CA1152907A - Out of contact highly geometrical crack arrestor - Google Patents

Abstract

OUT OF CONTACT HIGHLY GEOMETRICAL CRACK ARRESTOR

ABSTRACT
A system for arresting cracks in a pressurized natural gas fluid pipeline comprising a plurality of bands encircling the pipeline each having a highly geometrical rim cross section and being out of contact with the pipeline along an arc of the inner clrcumference of the encircling band.

Description

~S2907 1 BACKGROUND OF THE I~VENTION

2 The present invention relates to a method and apparatus

3 for llmiting the propagation of ductile fractures in pipelines used

4 to transport fluids such as natural gases, compressed gas liquids or liquid-natural gas mixtures under pressure (hereinafter "natural 6 gas fluids"). More particularly, it relates to crack arrestors 7 that may be put in place during fabrication of a pipeline or added 8 to existing pipelines without degrading their quality. The system 9 is particularly useful for long distance pipelines such as a c`on-ter~plated for the transport of natural gases and hydrocarbons from 11 the Artic.
12 It is known that it is very costly to fabricate and as-13 semble hlghly crack resistant large diameter pipe suitable for use 14 as pressurized natural gas transmission lines. In any event, it is not practical to design a pipeline that will under all condi-16 tions withstand the internal stresses or external shocks that may 17 cause cracks in a pipeline. In addition, it is known that a crack 18 of substantial size in such pipeline will rapidly enlarge such that 19 its ends may propagate along the pipeline for extremely long dis-tances. These propagating cracks are Or two types: brittle and 21 ductile. Brittle cracks propagate at high veloclties, typically, 22 -1,300-3,000 ft./sec. without much apparent deformation of the pipe-23 11ne near the crack. The pipeline appears to break open and;
24 thereby relieve the stress causing the original crack. Ductile fractures propagate at lower velocities, typically, 250-1,000 ft./
26 sec. and are associated with substantial plpeline deformation.
27 Ductlle fractures can propagate for substantial distances, up to 28 several miles, ripping open the pipeline as if lt was unzipped.
29 Typically, ductile fractures run axially along the pipeline. Duc-tile fractures predominate above a critical temperature termed the -2- ~

brittle-ductile transition temperature, althougll, dcpending upon tlle means of fabrlcation, brittle fractures may be found at elevated temperatures.
There is a belief that the propagation of ductile fractures ls made possiblé by tile pressure of tlle natllral gas in the pipcllne against the flaps forming the edges of the crack. ~y that mechanism the crack may be driven forward at a velocity equal to the velocity of the low pressure front caused by the escaping gas. In this manner, although the gas pressure in the pipeline may rapidly drop as a result of gas escaping through the crack, there i5 sufficient pressure at the flap to cause the crack to propagate until it is arrested in some manner.
Many devices and methods have been discussed to arrest the propagation of ductile fractures. Some suggest attaching large masses to the pipeline to cause the fracture to deviate fromits straight line path into a helical path in tlle hope that that will permlt the low pressure front to catch up with the crack and result in an arrest.
Another method that has been suggested in Loncaric Patent No. 3,870,350 is to periodically interpose along the pipeline more brittle sections.
This is believed to cause arrest at the end of the brittle section where the fracture again becomes ductile, due to the absence of the flap driving force. These latter arrestors would seem to have the disadvantage of artificiallylowering the crack reslstance of the pipeline: crack initiation becomes mDre probable although less destructive.
Crack arrestors have been suggested to interfere mechanically with the dynamic mechanism believed to sustain crack propagation. Thus, flexible sleeves or hoops which are an integral part of the pipe have been suggested to restrain flap formation - ~ ~ ~~ ~

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~5Z907 l See "E~ulletin7' Research Laboratory, U S. Steel Corporation, April 2 10, 1~74. Such flexible sleeves or hoops have been suggested to 3 encricle the pipeline, in contact, and at zero or greater tension.
4 Typically, it is suggested that such arrestors should be made of

