The present disclosure relates generally to techniques for equalizing pressure. More particularly, the present disclosure relates to systems and methods for controlled equalization of fluid pressure in subsea equipment.
A current method for pressure equalization is to install two or more valves 202 and 204 in series with a small volume between the valves. The pressure can be equalized in small steps by first opening and then closing the valve closest to the high-pressure side. Then opening and closing the valve closest to the low-pressure side. The pressure can in this manner by repeating the process several times be fully equalized between the high- and low-pressure sides.
The above-described method will cause a series of instantaneous pressure changes which magnitude will depend on the size of the volume between the two valves. A valve can usually only be operated a certain number of times before it starts leaking/malfunctioning. Hence, there is also a lifetime limitation regarding how often a valve can be opened and closed before the valve will malfunction. Traditional nozzle designs should not be used to slow down the pressure change due to high velocities, cavitation danger, unknown/uncertain behaviour at low Reynold numbers (transient and laminar zone) etc.
According to some embodiments, a system is described for controllably decreasing a pressure difference between a higher pressure volume (such as a fluid conducting pipe) and a lower pressure volume (such as a fluid conducting pipe). The system includes: a fluid inlet configured for fluid attachment to the higher pressure fluid conducting pipe; a fluid outlet configured for fluid attachment to the lower pressure fluid conducting pipe; a fluid flow line running from the fluid inlet to the fluid outlet; an accumulator or other pressure storage reservoir in fluid communication with the fluid flow line and configured to store and release fluid from the fluid flow line; and a flow restricting nozzle positioned between the fluid inlet and fluid outlet. According to some embodiments, the pressure storage reservoir is a gas or liquid contained in the higher pressure volume. In the case of a liquid, if close to its boiling point, will start evaporating to gas when the pressure starts dropping, thereby indirectly acting as an accumulator. This effect is due to the evolving gas preventing the pressure from dropping in the same manner as gas expansion from an initially trapped gas volume. According to some embodiments the system is configured for deployment in a subsea location wherein the higher and lower pressure fluid conducting pipes are located that in that subsea location. According to some embodiments, the pressure between the higher and lower pressure fluid conducting pipes is equalized. According to some embodiments, one of the conducting pipes forms part of a subsea processing station and the other is a subsea flow line. The subsea processing station, according to some embodiments, is of one of the following types: multiphase subsea pump; single-phase subsea pump; hybrid subsea pump; subsea multiphase compressor; subsea multiphase meter; subsea multiphase measurement system; subsea sampling system; subsea separation system; or a subsea manifold. According to some embodiments, the accumulator is in fluid communication with the fluid flow line between the nozzle and the fluid inlet.
According to some embodiments, the system further includes: a second accumulator in fluid communication with the fluid flow line and configured to store and release fluid from the fluid flow line; and first and second isolation valves positioned and configured to isolate the accumulator and the second accumulator, respectively, from fluid communication with the fluid flow line. The system can also include one or more additional flow restricting nozzles positioned between the fluid inlet and fluid outlet and each in parallel with the flow restricting nozzle.
According to some embodiments, the system is configured to be retrievable from a subsea location using a remotely operated underwater vehicle. According to some embodiments, the system is housed within and configured to be deployed upon a skid of a remotely operated underwater vehicle. According to some other embodiments, the system is integrated with the subsea station and is configured to be retrieved only along with the subsea station. The capacity of the accumulator and frictional loss characteristics of the nozzle can be selected to provide a desired smoothness in pressure change during operation.
According to some embodiments, a method is described for controllably decreasing a pressure difference between a higher pressure fluid conducting pipe (or other volume) and a lower pressure fluid conducting pipe (or other volume). The method includes: opening a first valve thereby allowing fluid communication along a fluid flow line between the higher pressure fluid conducting pipe and an accumulator such that pressure in the accumulator matches pressure in the higher pressure fluid conducting pipe; and then, opening a second valve thereby allowing fluid communication along the fluid flow line between the lower pressure fluid conducting pipe, a flow restricting nozzle and the accumulator. The combination of the accumulator and the nozzle providing a smooth transition towards equalization of the pressures of the higher and lower pressure fluid conducting pipes. According to some embodiments, the opening of the first valve, which can be connected to a subsea processing station, takes place following a high pressure procedure in the subsea processing station. According to some embodiments, prior to opening the first valve, one or more accumulator selection valves are operated in order to provide fluid communication of the accumulator with the fluid flow line and to isolate a second accumulator from the fluid flow line. According to some embodiments, one or more other flow restricting nozzles are positioned in parallel with the flow restricting nozzle, and prior to opening the second valve, one or more nozzle selection valves are operated so as to select the flow restricting nozzles that will be used in equalizing the pressure. The nozzle selection can be made, for example, to attain a desired smoothness in pressure equalization, and/or reduce detrimental effects due to particulate clogging of nozzles. In some cases, prior to the opening of the first valve, the accumulator can be pre-charged using a source fluid.
