GB2470369A

GB2470369A - Energy absorbing apparatus comprising weakening feature to promote predetermined failure mode

Info

Publication number: GB2470369A
Application number: GB0908580A
Authority: GB
Inventors: Andrew Michael Selman
Original assignee: Individual
Current assignee: Individual
Priority date: 2009-05-19
Filing date: 2009-05-19
Publication date: 2010-11-24
Also published as: WO2010133878A1; GB0908580D0

Abstract

An energy absorbing apparatus, which is connectable to a structure, including: at least one weakening feature 20 adapted to promote failure at a predetermined peak load, thereby limiting the load transmitted to the structure during progressive failure of the energy absorbing apparatus. There may be a plurality of weakening features 20, which may be fold lines parallel to the impact loading or apertures, arranged in series to promote a progressive collapse failure mode. The energy absorbing apparatus may be a planar member having a corrugated profile. The apparatus may be used as decking to support an immobile installation or building, such as an oil rig. A method of protecting a structure using the energy absorbing apparatus is also disclosed.

Description

Energy Absorbing Apparatus The present invention relates to energy absorbing apparatus. In particular, but not exclusively, the invention relates to energy absorbing apparatus used on oil rigs.

Various apparatus are used to absorb impact energy, typically in moving objects such as vehicles. This typically involves structural members often formed from steel and having a consistent geometric profile. The structural members progressively collapse when impacted (or progressively crush in the case of composite structural members). The geometric profiles include cylinders, box beams, I sections and top hat sections. The structural members can be arranged as beams or columns with respect to the loading. As with any structural member, weakening features are generally avoided where possible to maximise the load capacity of the member.

When the impact loading is simple, and the geometry of the structural member is suitable, the impact behaviour of the structural members is fairly predictable and repeatable. During an impact, the load rises linearly as the structural member elastically deforms until a peak load is reached. This peak load corresponds to the yield stress of the material. However, the structural member can still absorb considerably more impact energy during the subsequent plastic deformation stage. An ideal structural member will progressively collapse in an accordion manner as multiple local regions of the member reach the yield stress and then form plastic hinges' about which the adjacent material can pivot, absorbing energy as it does so.

It is therefore important that the structural member behaves in this predictable manner involving local plastic deformations. For instance, a loaded column which fails in a different mode such as global buckling will absorb far less energy.

This can happen when the geometry of the structural member is unsuitable, such

I

A-

as when the thickness of the material is too great. It can also happen when the loading is more complex. For instance, a compressively loaded column which also receives a small lateral load along its length is more likely to fail in a global buckling mode.

It is desirable to provide energy absorbing apparatus in which the failure mode is more predictable and/or repeatable.

Energy absorbing apparatus are components of an overall structure and it is desirable that the energy absorbing apparatus can prevent or limit the impact load which is transmitted via the energy absorbing apparatus to the overall structure. The structure will also have members which will fail when the loading exceeds a peak loading of the members. It is therefore desirable that the energy absorbing apparatus can limit the impact load transmitted to the structure to a value less than the peak load of the members of the structure.

Oil rigs have a main deck and typically have one or more secondary decks mounted above the main deck. The decking typically has a corrugated profile and is supported by various beams and columns of the rig. On the rig, there are a limited number of support points for mounting the secondary decks. Structural members of the rig can be subject to a range of impacts including from dropped or swinging objects (such as containers), helicopter crashes, or blast loading in the event of an explosion. The impact loading can therefore be severe and complex involving multiple impact loads in multiple directions.

It has been found by the inventor that the secondary decks can be used as energy absorbing apparatus for absorbing impact loads and preventing these loads from being transmitted to the rest of the rig. This increases safety and provides cost savings since it is less expensive to repair or replace a secondary deck than the rig as a whole.

According to a first aspect of the present invention there is provided an energy absorbing apparatus which is connectable to a structure, the energy absorbing apparatus including: at least one weakening feature adapted to promote failure at a predetermined peak load, thereby limiting the load transmitted to the structure during progressive failure of the energy absorbing apparatus.

The predetermined peak load may be adapted to be greater than a peak load of the structure.

