Pumped mud return system with an Anti-clogging system used in connection with offshore drilling operations

Abstract

The present invention concerns a pumped mud return system used in connection with offshore drilling operations. An outer pipe with an inner diameter C, forms a duct extending from a blowout preventer (11). An outer pipe outlet (3) is located in a wall of the outer pipe (1). A branch pipe (13) with an inner diameter A, is branched off from the outer pipe and (1), extends from the outer pipe outlet (3). A pump is provided to pump liquid through the branch pipe (13). An inner pipe (2) with an outer diameter B, is located inside the outer pipe (1) and pass the outer pipe outlet (3). An annulus 4 is formed between the outer pipe (1) and the inner pipe (2). The relationship between the riser C, the inner pipe B and the branch pipe A is such that B≥0.6C and A≤0.4B. A geometry provides a flow path between the annulus (4) and the branch pipe (13). The geometry includes at least one of a transition portion (14) between the annulus and the branch pipe, surrounding the outer pipe outlet (3), and a separate bypass duct (15) between an outer pipe second outlet (17) in the outer pipe (1) and a branch pipe wall inlet (18).

Description

PUMPED MUD RETURN SYSTEM WITH AN ANTI-CLOGGING SYSTEM USED IN CONNECTION WITH OFFSHORE DRILLING OPERATIONSTechnical Field

[0001] The present invention relates to a pumped mud return system used when performing exploration or production of oil and gas.

[0002] More specifically the invention relates to a so-called pumped mud return system, with a pipe in pipe including an outer pipe having an outlet located below the diverter, connected to a return pump to return fluid from the outer pipe to the rig circulating system.Background Art

[0003] Offshore drilling operations are performed with a pipe in pipe arrangement including an outer pipe such as a marine riser from a drilling vessel down to the BOP on the seabed and an inner pipe such as a casing or a drillpipe, reaching from a drilling vessel on the surface down into a well, utilizing the various aspects of fluid properties and equipment setup to achieve the desired operational conditions. The casing can also be other large pipe assemblies such as bottom hole assemblies.

[0004] In pumped mud return operations, the fluid level in the annulus between the inner pipe and the outer pipe is lowered below the diverter level to create desired operational conditions. The fluid level in the annulus is controlled by a pump connected to an outlet and a branch pipe on the outer pipe. For a certain type of riserless drilling operations, no RCD is installed and the fluid level interface between the drilling mud and the seawater at top of the BOP is maintained by controlling the pump.

[0005] When performing pumped mud return operations, the inner pipe may for different reasons obstruct flow from the annulus between the inner pipe and the outer pipe, out through the branch pipe outlet.

[0006] During operation, the inner pipe will move around inside the outer pipe. If the inner pipe is gets close to the outlet to the pump branched from the outer pipe, the flow from the annulus between the inner pipe and the outer pipe and out through the outlet will tend to draw the inner pipe towards the outlet. This is caused by thereduced area available for flow which causes a reduced pressure near the outlet due to a combination of the Bernoulli’s principle and the chocking effect of the reduced flow area. If the inner pipe gets close enough to the pump outlet, this effect is selfreinforcing, since the flow area is reduced as the closer the inner pipe gets to the outlet, which in turn further increases the pull on the inner pipe towards the outlet. In such situations the driller may find that the inner pipe stops moving laterally and gets stuck in this position.

[0007] This results in an obstructed flow through the outlet with a resulting large pressure drop. In other operational scenarios when there is e.g., a drill pipe with a smaller diameter, instead of the larger diameter inner pipe inside the outer pipe, this phenomenon does not occur to the extent that it is a problem, as the pipe is not able to obstruct the flow to such an extent that a sufficiently large pull towards the outlet is created.

