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(11) | EP 4 431 671 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A TRUSS FOR AN OFFSHORE STRUCTURE AND A METHOD OF ASSEMBLING THE TRUSS |
| (57) A truss for an offshore structure (1) comprises a plurality of beams (3). At least
one of the beams (3) includes a first hole (5) and an other structural element (4)
of the truss includes a second hole (6). The at least one of the beams (3) and the
other structural element (4) are connected to each other through a metal insert (7)
in the first and second holes (5, 6). The insert (7) forms an interference fit with
at least one of the at least one of the beams (3) and the other structural element
(4). The at least one of the beams (3) is made of aluminium, an aluminium alloy or
a reinforced plastic composite.
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[0001] The present invention relates to a truss for an offshore structure, comprising a plurality of beams.
[0002] Such a truss or space frame is known in the prior art. In the known truss the beams comprise pipes of (stainless) steel which are welded to each other at nodes where several beams meet each other. The outer surface of the known truss is treated in order to minimize corrosion under offshore conditions, for example by means of a coating.
[0004] For this purpose, in the truss according to the invention at least one of the beams includes a first hole and an other structural element of the truss includes a second hole, wherein the at least one of the beams and the other structural element are connected to each other through a metal insert in the first and second holes, which insert forms an interference fit with at least one of the at least one of the beams and the other structural element, wherein the at least one of the beams is made of aluminium, an aluminium alloy or a reinforced plastic composite.
[0005] An advantage of the truss according to the invention is that the mentioned materials of the at least one of the beams do not need an anti-corrosion treatment. For example, aluminium forms an outer skin of aluminium oxide which provides a natural corrosion protection; for example, aluminium alloy 6082 has a high corrosion resistance. Furthermore, connecting the at least one of the beams and the structural element through the metal insert avoids local weakening of the material of the at least one of the beams and the other structural element such as in case of welding. Besides, welding may lead to increase of material fatigue because of generating internal cracks caused by the welding process. This is typically disadvantageous in case of a floating offshore structure where waves generate cyclic forces on the structure. Fatigue capacity is relevant for an offshore structure because of its cyclic movement caused by waves during its lifetime of for example 20-25 years. Other dynamic load effects may be caused by wind, temperature fluctuations, eigenfrequencies or other impact from outside.
[0006] It is also noted that an interference fit provides residual stress of the insert onto material around the insert, which improves fatigue capacity. An interference fit avoids relative motion or clearance between the insert and the at least one of the at least one of the beams and the other structural element. In case of using an insert which fits in the first and/or second hole, but which does not form an interference fit, there may still be clearance or clearance may arise during lifetime.
[0007] It is noted that all beams or at least most of the beams of the truss are fixed to each other through respective inserts in a similar way as described above.
[0008] When the beams are made of aluminium or an aluminium alloy, bolting the at least one of the beams and the other structural element together would require strong bolts, for example made of steel. This makes the application of an aluminium beam less attractive since in offshore applications this would require galvanic separation, hence increasing costs. Besides, it would require repeated checks during the lifetime of the offshore application whether pretension of the bolts is still sufficient. Furthermore, in case of using bolts a higher number of them should be used than the number of inserts in case of applying interference fits. Aluminium bolts are undesired because of limited strength thereof.
[0009] The mentioned materials have a relatively low weight which is advantageous when transporting a plurality of such beams to an offshore site where the truss must be assembled. The beams may be transported by using standard transportation methods such as 40ft containers, for example.
[0010] The at least one of the beams may be made of an extruded aluminium or an extruded aluminium alloy in which the first hole is machined. This means that the hole is machined into the at least one of the beams after it has been extruded. An advantage of making the at least one of the beams of an extruded aluminium or an extruded aluminium alloy is that it provides a relatively high strength to weight ratio. It may have a very high yield strength, for example 260MPa. Extrusions can be manufactured in complex shapes that can be optimized easily to allow for optimal material usage. It is undesired to weld extruded parts because of resulting loss of strength or distortion.
[0011] If the at least one of the beams comprises a reinforced plastic composite it may be made of a fibre reinforced composite.
[0012] Preferably, the insert forms an interference fit with both the at least one of the beams and the other structural element in order to restrain all degrees of freedom between the at least one of the beams and the other structural element. It is also possible that the insert forms an interference fit with the at least one of the beams, the other structural element and still one or more additional structural elements.
[0014] The other structural element may comprise a non-extruded body. Such a body may form a node member to which at least two beams of the truss are mounted. It is also conceivable that the other structural element comprises an other beam, which is possibly similar to the at least one of the beams. In the latter case a plurality of similar beams may meet each other at a node of the truss.
