Field of the invention

The present invention relates to subsea pumps for the petroleum industry, instrumentation, control, reliability and cost thereof.

Background of the invention and prior art

Reliability is a key issue for subsea equipment.

A subsea pump system, that is a pump system arranged on or at the seabed for pumping multiphase fluid or one phase fluid, typically comprises electronic instrumentation and electronic control modules arranged subsea. Subsea electronics can be a limitation with respect to reliability, since a large number of components and connections are included. Even though each component has very high reliability, the reliability of each component typically must be multiplied with the reliability of other components. With sometimes thousands of components, the resulting reliability can be a limitation to uptime of the equipment.

Smart Fibres Ltd of UK has suggested a subsea pump with a condition monitoring system with fiber optical sensors, published by the url:

http://smartfibres.com/docs/Subsea Rotating Machine Condition Monitoring Svstem.pdf

According to Smart Fibres, optical Fiber Bragg Grating (FBG) technology is used for condition monitoring of a subsea pump and motor, by arranging optical fibers with FBG sensors to the subsea motor and pump. All optical sensors are suggested subsea, with electronic instrumentation at surface, connected via optical fibers in an umbilical. Only FBG sensors are suggested and only as arranged to the subsea pump and motor.

For a downhole pump, a so-called electric submersible pump (ESP), distributed fiber optic sensing devices for monitoring the health of an ESP downhole is described and illustrated in the patent publication US 20 5/01 10439 A1. The method and system described in said publication relate to determining a parameter of at least one component of an artificial lift system located in a subterranean formation. It is not explicitly described where processors and electronic modules are arranged; topsides or downhole. From said publication, is not clear whether components or parameters in addition to the at least one parameter and component are monitored with electronic sensors or optical fiber sensors, downhole or topsides, or where electronics are arranged. A demand exists for improved reliability and reduced cost for subsea pumps and subsea pump systems.

Summary of the invention

The invention meets the demand by providing a subsea pump system, comprising

a subsea pump,

a fluid conditioner tank,

a liquid conservation tank,

a line arranged for liquid recirculation from the liquid conservation tank to upstream the subsea pump, and

an umbilical for power, monitoring and control,

wherein the fluid conditioner tank is arranged upstream to the subsea pump which is arranged upstream to the liquid conservation tank.

The subsea pump system is distinctive by that it further comprises:

a first buoyancy element suspended in the fluid conditioner tank, a second buoyancy element suspended in the liquid conservation tank, optical fiber sensors, at least arranged to a suspension of the buoyancy element in the fluid conditioner tank and to a suspension of the buoyancy element in the liquid conservation tank, and

electronics,

preferably all subsea sensors consist of optical fiber sensors and all electronics for monitoring and control consist of electronics arranged topsides.

In the most preferred embodiments, all subsea sensors consist of optical fiber sensors and all electronics for monitoring and control consist of electronics arranged topsides. Topsides means above the water, on a platform or vessel or onshore. Preferably, no electronics is arranged subsea, especially no

electronics for monitoring and control. A possible exception, not for monitoring and control however, and definitely not in contact with process fluids or subsea pump motor compartment fluids, is a possible subsea electronics module, conveniently arranged at the subsea umbilical termination, for analog to digital conversion of optical signals, for allowing longer distance transmission. Such subsea electronics module, if present, is however not for monitoring and control, only for conversion of one type of optical signals to another type of optical signals for better transmission of the optical signals. Accordingly, no electronics is operatively coupled to the process equipment for monitoring and control in the most preferred embodiments. The subsea pump system preferably comprises a single wet mate connector, connecting the umbilical to the subsea pump and the optical fiber sensors of the subsea pump, the fluid conditioner tank and the liquid conservation tank.

Preferably, all other connections are by dry mate connectors or pre-installed fusion splices made before the subsea pump system installation.

In a preferable embodiment, the subsea pump system comprises fiber optical sensors in the umbilical for measuring both temperature and strain of dynamic loading of the umbilical. In a preferable embodiment, the subsea pump system comprises at least one Fabry Perot optical fiber pressure and temperature sensor.

In a further preferable embodiment, the subsea pump system comprises fiber optic sensors for liquid level monitoring using differential pressure, in the fluid conditioner tank and the liquid conservation tank. In a further preferable embodiment, the subsea pump system comprises fiber optic Bragg grating sensor arrays attached to the suspension of the buoyancy element in the fluid conditioner tank and to the suspension of the buoyancy element in the liquid conservation tank. The buoyance elements used in the system of the invention may have positive or negative buoyancy as submerged in liquid, however, said elements must have known weight and volume or the measured values must be calibrated to values of at least one of the parameters: level, flow rate and fluid composition.

The subsea pump system preferably comprises one or more of fiber optic Bragg grating or Distributed Acoustic Sensing, as optical fibers operatively arranged to the subsea equipment structure or subsea rotating equipment. Preferably, the pump system of the invention also comprises fiber optical Distributed

Temperature Sensing (DTS), particularly as arranged in the process fluid flow path or volumes and downstream a bypass choke, which is particularly useful for detecting risk of hydrate formation during shut-in or bypass choking. In addition, the subsea pump system preferably comprises fiber optic current sensors using Faraday Effect to modulate polarization in the presence of a magnetic field, wherein said sensors are arranged outside conducting elements or inside conducting elements, including the umbilical.

