Method and system for mercury removal and capture in a hydrocarbon feed stream

Abstract

A method of producing a reduced mercury content hydrocarbon feed stream downstream of a subsea well involves removing at least a first fraction of mercury from a hydrocarbon feed stream at a subsea or subsurface location within or adjacent a subsea wellhead or well 5 completion connected with the subsea well. The mercury in the hydrocarbon feed stream is coalesced and may be stored as liquid or converted to an inert particulate solid. Due to this removal of the mercury, the hydrocarbon feed stream downstream of the subsea or subsurface location has a reduced mercury content in comparison to the hydrocarbon feed stream, up stream of the subsea location. The removed mercury may be held in subsea 0 tanks that can be recovered to the surface. Alternately at least some of the mercury may be removed in and always remain within the well never leaving the subsurface environment.

Description

[0001] METHOD AND SYSTEM FOR MERCURY REMOVAL AND CAPTURE IN A HYDROCARBON FEED STREAM

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a method and system for mercury removal and capture in a hydrocarbon feed stream.

[0004] BACKGROUND ART

[0005] Hydrocarbon production feed streams contain numerous contaminants. Where and how a particular contaminant is removed from a production wellhead to downstream processing and refining plant is dictated by many factors. These include the type of containment and accessibility to the feed stream, for example at subsea, offshore or onshore. It may be most desirable yet most difficult to remove a particular contaminant as upstream as possible. This avoids contamination of downstream equipment and facilities. This is particularly significant where the contaminant is hazardous to life forms and / or has substantial cost implications for recovery, disposal or otherwise making safe contaminated equipment and plant. An example of such a contaminant is mercury.

[0006] The above references to the background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art.

[0007] SUMMARY OF THE DISCLOSURE

[0008] In one aspect there is disclosed a method of. reducing mercury concentration in a hydrocarbon feed stream downstream of a subsea well comprising: removing at least a first fraction of mercury from a hydrocarbon feed stream at a subsea or subsurface location within or adjacent to a subsea well or well completion connected with the subsea well, wherein the hydrocarbon feed stream downstream of the subsea location has a reduced mercury content in comparison the hydrocarbon feed stream up stream of the subsea or subsurface location.

[0009] In one embodiment removing at least the first fraction of mercury includes coalescing mercury contained in the hydrocarbon feed stream at the subsea location.

[0010] In one embodiment the method comprises storing the first fraction of removed mercury within a subsea storage facility. In one embodiment storing the first fraction of removed mercury comprises storing the first fraction as a liquid.

[0011] In one embodiment storing the first fraction of removed mercury comprises absorbing the mercury in a solid sorbent or converting the mercury into a solid or particulate material.

[0012] In one embodiment removing at least a first fraction of mercury from the hydrocarbon feed stream includes dividing the hydrocarbon feed stream into a plurality of parallel flow streams and coalescing mercury in each of the plurality of parallel flow streams separately from each other.

[0013] In one embodiment removing at least a first fraction of mercury from a hydrocarbon feed stream includes passing the hydrocarbon feed stream sequentially through two or more mercury coalescing systems.

[0014] In one embodiment removing of at least a first fraction of mercury is performed in one or more first vessels and the storing of the removed mercury is performed in one or more subsea mercury storage vessels, wherein the first vessels are separate from the subsea mercury storage vessels.

[0015] In one embodiment the method comprises swapping the subsea storage facility in situ with a fresh or reconditioned subsea storage facility.

[0016] In one embodiment the method includes returning mercury vapour formed in the subsea storage facility to the mercury removal system.

[0017] In one embodiment the method includes pressurising the subsea storage facility with an inert gas.

[0018] In one embodiment the method includes swapping the one or more first vessels in situ with one or more fresh or reconditioned first vessels.

[0019] In one embodiment removing the at least a first fraction of mercury and the storing of the first fraction of removed mercury is performed in one or more common vessels, wherein a portion of the first fraction of mercury removed from the hydrocarbon feed stream flowing through a specific common vessel is stored in the same common vessel. In one embodiment the method comprises sensing a volume of mercury in the common vessel.

[0020] In one embodiment the method comprises swapping a common vessel in situ with a fresh or reconditioned common vessel.

[0021] In one embodiment the method includes selectively connecting the hydrocarbon feed stream upstream of the subsea location to a production line upstream of the subsea location to bypass the mercury removal system.

[0022] In one embodiment the method includes bypassing a flow of the hydrocarbon about one or more common vessels.

[0023] In one embodiment the first fraction comprises substantially all the mercury.

[0024] In one embodiment the method comprises removing at least a portion of solid particulate materials from the hydrocarbon feed stream before removing the at least a first fraction of mercury from a hydrocarbon feed stream.

[0025] In one embodiment the subsea location is within the subsea wellhead or a xmas tree mounted on the wellhead and the first fraction is less than substantially all the mercury to provide a reduce mercury fraction hydrocarbon feed stream.

[0026] In one embodiment the method comprises forming the mercury removal system as a distributed system with a first portion of the mercury removal system that removes a portion of the first fraction of mercury within the wellhead or the xmas tree and a second portion of the mercury removal system that removes a remaining portion of the first fraction of the mercury from the reduced mercury fraction hydrocarbon feed stream upstream of the wellhead or the xmas tree.

[0027] In a second aspect there is disclosed a method of removal and capture of mercury from a hydrocarbon feed stream comprising: connecting a mercury removal system at a subsea location to a hydrocarbon feed stream wherein the mercury removal systems is arranged to remove mercury in the hydrocarbon feed stream and allow a flow of the hydrocarbon feed stream substantially free of mercury to and through downstream infrastructure. In one embodiment the subsea location is within a subsea well completion.

