Abstract:
A mudline riser annulus flow meter includes a liner configured to be attached to a riser to cover a hole; a cover configured to be attached to the riser to cover the liner such that a cavity is formed by the liner and the cover; a sensor rod configured to be attached to the liner and to extend inside cavity, the sensor rod having a bore; a magnet assembly configured to be fixedly attached to the sensor rod; and a waveguide tube attached to the cover. The bore of the sensor rode is configured to receive the waveguide tube.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for measuring a fluid flow in a pipe. 
         [0003]    2. Discussion of the Background 
         [0004]    During the past years, with the increase in price of fossil fuels, the interest in developing new production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel. One characteristic of the offshore locations is the high pressure to which the drilling equipment is subjected. For example, it is conventional to have parts of the drilling equipment designed to withstand pressures between 5,000 and 30,000 psi. In addition, the materials used for the various components of the drilling equipment are desired to be corrosion resistant and to resist high temperatures. 
         [0005]    Existing technologies for extracting oil from offshore fields use a system  10  as shown in  FIG. 1 . More specifically, the system  10  includes a vessel (or rig)  12  having a reel  14  that supplies power/communication cables  16  to a controller  18 . The controller  18  is disposed undersea, close to or on the seabed  20 . In this respect, it is noted that the elements shown in  FIG. 1  are not drawn to scale and no dimensions should be inferred from  FIG. 1 . 
         [0006]      FIG. 1  also shows that a drill string  24  is provided inside a riser  40 , that extends from vessel  12  to a BOP  28 . A wellhead  22  of the subsea well is connected to a casing  44 , which is configured to accommodate the drill string  24  that enters the subsea well. At the end of the drill string  24  there is a drill bit (not shown). Various mechanisms, also not shown, are employed to rotate the drill string  24 , and implicitly the drill bit, to extend the subsea well. The dirt and debris produced by the drill string  24  are removed by circulating a special fluid, called “mud”, through an inside of the drill string  24  and then through an annulus formed between the outside of the drill string  24  and an inside of the riser  40 . Thus, the mud is pumped from the vessel  12  through the drill string  24  down to the drill bit and back through the annulus of the riser  40  back to the vessel  12 . 
         [0007]    However, during normal drilling operation, unexpected events may occur that could damage the well and/or the equipment used for drilling. One such event is the uncontrolled flow of gas, oil or other well fluids from an underground formation into the well. Such event is sometimes referred to as a “kick” or a “blowout” and may occur when formation pressure inside the well exceeds the pressure applied to it by the column of drilling fluid (mud). This event is unforeseeable and, if no measures are taken to prevent it, the well and/or the associated equipment may be damaged. Although the above discussion was directed to subsea oil exploration, the same is true for ground oil exploration. 
         [0008]    Thus, a blowout preventer (BOP) might be installed on top of the well to seal the well in case that one of the above events is threatening the integrity of the well. The BOP is conventionally implemented as a valve to prevent the release of pressure either in the annular space, i.e., between the casing and the drill pipe, or in the open hole (i.e., hole with no drill pipe) during drilling or completion operations. Recently, a plurality of BOPs were installed on top of the well for various reasons.  FIG. 1  shows two BOPs  26  or  28  that are controlled by the controller  18 . 
         [0009]    However, deep water exploration presents a host of other drilling problems, such as substantial lost circulation zones, well control incidents, shallow-water flows, etc. Thus, many of these wells are lost due to significant mechanical drilling problems. A common characteristic of these problems is the abnormal flow of the mud. For example, the flow rate at the surface pump may become larger than the flow rate of the return mud at the ship. This suggests that the integrity of the well is compromised and the mud is escaping into the environment. Another possibility which is more dangerous for the safety of the personnel working on the rig is when the flow rate of the returning mud is larger than the flow rate of the surface pump. This event suggests that the integrity of the well may be compromised, and/or a high pressure intrusion into the well has taken place. This high pressure gas or fluid then may make its way up the riser and blowout the rig. If these events take place, it is noted that the operator of the BOP does not have the time to react and close the BOP. These events not only may lead to loss of lives but also increase the cost of drilling and reduce the chances that oil would be extracted from those wells, which is undesirable. 
