GB2255825A - Ultrasonic inspection of variable diameter platform legs - Google Patents

Abstract

Apparatus for ultrasonic inspection of the interior of a tubular structure, such as a tension leg of an offshore platform, includes an assembly 45 with ultrasonic transducers 47 on articulated arms 48, communicating with a remote control station 25 which can be in excess of 600m from the assembly 45. The transducer housings have integrally fed couplant, and are shaped and spring loaded for close contact with the interior surface. A dual-duplex transducer is provided for sizing flaws on an outer surface of the tubular structure. The apparatus is clamped by 37 and 38, then the assembly 45 is rotated and translated to scan e.g. the vicinity of a weld. Video cameras 51 permit observation of the arms 48. <IMAGE>

Description

APPARATUS AND METHOD FOR ULTRASONIC INSPECTION The invention relates to an apparatus and method for ultrasonic inspection, and more particularly, to an apparatus and method for ultrasonic inspection of a tubular structure such as a pipeline, flowline, tubular structural member or tension leg from its interior.

Compliant structures are used for exploration and production of petroleum reserves in deep subsea subterranean reservoirs at water depths greater than 500 feet.

One type of compliant structure is a tension leg platform which includes a semi-submersible floating platform anchored to piled foundations on the sea bed through substantially vertical members or mooring lines called tension legs. The tension legs are maintained in tension at all times by insuring that tension leg platform buoyancy always exceeds its operating weight. The tension leg platform is compliantly restrained by this mooring system against lateral motion while vertical motions are stiffly restrained. A description of tension leg mooring apparatus installation for deep water tension leg platforms is contained in U.S. Patents Nos. 4,784,529 and 4,844,659 to Hunter and Hunter et al. which are herein incorporated by reference.

Tension legs consist of tubular pipe sections which can have different interior diameters interconnected to produce a tension leg having a desired taper. Tubular pipe sections are welded together to form a tension leg of the required length. The tubular pipe sections can be welded directly together or the pipe sections can be welded to mechanical connectors which serve to interconnect and join individual pipe sections.

FIG. 1 shows tension leg 10 consisting of thin-walled tubular central section 12 with smaller diameter, thickwalled upper and lower tension leg coupling sections 14, 16 respectively interconnected with central section 12 by upper and lower tapered sections 18, 20 respectively. To fabricate tension leg 10, individual short lengths of tubing are welded together to form a unitary structure which is preferably assembled completely on shore and towed out to an installation site as a fabricated, unitary structure.

Regulatory agencies such as the U.S. Coast Guard may require that tendon legs be examined in service while they are submerged in a body of water such as a sea or ocean where drilling, exploration or production is taking place in order to detect damage, corrosion and size flaws in actual tension leg welds and in heat affected areas surrounding actual welds.

Tension leg inspection creates unique requirements and imposes severe restrictions on inspection tool design. An inspection tool is required for placing ultrasonic, video and other inspection apparatus in close proximity to a tension leg, its welds, and weld areas. The inspection tool must be able to travel through and inspect varying inner diameter tension leg sections and constrictions. It must be thin enough to pass through the smallest tension leg inner diameter, yet capable of bringing inspection apparatus into close contact with the walls of larger inner diameter tension leg sections. Furthermore, the tool must be remotely operable over the entire tension leg length and be capable of absolute and repeatable positioning along this length to accurately locate, size and monitor areas of interest.

To fulfill this need, a tool containing all power, control and inspection apparatus necessary to examine tension leg integrity within a tooling envelope sufficiently small to allow the tool to pass through tension leg smallest diameter constrictions and capable of engaging tension leg largest diameter section interior walls is required.

According to one aspect of the invention, an apparatus for inspecting the interior of a tubular structure which can be a pipeline, flowline, tubular structural member, or a tension leg includes a tool body equipped with an integral clamping and centralizing assembly which can also be referred to as a clamping assembly capable of dual mode remotely controlled deployment, an inspection assembly connected to the tool body, a control station at a remote location from the tool body and inspection assembly, communicating means for communicating between the control station and the tool body and a means connected to the tool body for raising and lowering the tool body in the tubular structure.The integral clamping and centralizing assembly can be operated in two remotely controlled deployment modes, a first mode which allows the apparatus to move relatively freely and roll along an interior surface of the structure and a second mode which locks the apparatus in a stationary position as the integral clamping assembly engages the interior surface of the tubular structure.

The tool body can be provided with two integral clamping assemblies positioned about a center of gravity of the tool body which protrude radially from a longitudinal axis of the tool body which can be approximately cylindrically shaped. The integral clamping and centralizing assembly can include articulated members which project radially from the tool body and are connected to equiangularly spaced engagement members which can be three integral wheel and shoe assemblies. The integral clamping and centralizing assembly can be continuously remotely deployed by adjusting fluid pressure.

