WO2015143544A1

WO2015143544A1 - Energy absorber for fall arrest system

Info

Publication number: WO2015143544A1
Application number: PCT/CA2015/000186
Authority: WO
Inventors: Maurice William Slack
Original assignee: Noetic Technologies Inc.
Priority date: 2014-03-27
Filing date: 2015-03-26
Publication date: 2015-10-01

Abstract

An energy absorber for a fall arrest system comprises at least one serpentine structure of contiguous ductile segments elements acting in a load path of a fall arrest system. The ductile elements include a plurality of curved segments folded upon each other to form the serpentine structure, and optionally may include transverse straight segments contiguously interconnecting adjacent curved segments. In the event of a downward load being applied to a lower region of the energy absorber, such as from a falling object tethered thereto, the energy absorber will elongate as the serpentine structure unfolds due to the formation of plastic hinges in the curved segments, thereby dissipating the energy of the falling object and minimizing the impact loading effects on the fall arrest system.

Description

ENERGY ABSORBER FOR FALL ARREST SYSTEM

FIELD OF THE DISCLOSURE

The present disclosure relates in general to apparatus for absorbing energy generated from falling equipment. In particular, the disclosure relates to apparatus for absorbing energy and reducing shock loading in fall arrest systems for casing running tools and other oilfield equipment.

BACKGROUND

Typical construction of an oil and gas well includes the operation of assembling a casing string and inserting it into the wellbore. It is increasingly common in the drilling industry for a drilling rig to be equipped with a "top drive" (rather than a conventional rotary table) for rotating the casing string. Many tools exist that are for use on top-drive- equipped rigs to facilitate insertion of a casing string into the wellbore, as well as for other casing-related operations. One example of such a tool is the "Gripping Tool" described in U.S. Patent No. 7,909,120. Such tools are known in the industry as casing running tools ("CRTs").

Prior to use, a CRT must be rigidly attached to the top drive. A shouldering threaded connection, known as a tool joint, is the primary means of connection. This process is known as "rigging in" the tool. As with all threaded connections, a risk exists that the connection might be unintentionally disengaged. In the case of CRTs and other equipment attached to the top drive, unintentional disengagement of the connection can result in a significant safety hazard from falling objects. A fall arrest system attached to the connected equipment can be a suitable means of reducing this safety hazard. A necessary feature of any fall arrest system is a means of dissipating the kinetic energy of the falling object in a controlled manner, to prevent possible overload of the fall arrest system should the falling object be stopped abruptly. Known energy absorbers, such as fluid-based absorbers and crumpling absorbers, can have many components, such as seals, pistons, and rods, and can have complicated architectures. Accordingly, there is a need for a comparatively inexpensive energy absorber that is of comparatively simple construction and is comparatively simple to manufacture, and which can dissipate energy over a defined stopping interval.

BRIEF SUMMARY

The present disclosure teaches apparatus for absorbing energy of a falling object as part of a fall arrest system ("energy absorbers").

The architectures of the disclosed energy absorbers feature at least one energy- absorbing section comprising a serpentine structure of contiguous "folded" elements of uniform or non-uniform thickness which acts in the load path of a fall arrest system. The energy absorbers are constructed from a ductile material such as but not limited to steel. The folded elements include curved segments in which plastic hinges can form when flexural stresses in the curved segments due to vertical loads acting on the energy absorbers exceed the elastic limit (i.e., yield stress) of the material from which they are made (e.g., steel). Optionally, the folded elements may also include generally transverse straight segments contiguously joining curved segments; i.e., with a straight segment being contiguously interposed between each sequentially-adjacent pair of curved segments. The serpentine structure extends between upper and lower mounting points, with the curved segments extending in alternate lateral directions from a longitudinal centerline or axis of the serpentine structure. In other words, when looking at the serpentine structure in front profile, if a given curved segment extends leftward from the axis, the curved segments above and below the first curved segment will extend rightward from the axis, and so on. Accordingly, in embodiments having transverse straight segments joining sequentially-adjacent curved segments, the longitudinal axis will bisectingly pass through the straight segments. In alternative embodiments not incorporating transverse straight segments, the longitudinal axis of the serpentine section will pass through the inflection points (i.e., points of curvature reversal) between contiguously adjacent curved segments.