[0016] If the at least one of the beams, the other structural element and the insert are made of aluminium or an aluminium alloy, the assembly of these parts is highly corrosion resistant.
[0017] The insert may comprise a hollow pin. It may be tubular including a rectangular cross-section, a circular cross-section, an oval cross-section or the like.
[0018] In a particular embodiment the insert is a first insert which forms an interference fit with the at least one of the beams and the other structural element, wherein the at least one of the beams includes a third hole at a distance from the first hole and the other structural element includes a fourth hole at a distance from the second hole, wherein the at least one of the beams and the other structural element are connected to each other through a metal second insert in the third and fourth holes, which second insert forms an interference fit with the at least one of the beams and the other structural element. A series of two interference fits provides a strong structural joint since the first insert as well as the second insert transfer forces between the at least one of the beams and the other structural element. If the inserts did not form interference fits, it would be possible that only one of the first and second inserts would transfer forces between the at least one of the beams and the other structural element. The second insert may have the same properties as the first insert.
[0019] The invention is also related to a method of assembling a truss as described hereinbefore, comprising the steps of supplying the metal insert, the at least one of the beams and the other structural element, positioning the at least one of the beams and the other structural element such that the first and second holes are aligned with respect to each other, cooling the insert and/or heating at least one of the at least one of the beams around the first hole and the other structural element around the second hole, and subsequently inserting the insert into the first and second holes, after which thermal equilibration is allowed so as to create the interference fit with at least one of the at least one of the beams and the other structural element. Introducing the insert into the first and second holes may be performed at relatively high speed in order to prevent the insert from being prematurely stuck in at least one of the first and second holes.
[0020] In a particular embodiment the first and second holes are through-holes which are aligned with respect to each other through introducing a guide pin into the respective first and second holes, which guide pin slidably fits in the first and second holes. It is conceivable to heat the guide pin before introducing it into the respective first and second holes.
[0021] During inserting the insert into the first and second holes the guide pin may be moved in the same direction as the insert away from the first and second holes.
[0022] In an advantageous embodiment the guide pin is pushed by the insert, since the removal of the guide pin and the introduction of the insert is performed simultaneously which facilitates maintenance of alignment of the first and second holes.
[0023] In the event that the insert is cooled, a thermal insulator may be applied between the insert and the guide pin or the guide pin may be made of a material which has a lower heat conductivity than the metal of the insert. This minimizes heat transfer from the guide pin to the insert, thus avoiding too fast heating of the insert which could lead to creating a premature interference fit.
[0024] The guide pin may have a lower thermal expansion rate than the insert such that in case of heat transfer from the guide pin to the insert, change of dimensions of the guide pin will be limited.
[0025] The invention will hereafter be elucidated with reference to the schematic drawings showing an embodiment of the invention by way of example.
Fig. 1 is a perspective view of two platforms, each having an embodiment of a truss according to the invention.
Fig. 2 is an enlarged part of Fig. 1, which is indicated by II in Fig. 1.
Fig. 3 is a sectional view along line III-III in Fig. 2 at a larger scale.
Figs. 4-8 are similar views as Fig. 3, illustrating steps of an embodiment of a method of assembling a truss according to the invention.
[0026] Fig. 1 shows a part of an offshore floating solar farm, which comprises a plurality of interconnected triangular platforms 1 including floating members 2. Each of the platforms 1 comprises a space frame or truss which supports a number of solar panels (not shown). Fig. 2 shows a part of the truss in more detail. It comprises beams 3 and other structural elements in the form of node members 4 to which several beams 3 are connected. In an alternative embodiment the beams 3 may be coupled directly to each other.
[0027] In the embodiment as shown in Figs. 1 and 2 the beams 3 are made of extruded aluminium and each of the node members 4 comprises a machined body of an aluminium alloy, for example 5083 or 6082. Fig. 2 shows that four beams 3 in a horizontal plane are coupled to the node member 4 and four beams 3 are coupled to the node member 4 by an angle to the horizontal plane, but this may be a different number at other node members 4.
[0028] Fig. 3 shows a connection between one of the node members 4 and one of the horizontally oriented beams 3. The node member 4 includes two pairs of first holes 5, being through-holes, and the beam 3 includes two pairs of second holes 6, being through-holes. Each pair of first holes 5 are located in opposite walls of the node member 4 and aligned with respect to each other. Similarly, each pair of second holes 6 are located in opposite walls of the beam 3 and aligned with respect to each other. The pairs of second holes 6 are machined in the beam 3 after it has been extruded. The beam 3 and the node member 4 are connected to each other through inserts 7 which are mounted in the pairs of first and second holes 5, 6. Each of the inserts 7 forms an interference fit with the beam 3 and the node member 4. In this case each of the inserts 7 comprises a tubular pin 7 having a circular cross-section. The inserts 7 are made of an aluminium alloy. They may be made by means of extrusion and machined afterwards. Machining provides the opportunity to obtain accurate dimensions of the insert 7. It is noted that two interference fits in series guarantee that both inserts 7 transfer forces between the beam 3 and the node member 4.
