NO20191083A1 - Subsea flushing System and Method - Google Patents

Description

Description

Title: SUBSEA FLUSHING SYSTEM AND METHOD

Field and Invention

The present invention regards a system and method for flushing components of subsea hydrocarbon production systems using high velocity circulation in a flow loop through the components. The hydrocarbon-rich fluid is sent through the flow loop adding some small amounts of the “clean” displacing fluid, which also act as power fluid for round circulation, while the same amounts of fluids (mixed oil/gas and clean fluid) are diverted from the flow loop.

Background of Invention

Offshore oil and gas fields are often developed using subsea systems. A subsea system typically comprises of subsea wells, manifolds, flowlines, pipelines, jumpers, valves, boosters, pumps, compressors, separators, chokes, power lines, signal lines, instrumentation, injection system for production chemicals, etc.

Most parts of the subsea system are placed in subsea structures called "templates", for ease of access, retrieval, installation and maintenance. For example, a subsea manifold template might have a piping manifold, on-off valves and the hydraulic system to operate the valves. The components are typically located within modules in a template (as indicated in Fig. 1). The module has isolation valves and connectors to interface with or to isolate it from other modules (as shown in Fig.2).

In subsea systems it is often necessary to remove, replace or upgrade parts of the subsea system for example jumpers, manifolds, multiphase meters, pumps, etc. The system is typically flushed with a hydrateinhibiting substance such as mono ethylene glycol (MEG) or Nitrogen . This is done to reduce the content of a pre-existing fluid in the system or component.

For example, for subsea systems that are producing hydrocarbons, the pre-existing fluid will likely be a mixture of liquid and gaseous hydrocarbons and water. If a component is to be removed, the concentration of hydrocarbons in the component should be reduced by flushing to a minimum such that harmful discharges to sea are minimized when the component is isolated and disconnected from the rest of the system.

For example, for subsea systems that are in the commissioning stage, the pre-existing fluid could be salt water, that has been used earlier to pressure test the system. In some components, the prolonged presence of salt water could lead to corrosion and ultimately compromise the structural integrity of the component. Therefore, flushing is often used to remove the salt water from the system.

Flushing typically involves pumping/compressing flushing fluids from one end of the system, through an access port and discharging from another end. It is typically performed through the existing flowlines for chemical inhibition (e.g. pumping from the receiving platform), from a ship, or a combination of both.

The flushing from the vessel is typically made by connecting an umbilical (with a diameter line of 2-4 inches in diameter) and sending displacing fluid into the system. The umbilical is connected through an access port in the template upstream or downstream the isolation valves of the component. The displacing fluid is then pumped from the vessel and "pushes" the unwanted fluids upstream (e.g. into the well) or downstream (e.g. towards the flowline) the component. As a rule of thumb, injecting 4 times the total volume of the geometry and using a velocity higher than 1 m/s, to ensure turbulent flow, will reduce the concentration of unwanted fluids to acceptable levels.

In some occasions flow-back to the ship is performed, for example when draining a high point in the system. In this case, a return line/umbilical is installed in an access port. However, this is usually not desirable because the oil that flows back represents an economic loss and the deck of the ship must be equipped with a hydrocarbon handling system.

Novel methods that make the flushing process more efficient, either by requiring smaller volumes of cleaning fluids, i.e., mono-ethylene glycol (MEG) or using lower rates, or simplification of the system are of high interest to the industry because of cost reduction:

● Less vessel time is required for the operation

● It enables using a smaller flushing umbilical

● Reduce the amounts of MEG required, and therefore, costs

In the prior art, US8978767 describes a subsea well service system and a method for use with a subsea pump capable of flushing the well intervention lubricator of an underwater hydrocarbon production facility. This method requires tanks subsea for storage of fluids which are flushed through the system. According to the information presented in sections / columns 7 and 8, methods A-1, A-2, B-1 and B-2, the fluid is not intentionally recirculated in a closed loop. In Fig. 1, 120 is the pump and 100 is a tank with some amount of the displacing fluid. The fluid displaced, coming out of the lubricator 20 in "B" (rich in hydrocarbons) can go either to 110 or to the destination market "sea, flowline or well". However, it is not explicitly described anywhere in the patent that this hydrocarbon-rich fluid entering the tank 110, will be intentionally pumped through the lubricator (20) again

EP1565277 describes a method to clean contaminants, foulants, sludge, deposits from equipment. This is done by circulating in a closed loop hydrocarbon-based fluid at high pressure and temperature. In this patent, the circulation is made with HC based fluids that can dissolve the deposits (usually harmful to the environment) The system is initially filled (in one step or sequentially) with HC based fluids and then circulated in a closed loop, without adding any more fluid while circulating. Furthermore, for disconnection or dismounting of an equipment or sub-system, this method will not remove hydrocarbons from the system.