Each curved segment (i.e., on either side of the associated longitudinal axis) can be considered as having an upper half and a lower half. In the serpentine section in its initial form (i.e., prior to being subjected to loading from a falling object tethered to energy absorber), the portions of the transverse straight segments contiguously joined to upper halves of curved segments slope at an acute positive angle, and the portions of the transverse straight segments contiguously joined to lower halves of curved segments slope at an acute negative angle (of the same magnitude). The same angular relationship would apply to a tangent line at the inflection point of contiguously joined curved segments in serpentine sections that do not incorporate transverse straight segments.

The curved segments are sized and configured such that over a desired operational deformation range, they will not reach a critical material failure stress/strain state. As noted above, the transverse straight segments are initially orientated at an acute angle relative to the direction of falling load (which would be vertical in the typical case), such that upon rotation due to the extension (or "unfolding") of the folded structure under load, the transverse segments will rotate, thereby increasing the effective length of the moment arm between the plastic hinges, to a maximum length as the transverse segments reach an orientation perpendicular to the load path, and thus tending to reduce the otherwise stiffening effect occurring as plastic hinges form.

The angular orientation of the transverse straight segments (or inflection point tangent lines in embodiments not having transverse segments) is preferably optimized for material properties to promote a balanced load displacement profile. The load deflection profile can be selected through several means, including but not limited to:

· varying the overall thickness or selectively varying the thickness of the energy absorber;

• adjusting the radius and in-plane width of the folded curved segments;

• adjusting the in-plane width and angle of the transverse segments; • adjusting the number of parallel loaded folded segments in the energy-absorbing section; and/or

• adjusting the number of curved segments in the energy-absorbing section.

Varying the profile depth along the length of the energy absorber may result in an optimized load displacement and stress / strain response. However, a uniform depth profile will facilitate manufacture using known techniques involving two-dimensional planar cutting systems.

Energy absorbers as disclosed herein are designed for, but not limited to, use as (or as a component of) a fall arrest system associated with a top-drive-mounted CRT. In the event that a downward vertical load is applied to an energy absorber, such as would occur if the energy absorber is attached to a rigid support and to a falling object, the energy absorber will elongate due to elastic deformation, followed by plastic deformation if the induced stresses exceed the elastic limit of the material, thereby dissipating the energy of the falling object and minimizing the impact loading effects on the fall arrest system. In this context, the preferred load/displacement response is one that will stop the falling object in as short a vertical distance as possible without exceeding a maximum applied load, based on load capacity ratings of other components in train with the applied load.

Energy absorbers in accordance with the present disclosure are primarily intended for one-time use only, in contemplation that the energy absorbers will be permanently (i.e., plastically) and maximally deformed after being subjected to their maximum rated load capacities. However, this does not rule out the possibility that an energy absorber might retain a residual energy-absorbing capability after undergoing less-than-maximal permanent deformation after being subjected to loads less than its rated capacity.

In accordance with a first aspect, the present disclosure teaches a serpentine structure made from a ductile material and having an upper end, a lower end, and a longitudinal axis, in which the serpentine structure comprises: • a plurality of curved segments alternately arrayed on either side of the longitudinal axis, with each curved segment comprising an upper half and a lower half; and

• one or more straight segments, each of which contiguously joins two of the plurality of curved segments, with the straight elements being bisected by the longitudinal axis into two portions, and in which: o the portions of transverse straight segments contiguously joining the upper halves of curved segments are oriented at an acute positive angle; and o the portions of the transverse straight segments contiguously joining lower halves of curved segments are oriented at an acute negative angle.

In accordance with a second aspect, the present disclosure teaches a serpentine structure made from a ductile material and having an upper end, a lower end, and a longitudinal axis, with the serpentine structure comprising a plurality of curved segments alternately arrayed on either side of the longitudinal axis, with each curved segment comprising an upper half and a lower half, in which;

• the upper half of each curved segment contiguously joins the lower half of an adjacent curved segment at a point of inflection; and

• a tangent line at the point of inflection where the upper half of a first one of the plurality of curved segments contiguously joins the lower half of a second one of the plurality of curved segments is oriented at an acute positive angle on the side of the longitudinal axis corresponding to the upper half of said first curved segment, and at an acute negative angle on the side of the longitudinal axis corresponding to the lower half of said second curved segment.