5 the same material as the pipeline. Crack arrestors have been sug- -

6 gested to interfere with the thinning of the pipeline in the region of the propagating crack tip. These arrestors have been designed 8 to tightly restrain the circumference of the pipeline. See U.S.
9 Patent Nos. 2,401,092 (wire wound at high tension), 3,349,807 (steel strap or band at high tension), and 3,631,897 (tensioning ll strands).
12 Each of the above-mentioned crack arrestors suffers from 13 the defect of requiring intimate contact ~s an integral part of the 14 pipellne. See also Risley U.S. Patent No. 3,096,105 (arrestor welded to pipe seCtion). This defect is enhanced by the fact that 16 the pipeline itself undergoes circumferential expansion as a result 17 of the internal pressure of the transported gas. Thus, a balance 18 must be maintained between the size of the arrestor and the pipe-l9 line's response to being pre~surized. A sleeve type crack arrestor that is tight when the pipeline is brought up to operating pressure 21 will introd~ce bending stresQes into the pipe at the edge of the 22 arrestor that could contribute to the formation of a crack in the 23 pipe which is detrimental.
24 Plain sleeve type members have also been suggested in connection with layipg deep underwater pipelines. See, e.g., Ells 26 U.S. Patent No. 3,860,039. There outside pressure on the pipeline 27 predominates over internal pressures durin~ the laying operation 28 and the major problem is to restrain buckling associated with pipe 29 oollapse and not crack propagation. For that purpose it has also been suggested to weld reinforced sections into the pipelina.

~15~907 l SUM~1ARY OF THE INVE~TION
2 According to this invention there is provided a pipellne 3 craok arrestor system designed to restrain the tendency of ductile-4 fracturing pipeline to develop an oval cross-section in advance of a propagating ductile crack tip.
6 Briefly, a crack arrestor full encirculement band is

7 fabricated from a material of high tensile strength, which prefer-

8 ably is steel. Such steel should preferably be ductile fracturing ' 9 under the conditions of use, although that is not essential. Al-ternatively, the arrestor may be made up of several sections that ll are assembled in place at the pipeline.
12 The arrestor preferably has a highly geometrical shape 13 suoh as that of a formed channel wlth its sides orthogonal to the 14 pipeline surface to impart rigidity against ovaling. These bands are applied ~u~ficiently out of contact with the pipeline so as 16 to provide a space between the arrestor and the pipeline at least 17 along some arc of the inner circumference of the bands, and,not 18 to provide a condition of full contact at zero tension. One result ~9 of such application is that no secondary longitudinal disoontinuity stresses are imposed at the edges of the band and the radial expan-~21 sion'of the pipeline is not restrained as the gas internal to the 22 pipeline is brought up to its high pressure state. In the pre-23 ferred embodiment, the average radial spacing between the arrestor 24 and the pipe is on the order of 10 times the radial expansion that the pipe undergoes when being brought up to pressure. The length 26 of the band longitudinally along the pipeline may be substantially 27 shorter than with conventional crack arrestors, typical valves 28 being ~rom 1/8 to 1~2 the diameter of the pipeline.
29 Accordingly, it ls an ob~ect of this ~nvention to provide a new and improved orack arrestor ~ystem for press'urized pipelines.

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l It i~ a further object of this invention to provide an improved ap-2 paratus that will retard and restrain ductile propagating cracks 3 in pressurized pipeline. It is a further object of this invention 4 to provide an improved pipeline for the transport of natural gas compressed gas liquids or liquid-natural gas mixtures under preq-6 sure that will restrain the propagation of ductile fracures along 7 its length.
8 Other aspects and advantages of this invention will be

9 apparent from the disclosure and the appended claims.
B~IEF DESCRIPTION OF THE DRAWINGS
ll In the drawings, Fig. 1 represents a fragmented side view 12 of a section of the preferred embodiment comprising a pipeline hav-13 ing a U-shaped formed channel ln position.
14 Fig. 2 is a vlew of a cross-section of the same embodi-ment as Fig. 1 including a spacing material between the arrestor 16 and the pipeline.
17 Fig. 3 is a view of the cross-section of the rim of the 18 U-shaped preferred embodiment of the arrestor.
l9 Fig. 4 is a side view of a pip~eline after a propagating ductile crack has been arrested.
Zl Fig. 5 is a fragmented side view of a section of a pipe-22 line having a T-shaped ring in position.
23 Fig. 6 i9 a partially cut-away side view of a section of 24 a pipeline having an I-beam-shaped ring in position.
Fig. 7 is a fragmented side view of a section of a pipe-26 llne having an I-beam-shaped formed channel in position.

