GB2114046A

GB2114046A - Submersible flotation collar

Info

Publication number: GB2114046A
Application number: GB08201190A
Authority: GB
Inventors: Andrew Charles Ratcliffe Tyrer; David Gordon Lace
Original assignee: Crp Marine Ltd; CRP Marine Ltd
Current assignee: Crp Marine Ltd; CRP Subsea Ltd
Priority date: 1982-01-16
Filing date: 1982-01-16
Publication date: 1983-08-17

Abstract

A method of forming a two- component plastics structure, having an outer shell and an internal core is described. The internal core of the final structure is formed in a first moulding process and the outer shell is then moulded around the pre- formed core.

Description

SPECIFICATION Improvements in and relating to the moulding of plastics parts The present invention is concerned with plastics moulded parts and in particular with the manufacture of plastics submarine flotation collars of the type used for giving bouyancy to submarine hoses installed at offshore single point moorings.

Such flotation collars have a body comprised by a plastics shell housing an inner core of relatively low density foamed plastics material.

The body is formed in two parts of generally semicircular form which, in use, are attached to one another to form an annular collar configuration which is positioned so as to embrace a section of hose extending from the seabed to the surface. A plurality of such collars are fixed at a number of different points along the hose so as to achieve a predetermined stable bouyancy for the hose in operation.

Previously, the two body parts of flotation collars of this type have been manufactured using a rotational moulding process for first forming the shell, the foam material being subsequently injected into the internal cavity of the shell. The rotational moulding technique involves heating a continuously moving metal mould which is charged with a predetermined quantity of, for example, polyethylene powder. The powder melts as a result of the heat and the now liquid polyethylene is distributed evenly around the mould by suitable rotation and lateral movement of the mould whereby a gradually increasing material thickness is built up to form the shell.

After formation of the shell,the necessary chemicals to form a foam core, usually of polyethylene, are inserted through a suitable hole and allowed to react.

The techniques for performing the aforegoing rotational moulding process are well established and it can be performed relatively inexpensively.

However, for the manufacture of submarine flotation collars of the type in question, the performance of the technique requires strict control of the rotation pattern, cycle times and heat distribution in order to obtain uniformity of shell thickness. The collar parts incorporate at their ends relatively complicated hinges and the moulding of these portions to achieve the necessary uniformity is difficult.

Another practical problem associated with the rotational moulding technique resides in the fact that shrinkage of the shell occurs inevitably during the cooling cycle. As a consequence of this, the dimensions of the moulding tool have to be slightly greater than the finished collar size and strict control of cooling rates and material grades is important in order to obtain uniformity of dimensions between different collars.

It is an object of the present invention to provide a method of manufacturing flotation collars of the initially described type which avoids orat least mitigates the problems associated with the rotational moulding technique.

In accordance with the present invention there is provided a method of forming a two-component plastics structure, such as a submersible flotation collar part, having an outer shell and an internal core, wherein the internal core of the final structure is formed in a first moulding process and the shell is then moulded around the pre-formed core.

Advantageously, the core is pre-formed with a plurality of blind bores at predetermined locations and, during the second moulding process for the formation of the shell around the core, these blind bores are used to receive respective pins which support the core in a predetermined spaced relationship with a shell mould to define the eventual shell thickness.

The holes left in the moulded shell by the latter pins are preferably filled by means of respective plugs, for example of moulded nylon.

Advantageously, each pin carries an O-ring seal in its peripheral surface to assist the prevention of the ingress of water during operation.

The invention is described further hereinafter, by way of example, with reference to the accompanying drawings, in which:~ Fig. 1 is a perspective view showing the two moulded halves of a flotation collar prior to connection together; Fig. 2 is a perspective view illustrating the method in accordance with the present invention, of supporting a pre-formed core in a non-rotating mould prior to formation of the shell; and Fig. 3 is a side view of the arrangement shown in Fig. 2.

Fig. 1 shows the principal parts which are used to make up a flotation collar of the type in question. In fact, this collar differs slightly from hitherto known collars of this type in that the two body halves 10, 12 are of identical shape and configuration whereby any one moulded body part can be used with any other body part to form a complete annular collar.For this purpose, each body part 10, 12 comprises a part-circular shell 14 having a first end which has a pair of projections 15, 1 5a disposed symmetrically relatively to the longitudinal axis of the shell and a second end which carries two projections 16, 18 disposed assymetrically relative to said longitudinal axis whereby the projections 16, 18 on the second end of the body part 10 will fit into the spaces 20, 22 formed by the projections 1 6, 1 8 on the second end of the body part 12 for forming a hinge. The hinge is completed by the formation of aligned holes 24, 26 in the projections 16, 18 which receive a metal pin 28 fixed in position by end nuts (not shown). For added strength, the holes 24, 26 preferably contain metal tubular liners moulded into each shell 14 during its formation.

The projections 1 5, 1 5a at the first end of the body parts are.also provided with aligned holes 30, 30a which again can include metal liners moulded in during formation of the shell. Each pair of holes 30, 30a receives a respective metal pin 32 held in place by end nuts (not shown). In order to enable the. collar to be fixed in place around a submarine hose, an adjustable securing mechanism is attached between the two pins 32 to enable the first ends of the collar halves to be drawn together. Since this latter mechanism is not part of the present invention, however, it has been omitted from the present drawings for ease of illustration.

