WO2018042206A1

WO2018042206A1 - Impact resistant structures

Info

Publication number: WO2018042206A1
Application number: PCT/GB2017/052574
Authority: WO
Inventors: Stephen John Kennedy
Original assignee: Intelligent Engineering (Bahamas) Limited; Leeming, John Gerard
Priority date: 2016-09-05
Filing date: 2017-09-05
Publication date: 2018-03-08
Also published as: GB2558528A; GB201615029D0

Abstract

A protective structure for protecting a protected structure from an impact load comprises: an impact receiving member configured to receive the impact load and formed by a structural sandwich plate member (10); and a collapsible structure (20) coupled to the impact receiving member at a plurality of locations and configured to undergo controlled progressive collapse under the impact load.

Description

IMPACT RESISTANT STRUCTURES

[ 0001 ] The present invention relates to impact resistant structures, in particular maritime and civil structures.

[ 0002 ] It is often a requirement of a structure that it be capable of withstanding impacts, the magnitude and direction of which will depend on the type of structure and its intended use. Examples of large impacts that a structure might need to be protected against include dropped objects and collisions by vehicles. Dropped objects are of concern in maritime and off-shore environments and building construction where assets and/or people need to be protected against the accidental release of heavy objects that are lifted to significant heights by cranes over a given area. Collisions from moving vehicles can be a particular problem in situations where heavy vehicles are moving at high speeds, such as in roads, or where very heavy vehicles are moving near costly and fragile infrastructure, such as in airports. The impact energy of an impact might be of the order of several hundred kiloJoules (kJ) or more.

[ 0003 ] Where damage to a structure in the event of an impact must be prevented, it is common to provide a protective structure between the protected structure and the potential impact. The protective structure is designed to absorb enough of the energy of the impact to prevent damage to the protected structure and to limit the force transmitted to the supporting structure. The protective structure may be damaged in the process of absorbing the impact energy and can therefore be regarded as a sacrificial structure. A protective structure may also be required to carry construction or operational loads.

[ 0004 ] Conventionally protective structures are stiffened steel structures. A steel plate or plates provides the primary protection from impacting objects and is strengthened by stiffeners to provide the required level of protection. However, the result is a complex structure that is heavy and vulnerable to puncture as a result of impacts adjacent to the stiffeners and to weld rupture, both of which could curtail the energy absorption capacity. Conventional protective structures must be replaced once damaged.

[ 0005 ] There is therefore a need for improved impact resistant structures.

[ 0006 ] According to an aspect of the invention, there is provided a protective structure for protecting a protected structure from an impact load, the protective structure comprising:

an impact receiving member configured to receive the impact load and formed by a structural sandwich plate member; and

a collapsible structure coupled to the impact receiving member at a plurality of locations and configured to undergo controlled progressive collapse under the impact load.

[ 0007 ] With such a construction, protection against impacts of given magnitude can be provided at lower weight and lower cost and with greater robustness against puncture. This is achieved through the combination of excellent membrane capacity in the structural sandwich plate member and the controlled progressive collapse mechanism of the collapsible structure. [0008 ] Embodiments of the invention will be described below with reference to the accompanying drawings, in which:

[0009] Figure 1 depicts in cross-section a structural sandwich plate member usable in an embodiment of the invention;

[0010 ] Figure 2 is a horizontal cross-section of a protective structure according to a first embodiment of the invention;

[0011] Figure 3 is a perspective view of the protective structure of Figure 2 installed to protect a pillar or column;

[0012 ] Figure 4 is a perspective view of the protective structure of Figure 2 illustrating the progressive collapse of the collapsible structure after an impact;

[0013] Figure 5 is a plan view of a protective structure according to a second embodiment of the invention;

[0014] Figure 6 is a cross-sectional view of a part of the protective structure of Figure 5 ;

[0015] Figure 7 is an enlarged cross-sectional view of a part of the protective structure of Figure 5;

[0016] Figure 8 is a further enlarged cross-sectional view of a part of the protective structure of Figure 5;

[0017 ] Figure 9 is a further enlarged cross-sectional view of a part of the support structure of the protective structure of Figure 5;

[0018 ] Figure 10 is a plan view of a protective structure according to a third embodiment of the invention;

[0019] Figure 11 is a cross-sectional view of a part of the protective structure of Figure 10.

[0020 ] In the Figures, like parts are indicated by like references.