28 Fig. 1 of the drawing ~how a plpeline 13 formed from a 29 plurallty of pipe sections having in place thereon a U-shaped ~ormed channel crack arrestor 11 spaced away from the arrestor 1 along an arc. The arrestor is preferably formed from standard hot 2 rolled U-channel stock rolled into a band and full penetration butt 3 welded. The arrestor can be prefabricated and slipped over the end 4 or a gap in the pipeline during assembly or repair. Alternatively the arrestor can be placed on an existing pipeline by splitting it 6 into two halves and then welding these into a ring with full pene-7 tration butt welds.
8 The rim cross-section should have a highly angular shape 9 (i.e., a so-called highly geometrical shape) and substantial bulk to inhibit deformation. An effective configuration for the rim ll cross-section is shown in Fig. 3 where the base 3 which faces the 12 pipe is 1/6 of the pipeline diameter, the rim 7 has a width of 1/48 13 dlameters, the notch 5 has a width of 1/10 diameters, the side wall 14 10 has a height of 1/12 diameters and the trough 9 has a thic~ness of 1/48 diameters.
16 A spacer material 12 may be inserted between the arrestor 17 and ~he pipeline to prevent destruction of any protective coatings lB applied to the pipeline. The inner diameter of the arrestor should l9 provide an average gap between the arrestor and the pipeline at least about 1 1/2 times larger than the expansion of the pipeline 21 due to pressure exerted by the contents of the pipeline. The aver-22 age gap may be as large as 15 to 20 times the expansion.
23 When assembled along the pipeline a plurality of arres-24 tors i8 employed. This not only a~sures that a crack meets an ar-restor before it covèrs a large distance, but also permits closely 26 spaced arrestors to cooperate by slowing a crack prior to arresting 27 it subsequently. As will be explained below, the present invention 28 has the unexpected feature of retarding the velocity of cracks be-29 fore they reach an arrestor, and it is therefore possible for a more remote arrestor to have its cooperative effect and ~id an . ~ . , ~ i ~1529~7 l earlier encountered arrestor to arrest a crack. This can permit a2 saving of material and weight as a multiplicity of lighter ar-3 restors may replace a single larger conventional arrestor. It is 4 possible to use the arrestors of the present invention in combina-tion with other types of arrestors to form a mixed arreqtor system.
6 Fig. 4 shows the observed manner in which the crack ar-7 res~or of the preferred embodiment arrests a propagating ductile B fracture 14. It will be noted that the tip of the fracture may 9 come to rest past the edge of the arrestor furthest from the direc-tion of approach of the crack.
ll Figs. 5, 6 and 7 show alternate embodiments of highly 12 geometrical crack arrestor systems in which the individual arrestor 13 has T and I configurations. In eaoh~, the members 15 and 19 rising 14 from the part closest to the pipeline supply rigidity to the ar-restor to restrain ovaling. It should be noted that unlike stan-16 dard anchor flange reinforcements, the arrestors of Fig. 5 of pre-17 sent invention are not welded to the pipeline.
18 In operation, the arrestors according to the present in-l9 vention do not degrade the pipeline or alter its operating charac-teristics. When a propagating ductile fracture approaches such 21 an arrestor, the energy release into the ovaling mode is absorbed 22 and as result the crack slows down. This slowing together with 23 absortion of the incident energy from the fractural pipe causes 24 the crack to arrest.
Experiment~s on a laboratory scale indicate that su¢cess-26 ful arrests of propagating ductile fractures can be accomplished 27 with a loose-fitting formed channel having a length 1/6 of the pipe 2~ diameter and a yield strength 67% that of the pipe. The preferred 29 range of yield strength is between about 50 and about 150~ of the yield strength of the pipeline. The preferred range of tensile . `

