The present invention relates to a subsea cooling assembly adapted to cool warm components of equipment relating to subsea hydrocarbon production, such as the flow of warm hydrocarbons produced from a subsea well or warm components in a subsea hydrocarbon compression facility

Background

A well stream flowing out of seabed wells can have a large amount of thermal energy. Hence it is known to cool the well stream in order to protect piping and associated subsea equipment. In addition other subsea equipment may need cooling, such as electric motors and components of a subsea compression facility. One known way of cooling is to employ the cold ambient sea water, either as a passive coolant or by forced cooling wherein the sea water is forced past the equipment or an associated heat exchanger.

The use of ambient sea water as coolant has an advantage in that the coolant is always available and cool, such as about 4 °C. A disadvantage is however that sea water contains salts and organisms which may lead to corrosion of the cooling equipment and scaling and fouling. Countermeasures include coating of the surfaces in contact with the sea water. However, this reduces the heat transfer between the coolant and the warm equipment.

Prevention of hydrate formation is always compulsory, and in some cases also prevention of wax formation Both hydrate formation and waxing can be prevented by chemical injection.

The chemicals injected to prevent hydrates are anti-freeze liquids, of which MEG (mono-ethylene-glycol) is the most common. Typically 70 %-weight of free water can be injected. Consumption of MEG would represent a significant OPEX element as well as an environmental problem MEG is therefore normally regenerated in a MEG regeneration plant that represents a significant CAPEX element, and some environmental concern still remains. Another method for flow assurance is Direct Electric Heating (DEH). Current technology is however limited to in the range of 30 km step out, and significant development and qualification work is remaining for step out in the range of 50 to 200 km, if at all technically obtainable.

The invention

According to a first aspect of the present invention there is provided a subsea cooling assembly comprising a cooling heat exchanger to which a coolant is provided through a cooling line, and a coolant pump adapted to pump the coolant through the cooling line. The cooling heat exchanger is arranged in association with a warm subsea element, thereby cooling the warm subsea element

According to the invention the coolant is circulated in the cooling line between the cooling heat exchanger and a heat disposal heat exchanger which is arranged in association with a hydrocarbon pipeline. In the heat disposal heat exchanger thermal energy is transferred from the coolant to a hydrocarbon flow in the hydrocarbon pipeline.

Thus, by using a cold flow of hydrocarbons to cool the coolant, the coolant can be cooled in a forced cooler. This results in a significantly reduced cooler size than a passive cooler, as in the prior art. With a hydrocarbon pipeline which is not thermally insulated, it will be efficiently cooled when extending a sufficient distance through the cold sea water. In some embodiments, however, it is desirable to thermally insulate the hydrocarbon pipeline in order to maintain an appropriate temperature in the hydrocarbon flow along a longer distance.

The term heat exchanger should be appreciated as any type of equipment suitable for transferring thermal energy between two mediums of different temperatures. It may for instance be a shell and tube type, or a pipe-in-pipe type, preferably with a counter flow configuration. The heat exchanger may also be in form of a coolant conducting pipe or line which is arranged with thermal contact with the warm subsea element, such as the hydrocarbon pipeline.

The warm subsea element can be any type of subsea equipment related to the production of hydrocarbons from a subsea well, which needs or benefits from cooling. Such equipment can for instance be an electric motor, a pump, or a compressor. The warm subsea element may also be a warm part of a

hydrocarbon pipe that conducts a warm flow of hydrocarbons. This is a typical situation close to a well head where the produced hydrocarbons tend to be excessively warm.

In one embodiment of the invention, the warm subsea element comprises a warm section of the hydrocarbon pipeline. As described above, this is typically close to the well / Xmas tree out of which a flow of warm hydrocarbons is produced.

