Subsea Wellhead Connector

This invention pertains to well apparatus, and more particularly to connectors for joining marine risers to subsea wellheads.

When preparing to produce fluids from an offshore well that has been completed with a subsea wellhead, a marine riser pipe is lowered from the surface platform and connected to the wellhead to provide guidance and protection for internally run production casing. In some instances the connection must be accomplished remotely and automatically, i.e., without the assistance of a diver, and the connector also must be capable of being rigidly yet releasably locked to the wellhead with diver assist. Such a connector must have an outside diameter small enough to pass through the platform jacket guides, an ability to adjust for any angular and/or horizontal misalignment with the wellhead, and an inside diameter large enough to pass successive tie-back strings. These requirements seriously restrict the type and design of connector suitable for this purpose, and though many connectors are known none possess all of the required features.

The present invention includes features that satisfy each of the foregoing requirements. In its preferred embodiment, the present invention comprises an annular connector body that telescopes over the outside of a wellhead and is securable thereto by a plurality of lockdown screw assemblies that automatically engage and securely grip the wellhead as the connector is lowered upon it, that can be locked rigidly in their gripping position by a diver, and that can be unlocked and withdrawn from their gripping position to facilitate removal of the connector from the wellhead. Broadly considered, each of the screw assemblies comprises a lockdown dog, a lockdown screw, a dog retainer element secured to the dog and slidably connected to the screw, an a spring system between the dog and the screw to bias the dog towards its gripping position. The lockdown screw is threaded into a lateral port through the connector body, thereby facilitating position adjustment of the screw to enable the dog to move outwardly against spring pressure sufficiently to pass by the wellhead hub flange as the connector descends onto the wellhead, and then move sufficiently inwardly into the recess below the hub flange in response to spring pressure to grip the wellhead and secure the connector to it. This threaded relationship between the lockdown screw and the wellhead also permits rigidly locking the dog into the recess by rotating the screw to move it inwardly until the spring system has been fully compressed, and then fully withdrawing the dog from the recess, by rotating the screw in the other direction, when release and removal of the connector is desired.

FIG. 1 is a fragmentary vertical section through a wellhead connector according to the present invention, and through the upper portion of a wellhead onto which the connector has been partially lowered.

FIG. 2 is a view taken along the line 2--2 of FIG. 1.

FIG. 3 is a view like FIG. 1 but showing the connector landed on the wellhead and secured thereto by one of its lockdown dogs.

FIG. 4 is a view like FIG. 3 but showing the lockdown dog rigidly locked in its functional position.

FIG. 5 is a view like FIG. 3 but showing the lockdown dog retracted for removing the connector from the wellhead.

As illustrated in the drawings, the preferred embodiment of a wellhead connector 10 in accordance with the present invention comprises: an upper annular body 12 that, in use, is threaded, bolted, welded or otherwise attached to the lower end of a marine riser or other pipe or well element (not shown) that is to be connected to the upper end or hub of a wellhead 14; an annular lower body 16 that is secured to the lower end of the upper body 12 by, for example, a plurality of circumferentially spaced studs 18 and nuts 20 (only one of each shown); and a plurality (preferably eight) of lockdown screw assemblies 22 (only one shown) circumferentially spaced about and extending radially through the lower body 16. To correct possible misalignment as the connector 10 is being lowered onto the wellhead 14, the connector preferably also includes a guide assembly comprising a plurality of axially-oriented guide fins 24 (only one shown) welded or otherwise fixed to an annular frame including an annular base member 26 and a sleeve-like side member 28, and this guide assembly is secured to the connector lower body 16 by means of a plurality of circumferentially spaced cap screws 30. An O-ring or other suitable annular seal means 32 provides a fluid-tight seal between the upper and lower bodies 12, 16, and when the connector is properly installed on the wellhead 14 (FIGS. 3-5) a pair of O-rings 34, 36 provide a fluid-tight barrier between the upper body and the wellhead.

Each of the lockdown screw assemblies 22 comprises a lockdown dog 38, a dog retainer bolt 40 threaded into the dog 38, a lockdown screw 42 with a central bore 42a through which the retainer bolt 40 extends and a counterbore 42b in which the retainer bolt head 40a resides, a back-up plug 44 threaded into the screw counterbore 42b, and a plurality of annular Belleville springs 46 surrounding the shank of the retainer bolt 40 between the dog 38 and the lockdown screw 42. The retainer bolt head 40a is of larger diameter than the lockdown screw central bore 42a, and thus the annular radial surface 42c between the central bore 42a and the counterbore 42b functions as a stop that prevents axial movement of the bolt 40 towards the center of the connector when the head 40a is against that surface 42c. Accordingly, in its assembled condition the Belleville springs 46 of the screw assembly 22 are in compression, thereby constantly biasing the dog 38 away from the lockdown screw 42.

When the lockdown screw assembly 22 is properly installed in the connector 10 (FIG. 1) the lockdown screw 42 extends through a threaded lateral port 48 in the lower body 16, and the lockdown dog 38 slidably resides in an annular groove 50 in the inner surface 16a of the lower body. As the connector 10 is lowered from its position shown in FIG. 1 into its landed position shown in FIG. 3 the dog 38, together with the bolt 40, moves radially outward in response to a camming force exerted on the dog's lower inner tapered surface 38a as this surface contacts the upper outer edge 14a of the wellhead 14. This outward movement of the dog 38 further compresses the Belleville springs 46, and these springs force the dog inwardly into the annular recess 14b beneath the wellhead hub flange 14c as the connector is landed (FIG. 3). In this landed position the springs 46 exert pressure against the dog 38 sufficient to prevent it from moving outwardly in response to a force tending to lift the connector 10 off the wellhead 14, thereby securing the connector to the wellhead.

The outer end portion 42d of the lockdown screw 42 is hexagonal or otherwise shaped to accept a wrench, whereby the screw 42 can be threaded inwardly into the position shown in FIG. 4, thereby fully compressing the springs 46 and rigidly locking the dog 38 in its wellhead gripping position.

In order to protect the exposed portion of the screw assembly 22 from damage or contamination, a cap 52 can be threaded onto the screw 42 until an O-ring 54 in the cap's forward end face establishes a seal with the outer surface 16b of the lower body 16.

When removal of the connector 10 from the wellhead 14 is desired, the protective cap 52 is removed from the lockdown screw 42 and the screw 42 then backed out of the connector port 48 into the position shown in FIG. 5, thereby withdrawing the lockdown dog 38 from the wellhead recess 14b and pulling it fully into the connector groove 50. With the dog 38 in this position, the connector 10 can then be lifted free of the wellhead 14 for retrieval at the surface platform.