FIELD OF THE INVENTION

This invention relates to insulating liners for submerged equipment, such as submarine telecommunication amplifiers, switches, multiplexers or demultiplexers.

BACKGROUND

The evolution towards higher capacity submarine systems increases the electrical power dissipation in submerged plant, which in turn demands that the heat transfer efficiency of all parts of the equipment be optimised in order to minimise the working temperatures of the critical components.

The highest long-term working temperatures of critical components in equipment such as submarine repeaters, particularly components such as pump lasers, need to be minimised and defined in order to guarantee the high reliability required for submerged equipment. For the internal electronic units, the increase in temperature above the seabed ambient level is proportional to the electrical power dissipated.

A thermal flow path of particular concern is that between the outer housing of the repeater, which is at local earth potential, and the repeater internal units, which may operate at voltages which are many thousands of volts different to the outer housing.

The current solution uses an electrically insulating sleeve located between the outer housing of the repeater and the repeater internal electronics module. This is generally provided by an extruded cylindrical shell of low-density polyethylene (LDPE). LDPE has a dielectric strength of approximately 20 KV/mm and a thermal conductivity in the region of 0.30 to 0.33 W/mK. Alternatively, high density polyethylene (HDPE) can be used which provides marginal improvement, with a dielectric strength of approximately 70 KV/mm and a thermal conductivity in the region of 0.40 to 0.45 W/mK. Therefore, whilst such a shell has good electrical insulating properties, its thermal conductivity is low, resulting in a significant thermal gradient through the repeater structure.

Manufacture of the sleeve is difficult and control of size is critical for assembly and performance efficiency. The current manufacturing method for PE insulation sleeves is to extrude the material as a tube. Such a process requires the polymer to be heated until molten and then extruded through a die under vacuum to form a tube which is then pulled through a series of water cooling baths to create a continuous cylinder. As high capacity repeaters grow in size to accommodate more electronics, forming of larger diameter thin walled structures becomes more difficult to achieve.

Furthermore, the contact area between the internal electronics module and the insulation sleeve and the insulation sleeve and the outer pressure housing is limited due to the rigidity of the polymer material. The current PE liner is manufactured to ensure an interference fit between it and the outer pressure sleeve once inserted into the repeater housing. The electronics module is then dynamically expanded to clamp against the internal diameter of the insulation sleeve to form an intimate contact for thermal transfer. Despite the radial clamping force applied by the electronics module, the percentage contact area is driven by the dimensional accuracy of the mating parts and the compliance of the insulating sleeve material itself. This can result in a contact efficiency of less than 50%.

The limitation of the thermal conductivity of the liner material has previously been tolerated, given the traditionally low electrical power output of equipment such as repeaters, which typically operate at a line current of 0.65A and a system voltage of 12KV DC, with each amplifier generating 35W. However, new higher capacity requirements are now placing greater demands on the thermal performance of submerged equipment.

The current trend for high functionality/high fibre count systems is driving the demand for high power repeaters running typically at line currents of 0.65 to 1 .5 A and voltages of up to 20KV DC, with each repeater generating 80W or more. The temperature rise within these new repeater designs needs to be reduced as much as possible to ensure the performance and reliability of critical components over the lifetime of the system.

In order to achieve the necessary reduction in critical component temperatures in higher capacity systems, thermal conductivities in excess of 1.5 W/mK are needed. This cannot be achieved by the currently used insulating materials such as LDPE.

It is known that higher thermal conductivities can be achieved by the addition ceramic fillers such as boron nitride. Such fillers can be difficult to distribute within the polymer base material and can have detrimental effects on the manufacturing process of components. The addition of fillers to insulating polymers such as polyethylene compounds the manufacturing difficulty. Dispersion and distribution of the filler is randomised due to the extrusion process thus material thermal and electrical properties can vary throughout the cylinder structure. Fillers dramatically change the extrusion process parameters of the polymer due to changes in viscosity and specific heat. This can result in poor dimensional stability of the tube, poor surface finish, excessive stress within the material and premature cooling within the die, resulting in clogging and binding. Fillers can also be abrasive, causing excessive die and tool wear.

The use of a high percentage of filler material can also reduce the elasticity of the base material, making the sleeve vulnerable to damage once manufactured, to the extent that the sleeve is too fragile to handle.

It is desirable to develop a sleeve with improved heat transfer efficiency for submerged telecom repeater electronics, whilst maintaining high voltage insulation.

