NO347676B1 - Subsea Control Unit - Google Patents

Description

The following invention relates to a control unit for controlling operations in a subsea well.

In the exploitation of subsea oil and gas reserves a well is drilled from a surface rig into a formation. On top of the well there is established a wellhead, from which various casings are suspended into the well. To keep control of the well during operations a blowout preventer (BOP) is placed on top of the wellhead and a marine riser extends from the BOP up to the rig.

After drilling of the well various steps are used to complete the installation. Normally this consists of first connecting a wellhead adapter to the wellhead. Next, a Christmas tree (XT) is installed and the BOP reconnected to the top of the Christmas tree. Operations can now be performed safely with the BOP ensuring control of the well during operations.

During production there may be times where it is necessary to perform a workover or intervention operation in a well. To this end, a simplified BOP is connected to the Christmas tree and a so-called lubricator is connected on top of the BOP. On top of the lubricator is installed a pressure control head (PCH). To perform a workover operation an instrument is lowered in the well with coil tubing, a wireline or a slick line. Pressurized grease is supplied to the PCH to seal around the wireline.

This operation has traditionally been performed from a topside facility, such as a drilling rig. A umbilical is connected between topside and the well to provide grease supply and for control and monitoring. This is costly and can be a difficult operation in for example bad weather.

The object of the invention is to provide a method for controlling the operation of a workover or intervention operation by locating a control unit proximate a well, comprising at least one battery module storing electric power, at least one electric motor powered by the battery, at least one pump driven by the electric motor, at least one reservoir for hydraulic fluid, at least one control valve module receiving hydraulic fluid from the pump, and a data processing module controlling the operation.

In US Patent Application No.2013153242 there is described a turbine power generator located subsea and operationally connected to a subsea power device. It further comprises a control unit comprising a housing for controlling operations in a subsea well, a battery for storing electric power, an electric motor powered by the battery, the motor driving a pump. The pump is arranged to supply hydraulic fluid from a reservoir to functions in the subsea well.

Such a system is described in US Patent Application No.2003150619. However, this system is concerned with control of functions in a well and not a workover operation.

In US Patent Application No.2003150619 there is described a hydraulic control assembly for actuating a hydraulically controllable downhole device where a hydraulic fluid source located on a surface installation supplying a low pressure hydraulic fluid and a subsea intensifier operably associated with a subsea wellhead.

The aim of the invention is to provide a control system for providing grease to a PCH in a subsea stack. This is achieved by having a control unit located proximate the well having a grease reservoir connected to the suction side of an additional pump such that grease is fed to a line that extends from the control unit to a pressure control head located at the top of the well for providing pressurized grease to the pressure control head.

In one aspect of the invention the control system comprises a data processing module controlling the operation.

In another aspect of the invention the control system comprises a communication unit.

In yet another aspect of the invention the control system comprises a flow meter. This is used to both monitor grease reservoir volume during operations and to monitor valve movements.

The invention will now be described in more detail with reference to the encloses drawings where:

Fig.1 is a drawing of a control unit, and

Fig.2 is a drawing of a subsea installation with workover equipment.

In Fig.1 there is shown control unit 10 according to the invention. It can be used in different configurations to enable its use in different operations but in this context, it will be used in a subsea workover operation. In the drawing the various parts are shown in exploded mode but it should be understood that all the various modules are contained within the unit 10.

The control unit 10 has a housing 20 containing the various modules.

A pump 21 is provided for supplying hydraulic fluid under pressure to a subsea stack 2. There may be more than one pump installed, for example one pump that supplies low pressure hydraulic fluid to the stack for operating the various actuators in the XT, a second pump that supplies high pressure hydraulic fluid downhole to control a subsurface safety valve and a third pump for supplying grease to the pressure control head (PCH). These pumps deliver hydraulic fluid to functions according to the need.

Reservoirs 19 stores hydraulic fluid to feed the suction side of the subsea pumps 21 as well as grease to the PCH. Preferably, it consists of a special made rubber bladder pipe clamped and sealed at the top and bottom where the inner side is filled with hydraulic control fluid and the external side is exposed to the static head pressure. This way the hydraulic system is always compensated with the static head pressure so that the pressure build by the pumps will escalate from the static head to the desired operational value.

A battery module 27 consists of a pack of rechargeable lithium battery capable of powering the pump 21 as well as other functions. The battery will have the necessary charge to power all functions during an operation.

In a preferred embodiment the communication between the unit and topside is an open water communication system, eliminating the need for communication cables between topside and the subsea control unit. To this end the housing 20 has an antenna 23 that can send and receive acoustic signals to and from a second antenna 24 located topside, for example a ship, as shown on Fig.1. The topside antenna 24 topside can be part of a local area network (LAN) where signals can be picked up by a receiver on an onboard computer 25. A human operator 26 can interface with the computer for monitoring the operation and sending commands to the processor in the control unit 10. Such commands may be to change pre-defines logics, read sensors, and perform manual actions such as valve commands. It can also be used as a bridge to expose system data to an external server or into the cloud so that the unit can be fully operated from land.

