NO20120489A1 - Braces for a flexible pipe - Google Patents

Abstract

The present invention describes a stiffener (12) for a flexible tube 810) made from a polyurethane elastomer blend obtained from a prepolymer blend of methylene diphenylene diisocyanate (MDI prepolymer), which is then crosslinked with a polyamine to give the polyurethane blend capable of being formed .

Description

STIFFENERS FOR A FLEXIBLE PIPE

The invention relates to a leg stiffener for a flexible pipe.

Bend braces are placed around the pipeline at locations where the flexible pipe may be exposed to high loads, induced by movements of the boat or swells. A leg brace consists of a molded plastic construction with a conical shape with a length of a few meters and which is intended to surround the pipeline in question. By reinforcing the flexible pipe with a leg stiffener at the most stressed point, bending beyond the minimum radius of curvature, beyond which the pipeline would undergo irreversible damage to the structure, is prevented.

The flexible pipes that bring fluids (for example hydrocarbons) from the wellhead are connected to the surface installation by passing through a steel guide that can be straight or bent, "I-pipe" or "J-pipe". Several guide or guide pipes are provided on the surface installation. During pulling of the flexible pipe, it is guided and pulled through a guide pipe for connection to the end of the pipe for the surface installation. In operation, the amplitude of the movements of the flexible pipe is generated by the movement of the following which can lead to a deterioration of the construction of the flexible pipe near, for example, the guide pipe. To prevent such damage to the flexible pipe, a flex brace surrounds the flexible pipe near the guide pipe. The leg brace is then attached to the end of the guide tube using means known per se.

The leg brace is generally located under the workstation for the surface installation in a submerged or partially submerged medium. Furthermore, the leg brace is subjected to numerous extension and compression loads generated by the movements of the surface installation and of the flexible pipe under the influence of ocean currents, wind and/or swells. The conditions prevailing for the leg brace can lead to premature aging of the plastic, which can damage the integrity of the flexible tube.

In order to limit these phenomena, the stiffeners are manufactured partly from a plastic material which exhibits good properties in terms of resistance to hydrolysis and elongation. One material that exhibits the best properties is polyurethane. Today, bone braces are manufactured, for example, from an Adiprene L167 TDI polyether with a Shore A hardness of 95. For a lifetime of 20 to 30 years, the maximum hydrolysis resistance temperature for this material is 50 °C.

Leg braces as they are conventionally manufactured can have a length of several metres, with thicknesses greater than 200 mm and a weight that can exceed 2 tonnes. To limit

the size and weight of the bone stiffeners, polyurethanes are chosen with relatively high hardness, typically above 90 Shore A. Within this hardness range, the difficulty of processability for the polyurethane increases with hardness. In particular, this applies that the higher the hardness of the polyurethane, the shorter the "pot life" of the mixture produced from a prepolymer based on diisocyanate and a chain extender such as polyamines or polyalcohols, which necessitates the use of machines whose casting speed is high, especially when it applies to the casting of large parts.

One purpose of the invention is to propose a bone stiffener which has an aging resistance improved by at least 10 °C compared to bone stiffeners according to the known technique, while at the same time ensuring a good homogeneity of the mechanical properties.

A further ability of the invention is to allow the production of a stiffener which exhibits an increased hardness compared to stiffeners according to the known technique, with Shore 60 D as an example, while ensuring a good homogeneity of the mechanical properties.

The third advantage is to allow the production of a leg brace, the production of which is improved compared to the known technique.

Surprisingly, it has been found that a polyurethane elastomer, prepared from a diphenylmethane diisocyanate (MDI) prepolymer and a chain extender selected from polyamines can be used to produce such a bone stiffener. The bone stiffener produced in this way then has an even better aging resistance compared to the known stiffeners and also an improved processability. This processability improvement allows a stiffener with increased hardness to be produced.

According to a particular embodiment of the invention, the polyurethane elastomer is produced from a prepolymer of diphenylmethane diisocyanate (MDI) and from a methylene dianiline salt. This embodiment is particularly advantageous for producing a cold cast of the leg brace. The cold casting of the leg brace is particularly advantageous because no complex equipment is needed.

The description will be better understood from the detailed description of the invention that follows below and with reference to the attached figure, where: Figure 1 is a section of a flexible pipeline equipped with a leg stiffener according to

the invention.