The shell 14 of each collar part 10, 12 completely surrounds an inner core part (not visible in Fig. 1 but indicated at 34 in Figs. 2 and 3) made of a relatively low density material, such as monocellular rigid polyurethane foam. In accordance with the present method, this inner core is formed as a preliminary step prior to the formation therearound of the outer shell 1 4. The formation of the foam core 34 is achieved by means of a moulding process, the appropriate chemical constituents being injected into a steel mould (not shown) which is then closed except for small breather holes left open near the top of the mould.Projecting into the interior of the mould at each of a number (six is convenient) of places is a respective metal pin, the purpose of which is to form corresponding blind holes in the moulded core 34 for later receiving supporting pins during formation of the shell as described below. The chemical reaction resulting from the mixing of the chemicals causes the foam to rise inside the mould to totally fill the mould cavity. After an appropriate time enabling the foam to cure, the mould is opened leaving the now rigid core shaped to the required configuration and having the aforementioned plurality (six in this case) of blind holes therein.

After surface preparation of the rigid foam core 34, the shell is cast around the core to produce the finished flotation collar. This stage is illustrated in Figs. 2 and 3 which show the premoulded core 34 positioned in a part 36 of a steel shell mould (not otherwise shown). As indicated by the six arrows P in Fig. 2, six metal pins project into the interior of the mould part 36 and engage in respective ones of the aforementioned blind holes in the core 34 whereby to support the core in a predetermined spaced relationship from the mould part 36 and the other part of the shell mould (not shown).

In the present method, unlike the rotational moulding technique, no external heat source and no continuous motion is required. The production of the shell is much simplified by the use of the present process and mould life is extended considerably since stressing due to heat and rotation is not incurred. Furthermore, the final shape and dimensions of the collar are accurate reproductions of the mould employed since there is no shrinkage during the curing cycle.

It will be appreciated that by using the present method, the thickness of the shell can be varied and controlled easily and greater uniformity and accuracy can be achieved. This is a particularly useful aspect of the present process since regions of the collar which are under the greatest stress during operation, such as the points of attachment of the securing mechanism and other metalwork, can be reinforced by an additional thickness of shell material.

The final step in the production of the collar is the sealing against water ingress of the holes remaining in the core and shell which received the core support pins. One way of sealing these holes is to insert in each a respective shaped nylon plug having an O-ring seal. These pins may be formed as moulded parts.

As regards the materials used for the construction of the collar, the preferred material for the core is monocellular rigid polyurethane foam which provides the crush resistance to enable the collar to withstand the hydrostatic pressures at the operational water depth and the resistance to water ingress in the event that the shell should become punctured. The invention is however, not limited to this core material and any other suitable mouldable plastics material which achieves satisfactory operational characteristics can be used.

The material of the shell is preferably a polyurethane elastomer, but again the invention is not limited to the use of this particular material.

The overall result of the new moulding process is the achievement of greater accuracy in reproducing the end product, greater consistency in achieving both wall thickness requirements and external dimensions, and the facility easily to provide reinforced portions having greater shell thickness, such as at the ends where the fixing and hinging mechanisms are to be attached.

Claims

1. A method of forming a two-component plastics structure, such as a submersible flotation collar part, having an outer shell and an internal core, wherein the internal core of the final structure is formed in a first moulding process and the shell is then moulded around the pre-formed core.

2. A method as claimed in claim 1, wherein the core is pre-formed with a plurality of blind bores at predetermined locations and, during the second moulding process for the formation of the shell around the core, these blind bores are used to receive respective pins which support the core in a predetermined spaced relationship with a shell mould to define the eventual shell thickness.

3. A method as claimed in claim 2, wherein the holes left in the moulded shell by the latter pins are filled by means of respective plugs.

4. A method as claimed in claim 3, wherein the plugs are made of moulded nylon.

5. A method as claimed in claim 3 or 4, wherein each plug carries an O-ring seal in its peripheral surface to assist the prevention of the ingress of water during use of the structure.

6. A submersible flotation collar part when made by the method of any of claims 1 to 5.

7. A submersible flotation collar part as claimed in claim 6 wherein said final structure is of generally part circular configuration having a first end which has a pair of projections disposed symmetrically relative to the longitudinal axis of the structure and a second, opposite end which carries two projections disposed asymmetrically relative to said longitudina! axis, whereby the projections on the second end of the structure will fit into the spaces formed by the projections on the second end of a further, identical such structure for use in forming a hinge therebetween.

8. A submersible flotation collar as claimed in claim 6 or 7 wherein the inner core is made of monocellular rigid polyurethane foam.

9. A submersible flotation collar as claimed in claim 6, 7 or 8 wherein the outer shell is made of a polyurethane elastomer.

10. A method of forming a two-component plastics structure, substantially as hereinbefore described with reference to the accompanying drawings.

11. A submersible flotation collar constructed substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.