US8684089 describes a system for circulating fluid in a subsea intervention stack. The present invention relates to a fluid circulation system for circulating fluid in a subsea cavity, the cavity being filled with a first fluid and having first and second end ports. The system comprises a container containing a second fluid, fluid lines extending from the container to the first and second end ports of the cavity, respectively, and a pump for exchanging the second fluid provided in the container and the first fluid provided in the subsea cavity . The invention also relates to a method for circulating fluid in a subsea cavity. This method requires tanks subsea for storage of fluids which are flushed through the system.

US20030056954 describes methods and apparatus for a subsea tieback.

Hot liquids are pumped through the inner pipe to control the temperature of the fluids flowing through the outer pipe. In an open circuit, the fluids pumped through the inner pipe are compatible with the fluids in the outer pipe so that they may be mixed and commingled. In a closed circuit, the liquids passing through the inner pipe are compatible with the environment around the outer pipe. In still another closed circuit, the hot fluids may be any available fluids that can be circulated through an inner pipe and a return pipe. This is not considered relevant for the present invention.

The object of this the present invention is to provide a method and device to save costs and to simplify subsea flushing of components prior to retrieval to surface;

● Less vessel time required for the operation

● Enables using a smaller flushing umbilical

● Reduce the amounts of MEG required

SUMMARY OF THE INVENTION

The present invention is applicable to parts of the subsea systems that transport fluids and that are somewhat "localized" and not spread over long distances, for example below 200 meters. It must be possible to connect the two ends of the component or "part" either with a permanent structure or with a retrievable tool, and to induce a high flow rate fluid circulation through it.

The invention is a system and method to perform flushing in subsea systems. The system consists of:

● A bypass line that runs parallel to the equipment/part to flush and is connected in one end upstream the equipment and on the other end to downstream the equipment. ● A pressure boosting device installed on the bypass line able to create a high-rate fluid circulation.

● An inlet port that allows to inject the flushing fluid into the bypass line, or upstream or downstream the equipment.

● Isolation valves upstream or downstream the equipment.

The method consists of, having one of the isolation valves closed (either on the upstream end or downstream end) injecting flushing fluid into the system while simultaneously inducing a high circulation rate using the pressure boosting device. The fluid is then recirculated in a closed loop consisting of bypass line and equipment, while part of the mix is diverted through the isolation valve which remains open.

The recirculation creates uniform mixing of fluids in the domain. Flushing fluid is added gradually to the domain, and the mixture is also gradually removed from the domain. Under these conditions, the volume fraction of the unwanted substance in the domain will exhibit an exponential decay with time. To achieve a target concentration of the unwanted substance in the domain, one could use a higher injection rate or wait longer times.

In the new invention, the recirculation is performed in a loop which is not completely closed because there is inlet of displacing fluid (non-HC fluid) and outlet of the mixture (HC-rich fluid).

To our opinion and technical judgment, and after reviewing the patents we have identified, it seems our idea and method has enough distinct and novel characteristics to merit patenting, namely:

- Create flow circulation in an "almost" closed loop system.

The closed loop system consists of piping parallel to the equipment to flush, a set of valves to allow communication between the equipment and the parallel piping, a pump (preferably a jet pump) to induce circulation, a port of inlet for the cleaning/flushing fluid and valves to isolate the equipment. It does not require tanks, reservoirs, or containers of volumes comparable with the equipment to flush.

-Inject gradually a clean and environmentally friendly fluid while circulating. This is done with a separate hydraulic line from a vessel, a platform, etc. that is connected to the inlet port.

- Gradually remove a dirty/hydrocarbon-rich fluid from the domain. This is achieved by simply opening one of the valves that isolate the equipment, e.g. upstream or downstream.

- Flow circulation means that most of the hydrocarbon-rich fluid removed from the equipment will be reinjected repeatedly into the equipment but with small amounts of a flushing (clean) fluid. Due to the fact that one is injecting gradually small amounts of a clean fluid and is removing small amounts of dirty fluid, the hydrocarbon content in the equipment will ultimately be reduced.

Abstract

The present invention relates to a fluid flow loop circulation system for flushing of equipment (170) using a bypass line containing a jet pump (150) in a subsea module (220).

High pressure power fluid is supplied to the jet pump through the access port (160). The power fluid, i.e., mono ethylene glycol (MEG), is injected into the bypass line generating high flow velocity and round circulation through the equipment (170). The hydrocarbon-rich fluid is sent through the flow loop adding some small amounts of the "clean" displacing fluid (power fluid), while the same amounts of fluids (mixed oil/gas and clean fluid) are diverted into the flow line (240 ) when the well (230) is isolated, or into the well (230) when the flow line (240) is isolated.

