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Publication numberUS3686887 A
Publication typeGrant
Publication dateAug 29, 1972
Filing dateJan 13, 1970
Priority dateJan 17, 1969
Publication numberUS 3686887 A, US 3686887A, US-A-3686887, US3686887 A, US3686887A
InventorsBruce Peter
Original AssigneeBruce Peter
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Scour control system for submerged structures
US 3686887 A
Abstract
A scour control system for establishing and maintaining convergent fluid flow conditions at the surface of a submerged particulate bed circumjacent to a submerged structure seated on the bed so that particle loss from the surface of the bed, within a closed region surrounding the structure, is eliminated as being the normal consequence of externally incident unidirectional fluid flow past the submerged structure at the particulate bed surface.
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Description  (OCR text may contain errors)

United States Patent Bruce 1 Aug. 29, 1972 [54] SCOUR CONTROL SYSTEM FOR SUBMERGED STRUCTURES [72] Inventor: Peter Bruce, 1O Torphichen Place,

Edinburgh, EH3 8DU, Scotland I22] Filed: Jan. 13, 1970 [2! 1 Appl. No: 2,501

[30] Foreign Application Priority Data Jan. 17, 1969 Great Britain ..2,927/69 Jan. 25, 1969 Great Britain ..4,351/69 [52] US. Cl ..61/63, 61/2 [51] Int. Cl. ..E02b 3/00 [58] Field of Search ..61/2, 53.74, 1, 63; 37/58, 37/75, 78, 79

[56] References Cited UNITED STATES PATENTS 7 2,854,825 10/1958 Crake ..61/53.74 X

3,312,069 4/1967 Jorda ..61/1 3,449,915 6/1969 Cummings ..61/2 3,452,966 7/1969 Smolski ..61/1

Primary ExaminerDavid J. Williamowsky Assistant Examiner-David H. Corbin [5 7] ABSTRACT A scour control system for establishing and maintaining convergent fluid flow conditions at the surface of a submerged particulate bed circurnjacent to a submerged structure seated on the bed so that particle loss from the surface of the bed, within a closed region surrounding the structure, is eliminated as being the normal consequence of externally incident unidirectional fluid flow past the submerged structure at the particulate bed surface.

17 Claims, 15 Drawing; Figures PATENTEDmszs m2 sum 1 0F 5 Figure I SCOUR CONTROL SYSTEM FOR SUBMERGED STRUCTURES Preferably the vertical position of an upper extremity of an accumulation of particles against the submerged structure, and hence the magnitude of the accumula tion of particles, is regulated by automatic control means.

Preferably the automatic control means for regulating the magnitude of the accumulation of particles includes: at least one sensing element capable of providing a signal related to the vertical distance separating a reference point on the shroud The present invention relates to a pump and shroud ducting system for modifying fluid flow patterns around the intersection of a submerged structure with a submerged particulate supporting bed in order to mitigate against and control the local erosion of the bed adjacent to the structure. Such erosion is generally known as scour.

The submerged structure could, for example, be a supporting leg of a jack-up sea drilling platform or rig.

Scour can occur at a submerged particulate bed when a submerged structure presents an obstruction to fluid flow over the bed and causes an increase in fluid flow velocity at locations on the bed surface adjacent to the structure. These locations then experience increased particle entrainment with the moving fluid. Particle loss from the locations ensues with the result that progressive erosion of the surface of the bed within these locations occurs.

If the fluid velocity is sufficiently high, surface erosion or scour adjacent to the submerged structure may proceed until the structure is undermined to such an extent that a settlement of the structure into the surface of the submerged bed may occur.

The supporting legs of sea drilling rigs are particularly prone to such settlement due to scour and resultant undesirable stresses in the overall rig structure are presently commonplace.

Since particle movement at the surface of a submerged particulate bed in the presence of a fluid flow in the local area adjacent to a submerged structure will generally follow the fluid flow paths at the surface of the bed, it follows that particle loss from such an area will occur if the flow paths pass out of the area.

Thus, if the flow paths at the particulate bed surface can be constrained to converge at all times on the sub merged structure, particle accumulation against the structure will occur and no scouring action, with resultant undermining of the structure, will be possible.

Such convergent fluid flow paths can be established by providing a vertically extending tubular shroud adjacent to the structure and pumping means whereby fluid may be conducted from the particulate bed adjacent to the structure to a level so spaced above the plane of the particulate bed that discharge of the ducted fluid causes substantially no disturbance at the particulate bed.

It is an objective of the present invention to mitigate and control the scouring action that tends to occur at a submerged particulate bed adjacent to a submerged structure by altering the fluid flow paths at the submerged particulate bed such that an accumulation of particles, transported by fluid flow, tends to occur against the submerged structure.