[0028] In one embodiment the subsea location is between a Xmas tree of the subsea well completion and the downstream infrastructure.

[0029] In one embodiment the method includes transferring the removed mercury to a subsea storage facility.

[0030] In one embodiment the transferring includes transferring the removed mercury to one or more subsea storage tanks.

[0031] In one embodiment the connecting comprises releasably connecting the mercury removal systems to the hydrocarbon feed stream.

[0032] In one embodiment the method includes connecting a bypass line between the subsea location and the downstream infrastructure enabling the hydrocarbon feed stream to bypass the mercury removal systems.

[0033] In one embodiment the method includes releasably connecting the mercury removal systems to the subsea storage facility wherein the subsea storage facility can be retrieved from the subsea location.

[0034] In one embodiment the method includes in situ swapping of the subsea storage facility with fresh subsea storage facility.

[0035] In one embodiment the method includes providing a mercury vapour return flow path between the one or more subsea storage tanks and the mercury removal systems whereby mercury vapour forming in the subsea storage facility is directed to flow back to the one or more mercury coalescing systems.

[0036] In one embodiment the method comprises holding a volume of an inert gas in each of the subsea storage facility.

[0037] In one embodiment the method includes absorbing the removed mercury in a solid medium in the subsea storage facility or conducting a process in the subsea storage facility to convert the removed mercury into a solid material. In one embodiment the method includes mounting the mercury removal systems and the subsea storage facility on a common skid and deploying the skid to the subsea location.

[0038] In one embodiment the connecting includes connecting the mercury removal system within a Xmas tree or well head of the completion.

[0039] In one embodiment the method includes coalescing a first fraction of mercury from the feed stream within the Xmas tree or well head wherein the feed stream leaving the well head has a reduced fraction of mercury.

[0040] In one embodiment the method includes feeding the feed stream with the reduced fraction of mercury to one or more additional mercury coalescing systems and transferring the coalesced mercury to a subsea storage facility.

[0041] In a third aspect there is disclosed system for subsea removal and capture of mercury from a hydrocarbon feed stream comprising: a mercury removal system capable of being fed a hydrocarbon feed stream the mercury removal systems arranged to removal at least a fraction of mercury carried in the hydrocarbon feed stream and produce a downstream flow of hydrocarbon feed stream substantially free of mercury; and a mercury storage facility in fluid communication with the mercury removal system for receiving and storing the removed fraction of mercury.

[0042] In one embodiment the mercury removal system is arranged to remove the at least a fraction of the mercury by coalescing mercury carried in the hydrocarbon feed stream.

[0043] In one embodiment the mercury collection system is in fluid communication with the mercury removal system to received and store the at least a fraction of the mercury.

[0044] In one embodiment the system comprises a bypass line, and a remotely operable valve system enabling selective opening and closing of the bypass line, wherein when bypass line is closed, the hydrocarbon feed stream is directed through the mercury removal system, and when the bypass line is opened the hydrocarbon feed stream is directed to flow past and avoid contact with the mercury removal system.

[0045] In one embodiment the system includes a mercury vapour return line enabling mercury vapour formed in the subsea storage facility to be returned to the mercury removal system. In one embodiment the system for subsea removal and capture of mercury from a hydrocarbon feed stream includes a plurality of storage facility isolation connectors connecting the mercury storage facility to the mercury removal system, and wherein the storage facility isolation connectors are remotely operable to enable the subsea storage facility to be disconnected from the mercury removal system without leakage from the mercury removal system or the mercury storage facility.

[0046] In one embodiment system for subsea removal and capture of mercury from a hydrocarbon feed stream includes a plurality of removal system connection valves connecting: an inlet of the mercury removal system to the hydrocarbon feed stream, a hydrocarbon outlet of the mercury removal system to a production pipeline, a mercury outlet of the mercury removal system to the mercury storage facility, and a mercury vapour return inlet of the mercury removal system to the subsea storage facility; wherein the plurality of removal system connection valves are remotely operable to enable the mercury removal system to be disconnected from the system for subsea removal and capture of mercury for in situ replacement.

[0047] In one embodiment the mercury removal system and the mercury storage system are disposed in a common vessel.

[0048] In one embodiment the system for subsea removal and capture of mercury from a hydrocarbon feed stream comprises a plurality of mercury coalescers and each mercury coalescer is disposed in a respective common vessel and mercury that is coalesced by a mercury coalescer in a respective common vessel is stored in the same respective common vessel.

[0049] In one embodiment the system for subsea removal and capture of mercury from a hydrocarbon feed stream comprises a connection system arranged to enable a user selectable connection between the common vessels and the hydrocarbon feed stream flowing whereby the connection can be switched at least between a parallel connection and a series connection, wherein in the parallel connection the hydrocarbon feed stream is split into a plurality of streams wherein respective streams flow through respective common vessels, and in the series connection the hydrocarbon feed stream sequentially flow through each of the series connected common vessels. In one embodiment the system for subsea removal and capture of mercury from a hydrocarbon feed stream comprises a connection system arranged to enable a standby connection for at least one common vessel wherein a connection of the common vessel to the hydrocarbon feed stream is capable of being switched between an isolated mode wherein the at least one common vessel is physically connected to the system for subsea removal and capture of mercury from a hydrocarbon feed stream but fluid ically isolated from the hydrocarbon feed stream, and an active mode wherein at least a portion of the hydrocarbon feed stream is flu idically connected to and flow through the at least one common vessel.

