CA1217389A - Helical seamed structural vessel, method and apparatus of forming same - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A double-wall structural vessel includes an inner tubular liner and an outer tubular shell surrounding the liner in spaced relation, both the liner and the shell comprising helically curved strips of material, and a continuous and helically curved spacer/stiffener therebetween. The liner and shell may fully overlie one another, or may be disposed relatively axially such that the shell edges are out of alignment with the liner edges. And, the spacer spans contiguous edges at successive turns of the liner as well as at successive turns of the shell, and is secured to the strips as well as to the liner edges and to the shell edges.
A floatable work housing is provided for vessel fabrication, the housing having a sealed opening through which the completed portion of the vessel progressively extends and is floated on a body of water to avoid a need for external supports and bearings.

Description

1 ~21738g i BACKGROUND OF THE INVENTION
This invention relates generally to a structural vessel as well as to an apparatus and a method of forming same. More l particularly, the vessel is double-walled, is wound helically 51 ana has a helically wound stiffening spacer between its walls.
Long cylindrical vessels of large diameter are known for , use as platform supports for offshore oil drilling operations, and are normally fabricated from a plurality of cylindrical plate sections seamed welded along contiguous or overlapping edges between sections to form a single-walled or a double-walled vessel. Circular reinforcing ribs are needed as well as longitudinal ribbing, and sometimes spaced bulkheads are required to insure structural integrity. The fabrication of especially such large and heavy vessels is, however, extremely time consuming and costly requiring numerous labor-consuming steps during the ¦welding and weld inspection operations.
On the other hand, cylinder conduits, tubes and piping have been fabricated more quickly and less laboriously by l helically winding sheets of material and sealing together 20 ¦ contiguous edges or surfaces of each pair of successive windings.
! Normally, however, such a helical seam pipe or tube is designed principly for use as a single-walled cond~it for fluids (pressurized or not), communication lines, and the li~e, requiring a relatively small diameter of sufficient shell strength without additional reinforcements.
~ SU~MARY OF THE INVENI IGN
It is therefore an object of the present invention to provide a structural vessel, double-walled, helically ~ound, rib-l ,~ i .1 ~Z17389 . I
stiffened between walls along helical turns and seamed weldedalong contiguous edges. Such a vessel is relatively easy to fabricate at substantially less cost and reduced labor than known structures, yet has improved strength characteristics permitting use of relatively thin shells. The unique approach utilized in making such vessel is also made part of this invention.
Another object of this invention is to provide a double-walled and rib-stiffened helically wound structural vessel in which structural beams are utilized as stiffeners secured tc the spaced walls via the seaming along such contiguous edges, together with a method and apparatus for carrying out the fabrication in a simple and economical manner.
A further object of this invention is to provide such a structural vessel which is fabricated by applying to the lead-ing edge of the inner wall a first section of a longitudinallysplit stiffening rib (which may be a honeycomb I-beam) before helically winding the inner wall, contiguous edges of each pair of successive turns being seamed, the remaining split section of the rib being welded to the first wound rib and the outer wall wound helically over and welded thereto as well as along contiguous edges.
A still further object of this invention includes, in the method of fabrication, the use of a water floatable housing for carrying out the fabrication steps, the forming vessel initiated inside the housing, end-capped, progressively completed, extending outside the housing through a sealed opening, l the housing ballasted down and the vessel being supported by I floating same in a body of water. A ballast control of the ~21~389 housing is provided for adjusting the inclination thereof to any change in inclination of the formed vessel extending outside the housing.
l A still further object of the present invention to 5l provide a aouble-walled structure vessel of spaced helically wound strips with a helically wound spacer/stiffener therebetween which presents another option to the aforedescribed design, while at the same time retaining a simplified fabricating approach which produces a vessel of large diameter having improved strength lOI characteristics and structural integrity.
A still further object of this invention is to provide such a double-~alled structural vessel in which the liner and the shell are relatively shifted axially such that the shell edges are out of alignment with the liner edges, and includins means such as welding for securing spacer means to the strips and the contiguous liner edges together as well as the contiguous ! shell edges together.
A still further object of this invention is to !Iprovide such a double-walled structural vessel in which the spacer 2011means which may comprise beam sections having flanges extending along the liner edges and along the shell edges, are likewise secured to the confronting surfaces of the strips between opposing ¦edges thereof.
ll A still further object of this invention is to provide .5 such a double-walled structural ~essel wherein one of the beam ~sections is secured to one of the edges of one of the strips before wir.ding same to fcrm the liner, and the other of contiguous edges at the successive turns of the liner is secured to such lZ17389 one edge and to the one section, and the other of the beam sections is secured at one of the edges to the other strip before winding same to form the shell, and the other of the l contiguous edges at the successive turns of the shell is 5l secured to such one edge of the other strip and to such other beam section, and the ends of the beam sections opposite their flanges are secured to confronting surfaces of the strips.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure l is a side elevational view of a helically formed structural vessel according to the invention;
15¦Figure 2 is a perspective view of the structural vessel in the process of being fabricated within a floating work housing;
I Figures 3 and 4 are perspective views of a rib stiffener ¦disposed between the walls of the present vessel, and being formed from a structural I-beam of honeycomb constructions;
201Figure 5 is a side view of the structural vessel accord-ing to one embodiment of the invention in the process of fabrication, taken substantially along the line 5-5 of Figure 2;

