The technical field of the present invention relates to conduits or pipelines submerged at seabed level and moving effluents.
It is well known to use single-shell or multiple-shell pipelines to move effluents at seabed level.
Most double-shell pipelines are fitted with an insulator between the two shells in order to reduce thermal losses. One illustrative design is disclosed in the French patent document 2,746,891 A previously filed by applicant. The pipeline described therein offers excellent insulation. However this pipeline still is heavy because comprising two steel shells in its preferred embodiment mode. Such a feature is a drawback in conveyance and especially when installing, the weight supported by the ship at sea possibly becoming prohibitive for the equipment, for instance tightening equipment, mounted on said ship. In the case of deep installations, the weight problem also may be limiting as regards single shell pipelines.
Whatever the pipeline design, it is known that for an ideal round tube or shell, the hydro-static crushing pressure always is much larger than the actually observed buckling pressure. This drop is due to manufacturing imperfections such as the tube being rendered oval or exhibiting flattened parts, thickness variations etc.
There is a present need for lower-weight pipelines without thereby incurring a loss in mechanical strength and without degrading their buckling behavior under hydrostatic pressure.
It is also obvious that making the tube walls thinner may offer significant, direct or indirect cost savings.
The present state of the art suggests using bracing means (xe2x80x9cbuckling arrestorsxe2x80x9d) when the tube wall thicknesses are less than mandated by the codes. In general such bracing means are apposed parts which must be welded onto the main tube, and they cannot be economically used to preclude buckling: their purpose is to prevent buckling from spreading after it was initiated by an unexpectedly large oval deformation (for instance excessive oval warping during tube laying).
Accordingly the objective of the present invention is to create a novel pipeline fitted with a double or multiple shell and offering a much lower weight at the same crushing strength, or vice-versa offering a substantially increased buckling strength for the same overall shell thickness.
The objective of the invention is a pipeline moving effluents at seabed level and comprising an inner shell and at least one outer shell which is separated from the inner one by an annular space into which is fitted a layer of insulating material, said pipeline being characterized in that the outer shell comprises flutings which are mutually apart and which are arrayed over at least part of its surface, the fluting bases resting against a spacer element mounted around the inner shell.
In a first embodiment the flutings are configured over the full length of the outer shell.
In another pipeline embodiment, the fluting depth is between 1 and 10% of the outside diameter of the outer shell.
In another feature of the pipeline of the invention, the fluting width at its inside base is substantially between 1 mm and 30% of the outside diameter of the outer shell.
In another feature of the pipeline of the invention, the flutings are configured concentrically with the longitudinal axis XX of said pipeline.
In yet another feature of the invention, the flutings when seen cross-sectionally are in the form of a circle.
In one embodiment variation of the invention, the flutings are mutually apart by a distance d substantially between 0.25 and 3 times the outside diameter of the outer shell.
In another feature of the invention, the flutings run helically at a pitch substantially between 0.25 and 3 times the outside diameter of the outer shell.
In another embodiment variation of the invention, the spacer element may be made of an insulating material, illustratively an open-pore micro-pore material.
The pipeline may comprise a metal lamella forced against the insulator, the flutings of the adjacent outer shell resting on said lamella.
In another variation of the invention, the spacer element is an elastically deforming plastic ring.
A foremost advantage offered by the invention is the increase in the pipeline""s buckling strength and hence the feasibility of making the shell(s) used thinner.
Another advantage of this design is that shear stresses are absorbed between the two shells when the insulating material is being compressed between the outer shell flutings and the inner shell.
The invention offers still another advantage in being devoid of accessory means when transmitting the axial stresses between the shells.