[0008] Bernoulli’s principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure. This causes the inner pipe to be subject to a force pulling it towards the outlet. As the available flow area decreases, the fluid velocity increases, hence self-reinforcing the phenomenon. The smaller the available flow area gets, the higher the probability is for particles to clog the outlet, leading to an even further increased fluid velocity and pulling force. As the available flow area gets smaller, there will also be a chocking effect and an under-pressure will be created in the outlet pipe. As the inner pipe gets closer to the outlet this force is selfreinforced and eventually the inner pipe will reach a position where the inner pipe OD is in contact with the outer pipe ID above and below the outlet. Moving the inner pipe from this position would require a significant force.

[0009] The combined effects of the above-mentioned phenomena are comparable to a vacuum cleaner creating a suction force pulling objects into the mouthpiece. A noticeable force must be imposed to remove objects blocking such a mouthpiece.

[0010] The effect of the inner pipe blocking the outlet may also cause significant stresses on system components caused by e.g., sudden pressure drops or surges. This stress may again inflict damage to pump motors, pumps, pipes, and seals as the pressure rating of several components may be exceeded.

[0011] Further, in scenarios where the inner pipe and the outer pipe are subject to relative movement due to operational scenarios or other external conditions, the inner pipe could enter a position blocking, or partly blocking, the outlet from the outer pipe to the branch pipe, reducing the available flow area and resulting in an obstructed flow through the outlet. As an example of such a situation is on a floating drilling rig that is dynamically positioned, where the drilling rig is operating slightly offset to the BOP. There will then be an inclination on the riser from the BOP up to the drilling rig, and the inner pipe may be forced against the inner wall of the outer pipe. This is particularly relevant for when the inner pipe has a large diameter, and the radial clearance is small. The problem solved with the present invention is particularly relevant for situations where the inner pipe has a large diameter relative to the outer pipe.

[0012] A typical outer pipe has a rating of up to 3 million Ibf (13345 kN) and is sensitive to all modifications to the wall thickness or flow area or design to maintain the rating. This poses challenges and limitations when attempting to solve the problem described above.

[0013] Further there are some limitations when manufacturing an outer pipe, such as that the different sections in each riser joint are welded together. For manufacturing considerations, the outlet is typically perpendicular to the riser.

[0014] When the inner pipe is in its most obstructive position in front of the outlet, there is only a small opening allowing flow through the outlet. If using a clock reference where 6 o’clock and 12 o’clock represent the longitudinal axis of the outer pipe and 3’o clock and 9 o’clock are perpendicular to the longitudinal axis, mathematically there is a small opening is all the way around the outlet, except at 12 and at 6, with the small opening being at its largest at 9 and 3 o’clock. However, the absolute major part of the available flow area is near 9 and 3 o’clock.

[0015] The effect of pulling the inner pipe towards the pump outlet will strictly speaking be happening whenever the inner pipe is offset from the center of the riser, but it is only when it gets close to the outlet that the resulting force is sufficient to have any noticeable effect.

[0016] Streamlining an outlet in a tube wall is frequently used to facilitate flow by reducing turbulence and pressure drop out of the outlet. Such streamlining may bedirectional to guide the flow in a particular direction. Directional streamlining is also frequently used to guide fluid flowing from a branch pipe end into an inlet in a pipe to guide the flow in the direction of the flow of the pipe. Such solutions have, however, not been found relevant to consider and they deal with a different problem than the present invention.Summary of invention

[0017] In the following specification, the term “inner pipe” is used to describe the pipe inside the outer pipe. The inner pipe is typically casing, drill pipe, coiled tubing or drill string. The latter inner pipes, however, often have a small diameter that is unproblematic thus making the present invention superfluous.

[0018] The present invention solves the problem of an inner pipe obstructing flow through an outlet port in an outer pipe without using centralizers or other additional equipment controlling the position of the outer wall of the inner pipe relative to the inner wall of the outer pipe. The key aspect of the invention is to ensure sufficient available flow area from the annulus through the outlet from the outer pipe to the branch pipe. There are several ways to achieve a sufficient available flow area.

[0019] The key object of the present invention is to reduce the choking effect for flow through the outlet, even in the most unfavorable situations. This is achieved by increasing the available area available for the flow through the outlet, even in the most unfavorable conditions.

