The present invention relates to a riser assembly, in particular to a riser assembly having a riser connector which provides support for at least one auxiliary flow line which extends along the exterior of and parallel to the riser.

A riser, for example a riser which surrounds the drill string in a subsea drilling operation, is made up of a plurality of tubular sections (often referred to as riser joints), the adjacent ends of which are connected by a riser connector. These sections are joined together using the riser connector on board a vessel such as a drilling rig, as the riser is lowered towards a subsea wellhead. Each riser joint normally comprises a main cylindrical pipe, and at least one external auxiliary, smaller diameter, cylindrical pipe (generally referred to as an auxiliary line), which is attached to the main pipe so that it is spaced from and extends parallel to the main pipe. Typically, two auxiliary lines are connected to the riser, diametrically opposite to one another. In some cases, more than two auxiliary lines are provided.

The main pipe forms an annular space around the drill string , along which drilling fluid is returned from the well bore. The auxiliary lines are used to circulate fluids between the vessel and a subsea blowout preventer (BOP) on the wellhead, and may comprise a choke line, a kill line, a booster line or hydraulic lines.

The riser is suspended from the vessel, and it will be appreciated that, as the wellhead could be located 3000m or more below the surface of the sea, the weight of the riser and string associated riser connectors can become extremely high. As such, it is known for the resulting load to be shared with the auxiliary lines. This load sharing is achieved by securing the ends of each section of auxiliary line to the riser, via the riser connector. For example, it is known for the riser connector to comprise annular flanges which extend radially outwardly from the riser connector, one being provided adjacent each end of each riser joint. The auxiliary riser sections extend between and are secured to the two flanges mounted on one riser joint, with a box and pin joint being located between the two flanges at the ends of adjacent riser joints to provide a fluid flow path between two adjacent auxiliary line sections. The adjacent riser joints are secured together by means of an internal locking ring which is mounted around the two adjacent riser joints in between the two flanges.

Examples of such riser connectors are illustrated in US 4,043,575 and US2016/0258562.

Figure 1 illustrates the riser connection described in US 4,043,575. This shows a joint between the lower end of a first riser joint 10, and the upper end of a second riser joint 12, there being two auxiliary lines 14, 16 mounted on either side of the riser joints 10, 12. The auxiliary lines 14, 16 are supported by flanges 18 and 20, and connected together via a connection comprising a box 22 and pin 24 which is located between the flanges. The riser joints 10, 12 are secured together using locking ring 25.

Figure 2 illustrates the riser connection described in US 2016/0258562. In this case, three auxiliary lines are 26, 28, 30 are shown mounted on the flanges 32, 34, the sections of auxiliary line attached to each of the two adjacent riser joints being connected by a box 36, 38, 40 and pin 42, 44, 46 located between the two flanges 32, 34. Again, the riser joints 10, 12 are secured together using a locking ring 48.

Other riser connector configurations are illustrated in US 4, 487,434, GB 2320541,

WO2011/104629, and US 4,280,719.

In the type of connector illustrated in Figures 1 and 2, the load transfer of a portion of the weight of the riser to the auxiliary lines can cause the flanges to deflect slightly, as illustrated in Figure 3 in relation to the embodiment of prior art riser connector described in US 2016/0258562. This causes angular misalignment of the box and pin connection between the adjacent auxiliary lines, and, as a result, there is contact stress and wear at the radiallyinward edge of the boxes and the radially outward edge of the pins at the radially outward sides of the assembly. Although the degree of the likely deflection is significantly exaggerated in Figure 3, repeated deflections can, over time, cause significant wear of the box and pin, and this could compromise the integrity of the seal between the adjacent sections of auxiliary line.

In the system described in WO 2011/104629, the adjacent ends of the riser joints are secured together by means of an external locking ring which surrounds and engages with the outer edges of the two flanges. This may assist in reducing the deflection of the flanges, but the use of such a larger diameter locking ring will have a significant impact on the weight of the riser connector.

In GB 2320541, additional locking mechanism are provided to lock the adjacent ends of the auxiliary lines together, independently of the locking of the riser joints, in order to prevent the adjacent sections of auxiliary line from separating vertically when the connector is subject to bending stresses caused by the pressure of fluid in the riser.

It is an object of the present invention to provide an alternative configuration of riser connector which provides for load transfer to the auxiliary lines, but in which wear of the box and pin connection between adjacent auxiliary line sections is reduced without significantly increasing the weight of the riser connector.