1 According to the present invention, a scour control system includes: an elongate, hollow, cylindrical shroud means, of substantially vertical axial orientation and with an open lower extremity, for conveying fluid from a location at the surface of a particulate submerged bed adjacent to a submerged structure seated thereon, to an upper location at a level so spaced vertically from the particulate bed that discharge of the conveyed fluid at the upper location causes substantially no disturbance of the particulate bed surface; fluid pumping means connected to the shroud whereby fluid flow may be caused within the shroud means; and mounting member means connecting said shroud means to said submerged structure whereby the shroud means is vertically substantially coaxial with the submerged structure and the open lower extremity of the shroud means is maintained in a position adjacent to the submerged structure and spaced from the particulate bed surface such that upwards forced fluid flow within the shroud means, due to the action of the fluid pumping means, causes a convergent fluid flow at the surface of the particulate bed adjacent to the submerged structure so that an accumulation of particles, transported by the convergent fluid flow, tends to occur against the submerged structure.

Preferably the shroud means is located coaxial with the vertical or substantially vertical axis of the submerged structure.

Preferably the shroud means is located outside, ad jacent to, and surrounding the submerged structure.

Preferably the shroud means is located inside the submerged structure.

Preferably the shroud means is constructed as an integral part of the submerged structure.

Preferably the mounting member means includes a movable mounting which permits relative vertical movement of the shroud means with respect to the submerged structure.

Preferably the vertical position of the shroud means with respect to the submerged structure is regulated by automatic control means.

Preferably the mounting member means includes an automatic control means comprising at least one sensing element capable of providing a signal related to change in the vertical distance separating a reference point on the shroud means from the plane of the undisturbed submerged particulate bed; signal amplification means; actuator motor driving means capable of changing the vertical position of the shroud means; and signal transmission means between the sensing elements, the amplifier means, and. the actuator motor driving means whereby negative feedback control of the position of the shroud means with respect to the undisturbed particulate bed plane may be established.

Preferably the vertical position of an upper extremity of an accumulation of particles against the submerged structure, and hence the magnitude of the accumulation of particles, is regulated by automatic control means.

Preferably the automatic control means for regulating the magnitude of the accumulation of particles includes: at least one sensing element capable of providing a signal related to the vertical distance separating a reference point on the shroud means from an upper extremity of an accumulation of particles against the submerged structure; signal amplification means; actuator means capable of changing the rate of fluid flow through the shroud means; and signal transmission means between the sensing elements, the amplifier means, and the actuator means whereby the rate of fluid flow within the shroud means and over the adjacent particulate bed surface, and hence the rate of accumulation of particles against the submerged structure, may be so regulated by negative feedback that position control of the vertical location of the upper extremity of the particle accumulation with respect to the reference point on the shroud means may be established.

According to a preferred aspect of the present invention, the fluid pumping means includes a rotatable impeller means mounted coaxially within the shroud means, and turbine means, drivable by externally incident fluid flow and coupled to the rotatable impeller means such that rotation of the turbine means due to the action of externally incident fluid flow results in an upward fluid movement within the shroud means.

Preferably the turbine means comprises a tangentially driven axial-bladed turbine wheel coupled to and mounted coaxially above the impeller means such that externally incident fluid flow in any direction tangential to the turbine wheel and in the plane of rotation of the turbine wheel causes unidirectional rotation of the turbine wheel.

According to another preferred aspect of the present invention, the fluid pumping means includes at least one nozzle located inside and substantially adjacent a lower extremity of the shroud means; at least one pipe capable of supplying a fluid to the nozzles and connected thereto; and a supply of a fluid deliverable through the pipe to the nozzles such that release or discharge of the fluid from the nozzles causes upward fluid movement within the shroud means by entrainment with the discharge, in the case of a liquid, and by entrainment with rising buoyant bubbles, in the case of a gaseous or a vaporous fluid delivered by the pipe. It will be appreciated that in the case of a liquid discharge from the nozzles, the nozzles must be pointing upwards.

The principle of operation of the scour control system may be described best with reference to the diagrammatic representations of FIGS. 1, 2 and 3 of the accompanying drawings.

FIG. 1 shows a sectional view through a vertically orientated tubular shroud 1 containing a coaxially mounted impeller 2 driven by a motor 3 and vertically separated from the surface of a submerged particulate bed 4. Rotation of the impeller 2 by the motor 3 causes upward fluid flow within the shroud l which, in turn, causes a particular fluid flow pattern at the surface of the particulate bed 4. The arrowed lines indicate fluid flow paths. A plan representation of the fluid flow paths 5 lying in the plane indicated by A-A in FIG. 1 is given in FIG. 2. For no external disturbances in the fluid overlying the submerged particulate bed 4, the fluid flow lines 5 in the plan representation of FIG. 2 will be radially convergent on the axis of the tubular shroud 1.

If a unidirectional fluid flow at the surface of the submerged particulate bed 4 exists additionally, as an external disturbance, the resultant combination of fluid flow patterns may be depicted by the fluid flow lines 5 shown in the plan representation of FIG. 3.

