A subsea riser is installed by lowering at least one riser conduit to the seabed when piggybacked to an elongate support that comprises at least one flowline. The elongate support may be a pipeline bundle, which may be attached to one or more towheads in a towable bundle unit. The riser conduit may be in fluid communication with the flowline. At the seabed, a free end portion of the riser conduit is detached from the elongate support by releasing subsea-releasable fastenings. Then, with the elongate support and a root end of the riser remaining at the seabed, the detached free end portion of the riser conduit is lifted away from the elongate support to a riser support, such as a platform, an FPSO (floating production, storage and offloading vessel), or a buoy.

A subsea catenary riser comprises a rigid riser pipe that is suspended from a floating support on the surface and extends through a sagbend to the seabed. A hang-off interface allows rotation or pivoting of the riser pipe relative to the support about mutually orthogonal horizontal axes. A subsea buoy is positioned on the riser pipe above the sagbend. The buoy applies buoyant upthrust force to the riser pipe via an attachment interface that allows rotation or pivoting of the riser pipe relative to the buoy about mutually orthogonal horizontal axes. In response to movement of the support, the riser pipe deflects with S-shaped curvature to vary the inclination, relative to the buoy, of the portion of the riser pipe to which the buoy is attached. The curvature comprises mutually opposed curves respectively above and below the buoy, joined by a region of inflection that coincides with the buoy.

A buoyancy module (10, 12, 14) configured for mounting on an elongate underwater member (16) and provided with a sensor module dock (32) for receiving and releasably retaining a sensor module (30).

A buoyancy module (10, 12, 14) configured for mounting on an elongate underwater member (16) and provided with a sensor module dock (32) for receiving and releasably retaining a sensor module (30).

A method of installing a subsea riser comprises placing an elongate negatively-buoyant support on the seabed and, when laying the riser on the seabed, guiding a riser portion onto the support to extend along and be cradled by the support. A hogbend region of 5 the riser is then formed by conferring positive buoyancy on the support to lift the support and the riser portion away from the seabed. An element of the support comprises a riser support disposed in a longitudinally-extending open-ended gap between buoyancy volumes disposed on opposite sides of 0 the gap. Coupling formations such as hinge portions can couple the element to a like element. When so coupled, the gaps of those elements align to define an upwardly-opening, longitudinally-extending groove to receive the riser.

A method of installing a subsea riser comprises placing an elongate negatively-buoyant support on the seabed and, when laying the riser on the seabed, guiding a riser portion onto the support to extend along and be cradled by the support. A hogbend region of 5 the riser is then formed by conferring positive buoyancy on the support to lift the support and the riser portion away from the seabed. An element of the support comprises a riser support disposed in a longitudinally-extending open-ended gap between buoyancy volumes disposed on opposite sides of 0 the gap. Coupling formations such as hinge portions can couple the element to a like element. When so coupled, the gaps of those elements align to define an upwardly-opening, longitudinally-extending groove to receive the riser.

A method of constructing a sliding subset foundation comprises embedding at least one mass of rocks in seabed soil and placing a sliding mudmat on the seabed over the or each mass of rocks. The rocks may be lowered into a cavity in the seabed by dumping the rocks into the cavity or when contained within a gabion that is inserted into the cavity. Alternatively, a gabion containing the rocks may penetrate the seabed soil, driven by self-weight or additionally by a deadweight bearing on the gabion. The gabion may be lowered toward tie seabed suspended from the deadweight. The deadweight may be removed and recovered after the gabion has been embedded in the seabed. The mass of rocks are embedded within an area of excursion of the mudmat to ensure that the mudmat will be directly above the mass of rocks at any position within the area of excursion.

A supporting device configured to support a pipeline on the bed of a body of water having a frame connectable in a sliding manner to a pipeline extending along a longitudinal axis between a laying vessel and the bed of a body of water; at least one floating body connected to the frame; and at least one connecting mechanism connecting the frame to the floating body and configured to define a distance between the axis of the pipeline and the floating body, so as to apply a torque to the portion of pipeline at the frame.

A supporting device configured to support a pipeline on the bed of a body of water having a frame connectable in a sliding manner to a pipeline extending along a longitudinal axis between a laying vessel and the bed of a body of water; at least one floating body connected to the frame; and at least one connecting mechanism connecting the frame to the floating body and configured to define a distance between the axis of the pipeline and the floating body, so as to apply a torque to the portion of pipeline at the frame.

A method of manufacturing marine riser buoyancy elements includes providing a master mold and mold inserts such that a range of buoyancy elements may be manufactured from one master mold and providing the mold inserts such that an annular space between the riser main conduit and the buoyancy elements, or a groove width between the buoyancy elements may be varied during manufacture.