NZ754923B - A subsea installation - Google Patents

Abstract

Some previously proposed subsea installations have lower pressure compensator lifetime and are less reliable. Disclosed here in is a subsea installation that (2) comprises a tank (3) containing an insulation fluid (4) or other fluid, a heat generating electric apparatus (7) positioned at least partly within the tank, and a pressure compensator (1) being in fluid communication with the tank and being configured to compensate volume variations of the insulation fluid or the other fluid by performing an expansive and a contracting movement. The subsea installation comprises further means for heating (5) the insulation fluid or the other fluid, said means for heating being configured to provide heating to the insulation fluid or the other fluid when the heat generating electric apparatus is in a non-operating state in order to reduce the volume variations of the insulation fluid or the other fluid. y within the tank, and a pressure compensator (1) being in fluid communication with the tank and being configured to compensate volume variations of the insulation fluid or the other fluid by performing an expansive and a contracting movement. The subsea installation comprises further means for heating (5) the insulation fluid or the other fluid, said means for heating being configured to provide heating to the insulation fluid or the other fluid when the heat generating electric apparatus is in a non-operating state in order to reduce the volume variations of the insulation fluid or the other fluid.

Description

A SUBSEA INSTALLATION FIELD OF THE INVENTION The present invention relates to a subsea installation.

BACKGROUND OF THE INVENTION Subsea installations are assemblies comprising several apparatuses used under water. Said assemblies can be installed for example on the bottom ground of a sea. As an example of said subsea installations power transformers used under water can be mentioned. Typically these power transformers comprise a tank filled with insulation and/or cooling fluid to encounter for the high pressure due to deep water depth. Examples of the other subsea liquid filled objects are sub- sea motors, subsea switchgears, subsea frequency converters, rectifiers and hy- draulic store tanks.

The subsea installations are used for example in modern oil and gas pro- duction in which the collection, separation, boosting and transport of production fluids takes place on the seabed. These processes require large amounts of power that has to be transferred from a remote location at suitable voltages and currents for minimum power loss, to the subsea installations.

During operation of the subsea installation, temperature and volume of the insulation and/or cooling fluid varies, whereby pressure compensation of the fluid is needed. This involves use of a pressure compensator, which is in fluid com- munication with the tank of the installation. The pressure compensator receives excess fluid when its temperature and volume increase, and returns the fluid back to the vessel when it cools down. The pressure compensator may also have a vari- able volume when it performs an expansive and a contracting movement for com- pensating the volume variation of the insulating fluid. An example of this type of pressure compensator is a bellow compensator.

Large pressure compensator movements due to large insulation fluid volume variations in a high hydrostatic pressure decrease mechanical cycling life- time of the pressure compensator. Therefore only a small part of the full compen- sation capacity of the pressure compensator is used when the water depth is high (1...3 km) having 100...300 bar pressure in order to improve the cycling lifetime of the pressure compensator. This, in turn, will generate a need of using a high num- ber of pressure compensators leading to an expensive price of the large subsea in- stallations and other related subsea components.

EP 2 738 780 discloses a subsea pressure compensation arrangement comprising a vessel filled with insulating fluid, and a pressure compensator for compensating pressure variations of the insulating fluid. The arrangement com- prises further a fluid connection pipe for fluid communication between the vessel and the pressure compensator, wherein the fluid connection pipe is connected to a bottom wall of the pressure compensator. A pipe opening arranged at an end of the fluid connection pipe extends above a level of the bottom wall of the pressure com- pensator.

BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is to provide a subsea installation to solve the above problems. The objects of the invention are achieved by a subsea installation which is characterized by what is stated in the independent claims. The preferred embodiments of the invention are disclosed in the dependent claims.

The invention is based on a subsea installation comprising a tank with an insulation fluid or other fluid. A pressure compensator is in fluid communication with the tank. The pressure compensator is configured to compensate volume var- iations of the insulation fluid or the other fluid by performing an expansive and a contracting movement. The subsea installation comprises further means for heat- ing the insulation fluid or the other fluid for reducing the volume variations of the insulation fluid or the other fluid.

An advantage of the subsea installation of the invention is that it pro- vides a longer pressure compensator lifetime and better reliability.

BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail by means of preferred embodiments with reference to the attached drawings, in which Figure 1 shows a pressure compensator of a subsea installation; Figure 2 shows a bellow compensator; Figure 3 shows a cylinder compensator; Figure 4 shows a pressure compensator of a subsea installation; Figure 5 shows a pressure compensator of a subsea installation; Figure 6 shows a heat pump.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a pressure compensator 1 of a subsea installation 2. The subsea installation 2 comprises a tank 3 comprising an insulation fluid 4 or other fluid. The pressure compensator 1 is in fluid communication with the tank 3, and the pressure compensator 1 is configured to compensate volume variations of the insulation fluid 4 or the other fluid by performing an expansive and a contracting movement. The subsea installation 1 comprises means for heating 5 the insulation fluid 4 or the other fluid for reducing the volume variations of the insulation fluid 4 or the other fluid.

The fluid communication between the pressure compensator and the tank 3 may be provided by a connection pipe 10 as shown in Fig.1. The pressure compensator receives excess insulation fluid 4 from the tank 3 when fluid’s tem- perature and volume increases, and returns the insulation fluid 4 back to the tank 3 when the fluid’s 4 temperature and volume decreases.

The heating of the insulation fluid 4 or the other fluid inside the tank 3 of the subsea installation 2 reduces the temperature variation of the fluid 4. The other fluid may be a cooling fluid, for instance. The reduction of temperature vari- ation of the insulating fluid 4 or other fluid results in reduction of volume variation of the insulating fluid 4 or other fluid in the tank 3. The reduction of the volume variation reduces the number of the mechanical movements of the pressure com- pensator 1. Further, also the extent of the mechanical movements is reduced. This means that large amplitude cycling of the pressure compensator 1 decreases. This decreases mechanical stress and strain on the pressure compensator 1 parts in the presence of hydrostatic pressure of the sea water 6. Further, the risk of fatigue failures in the moving parts of the pressure compensator 1 due to a repetitive stress diminishes.

The reduction of the volume variations of the insulation fluid 4 or the other fluid in the tank 3 enables a better utilization of the compensation capacity of the subsea pressure compensators 1. As the number of large amplitude cycling of the pressure compensator 1 decreases larger pressure compensator axial move- ments in high hydrostatic pressure can be allowed leading the need of fewer pres- sure compensators 1, longer pressure compensator cyclic lifetime and more com- petitive price of the subsea installation 2.

The dimensioning of a pressure compensator 1 system of a subsea in- stallation 2 is preferably made to allow a full volume variation of the insulation fluid 4 or the other fluid. This prevents damages in the subsea installation 2 if there is a temporal failure in the heating and/or cooling of the insulation fluid 4 or the other fluid in the tank 3. The full variation means the difference of the volumes be- tween the volume of the insulation fluid 4 or the other fluid at the temperature reached with the full heat load produced by the heat generating electric apparatus 7 and at the temperature of the surrounding water 6.

The pressure compensator 1 of a subsea installation 2 can be used in a subsea environment, e.g. exist on the sea bed. The pressure compensator 1 of a subsea installation 2 is also suitable for deep waters, where the water depth is high, 1000...3000 m, and the prevailing pressure is 100...300 bar. The water temperature in an ocean is typically 5-6°C in the depth of 1000 m and 0-3°C in the depth of 3000 m.

Examples of pressure compensators 1 performing an expansive and a contracting movement are a bellow compensator 8 and a cylinder compensator 9.

A bellow compensator 8 has one or more foldable and/or flexible side walls 14 for allowing volume variations of the bellow compensator 8. For instance, a top wall 13 or a bottom wall 11 may be arranged to move vertically y to compen- sate for the volume variations of the insulating 4 or cooling fluid.

Fig.2 shows a bellow compensator 8. The compensator is in fluid com- munication with the tank 3. The fluid communication between the bellow compen- sator and the tank 3 is provided by a connection pipe 10. The connection pipe 10 may enter the bellow compensator 8 through the bottom wall 11 of the compensa- tor 8. The bottom of the bellow 12 may be arranged fixedly with respect to the tank 3. The compensator 8 is thus allowed to expand and contract such that its top wall 13 may move in the vertical direction y, and its foldable side walls 14 may straighten and fold depending on the fluid volume within the compensator 8.