SUMMARY OF THE INVENTION

According to a first aspect there is provided a sleeve for providing a barrier between internal electronic units of submerged equipment and an outer case of the submerged equipment, wherein the sleeve comprises or is composed of a composite material comprising an elastomeric matrix material and a particulate filler material.

The ratio of the particulate filler material to the elastomeric matrix material may be selected such that the thermal conductivity of the composite material is at least 1 W/mK and the dielectric strength of the composite material is at least 80 KV/mm. The resulting sleeve may provide dual functionality, acting as both a high voltage DC electrical insulator and resulting in a reduced thermal gradient between the internal electronics assembly and the outer repeater housing over a service life of 25 years.

The elastomeric matrix material may be a silicone elastomer and the particulate filler material may be boron nitride. This combination of materials has been shown to result in a composite material with good properties for this application. The use of this material reduces the steady state operating temperature of critical components within the repeater for a given power dissipation whilst allowing operation at increased higher system voltages with improved HV ageing / life characteristics and reliability.

The amount of particulate filler material may be less than or equal to 45 percent (%) by weight of the composite material. This ratio of components has been shown to result in particularly good properties. The sleeve may be fabricated using compression moulding. Using compression moulding to produce the sleeve may result in improved distribution of filler material in the composite.

The submerged equipment may be or comprise a submarine telecommunications amplifier, switch, multiplexer or demultiplexer. Therefore, the sleeve is compatible for use with a variety of wet plant. For example, submarine repeaters, branching units and reconfigurable optical add-drop multiplexers (ROADM).

The submerged equipment may be configured to receive a system voltage of at least 0.5 KV. The sleeve may therefore provide a barrier for components operating at high voltages over a service life of 25 years.

The thermal conductivity of the composite material may be at least 1.5 W/mK.

The dielectric strength of the composite material may be at least 90 KV/mm.

The sleeve may be in the form of a hollow tube that is circumferentially continuous. This is a convenient configuration for protecting the internal electronics units of submarine equipment.

According to a second aspect there is provided a method of manufacturing a sleeve for submerged equipment, the sleeve comprising or being composed of a composite material comprising an elastomeric matrix material and a particulate filler material, the method comprising: placing a preform of the composite material into a mould cavity; and applying pressure to the preform to plastically deform the preform to form the sleeve.

This method may result in improved distribution of filler material in the composite compared to extruding the material. As the material is not extruded to form the tube, and neither is the material injected into a mould cavity, the elastomer material behaves more homogenously. This results in less residual stress within the material and a more dimensionally stable product.

The step of placing the preform of material into the mould cavity may comprise: placing the preform of the composite material into a lower part of a mould cavity; wrapping the preform around a core; and applying an upper half of a mould cavity over the wrapped core. This may allow sleeve components to be produced that are hollow and continuous. The method may further comprise, subsequent to the step of applying pressure to the preform to plastically deform the preform to form the sleeve: removing the formed sleeve from the mould cavity; and curing the plastically deformed material. This may stabilise the moulding.

The method may further comprise applying heat simultaneously with the pressure to plastically deform the preform to form the sleeve. This may allow for more efficient deformation of the preform.

The formed sleeve may be in the form of a hollow tube that is circumferentially continuous. This is a convenient configuration for protecting the internal electronics units of submarine equipment.

The elastomeric matrix material may be a silicone elastomer and the particulate filler material may be boron nitride. This combination of materials has been shown to result in a composite material with good properties for this application.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings:

Figure 1 (a) shows an example of a submarine repeater. Figure 1 (b) shows an expanded view of the central region of the repeater with an insulating sleeve between the internal electronics module and the outer housing of the repeater.

Figure 2 shows a logarithmic plot of time to breakdown vs breakdown voltage for an example of a 45% BN filled silicone elastomer composite material.

Figures 3(a)-(f) illustrate an example of a manufacturing method for forming a sleeve.

Figures 4(a)-(d) show schematic illustrations of combinations of good and bad dispersion and distribution of filler particles in a matrix material.

Figure 5 shows an example of a method for forming a sleeve.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 (a) shows a submarine repeater 1 . Figure 1 (b) shows an enlarged view of the central section of the repeater 1 . The outer pressure housing of the repeater, which is at sea earth potential, is shown at 2. Inside the housing is an internal electronics module, indicated at 3. The electronics module 3 operates at a high voltage of up to approximately 20 kV DC. A sleeve 4 provides a barrier between the internal electronics unit 4 and the outer case 2 of the repeater 1 . The sleeve 4 is composed of a composite material comprising an elastomeric matrix material and a particulate filler material. The sleeve is preferably circumferentially and/or axially continuous.