[0021] An embodiment of the present invention provides a protective structure to protect a protected structure from impacts. A given embodiment may be designed to protect the protected structure from impacts having an impact energy up to a design impact energy, with a safety margin. The design impact energy may be 100 kJ or more, desirably 500 kJ or more. The protective structure comprises an impact receiving member formed by a structural sandwich plate member and a collapsible structure coupled to the impact receiving member at multiple locations and configured to undergo controlled progressive collapse under an impact load.

[0022 ] The present invention derives its advantageous performance under impact loads, compared to a stiffened steel structure of comparable weight, through a combination of the behaviour of the structural sandwich plate member in membrane action (plastic deformation within the plane of the plate rather than flexural action) without puncture or rupture and the progressive collapse of the collapsible structure. The progressive collapse of the collapsible structure absorbs energy from the impact whilst the structural sandwich plate member spreads the impact load to reduce force concentrations in the collapsible structure. In particular, the structural sandwich plate member has high puncture resistance. The structural sandwich plate member also protects against high strain rate loading and therefore reduces the likelihood of weld rupture. [ 0023 ] In addition to the ability to withstand higher impact loads than a stiffened steel structure of comparable weight, a protective structure according to an embodiment of the invention may be simpler and cheaper to construct than a stiffened steel structure.

[ 0024 ] An embodiment of the invention includes folded plate stiffening elements that minimise the number of welds. Advantageously the folded plate stiffening elements are located in areas where they will be less susceptible to high strain rate loading and rupture.

[ 0025 ] An embodiment of the invention includes Y-shaped steel stiffeners that are configured to collapse in different configurations depending on the impact direction. Such steel stiffeners can limit the force transmitted into the supporting structure. Such stiffeners can also promote the development of membrane action in the structural sandwich plate members to improve the energy absorption capacity of the system.

[ 0026 ] An embodiment of the invention includes steel hooks which are configured to enable the impact receiving structure to be made separately from the supporting or reacting structure so as to permit easy and quick replacement of the impact receiving structure after an impact event.

[ 0027 ] The structural sandwich plate members can be manufactured off-site in factory conditions to ensure reliability and predictable behaviour under load.

[ 0028 ] Protective structures according to the present invention can be employed in all types of civil and maritime applications where a structure requires protection from impacts. By providing a sacrificial structure that fully absorbs the impact energy, the integrity of the protected structure can be fully assured. The present invention is especially effective in protecting against impacts having an impact energy of 100 kJ or more. Examples of applications in which such impacts can occur include : off-shore structures where cranes operate over machinery, such as wellheads; airports where heavy planes must manoeuvre close to buildings and machinery such as jetties; and construction areas, where structural members such as steel beams or columns are lifted by cranes. Protective decks, especially in construction sites, may also be required to support static loads, such as stacked construction material, and moving loads, such as trucks and plant. The present invention can provide protection decks formed of units of standard sizes that are bolted together to maximise re-use possibilities.

[ 0029 ] Fig. 1 is a cross-sectional view of a structural sandwich plate member 10 that can be used in embodiments of the present invention. The structural sandwich plate member 10 comprises a first outer layer 1, an intermediate, or core, layer 2 and a second outer layer 3. The intermediate layer 2 is bonded to each of the first and second outer layers 1, 3 with sufficient strength to transfer shear loads between the outer layers so as to form a composite structural member capable of bearing loads significantly greater than self-weight.

[ 0030 ] The first and second outer layers 1, 3 are made of metal and the intermediate layer 2 is made of a plastic or elastomeric material. The absolute and relative dimensions of the structural sandwich plate member 10 and the precise materials employed will depend on the application to which the member is to be put and in general may be as described in US-5,778,813 and US-6,050,208. Steel or stainless steel may be used in thicknesses of 3 to 30 mm. Similarly, the plastics or polymer core may be any suitable material, for example an elastomer such as polyurethane, as described in US-5,778,813 and US-6,050,208 and is preferably compact, i.e. not a foam. The core is preferably a thermosetting material rather than thermoplastic. The intermediate layer may have a thickness of 5 mm to 200 mm, desirably 10 to 100 mm.

[ 0031 ] Desirably, the intermediate layer has a modulus of elasticity, E, of at least 250 MPa, preferably 275 MPa, at the maximum expected temperature in the environment in which the member is to be used. In maritime applications this may be 100 °C. Desirably, the elastomer is not too stiff so that E should be less than 2,500 MPa at the lowest expected temperature, -40 or -45 °C in maritime applications.