The warm subsea element can comprise a warm component of a subsea compression facility. This can for instance be an electric motor or a pump, a compressor discharge cooler, or other types of equipment. It is noted, however, that the warm subsea element may also be a warm component which is not part of a compression facility

According to a preferred embodiment of the invention the coolant is freshwater. Moreover, the coolant can comprise an anti-freeze chemical In another embodiment the coolant is an oil

Preferably, in an embodiment of the invention, the cooling line comprises a coolant delivery line that guides coolant to the cooling heat exchanger, and a coolant return line that guides coolant away from the cooling heat exchanger. In this embodiment, the coolant return line guides coolant from the cooling heat exchanger to a plurality of heat disposal heat exchangers which are arranged with a mutual distance along and in association with the hydrocarbon pipeline. Thereby the assembly is adapted to heat the hydrocarbon pipeline at various locations. Moreover the assembly according to the invention may comprise a plurality of cooling heat exchangers which are provided with coolant through the coolant delivery line. The cooling heat exchangers are arranged in association with warm subsea elements which are thus cooled. In one embodiment the cooling heat exchanger is of a shell and tube type, wherein hydrocarbon flows in the inner pipes and the coolant flows in the space between the inner pipes and an outer shell.

In yet an embodiment, the coolant return line is arranged in association with flow conducting elements of a compression facility in order to assure flow in the flow conducting elements. That is, the coolant return line, or a branch of the same, is arranged to heat the flow conducting elements, for flow assurance in these

In one advantageous embodiment there is arranged a subsea buffer tank for heated coolant in that in the cooling line. The buffer tank may comprise an electric heater. Thus, during a shut down, the operator may still be able to provide warm coolant to the coolant return line

According to a second aspect of the present invention, there is provided a method of cooling a warm subsea element associated with a hydrocarbon flow flowing in a hydrocarbon pipeline. The method comprises the following steps: a) arranging a cooling heat exchanger in association with the warm subsea element and delivering a coolant to the cooling heat exchanger through a cooling line; and

b) cooling the coolant in a heat disposal heat exchanger arranged in

association with the hydrocarbon pipeline, by transferring thermal energy from the coolant to the hydrocarbon flow in the hydrocarbon pipeline.

The said warm subsea element which is associated with a hydrocarbon flow flowing in a hydrocarbon pipeline shall be understood as any type of subsea equipment that relates to subsea hydrocarbon production and which needs or benefits from cooling. Thus, it is not limited for instance to warm subsea elements which are in contact with such a hydrocarbon pipeline.

Advantageously the coolant can be fresh water.

In some embodiments the warm subsea element can be a component of a subsea compression facility. According to a third aspect of the present invention there is provided a method of heating a hydrocarbon pipeline, which hydrocarbon flowline extends from a subsea Xmas tree and several kilometers along the seabed. The method comprises

a) delivering a coolant to a cooling heat exchanger at a location where the hydrocarbon pipeline shall be cooled;

b) guiding heated coolant exiting the cooling heat exchanger in an insulated coolant return line to a downstream location of the hydrocarbon pipeline; c) delivering the heated coolant to a heat disposal heat exchanger arranged in association with the hydrocarbon pipeline at said downstream location, thereby heating the hydrocarbon pipeline and cooling the coolant;

wherein the coolant is flowed in a coolant delivery line extending from the heat disposal heat exchanger to the cooling heat exchanger and in the coolant return line extending from the cooling heat exchanger to the heat disposal heat exchanger. The coolant can be fresh water.

In order to avoid heat loss, the hydrocarbon pipeline should be thermally insulated.

A well stream that is flowing out of a seabed well has large amount of thermal energy. For a well stream that is conducted into a compression system with a 6 MW compressor, it has been calculated that it is necessary to cool such a well stream in an amount of typically 7 MW. That is, heat is removed from the well stream by cooling it typically from 100 to 35 °C. By use of fresh water that circulates between subsea heat absorbing heat exchangers that receives heat from heat sources and heat emitting/cooling heat exchangers that transfers heat to cold fluids, an effective cooling system is achieved. Furthermore, these for instance 7 MW can be recovered as large quantity of hot water at e.g. 70 - 80 °C. From the discharge cooler of a compressor of a subsea compression facility typically 5 MW can be recovered. This hot water can then be- utilised for flow assurance, i.e. prevention of hydrate formation and in other cases also of wax formation and perhaps also prevention of formation of other damaging