iFigure 6 is a typical section of the double-walled l internally stiffened vessel according to the invention, take 25~ substantially along the line 6-6 of Figure 5;
Figure 7 is a perspective view illustrating the technique used in fabricating the structural vessel according to the invention;

~2~73~9 Figures 8, 9, 10 and 11 are sectional views showin~ the steps employed in fabricating the Figur~e S vessel, taken sub-stantially along the line 8-8, 9-9, 10-10 and 11-11 of Figure 7;
Figure 12, is a slightly enlarged detail view of a ¦ typical pair of support and drive rolls provided for the vessel during fabricat ion;
Figure 13 is an enlarged' detail view of one of the several drive support rolls provided for rotating the structural vessel during fabrication;
Figure 14 is an enlarged detail view of a liquid seal between an opening in the work housing through which the formed structural vessel extends;
Figures 15 and 16 are respectively side and top views of.
the vessel during fabrication, the work housing being shown in section;
Figure 17 is an end view of the completed vessel;
Figure 18 is a view showing the vessel of Figure 17 altered in shape to form a ship's hull of different widths.
Flgure 19 is a perspective view illustrating the technique used in fabricating the structural vessel accordin~ tc another embodiment of the invention;
l Figure 20 is a cross-sectional view of the formed jvessel taken substantially along the line 20-20 of Pigure 19;
j~ Figure 21 is a cross-sectional view of the innex liner ¦I~ith a spacer section secured thereto, taken substantially along the line 21-21 of Figure 19; and Figure 22 is a cross-sectional view of the outer shell with a spacer section secured thereto, taken substantially along ; _5_ ~Z17389 the line 22-22 of Figure 19.
DETAILED DESCRIPTION OF THE INVENTION
Turning now to the drawings wherein like reference l characters refer to like and corresponding parts throughout the 5 1 several views, a completed helical seam structural vessel is shown in Figure 1, generally designated 20, as having end caps 21 of, for example, conical shape, which may be secured in place in any conventional manner for closing the opposed ends of the formed vessel. The vessel is hollow and preferably 10 cylindrical, although it may be of varying diameter without departing from the invention, and may thus be towed in a body of water by means of a tow line 22 hooked as at 23.
Reference is now made to Figure 7 which illustrates the essential features used in fabricating the helical seam 15 structural vessel according to one embodiment of the invention.
An inner strip 24 of material, such as plate steel, is wound from a supply (not shown) in a helical manner about a series of forming rolls 25 to form an inner tubular liner. Appropriate tube forming means will be utilized for forming the helical 20 turns from a continuous or laterally spliced strip. A first longitudinal half 26a of a rib stiffener produced from a structural I-beam (or double T-beam) 26 (Figure 3) is welded ~along a leading edge 27 (Figure 8) of the inner strip. First and second halves 26a and 26b are produced in a known manner 25 by separating the webs of beams along undulating or trapezoidal lines 28 to form individual heam sections 26_ and 26b, the pro-jecting regions being oppositely disposed and subsequently welded together, in a manner to be hereinafter described, so as to 12~7389 l ~ .