[ 0032 ] The tear, compression and tensile strengths as well as the elongation are desirably maximised to enable the structural sandwich plate member to absorb energy in unusual load events, such as impacts. In particular, the compressive and tensile strengths of the elastomer should be at least 20, and preferably 40, MPa. The compressive strengths can, of course, be considerably greater than these minima.

[ 0033 ] The metal layers are preferably structural steel though may also be aluminium, stainless steel or other structural alloys in speciality applications where lightness, corrosion resistance or other specific properties are essential. The metal should preferably have a minimum yield strength of 240 MPa and an elongation of at least 20%. For many applications, especially shipbuilding, it is desirable that the metal is weldable.

[ 0034 ] The ductility of the elastomer at the lowest operating temperature is desirably greater than that of the metal layers, which is about 20%. A preferred value for the ductility of the elastomer at lowest operating temperature is 50%. The thermal coefficient of the elastomer is desirably sufficiently close to that of the steel so that temperature variation across the expected operating range, and during welding, does not cause delamination. The extent by which the thermal coefficients of the two materials can differ will depend in part on the elasticity of the elastomer but it is believed that the thermal expansion coefficient of the elastomer may be about 10 times that of the metal layers. The coefficient of thermal expansion may be controlled by the addition of fillers to the elastomer.

[ 0035 ] The bond strength between the elastomer and metal layers is desirably at least 3 MPa, preferably 6 MPa, over the entire operating range. This is preferably achieved by the inherent adhesiveness of the elastomer to steel but additional bond enhancers may be provided.

[ 0036 ] In an embodiment, the elastomer essentially comprises a polyetherol (e.g. polyester or polyether) together with an isocyanate or a di-isocyanate, a chain extender. A filler can be provided, as necessary, to reduce the thermal coefficient of the intermediate layer, reduce its cost and otherwise control the physical properties of the elastomer. Further additives, e.g. to control hydrophobicity or adhesion, and fire retardants may also be included.

[ 0037 ] The ratio of the total thickness of the outer layers to the thickness of the elastomer, (Tl + T3) / T2, is in the range of from 0.1 to 2.5. [ 0038 ] Coatings, e.g. for cosmetic or corrosion resistance reasons, may be applied to the outer surfaces of the metal layers either before or after fabrication of the laminate.

[ 0039 ] The intermediate layer 2 may also include a plurality of hollow box-shaped forms enclosing voids. The size and material of the forms are chosen so that the overall density of the forms is lower than the density of the material of the intermediate layer 2, preferably less than 50% of the density of the material of the intermediate layer 2, or preferably less than 25% and most preferably less than 10%. One purpose of the forms is to take up space within the intermediate layer 2 and thus reduce the amount of the main core material required whilst maintaining or even increasing the desired spacing between first and second outer layers 1, 3. This reduces cost both directly as the forms are less expensive by volume than the main core material and secondly because the weight of the panels is reduced. The forms do not need to contribute to the overall structural strength of the structural sandwich plate member 10 but if structural sandwich plate member 10 is formed by injection of the intermediate layer 2, the forms must have physical properties sufficient to withstand pressures and temperatures arising during casting and curing of the intermediate layer 2.

[ 0040 ] The size, shape and distribution of forms within the intermediate layer 2 is chosen so that a sufficient number of ribs and/or columns of main core layer material extend between and bond to first and second outer layers 1, 3 at regular intervals across the length and width of the structural sandwich plate member 10. The forms do not have to be hollow, e.g. if made of a suitable lightweight material such as a foam, or may be filled with lightweight material, which may be insulating and/or fire resistant. A particularly useful material for the forms is expanded polystyrene, having a density of 20 to 40 kg/m³, which may be provided, e.g., either as spheres or ribs. Another particularly useful type of lightweight form is hollow plastic balls, e.g. of diameter similar to the spacing between plates.

[ 0041 ] In an embodiment of the invention, one or more deformable steel boxes are provided in the intermediate layer 2. Such a box can contribute to the energy absorption capacity and puncture resistance of the structural sandwich plate member.

[ 0042 ] The structural sandwich plate member 10 is substantially stronger and stiffer than a member of the same thickness of metal but no intermediate layer. This is because the member acts in an analogous manner to a box girder or I-beam with the intermediate layer performing the function of the web(s). To so function, the intermediate layer itself and the bonds to the outer layers must be sufficiently strong to transfer the forces that will arise in use of the member.

[ 0043 ] Another advantageous property of such a structural sandwich plate member, is that the intermediate layer acts to prevent crack propagation between the inner and outer layer. The elasticity of the intermediate layer prevents the stress concentration at the tip of a crack in one outer layer being transmitted to the other as a structure made of a single material may do.