[0020] This reduced choking effect even in the most unfavorable conditions, also means that in addition to reducing the negative effects of the inner pipe blocking the pump outlet to a manageable level, the forces acting on the inner pipe in these most unfavorable conditions are significantly reduced, which in turn reduces the probability that the inner pipe remains in the most unfavorable position.

[0021] In a first scenario, where the inner pipe is pulled towards the outlet by the differences in pressure forces, the invention introduces an increased available flow area from the annulus between the outer pipe and the inner pipe and through the outlet from the outer pipe to the branch pipe. The available flow area is increased and exceeds a cross-sectional area of the branch pipe. This will reduce the Bernoulliand choking effect and reduce risk that the inner pipe enters a position obstructing flow through the outlet. It also reduces the forces acting on the inner pipe and reduces the probability that it will remain in a position where it has some blocking effect on the flow.

[0022] In a second scenario, the inner pipe is mechanically pushed or pulled towards the outlet due to operating issues, and the invention ensures that there will be sufficient available flow area also in this situation. Without the invention, the flow is obstructed, resulting in undesired consequences.

[0023] The present invention thus concerns a pumped mud return system used in connection with offshore drilling operations. An outer pipe with a central longitudinal axis and an inner radius, forms a duct from a blowout preventer and extends to a drilling rig unless the pumped mud return system is a riser-less system. An outer pipe outlet is located in a wall of the outer pipe. A branch pipe is branched off from the outer pipe and extends from the outer pipe outlet. The pumped mud return system furthermore includes a pump adapted to pump liquid through the branch pipe. An inner pipe passing the outer pipe outlet is located inside the outer pipe, whereby an annulus is formed between the outer pipe and the inner pipe. A geometry forming an anti-clogging system ensuring a flow path and a sufficient available flow area, is provided between the annulus and the branch pipe. The geometry includes at least one of a transition portion surrounding the outer pipe outlet, where a transition distance Dt between the central longitudinal axis of the outer pipe and the transition portion is greater than the inner radius of the outer pipe, and a separate bypass duct between an outer pipe wall second outlet in the outer pipe and a branch pipe wall, liquid inlet.

[0024] The branch pipe has an inner diameter A, the inner pipe has an outer diameter B, and the outer pipe has an inner diameter C. The pipe diameters are such that B>0.6C and A<0.4B.

[0025] The available flow area may be increased by removing material in a perpendicular direction in relation to the longitudinal direction of the outer pipe, at the location of the outlet to the branch pipe.

[0026] The at least one transition portion may include a frustoconical recess surrounding the outer pipe outlet.

[0027] The at least one transition portion between the annulus and the branch pipe may include grooves providing an increased available flow area.

[0028] The separate bypass duct allow flow to take a different path from the annulus to the branch pipe through the branch pipe wall inlet for liquid, bypassing the first outlet.

[0029] The separate bypass duct may be located on the outside of the outer pipe, entering the branch pipe wall, liquid inlet.

[0030] The direction of the bypass duct would typically initially upon exit from the outer pipe be routed upwards at an angle to ensure there is no settling of material inside the pipe during the time that this pipe has no active flow through it.

[0031] The outer pipe wall second outlet may be located circumferentially outside the tangent lines of the outer pipe outlet as seen from the center of the outer pipe.

[0032] The pump may be utilized to reduce an annulus fluid level below a diverter.

[0033] The outer pipe outlet from may be at a depth below the surface of the body of water, and the pump may be a return pump to return fluid from the outer pipe to the surface.

[0034] The outer pipe may be a marine riser extending to a rig.

[0035] The outer pipe may extend to a position below an ocean surface S and may be open towards seawater at a location above the pipe outlet.

