GB2453168A - Frame for connecting a jumper to a riser - Google Patents

Abstract

A riser top assembly includes a structural frame 10, 12 mounted on the top of a vertical riser 18, the frame having a socket 30 to which a jumper 7 can be connected. The frame 10, 12 can be formed from two separate elements (fig 2b) connected together and is offset from the axis of the riser 18 such that the socket 30 is accessible from one side of the frame 10, 12.

Description

RISER TOP ASSEMBLY

BACKGROUND

a. Field of the Invention

The present invention relates to the top assembly and flexible jumper offtake for a free standing riser used in the extraction of hydrocarbons and in particular to risers that are used to extract oil or gas from offshore and deepwater fields.

b. Related Art Risers are high pressure dynamic tubular structures used in the extraction of oil and gas from offshore fields. They extend from the seabed to the surface production vessel and are used to transport oil, gas and injection fluids.

011 Figure 1 shows a schematic depiction of a riser which comprises a near-vertical steel pipe section which is tensioned by a near-surface buoyancy module. The S...

connection to the production vessel is made via a compliant, flexible jumper pipe catenary section. At the seabed the vertical tension is reacted by a foundation. The riser pipe, buoyancy module and flexible jumper all meet in a *: riser top assembly.

A key design issue is the design of the assembly at the interface between the buoyancy module, the vertical riser and the connection to the flexible jumper.

The design issue is a complex balance between structural capacity, fatigue performance, component size to assist handling and flexible jumper installation which typically occurs after the vertical riser section is installed. The challenge is often compounded by the preference to conduct the flexible jumper connection after the vessel end is connected, commonly called a second end connection. This further increases complexity of the design and criticality of the installation due to higher loads and positioning criticality and need to establish and run installation wires and aids.

As water depths in which oil extraction increase this problem becomes greater as design loads increase with riser and jumper lengths.

SUMMARY OF THE INVENTION

According to the present invention there is provided a riser top assembly comprising a structural frame adapted to be mounted at the top of a vertical riser and having a socket by means of which a flexible jumper can be connected to the top of the riser, wherein the frame is offset from the riser axis and the socket is accessible from one side of the frame.

:.. The frame preferably comprises two parallel tubular elements connected to one another by spacers, the socket being offset from a plane containing the axes of the two elements. The socket can be equidistant from the two elements.

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* 20 An exposed peg is preferably located above the socket to receive a jumper S...

supporting element and to support the jumper while the jumper is being deployed and connected to the socket. The peg can be mounted between the two elements and equidistant from the two elements, and vertically above the socket.

The tubular elements can be hollow and closed at both ends to provide buoyant chambers, to augment the buoyancy provided by the separate buoyancy module.

The riser top assembly is deigned to be used in conjunction with a jumper assembly, the jumper assembly comprising a jumper connector and a supporting means made up of a suspension member and a supporting link, where the link can mate with the pin so that the weight of the jumper is supported off the link while the connection to the socket is being made.

The link can be a plate with a cut out region which can be placed over the peg to support the jumper connector from the peg, and the peg can have a large diameter region and a smaller diameter region, a part of the cut out region being sufficiently large to pass over the larger diameters region of the peg, and another part of the cut out region being too small to pass over the larger diameter region. The cut out region can be a key-hole shaped cut-out.

The suspension member can be adjustable for length, and may include a turnbuckle to adjust its length, the turnbuckle being operable by a ROy. a...

Thus, the frame assembly is non-symmetrical about the riser pipe centreline providing free unobstructed access of equipment and installation aids over the S...

riser centreline.

*S S. 45 * S The non symmetrical structural arrangement of the frame results in additional bending loads in the frame members compared to a symmetrical frame arrangement. However this negative design feature is balanced by the benefit of improved accessibility directly to the riser centre-line.

To accommodate the higher bending loads the frame is fabricated from large diameter tubular members or other structural sections. The use of sealed tubular sections allows the in-water weight to be greatly reduced, improving the response of the riser and reducing the buoyancy can requirements.

Installation of the flexible jumper is accomplished by first connecting the first end to the vessel and then spooling out the jumper in a catenary configuration towards the pre-installed riser top assembly. Once the jumper is fully deployed it is lowered on a deployment wire until the jumper end spool and connector is at the elevation of the riser top assembly but laterally displaced by a few metres so as not to interfere or clash with the riser. The lowering operation requires careful re-positioning of the vessel to maintain the required catenary shape of the flexible jumper.