[ 0044 ] Figure 2 depicts an embodiment of the invention in which a protective structure, comprising impact receiving member 10 and collapsible structure 20, protects a pillar or column 30 as a protected structure. In plan view, impact receiving member 10 has the shape of an arc, e.g. a semicircle, concentric with the pillar 30. In an embodiment, a plurality of impact receiving members are provided to protect the protected structure from a wider range of angles than is possible with a single impact receiving member. In an embodiment, an impact receiving member completely surrounds the protected structure. The radius Ri of the impact receiving member is desirably as small as possible so that the protective structure is not unnecessarily large but it needs to be sufficient to accommodate the plastic deformation of the impact receiving structure .

[ 0045 ] Impact receiving member 10 comprises a structural sandwich plate member formed by first and second outer layers 1, 3 and intermediate layer 2. First and second outer layers may be made of steel and have thicknesses Ti, T3 in the range of from 3 to 30 mm. Thicknesses Ti, T3 need not be the same. Intermediate layer 2 may have a thickness T₂ in the range of from 5 to 50 mm. Desirably, T₂ is greater than each of Ti and T3.

[ 0046 ] In this embodiment, collapsible structure 20 comprises a plurality of collapsible members 21 formed of folded or bent metal, e.g. steel. Collapsible members 21 may alternatively be formed by extrusion. In cross-section, each collapsible member 21 resembles a V formed by first branch 21c and second branch 21d, with a third branch 21b extending from the end of the first branch 21c. A hook 2 la is formed at the end of third branch 2 lb. The fold of the V, i.e. the junction between first branch 21c and second branch 2 Id, is welded to outer layer 3 of impact receiving member 10, on the inner side of the arc. The free end of second branch 2 Id of each collapsible member 21 is welded to an adjacent collapsible member 21, e.g. at the fold between first branch 21c and third branch 21b. The welds between the collapsible members and between the collapsible members and the impact receiving member 10 can be continuous full penetration double-sided fillet welds or intermittent double-sided fillet welds.

[ 0047 ] In this embodiment, the complete collapsible structure 20 comprises a corrugated structure (the V shaped parts of the collapsible members 21) fixed to the impact receiving member 10 at the tips of one set of corrugations and provided with attachment portions (the third branches 21b and hooks 21a) projecting from the other set of corrugations. It will be appreciated that this structure can be formed using different combinations of members, e.g. Y-shaped members. The collapsible members 21 are simple and therefore easy to manufacture. The collapsible structure can be formed from a plurality of identical members. In the illustrated embodiment, two types of collapsible member 21 are provided, differing only in the direction of the hook 21a. Minimising the number of different parts required reduces cost.

[ 0048 ] End plates 22, 23 are provided to close the ends of the collapsible structure and to ensure it has consistent structural properties. End plate 22 has a hook at its distal end and is welded to the free end of second branch 2 Id of one outermost collapsible member 21. End plate 23 extends only to the fold between first branch 21c and third branch 21b of the other outermost collapsible member 21, to which it is welded. End plates may not be necessary if the protective structure forms a complete ring about the protected structure (pillar or column 30). [ 0049 ] Figure 3 is a perspective view of the protective structure in situ around pillar 30. It will be seen that the hooks 21a engage with channels 25 provided on reaction structure 24. Reaction structure 24, which can also function as a support structure, is an arcuate steel body, or steel-concrete sandwich, firmly anchored to the ground. The protective structure can be manufacture off-site and simply slotted into the channels of the reaction structure 24.

[ 0050 ] Figure 4 depicts the effect of an impact I in the location indicated. Impact I might be caused by a collision with a low speed but high mass vehicle, such as an aircraft tug. It will be seen that the impact receiving member 10 is deformed but not punctured or ruptured. The impact energy has been spread around the collapsible structure and most if not all of the collapsible members 21 have been deformed, contributing to the absorption of the impact energy. Deformation of the collapsible members 21 primarily occurs in third branches 21b and hooks 21a, whose behaviour is predictable. The number and thickness of the collapsible members 21 can be varied according to the design impact energy to be absorbed.

[ 0051 ] After a collision, the protective structure can be replaced swiftly. The reaction structure 24 is undamaged. Some of the channels 25 may need to be replaced. In any event, the downtime, i.e. the time during which the protected structure is vulnerable, is minimised.