According to the invention we provide a riser assembly comprising a riser having a longitudinal axis and comprising first riser joint with an end, and a second riser joint with an end which is adjacent to the end of the first riser joint, the riser assembly further comprising an auxiliary line having a first auxiliary line section and a second auxiliary line section which are connected via an auxiliary line joint comprising a first joint assembly connected to the first auxiliary line section and having a bearing surface and a second joint assembly connected to the second auxiliary line section and having a bearing surface, the riser assembly further comprising a first flange which extends radially outwardly from an exterior surface of the first riser joint adjacent to the end thereof and a second flange which extends radially outwardly from an exterior surface of the second riser joint adjacent to the end thereof, the first and second flange each having a bearing surface, the bearing surface of the first flange engaging with the bearing surface of the first joint assembly and the bearing surface of the second flange engaging with the bearing surface of the second joint assembly, wherein one of the bearing surfaces of the first flange and the first joint assembly has a rocker formation which is shaped to provide a point, line or area of contact between the two bearing surfaces whilst permitting relative angular movement between the two bearing surfaces.

Advantageously, one of the bearing surfaces of the second flange and the second joint assembly also has a rocker formation which is shaped to provide a point, line or area of contact between the two bearing surfaces whilst permitting relative angular movement between the two bearing surfaces.

The or each rocker formation may have an arcuate shape in transverse cross-section.

The or each rocker formation is advantageously shaped to permit relative angular movement between the two bearing surfaces about an axis which is generally perpendicular to the longitudinal axis of the riser.

The bearing surface of one or both of the first and / or second flange may be provided on an insert which is not integral with the remainder of the flange.

The bearing surface of one or both of the first and / or second joint assembly may be provided on a bearing part which is not integral with the remainder of the joint assembly. The first flange and second flange may be separated by an annular space around the ends of the first riser joint and second riser joint, the first joint assembly being connected to the second joint assembly in the annular space.

The first flange has a first side which forms a first end of the annular space and a second opposite side, whilst the second flange has a first side which forms a second end of the annular space, and a second opposite side. In this case, each bearing surface may form part of the first side of its respective flange.

The first joint assembly may extend through an opening provided in the first flange from the first side to the second side thereof.

The second joint assembly may extend through an opening provided in the second flange from the first side to the second side thereof.

The first auxiliary line section may be connected to the first joint assembly at the second side of the first flange.

The second auxiliary line section may be connected to the second joint assembly at the second side of the second flange.

The first joint assembly may comprise a pin and the second joint assembly may comprise a box, the pin being located in the box to provide the connection between the first auxiliary line section and the second auxiliary line section.

Embodiments of the invention will now be described, by way of example only, with reference to the following figures:

FIGURE 4 is an illustration of the longitudinal cross-section through a riser assembly according to the invention in a plane which includes the longitudinal axis of the riser,

FIGURE 5 is a longitudinal cross-section through one of the auxiliary lines and associated flanges of the riser assembly illustrated in Figure 4, this longitudinal cross-section being in a plane which includes the longitudinal axis of the auxiliary line and which is perpendicular to the plane of the longitudinal cross-section illustrated in Figure 4,

FIGURE 6 is a perspective illustration of the riser assembly illustrated in Figure 4,

FIGURE 7 is a side view of the riser assembly illustrated in Figure 4, FIGURE 8 illustrates a longitudinal cross-section through a two of the bearing surfaces in the riser assembly illustrated in Figures 4, 5 and 6, this longitudinal cross-section being in a plane which is parallel to but off-set from the plane of the longitudinal cross-section illustrated in Figure 5,

FIGURE 9a is a perspective illustration of part of the first flange of the riser assembly illustrated in Figures 4, 5 and 6, and

FIGURE 9b is a perspective illustration of part of the second flange of the riser assembly illustrated in Figures 4, 5 and 6.

Referring to Figures 4, 5 and 6, there is shown a riser assembly 110 comprising a riser 112 having a first riser joint 114 with an end, and a second riser joint 116 with an end which is adjacent to the end of the first riser joint 114. In this embodiment, the riser joints 114, 116 each have a circular transverse cross-section, and are joined together at their ends to enclose a generally cylindrical main passage 118 with a longitudinal axis A, by means of a locking ring 115, which is located around the exterior of both the ends, as is known to a person skilled in the art. It will be appreciated, however, that the invention is not restricted to the use of a locking ring 115, and another method of connecting the ends of the riser joints 114, 116 could be used, such as breech lock technology or actuated locking dogs.