The weakening feature may be adapted to promote a desired failure mode.

The weakening feature may be adapted to limit the load transmitted to the structure to a load which is substantially the same order as the peak load during progressive failure of the energy absorbing apparatus.

The weakening feature may be adapted to limit the load transmitted to the structure to a load which is substantially no greater than the peak load during progressive failure of the energy absorbing apparatus.

The energy absorbing apparatus may include a plurality of weakening features.

The plurality of weakening features may be arranged in series to promote the failure mode of progressive collapse.

The weakening feature may comprise a fold line. The fold line may be provided at a portion of the energy absorbing apparatus which is substantially parallel to an impact loading.

Alternatively or in addition, the weakening feature may comprise one or more apertures. The or each aperture may be provided at a portion of the energy absorbing apparatus which is at or near a support of the energy absorbing apparatus provided by the structure.

Alternatively or in addition, the weakening feature may comprise one or more of a slot, recessed portion, crack, score line, tapered portion or the like.

The energy absorbing apparatus may comprise a planar member. The planar member may have a corrugated profile. Each corrugation of the planar member may comprise a top hat section.

The structure may comprise an immobile installation or building. The structure may comprise an oil rig. The planar member may comprise a decking member of the oil rig.

According to a second aspect of the present invention there is provided a method of protecting a structure from an impact, the method comprising: connecting an energy absorbing apparatus to the structure, providing the energy absorbing apparatus with at least one weakening feature adapted to promote failure at a predetermined peak load, thereby limiting the load transmitted to the structure during progressive failure of the energy absorbing apparatus.

The method may include adapting the energy absorbing apparatus such that the predetermined peak load is greater than a peak load of the structure.

The method may include adapting the weakening feature to promote a desired failure mode such as progressive collapse.

The method may include limiting the load transmitted to the structure to a load which is substantially the same order as the peak load during progressive failure of the energy absorbing apparatus.

The method may include limiting the load transmitted to the structure to a load which is substantially no greater than the peak load during progressive failure of the energy absorbing apparatus.

The method may include providing a plurality of weakening features. The plurality of weakening features may be arranged in series to promote the failure mode of progressive collapse.

The method may include providing means for preventing or delaying contact between a portion of the deforming energy absorbing apparatus and the structure. The means may comprise a spacer member.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a perspective view of an energy absorbing decking and it's support; Figure 2 is a graph of stress against strain for the material used to form the decking of Figure 1; Figure 3 is a finite element model of a portion of the decking and support of Figure 1; Figure 4 is the results of the finite element model of Figure 3 at a deflection of 0 mm with contact; Figure 5 is the results of the finite element model of Figure 3 at a deflection of 20 mm with contact; Figure 6 is the results of the finite element model of Figure 3 at a deflection of 162 mm with contact; Figure 7 is the results of the finite element model of Figure 3 at a deflection of 176 mm with contact; Figure 8 is the results of the finite element model of Figure 3 at a deflection of 198 mm with contact; Figure 9 is the results of the finite element model of Figure 3 at a deflection of 227 mm with contact; Figure 10 is the results of the finite element model of Figure 3 at a deflection of 250 mm with contact; Figure 11 is the results of the finite element model of Figure 3 at a deflection of 0 mm with no contact; Figure 12 is the results of the finite element model of Figure 3 at a deflection of mm with no contact; Figure 13 is the results of the finite element model of Figure 3 at a deflection of 78 mm with no contact; Figure 14 is the results of the finite element model of Figure 3 at a deflection of 143 mm with no contact; Figure 15 is the results of the finite element model of Figure 3 at a deflection of 207 mm with no contact; Figure 16 is the results of the finite element model of Figure 3 at a deflection of 250 mm with no contact; Figure 17 is a graph of force against deflection for the finite element model of Figure 3 with contact; Figure 18 is a graph of energy against deflection for the finite element model of Figure 3 with contact; Figure 19 is a graph of force against deflection for the finite element model of Figure 3 with no contact; and Figure 20 is a graph of energy against deflection for the finite element model of Figure 3 with no contact.