         [0010]    Accordingly, it would be desirable to provide systems and methods that avoid the afore-described problems and drawbacks. 
       SUMMARY 
       [0011]    According to an exemplary embodiment, there is a mudline riser annulus flow meter. The flow meter includes a liner configured to be attached to a riser to cover a hole; a cover configured to be attached to the riser to cover the liner such that a cavity is formed by the liner and the cover; an insert configured to be disposed within the liner; a base configured to be attached to the insert; a sensor rod configured to be attached to the base and to extend inside cavity, the sensor rod having a bore; a magnet assembly configured to be fixedly attached to the sensor rod; a position sensor attached to the cover; and a waveguide tube attached to the position sensor. The bore of the sensor rode is configured to receive the waveguide tube. 
         [0012]    According to another exemplary embodiment, there is a mudline riser that includes a first flow meter. The first flow meter is configured to include a liner configured to be attached to the riser to cover a hole; a cover configured to be attached to the riser to cover the liner such that a cavity is formed by the liner and the cover; an insert configured to be disposed into the liner; a base configured to be attached to the insert; a sensor rod configured to be attached to the base and to extend inside cavity, the sensor rod having a bore; a magnet assembly configured to be fixedly attached to the sensor rod; a position sensor attached to the cover; and a waveguide tube attached to the position sensor. The bore of the sensor rode is configured to receive the waveguide tube. 
         [0013]    According to still another exemplary embodiment, there is a mudline riser annulus flow meter that includes a liner configured to be attached to a riser to cover a hole; a cover configured to be attached to the riser to cover the liner such that a cavity is formed by the liner and the cover; a sensor rod configured to be attached to the liner and to extend inside cavity, the sensor rod having a bore; a magnet assembly configured to be fixedly attached to the sensor rod; and a waveguide tube attached to the cover. The bore of the sensor rode is configured to receive the waveguide tube. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
           [0015]      FIG. 1  is a schematic diagram of a conventional offshore rig; 
           [0016]      FIG. 2  is a schematic diagram of a riser according to an exemplary embodiment; 
           [0017]      FIG. 3  is a schematic diagram of a riser having a flow meter according to an exemplary embodiment; 
           [0018]      FIG. 4  is a schematic diagram of a flow meter according to an exemplary embodiment; 
           [0019]      FIG. 5  is a schematic diagram of a flow meter according to an exemplary embodiment; 
           [0020]      FIG. 6  is a schematic diagram of a riser having plural flow meters according to an exemplary embodiment; and 
           [0021]      FIG. 7  is a flow chart illustrating steps of a method for attaching a flow meter to a riser according to an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a riser connected to a subsea BOP. However, the embodiments to be discussed next are not limited to these systems, but may be applied to other systems that require the detection of a fluid flow undersea. 
         [0023]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0024]    According to an exemplary embodiment, a flow meter is provided not only at a surface pump that pumps a mud through a riser but also at a location closer to a BOP or closer to a seabed surface. In this way, a flow rate difference between the flow rate of the pump and the flow rate of the return mud at the seabed surface may be detected earlier than in a case in which the flow rate of the return mud is determined at the sea surface. In this way, some abnormal behaviors in the riser may be detected prior to those abnormal behaviors surfacing with potential devastating effects. As a given amount of mud that starts its journey from the seabed or the bottom of the well to the rig may take about 20 minutes to arrive at the rig, by early detecting the abnormal flow of the mud in the well may give the personnel operating on the rig a time window enough for shutting down the well or abandoning the well depending on the situation. 
         [0025]    As discussed above with regard to  FIG. 1 , a plurality of risers  40  are used to connect an undersea BOP  26  to a rig  12  at the sea surface. The risers are traditionally made of steel or other resistant material that can withstand high pressures, corrosive environments and some bending due to the constant movement of the rig. Thus, each riser has a continuous outer skin for preventing a leakage of the mud flowing inside the riser. 
         [0026]    According to an exemplary embodiment illustrated in  FIG. 2 , there is a riser  50  having first  52  and second  54  flanges that are configured to connect to another riser or piece of equipment, e.g., BOP  56 .  FIG. 2  shows that riser  50  connected via flange  54  to a BOP  56 . For example, the riser  50  may be deployed away from the BOP. However, the riser  50  does not has to be directly connected to the BOP  56 . Bolts  58  are used to connect flange  54  to BOP  56 . An outer surface or skin  60  of the riser  50  is configured to prevent a leak of a fluid flowing through an inside  62  of the riser  50 . A flow meter may be attached to the riser as discussed next. 