The expansive and contracting movement of the bellow 12 for compen- sating the volume variation of the insulating fluid 4 may also be provided by the bottom wall 11. Then the bellow compensator 8 has a top wall 13 which is rigidly arranged with respect to the tank 3.

In Figure 2, the bellow compensator 8 is mounted to the side wall of the tank 3 but the bellow compensator 8 may be arranged to the roof of the tank, for instance.

The expansive and contracting movement of the bellow 12 part of the pressure compensator 1 provides repetitive stress causing fatigue to the material of the bellow 12 part. As the cycling is reduced by controlling the volume variation the fatigue risk is diminished.

Figure 3 shows a cylinder compensator 9. A cylinder compensator 9 has a movable piston 15 which is arranged to compensate for the volume variations of the insulating fluid 4 and which separates the insulating fluid 4 from exterior sea- water 6. For instance, a cylinder compensator 9 is a vertically arranged cylinder housing a piston 15, which can move vertically y due to the volume variations of the insulating 4 or another fluid, e.g. cooling fluid. Below the piston 15 is arranged a fluid space 16 for the insulating fluid 4 and above the piston 15 is a sea water space 17. The piston 15 is sealed such that mixing of the insulating fluid 4 and sea- water 6 is prevented.

In the shown Figure 1 there is shown one pressure compensator 1 but the number of the pressure compensators is naturally not limited to one but can be any number greater than one.

In an embodiment, the subsea installation comprises a heat generating electric apparatus 7. In Fig.1 the heat generating electric apparatus 7 is a subsea transformer and the insulation fluid 4 is transformer oil. The transformer oil is min- eral oil or silicon oil, for instance. Examples of other heat generating apparatuses 7 are motors, switchgears, frequency converters, rectifiers and hydraulic store tanks.

In another embodiment, parts of the heat generating electric apparatus 7 or the total apparatus can be located within the tank 3. As shown in Fig.4, the active parts of the transformer, i.e. the transformer windings and a transformer core, can be located within the tank 3. Fig.4 shows also the transformer input con- nections 18 and the load 19 to the transformer. Same reference signs in the Figures refer to corresponding parts in Figures.

Further, the heat generating electric apparatus 7 may comprise an elec- tric power switch and /or a variable speed drive which can be located within the tank 3, for example.

In yet another embodiment, the means for heating 5 may be configured to provide heating to the insulation fluid 4 or the other fluid when the heat gener- ating electric apparatus 7 is in a non-operating state. The heating of the insulation fluid 4 or the other fluid prevents the cooling of the insulation fluid 4 or the other fluid close to the temperature of the surrounding water 6 when the heat generating electric apparatus 7 is not providing heating to the insulation fluid 4 or the other fluid in the tank 3. As the temperature decrease remains smaller the contracting movement of the pressure compensator 1 due to the volume reduction of the insu- lation fluid 4 or the other fluid remains also smaller.

Another advantage of keeping the insulation fluid 4 or the other fluid temperature higher than the surrounding water 6 is that it keeps also the parts of heat generating electric apparatus 7 located within the tank 3 at a higher tempera- ture. For instance, the lifetime of a transformer is increased by keeping the hot spot area of the transformer winding drier.

In a further another embodiment, the means for heating 5 may be con- figured to gradually close down the heating to the insulation fluid 4 or the other fluid during the start-up of the heat generating electric apparatus 7.

In a still another embodiment, the subsea installation 2 may comprise means for cooling 20 the insulation fluid 4 or the other fluid for reducing the vol- ume variations of the insulation fluid 4 or the other fluid. Fig.5 shows a pressure compensator of a subsea installation comprising means for heating 5 and means for cooling 20.

In an embodiment, the means for cooling 20 is configured to provide cooling when the heat generating electric apparatus 7 is in an operating state. As the temperature increase remains smaller the expanding movement of the pres- sure compensator 1 due to the volume increase of the insulation fluid 4 or the other fluid remains also smaller.

Cooling can be provided through natural or forced convection, for in- stance.