The region denoted by the broken line 6 may be seen to experience only convergent flow at the particulate bed surface despite the existence of an external unidirectional fluid flow. Since no net outflow of fluid occurs across the closed boundary 6 at the surface of the particulate bed 4, no outward transportation of particles with consequent particle loss from the region can occur. Hence, scour, involving particle loss from a closed bounded region of a submerged particulate bed due to particle transportation by overpassing fluid flow, may be mitigated and prevented by the application of a superimposed convergent fluid flow within the region prone to scour.

This then is the principle of operation of the present invention.

Embodiments of the present invention will now be described by way of example with further reference to the accompanying drawings in which:

FIG. 4 shows a sectional side view of a submerged structure seated on a particulate bed with a scour control system, according to a first embodiment of the present invention, located inside the structure;

FIG. 5 shows a sectional plan view through B-B of FIG. 4 with the fluid flow patterns at the surface of the submerged particulate bed shown for the system in operation;

FIG. 6 shows a sectional side view of a submerged structure with a scour control system, according to a second embodiment of the present invention, located external to the structure;

FIG. 7 shows a sectional plan view through CC of the structure and scour control system depicted in FIG.

FIG. 8 shows a sectional side view of a submerged structure seated on a particulate bed with a scour control system, according to a third embodiment of the present invention, located externally on the submerged structure and including a tangential-drive axial-bladed turbine driven by external fluid flow and coupled coaxially to an impeller within the shroud part of the scour control system;

FIG. 9 shows a sectional plan view through D-D of the submerged structure and scour control system depicted in FIG. 8;

FIG. 10 shows a perspective view of the third embodiment of the present invention shown in FIG. 8;

FIG. 11 shows a sectional side view of a submerged structure with a scour control system, according to a fourth embodiment of the present invention, located externally on the structure and including fluid discharge nozzles mounted inside the shroud part of the scour control system;

FIG. 12 shows a sectional side view of a submerged structure with a scour control system, according to a modification of the fourth embodiment of the present invention, constructed integral with the submerged structure and with powering fluid discharge nozzles mounted within the submerged structure wall which acts additionally as the shroud part of the scour control system;

FIG. 13 shows a diagrammatic representation of a scour control system, according to the fourth embodiment of the present invention as shown in FIG. 11, including a position control system for regulating the vertical position of the shroud part of the scour control system;

FIG. 14 shows a diagrammatic representation of a scour control system, according to the fourth embodiment of the present invention as shown in FIG. 11, including a position control system for regulating the proximity of the upper extremity of the particle accumulation against the submerged structure to the lower extremity of the shroud part of the scour control system;

FIG. shows a diagrammatic representation of an electrical circuit configuration included in the position control system depicted in FIG. 14.

Referring to FIGS. 4 and 5, in a first embodiment of the present invention, a scour control system is located within a submerged open-frame structure 1a and includes an elongate hollow circular cylindrical openended shroud 1, substantially vertically orientated axially and with an internal centrally mounted bladed impeller 2 driven by an electric motor 3, movably mounted by brackets7 and pinions 8 engaging with rails 9 fixed to the submergedstructure 1a which is seated on a submerged particulate bed 4 and penetrates below the undisturbed surface plane 4a of the bed 4. An adjustable cable 10, attached by one end to an uppermost one of the brackets 7 and by the other end to an anchorage point on an upper part of the structure la, serves to locate the shroud 1 vertically with respect to the structure la in a position above the plane 4a such that upwards movement of fluid within the shroud 1 due to rotation of the impeller 2 by the electric motor 3 establishes flow paths 5, despite the presence of external unidirectional fluid flow paths 5a, whereby a particle accumulation 4b tends to occur against the struc ture la.

Referring to FIGS. 6 and 7, in a second embodiment of the present invention, a scour control system is located externally around a submerged circular cylindrical structure lg and includes an elongate hollow circular cylindrical annular shroud lb, open at a lower extremity lie and closed at an upper extremity 1d. A horizontal tubular duct 1e communicates the interior space of the annular shroud lb with the external fluid. A bladed impeller 2 driven by an electric motor 3 is mounted within the duct 1e at the open end remote from the shroud lb. A planar fin 1f, vertically orientated, forms a web between the duct 1e and the shroud lb to act as a direction orienting vane. The annular shroud lb is movably mounted on the structure lg by captive bearing balls 11 which are held captive with respect to the shroud lb and space apart the shroud lb and the structure lg. An adjustable cable 10, attached by one end to the upper external surface of duct 1e and by the other end to an anchorage point on an upper part of the structure lg, serves to locate the shroud lb vertically with respect to the structure lg such that upwards movement of fluid within the shroud 1g due to the rotation of the impeller 2 by the electric motor 3 establishes flow paths 5, despite the presence of external unidirectional fluid flows along flow paths 5a, whereby a particle accumulation 4b may occur against the structure 1g.

