GB2088513A - Pipe for use as part of a pipeline - Google Patents

Abstract

A pipe for use as part of a subsea pipeline has two end sections (a) separated by a central section (1),the latter section having a wall thickness (s) greater than the wall thickness (s1) of the former sections. The length (L) of the pipe is from 0.5 to 10 times its nominal external diameter, and the length (a) of each end section is at least 1.5 times the wall thickness (s) of the central section. The pipe, when used as part of a pipeline, reduces the tendency for fractures to be propagated along the pipeline. <IMAGE>

Description

SPECIFICATION Pipe for use as part of a pipeline This invention relates to a pipe for use as part of a pipeline, in particular a subsea pipeline laid at a considerable depth.

Subsea pipelines are subjected to stresses and loads of a different nature, both during laying and during use. These stresses are caused by external pressure, by internal pressure, by the influence of the environment and by the irregular configuration of the sea bed. Moreover, unforseen conditions may occur, such as defects in the material from which the pipelines are made, subsea landslides, bumps, and scraping by anchors.

More particularly, in the case of fracture of the pipeline, the fracture can be propagated, under certain conditions, for lengths of up to several kilometres. A similar propagation of damage may occur, due to the effect of external pressure, in the case of accidental kinking of the pipeline.

It is thus important to insert in the pipeline, at regularly spaced apart intervals, devices which, by increasing the resistance, are capable of stopping the propagation of the damage so as to restrict it to a short section.

It is known to equip pipelines, at greater or lesser intervals, with collars or outer rings which are positioned on the pipeline upon assembling, and which are secured by means of working spots. Another approach which has been suggested is to insert sleeves in the interior of the pipelines after assembly. Lastly, another approach is to insert, during assembly of pipelines, short pipes having the same outside diameter as the remainder of the pipeline, but thicker.

These approaches have defects. The method in which external collars are applied to the pipeline after assembly requires that supplementary operations be effected beyond the laying ramp because the collars so applied would not permit convenient sliding of the pipeline along the laying ramp and through the device for tensioning the pipeline. To insert internal sleeves in the assembled pipeline has the defects that pressure drops are increased, that cleaning of the pipeline is made more difficult and that the sleeves do not prevent fracture in the event that the unexpected stress is an internal over-pressure such as results from bursts, shock waves or water-hammer.To insert pipes having the same outside diameter as the remainder of the pipeline but with an increased thickness does not cause any difficulty in the assembling and laying operations, but has the defects given above in connection with the increase of the pressure drop and the difficulties of cleaning. There are, moreover difficulties as to internal alignment in correspondence with the welding seals and, in addition, there are shortcomings caused by the fact that pipes of different thicknesses are to be welded. Thus, under working conditions, there is an irregular distribution of the streases in the welded area due to the effect of the abrupt thickness variation.

As regards the internal cleaning, it is common practice to carry out such an operation by devices commonly called "pigs". These "pigs", in the most widely used version, consist of plugs equipped with brushes or like implements for removing incrustations and deposits, such devices being introduced into the pipeline and being propelled by fluid pressure. The variations of the inside diameter, limited as they may be, but repeated many times along the pipeline, may damage the gaskets of such "pigs" and hinder their forward motion.

According to the present invention, there is provided a pipe for use as part of a pipeline, the pipe consisting of two end sections separated by a central section, the central section having a wall thickness greater than the two end sections but having the same internal diameter as the two end sections, the length of the pipe being from 0.5 to 10 times its external diameter, and the length of each end section being at least 1.5 times the wall thickness of the central section.

For a better understanding of the invention, reference will now be made, by way of example, to the accompanying drawing which shows a pipeline including a pipe of the invention.

The total length L of the pipe shown in the drawing is between 0.5 times and 10 times its normal external diameter. The pipe is divided into two identical end sections "a" and a central section "1".