[ 0052 ] Figures 5 and 6 depict in plan and cross-section respectively a protective structure according to an embodiment of the invention for protecting a maritime structure. Figures 7, 8 and 9 show further details of this structure. In Figure 9, part of the collapsible structure is shown without the impact receiving member and the support structure. In this example, the protective structure forms an impact protection deck 100 for the well bay deck area of a Tension Leg Platform (TLP) structure. Impact protection deck 100 is a relatively shallow structure that can be mounted directly on a surface to be protected, e.g. the main deck of an off-shore structure. The impact receiving member of impact protection deck comprises a plurality of structural sandwich plate members, as described further below. The collapsible structure comprises a plurality of channels to which the structural sandwich plate members are welded. The collapsible structure is attached to the main deck which is both the protected structure and the support structure. The impact protection deck 100 provides improved puncture resistance compared to a stiffened steel structure, which is highly desirable given the vulnerable equipment to be protected.

[ 0053 ] The structural sandwich plate members forming the impact receiving structure include three types of interchangeable prefabricated modular panels: protective hatch covers 120, transverse deck panels 1 10 and edge deck panels 130, as illustrated in Fig. 5. Protective hatch covers 120, transverse deck panels 1 10 and edge deck panels 130 together provide an impact receiving member designed in particular to protect against dropped loads from cranes operating above the well bay deck area.

[ 0054 ] Protective hatch covers 120 are formed of structural sandwich plate members 10 as described above. In an embodiment, the protective hatch covers 120 have outer layers 1, 3 formed of 6 mm steel plate and an intermediate layer 2 of 25 mm elastomer. Protective hatch covers 12 are located directly above the well bay hatch covers in the main deck and provide access to the well bay hatch covers and well heads below.

[ 0055 ] Transverse deck panels 1 10 and edge deck panels 130 are fixed and box in the protective hatch covers 120 to provide restraint from lateral movement. Transverse deck panels 110 are formed of structural sandwich plate members 10 as described above. In an embodiment, the transverse deck panels 1 10 have outer layers 1, 3 formed of 8 mm steel plate and an intermediate layer 2 of 19 mm elastomer. Edge deck panels 130 are formed of structural sandwich plate members 10 as described above. In an embodiment, the edge deck panels 130 have outer layers 1, 3 formed of 6 mm steel plate and an intermediate layer 2 of 25 mm elastomer.

[ 0056 ] The various deck panels forming the impact receiving structure are welded to a collapsible structure 200 formed of a perforated cold-formed channel steel frame formed by channels 202, 203. The collapsible structure 200 is configured to collapse in a prescribed manner to absorb energy and limit force transfer to the main deck. Apertures 203a in the webs of channels 202, 203 are one method to control the collapse of the channels and the force transfer to the supporting structure 300.

[ 0057 ] As shown in Figures 8 and 9, channels 202, 203 bear on a thick resilient pad 205, e.g. of EPDM, to accommodate variations in the flatness of the bottom flange of the channels and the main deck. The resilient pad can have a thickness in the range of from 10 to 50 mm. The resilient pad desirably covers substantially the entire width and length of the bottom flange of the channels 202, 203.

[ 0058 ] As shown in Figures 7 and 9, channels 202, 203 are provided with cantilevered tapered ends 203b at the corners of the panels to which they are attached. At the distal ends of the tapered ends 204, the depth of the web of the channel may be between 20% and 50% of the depth of the web of the channel in the untampered part. A diagonal plate 204 connects the webs of the two perpendicular channels and is not connected to the deck panels. This arrangement provides a soft corner that eliminates hard spots associated with the intersections of two or more panels. The soft corner is therefore ductile and deforms plastically under impact loads.

[ 0059 ] The soft corner provides a collapsible feature for adjacent panels coming together at a common point (e.g. A and B in Figure 6). It is therefore possible to prevent the corner being too stiff, limit the load transfer and reduce the vulnerability of the structural sandwich plate members to puncture by allowing them to develop membrane action. Where two hatch cover panels meet in the middle of the edge of a transverse panel (B), the combination of the two soft corners and the channel with openings along the long edge permits a progressive collapse of the channel at the point of impact and the development of membrane action of the structural sandwich plate members acting as a cantilever off the soft noses.

[ 0060 ] The impact protection deck 100 is configured to support specified loads associated with both the static operation loads and accidental (extreme) loads from dropped objects. The impact protection deck 100 can prevent rupture of the main deck from critical dropped objects. [ 0061 ] The provision of three different panel types (hatch cover, transverse and edge) enables the impact receiving member to be configured to fit required areas, such as the well bay area. As the panels are interchangeable, damaged panels can be replaced with another panel from a non-critical area to maintain operations until a replacement panel can be provided. The protective hatch covers 120 are removable and provide full access to well bay hatch covers in the main deck. The edge and transverse panels 1 10, 130 are fixed to the main deck to restrict the movement of protective hatch covers 120 for sea and operational loads.