Referring to FIGS. 8, 9, and 10, in a third embodiment of the present invention, a scour control system is located externally around a submerged circular cylindrical vertical structure lg and includes an elongate, hollow, annular, circular cylindrical shroud 1b, open at a lower extremity 1c and with an inner wall 14 slidably bearing against the exterior of the structure 1g. A conical hollow annular duct 12 closes the upper extremity 1d of the shroud 1b and communicates the annular interior space of the shroud lb with a hollow cylindrical annular duct 13 containing a bladed impeller 2 rotatably mounted on bearings 16 around the inner wall 14 of the shroud lb. A tangentially driven axialbladed circular cylindrical turbine wheel 15, carrying axial blades 17, is mounted coaxially above the impeller 2 and is directly coupled to it while additionally being rotatably mounted coaxiaily around the inner wall 14 of the annular shroud lb such that external fluid flow incident on the blades 17 of the turbine wheel 15 causes unidirectional rotation 18 of the turbine wheel 15 and consequent unidirectional rotation 18 of the impeller 1 which, in turn, causes upward fluid movement within the shroud lb, the duct 12, the duct 13, and the turbine wheel 15 whereat centrifugal force causes ejection of the ducted fluid out between the rotating turbine blades 17 as an effluence 19. This upward fluid movement causes ingestion of fluid at the open ,lower extremity 1c of the shroud lb such that a convergent fluid flow pattern combines with any external unidirectional flow pattern at the surface of the particulate bed to form a combination flow pattern,as indicated in FIG. 10, which mitigates against and preventsthe occurance of particle removal, or scour, in the region of the particulate bed 4 circumjacent to the structure lg. As shown in FIG. 8, the horizontal crosssectional area of ducted fluid at the impeller 2 is greater than the horizontal cross-sectional area of ducted fluid at the lower extremity 1c of the shroud lb so that the velocity of fluid ingestion exceeds the velocity of ducted fluid at the impeller 2. This con stitutes a velocity ratio which allows ingestion fluid velocities at the particulate bed surface circumjacent the structure lg to exceed the velocity of the incident turbine-driving external fluid flow so as to maximize the particle transportation into the bed region circumjacent the structure lg for a given external fluid flow rate and a given total turbine blade area.

Referring to FIG. 11, in a fourth embodiment of the present invention, a scour control system includes an elongate, hollow, circular cylindrical, annular shroud 1b, open at both upper and lower extremities and slidably located externally surrounding a submerged circular cylindrical structure 1g. A hollow toroidal ring 20 is located and mounted in the internal annular space within the lower extremity 1c of the annular shroud 1b and encircling the inner wall 14 of the shroud lb. The hollow ring 20 is equipped with upwards pointing nozzles 21 equally spaced around the ring such that pressurized air delivered into the ring 20 may be discharged upwards in an even distribution within the annular space inside the shroud 1b. A pipe 22 supplies pressurized air from an upper location on the submerged structure lg to the interior of the ring 20. An adjustable cable 10, attached by one end to the inner wall 14 of the shroud 1b and by the other end to an anchorage point located on an upper part of the structure lg, serves to locate the shroud 1b vertically with respect to the structure 1g such that upwards movement of fluid within the shroud lb, due to entrainment with rising air bubbles 25 issuing from the nozzles 21, establishes flow paths 5, despite the presence of external unidirectional fiows, whereby a particle accumulation 4b may tend to occur against the structure lg.

Referring to FIG. 12, in a modification of the fourth embodiment of the present invention as shown in FIG. 11, a scour control system includes a shroud constructed as an integral part of a submerged cylindrical structure lg. Horizontal rows of circumferencially disposed equally spaced slanted inlet holes 23 are located in the wall of the structure lg at several equally spaced vertical locations. A movable cylindrical sleeve 24 capable of closing off all of the holes 23 is suspended within the structure lg by an adjustable cable 10 of which one end is attached to the sleeve 24 and the other end is attached to an anchorage point located on an upper part of the structure lg.

The purpose of the sleeve 24 is to uncover successively higher inlet holes 23 as the structure 1g settles into the submerged bed 4, under the force of its own weight, so that the scour control system can effectively maintain a working separation from the particulate bed surface plane 4a.

A hollow toroidal ring is fixed to the inside of the wall of the structure lg above the uppermost reach of the sleeve 24 and is equipped with equally spaced upwards pointing nozzles 21 whereby pressurized air supplied by a pipe 22 attached to the ring 20 may be discharged within the structure lg so that rising buoyant bubbles 25 are formed which entrain fluid while rising up within the structure lg to escape from exhaust holes 26 located above the ring 20. The resultant upwards fluid movement causes fluid ingestion at the uncovered inlet holes 23 whereby a scour mitigation and prevention effect is established at the surface of the particulate bed circumjacent to the submerged structure lg as hereinbefore described.