produce a known honeycomb beam se~tion of the type shown in Figure 1 4. Before the helical winding of inner strip 24, ~eam section i 26a is welded as at 28 and/or 28a (Figure 8) along leading edge l 27 of the strip, substantially one-half of the flange of section 5 ! 26a extending outwardly of the edge as shown. This extended l flange portion is lapped over a trailing edge 31 of the next i~adjacent formed helical turn (Figure 9), as at station 32, and a fillet weld 33 and/or 33_ is applied at such station for joining the contiguous leadlng ~nd trailing edges of the turns together 101 via beam section 26a. In the meantime, ~eam section 26b is wound over web section 26a in such a manner that projecting web regions are oppositely disposed to form a honeycomb beam structure of the ¦type shown in Figure 4. These projecting web regions are sub-¦sequently welded together as at 34 (Figures 6 and 10~ at station 15l¦37 for completing the weld joint between the leading and trailing ¦¦edges and for filing the gap therebetween. An Automatic Ultrasonic Inspection Station 30 of the inner strip major weld is located as l¦shown in Fig. 7.
jl An outer strip 38 of material, such as plate steel, is 201 then helically wound, to form an outer tubular shell, over beam llsection 26b with its trailing edge 39 (Figure 10) overlapping !~ the flange of this section, whereafter a fillet weld 41 is applied at station 42. The leading edge 43 of the outer strip is then llfillet welded as at 44 (Figure 11) to the contiguous edge of the 25 ¦next adjacent turn via beam section 26b, at station 4~, and a ~¦major weld 40 completes the welded seam and fills the gap between ¦¦contiguous edges of the outer strip turns, at station 46. An Automatic Ultrasonic Inspection Station 30a of the outer strip lZ17389 major weld is disposed as in Fig. 7. A sandblast chamber 47 as well as a paint and drying chamber 48 are provided for carrying out the exterior finishing operations of the formed vessel.
l Support roller assemblies 49 (Figures 5 and 7) are 5 1 provided at opposite lower sides of the formed vessel and form a cradle support within work housing 51 (Figure 2), these roller assemblies being mounted on transverse box beams 52, or the like, support on a partition floor 53 (Figure 15) provided within '¦the work housing. Each support roller assembly 49, shown in 10 ¦more detail in Figures 12 and 13, comprises a pair of pneumatic wheels or tires 54 each containing an internal hydraulic drive motor unit 55 as including a hub 56 on which the wheel is mounted for rotation therewith, and a hydraulic drive motor 57.
I The motor is stationary anc is supplied via hydraulic lines 58 15 ¦ so that its output drives hub 56 which in turn rotates wheel 54.
These drive motors are relief valved for establishing a synchronized speed between the drive rolls, and are suitably controlled for variation and reversability, if necessary. A
support bracket 59 for the wheel pair, on which a wheel arm 20 support 60 is attached, has flanges mating with flanges of an underlying bracket 61, with fasteners extending through elongated slots in one of the flanges for adjusting the wheels toward and away from the rotational axis of the formed vessel to adjust for any diameter variation of the wheels. And, s~aced holes 62 25 1l along each box beam permit the roll support assembly to be ¦adjusted to accommodate different sized vessel diameters Pairs ¦of support wheels of each assembly are typically located along the seamed edges of the outer helical turns (Figure 7) for -8- ` `