A serpentine structure in accordance with either of the first and second above- described aspects may comprise mounting points at the upper and lower ends of the serpentine structure. In accordance with a third aspect, the present disclosure teaches an energy- absorbing apparatus comprising two serpentine structures in accordance with either of the first and second above-described aspects, in which wherein the upper ends of the two serpentine structures are connected to an upper mounting point, and the lower ends of the two serpentine structures are connected to a lower mounting point.

In one embodiment of the energy-absorbing apparatus, the two serpentine structures are laterally juxtaposed and coplanar.

In a second embodiment of the energy-absorbing apparatus, the two serpentine structures overlie each other, and may be spaced apart from each other. In a third embodiment, the two serpentine structures are curved such that the energy-absorbing apparatus is of a cylindrical configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described with reference to the accompanying Figures, in which numerical references denote like parts, and in which:

FIGURE 1 is a cross-sectional view of a first embodiment of an energy absorber in accordance with the present disclosure, shown connected (through intermediary structure) to a casing running tool.

FIGURE 2 is a front view of an energy absorber as in FIG. 1.

FIGURE 3 is a load-versus-displacement graph for an energy absorber as in FIG. 1 with and without an initially acute angle on the transverse straight segments.

FIGURE 4 is a cross-sectional view of an energy absorber as in FIG. 1 , shown in a deformed configuration as it would appear after plastic deformation in response to vertical loading. FIGURE 5 is a front view of a second embodiment of an energy absorber in accordance with the present disclosure, having a single serpentine energy-absorbing section.

FIGURE 6 is an isometric view of a third embodiment of an energy absorber in accordance with the present disclosure, having two overlaid and interconnected serpentine energy-absorbing sections.

FIGURE 7 is an isometric view of a fourth embodiment of an energy absorber in accordance with the present disclosure, having serpentine energy-absorbing sections arranged in a cylindrical configuration.

DETAILED DESCRIPTION

FIG. 1 illustrates a first embodiment 100 of an energy absorber in accordance with the present disclosure. More specifically, FIG. 1 shows two energy absorbers 100 as they would appear as components in the load train of a fall arrest system. Energy absorbers 100 are shown attached to a casing running tool (CRT) 300 by means of a swivel assembly 200 mounted to CRT 300, with swivel assembly 200 comprising shackles 201 that connect to mounting points 103 associated with lower regions of energy absorbers 100. The upper end of CRT 300 is attachable to a top drive quill (not shown) by means of a rotary shouldered tool joint connection 301. CRT 300 can rotate freely while holding swivel assembly 200 and energy absorbers 100 rotationally fixed relative to the top drive mount points, with relative motion being accommodated by means of a swivel bearing 202. In the event that connection 301 fails or is unintentionally released and CRT 300 falls from the top drive, thereby loading the fall arrest system, energy absorbers 100 will elongate plastically by the mechanism described below with reference to FIG. 2, thus dissipating the kinetic energy of the falling CRT 300 and any attached equipment.

FIG. 2 illustrates energy absorber 100 in a front profile view. Energy absorber 100 has upper and lower mounting points 101 and 103, in the illustrated case symmetrically located on either side of (i.e., above and below) a middle interval 102. Middle interval 102 comprises two laterally adjacent serpentine energy-absorbing sections 108 and 109, each connected at its upper and lower ends to respective upper mounting points 101 and lower mounting points 103. Each of energy-absorbing sections 108 and 109 comprises a plurality of opposing external curved segments 104 and internal curved segments 107 contiguously joined by initially acute-angled transverse straight segments 105 (with reference number 110 in FIG. 2 indicating the initial acute angle of transverse straight segments 105). Optionally, laterally-adjacent internal curved segments 107 of serpentine sections 108 and 109 may be connected by center tie links 106, as shown in FIG. 2.