Figure 1 schematically shows a flexible tube (10) provided with a leg stiffener (12). The flexible pipe is inserted through the guide pipe (14) on the (not shown) surface installation. The leg brace is arranged level with the pipe so that during the connection of the flexible pipe to the surface installation, the leg brace is level with the mouth of the guide pipe and thereby protects the flexible pipe from possible damage during movements due to the boat. The leg brace is then fixed by means of a clamp (not shown) or in any other known way around the end of the guide tube.

According to the invention, the leg brace is produced with a new polyurethane mixture. The polyurethane used to manufacture the bone stiffener is obtained from a diphenylmethane diisocyanate prepolymer (MDI prepolymer) as a preparation. This prepolymer is then crosslinked with a polyamine to form the polyurethane elastomer blend which is capable of being molded to produce the bone brace.

The polyurethane elastomer is thus produced according to a two-stage process. In a first step, a first prepolymer is produced by condensing the alcohol functions of a polyol with the isocyanate functions on the polyisocyanate. In a second step, the prepolymer and crosslinking agent are preheated and mixed in a molding machine and then the mixture is injected into a preheated mold shaped like the leg brace, and finally this mold is held at a temperature in an oven for the duration of the crosslinking.

This manufacturing process makes it possible to control the properties of the final material to a greater extent, which has an advantage when it comes to casting parts that require large amounts of material, as is the case with leg braces.

According to a first step, an intermediate diphenylmethane diisocyanate prepolymer (MDI prepolymer) is synthesized.

The MDI prepolymer is obtained by reacting, at a temperature between 30 and 150 °C, an MDI diisocyanate with a polyol with a high molecular weight between, for example, 250 and 10,000 g.mol"<1>. The ratio between the number of N=CO functions and the number OH functions in the reaction mixture can be between 1.5 and 20. The reaction temperature is, for example, between 30 and 150 °C.

The MDI diisocyanate used for the production of the prepolymer of MDI is selected from 2,4'-diphenylmethane diisocyanate and 4,4'-diphenylmethane diisocyanate or also 2,2'-diphenylmethane diisocyanate.

Alternatively, 0 to 20% by weight of an aliphatic diisocyanate can be added to the MDI prepolymer mixture. The aliphatic diisocyanate is then selected from the group consisting of 1,1'-methylenebis-(4-isocyanatocyclohexane), 1,4-cyclohexane diisocyanate, isophorone diisocyanate (IPDI), 1,3-xylene diisocyanate, hexamethylene diisocyanate and 1,1,4,4-tetramethylxylene diisocyanate or a mixture of one of these compounds.

Preferably, the high molecular weight polyol is selected from the group of polyether glycols and polyester glycols.

In the remaining part of the description, the term "diphenylmethane diisocyanate prepolymer preparation" is defined as a preparation containing at least 80% diphenylmethane diisocyanate prepolymer, obtained by reaction of at least one polyol and a diisocyanate monomer and which also includes an amount of free MDI monomer or another solvent or softener.

According to a second step, the MDI prepolymer preparation is then reacted with a crosslinking agent in a ratio of which the order of magnitude of the stoichiometry is selected from polyamines and preferably aromatic polyamines. The ratio between the total number of N=CO functions in the MDI prepolymer preparation and the number of NH2 functions in the reaction mixture is essentially in the order of stoichiometry. It can, for example, be between 0.7 and 1.2.

Preferably, the polyamines are selected from among aromatic polyamines. As an example, the polyamine can be 4,4'-methylenebis(2-chloroaniline), 3,5-diamino-4-chlorobenzoic acid isobutyl ester or even 4,4'-methylenebis(3-chloro-2,6-diethylaniline).

The mixture consisting of the prepolymer preparation and the crosslinking agent is injected into a pre-heated mold with a conical shape typical of a bone brace. This mold is kept at a temperature in an oven for the duration of the crosslinking and finally the leg brace is removed from the mold.

EP 1 950 234 typically describes a new polyurethane elastomer which can be used to produce a bone stiffener according to the first embodiment of the invention.