When the end of the flexible jumper spool is correctly positioned with respect to the riser as viewed from the side elevation by the ROV the jumper assembly is moved laterally to the plane of the catenary such that the installation wire and rigging interfaces with the riser frame.

Correct positioning of the jumper allows a large diameter steel rigging link to be :... located over a peg or pin mounted off the riser frame. The pin is positioned such that when the rigging link is connected the flexible jumper and its connector are positioned safely above its mating hub. At this point the tension in the I...

installation wire connected between the jumper and the jumper installation * * S...

* 20 vessel can be slacked off and the load of the jumper transferred onto the riser via the peg. As the tension in the installation reduces, the riser top assembly moves laterally to a new equilibrium position as defined by the revised load condition.. This position will typically be the normal mean offset position where the jumper will exhibit minimum bending once connected to the riser. This assists alignment and connection to the vertical riser section.

Once the riser has moved to its new equilibrium position and settled an ROV is used to lower the flexible jumper spool and connector down onto its mandrel in a slow and controlled manner. This is achieved via a turnbuckle system incorporated between the large diameter rigging link and the steel jumper spool located on the end of the flexible jumper.

As the ROV actuates the turnbuckle using a torque tool the turnbuckle extends and the connector is lowered onto the mandrel whence it can be locked and sealed. After locking and sealing the tension in the turnbuckle is adjusted to a final installation level selected to ensure that the majority of the flexible jumper loads is directed through the turnbuckle rather than through the jumper to riser connector. This allows the specification of the jumper connector to be greatly reduced.

Removal of the flexible jumper is the reverse of that described above.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, with reference to the following Figures in which: * S *S.. . . . Figure 1 shows a schematic depiction of a known riser arrangement; *SS** * 20 Figure 2a shows a front view of a riser top assembly according to the . : invention; S. Figure 2b shows a side view of the riser top assembly according to the invention; Figure 3 shows a front view of the riser top assembly with a flexible jumper in the final installed position: and Figures 4a to 4f show an installation sequence for connecting the flexible jumper to the riser:

DETAILED DESCRIPTION

Figure 1 shows a riser 1 tensioned between a foundation 2 in the sea bed 3 and a buoyancy member (or aircan) 4. A production vessel 5 is shown at sea level 6, and a jumper pipe 7, lying in a catenary curve, is connected between the vessel and a riser top assembly 8. Fluids will pass up the riser pipe and then into the jumper pipe 7, to be directed through the pipe 7 to the vessel 5.

Figures 3 show the arrangement of the riser top assembly 8. It consists of two 40inch diameter vertical steel tubulars 10, 12 with a 60inch distance between centrelines. The 40inch tubulars are braced in four places by 30 inch horizontal tubulars 14. All tubulars are sealed with atmospheric pressure internally and designed to resist hydrostatic collapse at the application depth. *SS. * S

A vertical 30inch diameter tubular 16 is connected via a plate structure 17 to the **5* * lower end of the two 40inch tubulars. The riser tubular 18 is located at the 1*15.5 * 20 centre of the 30inch tubular 16 and is axially supported at the top end of the 30 inch and laterally guided at the bottom end. The length of this 30inch cylinder is I...

selected based on specific application but is typically 5m.

A similar 30inch diameter tubular 20 is used to connect the buoyancy can 4 to the frame via a flex connector 21 which is fabricated using a similar plate structure and is fitted to the upper end of the two 4Oinch tubulars. This upper 30inch tubular 20 is concentric with the lower riser support tubular 16 ensuring that the buoyancy load is applied concentric with the riser minimising riser pipe bending loads.

The riser pipe 18 passes up through the lower 3Oinch tubular 16 and is rigidly connected to the riser top assembly 8 via a weldment or via a load shoulder and reaction bushing.

Above the riser connection point a forged Y spool may be included for future access during production mode for coiled tube intervention for wax and hydrate remediation. This is located between the two vertical structural members which naturally provide protection from damage whilst facilitating access to the reentry hub.