A data processor (not shown) is provided for control of the unit and for communication to topside. The processor includes a programmable device as well as a data storage unit. The processor monitors the various functions of the unit, such as battery status (state of charge) and the volume level in the reservoirs. It is also responsible for reporting its status such as state of charge and faulty alarms to the topside server computer. The processor can also perform operations diagnostics and internal pressure reading.

The processor can be programmed to take autonomous actions based on pre-defined system triggers such as loss of communication or sudden increase or decrease of pressure in a line.

The processor may also include means to put equipment in deep sleep mode while the operation is held, for example if it becomes necessary to put the operations into a safe mode or during trouble shooting or other scenarios caused not directly related to the tool. This feature includes reducing power consumption to its minimum to save battery for when the operation resumes.

The unit may be equipped with pressure and temperature transmitters. This can be used to perform self-monitoring and/or pressure testing.

The unit may further be equipped with flow meters that can interact directly to the topside server computers. They can be used to calculate volume displacement of a specific functions estimating valve stroke position of opening or closing. The flow meter is equipped with a revolution (volume) counter that is seen real time by the data processing unit locally at subsea. This allow a very precise volume calculation to be reported periodically to the topside computer 25 even when a rapid flow spike happens when a valve is triggered or the pump runs.

The flow meters may also be used to calculate reservoir volume level during operation.

In a preferred embodiment the control unit 10 comprises a valve module 22 that is a multiplexed valve pack that is fed by hydraulic fluid from the outlet of the pump and is arranged to distribute hydraulic fluid at several pressure levels to various output functions lines linked to the workover stack via the jumper 32.

In another embodiment the valves are operated upon commands and power from the processor.

In one scenario the control valves are fail safe valves, meaning that when the header pressure drops to static head, every function line will also drop to that pressure without any electronic action required.

In another scenario the control valves can be placed in a middle position where they will hold the pressure in the function line without linking it to either supply header or return header. This feature adds capability of the control valve to perform a leak test on any of the stack up functions controlled by the control system.

A remotely operated vehicle (ROV) 30 may be utilized to facilitate the operation. A tether connects the ROV to topside via a cable 31. The ROV can recharge the battery 20 if necessary and can also take control of the valves in an emergency.

The control unit 10 is located on the seabed 1, preferably on a mudmat. The unit is located in close proximity to the subsea well. Hydraulic and electric lines 32 and 33 are connected to the XT 2 and the system tested to see that all is working properly. One line 32 may carry hydraulic fluid and another 33 may carry communication and, in case, electric power.

In Fig.2 there is shown how the control unit 10 is used to control workover and/or intervention operations. In such operations a so-called lower riser package and disconnect unit (LRP/EDP) 40 is connected onto the BOP. A lubricator 42 and a pressure control head 41 completes the setup. This is well known to persons skilled in the art. In such operations, it is necessary to supply grease to the pressure control head 41. To this end one pump is utilized to pump grease from the grease accumulator to the pressure control head 41 through additional line 34.

When necessary, for example in an emergency, the system may be set in safety mode where a umbilical is lowered to the seabed and the ROV is used to attach the end of the umbilical to the control unit 10.

Claims (8)

1. A control unit comprising a housing (20), for controlling operations in a subsea well where said control unit (10) is located on the sea bed (1) next to a subsea a well (2), at least one battery module (27) storing electric power, at least one electric motor powered by the battery module (27), at least one pump (21) driven by the electric motor, at least one hydraulic fluid reservoir (19) connected to the suction side of the pump such that hydraulic fluid is fed to a hydraulic line (32) that extends from the control unit (10) to the subsea well (2) providing hydraulic fluid to a function in the well (2), characterized in that the control unit (10) further comprises a grease reservoir connected to the suction side of an additional pump such that grease is fed to a line (34) that extends from the control unit (10) to a pressure control head (41) located at the top of the well (2) for providing pressurized grease to the pressure control head (41).

2. A control unit according to claim 1, characterized in that it comprises at least one control valve module (22).

3. A control unit according to claim 1, characterized in that it comprises a data processing module controlling the operation.

4. A control unit according to claim 1, characterized in that it comprises a communication unit for communication with a topside facility.

5. A control unit according to claim 4, characterized in that the communication unit comprises an antenna (23) for receiving acoustic signals.

6. A control unit according to claim 1, characterized in that it comprises a at least one flow meter.

7. A control unit according to claim 6, characterized in that the flow meter is arranged for calculating grease reservoir volume level during operations

8. A control unit according to claim 6, characterized in that the flow meter is arranged for monitoring valve movements.