The section "1" has increased thickness "sol", whereas the sections "a" have the same thickness "s" as the other pipes forming the pipeline. The increased thickness "s," is between 1.1 times and 3 times the thickness "s" of the end sections of the pipe. The length of the end sections "a" is dictated by the fact, mentioned above, that variations of the thickness cause a considerable local increase of the stresses in the steel mass. In the pipe of the invention, the region in which stresses are irregularly distributed is kept far away from the welded joint. The length "a" of the end sections must be at least 1.5 times the thickness "ski", and is preferably from 2 to 5 times as much.The region in which there is the transition from the thickness "sl" to the thickness "s" is preferably in the form of a shoulder portion having a radius of curvature which preferably exceeds 5 millimetres. Such a radius limits the local accumulation of stress. In addition, the gradual transition from "s" to "s," facilitates pipelaying operations.

When assessing the advantages of the pipe according to the present invention, it should be borne in mind, moreover, that standard specifications stipulate the outside diameter as the characteristic diameter so that, for a given nominal diameter, there are different internal diameters depending upon the thickness. In addition, it should be taken into account that the constructional allowances, in their turn, have a bearing on the internal diameter.

In the present invention, the reference diameter is the internal diameter and the thickness increase will influence only the external diameter.

Assembly of a pipeline include a pipe of the invention is not altered in the slightest, inasmuch as no supplementary operations or tools are required.

A very satisfactory internal alignment is obtained in correspondence with the welding seam. This fact, which eliminates internal unevenness arising from the allowance for the thickness, makes it possible to effect the welding operations better and more reliably. There are neither difficulties in cleaning the pipeline nor any hinderance to fluid flow arising from whirling or pressure drops.

An improvement of the resistance of the pipeline to tangential stress is obtained in that the increased thickness "sol" reduces the specific tangential stress, which stress is expressed by the equation: AP D a 251 wherein a is the stress, AP is the difference between the internal and the external pressures, D is the diameter and s1 is the thickness.

The thickness increase stops the propagation of any longitudinal fractures. Likewise, a local improvement is obtained in the resistance of the pipeline to ovalization, thereby reducing the tendency of the pipeline to kink.

The stiffness against ovalization of a pipe having a given length is proportions to the parameter: s3 12 It is apparent that the thickness increase considerably improves the resistance to ovalization, and prevents the propagation of such a phenomenon.

In addition to solving in a simple and cheap way the technical problem of locally increasing the resistance of pipelines both to breaking, by preventing the propagation of breaks to adjoining pipe sections, and to kinking, the pipe according to the invention has the advantages that its installation is easy since the pipe is welded like a conventional pipe and no junction problem exists, the pipelaying and launching operations being unaltered, that it does not restrict fluid flow, that it does not hinder cleaning, and that it limits local increase of stresses and localizes these to a region away from the welding seam.

The length of pipeline between pipes according to the invention varies according to the depth of the sea bed and other parameters. As an indication, however, the pipes are spaced apart from each other by about 30 metres to about 1000 metres. Since the pipes of the invention afford a considerable benefit in the stiffness of pipelines and their resistance to longitudinal bending, they should be installed at closer intervals where the sea bed has a more irregular outline or more difficult environmental conditions are encountered.

Claims (6)

1. A pipe for use as part of a pipeline, the pipe consisting of two end sections separated by a central section, the central section having a wall thickness greater than the two end sections but having the same internal diameter as the two end sections, the length of the pipe being from 0.5 to 10 times its external diameter, and the length of each end section being at least 1.5 times the wall thickness of the central section.

2. A pipe as claimed in claim 1, wherein the length of each end section is from 2 to 5 times the wall thickness of the central section.

3. A pipe as claimed in claim 1 or 2, wherein the central section has a wall thickness which is from 1.1 to 3 times the wall thickness of the end sections.

4. A pipe as claimed in claim 1, 2 or 3, having a curved shoulder portion around its periphery at the junction between each end portion and the central portion.

5. A pipe as claimed in claim 4, wherein the curved shoulder portion has a radius of curvature greater than 5 millimetres.

6. A pipe for use as part of a pipeline, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.