[0036] The pumped mud return system may include a plurality of separate bypass ducts.Brief description of drawings

[0037] Figure 1 is a schematic representation of a conventional drilling system of the prior art;

[0038] Figure 2 is a schematic representation of a pumped mud return system of the prior art where the invention may be implemented;

[0039] Figure 3 is a schematic representation of a riser-less pumped mud return system of the prior art where the invention may be implemented;

[0040] Figure 4a is a cross-section, of a pumped mud return system of the prior art in a perspective view with an inner tube centrally located in an outer pipe, and a branch pipe extending from an outer pipe outlet;

[0041] Figure 4b corresponds to fig. 4a, and shows the inner tube obstructing the outer pipe outlet;

[0042] Figure 4c shows the situation of fig. 4b and is a schematic representation of the cross-section showing how the inner tube obstructs the outer pipe outlet;

[0043] Figure 5a corresponds to fig. 4b, furthermore showing an anti-clogging system formed by a transition providing a geometry with an increased available flow area in a first embodiment of the invention;

[0044] Figure 5b shows the situation of fig. 5a and is a schematic representation of the cross-section the increased the flow area of the first embodiment of the invention;

[0045] Figure 5c shows the inside the outer pipe in a view towards the outer pipe outlet in the first embodiment of the invention;

[0046] Figure 5d shows the situation of fig. 4b and is a schematic representation of a perspective view of the cross-section showing recesses forming grooves to increase the flow area in a second embodiment of the invention;

[0047] Figure 5e shows the situation of fig. 5d and is a schematic representation of the cross-section showing the increased the flow area of the second embodiment of the invention;

[0048] Figure 5f shows the situation of fig. 5e and is a schematic representation of a view of the invention from an inside of the outer pipe and towards the outer pipe outlet according to the second embodiment of the invention, showing favorable positions of recesses to facilitate flow;

[0049] Figure 5g shows the situation of fig. 5e and is a schematic representation of the view of the invention from the inside of the outer pipe and towards the outer pipe outlet area according to the second embodiment of the invention, showing unfavorable positions of recesses to facilitate flow;

[0050] Figure 6a is a view corresponding to the view of fig. 4c showing an anticlogging system with a geometry including a bypass duct of a third embodiment of the invention; and

[0051] Figure 6b shows the limits of where such a bypass duct as shown in fig. 6a may be located.Detailed description of the drawings

[0052] Figs. 1 -4c show examples of the prior art.

[0053] Figure 1 is a schematic representation of a conventional drilling system of the prior art and shows a conventional drilling setup. In both conventional drilling and when drilling with a pumped mud return system can drilling fluid be pumped from a vessel I rig on the ocean surface S into the outer pipe 1 either through a drill pipe / casing (an inner pipe 2) inside the outer pipe 1 (the flow coming up from the well located below the BOP 11 ), or through an external boost line 12, entering the outer pipe 1 just above the blow out preventer (BOP) 11 at the seabed B. The fluid flows upward in the annulus 4 between the drill pipe / casing (inner pipe 2) and the outer pipe 1 .

[0054] In conventional drilling the drilling fluid exits from the outer pipe 1 through the diverter 10 and into the flow line 8. In some Managed Pressure Drilling (MPD) applications using a Rotating Control Device (RCD), often referred to as Surface Back Pressure (SBP) the outlet from the outer pipe is located subsea, but the setup differs from pumped mud return applications as shown in fig. 2; In SBP there is not a subsea pump present connected to the outer pipe outlet; In SBP there is a Rotating Control Device (RCD) present in the string, which centralizes the inner pipe inside the outer pipe. The problem solved by the present solution will therefore not be present.

[0055] Figure 2 describes a pumped mud return system where the drilling fluid exits the outer pipe 1 through an outer pipe outlet 3 located below the flow line 8. Here the outlet is shown below a telescopic joint 9. This outlet can also be above the telescopic joint 9, just below the diverter 10. A branch pipe 13 is connected to the outer pipe outlet 3 leading the fluid, via an isolation valve 6, to a pump 5, whichpumps the fluid topside to the vessel through a mud return line 7. When the outer pipe outlet 3 is placed above the telescopic joint 9, the person skilled in the art will recognize that the pump 5 will be located topside on the drilling vessel. In the system of fig. 2 the outer pipe 1 is a marine riser extending between the BOP 11 and the rig.