Referring to FIG. 13, in an embodiment of a first aspect of the present invention, a scour control system as shown in FIG. 11 includes a means of automatically positioning the shroud lb with respect to the undisturbed particulate bed plane 4a which comprises: a depth measuring device 27, of the acoustic echo sounder type, capable of giving an output voltage proportional to the vertical distance between the device 27 and the particulate bed plane 4a; a differential amplifi er 29 to compare the output voltage of device 27 with a remotely selectable reference voltage and amplify the difference between these two voltages; an actuator 30 driven by the output of amplifier 29; an electric motor 31 controlled by the actuator 30; a winch drum 32 mounted on the spindle of the electric motor 31; and a cable 10 anchored to and wrapped round the drum 32 at its upper end and attached to an upper part of the movable shroud lb of the scour control system by its lower end. The depth measuring device 27 is mounted, together with the amplifier 29, on one extremity of a horizontal elongate member 28 attached rigidly to the shroud 1b so that the device 27 operates over an undisturbed area of the surface of the particulate bed 4. The actuator 30, motor 31, and drum 32 are mounted on an upper part of the submerged structure lg. The shroud lb is drawn upwards by the cable 10 winding on the drum 32 when the voltage from device 27 is less than the reference voltage and is lowered downwards when the voltage from device 27 exceeds the reference voltage. Thus variation of the reference voltage may be used to set the steady state vertical position of the shroud, 1b with respect to the plane 4a. Flexible interconnecting wires 32a electrically link the amplifier 29, the depth measuring device 27, and the actuator 30 such that, together with the motor 31, the drum 32, the cable 10, and the movable shroud lb, a negative feedback closed loop automatic position control system is formed capable of maintaining the shroud lb at a selectable level above the undisturbed particulate bed plane 4a.

Referring to FIGS. 14 and 15, in an embodiment of a second aspect of the present invention, a scour control system as shown in FIGS. 11 and 13 includes a means of automatically regulating the vertical position of the upper extremity 4d of the accumulation of particles 4b, occurring against the submerged structure lg supporting the shroud lb of the scour control system, which includes: a photo-electric level sensing device 33, which produces a voltage proportional to the vertical separation between the lower extremity of the shroud 1b and the upper extremity of the particle accumulation 4b, the lastmentioned upper extremity being designated 4d; a differential amplifier 34, mounted on the shroud lb, which compares the output voltage of device 33 with a selectable reference voltage and amplifies the difference between the two voltages; an electric lamp 35 mounted on the lower extremity 1c of the shroud lb and orientated to illuminate the entire sensing device 33 in the absence of obscuration of device 33 by particle accumulation; an electromagnetic actuator valve 36 capable of varying the supply of air through pipe 22 to the ring 20 and nozzles 21 of a scour control system according to the fourth embodiment of the present invention hereinbefore described; and interconnecting wires 37 linking the sensing device 33, the amplifier 34, and the actuator valve 36 together to form a closed loop negative feedback automatic position control system capable of maintaining the level of the upper extremity 4d of the particle accumulation 4b at a selectable distance below the lower extremity of the shroud lb.

The photo-electric level sensing device 33 comprises an elongate rigid member 38 attached to and depending vertically from the inner wall 14 of the shroud lb. An array of photodiodes 39, equally spaced vertically along the member 38, are connected together by the cathodes to a negative voltage supply 40. The anode of each photodiode 39 is connected between successive resistors 41 in a chain of equal resistors which in turn is connected between the negative voltage supply 40 and the positive voltage supply 42. Illumination of any of the photodiodes 39 causes it to drop to a low value of reverse impedance and hence to short-circuit the corresponding resistor in the resistor chain. The photodiodes 39 adjacent to the negative voltage supply 40 are located at the uppermost part of the support member 38 so that illumination by lamp 35 of those photodiodes 39 unobscured by the particle accumulation 4b causes short-circuiting of the corresponding resistors 41 in the resistor chain such that an output voltage from a point 43 on the resistor chain exhibits a stepped variation in proportion to he separation of the level of obscuration of the photodiodes 39 from the lower extremity 1c of the shroud 1 lb.

The output of the level sensing device 33 is connected to one input of a differential amplifier 44, incorporating a stabilization network, where it is compared with a reference voltage selectable by means of a potentiometer 45 connected across the positive and negative voltage supplies 42 and 40. The output of the amplifier M is connected to drive the actuator valve 36 which in turn varies the rate of supply of air passing through the pipe 22 and, hence, the rate of fluid flow entrained within the shroud 1b. The setting of the potentiometer 45 will determine the steady state position of the level of obscuration of the photodiodes 39 and hence will determine the steady state level of the upper extremity 4d of the particle accumulation 4b.