The means for cooling may be a heat pump 21 as shown in Fig.6. The part of the heat pump 22a which is in heat transferring contact with the surround- ing water 6 operates as a condenser and the part of the heat pump 22b which is in heat transferring contact with the insulating fluid 4 or the other fluid operates as an evaporator. Thus the heat pump transfers heat from the insulating fluid 4 or the other fluid in the tank 3 to the surrounding water 6.

The part of the heat pump 22a which is in heat transferring contact with the surrounding water 6 may be an external unit heat exchanger outside of the tank 3. The part of the heat pump 22a which is in heat transferring contact with the sur- rounding water 6 may also be integrated to an outer wall 23 of the tank 3. An ex- ample of the integration is shown in Fig.6 where an outer wall of the tank 23 is forming a part of the condenser housing. Surrounding water 6 freely flows across an outer wall 23 of the tank 3 and cools the working fluid circulating in the heat pump 21 as the heat is dissipated through the outer wall of the tank 23 to the sur- rounding water 6. The part of the heat pump 22a forming the condenser is sepa- rated from the insulation fluid 4 with a cover 24 which prevents the heat transfer from the condenser to the insulation fluid 4. Another example of the integration is where the outer surface of the condenser housing may be attached, e.g. connected with thermal grease, to the outer wall 23 of the tank 3.

An advantage of controlling the insulation fluid 4 or the other fluid tem- perature increase by providing cooling is that it keeps also the parts of heat gener- ating electric apparatus 7 located within the tank 3 at a lower temperature. For instance, the lifetime of a transformer is increased by keeping the temperature rise of the transformer winding within limits. Another advantage of providing cooling to a transformer is that it increases the loading capability of the transformer which is limited mainly by winding temperature. A further advantage of providing cooling to a transformer is the possibility of obtaining a higher power density in a low vol- ume subsea transformer.

In another embodiment, the subsea installation comprises a tempera- ture sensor 25 for measuring the temperature of the insulation fluid 4 or the other fluid, and a temperature controller 26 comprising a temperature range. The tem- perature sensor 25 and the temperature controller 26 are shown in Fig.5 only. The temperature sensor 25 and the temperature controller 26 may be installed in any pressure compensator of a subsea installation presented in the Figures.

In yet another embodiment, the temperature range of the temperature controller 26 is a pre-set range or the temperature range is adjustable by remote control through data transmission.

In a further another embodiment, when the heat generating electric ap- paratus 7 is in an operating state the means for heating 5 is configured to provide heating if the measured temperature goes below a set temperature range and the means for cooling 20 are configured to provide cooling if the measured tempera- ture exceeds the set temperature range.

The heat load to the insulating fluid 4 or the other fluid produced by the heat generating electric apparatus 7 depends on the loading 19 of the apparatus.

Thus the need of heating or cooling the insulating fluid 4 or the other fluid in the tank varies when the heat generating electric apparatus 7 is in an operating state.

An advantage of controlling the insulation fluid 4 or the other fluid tem- perature to remain in a set temperature range is that the volume variations of the insulation fluid 4 or the other fluid remain in a volume range defined by the tem- perature variation range.

In a still another embodiment, the means for heating 5 and the means for cooling 20 is a reversible heat pump working in direction either to provide heat- ing or to provide cooling.

The heat pump 21 shown in Fig.6 may also present a reversible heat pump. In the heating mode the part of the heat pump 22a which is in heat transfer- ring contact with the surrounding water 6 operates as an evaporator and the part of the heat pump 22b which is in heat transferring contact with the insulating fluid 4 or the other fluid operates as a condenser. Thus the heat pump 21 transfers heat from the surrounding water 6 to the insulating fluid 4 or the other fluid in the tank.

In the cooling mode the part of the heat pump 22a which is in heat trans- ferring contact with the surrounding water 6 operates as a condenser and the part of the heat pump 22b which is in heat transferring contact with the insulating fluid 4 or the other fluid operates as an evaporator. Thus the heat pump 21 transfers heat from the insulating fluid 4 or the other fluid in the tank 3 to the surrounding water 6.

In an embodiment, the means for heating 5 comprises a resistive heater, an inductive heater or a heating cable with an internal temperature control or a heat pump 21.