lZ17389~

driving the vessel during fabrication in the direction of the major arrow thereof. Such a support and drive of the vessel ¦allows for several anticipated minor irregularities, such as ,~out-of-round, weld profiles, chording from the castellated beam 5,lreinforcing ribs as well as to transmit the expected loads without distortion of the outer shell.
And, synchronized steering motors 63 may be provided for steering the wheels along the outer seams for positively llmaintaining the support and drive wheels 54 along the reinforc-10 ling ribs of the structure with precise forward feed for well gap control.
¦ Work housing 51 has an access opening 64 in a side wall ¦thereof (Figure 2) through which the inner and outer strips and l the reinforcing ribs are introduced from their respective storage supplies for the fabrication of the structural vessel according to the invention. And, an end wall 65 of the l housing has an opening 66 (Figures 14 to 16) through which the ! fabricated end of the vessel progressively extends. A water ~ lock in the form of a pneumatic seal is provided along at least 20l the bottom half edge of this opening for sealing the interior of l the housing against water entry. Such a seal may be in the form I of a closed-ended arcuate tube 67 of elastomeric material supplied with air under pressure from an air supply 68 and l having a plurality of air openings 69 along the inner wall 25, thereof through which air is emitted for creating both a pneuma-tic seal as well as an antifriction bearing. Along the upper ¦half of the edge of opening 66 are provided a pluralit~ of support and drive roller assemblies 49 similar in construction "

_ () _ lZ17;~89 and operation to that descr1bed with reference to Figure 12.
These rollers function to compensate for the nose-down or nose-up loads created during fabrication of the vessel depending on the extent of the vessel outwardly of the housing holding down the vessel against the housing seal should the housing and the vessel be ballasted out of synchronization.
The work housing floats on a body of water (Figure 2) which may be the same as that in which the formed vessel floats as it extends from the housing. And, as the formed vessel progessively extends outwardly of end walls 65, the housing ¦will tend to incline slightly downward at this end. In order to avoid any damage to the pneumatic seal and/or to the assemblies 49 surrounding the end wall opening, the housing is water ballast controlled by the provision of ballast tanks 71 (Figures 15, 16) connected to a suitable compressed air supply through valving for positively adjusting the inclination of the housing to conform to the vessel inclination. Openings 70 in the bottoms of the tanks facilitate ballast control in a known manner.
It should be pointed out that opening 66 can be varied to accommodate the fabrication of vessels of different diameters Iby provision of fish plates, or the like, at the opening on which the pneumatic seal and the roller 49 are mounted.
A helical seamed, double-walled structural vessel according to another embodiment of the invention is shown in Figurè
l9, generally designated 80, in the process of being fabricated with a portion thereof, shown at the left in the Figure, having been completed. The vessel is hollow and preferably cylindrical, although it may be of varying diameter, and may have conical . ''.
~ -10- ~

~Z~73~9 ~end caps to facilitate easy towing in a body of water, similarly as described with reference to Fi~. 2.
As will be seen, the structural vessel according to l this other embodiment of the invention is similar to the 51 aforedescribed one embodiment of the vessel in that it comprises l a helically wound inner liner surrounded in spaced relation by ¦ia helically wound outer shell with helically wound spacer means jlbetween the liner and shell secured thereto via means which also secures the contiguous liner edges together and which secures the contiguous shell edges together. Such spacer means, which is continuous and also functions as a stiffener, may for example be produced in a known manner from a structural I-beam, double ~T-beam, channel-shaped beam or Z-shaped beam by separating the beam web along an undulating or trapezoidal line to form individual 15 ~beam sections 81 and 82 having castellated webs which facilitate Ibending while being helically wound. Otherwise, continuous bars iof rectangular cross-section capable of bending when helically ¦wound/ can be utilized without departing from the invention.
In the process of fabricatina the vessel, a continuous inner strip 83 of material, such as plate steel, is wound from a supply (not shown) in a helical manner about a series of form-ing rolls such as shown in Fig. 7, to thereby form an inner tubular liner. Appropriate tube forming means will be utilized Ifor forming the helical turns from a continuous or laterally spliced strip. Before winding, beam section 81 is fillet welded for example along a leading edge 84 of the inner strip with approximately one-half its flange 81a extending outwardly of this ¦edge. This extended flange portion is lapped o~er a trailinq ed~e 85 of the next adjacent formed helical turn and a butt weld 86 is li .