[0050] In one embodiment of the system for subsea removal and capture of mercury from a hydrocarbon feed stream at least a portion of the mercury removal system is located within a wellhead or Xmas tree and the portion of the mercury removal system removes at least a portion of mercury from a well production stream prior to the well production steam flowing downstream of the wellhead or Xmas tree.

[0051] In one embodiment the system for subsea removal and capture of mercury from a hydrocarbon feed stream includes a desander arranged to remove at least a portion of solid particulate materials from the hydrocarbon feed stream before removing the at least a first fraction of mercury from a hydrocarbon feed stream.

[0052] BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Notwithstanding any other forms which may fall within the scope of the system and method as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to becoming drawings in which:

[0054] Figure 1 is a schematic layout drawing of an embodiment of a system for, and that facilitates a method of, subsea removal and capture of mercury from a hydrocarbon feed stream.

[0055] Figure 2 is a schematic layout drawing of a second embodiment of a system for, and that facilitates a method of, subsea removal and capture of mercury from a hydrocarbon feed stream.

[0056] Figure 3 is a schematic layout drawing of a third embodiment of a system for, and that facilitates a method of, subsea removal and capture of mercury from a hydrocarbon feed stream. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0057] Specific embodiments of the disclosed system and method will now be described by way of example only. The terminology used herein is for the purpose of describing embodiments only and is not intended to limit the scope of the disclosed system and method. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to pertaining the system and method. In the drawings, like reference numbers refer to like parts.

[0058] The general idea behind the present disclosure is to remove contaminants that not only contaminate the hydrocarbon feed stream and need removal to produce a target product, but also contaminate the infrastructure from the well to the place of removal. The infrastructure includes subsea pipelines and solids separators. When the nature of the contaminant is such that it requires special procedures to protect workers and the environment from harm the cost in handling the infrastructure after production ceases may be in or exceed the range of USD$10sM - $100sM.

[0059] In the case of the containment being mercury, the idea of the present disclosure is to remove substantially all the mercury as close to a production well as possible, so that the hydrocarbon feed stream form that point is substantially free of mercury. Here the expressions “substantially all the mercury” or “substantially free of mercury” is intended to mean removal of mercury to a level deemed to safe by health authorities. For example, the Australian & New Zealand Guidelines For Fresh & Marine Water Quality prescribe a marine guideline of no more than 0.4 pg / L of water.

[0060] In one embodiment the disclosed method involves coalescing mercury in the hydrocarbon production feed stream at a subsea or subsurface location so that a flow of the hydrocarbon production feed stream is substantially free of mercury prior to reaching downstream infrastructure. Such infrastructure may include but is not limited to subsea pipelines, manifolds, subsea sand and solids separation equipment, gas / liquid separation equipment, compressors, and pumps, as well as offshore topsides and onshore plant and equipment.

[0061] The subsea location may be for example within a subsea or subsurface completion. This includes for example within a production flow line, a manifold or even within a well head or Xmas tree. When within a well head or Xmas tree substantial all or a reasonable fraction of the mercury may be coalesced and thus removed from the production feed stream prior to feed stream reaching the production flow line or manifold. In that event the coalesced mercury remains in effect within the subsurface environment, within the well and never reaches the external environment.

[0062] Figure 1 shows an embodiment of a system 10a for performing a method of subsea removal and capture of mercury from a hydrocarbon feed stream. At the heart of the system 10a is a mercury removal system 12. The mercury removal system 12 may include for example one or more mercury coalescers 14 each comprising a vessel 16 and an internal metal mesh screen through with the hydrocarbon flows. Mercury droplets in the feed stream deposit on the surface of the metal mesh and by capillary action coalesce to form larger drops that fall by action of gravity to a lower end of the vessel. The hydrocarbon feed stream, now substantially free of mercury flows is flows out of the vessels 16 to constitute a substantially mercury free production feed stream.

[0063] Looking at the system 10a more closely, the system 10a is connected to a Xmas tree / manifold production line 18 through an inlet isolation connection system 20. The inlet isolation connection system 20 includes a first isolation connector / hub 22a, a first gate valve 24a, a second gate valve 24b and a second connector / hub 22b all connected in series in a branch line 26. The branch line 26 at the second connector / hub22b connects with a feed line 28 that leads to the mercury removal system 12. At a branch point 30 located between the two gate valves 24a and 24b, the branch line 26 connects to a bypass line 32 via an intermediate gate valve 34a.

[0064] The feedline 28 directs the hydrocarbon feed stream through an inline valve 35 to inlets 36 of the mercury removal systems 12. A substantially mercury free hydrocarbon feed stream flows from outlets 38 of coalescers 14 of the mercury removal system 12 to respective branches of a branched flow line 40. The flow line 40 is connected through an inline valve 39 to an outlet isolation valve connector / hub system 42 to a production pipeline 44. The production pipeline 44 carries a substantially mercury free hydrocarbon feed stream to further downstream infrastructure. The production flowline / jumper 44 itself also forming part of the downstream infrastructure.

[0065] The outlet isolation connection system 42 includes a first connector / hub 46a, a first gate valve 48a, a second gate valve 48b and a second connector / hub 46b all connected in series in a branch line 50. The branch line 50 at the connector / hub 46b connects to the production flowline 44. At a branch point 52 located between the two gate valves 48a and 48b, the branch line 50 connects to the bypass line 32 via an intermediate gate valve 34b. Mercury that has coalesced in the removal system 12 flows from outlets 54 at the lower ends of the vessels 16 through a branched mercury feed line 56 to a subsea storage facility 60. In the mercury feedline 56 downstream of the outlets 54 and upstream of the storage facility 60 there are, in series connection, a pair of gate valves 62a and 62b and an isolation connector 64. Further gate valves 66a and 66b are disposed in a conduit 68 leading from the isolation connector 64 to the storage facility 60.