As mentioned, the advantages of the subsea pump system of the invention mainly relates to cost and reliability. For each sensor or each parameter to be measured at a specific location subsea, a rough estimate is that 0.1 to 1 million USD will be saved in capital cost, before installation. The subsea pump system of the invention preferably comprises several optical fiber sensors in each fiber, preferably comprises at least three optical fiber sensors operatively arranged through the umbilical and to equipment subsea, and preferably at least one redundant fiber or sensor for each parameter and location. The result is a very significant improvement in reliability. In addition, since the installation subsea involves only one, optionally no, subsea wet connector matings, since all sensors are presinstalled and fusion spliced, a large simplification for installation will also be achieved. Faster and simpler installation means significant reduction in cost. Furthermore, the fiber optical sensors have advantage by not being affected by electromagnetism, allowing measurements at locations where electronic sensors may not function. Figures

Figure 1 illustrates an embodiment of a subsea pump system of the invention. Figure 2 illustrates details of said system.

Detailed description

Reference is made to Figure 1. A subsea pump (1) is connected to topside power and communication (24) system via an umbilical (23) containing power transmission cable (19) and optical fiber (15, 16, 17). The umbilical is

terminated at a topside umbilical termination unit (18) and subsea umbilical termination unit (14). The subsea umbilical termination unit (14) includes wet mateable connectors (13) for power cables and optical fibers. Subsea optical fibers (15a, 16a, 17a) forming the subsea instrumentation are connected to the subsea umbilical termination through one or more wet mate connectors (13). In this embodiment: a first fiber (15a) may be measuring vibration, for example using distributed acoustic sensing; a second fiber (16a) represents a fiber coupled to the pump station flow lines for distributed temperature measurement; a third fiber (17a) is a fiber with one or more Fabry Perot sensors for pressure measurements (10,11 ) with the same fiber extended to measure strain or pressure (8,9) for tank fluid density. More specifically, the system includes a fluid conditioning tank (5) which separates liquid and gas phases from a subsea well and provides an averaged gas volume fluid fraction at the pump input (3), achieved by having a perforated outlet pipe extending up into the tank volume. At the outlet of the pump (25) is a liquid conservation tank (6) which ensures a percentage of liquid is circulated back to the inlet of the pump, via a separate line from a liquid filled part of said tank to upstream the pump. The fluid level in both tanks can be estimated by the use of a test cylinder, also termed a buoyancy element, of known density (4, 7), which is located inside the tanks with either an optical strain or optical pressure gauge (8, 9) mechanically attached to load bearing suspension structure between test cylinders (4,7) and tanks (5,6), respectively. As the fluid level increases, the weight of the test cylinder (4, 7) will decrease due to the buoyancy from displacing fluid. This displacement is measured through gauges (8, 9). A benefit of this approach is that it provides a measurement of net fluid density in the tank from which level may be inferred by calculation. The net density measurement is also a useful input to the pump control algorithm as it is related to the density of fluid at the pump inlet.

Measurements such as those mentioned above may be included on the same fiber or additional fibers. In each case the fiber used for measurement is extended through the umbilical (23) and measurement taken topside without the need for subsea electronics. The subsea pump system and subsea pressure booster of the invention may include every feature or step as here described or illustrated, in any operative combination, which operative combinations are embodiment of the invention.

The invention also provides a subsea pressure booster, comprising a pump compartment or compressor compartment with impellers and diffusers and a motor compartment with a motor operatively coupled to rotate the impellers, and a lubrication arrangement for lubrication of the motor compartment bearings, seals and coil windings, distinctive in that the subsea pressure booster comprises at least one optical fiber sensor arranged in the motor compartment or in a lubricant circuit part arranged from and to said motor compartment, for monitoring a lubricant flow rate, and preferably all subsea sensors consist of optical fiber sensors and all electronics for monitoring and control consist of electronics arranged topsides. The flow rate of the lubricant, typically an oil or a water-glycol mixture, is a vital parameter for monitoring a subsea pressure booster, giving a direct monitored parameter providing early warning if the lubricant flow rate drops or increases outside a due operation window, which parameter is not mentioned or implicit by the teaching of Smart Fibres or the patent publication US 2015/0110439 A1. Preferably, the lubricant flow rate is measured at the lubricant inlet to and lubricant outlet from a bearing or other component, by using Fabry-Perot optical fiber pressure sensors, relating the lubricant pressure drop over the component to a lubricant flow rate and a motor speed. More specifically, a lubricant impeller or pump is driven directly by or is operatively coupled, typically with a 1 to1 coupling, to the motor, meaning that the lubricant flow rate is directly related to motor speed. For a known or measured motor speed, the lubricant pressure drop over a component is then directly related to the lubricant flow rate. Alternatively, Fabry-Perot optical fiber sensors are arranged to measure strain or stress to a restriction in a lubricant inlet or outlet or both inlet and outlet, the measured strain or stress relates to lubricant flow rate. FBG vortex flow meters can be used but are less feasible for measuring lubricant flow rate due to limitations with respect to vibrations, high lubricant viscosity at start up and too small dimensions at the locations for measurements. The lubricant flow rate is preferably measured for each bearing of a motor shaft. Fabry-Perot optical fiber pressure or differential pressure sensors, and other fiber optical sensors or arrangements, are arranged in a single optical fiber or in several optical fibers. In addition, pressure and temperature are preferably also measured, as well as vibration and other parameters, preferably with only optical fiber sensors subsea and electronics merely topsides.