[0056] Figure 3 shows a solution substantially corresponding to fig. 2. Fig. 3 however shows a typical riser-less version of a pumped mud return system. The difference from fig. 2 is that the outer pipe 1 does not extend all the way to the rig. Instead, the outer pipe 1 extends to a level above the BOP 11 , wherein the outer pipe outlet 3 is situated. (A Tig” in the content of this application is considered to include all sorts of suitable rigs, including drilling ships, jack-up rigs, semi-submersible rigs, gravity-based rigs etc.). In other words, the outer pipe 1 extends to a position below the ocean surface S and is open towards seawater at a location above the pipe outlet 3. The inner pipe 2 is exposed to the surrounding seawater and is not surrounded by the outer pipe 1 at the location above the outer pipe 1 .

[0057] The solutions shown in figs. 1 , 2 and 3 are schematic representations of the equipment present in a drilling set-up. Equipment such as e.g., kill / choke lines and topside arrangements are not relevant for this patent and are therefore left out for clarity. The person skilled in the art will be familiar with all the other components required for a drilling operation.

[0058] Figure 4a shows the inner pipe 2 (such as a casing) in a centered position inside the outer pipe 1 . The area shown is the outer pipe outlet 3 going from the outer pipe 1 to through the branch pipe 13 and to the pump 5 (Fig. 2). The inner pipe 2 is shown in an optimal position in flow terms at the center of the outer pipe 1 and a perfect annulus 4 is formed between the outer pipe 1 and the inner pipe 2. The available flow area equals a cross-sectional area of the outer pipe outlet 3 and of the branch pipe 13.

[0059] Figure 4b shows the worst position of the inner pipe 2 in flow terms, where the inner pipe covers the outer pipe outlet 3.

[0060] When the inner pipe 2 is in this position, an available flow area 16 is at a minimum and a Bernoulli effect and choking effect are at their maximum. This phenomenon typically results in undesired consequences when the ratio between outlet inner diameter ID (A), inner pipe outer diameter OD (B) and outer pipe innerdiameter ID (C), as shown in Figure 4c, are according to the following: B>0.6C (Inner pipe OD more than 60% of outer pipe ID) A<0.5B (Outlet ID less than 50% of inner pipe OD)

[0061] In Figures 4c, 5b and 6 the dotted lines represent the ID of the outer pipe 1 below and above the outer pipe outlet 3. This line is the limit for radial movement of the inner pipe 2 inside the outer pipe 1 . The inner pipe 2 is shown in the worst position, with the outer surface of the inner pipe aligned with the inner surface of the outer pipe in front of the outer pipe outlet 3.

[0062] Figures 5a and 5b show a first embodiment of the invention with the inner pipe 2 inside the outer pipe 1 covering the outer pipe outlet 3. The outer pipe outlet 3 leads to a branch pipe 13 leading liquid out of the outer pipe 1 as described above. The outer diameter of the inner pipe 2 is smaller than the inner diameter of the outer pipe 1 , thus forming an annulus 4 inside the outer pipe 1 . The anti-clogging system includes a transition portion 14 between the annulus 4 and the branch pipe 13 forming a geometry ensuring a flow path between the outer pipe 1 and the branch pipe 13, perpendicular to the direction of the outer pipe, surrounding the outer pipe outlet 3.

[0063] The transition portion 14 between the annulus and the branch pipe provides an increased available flow area 16, compared to prior art solutions without a transition portion 14 surrounding the outer pipe outlet 3, preventing the outside of the inner pipe 2 from obstructing flow out of the outer pipe outlet 3. The outer pipe 1 includes a central longitudinal axis Ax and an inner radius Ro. A transition distance Dt from the central longitudinal axis Ax and to the transition portion 14 exceeds the inner radius Ro (Dt>Ro). The frustoconical geometry ensuring a flow path is preferable from a machining perspective, but any geometry ensuring an increase in available area would be according to this invention. The available flow area exceeds the cross-sectional area of the branch pipe 13. An inner wall of the outer pipe 1 is cylindrical, and the available flow area and the transition portion 14 commences where the inner shape of the inner wall of the outer pipe 1 deviates from this cylindrical shape. If using the previously described clock reference, such recess would be most beneficial to enlarge at 3 and 9 o’clock, as this is where the flow would be increased the most from a given amount of material removal.