applied for joining the contiguous leading and trailing edges of the inner turns together via beam section 81.
Beam section 82 may then be fillet welded along a l¦trailing edge 87 of an outer strip 88 of material, such as plate steel, with substantially one-half flange 82a of section 82 ¦extending outwardly of this edge as shown. The outer strip is ¦then helically wound about the inner strip and is maintained axially shifted relative thereto such that the edges of the l outer strip are out of alignment with the edges of the inner 10 strip, as shown in Figs. 19 and 20, whereby the web tips of beam section 81 confront the inner surface of strip 88, and the web tips of beam section 82 confront the outer surface of strip 83. In such manner, the web tips of the beam sections are respectively-disposed between the opposing edges of the strips which confront them. And, a leading edge 89 of outer strip 88 overlaps the exposed portion of flange 82a on an already wound outer strip and is fillet welded to the trailing edge of that strip after which the seam at these edges is butt welded together 'as at 91 via beam section 82. The web tips of both beam sections 20 ¦lare then fillet or fusion welded to the respective surfaces of the wound strips which they confront.
Support roller assemblies are provided at opposite lower sides of the formed vessel and form a cradle support l~within a work housing, similarly as described with reference to 25 I¦Fig. 5. And, such cradle support may be located within a wor~
housing which is water bouyant and which has an opening in a side ¦wall thereof through which the formed structural vessel progressively extends in the direction of the arrow of Fig. 19 during the .