1~52907 1 strength ls +50% of that of the pipeline. Although the channel is ~ the preferred embodiment, other shapes such as I or T beams welded, 3 cast or rolled into a loose-fitting rigid section are also pro-4 posed. The channel arrestor is preferably formed from material rolled into a circu,lar ring having full penetration butt weld. As 6 already mentioned, the arrestors may be placed on an existing pipe-7 line by forming it into two or more sections and then welding these 8 $nto a ring with full penetation butt welds.
9 It is believed that the invention will provide the ad-vantage of inducing no secondary stresses in the pipeline. Because 11 it is out of contact along an arc when no crack is present, spiral 12 welded pipe may be used without the need to grind off the spiral 13 weld seam ln the vicinlty of the arrestor to avoid intense local-14 ized secondary stresses. Furthermore, because it is out of con-~5 tact, a liner to prevent damage to the protective coatings on the 16 pipeline could be fitted in place at fabrication and all field 17 coating could be eliminated. Also, since the geometry of the ar-18 restor need not conform closely to the pipeline, its ends may be 19 chambered to facilitate dollys rolling over it.
,To better understand the invention, it should be under-21 stood that the region just in advance of the propagating ductile 22 crack tip although not yet cracked is sub~ect to stralns and defor-23 mation that cause substantial ovaling of the pipellne's initially 24 circular cross-section. From the point of view of an observer see-ing an advancing crack, ovaling occurs prior to the arrival of the 26 crack tip. The other phenomena such as flap formation, although 27 the may provide the dynamic means to maintain the crack' 9 propaga-28 tlon, arrive at a position on the pipe only after the crack has 29 passed. Thus, an arrestor designed to restrain ovaling commences to function before the crack has already passed through and there-_9_ 1152gO~ , 1 fore acts before the larger stresses and pressures at the flap are 2 acting upon it. The action may be analogized to that of rock ~et-3 ties at the ocean which gently reflect and break up propagating 4 waves before they reach the shoreline and thereby eliminate the need for strong walls to protect beaches from crashing ocean waves 6 at full crest.
7 It is believed that an arrestor spaced from the pipe and 8 formed in a highly geometrical configuration sufficiently arrests 9 ovaling in high pressure gas pipelines to restrain crack propaga-tion and in addition sufficiently slows such propagation to a point 11 where it can be more easily restrained by other crack arrestors in 12 close proximity further down the pipeline. One unexpected result 13 observed during tests on a laboratory scale of full encirclement 14 loose-fitting formed channel arrestors is that as a fracture ap-proached such an arrestor, its speed was reduced substantially be-16 fore the crack tip reached the arrestor. Thus, on an experiment 17 where the steady state crack propagation speed was 690 ft./sec., 18 the appraoching crack was slowed to 515 ft./sec. prior to arriving 19 at ~he arrestor that assisted to arrest the rack.
It is believed that the retarding effect on crack propa-21 gation permits the cooperation between a plurality of arrestors on 22 the same pipeline where~y one slows down a propagating crack so 23 that even if the arre-stor then fails to bring about a complete ar-24 rest, the next arrestor in sequence may arrest completely. The ar-restor~ should be within 2 pipeline diameters of each other.
26 Experimentation on reduced s¢ale pipelines having 6 inch 27 diameters indicates that when loose arrestors are employed not hav-28 ing a highly geometrical rlm cross-section, such a9 loose-fitting 29 sleeves as small as 1/4 diametér long and having the same thickness and yield strength as ~the pipe, they undergo gross plastlc deforma-

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l tion for their entire length. These sleeve-type arrestors there-2 fore need to be manufactured from a ductile material to avoid frac-3 turing ~uring the arrest of a crack. In the present invention em-4 ploying a formed channel or other geometrically shaped arrestor, the arrestor may, however, be fabricated from ductile or brittle 6 cracking material. Also the present invention appears to arrest 7 a crack in a different manner-without the circumferential tearing 8 Of the pipe that occurs with the sleeve-type arrestors.
g - Therefore, in accordance with the invention, a crack ar-restor comprises in its preferred embodiment a steel channel having ll a length approximately 1/8 to 1/2 of the diameter of the pipeline 12 over which it is fitted so that the average radial spacing between 13 the pipe and the arrestor is between 1 1/2 and 20 tlmes the expan-14 slon due to bringing the pipeline up to operating pressure.

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Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a crack arrestor system for arresting ductile propagating cracks in a pressurized natural gas fluid pipeline, a first crack arres-tor band, encircling the pipeline and providing a radial spacing there-between of at least about 1 1/2 times greater than the expansion of the pipeline as the gas internal to the pipeline is brought up to its high pressure state whereby restraint of such expansion is precluded, said crack arrestor band having an internal diameter and tensile strength such as to restrain ovaling of the pipeline in advance of a propagating ductile crack tip, and a second corresponding crack arrestor band encircling the pipeline and spaced not more than about two pipeline diameters from said first crack arrestor band to cooperate with said first crack arrestor band in restraining of the ovaling of the pipeline.

2. In a crack arrestor system as claimed in claim 1 in which the radial spacing between each of said crack arrestor bands and said pipe-line is not greater than about 20 times the expansion of the pipeline due to pressure exerted by the contents of the pipeline when the pipe-line is brought up to pressure.