[0066] The storage facility 60 in this embodiment is in the form of one or more storage tanks 70.

[0067] The tanks 70 hold the coalesced mercury in a liquid form. A level transmitter 71 associated with the tanks 70 may be provided to inform an operator of the level of liquid mercury in the tanks 70. However, in an alternate embodiment the storage facility may include a sorbent material arranged to absorb the coalesced mercury into a solid medium or convert liquid mercury into a solid or particulate material, such as, but not limited to mercury sulphide.

[0068] The storage facility 60 / tanks 70 maybe pressurised by a volume of an inert gas. The inert gas may comprise nitrogen or argon. The gas acts as a blanket over the liquid mercury minimising the formation of mercury vapor. Nevertheless, mercury vapour that may form is recycled back to the mercury coalescing systems 12 though a mercury vapour return line 72. Numerous valves are disposed within the vapour return line 72. These include gate valves 74a and 74b, nonreturn valve 76, isolation connector 78 and gate valves 80a and 80b.

[0069] The system 10a includes numerous sensors to enable monitoring of the condition and status, as well as control, of components and equipment in the system 10a. The sensors include, but are not limited to:

[0070] • pressure and temperature sensor 82a between the isolation valves 24a, 24b and the gate valve 34a;

[0071] • pressure and temperature sensor 82b between the isolation valves 48a, 48b and the gate valve 34b;

[0072] • pressure and temperature sensor 84 in the vapour return line 72 between the storage facility 60 and the gate valves 74a, 74b;

[0073] • sand detectors, such as acoustic sand detectors may be deployed upstream on inlet spool 18 and outlet spool 44.

[0074] The system 10a also includes a plurality of ROV hot stabs / receptacles 86a, 86b (hereinafter “ROV hot stabs 86”) and 88. The ROV hot stabs 86a & 86b enable the use ROVs to allow access to the cavity's to permit leak testing of valves 34a, 24a 24b, and valves 36b 48a, 48b respectively By selectively closing valve 24b and valve 48a and opening 34a and 36b the bypass line 32 is open the production feed stream from the production line 18 flows through the bypass line 32 to the production pipeline 44, thereby bypassing the mercury removal system 12. This may be activated for example during well cleanup or other scenarios. The ROV receptacle 88 may be used to facilitate the injection of inert gases into the storage tanks 70 or to drain excess mercury from the collection tanks 70 via the ROV Hot stab 88.

[0075] The system 10a may also include a skid / mud mat 90 on which the mercury coalescing system 12 and the storage facility 60 are mounted. This facilitates the convenient deployment of the system 10a is a single unit from a vessel.

[0076] A subsea control module 92 and associated connected actuators 94 are operably coupled via electro and / or hydraulic control lines to the valves and other sub-systems and components of the system 10a. This SCM facilitates control of the various valves of the system 10a. The subsea control module 92 is connected to the actuators 94 by a physical electrical and / or electro / hydraulic communications and power network 96. The SCM 92 also provides power and communications to the various instruments and transducers on system 10a.

[0077] If it is necessary or desirable to do so to the storage facility 60 can be disconnected in situ by closing the isolation valves 66a, 66b, 74a, 74b, 62a, 62b, 80a ,80b and opening I disconnecting connectors 64, 78. Once disconnected, a fresh storage facility 60 can be connected in. This enables in situ swapping or replacement of the storage system 60 in the event of a system 60 reaching its storage capacity, needs reconditioning or has developed a fault. Similarly, if necessary or desirable to do so the mercury coalescing system 12 can be swapped over or replaced in situ by closing valve 24b, 35, 39, 48a, 62a, 62b, 66a, 66b, 80a, 80b, 74a, 74b and opening bypass valves 34a and 34b thereby opening bypass line 32 so that the hydrocarbon feed stream from the production line 18 which flows through the branch line 26, bypass line 32, branch line 52 into the production pipeline 44, then disconnecting connectors 22b, 46a, 78 and 64 to allow removal of unit 12.

[0078] In the embodiment shown in Figure 1 the mercury removal system 12 has a plurality of coalescers 14 connected in parallel and the subsea storage facility 60 has a plurality of storage facility has a plurality of storage tanks 70 connected in parallel. In a modified embodiment, with the installation of appropriate valving, any one of the parallel connected coalescers 14 can be isolated form a remainder of the coalescers 14. The isolated coalescer 14 does not receive any flow of the hydrocarbon feed stream. This allows for various operational scenarios which may be appropriate in prevailing conditions such as:

[0079] A) operating the coalescer vessels 14 in parallel;

[0080] B) operating the coalescer vessels 14 in series;

[0081] C) operating the coalescer vessels 14 Individually; by utilising the correct valve sequencing.

[0082] In a further variation for the embodiment shown in Figure 1 when the mercury removal system includes a plurality of coalescers 14, the coalescers 14 may be connected in series rather than parallel. Operating the coalescers 14 in series should result in a greater proportion of mercury removal than a parallel connection. However, this may come at the expense of reduced flow rate in the production pipeline 44.

[0083] In another embodiment of the system 10b shown in Figure 2 a coalescer 14 and a storge tank 70 may be integrated into a unitary mercury removal and storage unit 15. In the unit 15 both the removal of the at least a first fraction of mercury and the storing of the first fraction of removed mercury are performed in a common vessel 17. In this embodiment the mercury removal system 12 has two unitary mercury removal and storage units 15a and 15b (referred to in general as “unit 15” in the singular and “units 15” in the plural). In one form the unit 15 the coalesced mercury can be stored as a liquid. In an alternate form of the unit 15 the coalesced mercury may be absorbed in a solid particulate material, or otherwise converted into an inert solid material.