The riser assembly 110 further comprises two auxiliary lines 120, 120', each having a first auxiliary line section 122, 122' and a second auxiliary line section 124, 124' which are connected via an auxiliary line joint 126, 126'. In this embodiment, the auxiliary line sections 122, 122', 124, 124' each have a circular transverse cross-section, and are connected to enclose a generally cylindrical passage 128, 128' with a longitudinal axis B, B'. The auxiliary lines 120, 120' are arranged around the exterior of the riser 112 such the longitudinal axes of the auxiliary lines 120, 120' lie generally parallel to the longitudinal axis A of the riser 112. In this embodiment, the two auxiliary lines 120, 120' are located at diametrically opposite one another relative to the riser 112, so that the riser 112 lies directly between them. Whilst in this example, two auxiliary lines 120, 120' are provided, this need not be the case. The riser assembly 110 may comprise only one or more than two auxiliary lines.

Each auxiliary line joint 126, 126' may comprise a first joint assembly 130, 130' connected to the first auxiliary line section 122, 122' and a second joint assembly 132, 132' connected to the second auxiliary line section 124', 124'. Each joint assembly has a tubular body, which, in this embodiment has an end with an external thread, and each auxiliary line section 122, 122', 124, 124' is secured to its respective joint assembly 130, 130', 132, 132' by a threaded connection with this external thread. In this embodiment, each first joint assembly 130, 130' comprises a pin, and each second joint assembly 132, 132' comprises a box, each pin being located in the corresponding box to provide the connection between the first auxiliary line section 122, 122' and the second auxiliary line section 124, 124', as is known from the prior art described above. The pin and box both have a longitudinal axes which coincide when the pin is properly aligned in the box.

The riser assembly 110 further comprises a first flange 134 which extends radially outwardly from an exterior surface of the first riser joint 114 adjacent to the end thereof and a second flange 136 which extends radially outwardly from an exterior surface of the second riser joint 116 adjacent to the end thereof. The first flange 134 and second flange 136 are therefore separated by an annular space around the ends of the first riser joint 114 and second riser joint 116. The first flange 134 has a first side 134a which forms a first end of the annular space and a second opposite side 134b, and the second flange 136 has a first side 136a which forms a second end of the annular space, and a second opposite side 136b. In this example, the first and second sides 134a, 134b, 136a, 136b of the flanges 134, 136 extend generally perpendicular to the longitudinal axis A of the main passage 118.

The first joint assemblies 130, 130' each extend through an opening 138, 138' provided in the first flange 134 from the first side 134a to the second side 134b thereof. Similarly, the second joint assemblies 132, 132' each extend through a corresponding opening 140, 140' provided in the second flange 136 from the first side 136a to the second side 136b thereof. In this embodiment, the tubular body of each of the joint assemblies 130, 130', 132, 132' extends through the openings 138, 138', 140, 140'. Each first joint assembly 130, 130' connects to its corresponding second joint assembly 132, 132' in the annular space, whilst the first auxiliary line section 122, 122' of each auxiliary line 120, 120' is connected to the first joint assembly 130, 130' at the second side 134b of the first flange 134, and the second auxiliary line section 124,124' of each auxiliary line 120, 120' is connected to the second joint assembly 132, 132' at the second side 136b of the second flange 136.

Each joint assembly 130, 130', 132, 132' has a bearing surface 130a, 130a', 132a, 132a' which engages with a corresponding bearing surface provided on one of the flanges 134, 136. In this embodiment, a bearing surface provided on the first side 134a of the first flange 134 engages with the bearing surface 130a, 130a' of each of the first joint assemblies 130, 130', whilst a bearing surface provided on the first side 136a of the second flange 136 engages with the bearing surface 132a, 132a' of each of the second joint assemblies 132, 132'.

In order to ensure that deflection of the flanges 134, 136 illustrated in Figure 3 and occurring when the riser assembly 110 is suspended from a drilling vessel is not transmitted to the auxiliary line joint 126, 126', each of the bearing surfaces of the first flange 134, and second flange 136 has a rocker formations 150 which are shaped to provide a point or area of contact with the bearing surfaces 130a, 130a', 132a, 132a', of each of the joint assemblies 130, 130', 132, 132' whilst permitting relative angular movement between the two bearing surfaces. This is best illustrated in Figures 8, 9a and 9b, and is not visible in the cross-sections illustrated in Figures 4 and 5. The rocker formations 150 are arranged to permit relative angular movement between the two bearing surfaces about an axis which is generally perpendicular to the longitudinal axis A of the main passage 118. Moreover, in this preferred embodiment, the rocker formations 150 are also arranged so that the axis about which there is relative angular movement between the two bearing surfaces is also perpendicular to a line extending between the longitudinal axis A of the main passage 118 of the riser 112, and the longitudinal axis B/B' of the associated auxiliary line 120/120'.