~ ~2~.~389 fabrication process, all as described with reference to Fig. 5.
Moreover, w}lile the outer strip is being wound in ¦place, concrete may be pumped into the space between the strips l for structural purposes, weep holes being Drovided in the inner liner for known purposes. ~And, the inner liner may be covered with a thin concrete lining durlng the fabrication process to protect the steel vessel against corrosion when, for example, used as a water aqueduct.
l From the foregoing it can be seen that a double-walled lO ¦ web stiffened structural vessel is capable of being produced according to the invention in a simple and uniform manner of practically any predetermined size, and enjoys a wide variety of uses. For example, vessel 20 can easily be converted from its cylindrical shape of Figure 17 into a ship's hull shown at 20' or 20" in Figure 18 by simply slitting through the double wall longitudinally along a line 72 parallel to the central axis of the vessel. Free ends 73 and 74 thus formed may then be moved apart a predetermined distance, as by jacking, and a flat deck 75 spanning ends 73 and 74 may be secured thereto so as io convert the vessel into a submersible or a partly submersible ship's hull having a predetermined beam width without the need for many of the usual structural bulkheads and/or internal supports. The beam width can be varied significantly between ¦hull 20' and 20" without substantially reducing the overall 25 Idepth of the hull. During the conversion process, it should be pointed out that opposed ends 73 and 74 are spread apart while maintaining, in some conventional manner, a substantially spiral contour for the hull which may be utilized as a ~reighter, ore ! -13-12~7~89 ~boat, super barge, liquid tanker, pressurized sea-going vessel, i or the like. And, the structure vessel according to the invention may be utilized as a storage tank, grain elevator, l tunnel, transportation unit~ support for an offshore drilling 51 platform, etc. A minimal amount of welding is required to ,¦fabricate the present vessel since the continuous edges of the ¦jinner and outer strips may be seamed together via beam or rib ¦¦26, and position welding (directed downwardly) for the seaming l of both inner and outer strips is more reliable and less time 101 consuming. As the vessel progresses during fabrication, its ! dead weight is supported in a body of water requiring no external mechanical supports outside a floating housing thereby reducing the cost of fabrication and the need for replacement of friction bearings and supports externally of the floating work housing.
And, vessel 80 is likewise capable of being converted into a ship's hull in a similar manner to that aforedescribed.
Moreover, vessel 80 provides the advantage that, during the Ifabrication process according to the invention the narrow webs of the spacer ribs need not be tracked as in the first embodiment of the present design since the tips of these webs are now welded along confronting respective surfaces of the inner and outer strips.
And, since during the winding of the ribs the web tips of the l inner section slightly fan out and the web tips of the outer 251 section slightly contract, care must be ta~en in properly cutting ~ the beam web along an undulating line so that the tips of the ! castellated webs produced will match as closely as possible during the fabrication process. The presen~ invention avoids ' -14-173t~9 these potential difficulties because the web tips between beam i sections do not confront and need not match.
Inherent in the type of vessel construction of the l invention is an overall buckling strength sufficient to eliminate 5j the need for intermediate bulkheads or deep frames. Also, the ability to use thinner plates provides superior physical and mechanical properties and is less limiting in construction capabilities. The fabrication process is unique in that it provides for a continuous, automated and integrated operation.
101 Labor costs are reduced and required skills are minimal. Layout, cutting, shaping, welding and weld testing, blasting, and painting of metalizing occur sequentially. There is economy in the capitol expenditures since the requirement for costly items such as welding manipulators, ways and scaffolds, bending rolls and process for precision cutting, edge preparation equipment, and overhead cranes, are minimum or non-existent.
Internal bulkheading and endcapping of the vessel is enhanced by the ability to roll the vessel to any position for fitting and welding. l~ith the use of a body of water to support the completed portion of the vessel, the production facility is rendered portable and relatively inexpensive thereby leading to savings in freight on tunnel projects, supports, etc.
Unsinkable monolithic hulls can be produced by filling the space between inner and outer walls with a high buoyancy, low water absorption, rigid, urethane foam. As each helical segment of the vessel is completed, the foam system components may be metered, mixed, and dispensed as a liquid stream into the cavity, where they react and expand to fill the void. During e~pansion li -15-1; :17;189 l a permanent internal pressure is created as in a pneumatic tire ! additionally strengthening the vessel. Not only is the resultant vessel also reliably buoyant but the insulating ¦~efficiency of foam further increases the uses of the present 5¦ivessel for the storage of, or operation in extreme cold or j hot climates. For ultra-compressive strength and/or rigidity, various concrete formulations can be injected into the space ! between the walls using a similar technique. Further, by varying l the density of the concrete mixture, the weight/displacement 101 ratio can be adjusted to meet design requirements in floating arrangements. Moreover, conventional hulls can be produced by axially splitting the vessel and moving its sides apart a distance up to sixty percent beyond its original diameter. Thus, a light-weight hull is quickly fabricated at minimum costs.
lS Also, closed vessel 20 of the inve~ntion may be utilized for the transport and/or storage of both a liquid and a gas by the provision of a longitudinal flexible diaphragm separating the same, as generally disclosed in U.S. Patent No. 4,000,700.
Obviously, many other modifications and variations of 201 the present invention are made possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced therwise tnan as specifically described.

Claims (20)

The embodiments of the invention in which an ex-clusive property or privilege is claimed are defined as follows:

1. A structural vessel, comprising an inner tubular liner and an outer tubular shell surrounding said liner in spaced relation, said liner and said shell comprising helically curved strips of material, continuous and helically curved spacer means between said liner and said shell, said spacer means spanning contiguous edges at successive turns of said liner and spanning contiguous edges at successive turns of said shell, and means seaming together said liner edges and seaming together said shell edges, said spacer means being secured to said strips via said seaming means.

2. The vessel according to claim 1, wherein said spacer means comprise beam sections having flanges extending along said liner edges and said shell edges.

3. The vessel according to claim 2, wherein said strips overlie one another such that said shell edges are aligned with said liner edges.