         [0027]    As shown in  FIG. 2 , a hole  64  is formed on a side of the riser  50  and that hole may be covered with a liner  66  as shown in  FIG. 3 . The shape and size of the hole  64  and consequently the liner  66  may vary based on a location of the riser  50  relative to the seabed, an internal diameter of the riser, etc. However, various shapes and sizes may be used. The liner  66  may be made of an elastomeric material or a metallic material that has the property to withstand the pressure of the mud and also to deform without breaking or cracking when the pressure of the mud is different from the pressure of the ambient. Thus, in an application, any material that is flexible enough and pressure resistant may be used for the liner  66 . 
         [0028]    The liner  66  is configured to fully cover hole  64  so that no fluid from inside the riser  50  is allowed to exit the riser or the other way around. The liner  66  is fixed to the riser, e.g., by being bolted between a bracket  68  and a cover  70 . Bracket  68  may be attached to the riser  50 . The cover  70  may be made of steel or another material that can withstand high pressures and/or corrosive environment. The cover  70  may be part of a flow meter  71 . 
         [0029]    According to an exemplary embodiment, the liner  66  may include an insert  104 , made for example, out of a metal. Other materials are also possible. The insert  104  may be provided towards a center of the liner  66 . The insert  104  may also include a threaded hole  106  in which a base  108  having a matching threaded extension may be treaded.  FIG. 4  shows in more details the elements discussed above. Base  108  has the threaded extension  108   a  inserted into insert  104  and it is configured to connect to a sensor rod  110 . The sensor rod  110  may have a cylindrical shape with a bore  111  formed around the middle of the cylinder. The sensor rod  110  may be made integrally with the base  108  or as two different pieces that attach to each other, e.g., screw, weld, etc. A magnet assembly  112  may be attached to the sensor rod  110 , e.g., by a bolt  114 . The magnet assembly  112  may include one or more magnets having a disc form with a central bore configured to fit the central bore  111  of the sensor rod  110 . In one application, the magnet assembly  112  may include plural magnets connected or not to each other. 
         [0030]    A position sensor  116  (e.g., a transducer) may be attached to an outside of the skin  60  of the riser  50 .  FIG. 4  shows the position sensor attached to cover  70 , outside cavity  124 . The position sensor  116  may have many configurations. For simplicity, only a magnetostrictive sensor is discussed next. The position sensor  116  includes a waveguide tube  118  that is configured to enter the bore  111  in the sensor rod  110  so that the sensor rod  110  is free to move along an axis X as shown in  FIG. 4  when the insert  104  moves together with the liner  66  due to pressure changes inside the riser  50 . A principle of operation of the position sensor  116  is discussed next. However, this discussion is exemplary and not intended to limit the types of sensors that may be used with the riser  50  for determining the flow of the mud. 
         [0031]    A spacer  120 , such as an o-ring, may be placed between magnet assembly  112  and sensor rod  110 . Magnet assembly  112  may include two or more permanent magnets. In some embodiments, magnet assembly  112  may include three magnets; four magnets in other embodiments; and more than four magnets in yet other embodiments. 
         [0032]    The stationary waveguide tube  118  may be located within the sensor rod  110 . In one application, sensor rod  110  is radially spaced from the waveguide tube  118  so as not to interfere with the movement of liner  66  or to cause wear on waveguide tube  118 . Similarly, magnet assembly  112  may be radially spaced apart from waveguide tube  118 . In selected embodiments, magnets of the magnet assembly  112  may be in a plane transverse to waveguide tube  118 . 
         [0033]    Additionally, a conducting element or wire (not shown) may be located through the center of waveguide tube  118 . Both the wire and waveguide tube  118  may be connected to the position sensor  116 , located external to cover  70 , through a communications port  122 . Position sensor  116  (e.g., a transducer) may also include a suitable means for placing an interrogation electrical current pulse on the conducting wire. Appropriate O-rings or other seals (not shown) are located between the waveguide tube  118 , the cover  70  and the position sensor  116  to seal against leaks. 
