Pipes and tubular conduits – Distinct layers – With intermediate insulation layer
Reexamination Certificate
1999-11-23
2001-05-08
Brinson, Patrick (Department: 3752)
Pipes and tubular conduits
Distinct layers
With intermediate insulation layer
C138S129000, C138S144000
Reexamination Certificate
active
06227250
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a lagged pipe for transporting fluids and more particularly to a flexible pipe used for transporting hydrocarbons from a subsea well.
It is becoming increasingly necessary for flexible pipes to be well insulated because of the operating conditions under which offshore oil deposits are worked. This is because the heavy oils in these deposits tend to solidify while they are being transported between the subsea production well and the surface equipment, such as a platform, as a result of heat losses in the flexible pipe submerged in the sea. Good thermal insulation is also desirable in order to avoid the formation of hydrates to which certain crude oils are liable as they cool down.
The known prior solutions are described in FR-A-2,563,608. One of the improvements recommended by FR-A-2,563,608 consists in placing, around the internal core of the pipe, a plurality of annular partitions distributed along the length of the core and integral with the latter, in filling the annular chambers made between the successive partitions with an insulation material, in continuously extruding an outer sheath over the partitions, the space between two successive partitions being between 20 and 200 m depending on the service conditions. According to another improvement described in that document, the thermal insulation is obtained by spiralling hollow tubes around the central core, the tubes having a diameter of between
5
and 30 mm and a thickness of 0.5 to 4 mm. In all the embodiments, the bands of thermal insulation must offer a certain resistance to the hoop stress and a certain mechanical strength to be able to transfer the clamping loads to the tensile armour plies. Because of the short-pitch winding of these bands of thermal insulation, the corresponding layers tend to behave as rings and they therefore offer great resistance to radial deformation, such resistance being called the “vault effect” by the experts. When the number of thermal insulation layers is increased, as is the case in very deep applications, the force transferred to the tensile armour plies is relatively small compared with the force applied by the tensioners, as described below, which considerably reduces the capacity of the laying system. The vault effect becomes significant when nEe
3
>400 Nm, where n is the number of layers, E is the Young's modulus and e the thickness of one layer.
Pipes with such a thermal insulation exhibit what the experts call the “vault effect” and their applications are limited by the number of thermal insulation layers. It is important to note or recall that so-called rigid or flexible pipes, when they are being unwound from the holding reel or reels placed on a laying barge, pass through devices called tensioners whose purpose is especially to take up most of the weight of a pipe or of the tensile loads on the latter before it is submerged in the water. Such tensioner devices are well known and described, for example, in U.S. Pat. No 4,345,855. In order for these tensioners to fulfil their role fully, the maximum clamping force of the tensioners must be transferred, at least mostly, if not completely, to the central core of the pipe.
In a rigid or almost-rigid pipe, as in a pipe with an “external vault”, the resistance to the transfer of the application force of the tensioners is too high and only part of the applied force is transferred to the central this is insufficient to take up the weight of the pipe or the tensile forces in the latter.
In a pipe thermally insulated by means of superposed layers of thin bands wound with a short pitch, that is to say with a wind angle of greater than 55°, or in a pipe of the type described in FR-A-2,563,608, the application force of the tensioners deforms the insulating structure very considerably at the clamping pads of the tensioners while causing the insulating structure between the pads to creep or expand, thus forming kinds of bulges between the pads. Curves of the force (F
T
) transferred to the central core as a function of the force (F
P
) applied by the pads of the tensioners, for various thicknesses
e
of a compliant insulating structure, show that as
e
increases, the transferred force (F
T
) becomes relatively low. However, if a thin insulating structure is produced, then the thermal insulation obtained is unsatisfactory or not effective enough.
SUMMARY OF THE INVENTION'S
The object of the present invention is to propose a novel thermal insulation structure for pipes, which makes it possible to avoid the vault effect, ensure good thermal insulation and be easily mounted in a pipe, while not requiring a bulky and expensive winding machine such as a spiralling machine.
One subject of the present invention is a lagged pipe for transporting fluids having a thermal insulation structure placed around a central core, and it is characterized in that the thermal insulaton structure comprises at least one layer consisting of separate solid strips, each strip being wound with a very long pitch and a maximum wind angle around the said core of less than 30°.
One advantage of the present invention is that it is possible to use strips of greater thickness while still ensuring good transfer of the application force of the tensioners to the central core on the one hand, and good thermal insulation on the other. This comes from the fact that the osculating radius of the strips on the central core is, because of the relatively long pitch at which the strips are wound, much greater than the osculating radius of bands or tubes wound with a short pitch. Consequently, it is no longer necessary to wind a large number of insulating layers around the central core.
Another feature of the invention is that the strips are wound in an S/Z configuration. This allows a winding machine much less expensive than a spiralling machine to be used.
Another feature of the invention lies in the fact that the radially opposed faces of each profile are non-planar, and preferably slightly curved, so that the lower face of each strip well matches the curvature of the central core.
In another embodiment the strip has lateral faces which at least partially imbricate with the lateral faces of the following strip so as to improve the cohesion of the layer formed by the juxtaposed strips and avoid thermal bridges between turns.
REFERENCES:
patent: 2225333 (1940-12-01), Daniels
patent: 3804438 (1974-04-01), Humphries et al.
patent: 3853149 (1974-12-01), Stine
patent: 4492089 (1985-01-01), Rohner et al.
patent: 5722462 (1998-03-01), Dridi et al.
patent: 2563608 (1985-10-01), None
Brinson Patrick
Coflexip
Ostrolenk Faber Gerb & Soffen, LLP
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