[0029] In order to prevent the truss against galvanic corrosion the potential difference between the metals of the node members 4, the beams 3 and the inserts 7 at their contact surfaces is preferably as small as possible, for example smaller than 0.1 V.
[0030] Referring to Fig. 2, the four beams 3 which are coupled to the node member 4 by an angle to the horizontal plane pass through respective holes in an upper wall of the node member 4. This means that pairs of two beams 3 may be fixed to the node member 4 via respective inserts 7 at different location of the node member 4. Each of the inserts 7 may form interference fits with opposite walls of the node member 4 and opposite walls of two beams 3. Numerous alternative configurations are conceivable.
[0031] Figs. 4-8 show successive steps of an embodiment of a method of assembling one of the beams 3, the insert 7 and the node member 4. In order to assemble the truss these steps are repeated at all node members 4 of the truss.
[0032] Fig. 4 shows that the node member 4 and the beam 3 are positioned such that the pair of first holes 5 and the pair of second holes 6 are aligned with respect to each other. A guide pin 8 is inserted into the pair of first holes 5 and the pair of second holes 6 in direction X in order to obtain and maintain a proper alignment. The guide pin 8 fits slidably in the pair of first holes 5 and the pair of second holes 6. A leading end of the guide pin 8 is chamfered in order to avoid damage of the pair of first holes 5 and the pair of second holes 6 during insertion. A trailing end of the guide pin 8 is provided with a thermal insulator 9 and a protrusion 10 projects from the trailing end and is directed opposite to the direction X. The guide pin 8 may be made of stainless steel.
[0033] Fig. 5 shows that the insert 7 is mounted on a core 11. A leading end of the core 11 has a hole 12 in which the protrusion 10 of the guide pin 8 fits. Before the condition as shown in Fig. 5 the core 11 and the insert 7 are cooled in a liquid nitrogen bath (not shown) such that they are shrunk such that the outer diameter of the insert 7 is slightly smaller than the diameters of the pair of first holes 5 and the pair of second holes 6. Fig. 5 shows that the leading end of the core 11 and a leading end of the insert 7 contact the thermal insulator 9. The protrusion 10 and the hole 12 ensure proper alignment of the guide pin 8 and the core 11 including the insert 7.
[0034] The core 11 is preferably made of the same material as the insert 7 in order to have the same thermal expansion coefficient. Alternative materials are conceivable, for example titanium, nickel or alloys thereof, or non-metallic materials such as POM or PTFE. The protrusion 10 of the guide pin 8 has a relatively small diameter; the ratio between its length and diameter may lie between 2 and 4. This gives a better fit in the hole 12 because of relatively low shrinkage of the cooperating hole 12 in the core 11. Besides, a minimum heat transfer from the guide pin 8 to the core 11 is achieved. Further heat loss may be achieved by necking and/or hollowing the protrusion 10.
[0035] The leading end of the insert 7 is chamfered in order to avoid damage of the pair of first holes 5 and the pair of second holes 6 during inserting it. Similarly, edges of the holes 5 and 6 may also be chamfered.
[0036] It is advantageous when the material of the insert has a relatively high coefficient of linear thermal expansion, for example larger than 15.10-6 K-1 at 20°C, since a relatively small temperature decrease already provides a significant shrinkage.
[0037] Fig. 6 shows that the core 11 and the insert 7 push the guide pin 8 in direction X such that the guide pin 8 slides through the pair of first holes 5 and the pair of second holes 6 and the insert 7 is inserted into the pair of first holes 5 and the pair of second holes 6 simultaneously. The displacement terminates when the leading end of the guide pin 8 reaches a stopping member 13. The distance between the stopping member 13 and the node member 4, on the one hand, and the lengths of the insert 7 and the guide pin 8, on the other hand, are selected such that the insert 7 projects at opposite sides of the node member 4. This provides a tolerance to prevent the insert 7 from being inserted too far inside one of the pair of first holes 5, since edge loading of the insert 7 is undesired. Besides, the projections at opposite sides of the node member 4 may be used for other functions, for example for temporarily connecting tools or as lifting pins of the truss. Furthermore, it is relatively easy to machine the insert 7 out from the beam 3 and the node member 4, if necessary.