components at low temperature. Forced coolers (active coolers) of some kind, and favourably shell and tube coolers, may advantageously be applied in embodiments of the present invention. Heat exchangers in the form of forced coolers are very efficient compared to free convection coolers (passive coolers). The overall heat transfer coefficient (OHTC) of active coolers with fresh water as coolant can typically be 1000-1500 W/(m2 ^* K), and with seawater as coolant typically to 700 VW(m2 ^* K) (reduced due to necessary coating) For passive coolers the corresponding numbers are typically 150-200 W/(m2 ^* K). Hence, use of active coolers results in a much smaller cooler area, dimensions and weight, compared to passive coolers. Because the coolant of a preferred embodiment of the present invention, namely fresh water, is clean and non-corrosive, corrosion control will be easier than for passive coolers exposed to the saline seawater. Another important feature is that the heat exchangers of such a preferred embodiment are not subject to neither temperature induced nor CP (cathodic protection) induced carbonate scaling and coating, which reduces the OHTC significantly, is not necessary for the coolers of the invention.

Examples of embodiments

Having described the invention in general terms above, more detailed examples of various embodiments of the present invention will be given in the following with reference to the drawings, in which

Fig. 1 is a principle view of a subsea cooling assembly according to the

invention;

Fig 2 is a principle view of another embodiment of the subsea cooling assembly according to the invention;

Fig. 3 is a principle view of an embodiment of the present invention including a subsea compression facility;

Fig. 4 is a schematic view of the subsea cooling assembly shown in Fig 3;

Fig. 5 is an alternative embodiment resembling the embodiment shown in Fig. 3; Fig. 6 is an embodiment corresponding in part to the one shown in Fig. 4,

however adapted for reverse coolant flow; Fig. 7 is a principle view of the subsea compression facility shown in Fig. 3 and a cooling assembly according to the present invention.

Fig. 1 illustrates a first embodiment of the present invention. On the seabed 1 there is arranged a Xmas tree 3 which is in connection with a hydrocarbon well extending into the seabed 1. During production from the well, a flow of hydrocarbon-containing fluid flows out from the Xmas tree 3 and into a hydrocarbon pipeline 5. The hydrocarbon pipeline 5 is arranged on the seabed 1 and guides the hydrocarbons away from the Xmas tree 3, for instance to a receiving facility on land.

The flow of hydrocarbons being produced and guided through the Xmas tree 3 can be warm, for instance 100 °C or more. In order to protect associated equipment through which the hydrocarbons flow, it is cooled by means of a cooling heat exchanger 101. The cooling heat exchanger 101 can be of any appropriate type, for instance a shell and tube type in which a coolant is guided axially through a shell that is arranged about the hydrocarbon pipeline 5.

To the cooling heat exchanger 101 there is provided a cold coolant through a cooling line 103. More specifically the coolant is provided through a coolant delivery line 104. The outlet of the cooling heat exchanger 101 is connected to a coolant return line 106.

According to the invention, the coolant delivery line 104 and the coolant return line 106 are, at a location distant from the cooling heat exchanger 101 , connected to a heat disposal heat exchanger 105. The heat disposal heat exchanger 105 is arranged in association with the hydrocarbon pipeline 5 in such a distance from the cooling heat exchanger 101 that the flow of hydrocarbons has become cold. Thus it is an appropriate location for cooling the coolant in the cooling line 103, by using the cold flow of hydrocarbons as a cold source.

The heat disposal heat exchanger 105 may also be of the shell and tube type, however other configurations are also possible Preferably, the shell and tube type heat exchangers 101 , 105 described herein are counter flow heat exchangers. With such a configuration the coolant flowing out from the cooling heat exchanger 101 and into the coolant return line 106 can be significantly warmer than the hydrocarbons flowing out from the cooling heat exchanger 101 , in the hydrocarbon pipeline 5.

A coolant pump 107 is arranged in the cooling line 103 in order to pump the coolant through the cooling line 103.