With the scour control system in open loop opera tion, that is, without the negative feedback provided by the automatic control systems of the first and second aspects of the present invention, the accumulation of particles 4b may increase until the lower part of the shroud l or lb tends to become blocked. In this condition, the necessary convergent fluid flow at the surface of the particulate bed 4 will decrease to negligibly small proportions permitting the existing natural external fluid flow, tending to cause scour, to erode the particle accumulation 4b until the lower part of the shroud 1 or lb becomes cleared of particulate obstruction. An equilibrium balance state or an oscillatory state will tend to be established between these two opposing influences with the result that the upper extremity 4d of the particle accumulation 4b will be maintained within limits about a level below the lower extremity 1c of the shroud l or lb related to fluid flow rate within the shroud and to external fluid flow rate over the surface of the particulate bed 4 circumjacent to the submerged structure la or lg.

Both control systems hereinbefore described may advantageously be included in a single scour control system. I

Such open loop operation of the scour control system can, however, neither accommodate for settlement of the submerged structure due to causes unrelated to scour nor eliminate the possibility of reaching an unstable oscillatory state of the particle accumulation.

With the scour control system in closed loop operation, that is, including the negative feedback provided by the automatic control systems, the upper extremity 4d of the particle accumulation 4b will be maintained in a substantially fixed positional relationship to the shroud l and lb which, in turn, will be in a substantially fixed stable positional relationship with the undisturbed particulate bed plane 4a.

Any desired accumulation of particles 4b against the submerged structure la and lg may therefore be maintained in a stable manner irrespective of settlement of the supporting submerged structure la or 13 or the rate of flow of the external fluid past the structure la or lg at the particulate bed surface 40.

It will be appreciated that the pumping means of the scour control system may take a variety of additional forms, all with the object of causing upward fluid flow within the shroud of the scour control system.

Also, it will be appreciated that alternative locations of the shroud are possible. For example, the shroud may be located upstream and adjacent to the submerged structure; the shroud may alternatively be replaced by a plurality of shrouds, serving a single impeller or each with an impeller, disposed adjacent to the submerged structure and above the submerged particulate bed. In each of these last mentioned examples, the action of the scour control system is substantially as hereinbefore described.

It may be noted that reversal of the impeller motion or the impeller blade angle of attack will reverse the fluid flow in the shroud and so cause DlVERGENT fluid flow paths at the submerged particulate bed surface. The reversed fluid flow paths will now leave any local closed area of the particulate bed circumjacent to the submerged structure, at the surface of the bed, with the result that massive particle loss from the local area of the bed will occur constituting accelerated scouring. This mode of operation of the scour control system according to the present invention may be used to free a submerged structure penetrating a submerged particulate bed from the gripping action of the bed and so facilitate easy removal of the structure from its supporting bed.

In the operation of the scour control system according to the present invention, say for controlling scour at a leg of a jack-up bottom supported sea drilling rig, the shroud 1b is approximately located with respect to the sea bed surface 4a when the leg lg is positioned on the sea bed 4. The shroud llb is then lowered by the cable 10 until it is almost in contact with the surface 40 of the sea bed 4. The air supply to the nozzles 21 on ring 20 within shroud 1b is set at a low delivery rate until an accumulation of sand particles 4b builds up sufficiently, due to the water flow 5, to partially block the lower extremity 1c of the shroud 1b. The shroud lb is now raised slowly and the delivery rate of air to the nozzles 21 increased until the sand particle accumulation again partially blocks the lower extremity lc of the shroud lb with the air delivery rate finally set at half the max imum rate. This then is the normal operating state of the scour control system and the convergent water flow condition at the surface of the sea bed, necessary to mitigate against and prevent scouring, is established.

Where the scour control system includes the two automatic control systems for regulating the position of the shroud lb and the proximity of the upper extremity 4d of the sand accumulation 4b to the lower extremity 1c of the shroud lb, the operation of the scour control system is considerably simplified and consists merely of energizing both control systems and preselecting the two reference voltages to the differential amplifiers 29, 34. The shroud 1b will then be driven downwards by motor .31 automatically until device 27 produces a signal voltage output equal to the reference voltage selected for amplifier 29 whereupon the shroud lb will stop and be maintained in its vertical position, owing to the disappearance of the amplified difference voltage at-the output of the differential amplifier 29 causing the motor 31 to stop. The shroud will thus be maintained at a constant height above the sea bed surface according to the magnitude of the preselected reference voltage. The sensing device 33, of the second automatic control system, may have penetrated the sea bed surface or may still be wholly above the sea bed surface. in the latter case, the water flow rate within the shroud is at a maximum and in the former case at a lesser rate according to the degree of obscuration of the sensing device 33 by the sea bed material. The water flow into the shroud lb causes a build-up of sand against the submerged structure lg until increasing sand obscuration of the sensing device 33 changes its output voltage such that the reference voltage to the amplifier 34 is equalled and the water flow consequently cut off due to closure of the actuator valve 36 for zero signal out of the amplifier 34. Any external scouring action opposes the increase of the sand accumulation 4b, and hence the rise of the obscuration level of device 33, so that the dynamic equilibrium state involves the existence of a voltage error between the output of the device 33 and the reference voltage to amplifier 34. This error voltage, when amplified by amplifier 34, constitutes the demand signal to the actuator valve 36 of the automatic control system which maintains an appropriate water flow rate within the shroud lb such that the necessary convergent flow conditions for scour mitigation and prevention are always present during operation of the scour control system.