Figure 1 shows an energy absorbing apparatus in the form of decking 10 which provides a secondary deck mounted above the main deck on an oil rig. The decking 10 is planar and has a corrugated profile, each corrugation comprising an inverted top hat section 12. This top hat section 12 has a planar closing top surface 14, two side walls 16 and a bottom flange 18. The decking 10 is connected or mounted to a number of I section beams 100 of the rig which therefore support the decking 10, and the beams 100 are supported on a number of I section columns 102 which extend upwards from the main deck.

The decking lOis formed from steel. Figure 2 is a graph of the approximate stress against strain for the steel material. The stress rises linearly during elastic deformation until the yield stress of the material is reached. Subsequent to this is a plastic deformation stage in which the stress continues to rise linearly but at a significantly reduced rate.

This is the typical behaviour of steel material such as in a (tensile or compressive) coupon test. An actual structure will behave in a corresponding manner, particularly in the elastic phase, but the behaviour of the structure will also depend on geometric factors. In particular, the (post-collapse) behaviour in the plastic deformation stage may be quite different depending on the mode of failure.

The behaviour of the decking 10 subject to an impact has been modelled using finite element analysis to determine the loads which are transmitted to the rig (the beam 100 in the model). Due to symmetry of geometry and loading (assuming a uniform load applied to the top surface of the decking 10), and since only the load at the support is of interest, only a portion of the decking 10 needs to be modelled. Figure 3 is a finite element model of this portion of the decking 10 and beam 100.

Although not apparent in Figure 1, in can be seen in Figure 3 that each side wall 16 of the decking 10 includes a weakening feature in the form of a fold line 20.

This is provided to promote failure at a predetermined peak load, thereby limiting the load transmitted to the rig during progressive failure of the decking 10.

Referring again to Figure 1, it can be seen that the portion 30 of decking 10 directly above the column 102 also has a weakening feature in the form of a number of apertures 22. This is so that this portion 30 will fail at a lower load than the rest of the decking 10 and thus transmit less loading directly to the column 102.

Figures 4 to 10 show the results of the finite element analysis at a range of deflections. In this analysis, the beam 100 is included. A graph of the force on the beam against deflection is shown in Figure 17.

The load rises to the peak load 40 of around 60 kN at a deflection of only a few millimetres. At this point, a plastic hinge is formed at the fold line 20. Following this, the portions of the side walls 16 on either side of the fold line 20 progressively pivot towards each other. Plastic hinges also begin to form at the top and bottom portions of the side walls 16. The load gradually falls.

This continues until, at a deflection of about 150 millimetres, the folding middle portion of the side walls 16 makes contact with the top surface of the beam 100.

This provides a greater resistance and the load rises sharply to a maximum 42 of around 120 kN. Nevertheless, the decking 10 is still able to continue folding, absorbing further energy, and the load again falls during further deflection until the side walls 16 are substantially flattened as shown in Figure 10.

Although the maximum load 42 transmitted is greater than (about twice the value of) the peak load 40, it is in the same order of magnitude and the maximum load 42 only occurs after significant deflection.

Figure 18 shows that impact energy is absorbed in a fairly linear manner, which is generally desirable, although the rate of absorption increases following contact with the beam 100.

In an alternative embodiment, further fold lines could be provided. For instance, a fold line could be provided at a distance of three quarters of the length of the side walls 16 from the bottom flange 18 to promote folding following contact of the side walls 16 with the beam 100. This would reduce the load at this stage.

It is also possible to ensure that the side walls 16 do not contact the beam, or at least until significantly more deflection has occurred. For instance, spacers could be provided between the decking 10 and the beam 100. Figures 4 to 10 show the results of the finite element analysis at a range of deflections with the beam 100 excluded. This represents the situation when there is no contact with the beam 100, such as when spacers are used. A graph of the force on the beam against deflection is shown in Figure 19.

The load rises to the peak load 40 of around 65 kN at a deflection of only a few millimetres, which is similar to the previous case. A plastic hinge then forms at the fold line 20 and the portions of the side walls 16 on either side of the fold line then pivot towards each other. Plastic hinges also form at the top and bottom portions of the side walls 16. The load gradually falls.