Specifically, EP 1 950 234 describes a polyurethane elastomer which is obtained from a prepolymer comprising a 2,4'-diphenylmethane diisocyanate prepared according to a process

way that includes the following steps:

(a) Reaction of diphenylmethane diisocyanate (MDI) comprising more than 80% 2,4'-diphenylmethane diisocyanate with a high molecular weight polyol selected from the group consisting of polyalkylene ether polyols having a molecular weight between 250 and 10,000 g.mol"<1> and a temperature between 30 and 150 °C, for a period of time sufficient to give the 2,4'-MDI prepolymers and to convert the OH functions in the polyol with a ratio between the NCO functions and the OH functions between 1.5:1 and 20:1. ( b) Addition to the 2,4'-MDI prepolymers of an aliphatic diisocyanate selected from 1,1'-methylenebis(4-isocyanatocyclohexane), 1,4-cyclohexanediisocyanate, isophorone diisocyanate (IPDI), 1,3-xylene diisocyanate, hexamethylene diisocyanate and 1, 1,4,4-tetramethylxylene diisocyanate or a mixture of one of these compounds. (c) Addition of a necessary amount of an aliphatic and/or aromatic diamine or a polyamine whereby the components will react so that the ratio between the equivalent of NCO groups and the sum of free NCO groups is between 0.8:1 and 1.2:1.

The methods of synthesis described in EP 1 950 234 are one possible way to obtain a polyurethane elastomer by forming a 2,4'-MDI prepolymer mixture, cross-linked with an aliphatic or aromatic polyamine, and heat molded to produce a bone stiffener.

While the crosslinking previously of an MDI prepolymer with a polyamine was abandoned due to the very high reactivity with regard to MDIs, the document EP 1 950 234 describes a new route for the synthesis of polyurethane elastomers by reacting a preparation containing an MDI prepolymer and a polyamine in which the reaction parameters are controllable.

The bone stiffeners produced from a polyurethane elastomer, synthesized by reacting a mixture comprising an MDI prepolymer and a polyamine, have better hydrolyzability characteristics and have a better fatigue strength than bone stiffeners produced from polyurethane elastomers according to the prior art, while at the same time maintaining homogeneous mechanical properties in the areas with large thicknesses, typically 100 mm to 500 mm. Furthermore, it is possible, while still maintaining the homogeneity of the mechanical properties, to achieve bone stiffeners with greater hardness. Finally, a significant advantage is that there is better processability for the polyurethane in the case of the formulations based on MDI prepolymer and a polyamine makes it possible, during casting, to ensure a cohesion between the molded part that has already hardened and the rest of the formulation that has not yet is hardened.

The properties of the material thus obtained make it possible to achieve a hardness between 50 Shore D and 70 Shore D while ensuring homogeneous properties within the material. The hardness measurements are carried out in accordance with the standard ISO 868 or ASTM D2240. The resulting material also has a hydrolysis resistance of up to 60 °C for 30 years, or even 70 °C for a shorter time, and a fatigue strength that is increased by a factor of 2 to 10 depending on the working conditions. The hydrolysis resistance measurements are carried out by tensile tests according to standard ASTM D638 on test pieces that have been subjected to accelerated aging in water, and with additional polishing according to the Arrhenius law.

A second, special embodiment of the invention will now be described. In this particular embodiment of the invention, the diisocyanate prepolymer is crosslinked with a special family of amines where the reactive amine functions are blocked to reduce their reactivity with respect to the MDI prepolymer.

The polyurethane elastomer used to produce the bone stiffener is obtained from a diphenylmethane diisocyanate prepolymer (MDI prepolymer) as a mixture which is then cross-linked with a complex of methylene dianiline (MDA) salts to give the polyurethane elastomer mixture the ability to be cold-formed to obtain the bone stiffener. The term "cold forming" shall be understood here to mean that the filling of the mold can take place without the need for preheating the mold or the reactants (except to ensure degassing), and that the crosslinking reaction does not occur during filling of the mold. This enables a slow and controlled filling of the mold and thereby avoids the formation of bubbles as the cross-linking takes place in a second step during an increase in the temperature of the material introduced into the mold in the form of the leg brace.

The methylene dianiline (MDA) salt is a polyamine whose amine functions are blocked in order to reduce the reactivity of the amine with respect to the MDI prepolymer when it comes to crosslinking the MDI prepolymer mixture and thereby be able to advantageously carry out said cold casting of the bone stiffener. During the cold casting phase, the amine functions of the MDA salt will be unblocked with a controlled increase in temperature to react with the MDI prepolymer mixture. The gradual unblocking of the amine functions for the MDA salt complex makes it possible to reduce the reactivity of the cross-linking agent and advantageously carry out cold casting of the bone stiffener.

The polyurethane elastomer is manufactured according to a three-step process. In a first step, an intermediate prepolymer preparation is produced by condensing the alcohol functions of a polyol with the isocyanate functions of a polyisocyanate as previously described for the first embodiment of the invention.