Each serpentine section (108, 109) has a longitudinal axis X-l laterally separating external and internal curved segments 104 and 107, and substantially bisecting transverse straight segments 105. The load path of a falling object tethered to energy absorber 100 is defined by a load path axis X-2 extending between upper and lower mounting points 101 and 103.

As noted previously, it is optional, rather than essential, for serpentine sections in accordance with the present disclosure to include transverse straight segments (such as transverse segments 105 in energy absorber 100) interconnecting adjacent curved segments (such as curved segments 104 and 107 in energy absorber 100). Although all of the embodiments illustrated herein include transverse straight segments, in variant embodiments adjacent curved segments could be contiguous with each other. In such variants, adjacent curved segments would contiguously meet at a corresponding point of curvature reversal (inflection point). Such variant embodiments would function in generally the same way as embodiments that include straight segments between adjacent curved segments, because portions of the serpentine sections bridging the inflection points between contiguous curved segments would, under load, behave or function in essentially the same fashion as the straight segments in embodiments having serpentine sections incorporating straight segments. Accordingly, all descriptions herein of embodiments having serpentine sections that incorporate transverse straight segments are to be read with the understanding that transverse straight segments could be omitted from variants thereof without departing from the scope of the present disclosure. In the illustrated embodiment, the use of two symmetrically adjacent and interconnected serpentine energy-absorbing sections 108 and 109 promotes balanced and stable load displacement behaviour under load. It is to be understood that energy absorber 100 may comprise any number of adjacent serpentine energy-absorbing sections, and that when two or more adjacent energy-absorbing sections are required to react load in a parallel fashion, a slight positive slope to the load/displacement response is desirable to minimize or avoid instabilities that might develop as energy-absorber 100 deforms under load.

Optional center tie links 106 connecting adjacent internal curved segments 107 encourage a stable response under load, and manage small differences in segment thickness T (see FIG. 6 for illustration of thickness T) and load displacement stiffness that might bias one serpentine section to deform differently from the adjacent serpentine section. It is to be understood that similar stability improvements can alternatively be effected by other means, including (but not limited to) varying the thickness profile along the segment lengths such that curved segments 104, 107 nearer one end (e.g., upper end) of the serpentine section will preferentially deform before the curved segments 104, 107 nearer the other end (e.g., lower end).

The thickness and radius of curvature of curved segments 104 and 107 are selected to promote essentially complete straightening of these segments without exceeding characteristic material strain limits, while optimizing the load resistance provided during response to a fall event. In the illustrated embodiment, transverse straight segments 105 are curved back at a negative acute angle 110 to optimize the ratio of starting length to extended length, and to provide a balancing softening response such that the stiffening resulting from the formation of plastic hinges in curved segments 104 and 107 is counteracted by the increasing moment arm length as the initial acute angle of transverse segments 105 flattens out during deformation of energy-absorbing sections 108 and 109 under load.

In certain configurations, energy-absorbing sections 108 and 109 have an initially uniform depth D (see FIG. 6 for illustration of depth D). However, depth D could be varied within an energy-absorbing section without departing from the scope of this disclosure. Depth D is selected to achieve desired load displacement performance and lateral stability of the serpentine structure during deformation under load.

FIG. 3 illustrates load-versus-displacement profiles for an energy absorber 100 having an acute-angled traverse segment (curve 112) and for an energy absorber 100 having an obtuse-angled transverse segment (curve 113). For a given energy absorption requirement, the maximum load on the fall arrest system can by minimized by absorbing energy over a greater distance with a more uniform load reaction. The energy absorbers represented by the two curves absorb a similar amount of energy. Curve 112 corresponding to an energy absorber with acute-angled transverse segments exhibits preferable load/displacement characteristics, and has a more uniform load over a larger displacement interval, enabling it to absorb more kinetic energy than an energy absorber with an obtuse-angle design.

FIG. 4 shows energy absorber 100 as it would appear after loading, symmetrically elongated. The radius of curvature of curved segments 104 and 107 has generally increased and in some locations has reversed, and straight segments 105 have rotated through the maximum moment arm angle to form an obtuse angle 111. Through-depth strain results in a final geometry with variable depth.