         [0034]    As a pressure difference between the inside  62  of the riser  60  and a cavity  124  formed between the cover  70  and the liner  60  changes, the insert  104  with the sensor rod  110  and magnet assembly  112  move along axis X. Thus, by the operation of the magnetostrictive sensor disposed therein, it is possible to determine on a continuous basis the position of the liner  66  or insert  104  relative to a non-disturbed position. Based on this displacement of a portion the liner  66  and/or the insert  104 , a flow rate of the mud through the inside  62  of the riser  60  may be determined. 
         [0035]    With regard to the operation of the magnetostrictive sensor, magnetostriction refers to the ability of some metals, such as iron or nickel or iron-nickel alloys, to expand or contract when placed in a magnetic field. A magnetostrictive waveguide tube  118  may have an area within an external magnet assembly  112  that is longitudinally magnetized as magnetic assembly  112  is translated longitudinally about waveguide tube  118 . Magnetic assembly  112 , as described above, includes permanent magnets that may be located at evenly spaced positions apart from each other, in a plane transverse to waveguide tube  118 , and radially equally spaced with respect to the surface of waveguide tube  118 . An external magnetic field is established by magnetic assembly  112 , which may longitudinally magnetize an area of waveguide tube  118 . 
         [0036]    Waveguide tube  118  surrounds a conducting wire (not shown) located along its axis. The conducting wire may be periodically pulsed or interrogated with an electrical current in a manner well-known in the art, such as by position sensor  116  located on the outside of cover  70 . Such a current produces a toroidal magnetic field around the conducting wire and waveguide tube  118 . When the toroidal magnetic field intersects with the magnetic field generated by the magnetic assembly  112 , a helical magnetic field is induced in waveguide tube  118  to produce a sonic pulse that travels toward both ends of the waveguide tube  118 . Suitable dampers (not shown) at the ends of waveguide tube  118  may prevent echo reverberations of the pulse from occurring. However, at the transducer end or head, the helical wave is transformed to a waveguide twist, which exerts a lateral stress in very thin magnetostrictive tapes connected to position sensor  116 . A phenomenon known as the Villari effect causes flux linkages from magnets running through sensing coils to be disturbed by the traveling stress waves in the tapes and to develop a voltage across the coils. Position sensor  116  may also amplify this voltage for metering or control purposes. 
         [0037]    Because the current pulse travels at nearly the speed of light, and the acoustical wave pulse travels roughly at only the speed of sound, a time interval exists between the instant that the head-end transducer receives each pulse compared with the timing of the electrical pulse produced by the head-end electronics. This time interval is a function of the distance that the external magnet assembly  112  is from the transducer end of the tube. By measuring the time interval and dividing it by the sound velocity of propagation inside the tube, the absolute distance of the magnet assembly from the head end of the tube can be determined. By proper calibration, this distance may be mapped to a flow inside the riser  50 . For example, taking into account the internal diameter of the riser and the external diameter of the drill line, various pressure differences and implicitly displacements of the waveguide tube  116  may be correlated to the corresponding flows through the riser and stored in a predetermined table. Then, based on the predetermined table, a processor may identify the corresponding flow to a given pressure difference or displacement. 
         [0038]    Position sensor  116  may have an interface  126  that allows electrical signals to be sent to the waveguide  118  and also to transmit a measurement of the waveguide  118  outside the position sensor. In one application, the electrical signals are exchanged between the position sensor  116  and a processor on the MUX pod (not shown) or a processor on the vessel  12 . In another application, the position sensor  116  may include a processor  117  for determining the flow rate inside the riser  50 . 
         [0039]    The liner  66  is discussed now with regard to  FIG. 5 . Liner  66  may have a variable thickness with a central portion  66   a  having a larger thickness for accommodating the insert  104 . In one exemplary embodiment illustrated in  FIG. 5 , the insert  104  is fully embedded into the central portion  66   a  of the liner  66 . Further, only the threaded hole  106  of the insert  104  is configured to be exposed to cavity  124  when the base  108  is not in place. Otherwise, the insert  104  is not exposed to the inside  62  and cavity  124 . 