When the heat pump is used only for heating the insulating fluid 4 or another fluid the part of the heat pump 22a which is in heat transferring contact with the surrounding water 6 operates as an evaporator and the part of the heat pump 22b which is in heat transferring contact with the insulating fluid 4 or the other fluid operates as a condenser. Thus the heat pump transfers heat from the surrounding water 6 to the insulating fluid or the other fluid in the tank 3.

The required power to the heating or cooling the insulating 4 or the other fluid can be taken with several means, e.g. by separate heating transformer or reactor.

The inventive subsea installation provides a longer lifetime for the pres- sure compensator and better reliability, which are key factors for a deep water sub- sea transformer operation. The inventive subsea installation is advantageous in case the subsea installation has a high heat generation causing large volume varia- tions to the insulating fluid.

The inventive subsea installation can be applied to different types of subsea assemblies comprising insulation or other fluid in a tank. Examples of such subsea assemblies are subsea motors, subsea switchgears, subsea frequency con- verters, rectifiers and hydraulic store tanks.

It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The inven- tion and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

Part list: 1 a pressure compensator; 2 a subsea installation; 3 a tank; 4 an insulation fluid; 5 means for heating; 6 water; 7 a heat generating apparatus; 8 a bellow compensator; 9 a cylinder compensator;10 a connection pipe; 11 a bottom wall; 12 a bellow; 13 a top wall; 14 side walls; 15 a piston; 16 a fluid space; 17 a sea water space; 18 input connections; 19 a load; 20 means for cooling; 21 a heat pump; 22a-b a part of the heat pump; 23 an outer wall of a tank; 24 a cover; 25 a temper- ature sensor; 26 a temperature controller; y vertical direction.

Claims (16)

1. A subsea installation comprising a tank containing an insulation fluid or other fluid, a heat generating electric apparatus positioned at least partly within the tank, and a pressure compensator being in fluid communication with the tank 5 and being configured to compensate volume variations of the insulation fluid or the other fluid by performing an expansive and a contracting movement, wherein the subsea installation further comprises means for heating the insulation fluid or the other fluid, said means for heating being configured to provide heating to the insu- lation fluid or the other fluid when the heat generating electric apparatus is in a 10 non-operating state in order to reduce the volume variations of the insulation fluid or the other fluid.

2. A subsea installation according to claim 1, wherein the means for heating is configured to gradually close down the heating to the insulation fluid or the other fluid during the start-up of the heat generating electric apparatus. 15

3. A subsea installation according to claim 1 or 2, wherein the subsea installation comprises means for cooling the insulation fluid or the other fluid for reducing the volume variations of the insulation fluid or the other fluid.

4. A subsea installation according to claim 3, wherein the means for cooling is configured to provide cooling when the heat generating electric appa- 20 ratus is in an operating state.

5. A subsea installation according to any one of claims 1-4, wherein the subsea installation comprises a temperature sensor for measuring the temperature of the insulation fluid or the other fluid, and a temperature controller comprising a temperature range. 25

6. A subsea installation according to claim 5, wherein the temperature range of the temperature controller is a pre-set range or the temperature range is adjustable by remote control through data transmission.

7. A subsea installation according to claim 3 or 4, wherein when the heat generating electric apparatus is in an operating state the means for heating is 30 configured to provide heating if the measured temperature goes below the temper- ature range and the means for cooling are configured to provide cooling if the measured temperature exceeds the temperature range.

8. A subsea installation according to claim 7, wherein the means for heating and the means for cooling comprises a reversible heat pump working in 35 direction either to provide heating or to provide cooling.

9. A subsea installation according to any one of claims 3-4, wherein the means for cooling comprises a heat pump.

10. A subsea installation according to claim 1, wherein the means for heating comprises a resistive heater or an inductive heater or a heat pump.

11. A subsea installation according to claim 1, wherein the means for 5 heating comprises a heating cable with an internal temperature control.

12. A subsea installation according to any one of claims 1-11, wherein the heat generating electric apparatus comprises a subsea transformer.

13. A subsea installation according to claim 12, wherein the insulation fluid is transformer oil. 10

14. A subsea installation according to claim 12, wherein transformer windings and a transformer core are located within the tank .

15. A subsea installation according to any one of claims 1-14, wherein an electric power switch and /or a variable speed drive are located within the tank.

16. Use of a subsea installation according to any one of claims 1-15 in a 15 subsea environment.