In a second step, the MDI prepolymer and the MDA salt are mixed, preheated to around 70 °C to thereby ensure degassing of the mixture.

The salt of methylenedianiline (MDI) is preferably the tris(4,4'-diaminodiphenylmethane)-NaCl complex obtained between methylenedianiline (MDA) and the sodium chloride salt. Optionally, this salt may comprise a plasticizer. It is sold, for example, under the brands CAYTUR 31™ and CAYTUR 31-DA™.

The mixture is then injected into a mold shaped like a bone brace.

In a third step, the mold, which must be able to be preheated in advance, is introduced into an oven and the temperature is increased in a controlled manner. During this final step, the temperature rise profile of the oven is adjusted to gradually release the amine functions of the MDA salt and to cross-link the MDI prepolymer.

In this particular embodiment, the reaction time leading to the formation of the polyurethane elastomer is longer due to the controlled reactivity of the selected crosslinking agent. Thus, it is not necessary to use casting machines with a high throughput. Furthermore, the bone stiffener produced from the polyurethane elastomer obtained from an MDI prepolymer cross-linked with methylene dianiline (MDA) salt has mechanical characteristics comparable to those obtained with the bone stiffener obtained according to the first embodiment of the invention.

WO 209/108510 typically describes a new polyurethane elastomer which can be used to produce a bone stiffener by a cold casting technique according to the second embodiment of the invention. Therefore, what is described in WO 2009/108510 is considered part of the present description as a reference.

WO 2009/108510 describes a polyurethane elastomer mixture, obtained by reacting an MDI prepolymer and methylene dianiline, complexed with a sodium chloride salt, where the mixture is stable over time for several days with the degassing temperature of 70 °C. This mixture is then introduced into a mold shaped like a bone stiffener and heated to a crosslinking temperature determined to be between 115 and 130°C to release the blocked amines of the MDA complex and to initiate the crosslinking reaction for the MDI prepolymer mixture.

The cross-linking reaction takes place under stoichiometric conditions, so to speak. The stoichiometry is obtained by the ratio between the total number of N=CO functions in the MDI prepolymer mixture and the total number of NH2 functions which are released at the crosslinking temperature of the MDI prepolymer. This ratio can be easily determined and can, for example, be mel-

lom 0.7 and 1.2.

The leg brace obtained in this way has improved properties in a manner identical to that described for the first embodiment of the invention.

Claims (9)

1. Stiffener for a flexible pipe, made of polyurethane, where the polyurethane is obtained by reacting, in a ratio between 0.7 and 1.2, a mixture consisting of at least 80% of a diphenylmethane diisocyanate (MDI) prepolymer and a polyamine. 2. Bend stiffener according to claim 1, characterized in that the mixture of the diphenylmethane diisocyanate prepolymer also contains up to 20% by weight of an aliphatic diisocyanate selected from 1,1'-methylenebis(4-isocyanato-cyclohexane), 1,4-cyclohexane diisocyanate, isophorone diisocyanate (IPDI) , 1,3-xylene diisocyanate, hexamethylene diisocyanate and 1,1,4,4-tetramethyl xylene diisocyanate, or a mixture of one of these. 3. Bone stiffener according to claim 1 or 2, characterized in that the polyamine is selected from 4,4'-methylenebis-(2-chloroaniline), 3,5-diamino-4-chloro-benzoic acid isobutyl ester and 4,4'-methylenebis-(3- chloro-2,6-diethylaniline). 4. Bone stiffener according to claim 1 or 2, characterized in that the polyamine is the tris(4,4'-diaminodiphenylmethane)-NaCl complex and that the bone stiffener is cold cast. 5. Leg brace according to one of the preceding claims, characterized in that the diphenylmethane diisocyanate prepolymer is synthesized by reaction of a diphenylmethane diisocyanate and a polyol with a molecular weight between 250 and 10,000 g.mol"<1>, selected from the group comprising polyester glycol and polyether glycol and that the ratio between the number of N=CO functions and the number of OH functions in the reaction mixture is between 1.5 and 20. 6. Bone brace according to claim 5, characterized in that the polyol is selected from the polyether glycol family. 7. Leg brace according to claim 1, characterized in that the diphenylmethane diisocyanate is 2,4'-diphenylmethane diisocyanate. 8. Leg brace according to claim 3, characterized in that it is hot cast. 9. The bone brace according to one of the preceding claims, characterized in that the hardness of the bone brace is greater than or equal to 50 Shore D with a hydrolysis resistance of at least 50 °C over a period of 20 years.