Thus, the operation of embodiments of the present invention makes possible the elimination of scouring as a hazard to bottom supported submerged structures at sea.

I claim:

1. A scour control system including: an elongate, hollow, cylindrical shroud means, of substantially vertical axial orientation and with an open lower extremity, for conveying fluid from a location at the surface of a particulate submerged bed, adjacent to a submerged structure seated on the submerged bed, to an upper location at a level so spaced vertically from the surface of the particulate bed that discharge of fluid from the shroud at the upper location causes substantially no disturbance of the particulate bed surface; fluid pumping means connected to the shroud means whereby fluid flow may be caused within the shroud means; mounting member means connecting said shroud means movably to said submerged structure whereby the shroud means is vertically substantially coaxial with the submerged structure and the open lower extremity of the shroud means is maintained in a position adjacent to the submerged structure and spaced from the particulate bed surface such that upward forced fluid flow within the shroud means, due to the action of the fluid pumping means, causes a convergent fluid flow at the surface of the particulate bed adjacent to the submerged structure so that an accumulation of particles, transported by the convergent fluid flow, tends to occur against the submerged structure; and automatic control means whereby the vertical position of an upper surface of any accumulation of particles against the submerged structure arising from fluid ingestion into the shroud means, and hence the magnitude of such accumulation of particles, is substantially regulated and controlled.

2. A scour control system as claimed in claim 1, wherein the fluid pumping means includes: at least one nozzle located inside and substantially adjacent a lower part of the shroud means; at least one pipe connected to the nozzle and capable of supplying a fluid to the nozzle; mounting means whereby the nozzle is mounted within the shroud means; and a supply of fluid deliverable through the pipe to the nozzle such that release or discharge of the fluid from the nozzle causes upward fluid movement within the shroud means by entrainment with the discharge, in the case of a liquid supply fluid, and by entrainment with rising buoyant bubbles, in the case of a gaseous or vaporous supply fluid.

3. A scour control system as claimed in claim 2, wherein the automatic control means includes: at least one sensing element capable of providing a signal related to the vertical distance separating a reference point on the shroud means from an upper extremity of any accumulation of particles, within range of the sensing element, which may form against the submerged structure arising from fluid ingestion into the shroud means; signal amplification means; actuator means capable of regulating the pumping means to change the rate of fluid flow through the shroud means; and signal transmission means between the sensing elements, the amplifier means, and the actuator means whereby the rate of fluid flow within the shroud means and over the particulate bed adjacent surface, and hence the rate of accumulation of particles against the submerged structure, may be so regulated by negative signal feedback that position control of the vertical location of an upper extremity of the particle accumulation may be established with respect to the reference point on the shroud means.

4. A scour control system as claimed in claim 2, wherein the vertical position of the shroud means with respect to the submerged structure is regulated and controlled by automatic control means.

5. A scour control system as claimed in claim 4, wherein the automatic position control means of the shroud means includes: at least one sensing element capable of providing a signal related to change in the vertical distance separating a reference point on the shroud means from the plane of the undisturbed submerged particulate bed; signal amplification means; actuator motor driving means capable of changing the vertical position of the shroud means; and signal transmission means between the sensing element, the amplifier means, and the actuator motor driving means whereby negative feedback control of the position of the shroud with respect to the undisturbed particulate bed surface plane may be established.

6. A scour control system as claimed in claim 1, wherein the fluid pumping means includes a rotatable impeller means mounted coaxially within the shroud means, and a turbine means drivable by externally incident fluid flow and coupled to the rotatable impeller means such that rotation of the turbine due to the action of the external fluid flow results in an upward fluid movement within the shroud means.

7. A scour control system as claimed in claim 6, wherein the turbine means comprises a tangentially driven axial-bladed turbine wheel coupled to and mounted coaxially above the impeller means whereby externally incident fluid flow in a direction tangential to the turbine wheel causes unidirectional rotation of the turbine wheel and consequent unidirectional rotation of the coupled impeller means.

8. A scour control system as claimed in claim 7, wherein the automatic control means includes: at least one sensing element capable of providing a signal related to the vertical distance separating a reference point on the shroud means from an upper extremity of any accumulation of particles, within range of the sensing element, which may form against the submerged structure arising from fluid ingestion into the shroud means; signal amplification means; actuator means capable of regulating the pumping means to change the rate of fluid flow through the shroud means; and signal transmission means between the sensing elements, the amplifier means, and the actuator means whereby the rate of fluid flow within the shroud means and over the adjacent particulate bed surface, and hence the rate of accumulation of particles against the submerged structure, may be so regulated by negative signal feedback that position control of the vertical location of an upper extremity of the particle accumulation may be established with respect to the reference point on the shroud means.