When passing through the cooling heat exchanger 101 close to the subsea Xmas tree 3 in the embodiment shown in Fig. 1 , a substantial amount of thermal energy from the flow of hydrocarbons flowing out of the Xmas tree 3 is recovered in the coolant. As discussed above, the coolant can for instance be heated to about 80 °C. This now warm coolant is transferred to the heat disposal heat exchanger 105 through the coolant return line 106 in order to remover the thermal energy from the coolant.

In one embodiment the coolant return line 106 is provided with thermal insulation. In this manner, the coolant will remain warm until it reaches the location of the heat disposal heat exchanger 105 When flowing through the heat disposal heat exchanger 105, a significant amount of thermal energy will be transferred to the hydrocarbon flow in the hydrocarbon pipeline 5. In such an embodiment the cooling assembly according to the invention is used also for flow assurance in the hydrocarbon pipeline, in addition to the cooling of warm elements.

In order to monitor that the hydrocarbons in the hydrocarbon pipeline 5 exhibit a temperature within the allowed temperature range, hydrocarbon temperature gauges 109 are arranged at various points along the hydrocarbon pipeline 5. Moreover, in order to ensure a proper operation of the heat exchangers 101 , 105 and the coolant pump 107, coolant temperature gauges 111 are also arranged at various points along the cooling line 103. In the embodiment shown in Fig. 1 an adjustable coolant valve 113 is arranged in the coolant return line 106. The control of temperatures of coolant and cooled medium are established

technology and can be done in several ways. Hence this will not be described in detail herein. Fig. 2 shows an embodiment of the invention which in many respects is similar to the one shown in Fig. 1. However, in the embodiment shown in Fig. 2 there are arranged a plurality of heat disposal heat exchangers 105 and the hydrocarbon pipeline 5 is thermally insulated. A substantial distance exists between each heat disposal heat exchangers 105, for instance more than 30 km. Thus, the operator is able to heat the hydrocarbon flow in the hydrocarbon pipeline 5 at a proper location or more locations, and thereby assure flow in the hydrocarbon pipeline 5. For instance, the hydrocarbons may still be sufficiently warm at the location of the first heat disposal heat exchanger 105 (i.e. the one to the left in Fig. 2). The operator would then rather transmit heat from the coolant to the hydrocarbon flow at a location further downstream, for instance at the position of the next heat disposal heat exchanger 105 The case in Fig. 2 could also be that the distance between each of the heat disposal heat exchangers 105 is so large that the hydrocarbons need heating at the locations of every heat disposal heat exchanger 105.

The operator will be able to choose the appropriate heat disposal heat exchangers 105 which should be used by means of input from the hydrocarbon temperature gauges 109 and appropriate actuation of the coolant valves 113.

Fig. 3 is a principle drawing of a subsea compression facility 200 which is arranged in the hydrocarbon pipeline 5. On the left hand side of Fig. 3 the cooling heat exchanger 101 is arranged in association with the hydrocarbon pipeline 5 in a position where the hydrocarbons need cooling

Another significant heat source is the compressor discharge cooler. To the compressor discharge cooler there is connected a cooling heat exchanger 101 a. Other components that need cooling are provided with a cooling heat exchanger 101 b, 101 c, 101d, 101e. Such components may include an anti surge cooler to which cooling heat exchanger 101 b is connected and a liquid line pump 201 to which motor the cooling heat exchanger 101d is connected, and a compressor motor to which the cooling heat exchanger 101c is connected. Although such components may or may not contribute positive to production of hot water, the components to which the cooling heat exchangers are connected are preferably cooled. Preferably these components are efficiently cooled in compact indirect heat exchangers, such as the above mentioned shell and tube type.