[ 0062 ] In an embodiment, the various panels (transverse panels 1 10, edge panels 130 and protective hatch covers 120) of the impact receiving member are configured to provide a puncture impact resistant plating with excellent membrane capacity. The collapsible structure 200 is configured to provide: (1) a controlled progressive collapse mechanism for impact loads, and (2) soft support at the corners of the panels and panel junctions while maintaining the structural capacity for operational loads. [ 0063 ] Another embodiment of the invention is depicted in Figures 10 and 1 1. This is a raised protective structure which can sit on a deck or other generally flat surface. The protective structure comprises an impact receiving structure, formed by structural sandwich plate members 1 10, supported by a supporting structure 200, formed by steel beams 201 sitting on collapsible W-stubs 210.

Structural sandwich plate members 1 10 include perimeter bars 1 1 1 which are provided with bolt holes to enable the structural sandwich plate members to be bolted to steel beams 201.

[ 0064 ] Stubs 210 are designed to deform/collapse (which slows the object down through a prescribed distance) and limit the force transfer to the supporting structure below. This protection deck configuration can be used for offshore structures or for buildings as a construction protection deck. The W-shape stubs 210 generally are located on top of columns or piles and are configured not to transfer any force greater than the capacity of the supporting structure below.

[ 0065 ] The embodiment of Figures 10 and 11 may incorporate the soft corner feature of the embodiment of Figures 5 to 9.

[ 0066 ] Having described exemplary embodiments of the invention, it will be appreciated that modifications and variations of the described embodiments can be made. The invention is not to be limited by the foregoing description but only by the appended claims.

Claims

1. A protective structure for protecting a protected structure from an impact load, the protective structure comprising:

2. A protective structure according to claim 1 wherein the collapsible structure comprises a plurality of folded metal members.

3. A protective structure according to claim 2 wherein the folded metal members are welded to the impact receiving member at a fold line.

4. A protective structure according to claim 3 wherein a free edge of a folded metal member is welded to another metal member.

5. A protective structure according to claim 1 wherein the impact receiving member comprises a plurality of prefabricated modular panels.

6. A protective structure according to claim 5 wherein the prefabricated modular panels are interchangeable.

7. A protective structure according to claim 5 or 6 wherein there are from 2 to 10 different types of prefabricated modular panels.

8. A protective structure according to claim 5, 6 or 7 wherein the prefabricated modular panels include removable panels.

9. A protective structure according to claim 5, 6, 7 or 8 wherein the prefabricated modular panels include fixed panels that are welded to the collapsible structure.

10. A protective structure according to claim 9 wherein the fixed panels restrain the movable panels against lateral movement.

11. A protective structure according to any one of claims 5 to 10 wherein the collapsible structure comprises a perforated cold-formed steel frame.

12. A protective structure according to claim 11 wherein the perforated cold-formed steel frame comprises a cantilever section to support a corner of a panel.

13. A protective structure according to claim 12 wherein the cantilever section comprises a channel that tapers toward the corner of the panel.

14. A protective structure according to any one of claims 5 to 13 wherein the collapsible structure comprises a plurality of stubs configured to deform under a predetermined load to as to limit force transfer into a supporting structure.

15. A protective structure according to any one of the preceding claims wherein the protective structure is configured to absorb impacts having impact energy up to a design impact energy, the design impact energy being greater than 100 kJ.

16. A protective structure according to any one of the preceding claims wherein the structural sandwich plate member comprises first and second outer layers of metal and an intermediate layer of elastomer that bonds to the outer layers to transfer shear forces therebetween.

17. A protective structure according to claim 14 wherein the first and second outer layers have a thickness in the range of from 0.5 mm to 20 mm, desirably 2 to 20 mm.

18. A protective structure according to claim 14 or 15 wherein the intermediate layer has a thickness in the range of from 5 to 200 mm, desirably 10 to 100 mm.

19. A protective structure according to any one of the preceding claims wherein the collapsible structure is connected to the protected structure.

20. A protective structure according to any one of claims 1 to 16 wherein the collapsible structure is connected to the ground.

21. A civil engineering structure comprising a protective structure according to any one of the preceding claims.

A maritime structure comprising a protective structure according to any one of claims 1 to