring assembly 1, with the lifting lugs 30 attached, would be too large in diameter to be able to pass through the opening 4 in the rotary table 6. However the tension ring 12 without the lugs 30 can pass through the opening. As a result, the tension ring 12 can be located accurately on the pipe 10 whilst above the rotary table, and then the final fixing can be completed below the rotary table. This can give a great timesaving when installing a tension ring assembly 1 of the type described.

The construction of the lifting lugs 30 and the way in which these are connected to each collar segment 14, 16 is shown in Figures 8 to 15. Each collar segment has on its radially outer surface 26 a machined flat 51 in which is provided two recesses 52. Each recess is symmetrical about a central plane 53 through the corresponding lug 30 that extends radially inwards from the outer surface to intersect with the collar axis 2. This central plane 53 is the plane of Figure 11. Each recess 52 is substantially rectangular in cross-section in a plane perpendicular to the central plane 53 such that the recess has a pair of inwardly opposite side walls 55 that are parallel with the central plane.

Each lifting lug has a pair of tongues 56 with a similar rectangular cross-section to that of the recesses 52 including a pair of outwardly opposite side walls 54, such that the tongues make a close sliding fit with the corresponding recesses when the tongues slot into the recesses. When each tongue 56 is fully in its recess 52, a flat inner surface 61 of the lug 30 abuts flush against the flat 51 on the radially outer surface 26 of the collar segment 14, 16. The lifting lug 30 is therefore securely seated against the outer substantially cylindrical surface 26 of the collar segment.

Each collar segment 14, 16 has for each lifting lug 30 a pair of axially extending bores 57 each of which has a bore axis 58 that is aligned with the plane of one of the opposite side walls 55 of the recesses. A pair of connection pins 59 is inserted in an axial direction 60 into the bores when one of the lifting lugs is to be connected.

The bore 57 partially intersects the recesses 52 such that in cross-section a portion of the bore extends laterally into each recess and another portion of the bore in cross-section extends laterally into collar segment. In this example, the bore 57 is bisected where this intersects the recess, so that half the bore in cross-section is in the recess and the other half of the bore in cross-section is in the collar segment. So that the bore 57 for each pin 59 is continuous, each tongue therefore has a half-cylindrical bore surface 62 machined into each of the opposite side walls 54, which are substantially flat, apart from corner chamfers. The half-cylindrical bore surfaces 62 align with similar half-cylindrical bore surfaces 63 in the flat side walls 55 of the recess 52 within the collar segment. The tongue 56 therefore has a part-cylindrical recess which forms a portion of the bore 57 for the pin 59.

From this description it will be appreciated that each pin 59 has a portion 64 in cross-section which protrudes into each recess 52 where the bore partially intersects the recess, as illustrated by the dashed outline 64 in the cross-section of Figure 12. Each tongue 56 is therefore located and held in place when the protruding portion 64 of the axially inserted pin seats in the half-cylindrical bore surface of the tongue, which therefore provides a keyway 62 that engages with the protruding portion 64 of the inserted pin 59. The tongue therefore has a pair of keyways in a pair of outwardly opposite side surfaces of each tongue. The shape of the keyway need not be hemi-cylindrical and will depend on the shape of the pin and the way in which the bore partially intersects with the recess. The keyway may therefore be any type of groove, channel, or other type of recessed feature, preferably elongate in the axial direction relative to its width or dimension in a lateral direction, and which has an opening along its axial dimension where the bore partially intersects the recess. The keyway therefore extends axially to align with the bore where the bore partially intersects the recess, whereby the protruding portion of the inserted pin is a portion in cross-section of the pin.

This arrangement allows the pins and surfaces that engage with the pins, to lie entirely within the outer circumference of the tension ring, and as a result the radius of the tension ring can be kept sufficiently small that the tension ring during initial assembly or final removal can be pass through apertures which could block passage of a tension ring in which the lifting lugs were connected using known means.

Furthermore, because the tongue held within the recess locates the lug in the axial direction, the axial length of each lug can be reduced as compared with arrangements in which the lug must locate with top and bottom surfaces of the tension ring.

It should be noted that it would be possible to have just one tongue 56 and associated recess 52. It is, however, preferred to have two such tongues and recesses for each lifting lug in order to provide redundancy to the design, so that there is no single point of failure.

The configuration of the tongues, recesses and bores is such that each keyway 62 comes into alignment with the bore 57 when the tongue 56 is slotted into its recess 52 so that said when the pin 59 is inserted into its bore 57, the protruding portion 64 of the inserted pin engages with the keyway 62 to secure the lifting lug 30 into position.

The pins 59 each have a head 65, which ensures that each pin is inserted to a defined depth in the bore 57, which extends fully through the collar segment 14, 16 to the lower end 29 of the tension ring 12. An end 66 of each pin protrudes from its bore at the lower end 29 of the tension ring. Each pin end 66 has a through-hole 67 into which is inserted a retaining clip, such as an R-clip 68. Each lifting lug 30 is also provided with clips 69, and after installation all clips are preferably connected by wires (not shown) to ensure that none of the pins comes loose.

Each lifting lug 30 has a main body 70 having a pair of substantially rectangular cross-section bores 71, 72 from which the tongues 56 extend. The lug main body 70 and two tongues 56 have aligned clearance holes for a pair of bolts 73 which pass through the main body and tongues to be secured by nuts 74. The end of each bolt has a through hole 77 and/or an annular groove 75 for an R-clip 76, which is also preferably wired to the other retaining clips 68, 69 by wires (not shown) to ensure that none of the bolts 73 and nuts 74 securing the tongues 56 to the lug main body 70 come loose.

In addition to helping to keep the tension ring relatively compact in both radial and axial directions, the arrangement described above for connecting lifting lugs to collar segments is particularly convenient when the time comes to remove the tension ring assembly 1 from the conductor pipe 10. This may be necessary, for example, if different lifting gear is to be used to lift the conductor pipe 10, or if the tension ring assembly 1 is to be permanently removed from the conductor pipe 10.

The presence of the lifting lugs 30 naturally obscures access to the main connection bolts 36 and nuts 38 for the collar halves 7, 9. This is particularly the case here, as in order to help minimise the diameter of the tension ring 12, the main connection bolts 36 and nuts 38 are not positioned prominently on any outwardly protruding connection flanges, but are instead situated inside recessed channels 80 in the otherwise substantially cylindrical outer surface 26 of the tension ring 12.

The lifting lugs 30 may be removed from the tension ring 12 by withdrawing the pins 59, which may be hammered upwards from their protruding ends 66 allowing the head of the bolt 65 to be gripped and pulled upwards. Preferably, each pin 59 also has a threaded recess 78 in the head 65 into which a bolt or eye (not shown) may be screwed to provide a grip for pulling the pin out of its bore 57. When the pins 59 are removed from the bores 57, the pin protruding portions 64 then disengage from the keyways 62 in the lug tongues 56 which frees the lifting lug 30 for removal from the collar segment 14, 16.

The invention therefore avoids having to use hydraulic equipment to undo bolts connecting lifting lugs to a collar, which can present problems owing to the need to provide a support for reaction against the toque generated by such equipment.

After removal of the lifting lugs 30, there is ready access to the main connection bolts 36 and nuts 38, which are then removed using suitable hydraulic gear. In this case, the hydraulic gear is connected to both the bolts and nuts and so all generated torque is safely contained within the hydraulic tool.

The invention therefore provides a convenient apparatus and method for securing a tension ring assembly around a pipe in which the tension ring assembly has a radially compact collar to which at least one lifting or tensioning lug may be removably connected.