In the embodiment shown in Fig 3, the liquid from the gas-liquid separator 203 and the compressed gas are co-mingled at the downstream side of the compression facility (hydrocarbon pipeline 5)

The motor of the liquid pump 201 installed in the liquid line 205 is cooled with the cooling heat exchanger 101d which receives cold coolant through the cooling line 103d. The coolant is cooled in a heat disposal heat exchanger 105d which is arranged on the same liquid line 205, further downstream. The liquid in the liquid line 205 is cold and thus an advantageous cold source for cooling the coolant by means of forced convection. The cooling of the motor of the liquid pump 201 is thus provided with a stand-alone cooling assembly within the compression facility 200.

Contrary to this, the heated coolant in the coolant return line 106, which has been heated in the cooling heat exchanger 101 upstream of the compression facility 200, as well as the coolant delivered to the cooling heat exchanger 101a which cools the compressor discharge cooler 207, are both connected to the heat disposal heat exchanger 105 in association with the hydrocarbon pipeline 5 downstream of the compression facility 200.

The various components of the subsea compression facility 200 are known to the person skilled in the art and all of them will not be discussed herein.

Fig 4 shows a schematic view of the cooling heat exchangers 101 , 101a, 101b, 101c, 101d, 101e, connected to a common coolant delivery line 104 and coolant return line 106. The latter are connected to a heat disposal heat exchanger 105 which is arranged in association with the hydrocarbon pipeline 5 at a significant distance downstream of the compression facility 200. A difference between the configuration shown in Fig. 4 and the one in Fig. 3 is, however, that cooling heat exchanger 101 d which is arranged in association with the liquid pump 201 is shown connected to the common cooling line 103 in Fig. 4.

Fig. 5 shows a subsea compression facility 300 which is connected to a hydrocarbon pipeline 5 upstream. The compression facility 300 shown in Fig. 5 corresponds to the one shown in Fig. 3 except that the gas phase and liquid phase lines are not co-mingled on the downstream side of the facility. In stead, on the downstream side of the compression facility 300 there is a gas phase hydrocarbon pipeline 5a and a liquid phase hydrocarbon pipeline 5b. Both of these are provided with heat disposal heat exchangers 105 to dump heat. In addition the heat disposal heat exchangers 105 may be used to assure sufficient temperature in the hydrocarbons (flow assurance)

Fig. 6 schematically shows another embodiment of a subsea cooling assembly according to the present invention. A plurality of cooling heat exchangers 101 are adapted to cool various components with coolant delivered through the coolant delivery line 04. Coolant which is heated in the cooling heat exchangers is provided to the heat disposal heat exchanger 105 which is arranged in association with the hydrocarbon pipeline 5 in order to dump the thermal energy in the coolant (thereby heating the hydrocarbon flow in the hydrocarbon pipeline 5).

In addition, the embodiment in Fig. 6 has a coolant buffer tank 115. The coolant buffer tank 1 15 is heat insulated and exhibits an electric heater 1 7. If, for some reason, warm coolant cannot be delivered from the various cooling heat exchangers 101 - 101e, the coolant can be heated and stored in the buffer tank 115. This can take place for instance during shutdowns, in which case no heat can be recovered from the flow of hydrocarbons or compression facility. The assembly can then still be used to assure flow in various components 119 connected to a coolant return line branch 106i. The coolant return line branch 106i can deliver warm coolant to the various components 119 of the compression facility which may need heating, for instance an anti-surge valve or dead legs Such components 119 are typically flow conducting components. The flow of warm water is routed through the coolant return line branch 106i by throttling a throttling valve 401. In cases of production shut down, the heating element 17 in the buffer tank 115 keeps the coolant (fresh water) warm and the coolant pump 107 circulates warm water to components that need warming for flow assurance.

It is also noted that one or more coolant buffer tanks 115 as the one shown in Fig. 6, also can be arranged in combination with any other embodiment described herein, for instance the ones shown in Fig. 1 and Fig. 2.

Fig. 7 illustrates an embodiment of a combination of the one shown in Fig. 3, including the subsea compression facility 200, and the one shown in Fig. 2, including the plurality of heat disposal heat exchangers 105. For simplicity the associated valves 113, hydrocarbon temperature gauges 109, and the coolant temperature gauges 111 have been left out in Fig. 7.