In this example, the elastomeric matrix material is a silicone elastomer and the particulate filler material is boron nitride. However, other elastomers and filler materials may be used. For example, the filler material may be a particulate ceramic or refractory material. The filler material is dispersed in the elastomeric matrix. The matrix material forms the body of the sleeve element. The matrix material may be substantially continuously interconnected throughout the sleeve element. The matrix material may comprise greater than 50%, greater than 70% or greater than 80% by volume of the sleeve element.

The ratio of the particulate filler material to the elastomeric matrix material is preferably selected such that the thermal conductivity of the composite material is at least 1 W/mK and the dielectric strength of the composite material is at least 80 KV/mm.

In a preferred example, the amount of boron nitride filler is equal to 45% by weight of the composite material. Using this ratio of materials, empirical testing has shown that the dielectric strength of the composite material is increased from 20KV DC/mm for LDPE to approximately 100KV DC/mm for the 45% BN filled elastomer when uniformly dispersed and distributed within the elastomer. Furthermore, the thermal conductivity is increased from 0.3 W/mK for LDPE to greater than 1.6 W/mK and is achievable without detrimental effect to the high voltage insulation properties of the material. Therefore, the filled elastomer has been shown to provide electrical insulating properties similar to polyethylene whilst providing a much higher thermal conductivity.

Other ratios of filler material to elastomeric matrix material of less than 45% may also be used and have also been shown result in an improvement to the dielectric strength and the thermal conductivity over the use of LDPE as the sleeve material.

The composite material is applied as a single piece thin walled sleeve over the entire electronics assembly to provide a dual function, operating as both a high voltage DC electrical insulator exposed to a constant electrical field, whilst also providing a reduced thermal gradient between the internal electronics assembly. The long-term outgassing of hydrogen or any other degradation products from the composite material has also been quantified by accelerated testing to confirm that there is no adverse impact on any aspect of reliability over the system lifetime.

An important requirement for such submerged equipment is that the voltage is applied under steady state conditions such that the equipment should operate over a service life of 25 years. When looking at the high voltage ageing of the elastomer, the critical factor is to establish the “N” value for the material.

The empirical formula which predicts time T to failure at DC voltage V is:

T.V ^N = C (1 ) where C is a constant for a given material.

If the voltage varies with time, the formula becomes: i V ^N .dt = C (2)

For a steady linear ramp to failure at breakdown voltage VB and time on ramp to failure TB the above integration gives:

(VB ^N .TB )/(N + 1 ) = C (3)

Taking logarithms:

N.LOG(VB) + LOG(TB) = LOG(C.(N+1 )) (4)

Therefore, a plot of LOG(TB) against LOG(VB) at different ramp rates will have a negative slope with gradient N.

An example is shown in Figure 2 for a 45% BN filled silicone elastomer. Typically for the LDPE material, an“N” value of 4.75 is adopted within the submarine equipment and cable industry to predict the life of the polymer mouldings when exposed to an any applied voltage. From empirical testing, Figure 4 shows that the“N” value for the filled elastomer to be higher than 4.75 with a confidence of 99.7% with a predicted value of N=7.6. A convenient method for forming the sleeve is by compression moulding. Figure 3 shows an example of such a manufacturing method for producing the sleeve described herein.

The uncured elastomer to be milled into an uncured sheet 30 which is laid into a mould cavity 31 as shown in Figure 3(a). In Figure 3(b), a cylindrical core 32 is placed onto the sheet 30 which forms the inside diameter of the insulation sleeve. In Figure 3(c), the sheet material 30 is wrapped around the core 32 and crimped to form a seam 33. In Figure 3(d), the upper half of mould cavity 34 is then applied over the assembled material. In Figure 3(e), pressure P and optionally heat are then applied to the mould tool assembly 35 to plastically deform the material to form the tube to the correct dimensions. In Figure 3(f), the moulded sleeve 36 is removed from the tool post moulding and then cured to stabilise the moulding.