9. A scour control system as claimed in claim 8, wherein the vertical position of the shroud means with respect to the submerged structure is regulated and controlled by automatic control means.

10. A scour control system as claimed in claim 9, wherein the automatic position control means of the shroud means includes: at least one sensing element capable of providing a signal related to change in the vertical distance separating a reference point on the shroud means from the plane of the undisturbed submerged particulate bed; signal amplification means; actuator motor driving means capable of changing the vertical position of the shroud means; and signal transmission means between the sensing element, the amplifier means, and the actuator motor driving means whereby negative feedback control of the position of the shroud with respect to the undisturbed particulate bed surface plane may be established.

11. A scour control system as claimed in claim 1, wherein the automatic control means includes: at least one sensing element capable of providing a signal related to the vertical distance separating a reference point on the shroud means from an upper extremity of any accumulation of particles, within range of the sensing element, which may form against the submerged structure arising from fluid ingestion into the shroud means; signal amplification means; actuator means capable of regulating the pumping means to change the rate of fluid flow through the shroud means; and signal transmission means between the sensing elements, the amplifier means, and the actuator means whereby the rate of fluid flow within the shroud means and over the particulate bed adjacent surface, and hence the rate of accumulation of particles against the submerged structure, may be so regulated by negative signal feedback that position control of the vertical location of an upper extremity of the particle accumulation may be established with respect to the reference point on the shroud means.

12. A scour control system as claimed in claim 11, wherein the vertical position of the shroud means with respect to the submerged structure is regulated and controlled by automatic control means.

13. A scour control system as claimed in claim 12, wherein the automatic position control means of the shroud means includes: at least one sensing element capable of providing a signal related to change in the vertical distance separating a reference point on the shroud means from the plane of the undisturbed submerged particulate bed; signal amplification means; ac-

tuator mot r drivin means ca able of chan in the vertical position of tfie shroud means; and sign%l t ransmission means between the sensing element, the amplifier means and the actuator motor driving means whereby negative feedback control of the position of the shroud with respect to the undisturbed particulate bed surface plane may be established.

14. A scour control system as claimed in claim 1, wherein the fluid pumping means includes: rotatable impeller means located within an upper extension of the shroud means and coupled to motor means whereby rotation of the impeller means by the motor means causes an upward fluid movement within the shroud means.

15. A scour control system as claimed in claim 14, wherein the automatic control means includes: at least one sensing element capable of providing a signal related to the vertical distance separating a reference point on the shroud means from an upper extremity of any accumulation of particles, within range of the sensing element, which may form against the submerged structure arising from fluid ingestion into the shroud means; signal amplification means; actuator means capable of regulating the pumping means to change the rate of fluid flow through the shroud means; and signal transmission means between the sensing elements, the amplifier means, and the actuator means whereby the rate of fluid flow within the shroud means and over the particulate bed adjacent surface, and hence the rate of accumulation of particles against the submerged structure, may be so regulated by negative signal feedback that position control of the vertical lo cation of an upper extremity of the particle accumulation may be established with respect to the reference point on the shroud means.

16. A scour control system as claimed in claim 15, wherein the vertical position of the shroud means with respect to the submerged structure is regulated and controlled by automatic control means.

17. A scour control system as claimed in claim 16, wherein the automatic position control means of the shroud means includes: at least one sensing element capable of providing a signal related to change in the vertical distance separating a reference point on the shroud means from the plane of the undisturbed submerged particulate bed; signal amplification means; actuator motor driving means capable of changing the vertical position of the shroud means; and signal transmission means between the sensing element, the amplifier means, and the actuator motor driving means whereby negative feedback control of the position of the shroud with respect to the undisturbed particulate bed surface plane may be established.

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Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4300855 *Mar 13, 1980Nov 17, 1981Kenneth WatsonRotatable ice-formation-preventing device
US4505617 *Jan 4, 1983Mar 19, 1985National Research Development CorporationStabilizing bluff structures against oscillation
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US5149227 *May 6, 1991Sep 22, 1992Parks James MBeach stabilization with multiple flow control
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US5762448 *May 29, 1996Jun 9, 1998Continuum Dynamics, Inc.System for alleviating scouring around submerged structures
US5784338 *Sep 15, 1997Jul 21, 1998The United States Of America As Represented By The Secretary Of The ArmyTime domain reflectometry system for real-time bridge scour detection and monitoring
US5790471 *Nov 6, 1997Aug 4, 1998U.S. Army Corps Of Engineers As Represented By The Secretary Of The ArmyWater/sediment interface monitoring system using frequency-molulated continuous wave
US6100700 *Feb 5, 1998Aug 8, 2000U.S. Army Corps Of Engineers, As Represented By The Secretary Of The ArmyBridge scour detection and monitoring apparatus using time domain reflectometry (TDR)
Classifications
U.S. Classification405/15, 405/74, 405/211
International ClassificationE02B17/00
Cooperative ClassificationE02B17/00
European ClassificationE02B17/00