Figure 3

Short description of drawings

Fig. 1 shows a subsea template made of several modules, with the inlet and outlet flowline indicated

Fig. 2 shows a module and equipment within a module

Fig. 3 shows a flushing system as a permanent part of the subsea template employing a jet pump and external supply of flushing fluid

Fig. 4 shows a flushing system as a permanent part of the subsea template employing a jet pump and internal supply of flushing fluid

Fig. 5 shows a flushing system assembled by connecting a tool with a jet pump external to the subsea template.

Fig. 6. Flushing system assembled by connecting a tool with jet pump external to the subsea template. The equipment to flush (1 and 2) are spread in two modules.

Fig. 7. Shows a flushing system as a permanent part of the subsea template employing a rotordynamic pump with external supply of flushing fluid and power

Fig. 8 shows a flushing system as a permanent part of the subsea template employing a pump with internal supply of flushing fluid and power

Fig. 9 shows a flushing system assembled by connecting a tool with a power powered pump external to the subsea template

Fig. 10 shows oil volume fraction in jumper versus time using traditional flushing

Fig. 11 shows oil volume fraction in jumper versus time using the new method

Detailed description

An example of a possible flushing configuration is shown in Fig. 3. Consider a subsea module (220), with an upstream side (230) and a downstream side (240) with module isolation valves (120) and (180) and connectors ( 110) and (200). A bypass line has been placed between (250) and (260), upstream and downstream of the equipment (170). A Jet pump or ejector (150) has been placed in this bypass line. In this particular drawing the jet pump has been connected to induce circulation from (130) to (190), but it could also have been connected in the opposite direction. Two isolation valves (130) and (190) have been placed on the bypass line to be closed during normal operations. An on-off valve (140) and an access port (160) have been connected to the power fluid side of the jet pump (150).

The flushing method consists of:

1. Isolate the downstream side of the module, or the upstream side of the module. This is done either by;

● opening valves (120) and (100), and closing (180) and (210), or

● opening valves (180) and (210), and closing (120) and (100)

2. connect an umbilical or flowline to provide the flushing fluid to port (160) (e.g. from a vessel)

3. Open valves (130), (140), (190).

4. Inject flushing fluid through the umbilical and port (160). The flushing fluid activates the jet pump and creates a high circulation rate through (150), (190), (170), (130), in the clockwise direction. If the jet pump is connected in the opposite way, the circulation will be counter-clockwise.

5. Let some time pass. The mixture of flushing and pre-existing fluid will gradually exit the module (either through the upstream, (230) or downstream (240) side, whichever is open). For example, if the downstream side is open, the mixture will flow towards the processing facilities, invading pipe manifolds, flowlines and pipelines located downstream the module. If, on the contrary, the upstream side is open, then the mixture will flow towards wells, manifolds and jumpers located upstream the module. The total volume of the mixture that exits the module will probably be small compared with the total volume of the production system. Therefore, the flow induced towards the upstream or downstream side will probably be small as well. Some pressurization might occur during the flushing process.

In the present invention, besides recirculating in a closed loop, the loop is not completely closed, but there is inlet of displacing fluid (non-HC fluid) and outlet of the mixture (HC-rich fluid). In the present method, one intentionally sends the hydrocarbon-rich fluid again through the geometry adding some small amounts of the "clean" displacing fluid, while the same amounts of fluids (mixed oil and clean fluid) are diverted into the flow line or the well.

Another example of a similar configuration is shown in Fig. 4. In this case, the supply of flushing fluid (270) is provided by the existing chemical injection system pre-existing in the subsea template, thus the flushing operation can be performed without assistance from a vessel.

Another example of a flushing configuration is shown in Fig. 5. In this case, a tool with supply of cleaning fluid (MEG) to a jet pump is connected to the subsea module. After the circulation and cleaning of the component or module, the tool cab be disconnected and retrieved. This system does not require to have the pump on the subsea module, which might be attractive to reduce costs and maintenance frequency, but it does require to have two access ports with isolation valves, one connected upstream the equipment and one connected downstream the equipment. The ports could be connected with the tool through, for example, flexible hoses or umbilicals. This flushing system could be more attractive and flexible to flush equipment that is spread over two or more modules or templates, as indicated in Fig. 6.

Another example of a flushing configuration is shown in Fig. 7. In this case, a tool with supply of cleaning fluid (MEG) is connected to the circulation loop and an electric-powered pump is used for circulation and cleaning of the component. After the circulation and cleaning of the component or module, the tool can be disconnected and retrieved.

A variation of the system shown in Fig. 7 is shown in Fig. 8. It consists of using flushing fluid from the chemical distribution system of the template/module (100) and electrical power from the template/module (110).

Another variation of the system shown in Fig. 7 is shown in Fig. 9. It consists of the electric powered pump and bypass line to be part of the subsea tool and not in the module/template.