[0038] After inserting the insert 7 it will be heated up due to thermal equilibration, hence resulting in radial expansion of the insert 7. Consequently, the insert 7 will form an interference fit in the pair of first holes 5 of the node member 4 and the pair of second holes 6 of the beam 3.
[0039] Fig. 7 shows that the core 11 is retracted from the insert 7, whereas the insert 7 remains in the node member 4 and the beam 3. Preferably, the core 11 is removed as soon as possible such that it can be put into the liquid nitrogen bath again, for example before terminating the step of thermal equilibration.
[0040] Fig. 8 shows a final condition of the assembly of the beam 3, the node member 4 and the insert 7, in which the guide pin 8 is also removed.
[0041] Instead of or in addition to cooling the insert 7, it is also possible to heat the node member 4 and the beam 3 at least around the pair of first holes 5 and the pair of second holes 6, but this is less preferred because of the relatively high heat conductivity of the applied metals, i.e. more than 75 W/(mK). This makes local heating inefficient.
[0042] Furthermore, it is possible to check the tolerances of the aligned pairs of first holes 5 and the pair of second holes 6 by means of a fitting pin (not shown) before inserting the guide pin 8. It may also be used as a pre-alignment tool. In practice a plurality of fitting pins of different diameters may be applicable in order to obtain an indication what cooling temperature of the insert 7 and the core 11 is appropriate. Since the thermal expansion coefficient of all parts is more or less linear the dimensions and the temperature are directly related.
1. A truss for an offshore structure (1), comprising a plurality of beams (3), wherein
at least one of the beams (3) includes a first hole (5) and an other structural element
(4) of the truss includes a second hole (6), wherein the at least one of the beams
(3) and the other structural element (4) are connected to each other through a metal
insert (7) in the first and second holes (5, 6), which insert (7) forms an interference
fit with at least one of the at least one of the beams (3) and the other structural
element (4), wherein the at least one of the beams (3) is made of aluminium, an aluminium
alloy or a reinforced plastic composite.
2. A truss according to claim 1, wherein the at least one of the beams (3) is made of
an extruded aluminium or an extruded aluminium alloy in which the first hole (5) is
machined.
3. A truss according to claim 1, wherein the at least one of the beams (3) is made of
a fibre reinforced composite.
4. A truss according to any one of the preceding claims, wherein the insert (7) forms
an interference fit with both the at least one of the beams (3) and the other structural
element (4).
5. A truss according to any one of the preceding claims, wherein the other structural
element (4) is made of aluminium or an aluminium alloy.
6. A truss according to any one of the preceding claims, wherein the other structural
element (4) comprises a non-extruded body.
7. A truss according to claim 6, wherein more than one beam (3) is connected to the body
(4).
8. A truss according to any one of the preceding claims, wherein the insert (7) is an
extruded insert, preferably made of aluminium or an aluminium alloy.
9. A truss according to any one of the preceding claims, wherein the insert (7) comprises
a hollow pin.
10. A method of assembling a truss according to any one of the preceding claims, comprising
the steps of supplying the metal insert (7), the at least one of the beams (3) and
the other structural element (4), positioning the at least one of the beams (3) and
the other structural element (4) such that the first and second holes (5, 6) are aligned
with respect to each other, cooling the insert (7) and/or heating at least one of
the at least one of the beams (3) around the first hole (5) and the other structural
element (4) around the second hole (6), and subsequently inserting the insert (7)
into the first and second holes (5, 6), after which thermal equilibration is allowed
so as to create the interference fit with at least one of the at least one of the
beams (3) and the other structural element (4).
11. A method according to claim 10, wherein the first and second holes (5, 6) are through-holes
which are aligned with respect to each other through introducing a guide pin (8) into
the respective first and second holes (5, 6), which guide pin (8) slidably fits in
the first and second holes (5, 6).
12. A method according to claim 11, wherein during inserting the insert (7) into the first
and second holes (5, 6) the guide pin (8) is moved in the same direction (X) as the
insert (7) away from the first and second holes (5, 6).
13. A method according to claim 12, wherein the guide pin (8) is pushed by the insert
(7).
14. A method according to claim 13, wherein the insert (7) is cooled, and wherein a thermal
insulator (9) is applied between the insert (7) and the guide pin (8) or the guide
pin (8) is made of a material which has a lower heat conductivity than the metal of
the insert (7).
15. A method according to any one of the claims 11-14, wherein the guide pin (8) has a
lower thermal expansion rate than the insert (7).