The integral clamping and centralizing assembly includes wheels which can be made of non-metallic material mounted on flexible members which apply light rolling pressure on a tubular structure interior surface in the first deployment mode. Integrally mounted with the wheels and flexible members is a hard, non-metallic shoe which in the second deployment mode engages the interior surface of the tension leg. Use of non-metallic wheel and shoe avoids damage such as scratches which can become corrosion sites in the tubular structure.

The integral clamping and centralizing assembly is provided with a main fluid pressure source and a booster fluid pressure source which provides increased force for engaging the tubular structure interior surface at a smaller tubular structure inner diameter end of its dynamic range.

The integral clamping and centralizing assembly includes failsafe operational features which allow the integral clamping and centralizing assembly to collapse in the event of a loss of pressure or electric control power.

Integral clamping and centralizing assembly arms fracture in the event of an application of excess force. The booster component is so positioned that in the event of operator error during larger diameter operation, the booster component cannot inadvertently actuate a fuller extension of the integral clamping and centralizing assembly arms already extended in the second deployment mode, which could otherwise damage the tubular structure.

In another aspect of the invention, apparatus for inspecting the interior of the tubular structure includes a tool body, a first electronics module, a second drive module, an inspection assembly connected to the tool body, a control station at a location remote from both the tool body and inspection assembly, communicating means connected to the tool body for communication with the control station and raising means for raising and lowering the tool body in the tubular structure. The electronics include amplifiers to boost control signals from the control station and data signals from the inspection assembly. A third module contains fluid pressure controls.

An approximately cylindrical tool body is connected to an inspection assembly including a video camera and an ultrasonic inspection assembly. An axial drive assembly includes a servo motor and position encoder. A rotary drive assembly includes a rotary motor and position encoder Both provide inspection assembly motion whose extent is adapted according to the tension leg and tool body dimensions to insure that a particular volume under inspection is thoroughly scanned.

In an aspect of the invention, an apparatus for inspecting the interior of a tubular structure includes a tool body, an inspection assembly connected to the tool body, a control station at a remote location from both the tool body and the inspection assembly and an integral umbilical cable connected to the tool body which includes load-carrying armor to support, raise and lower the tool body in the tubular structure, and conduits to carry couplant medium, fluid pressure and power, control and data signals between the control station and the tool body and inspection assembly.

Shared conductors carry ultrasonic control signals from the control station and power from a remote on-tool pulser as well as data signals from the ultrasonic inspection assembly. The load-carrying armor can be torque-balanced steel wire wound contra-helically around couplant medium conduit, fluid pressure and power and control signal conduits. Armor can be covered by outer cable sheathing.

In another aspect of the invention, an apparatus for inspecting the interior of a tubular structure includes a tool body which can be cylindrical, an inspection assembly connected to the tool body, a control station at a remote location from the tool body, including an ultrasonic primary pulser trigger and video controls and communicating means connected to the tool body for power and signal communication between the control station and the tool body.

An electronics module is sized to fit interior dimensions of the tubular structure and contains electronics miniaturized to accommodate module dimensions. It allows a separation exceeding 500 feet between the remote control station and the inspection assembly electronics. Means are connected to the tool body for raising and lowering the tool body and inspection assembly in the structure.

The primary pulser trigger provides a timing pulse which can be a square wave pulse having an amplitude in the range of about 50V to 300V, and preferably about 180V to 200V and having a pulse width in the range of about 50ns to 500ns, and preferably about 200ns to 225ns.

The electronics module includes a remote pulser responsive to a signal from the primary pulser trigger which can be a signal in the range of about 100V to 150V. In response to the signal, the remote pulser generates a spike having an amplitude in the range of about 450V to 500V, to power a transducer in the inspection assembly.

In another aspect of the invention, an apparatus for inspecting the interior of a tubular structure includes a tool body, an inspection assembly further including ultrasonic inspection means deployed on more than one fluid pressure- activated articulated arm as well as video inspection means, a control station at a remote location from the tool body, an inspection assembly, means connected to the tool body for communication with the control station and means for raising and lowering the tool body and inspection assembly in the tubular structure. The apparatus can further include means for eddy current inspection to detect surface flaws.

The apparatus further includes means for accurately positioning the tool body in the tubular structure such as a winch. The tool body is connected to the inspection assembly by an axial drive shaft capable of axially positioning the inspection assembly over an axial distance scaled according to the length of the tubular structure. A rotary drive motor moves the inspection assembly for circumferential scanning of a tubular structure interior surface so that the entire circumference is examined.

Video inspection means can include cameras having fields of view along the tool body axis and along the tension leg interior surface lit by a variable light source enclosed in glass for submerged operation and provided with a heat sink for dry operation. Video inspection can proceed simultaneously with ultrasonic inspection.

An audio microphone can be positioned near the tool to pick up tool and other sounds within the tubular structure.