[0084] In describing the embodiment of the system 10b shown in Figure 2 the same reference numbers are used to denote the same features as in the embodiment of the system 10a shown in Figure 1. The substantive difference between the embodiments of systems 10a and 10b is the replacement of the of the separate coalescers 14a and 14b (referred to in general as “coalescer 14” in the singular and “coalescers 14” in the plural) and storage tanks 70 of system 10a with the units 15a, 15b. Thus, in system 10b the separate coalescer 14 and storage tank 70 of the first embodiment are integrated into a corresponding unit 15.

[0085] In the system 10b each unit 15 has an inlet 36 to receive hydrocarbon from the hydrocarbon feed stream flowing through the feedline 28. The hydrocarbon once it enters the vessel 17 of a unit 15 flows upwardly by virtue of flow pressure to a unit 15 outlet 38. Mercury contained in this flow coalesces in the coalescer 14 of the unit 15 and from where it subsequently drips by action of gravity to collect as a volume of mercury in the bottom of the vessel 17. In the system 10b a connection system 100 is provided to enable user selectable connection between the units 15 and the hydrocarbon feed stream flowing through the feedline 28. The connection system 100 can be selectively arranged to:

[0086] • connect the units 15a and 15b in parallel to between the feedline 28 and the flow line 40. This would result in substantially one half of the mass flow of the hydrocarbon being processed through each of the units 15;

[0087] • connect the units 15a and 15b in series so that all the mass flow of hydrocarbon is processed by unit 15a, and the mercury stripped hydrocarbons from the outlet of unit 15a then flows into the inlet 36 of the unit 15b. This will provide an improved degree of mercury removal in comparison to the above parallel connection;

[0088] • connect only one of units 15 between the pipeline 28 and the flow line 40. The unconnected unit 15 lays as a redundant unit to be fluidically connected between the pipeline 28 and the flow line 40 when some action other than mercury removal is to be performed on the previously connected unit 15 which is then fluidically isolated from the pipeline 28 and flow line 40. These actions may include but are not limited to recovery of the previously connected unit 15 for maintenance, replacement or collection of the stored mercury at the surface. In this scenario all the mass flow of the hydrocarbon is processed at any one time only by the unit 15 that is fluidically connected between the pipeline 28 and flow line 40.

[0089] The connection system 100 includes a plurality of valves 102, 104, 106, 108 and 110 in respective flow lines 112, 114, 116, 118 and 120. The valves can be opened and closed to control the flow of hydrocarbon through the units 15.

[0090] • The flow line 112 and associated valve 102 provide a fluid flow path between pipeline 28 and inlet 36 of unit 15a.

[0091] • The flow line 114 and associated valve 104 provide a fluid flow path between the outlet of unit 15a and flow line 40.

[0092] • The flow line 116 and associated valve 106 provide a fluid flow path between the outlet of u n it 15a and flow line 118.

[0093] • The flow line 118 and associated valve 108 provide a fluid flow path between pipeline 28 and inlet 36 of unit 15b.

[0094] • The flow line 120 and associated valve 110 provide a fluid flow path between the outlet of unit 15b and the flow line 40. A further difference between the systems 10a, and 10b is the inclusion, in system 10b of additional valves to enable recovery / drainage of collected mercury. The additional valves include gate valve pairs 126a, 128a; gate valve pairs 126b, 128b; and gate valve 130.

[0095] The valves 126a, 128a, 126b, 128b, and 130 together with an end stop connector 132, form a mercury drainage system 134 to facilitate drainage of mercury that is collected and stored in the units 15. The mercury drainage system 134 in this embodiment is configured to be opened only when the units 15 are decoupled from the subsea system 10b and returned to the shore. Draining the mercury from the units 15 will require the end connector 134 to be connected to a vessel (not shown) and gate valve 130 to be opened. Unit 15a can be drained by then opening gate valve pair 126a, 128a. Upon draining unit 15a, the gate valve pair gate valve pair 126a, 128a is closed. Unit 15b can be drained by then opening gate valve pair 126b, 128b.

[0096] Assuming the valves 35 and 39 are open the connection configuration of the units 15 can be changed by switching the valves 102-110 between open and closed states in various combinations as set out below.

[0097] Parallel configuration.

[0098] The units 15a and 15b are arranged in a parallel configuration when valves 102, 104, 108 and 110 are open and valve 106 is closed. The mass flow of hydrocarbon in pipeline 28 is split to flow through flow lines 112 and 118 into the inlets 36 of the units 15a, 15b; and the hydrocarbon with removed mercury flows from the outlets 38 of the units 15 through flow lines 114 and 120 to the flow line 40.

[0099] Series Configuration.

[0100] The units 15a and 15b are arranged in a series configuration when valves 102, 106 and 110 are open and valves 104 and 108 are closed. The mass flow of hydrocarbon in pipeline 28 flows through flow lines 112 into the inlet 36, and subsequently from the outlet 38 of the unit 15a, the hydrocarbon with removed mercury flows from through flow line 116 into the inlet 36 of unit 15b then from the outlet 38 of unit 15b through flow line 120 to the flow line 40. In this configuration the entire mass flow of the hydrocarbon flows sequentially through the unit 15a, then the unit 15b and is therefore subject to two separate sequential mercury removal steps.

[0101] Standby configuration.