As such, the angular deflection of the flanges 134, 136 illustrated in Figure 3 is not transmitted to the auxiliary line joint 126, 126', as the bearing surfaces of the flanges 134, 136 can pivot about the rocker formation 150 relative to the bearing surfaces of the auxiliary line joint 126, 126' during the deformation of the flanges 134, 136 caused by the forces transmitted along the auxiliary lines 120, 120'.The pin can remain properly aligned in the box, and wear of the pin and box resulting from repeated loading of the riser assembly 110 may therefore be reduced.

It will be appreciated that, whilst in this embodiment, the rocker formations 150 are provided on the bearing surfaces on the flanges 134, 136, this need not be the case. The rocker formations could, instead, be provided on the bearing surfaces 130a, 130a', 132a, 132a' on the joint assemblies 130, 130', 130. It should also be appreciated that whilst in this embodiment, there is a rocker formation 150 associated with every interface between a bearing surface of a flange 134, 136 and a bearing surface of a joint assembly 130, 130', 132, 132', this need not be the case. For example, there may only be a rocker formation 150 between the bearing surfaces of one of the flanges 134, 36 and the associated bearing surfaces of either the first joint assemblies 130, 130' or the second joint assemblies 132, 132'

The rocker formation 150 may have a curved surface. It may, for example, have an arcuate shape in transverse cross-section. In this example, the rocker formation 150 corresponds in shape to the curved surface of a portion of a cylinder formed by cutting the cylinder longitudinally along two of its radii. This need not be the case, however, and the rocker formation 150 could, for example be triangular or conical, or have the form of a triangular prism or cone with curved corners. In this embodiment, the bearing surfaces 130a, 130a', 132a, 132a' of both of the first and second joint assemblies 130, 130', 132', 132' are each provided on a bearing part 152, 152' which is not integral with the remainder of the joint assembly 126, 126'. Specifically, in this case, each of these bearing surfaces 130a, 130a', 132a, 132a' is provided on an annular insert 152, 152' which is located around the tubular body of the joint assembly 130, 130', 132, 132', and is clamped between the respective flange 134, 136 and a shoulder 142, 142', 144, 144' which is integral with the tubular body of the joint assembly 130, 130', 132, 132'.

Also in this embodiment, the bearing surface of both of the first flange 134 and second flange 136 is provided on an inserts 146a, 146b, 146a, 146b', 148a, 148b, 148a', 148b (not visible in Figure 4) which are not integral with the remainder of the flange 134, 136. In this example each flange is provided with a set of such inserts 146a, 146b, 146a, 146b', 148a, 148b, 148a', 148b for each auxiliary line 120, 120'. In this embodiment, each set comprises two inserts which are positioned generally diametrically opposite to one another around one of the first joint assemblies 130, 130' or second joint assemblies 132, 132'. It will be appreciated, however, that this need not be the case, and each flange 134, 136 could equally be provided with one insert for each auxiliary line 120, 120'.

The use of such inserts is not essential, but can be advantageous, as they can be made of a higher strength material than the rest of the joint assembly. It will be appreciated that by providing a rocker formation 150, the area of contact between the two engaged bearing surfaces is reduced compared to if the bearing surfaces were both flat. As such, when a force of a given magnitude is applied to the riser assembly 110, the pressure at the area of contact between the bearing surfaces will be much higher. As such, it is desirable to fabricate the bearing surfaces from a material with a high compressive strength to avoid plastic yield, deformation, and flattening of the rocker surface 150. It may not, however, be necessary for the remainder of the flange 134, 136 or joint assembly 130, 130' , 132, 132' to be made from such a high strength material, and using such a material for all these components could increase the cost and/or weight of the riser assembly 110 more than is necessary. This may be avoided by providing the bearing surfaces on such inserts.

The inserts may be detachable from the joint assembly 130, 130', 132, 132' or flange 134, 136, in order that the inserts may be replaced when worn to such an extent that the desired degree of angular relative movement is no longer provided.