Method and a system for controlling the temperature of a fluid in an unbonded flexible pipe

Abstract

The present invention relates to a method and a system comprising a floating unit for processing, handling or storing a fluid and at least one unbonded flexible pipe for transporting said fluid to the floating unit. The unbonded flexible pipe comprises an electric heating system, and the temperature of the fluid in the unbonded flexible pipe is measured and the measured temperature is used to control the electrical input to the electric heating system in the unbonded flexible pipe.

Claims

1. A method for processing, handling or storing a fluid via a floating unit, at least one unbonded flexible pipe for transporting said fluid to the floating unit, and an electric heating system connected to an electric power source, said unbonded flexible pipe comprising a first end terminated in an end-fitting, which is connected to a connector on the floating unit, and a second end connected to a source for fluid, and the first end extending from the end-fitting up to 1500 m towards the second end, the method comprising: the temperature of the fluid in at least a part of the first end of the flexible pipe being measured; the measured temperature being used as a parameter for partly or fully controlling the electric power input from the electric power source to the heating system; and the electric power input to the heating system being controlled according to the measured temperature parameter such that the fluid in the first end of the flexible pipe has a temperature substantially corresponding to a predetermined temperature.

2. The method according to claim 1, wherein the heating comprises direct heating of the fluid.

3. The method according to claim 1, wherein the heating system comprises a carcass.

4. The method according to claim 1, wherein the predetermined temperature is close to the optimal temperature for subsequent processing, handling or storing of the fluid on the floating unit.

5. The method according to claim 1, wherein the electric power input is provided in pulses to the electric heating system.

6. The method according to claim 5, wherein the pulses have a length in the range from about 10.sup.6 Hz to about 10 Hz (PWM mode) or from about 10 Hz to about 10.sup.3 Hz (switch mode).

7. The method according to claim 1, wherein the electric power input is provided as AC current.

8. The method according to claim 1, wherein the electric power input is provided as DC current.

9. The method according to claim 1, wherein the means for controlling the electric power input to the heating system comprise a transformer with variable turns ratio or a diode.

10. The method according to claim 1, wherein the means for controlling the electrical power input to the heating system comprise a thyristor switch operated either in PWM mode, switch mode or a combination hereof.

11. The method according to claim 1, wherein the temperature of the fluid in the unbonded flexible pipe is measured by at least one optical sensor.

12. The method according to claim 1, wherein the heating system is present in an unbonded flexible pipe made from multiple sections.

13. The method according to claim 1, wherein the heating system comprises a pressure armour.

14. The method according to claim 1, wherein the heating system comprises a tensile armour.

15. The method according to claim 1, wherein the unbonded flexible pipe comprises at least one insulating layer.

16. The method according to claim 1, wherein the predetermined temperature is in the range of about 30 C. to about 130 C.

17. The method according to claim 1, wherein the fluid is extracted from a well and having a first temperature, and the fluid is heated in the flexible pipe to a second temperature which is higher than the first temperature.

18. The method according to claim 1, wherein the second temperature is 30 C. or higher.

19. The method according to claim 1, wherein there is no heat exchange between the fluid and a heat exchanger on the floating unit before processing the fluid.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The invention will now be described in further details with reference to embodiments shown in the drawing in which:

(2) FIG. 1 shows an embodiment of the method according to the invention;

(3) FIG. 2 shows a diagram according to the invention;

(4) FIG. 3 illustrates how the temperature may be controlled according to the invention.

(5) The figures are not accurate in every detail but are only sketches intended to the show the principles of the invention. Details which are not a part of the invention may have been omitted. In the figures the same reference numbers are used for the same parts.

(6) FIG. 1 shows an embodiment in which the method is carried out. A floating unit 1 is floating on the sea surface 2. Via a connecting structure 3 the floating unit 1 is connected to an unbonded flexible pipe 4 having a first end 4a and a second end 4b, which extends from the connecting structure 3 to a sub-sea well 5 located at the seabed 6. The unbonded flexible pipe 4 hangs in an S-shaped configuration and is supported by floats 7 and a support rail 8 located on the seabed 6.