These together with other aspects, features, and advantages of the present disclosure, along with the various features of novelty, which characterize the invention, are pointed out with particularity in the claims annexed to and forming a part of this disclosure. The above aspects and advantages are neither exhaustive nor individually or jointly critical to the spirit or practice of the disclosure. Other aspects, features, and advantages of the present disclosure will become readily apparent to those skilled in the art from the following description of exemplary embodiments in combination with the accompanying drawings. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.
It should be noted that the term “controllable” here is used to express “to keep control” of the pressure decrease or increase during pressure equalization. That is, the pressure decrease or increase during the equalization should be predictable. Hence, the term “controllable” as used herein does not mean “adjustable”.
To assist those of ordinary skill in the relevant art in making and using the subject matter hereof, reference is made to the appended drawings, in which like reference numerals refer to similar elements:
In the following detailed description of the preferred embodiments, reference is made to accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention.
The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention; the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. Further, like reference numbers and designations in the various drawings indicate like elements.
An ROV 122 is shown being deployed from a location on the surface 104 of sea 102, such as intervention vessel 128. ROV 122 is tethered using main lift umbilical 126 to tether management system 124, which manages the free-swimming tether 127 to ROV 122. The ROV 122 is used to control the pressure equalization units 120 and/or 121, for example by manipulating valves on the units 120, 121 and/or station 108. According to some embodiments, ROV 122 can also be used to deploy and/or retrieve unit 120 by positioning and connect/disconnecting unit 120 to/from station 108.
In one example, the equalization unit 120 is used to reduce fluid pressure in station 108 in a controlled manner, for example following a pressure test of station 108. In this example the hot stab valves 302 and 304 are opened the pressure in the accumulator 310 is increased to match the pressure in station 108. The bleed valve 314 is opened and the combination of the accumulator 310 and nozzle 312 allow for depressurization of station 108 in a controlled manner. Note that the bleed valve 314 can lead back to the flow line 106, to the topside or into the sea, depending on the type of fluid and environmental considerations. Thus, the pressure gradient during depressurization is predictable according to the properties of the accumulator 310 and nozzle 312, as well as by the pressure differential.
According to some embodiments, the operation of the equalization unit 600 will be described in the context of several exemplary cases. In a first example case, a high pressure exists in the subsea station 108 following a procedure such as a pressure test of the station 108. Valves 650 and 652 are closed on both sides equalization unit 600 (which can be either permanent or retrievable). Using fluid supply valve 652, the MeOH supply system (or other fluid supply systems) can be used to pressurize the accumulator 610 (by opening valve 640 and closing valve 642) to a desired pressure. Valve 650 is then opened in order to take the initial pressure drop in the station 108. The pressure in the station 108 decreases as the pressure in accumulator 610 increases. One or more of the valves 632, 634 and 636 are opened which select which one or more of the nozzles 622, 624 and 626 are to be used. Valve 660 is then opened. The pressure in the station 108 is then gradually decreased towards pressure in flow line 662. The capacity of accumulator 610 and properties of one or more of the nozzle being used will determine the pressure drop gradient. Preferably, the pressure drop gradient is selected so as to be suitable for the station 108. For example, in the case where station 108 is a subsea pump, the nozzles are selected so as to match the response time for the barrier fluid system of the pump (not shown in
In a second illustrating case, the flow line 662 initially has a higher pressure compared to station 108. In this case, accumulator 612 is used (by opening valve 642 and closing valve 640) in combination with one or more of the nozzles 622, 624 and 626 (by opening or closing valves 632, 634 and 636) to avoid an overly abrupt pressure increase inside the station 108. Thereby avoiding undesirable effects, such as shifting the pump shaft or exceeding the maximum rate of pressure change for the pump barrier system.
In general, the system 600 can be used for all cases where a smooth pressure transition is desired between a high pressure and a low pressure section. Installing isolation valves on the accumulators allows (such as valves 640 and 642) the unit 600 to be used for equalization where the higher pressure is on either side of the unit 600. Furthermore, by using several nozzles in parallel (such as nozzles 622, 624 and 626), increased versatility (by accommodating various pressure differentials and pressure gradients) increase system robustness (by providing redundant nozzles and higher tolerance for particulate matter) is provided.
Thus the described techniques, according to some embodiments, solves problems associated with the prior art technique of repeatedly opening and closing two valves separated by a small volume, including: (1) the problem with the number of valve operations; and (2) abrupt and/or unpredictable pressure changes.
Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. For example, although many embodiments have been described herein in the context of various types of subsea applications, the techniques described herein are also applicable beyond the subsea environment. Examples of surface applications for the pressure equalization techniques described herein include refining and chemical processing, but in general the techniques described herein can be applied to any surface setting where pressure change and/or equalization is desirable.
It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words, which have been used herein, are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.