[0064] Fig. 5c shows the inside of the outer pipe 1 in a view towards the outer pipe outlet 3. The transition portion 14 forms an increased flow area of the first embodiment of the invention. The transition portion 14 may be machined as a frustoconical shape. Post machining, the transition portion 14 would then, as shown in fig. 5c, have a smaller height h than width w. A person skilled in the art understands that this relation between h and w where h<w is due to the curved geometry of the inside of the outer pipe 1 . Similarly, the transition portion 14 could have other geometries, such as the shape of a bellmouth. Again, an inlet of the bellmouth forms the available flow area exceeding the cross-sectional area of the branch pipe 13.

[0065] Figures 5d and 5e show a second embodiment of the invention where the increased available flow area 16 is provided by grooves 19 providing the transition portion 14. The grooves 19 are provided surrounding the 3 and 9 o’clock position for greater effectiveness. Grooves 20 are shown in the 6 and 12 o’clock positions and these positions are unfavorable as they are the least effective positions when the inner pipe 2 is directly in front of the outer pipe outlet.

[0066] Figures 5f and 5g show the inside of the outer pipe 1 in a view towards the outer pipe outlet 3 in the second embodiment of the invention. In fig. 5f, the grooves 19 are located on the side of the outer pipe outlet away from the 12 o’clock and 6 o’clock positions at the center of the outer pipe outlet 3 to provide the increased flow area. Increasing the available flow area with grooves also allow a required amount of material and the required mechanical properties to be maintained in the transition portion 14. The groves may provide a sufficient flow area and low manufacturing costs.

[0067] In fig. 5g, the transition portion 14 also forms the increased flow area of the second embodiment of the invention. The grooves 20 are located surrounding the 12 o’clock and 6 o’clock positions at the center of the outer pipe outlet 3 to provide the increased flow area. These grooves 20 are less effective than the side grooves 19 due to the blocking effect of the inner pipe below and above the center of the outer pipe outlet 3 to the branch pipe 13. The grooves 20 may however be effective if the inner pipe is located slightly off center in relation to the outer pipe outlet 3.

[0068] Figure 6a shows a third embodiment of the invention schematically as a cross-section. The outer pipe outlet 3 is substantially blocked by the inner pipe 2 but a geometry including a separate bypass duct 15 extends between an outer pipe second outlet 17 in the outer pipe 1 and a branch pipe wall inlet 18 to provide a separate flow channel that will reduce the tendency the flow out of the outer pipe outlet 3 has to pull the inner pipe towards the outlet. The outer pipe second outlet 17 is positioned in relation to the outer pipe outlet 3 so that it is ensured that the inner pipe 2 is unable to obstruct both outlets at the same time. The geometry with the separate bypass duct 15 may be integrated as one or several bores in a housing forming the outlet for the branch pipe. Alternatively, the bypass duct 15 may be formed as one or several external pipes or hoses between the second outlet 17 of the outer pipe 1 , and the branch pipe wall inlet 18 of the branch pipe 13.

[0069] Figure 6b shows the inside the outer pipe 1 in a view towards the outer pipe outlet 3. Two dotted lines X and Y in a longitudinal direction of the outer pipe 1 and extending tangentially from the outer pipe outlet 3, mark the limit of where an outer pipe outlet 17 may be positioned. An outer pipe second outlet 17 may be positioned either to the left of longitudinal axis X or to the right of longitudinal axis Y. The minimum distance between longitudinal axis X and longitudinal axis Y is the diameter A of the branch pipe 13. (fig. 6a). The outer pipe outlet 17 may thus be located along the inner circumference of the outer pipe between the left side of line X and the right side of line Y. The main issue is to locate the outer pipe outlet 17 away from the outer pipe outlet 3 and thus away from the inner pipe 2 (fig. 6a).