Since no contact is made with the beam 100, the load continues to fall and then flattens out. The maximum load is therefore the peak load 40. From around 150 mm deflection, the folding side walls 16 fold below the top of the beam 100.

Figure 20 shows that impact energy is absorbed in a highly linear manner.

The present invention therefore provides means for limiting the loading transmitted to the rest of the structure. The geometry, material and loading conditions can be selected to give a predetermined peak load 40 (determined from experiment or modelling). This can be adapted to be substantially greater than a peak load of the structure and so the likelihood of failure of a component of the structure is significantly reduced.

The weakening feature of the invention also makes the desired failure mode significantly more likely. For realistic loadings, regardless of their complexity, initial failure is highly likely to be at the weakening feature. Further collapse behaviour is then far more likely to be predictable.

Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.

Claims

Claims 1. An energy absorbing apparatus which is connectable to a structure, the energy absorbing apparatus including: at least one weakening feature adapted to promote failure at a predetermined peak load, thereby limiting the load transmitted to the structure during progressive failure of the energy absorbing apparatus.
2. An energy absorbing apparatus as claimed in claim 1, wherein the predetermined peak load is adapted to be greater than a peak load of the structure.
3. An energy absorbing apparatus as claimed in claim I or 2, wherein the weakening feature is adapted to promote a desired failure mode.
4. An energy absorbing apparatus as claimed in any preceding claim, wherein the weakening feature is adapted to limit the load transmitted to the structure to a load which is substantially the same order as the peak load during progressive failure of the energy absorbing apparatus.
5. An energy absorbing apparatus as claimed in claim 4, wherein the weakening feature is adapted to limit the load transmitted to the structure to a load which is substantially no greater than the peak load during progressive failure of the energy absorbing apparatus.
6. An energy absorbing apparatus as claimed in any preceding claim, wherein the energy absorbing apparatus includes a plurality of weakening features.
7. An energy absorbing apparatus as claimed in claim 6, wherein the plurality of weakening features is arranged in series to promote the failure mode of progressive collapse.
8. An energy absorbing apparatus as claimed in any preceding claim, wherein the weakening feature comprises a fold line.
9. An energy absorbing apparatus as claimed in claim 8, wherein the fold line is provided at a portion of the energy absorbing apparatus which is substantially parallel to an impact loading.
10. An energy absorbing apparatus as claimed in any preceding claim, wherein the weakening feature comprises one or more apertures.
11. An energy absorbing apparatus as claimed in claim 10, wherein the or each aperture is provided at a portion of the energy absorbing apparatus which is at or near a support of the energy absorbing apparatus provided by the structure.
12. An energy absorbing apparatus as claimed in any preceding claim, wherein the energy absorbing apparatus comprises a planar member having a corrugated profile.
13. An energy absorbing apparatus as claimed in any preceding claim, wherein the structure comprises an immobile installation or building.
14. An energy absorbing apparatus as claimed in claim 13, wherein the structure comprises an oil rig.
15. A method of protecting a structure from an impact, the method comprising: connecting an energy absorbing apparatus to the structure, providing the energy absorbing apparatus with at least one weakening feature adapted to promote failure at a predetermined peak load, thereby limiting the load transmitted to the structure during progressive failure of the energy absorbing apparatus.
16. A method as claimed in claim 15, including adapting the energy absorbing apparatus such that the predetermined peak load is greater than a peak load of the structure.
17. A method as claimed in claim 15 or 16, including adapting the weakening feature to promote a desired failure mode such as progressive collapse.
18. A method as claimed in any of claims 15 to 17, including limiting the load transmitted to the structure to a load which is substantially the same order as the peak load during progressive failure of the energy absorbing apparatus.
19. A method as claimed in claim 18, including limiting the load transmitted to the structure to a load which is substantially no greater than the peak load during progressive failure of the energy absorbing apparatus.
20. A method as claimed in any of claims 15 to 19, including providing a plurality of weakening features.
21. A method as claimed in any of claims 15 to 20, including providing means for preventing or delaying contact between a portion of the deforming energy absorbing apparatus and the structure.