Figures 4(a)-(d) show the difference between materials with good and bad dispersion and/or good and bad distribution. By adopting compression moulding, it is possible to ensure good dispersion and good distribution of the boron nitride filler, as illustrated in Figure 4(d), as this is achieved at the sheet milling phase. Unwanted turbulent disruption of the filler distribution within sleeve moulding is limited as the tool cavity is already 90% full prior to pressure being applied and so material has little distance to move.

Therefore, using compression moulding to produce the sleeve can result in improved distribution and dispersion of filler material in the composite, compared to extrusion of the composite, which is essential to ensure uniform thermal and electrical properties. As the material is not extruded to form the tube, and neither is the material injected into a mould cavity, the elastomer material behaves more homogenously. This results in less residual stress within the material and a more dimensionally stable product. Such a manufacturing approach provides scalability for future products. Furthermore, it has been found that the pattern of dispersion of particulate filler in the matrix remains substantially uniform after compression moulding, whereas if a particulate filler is incorporated in an injection moulding material it is difficult to assure even dispersion of the filler.

Figure 5 summarises an example of a method of manufacturing the sleeve. In step 501 , the method comprises placing a preform of the composite material into a mould cavity. The method then moves on to step 502, where pressure is applied to the preform to plastically deform the preform to form the sleeve. This method provides for the ability to manufacture a dimensionally adaptable single piece thin walled compliant sleeve using a 45% boron nitride filled silicone elastomer which is able to cover the whole electronics module within a submerged repeater. The resulting sleeve provides dual functionality, acting as both a high voltage DC electrical insulator and resulting in a reduced thermal gradient between the internal electronics assembly and the outer repeater housing over a service life of 25 years.

Substitution of the extruded LDPE sleeve with a 45% BN filled silicone elastomer composite has been found to increase the thermal conductivity of the sleeve from 0.3 W/mK for LDPE to >1.6 W/mK and has been found to be achievable without detrimental effect to the high voltage insulation properties of the material. The dielectric strength of the sleeve has been found to increase from 20kV/mm DC to 10OkV/mm, with an increased HV life exponent (“N” value), from 4.75 for LDPE to 7.6 for the composite material.

The use of this material can reduce the steady state operating temperature of critical components within the repeater for a given power dissipation whilst allowing operation at increased higher system voltages with improved HV ageing / life characteristics and reliability.

Another potential advantage is the increased contact area provided by the sleeve. The use of a silicone elastomer can provide a compliant interface between the both outer pressure sleeve and the internal electronics module removing the risk of air gaps due to material hardness or dimensional mismatch of mating components. Compression set of the material is limited over the operating and storage temperature ranges of the submerged equipment, ensuring the electronics module remains in contact with the insulation over its service life. Such an elastomeric interface is much more tolerant of radial contraction of the outer pressure sleeve when used in deep water applications up to 8000m.

A compliant elastomer interface also provides additional shock and vibration isolation of the electronics protecting critical components during the handling, deployment and recovery of the submerged equipment.

Additional benefits include lower hydrogen outgassing levels resulting from ageing of the sleeve material. Elevated temperature testing has shown the filled elastomer to evolve 50% less hydrogen than for similar volumes of polyethylene.

Simulations have suggested that the materials described above for the sleeve can in at least some applications provide significant advantages. For example, undersea communications housings are typically installed and then left in situ for a number of years, potentially whilst operating at relatively high voltages and continuous operation. Simulations have suggested that the materials described above can provide a barrier for components operating at relatively high voltages (e.g. greater than 400V) over a service life of 25 years.

Therefore, in summary, the sleeve described herein can in certain embodiments provide:

Dual functionality of high voltage insulator and low thermal gradient interface for subsea application

Increased dielectric strength from 20 to 100KV DC/mm

Increased thermal conductivity from 0.3 to >1 6W/Mk

Improved ageing characteristics with an“N” value above the industry accepted level of

4.75. The N value is typically 7.6 for a 45% BN filled silicone elastomer

50% reduction in hydrogen outgassing evolved from ageing of material

Compliant interface increasing thermal contact area efficiency

Provision of additional shock and vibration isolation to internal electronics of submerged equipment

Improved high voltage performance

A simplified and flexible method of manufacture allowing adaptability to future submerged equipment requirements

Although the sleeve has been described above with respect to the example of a submarine repeater, the sleeve is also applicable to other submarine telecommunications equipment, such as amplifiers, switches, multiplexers and demultiplexers. For example, branching units and reconfigurable optical add-drop multiplexers (ROADM).

The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.