[0070] The outer pipe outlet 17 will be located at substantially the same depth below the ocean S as the outer pipe outlet 3

[0071] The geometry of the third embodiment of the invention may include several bypass ducts 15 from the outer pipe 1 to the branch pipe 13. The bypass duct 15 will ensure that fluid will not be restrained from exiting the outer pipe 1 by the inner pipe 2 blocking the outer pipe outlet 3, as liquid will have the possibility to exit through the additional outlet(s) 15.

[0072] The outer pipe outlet 3 in the context of this specification means an outlet for fluid flowing from the inside of the outer pipe 1 , out through the outer pipe outlet 3 and into the branch pipe 13. The outer pipe outlet 3 should not be confused with aninlet allowing fluid into the outer pipe 1 . An inlet will not have the same problems as the outer pipe outlet 3 as flow through an inlet will tend to push the inner pipe 2 away from the inner wall surrounding the inlet of the outer pipe 1 .

[0073] The elements of the three embodiments of the invention may be combined to accommodate varying requirements in terms of flow rate and flow path dimensions.

[0074] An essential effect of the invention is that the force on the inner pipe towards the outer pipe outlet 3 is reduced, and the inner pipe may move away from the outer pipe outlet and further improve the conditions for fluid flow.

Claims

CLAIMS1 . A pumped mud return system used in connection with offshore drilling operations comprising: an outer pipe (1 ) with an inner diameter C, a central longitudinal axis Ax and an inner radius Ro, extending from a blowout preventer (11 ); an outer pipe outlet (3) in a wall of the outer pipe (1); a branch pipe (13) with an inner diameter A, branched off from the outer pipe (1 ), extending from the outer pipe outlet (3); a pump (5) adapted to pump liquid through the branch pipe (13); an inner pipe (2) with an outer diameter B, inside the outer pipe (1 ), passing the outer pipe outlet (3), whereby an annulus (4) is formed between the outer pipe (1 ) and the inner pipe (2), and wherein B>0.6C and A<0.4B; and a geometry ensuring a flow path between the annulus (4) and the branch pipe (13), and wherein the geometry includes at least one of a transition portion (14) surrounding the outer pipe outlet (3) between the annulus and the branch pipe (13), where a transition distance Dt between the central longitudinal axis Ax and the transition portion (14) is greater than the inner radius Ro of the outer pipe (1 ), and a separate bypass duct (15) between an outer pipe wall second outlet (17) in the outer pipe (1 ) and a branch pipe wall inlet (18).

2. The pumped mud return system of claim 1 , wherein the at least one transition portion (14) includes a frustoconical recess surrounding the outer pipe outlet (3).

3. The pumped mud return system of claim 1 or 2, wherein the at least one transition portion (14) between the annulus and the branch pipe (13) includes grooves (19) providing an increased available flow area (16).

4. The pumped mud return system of claim 1 2 or 3, wherein the outer pipe wall second outlet (17) is located outside a longitudinal axis X, Y of the outer pipe (1 ) extending from a tangential line extending from the outer pipe outlet (3) in the wall of the outer pipe (1 ), and wherein an inner diameter of the separate bypass duct (15) is smaller than an inner diameter A of the branch pipe (13).

5. The pumped mud return system of claim 1 , wherein the pump (5) is utilized to reduce an annulus fluid level below a diverter.

6. The pumped mud return system of claim 1 , wherein the outer pipe outlet (3) is at a depth below the surface of the body of water, and wherein the pump (5) is a return pump to return fluid from the outer pipe (1 ) to the surface.

7. The pumped mud return system of claim 1 , wherein the outer pipe (1 ) is a marine riser extending to a rig.

8. The pumped mud return system of claim 1 , wherein the outer pipe (1 ) extends to a position below an ocean surface S and is open towards seawater at a location above the pipe outlet (3).

9. The pumped mud return system of any of the preceding claims including a plurality of separate bypass ducts (15).

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