The deformed configuration of energy absorber 100 illustrated in FIG. 4 is representative of a typical specified deformed state under a prescribed load, and is not a deformation limit. Further displacement will result in a rapid increase in the stiffness of the load-to-displacement response of the energy absorber.

FIG. 5 illustrates a second embodiment 400 of an energy absorber in accordance with the present disclosure, shown in front profile. Energy absorber 400 comprises a single serpentine energy- absorbing section 403 comprising a plurality of curved segments 404 contiguously joined by transverse straight segments 406 oriented at acute angles 405 (similar to straight segments 105 of energy absorber 100 in FIG. 2), and having upper and lower mounting points 401 and 402. This embodiment has the practical advantage of being narrow in lateral width. Serpentine section 403 has a longitudinal axis X-3 laterally separating adjacent curved segments 404 and 407; axis X-3 also coincides with the load path axis of energy absorber 400.

FIG. 6 illustrates a third embodiment 500 of an energy absorber in accordance with the present disclosure. Energy absorber 500 comprises two serpentine energy- absorbing sections 503 and 504 in parallel juxtaposition and interconnected as shown, with serpentine energy-absorbing sections 503 and 504 each having an array of curved segments 505 and, in the illustrated embodiment, transverse segments 506. In the illustrated embodiment, upper and lower mounting plates 501 and 502 are sandwiched between respective upper and lower end plates 507 and 508 on the juxtaposed serpentine sections 503 and 504 so as to interconnect serpentine sections 503 and 504 by means of suitable fasteners installed through mounting holes 509 in upper and lower end plates 507 and 508 and matching fastener holes (not shown) in mounting plates 501 and 502.

Mounting plates 501 and 502 also serve as spacers between serpentine energy- absorbing sections 503 and 504 to ensure that during extension of the system in response to loading from a falling object, the curved segments 505 of serpentine sections 503 and 504 will not catch on adjacent curved segments 505. More specifically, if serpentine sections 503 and 504 are mounted too closely adjacent to each other, they could laterally interfere with each other as a consequence of through-depth thickening developing in curved segments 505 as they unfold due to flexure induced by the applied load. In the illustrated embodiment, serpentine sections 503 and 504 are installed such that the orientations of their respective curved segments 505 are reversed relative to each other, thereby enhancing lateral stability when the system is deforming under load. This embodiment provides the benefit of the increased load capacity available from two energy-absorbing sections but in an assembly having the width of a single energy- absorbing section.

In alternative embodiments, serpentine sections 503 and 504 could have matching orientations without departing from the scope of the present disclosure. Although the above-described embodiments having "flat" or planar serpentine sections may be particularly advantageous from a manufacturing perspective, embodiments in accordance with this disclosure are not limited or restricted to such configurations. FIG. 7 illustrates a fourth embodiment 600 of an energy absorber in accordance with the present disclosure, in which energy absorber 600 comprises serpentine energy-absorbing sections 601 and 602 each generally similar (or analogous) to serpentine section 403 of energy absorber 400 illustrated in FIG. 5, but rather than being of essentially flat or planar configuration like serpentine section 403, serpentine sections 601 and 602 are curved such that energy absorber 600 is generally configured like a cylindrical tube.

Serpentine sections 601 and 602 connect at their upper and lower ends to upper and lower collars 603 and 604 (shown by way of non-limiting example as cylindrical collars in FIG. 7), which have mounting means shown by way of non-limiting example as mounting holes 605 in FIG. 7. Collars 603 and 604, particularly when of cylindrical configuration as in FIG. 7, promote uniform displacement of the ends of energy absorber 600 when subjected to an axial load. Serpentine sections 601 and 602 may be designed and configured to manage the effects of load-displacement such that the stress/strain response is of a predictable nature generally as illustrated in FIG 3.

The cylindrical configuration of energy absorber 600 makes it easier to protect (such as by wrapping with sheet metal, heavy plastic film, or other suitable materials) than the "flat" energy absorber embodiments. Wrappng material for energy absorber 600 should ideally be non-confining so as to not interfere with the desired stress/strain response as outlined in FIG 3, while being robust enough to prevent significant damage from or to articles that might inadvertently come into contact with the wrapped energy absorber after installation. The cylindrical configuration of energy absorber 600 makes it more compact than other embodiments and prior art energy absorbers of comparable load capacity, making it particularly useful in environments with rotating equipment such as drilling rigs where snagging is a concern.