[0102] In the standby configuration one of the units 15 / vessels 17 is connected to provide mercury removal to the hydrocarbon while the other unit 15 / vessel 17 is connected in a standby mode. For example, the unit 15a is connected to provide mercury removal to the hydrocarbon while the unit 15b is connected in a standby mode. In the standby mode the unit 15a is physically connected to and is part of the system 10b but is fluidically isolated from the pipeline 28 and the flow line 40. In this configuration, valves 102 and 104 are open and valves 106, 108 and 110 are closed. Mercury removal is performed only by the unit 15a. If or when it is desired to switch the state of the units 15, so that unit 15a is isolated and unit 15b is active to remove mercury form the hydrocarbon feed stream, the valves 102, 104 and 106 are closed and the valves 108 and 110 are opened. Now all the hydrocarbon feed stream in pipeline 28 flows through the unit 15b where mercury is removed and the mercury stripped hydrocarbon flows from the outlet 38 and through the flow line 120 to the flow line 40. If it is desired to retrieve the unit 15a, valves 35 and 39 are also closed and the unit 15a can be disconnected from the system 10b.

[0103] When in the parallel or series configuration the units 15 can be retrieved together and taken to an onshore and subsequently drained together by operation of the mercury drainage system 134 as described above. However, for the standby configuration the mercury drainage system is slightly modified from the system 134 so that each unit 15 has its own and independently operable mercury drainage system. This can be achieved by removing the common gate valve 130 shown in Fig 2 and connecting an end stop connector 132 in series with gate valve 128a, and connecting another end stop connector 132 in series with gate valve 128b. This enables each unit 15 to be retrieved and drained while the other remains connected in and operational in the system 10b.

[0104] The system 10b also includes a subsea control module and associated connected actuators which are operably coupled via electro and / or hydraulic control lines to the valves and other sub-systems and components of the system, as well as an associated physical electrical and / or electro / hydraulic communications and power network as in the system 10a, but these are omitted in Fig 2 for the sake of visual simplicity.

[0105] Figure 3 shows a further embodiment of the system 10c that differs from the system 10b by the inclusion of a desander 140. The desander 140 removes at least a portion of solid particulate materials such as but not limited to sand entrained in the hydrocarbon prior to reaching the mercury removal and collection units 15. The desander 140 is connected in series in the pipeline 28 upstream of the units 15. Therefore, all the hydrocarbon in the pipeline 28 must first pass through the desander 140 prior to entering any one or both of the units 15. More specifically the desander 140 is connected in the pipeline 28 between the isolation connector 22b and the units 15 and downstream of the valve 35. Many types of commercially available desanders 140 may be used to remove sand from the wellhead flow. In the specific example in Figure 3 the desander 140 includes a cyclone separator 142, an inlet 144, a desanded hydrocarbon outlet 146 an accumulator 148 and a discharge valve arrangement 150.

[0106] Wellhead fluid enters the cyclone separator 142 of the desander 140 at inlet 144. The action of the cyclone causes a swirling fluid flow and induces centrifugal forces on the wellhead fluid forcing solids, such as sand, to move radially outwardly near the wall of the cyclone separator. The solids are pulled down by gravity to the accumulator 148 while a desanded flow of the wellhead fluid is induced to exit the desander 140 through outlet 146 at the centre of the swirling flow.

[0107] A desander 140 may be incorporated in all embodiments of a system for performing a method of subsea removal and capture of mercury from a hydrocarbon feed stream within the scope of the present disclosure.

[0108] Embodiments of the system 10a, 10b, 10c (hereinafter referred to in general as “system 10”) provide numerous benefits and advantages. Significantly the length of production pipeline(s) 44 from the system 10 adjacent or in the well completion to any downstream processing facility whether it is subsea, topsides of an offshore platform or FPSO, on onshore is free of mercury contamination and, barring the existence of any other contaminant, environmentally safe. Therefore, that structure or pipeline may be left on the seabed and potentially form an artificial reef. Alternately if there is still a requirement to retrieve the production pipeline, the cost for doing so will be substantially lower that for a mercury contaminated pipeline because of the less stringent health and safety handling requirements. Also, the mercury in a liquid form in the storage facility 60 of system 10a, or in the units 15 of systems 10b or 10c can be retrieved and sold as a commercial product. Naturally all other downstream equipment and processing facilities such as pumps, compressors, phase separators, sand / water filets / traps will be substantially free of mercury contamination.

[0109] While several exemplary embodiments have been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist.

[0110] For example, while embodiments of the system 10 shown in Figures 1-3 are connected to the production line 18 downstream of a well head or xmas tree, in other embodiments the system 10 may be incorporated within the wellhead or Xmas tree or otherwise upstream of the inlet isolation connection system 20. In one such an embodiment the system 10 may be arranged to absorb the mercury in a solid particulate material, or otherwise convert the coalesced mercury to an inert solid material that is left within the wellhead / xmas tree. In such an embodiment the removal system 12 also acts as the or a storage facility. In such an embodiment the removal system may be in the form of a gravel pack comprising gravel coated with a material to facilitate coalescing and / or absorbing at least a fraction of the mercury contained in the hydrocarbon feed stream. However, in an alternate embodiment where the system 10 may store coalesced mercury as a liquid which is retained within the well. Nevertheless, the system 10 may also be connected into the well head / Xmas tree upstream of the inlet connector 20 in a manner that it can be retrieved for drainage and recovery of the liquid mercury. In this event a bypass flow line and valve are provided to enable flow from the well head / Xmas tree through the production line 18 to the inlet connector 20 if or when the system 10 is removed.