(7) A control device and an electric power supply are located on the floating unit 1 and not visible on the figure. The unbonded flexible pipe 4 comprises a heating system and a temperature monitoring system. In this embodiment the heating system and the temperature monitoring system extend through the entire unbonded flexible pipe, and it is possible to heat and monitor the temperature in both the first end 4a and the second end 4b of the unbonded flexible pipe 4.

(8) The heating system comprises a metallic electrically conductive carcass, which is the primary heating unit. The electrically conductive carcass is the carcass in the unbonded flexible pipe 4. A metallic electrically conductive tensile armour in the unbonded flexible pipe 4 serves as a return path for the current which is sent to the carcass. Consequently, when an electric current is sent through the Joule heating will occur due to the electrical resistance in the metallic material. The carcass is in direct contact with the fluid conveyed in the unbonded flexible pipe 4, and the heat generated in the carcass will be transferred rapidly into the fluid.

(9) When the fluid is conveyed from the well 5 to the floating unit 1 via the unbonded flexibe pipe 4, the temperature is measured in the fluid in the bore of the unbonded flexible pipe by temperature sensors connected to the temperature monitoring system which forms part of the control device. The temperature monitoring system is based on optical sensors and is able to provide a rather precise temperature profile of the fluid conveyed in the unbonded flexible pipe 4.

(10) When the temperature monitoring system receives a signal from the temperature sensors, this signal is sent to and processed in the control device. The signal comprises information about a measured temperature of the fluid. If the measured temperature is below a predetermined temperature, the control device will send a signal to the power source causing an electrical input to be sent to the heating system in the unbonded flexible pipe, i.e. the carcass. If the measured temperature corresponds to or is above the predetermined temperature, no electrical input will be sent to the heating system.

(11) The principles are shown in FIG. 2, which is a simplified diagram illustrating the principles of the invention. The control device 10 is connected to the unbonded flexible pipe 4 via the temperature monitoring system 11 and the power source 12, and the lines 13, 14, 15 and 16.

(12) During operation the temperature monitoring system 11 receives temperature measurements from temperature sensors in the unbonded flexible pipe 4 via line 13. The temperature measurements correspond to the temperature of the fluid in the pipe at certain positions in the pipe. The temperature measurements are collected and processed in the temperature monitoring system 11 and the result is delivered to the control device 10 via line 14. In the control device 10 the result is compared to a predetermined temperature, and if the result is below the predetermined temperature, the control device 10 will send a signal to the power source 12 via line 15. In response to the signal, the power source 12 will send an electrical signal to the heating system in the unbonded flexible pipe. The electric power input is sent as pulses and the power source will continue to deliver these pulses of electric power input until the fluid at a certain position in the unbonded flexible has reached a desired temperature. In the first end of the pipe the desired temperature should substantially correspond to the predetermined temperature at which the fluid is delivered to the floating unit.

(13) FIG. 3 illustrates how the temperature of the fluid may be controlled. S is the position of the fluid source, e.g. a well, and the fluid has the temperature T.sub.S. T.sub.1 and T.sub.2 are the temperatures of the fluid, when it is delivered from the unbonded flexible pipe to the floating unit at position P with or without heating.

(14) Curve A shows how the temperature will decrease along the length of the pipe when no heat is applied. When the fluid reaches the floating unit, the temperature will change from T.sub.S to T.sub.1, which is a decrease in the temperature of T.sub.1. However, if heat is applied to the fluid during its passage through the unbonded flexible pipe, curve B illustrates that the temperature will reach T.sub.2 and the temperature will be increased by T.sub.2 when compared to the temperature T.sub.S at the source. The temperature T.sub.2 may be selected to correspond to the processing temperature of the fluid on the floating unit.