At the top end of the Y spool a connector mandrel 24 is machined to interface with a connector on the end of a rigid jumper spool 26 which is flanged onto the end of the flexible jumper 7. A 180 degree alignment funnel 28 is located around the Y spool mandrel 24 to aid alignment of the connection system A parking/load peg 30 is fabricated onto the middle 30inch cross brace 14. This *::::* peg is used to capture a rigging link 32 used during flexible jumper installation.

The peg extends from the face of the frame 8 a sufficient distance to ensure that at the point of capture of the rigging link 32 the jumper spool 26 and its connector 36 are spaced from the frame and mandrel (Figure 4b). This helps to ensure that inadvertent damage is prevented in the event that vessel station *.S.

keeping is variable. Once the rigging link is captured it is then slid along the pin to interface closely with the frame structure, and the jumper spool lines up with the Y spool.

The rigging link 32 is cut from plate and machined with holes 38, 40 at both ends to accept shackles connecting the installation wire back to the vessel and a clevis for a strop 34 which includes a turnbuckle 42.

The turnbuckle 42 is designed to take the full flexible jumper pull-in load typically 20-6OTe. It is configured using a lead screw, thrust bearing and pinion so that it can be adjusted using the ROV and standard torque tool. Once the flexible jumper is fully connected to the main riser the main jumper loads are designed to pass through the turnbuckle thereby reducing the required load capacity of the jumper connector.

In the event that the jumper requires replacement and the turnbuckle cannot be operated due to a failure from dirt ingress of corrosion it is noted that tension can be applied to the rigging link from the surface allowing it to be slid off the parking/load pin without first slacking off the tumbuckle.

Figures 4 show the sequence of events. In Figures 4a and 4b, the jumper 7 is lowered in the water to a position where the rigging link 32 lines up with the peg 30. At this point, the jumper is spaced from the riser top assembly. * * * *..

*.. In Figures 4c and 4d the link 32 has been placed over the page 30and the jumper spool 26 is offered up to the Y spool on the assembly 8. As the jumper is now supported off the peg 30, the installation cable 50 has gone slack.

**....

* * 20 The connection between the jumper and the riser is now made, in a known **** ** * manner. * * S * S.

Once the connection has been made. the installation cable 50 is removed, and the weight of the riser catenary is supported off the peg 30 so that the connection between the jumper and the riser is not under stress.

Because the main part of the frame (10,12) is offset from the riser axis, the components which are involved in the jumper/riser connection are readily accessible.

Claims (11)

1. A riser top assembly comprising a structural frame adapted to be mounted at the top of a vertical riser and having a socket by means of which a flexible jumper can be connected to the top of the riser, wherein the frame is offset from the riser axis and the socket is accessible from one side of the frame.

2. An assembly as claimed in Claim 1, wherein the frame comprises two parallel tubular elements connected to one another by spacers, the socket being offset from a plane containing the axes of the two elements.

3. An assembly as claimed in Claim 2, wherein the socket is equidistant from the two elements.

:.

4. An assembly as claimed in any preceding claim, including an exposed peg located above the socket to receive a jumper supporting element and to support the jumper while the jumper is being deployed and connected to the socket. : 20

5. An assembly as claimed in Claim 4, wherein the peg is mounted between the two elements and equidistant from the two elements.

6. An assembly as claimed in Claim 3, wherein the peg is vertically above the socket.

7. An assembly as claimed in any preceding claim, wherein the tubular elements are hollow and closed at both ends to provide buoyant chambers.

-10 - 6. In combination, an assembly as claimed in any preceding claim and a jumper assembly, the jumper assembly comprising a jumper connector and a supporting means comprising a suspension member and a supporting link, the link being adapted to mate with the pin.

7. A combination as claimed in Claim 6, wherein the link is a plate with a cut out region which can be placed over the peg to support the jumper connector from the peg.

8. A combination as claimed in Claim 6 or Claim 7, wherein the peg has a large diameter region and a smaller diameter region, a part of the cut out region being sufficiently large to pass over the larger diameters region of the peg, and another part of the cut out region being too small to pass over the larger diameter region.

9. A combination as claimed in any one of Claims 6 to 8, wherein the suspension member has an adjustable length. S..

10. A combination as claimed in Claim 9, wherein the suspension member : 20 includes a turnbuckle to adjust its length, the turnbuckle being operable by a ROy. P.*i S.

*

11. A riser top assembly substantially as herein described with reference to the accompanying drawings.