[0111] Depending on the percentage of extracted from the feed stream prior to leaving the wellhead / xmas tree and flowing in production line 18, at least one additional mercury coalescer 14 / mercury storage vessel arrangement may be provided in the system 10 downstream of inlet connector 20 and upstream of the outlet connector 42. Thus, more broadly the system 10 may be in the form of a distributed system with a portion of the mercury (for example up to 30%) removed from the hydrocarbon feedstock prior to leaving the wellhead / xmas tree (in which case this portion of the mercury in effect remains within the subsurface environment and thus never reaches the surface), with the remaining portion of the mercury (i.e., 70%) being removed between the production line 18 in production pipeline 44.

[0112] In yet a further variation an embodiment of the disclosed system may include: o a mercury coalescer and associated storage unit (for example (a) a mercury coalescer 14 and associated tank 70 of Fig 1 ; or (b) a unit 15 of Figs 2-3) upstream of the inlet connector 20, including within the wellhead / Xmas tree; o a desander 140 downstream of the inlet connector 20; and o one or more units 15, or coalescer 14 and associated storage vessel 60 combinations, downstream of the desander 140.

[0113] It should also be appreciated that the exemplary embodiments of the system and method are only examples, and are not intended to limit the scope, applicability, or configuration of the present disclosure in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the disclosed system and method. In the claims which follow, and in the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” and variations such as “comprises” or “comprising” are used in an inclusive sense, i.e., to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the system and method as disclosed herein.

Claims

CLAIMS1 . A method of producing a reduced mercury content hydrocarbon feed stream downstream of a subsea well comprising: removing at least a first fraction of mercury from a hydrocarbon feed stream at a subsea or subsurface location within or adjacent a subsea wellhead or well completion connected with the subsea well, wherein the hydrocarbon feed stream, downstream of the subsea or subsurface location has a reduced mercury content in comparison to the hydrocarbon feed stream, up stream of the subsea location.

2. The method according to claim 1 wherein removing at least the first fraction of mercury includes coalescing mercury contained in the hydrocarbon feed stream at the subsea location.

3. The method according to claim 1 or 2 comprising storing the first fraction of removed mercury within a subsea storage facility.

4. The method according to claim 3 wherein storing the first fraction of removed mercury comprises storing the first fraction as a liquid.

5. The method according to claim 3 wherein storing the first fraction of removed mercury comprises absorbing the mercury in a solid sorbent or converting the mercury into a solid or particulate material.

6. The method according to any one of claims 2 to 5 wherein removing at least a first fraction of mercury from the hydrocarbon feed stream includes dividing the hydrocarbon feed stream into a plurality of parallel flow streams and coalescing mercury in each of the plurality of parallel flow streams separately from each other.

7. The method according to any one of claims 2 to 5 wherein removing at least a first fraction of mercury from a hydrocarbon feed stream includes passing the hydrocarbon feed stream sequentially through two or more mercury coalescing systems.

8. The method according to any one of claims 3 to 7 removing of mercury is performed in one or more first vessels and the storing of the removed mercury is performed inone or more subsea mercury storage vessels, wherein the first vessels are separate from the subsea mercury storage vessels.

9. The method according to claim 8 comprising swapping the subsea storage facility in situ with a fresh or reconditioned subsea storage facility.

10. The method according to any one of claims 3 to 8 including returning mercury vapour formed in the subsea storage facility to the mercury removal system.

11. The method according to any one of claims 3 to 8 including pressurising the subsea storage facility with an inert gas.

12. The method according to any one of claims 8 to 11 including swapping the one or more first vessels in situ with one or more fresh or reconditioned first vessels.

13. The method according to any one of claims 3 to 7 where the removing of the at least a first fraction of mercury and the storing of the first fraction of removed mercury is performed in one or more common vessels, wherein a portion of the first fraction of mercury removed from the hydrocarbon feed stream flowing through a specific common vessel is stored in the same common vessel.

14. The method according to claim 13 comprising sensing a volume of mercury in the common vessel.

15. The method according to claim 13 comprising swapping a common vessel in situ with a fresh or reconditioned common vessel.

16. The method according to any one of claims 1 to 15 including selectively connecting the hydrocarbon feed stream upstream of the subsea location to a production line upstream of the subsea location to bypass the mercury removal system.

17. The method according to any one of claims 13 to 15 comprising bypassing a flow of the hydrocarbon about one or more common vessels.

18. The method according to any one of claims 1 to 17 wherein the first fraction comprises substantially all the mercury.

19. The method according to any one of claims 1 to 18 comprising removing at least a portion of solid particulate materials from the hydrocarbon feed stream before removing the at least a first fraction of mercury from a hydrocarbon feed stream.

20. The method according to any one of claims 1 to 19 wherein the subsea location is within the subsea wellhead or a xmas tree mounted on the wellhead and the first fraction is less than substantially all the mercury to provide a reduce mercury fraction hydrocarbon feed stream.

21. The method according to claim 20 comprising forming the mercury removal system as a distributed system with a first portion of the mercury removal system that removes a portion of the first fraction of mercury within the wellhead or the xmas tree and a second portion of the mercury removal system that removes a remaining portion of the first fraction of the mercury from the reduced mercury fraction hydrocarbon feed stream upstream of the wellhead or the xmas tree.

22. A method of removal and capture of mercury from a hydrocarbon feed stream comprising: connecting a mercury removal system at a subsea location to a hydrocarbon feed stream wherein the mercury removal systems is arranged to remove mercury in the hydrocarbon feed stream and allow a flow of the hydrocarbon feed stream substantially free of mercury to and through downstream infrastructure.

23. The method according to claim 22 wherein the subsea location is within a subsea well completion.

24. The method according to claim 23 wherein the subsea location is between a Xmas tree of the subsea well completion and the downstream infrastructure.