         [0040]    Returning to  FIG. 3 , a compensating system  130  is configured to communicate with cavity  124  for controlling a pressure inside the cavity. In this respect, a pressure inside the cavity  124  is allowed to be equal to an ambient pressure, e.g., sea water pressure at the depth where the riser  50  is deployed. When the riser  50  is at the surface, the pressure inside cavity  124  is the atmospheric pressure. However, once deployed undersea, the pressure inside cavity  124  is maintain at ambient pressure by using, for example, a diaphragm or a piston  132  for separating the medium inside the cavity  124 , e.g., air, and the sea water. The pressure inside the cavity  124  may be increased to account for the hydrostatic head applied by the mud column. For example, in 10,000 ft of water with 18 ppg mud the hydrostatic head may be around 4500 psi. This pressure applied a force to the liner that may need to be balanced. If this is the case, a piston  132  rather than a diaphragm may be preferred. The compensating system  130  may be fluidly linked to a pressure source, e.g., accumulators on the MUX pod, for providing an extra pressure inside cavity  124  for compensating the sea weight of the mud column. In one application, the pressure of the mud column being larger than the ambient pressure of the sea water at the level of the flow meter, a supplementary pressure may be applied to cavity  124  such that a sum of (i) the supplementary pressure and (ii) the ambient pressure of the sea water equals the pressure of the mud column. The supplementary pressure may be calculated based on the density of the mud column, density of sea water and the depth of the flow meter relative to the sea surface. These values may be stored in a storage device that is accessible either by the operator of the flow meter or by the processor determining the flow in the riser. Thus, in one application, the processor may automatically determine the supplementary pressure to be applied to the cavity  124 . 
         [0041]    Thus, when in use, the flow meter detects a flow rate of a fluid, e.g., mud inside the riser, based on the pressure difference of the mud at that depth and the sea water pressure at the same depth. If the flow though the riser is constant, the deformation of the liner  66  is constant and the position sensor  116  determines a single position. However, when the flow is irregular, the deformation of the liner  66  may change which determines the position of the sensor rod  110  to change. Thus, the position sensor  116  may determine a changing position and consequently, the processor analyzing this data may determine the fluid flow change inside the riser  50 . 
         [0042]    According to an exemplary embodiment illustrated in  FIG. 6 , multiple flow meters  71   a  to  71   c  may be added to the riser  50 . The number of flow meters may vary between one and ten. For providing an accurate reading, a size of the liner  66  may be correlated with a flow through the riser. Thus, for example, a first flow meter may be used for flows between 0 and 300 gpm (gallons per minute) and another one for flows between 300 and 600 gpm. The larger the area of the liner  66  the better the accuracy of the reading. 
         [0043]    The differential pressure flow meter  71  discussed above is suitable for measuring a fluid flow in a riser provided undersea for the following reasons. Because the fluid flowing through the riser is dirty, e.g., may include rocks, stones, soil particles, etc., existing turbine flow meters would fail as the turbines and/or paddle wheel may get stuck. The ultrasonic and thermal mass flow meters are not suitable as the fluid density may be changing and also the solids concentration in the flow. The Coriolis and oval gear meters are also not suitable because of the minimal flow restriction allowed. Neither the Doppler meters are suitable as the flow may have air bubbles or solids circulating within. A subsea magnetic flow meter is not appropriate due to its minimal envelope dimensions. 
         [0044]    According to an exemplary embodiment illustrated in  FIG. 7 , there is a method for attaching a flow meter as discussed in the previous figures to a riser. The method includes a step  700  of attaching a liner to a riser to cover a hole in the riser, a step  702  of attaching a sensor rod to the liner, a step  704  of providing a cover over the liner and the sensor rod, a step  706  of attaching a position sensor to the cover, and a step  708  of attaching a waveguide tube to the position sensor such that the waveguide tube extends inside the cover and partially enters a bore in the sensor rod. Optionally, the method includes a step of attaching an insert to the liner, a step of screwing a base to the insert, a step of screwing the sensor rod into the base, etc. 
         [0045]    The disclosed exemplary embodiments provide a flow meter, a riser and a method for measuring a flow through an inside of the riser. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
         [0046]    Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. 
         [0047]    This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.