In unillustrated variant embodiments, energy absorbers in accordance with the present disclosure could be formed in other three-dimensional forms such as (by way of non-limiting example) tube-like structures that have generally square, rectangular, or other polygonal cross-sectional configurations.

It will be readily appreciated by those skilled in the art that various modifications to embodiments in accordance with the present disclosure may be devised without departing from the present teachings, including modifications which may use structures or materials later conceived or developed. It is to be especially understood that the scope of the disclosure and the claims appended hereto should not be limited by any particular embodiment described and illustrated herein, but should be given the broadest interpretation consistent with the disclosure as a whole. It is also to be understood that the substitution of a variant of a claimed element or feature, without any substantial resultant change in functionality, will not constitute a departure from the scope of the disclosure or claims.

In this patent document, any form of the word "comprise" is intended to be understood in a non-limiting sense, meaning that any item following such word is included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article "a" does not exclude the possibility that more than one such element is present, unless the context clearly requires that there be one and only one such element. Any use of any form of any term describing a connection or other interaction between elements is not meant to limit the interaction to direct connection or interaction between the elements in question, but may also extend to indirect connection or interaction between the elements such as through secondary or intermediary structure.

Relational terms such as but not limited to "vertical", "horizontal", "straight", "bisected", and "symmetrical" are not intended to denote or require absolute mathematical or geometrical precision. Accordingly, such terms are to be understood as denoting or requiring substantial precision only (e.g., "substantially vertical" or "generally vertical") unless the context clearly requires otherwise. Any use of any form of the term "typical" is to be interpreted in the sense of being representative of common usage or practice, and is not to be interpreted as implying essentiality or invariability.

Claims

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

1. A serpentine structure made from a ductile material and having an upper end, a lower end, and a longitudinal axis, said serpentine structure comprising:

(a) a plurality of curved segments alternately arrayed on either side of the longitudinal axis, with each curved segment comprising an upper half and a lower half; and

(b) one or more straight segments, each of which contiguously joins two of the plurality of curved segments, said straight elements being bisected by the longitudinal axis into two portions, wherein:

(b.1) the portions of transverse straight segments contiguously joining the upper halves of curved segments are oriented at an acute positive angle; and

(b.2) the portions of the transverse straight segments contiguously

joining lower halves of curved segments are oriented at an acute negative angle.

2. A serpentine structure made from a ductile material and having an upper end, a lower end, and a longitudinal axis, said serpentine structure comprising a plurality of curved segments alternately arrayed on either side of the longitudinal axis, with each curved segment comprising an upper half and a lower half, wherein:

(a) the upper half of each curved segment contiguously joins the lower half of an adjacent curved segment at a point of inflection; and

(b) a tangent line at the point of inflection where the upper half of a first one of the plurality of curved segments contiguously joins the lower half of a second one of the plurality of curved segments is oriented at an acute positive angle on the side of the longitudinal axis corresponding to the upper half of said first curved segment, and at an acute negative angle on the side of the longitudinal axis corresponding to the lower half of said second curved segment.

3. A serpentine structure as in Claim 1 or Claim 2, further comprising mounting points at the upper and lower ends of the serpentine structure.

4. An energy-absorbing apparatus comprising two serpentine structures as in Claim 1 or Claim 2, wherein the upper ends of the two serpentine structures are connected to an upper mounting point, and the lower ends of the two serpentine structures are connected to a lower mounting point.

5. An energy-absorbing apparatus as in Claim 4 wherein the two serpentine structures are in laterally coplanar juxtaposition.

6. An energy-absorbing apparatus as in Claim 4 wherein the two serpentine structures overlie each other.

7. An energy-absorbing apparatus as in Claim 5 wherein the two serpentine structures are spaced apart from each other.

8. An energy- absorbing apparatus as in Claim 4 wherein the two serpentine structures are curved such that the energy-absorbing apparatus has a cylindrical configuration.