Any type of pump and power supply can be used. A Jet pump powered with high pressure fluid (cleaning fluid) through a small diameter umbilical can create high flow rate and sufficient boosting pressure for high circulation rate and velocity in the circulation loop. The power fluid (cleaning fluid) is mixed into the circulation loop. Alternatively, other types of hydraulic powered pumps (rotordynamic, positive displacement pumps, etc.) can be used with the power fluid injected into the circulation loop. Electric powered pumps can also be used. For the sample configurations described above, the equipment to flush was located within a module on the subsea template. However, the proposed method and system can also be applied when the equipment and system to flush is spread over several templates or modules, as long as a high circulation rate can be achieved. However, a larger system will require a larger and more powerful device to induce flow circulation, thus this is a limiting factor.

In regular flushing operations where the standard flushing procedure is employed there is usually a minimum rate required to achieve a certain residual volume fraction. If the right rate is not used, one could circulate for as long as possible, without achieving a significant variation in the residual amount of the unwanted substance.

Power supply and fluid injection for flushing can also be provided from a remote operated vehicle (ROV), from an autonomous operated vehicle (AUV) or from local storage and power supply located on the seabed.

The described system and method may also be used for flushing of any fluid containing systems located both onshore and offshore.

As an example case, numerical simulations were performed on a Jumper-like geometry (U-shaped piping) with 6” diameter and 2 m height, 4 m length of water flushing oil. The standard flushing method and the new proposed method were tested in the simulator. Simulations were performed with a transient multiphase simulator and verified with experimental data.

Fig. 10 shows the oil volume fraction in the domain versus time for the standard flushing method for several values of flushing water rates. It can be seen that, while the reduction of oil fraction is initially almost linear with time, after a short time, it reaches a minimum value and it is not possible to reduce it further.

Fig. 11 shows the oil volume fraction in the domain versus time for the new flushing method for several values of flushing water rates. It can be seen that higher injection rates of water the drop in oil volume fraction is quicker in time, but, given enough time, almost all curves reach low values of oil volume fraction.

For example, if one were to flush the jumper using the standard method (no recirculation), and it is necessary to reach a remaining oil volume fraction of 7% in the domain, then a flushing rate of 20.77 m3/h or higher is required (based on the data presented in Fig. 10). However, if the flushing method proposed in this application is used (with recirculation), then one could reach the same remaining oil volume fraction by injecting 8 m3/h only and waiting 700 s (based on the data presented in Fig. 11). This represents a reduction in the required flow rate of MEG of 61%. It the MEG is injected from a vessel with an umbilical, this reduction might allow to use a smaller MEG pump and umbilical diameter.

Claims (10)

Claims

1. A system to perform flushing of components in hydrocarbon subsea production systems comprising:

a. A line bypassing the component with at least one isolation valve. b. An access port for supply of cleaning fluid

c. A pump to create fluid circulation through the bypass line and the components

d. At least one isolation valve placed upstream or downstream the component.

Characterized in that:

The bypass line, the access port, the pump to create fluid circulation, the isolation valve and the component are connected in such a way that fluid round-circulation can be established so that the hydrocarbon-rich fluid can be sent through the geometry adding some small amounts of the “clean” displacing fluid, while the same amounts of fluids (mixed oil and clean fluid) are diverted into the flow line or the well, or any other fluid collecting device.

2. The system according to claim 1 in which the line bypassing the component, the access port and the pump to create fluid circulation are outside the subsea template and part of an intervention tool attached to the umbilical from the vessel or from the subsea system

3. The system according to claim 2 in which the pump to create flow rate circulation is a jet pump (ejector pump) and the power fluid is the flushing fluid.

4. The system according to claim 1 in which the flushing system is a permanent part of the subsea template.

5. The system according to claim 4 in which the device to create flow rate circulation is a jet pump (ejector pump) and the power fluid is the flushing fluid.

6. The system according to claim 2 in which the pump to create flow rate circulation is a rotordynamic pump that requires electrical power to be actioned.

7. The system according to claim 4 in which the device to create flow rate circulation is a rotordynamic pump that requires electrical power to be actioned.

8. A system according to claim 4 in which the flushing fluid comes from the existing chemical injection system in the subsea template.

9. A system according to claims 4 in which the flushing fluid comes from a vessel via an umbilical connected through an access port.

10. A method to perform flushing of component in hydrocarbon subsea production systems comprising the steps of

a. establishing a bypass from an upstream to a downstream side of the component,

b. Isolating the system downstream or upstream the component to flush, but allowing one of said downstream and upstream sides to be open, c. Injecting a flushing fluid through the access port.

d. Activating fluid circulation through the bypass and the component e. Allowing the mixture of undesired fluid and flushing fluid to exit through the non-isolated end of the system