25. The method according to claims 22 or 23 including transferring the removed mercury to a subsea storage facility.

26. The method according to claim 25 wherein the transferring includes transferring the removed mercury to one or more subsea storage tanks.

27. The method according to any one of claims 22 to 26 wherein the connecting comprises releasably connecting the mercury removal systems to the hydrocarbon feed stream.

28. The method according to any one of claims 22 to 27 including connecting a bypass line between the subsea location and the downstream infrastructure enabling the hydrocarbon feed stream to bypass the mercury removal systems.

29. The method according to any one of claims 25 to 28 including releasably connecting the mercury removal systems to the subsea storage facility wherein the subsea storage facility can be retrieved from the subsea location.

30. The method according to any one of claims 25 to 29 including in situ swapping of the subsea storage facility with fresh subsea storage facility.

31. The method according to any one of claims 26 to 30 comprising providing a mercury vapour return flow path between the one or more subsea storage tanks and the mercury removal systems whereby mercury vapour forming in the subsea storage facility is directed to flow back to the one or more mercury coalescing systems.

32. The method according to any one of claims 26 to 31 comprising holding a volume of an inert gas in each of the subsea storage facility.

33. The method according to any one of claims 25 to 32 including absorbing the removed mercury in a solid medium in the subsea storage facility or conducting a process in the subsea storage facility to convert the removed mercury into a solid material.

34. The method according to any one of claims 25 to 33 including mounting the mercury removal systems and the subsea storage facility on a common skid and deploying the skid to the subsea location.

35. The method according to claim 23 wherein the connecting includes connecting the mercury removal system within a Xmas tree or well head of the completion.

36. The method according to claim 35 including coalescing a first fraction of mercury from the feed stream within the Xmas tree or well head wherein the feed stream leaving the well head has a reduced fraction of mercury.

37. The method according to claim 36 including feeding the feed stream with the reduced fraction of mercury to one or more additional mercury coalescing systems and transferring the coalesced mercury to a subsea storage facility.

38. A system for subsea removal and capture of mercury from a hydrocarbon feed stream comprising: a mercury removal system capable of being fed a hydrocarbon feed stream the mercury removal systems arranged to removal at least a fraction of mercury carried in the hydrocarbon feed stream and produce a downstream flow of hydrocarbon feed stream substantially free of mercury; and a mercury storage facility in fluid communication with the mercury removal system for receiving and storing the removed fraction of mercury.

39. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to claim 38 wherein the mercury removal system is arranged to remove the at least a fraction of the mercury by coalescing mercury carried in the hydrocarbon feed stream.

40. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to claims 38 or 39 wherein the mercury collection system is in fluid communication with the mercury removal system to received and store the at least a fraction of the mercury.

41. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to claims 38 to 40 including a bypass line, and a remotely operable valve system enabling selective opening and closing of the bypass line, wherein when bypass line is closed, the hydrocarbon feed stream is directed through the mercury removal system, and when the bypass line is opened the hydrocarbon feed stream is directed to flow past and avoid contact with the mercury removal system.

42. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to any one of claims 38 to 41 including a mercury vapour return line enabling mercury vapour formed in the subsea storage facility to be returned to the mercury removal system.

43. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to any one of claims 38 to 42 including a plurality of storage facility isolation connectors connecting the mercury storage facility to the mercury removal system, and wherein the storage facility isolation connectors are remotely operable to enable the subsea storage facility to be disconnected from the mercury removal system without leakage from the mercury removal system or the mercury storage facility.

44. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to any one of claims 38 to 42 including a plurality of removal system connection valves connecting: an inlet of the mercury removal system to the hydrocarbon feed stream, a hydrocarbon outlet of the mercury removal system to a production pipeline, a mercury outlet of the mercury removal system to the mercury storage facility, and a mercury vapour return inlet of the mercury removal system to the subsea storage facility; wherein the plurality of removal system connection valves are remotely operable to enable the mercury removal system to be disconnected from the system for subsea removal and capture of mercury for in situ replacement.

45. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to claim 38 wherein the mercury removal system and the mercury storage system are disposed in a common vessel.

46. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to claim 38 wherein the mercury removal system comprises a plurality of mercury coalescers and each mercury coalescer is disposed in a respective common vessel and mercury that is coalesced by a mercury coalescer in a respective common vessel is stored in the same respective common vessel.

47. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to claim 46 comprising a connection system arranged to enable a user selectable connection between the common vessels and the hydrocarbon feed stream flowing whereby the connection can be switched at least between a parallel connection and a series connection, wherein in the parallel connection the hydrocarbon feed stream is split into a plurality of streams wherein respective streams flow through respective common vessels, and in the series connection thehydrocarbon feed stream sequentially flow through each of the series connected common vessels.

48. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to claim 46 comprising a connection system arranged to enable a standby connection for at least one common vessel wherein a connection of the common vessel to the hydrocarbon feed stream is capable of being switched between an isolated mode wherein the at least one common vessel is physically connected to the system for subsea removal and capture of mercury from a hydrocarbon feed stream but flu idically isolated from the hydrocarbon feed stream, and an active mode wherein at least a portion of the hydrocarbon feed stream is flu idically connected to and flow through the at least one common vessel.

49. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to claim 38 wherein at least a portion of the mercury removal system is located within a wellhead or Xmas tree and the portion of the mercury removal system removes at least a portion of mercury from a well production stream prior to the well production steam flowing downstream of the wellhead or Xmas tree.

50. The system for subsea removal and capture of mercury from a hydrocarbon feed stream according to any one of claims 38 to 48 including a desander arranged to remove at least a portion of solid particulate materials from the hydrocarbon feed stream before removing the at least a first fraction of mercury from a hydrocarbon feed stream.

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