4. The vessel according to claim 3, wherein said beam sections are joined together.

5. The vessel according to claim 3, wherein said beam sections have webs of toothed formation defining projections joined together at the tips thereof.

6. A method of forming a structural vessel, comprising the steps of:
helically winding one strip of material to form an inner tubular liner;
helically winding another strip of material and surrounding said liner in spaced relation to form an outer tubular shell;
disposing continuous, helically curved, separate spacer means between said liner, and said shell, said spacer means spanning contiguous edges at successive turns of said liner and spanning contiguous edges at successive turns of said shell;
securing said liner edges together and to said spacer means; and securing said shell edges together and to said spacer means.

7. The method according to claim 6, wherein said winding steps include maintaining said shell edges aligned with said liner edges.

8. The method according to claim 7, wherein said spacer means comprise beam sections having flanges extending along said liner edges and said shell edges, and joining said sections together.

9. The method according to claim 6, wherein said spacer means comprise beam sections having flanges, one of said sections being first secured at one of said edges of said one strip before being wound to form said liner, then securing the other of said contiguous edges at said successive turns of said liner to said one edge and to said one section, then joining the other of said sections to said one section, and thereafter carrying out the step of securing said shell edges together and to said other section.

10. The method according to claim 6, including the further steps of maintaining a constant curvature of said one strip and said another strip during said winding steps for producing a circular cylindrical vessel, cutting through said vessel along a single line parallel to the central axis thereof, spreading apart opposed sections of said vessel formed after cutting, and joining together said spread apart sections.

11. The method according to claim 10, including the further step of forming said sections into opposed spiral shapes during said spreading step.

12. The method according to claim 10, wherein said spread apart sections are joined together by flat decking material for forming a ship's hull construction from said vessel.

13. A structural vessel, comprising an inner tubular liner and an outer tubular shell surrounding said liner in spaced relation, said liner and said shell com-prising helically curved strips of material, continuous and helically curved spacer means between said liner and said shell, said spacer means spanning contiguous edges at successive turns of said liner and spanning contiguous edges at successive turns of said shell, said liner and said shell being relatively shifted axially such that said shell edges are out of alignment with said liner edges, and means securing said spacer means to said strips and said liner edges together as well as said shell edges together.

14. A vessel according to claim 13, wherein said spacer means comprise beam sections having the bases thereof extending along said liner edges and along said shell edges.

15. The vessel according to claim 13, wherein said spacer means are secured to confronting surfaces of said strips between opposing edges thereof.

16. The vessel according to claim 14, wherein ends of said beam sections opposite said bases are secured to con-fronting surfaces of said strips between opposing edges thereof.

17. The vessel according to claim 16, wherein said beam sections have webs of toothed formation at said ends thereof.

18. A method of forming a structural vessel, comprising the steps of:
helically winding one strip of material to form an inner tubular liner;
helically winding another strip of material surrounding said liner in spaced relation to form an outer tubular shell;
disposing continuous, helically curved, separate spacer means between said liner and said shelf, said spacer means spanning contiguous edges at successive turns of said liner and spanning contiguous edges at successive turns of said shell;
during said winding steps, maintaining said liner and said shell relatively shifted axially such that said shell edges are out of alignment with said liner edges;
securing together said spacer means and said strips including respectively securing together said liner edges and said shell edges such that said liner edges and said shell edges are secured together via said spacer means.

19. The method according to claim 18, wherein said spacer means comprises beam sections having bases extending along said liner edges and along said shell edges, and joining ends of said sections opposite said bases to surfaces of said strips which confront said ends.

20. The method according to claim 18, wherein said spacer means comprise beam sections having bases, securing one of said sections at one of said edges of said one strip before winding same to form said liner, securing the other of said contiguous edges at said successive turns of said liner to said one edge and to said one section, securing the other of said section at one of said edges of said another strip before winding same to form said shell, securing the other of said contiguous edges at said successive turns of said shell to said one edge of said another strip and to said other section, and securing ends of said sections opposite said bases to confronting surfaces of said strips.