Pressure-compensated dual marine riser

Claim 2

Marine riser system for use in offshore well drilling operations, said system comprising two risers that are adapted to be vertically arranged with a substantially annular space therebetween, each riser consisting of a plurality of sections that are sealingly interconnected in end-to-end relationship, the outer riser comprising connecting means at the lower end thereof, said connecting means being adapted for connecting and sealing the lower end of the outer riser to a wellhead, said outer riser further comprising supporting means near the upper end thereof, said supporting means being adapted for being connected to a heave compensator system carried by a floating drilling means, sealing means being arranged for sealing off the annular space at, at least, the lower end thereof, and closeable fluid pressure equalizing means arranged in the wall of the outer riser, said means being adapted for equalizing the fluid pressures prevailing in the annular space and in the space outside the outer riser.

Claim 3

Marine riser system according to claim 1, wherein the pressure equalizing means comprises at least one closeable passage arranged near the upper end of the annular space, and at least one closeable passage arranged near the lower end of the annular space.

Claim 4

Marine riser system according to claim 1, wherein the risers are interconnected near their lower ends to prevent a relative axial displacement between the lower parts of the risers.

Claim 5

Marine riser system according to claim 1, wherein the risers are interconnected near their upper ends to prevent a relative axial displacement between the upper parts of the risers and wherein the sealing means arranged near the lower end of the annular space allow a relative axial displacement between the lower parts of the risers.

Claim 6

Marine riser system according to claim 1, wherein buoyancy means are attached to the outer wall of at least one of the risers.

Claim 7

Method for installing the marine riser system according to claim 1 between a floating drilling means and a submerged wellhead, comprising the steps of: a. suspending the outer riser from the floating drilling means by means of a heave compensating system mounted on the floating drilling means, the lower end of the outer riser being above the submerged wellhead to which it is to be coupled; b. placing the lower end of the outer riser on the wellhead and coupling this lower end of the outer riser to this wellhead; c. lowering the inner riser through the outer riser to a position wherein the inner riser is enclosed by the outer riser, and sealing off the annular space between the outer and inner riser near both ends of this annular space; and d. manipulating the pressure equalizing means to equalize the fluid pressures prevailing in the annular space and outside the outer riser.

Claim 8

Marine riser system according to claim 1, wherein the sealing means that are arranged for sealing off the upper end of the annular space allow a relative axial displacement between the upper parts of the risers.

Claim 9

Marine riser system according to claim 7, wherein the sealing means arranged for sealing off the upper part of the annular space include a cylinder/piston arrangement adapted for tensioning the inner riser with respect to the outer riser by means of hydraulic fluid supplied to the cylinder means of the arrangement, said arrangement comprising at least one piston connected to one of the risers near the upper end thereof, and cylinder means connected to the other riser.

Claim 10

Marine riser system according to claim 8, wherein the piston is an annular piston mounted on the inner wall of the outer riser, and the cylinder means is connected to the inner riser by means of a remotely operable coupling.

Claim 11

Marine riser system according to claim 8, wherein the cylinder means are connected to the interior wall of the outer riser by means of a remotely-operable coupling.

Claim 12

Marine riser system according to claim 10, including a fluid passage way between the inner wall of the outer riser and the outer wall of the cylinder means, said passage way communicating at one end thereof with the cylinder means and at the other end with a conduit arranged at the outside of the outer riser.

Claim 13

Marine riser means according to claim 8, wherein the cylinder parts at opposite sides of the piston can be brought into communication with each other via a by-pass conduit.

Claim 14

Method for installing the marine riser system according to claim 12 between a floating drilling means and a submerged wellhead, the method comprising the steps of: a. suspending the outer riser from the floating drilling means by means of a heave compensating system mounted on the floating drilling means, the lower end of the outer riser being above the wellhead to which it is to be coupled; b. lowering a tubular member through the outer riser, coupling the tubular member to the cylinder means of which the parts situated at both sides of the piston are in communication with each other via a by-pass conduit, and landing the lower end of the tubular member on the wellhead, whereafter this end is coupled to the wellhead; c. gradually closing the by-pass conduit, thereby activating the heave compensating system and maintaining the lower end of the outer riser at a fixed distance above the wellhead; supplying pressure fluid to one side of the piston after the by-pass conduit has been closed, to displace the outer riser downwards against the action of the heave compensating system until the lower end of the outer riser contacts the wellhead; e. coupling the lower end of the outer riser to the wellhead; f. removing the fluid pressure from the cylinder means, detaching the coupling between the tubular member and the cylinder means and lifting the tubular member aboard the floating drilling means; g. supplying drilling mud to the interior of the outer riser, and carrying out drilling operations; h. subsequently lowering the inner riser through the outer riser, connecting the lower end of the inner riser to the outer riser, and the upper end of the inner riser to the cylinder means; i. admitting pressure fluid to one side of the piston in the cylinder means to tension the inner riser; and j. manipulating the pressure equalizing means after one of the steps (g) - (i) to equalize the fluid pressures prevailing in the annular space and outside the outer riser.