According to another aspect of the invention, a method for inspecting a variable diameter tension leg while in service in a body of water includes steps of lowering a collapsible inspection tool designed to pass through small diameter sections of the tension leg, packaging inspection tool electronics, fluid pressure controls, inspection equipment and inspection drive assemblies within a tooling envelope smaller than a smallest interior diameter of tSe tension leg, transmitting electric power, control signals and fluid pressure from a remote control station to the inspection tool, inspecting the tension leg interior with an inspection assembly and receiving data from the inspection assembly at the remote control station.

The tension leg can be subjected to simultaneous, video and ultrasound, audio and ultrasound or audio, video and ultrasound inspection. Additionally, eddy current inspection can be conducted to detect surface flaws.

An integral umbilical cable carrying couplant medium and fluid pressure as well as power and control signals from the remote control station can be used to raise and lower the inspection tool within the tension leg when the dual mode clamping assembly is deployed in a first mode permitting the tool to roll freely along the interior surface of the tension leg. When an inspection point is reached, the dual mode integral clamping assembly can be energized in a second deployment mode so that it engages the interior of the tension leg keeping the tool stationary during the inspection. The inspection tool can be divided into compartments to shield miniaturized electronics which can be used for amplifying control signals from the remote control station thus permitting a separation in excess of 500 feet between the remote control station and inspection tool electronics.

The tension leg interior surface can be scanned in a boustrophedonic pattern by computer controlling axial and rotary drive motors on the inspection tool. The tension leg interior surface can also be scanned in other patterns so long as such pattern permits a complete inspection of the area of interest.

In an aspect of the invention, an apparatus for inspecting the interior of a tubular structure includes a tool body, an inspection assembly including ultrasonic inspection means deployed on a fluid pressure-activated articulated arm, a control station at a remote location from the tool body and an inspection assembly, means-connected to the tool body for communication with the control station and means for raising and lowering the tool body and inspection assembly in the structure.

The apparatus can include an ultrasonic transducer inspection assembly and can have more than one transducer contained in an ultrasonic inspection assembly. Each transducer is contained in a housing with a spring-loaded housing face shaped to follow the tubular structure interior curvature and allow a thin film of couplant medium to intervene between the housing face and the tubular structure interior surface. The medium can be delivered to the tubular structure interior surface through internally fed couplant weeping ports in the housing. Transducers can include any one of or combination of tandem-duplex, dualduplex and single shear wave and dual 0 transducers.

Single shear wave transducers can have other angles such as 300, 600 and 700 depending on the geometry of the component under inspection. Tandem duplex transducers are described in U.S. Patent No. 4,658,649 to Brooks which is herein incorporated by reference.

In another aspect of the invention, an ultrasonic transducer includes an ultrasound generator, a wedge cut in a dual-duplex design capable of generating direct and indirect shear waves in a material, and an ultrasound pickup for receiving ultrasound which has interacted with the material. Incident and roof angles of the dual-duplex design wedge are calculated to be in the range of the first critical angle to produce direct and indirect shear waves and can be from 10 to 30 less than the critical angle. The dual-duplex transducer can be scaled so that the wedge projects sound beams into the material in two directions, 1800 apart.

In a further aspect of the invention, the dual-duplex transducer already described is incorporated into an apparatus for inspecting a tubular structure from the inside. The apparatus includes a tool body, an inspection assembly with a dual-duplex transducer, a control assembly, means for communication between the control assembly and inspection assembly and means for positioning the inspection assembly in the tubular structure.

Another aspect of the invention provides a nondestructive' inspection method for a tension leg while in service in a body of water including steps of moving flaw locating and sizing means into contact with an interior surface of the tension leg, coupling the flaw locating and sizing means with the interior surface of the tension leg, locating flaws on the tension leg, sizing flaws on the tension leg inner surface, and sizing flaws on a tension leg outer surface. The step of locating flaws can be performed with transducers having the designs already described.

An embodiment of the invention will now be described, by way of example only, with reference to the drawings; wherein: FIG. 1 is an elevation view of a tension leg.

FIGS. 2A, 2B and 2C comprise an elevation view of a tension leg inspection apparatus deployed within a section of a tension leg.

FIGS. 3A, 3B and 3C comprise an elevation view of the tension leg inspection apparatus deployed within another section of the tension leg.

FIG. 3D is a cross section view of an integral clamping assembly.

FIGS. 4A, 4B and 4C comprise an elevation view of the tension leg inspection apparatus deployed within another section of the tension leg.

FIG. 5 is a cross section view of an integral clamping and centralizing assembly including a booster cylinder.

FIG. 6 is a cross section view of an electronics module of the tension leg inspection apparatus.

FIG. 7 is a cross section view of a drive assembly module of the tension leg inspection apparatus.

FIG. 8 is a cross section view of an integral umbilical cable.

FIG. 9 is a partially cross section view of a module for connecting the integral umbilical cable to a tension leg inspection apparatus tool body.

FIG. 10 is an elevation view of the inspection assembly of the tension leg inspection apparatus.

FIG. 11 is another elevation view of the inspection assembly of FIG. 10.

FIG. 12A is a plan view of the ultrasonic inspection assembly with the transducer assemblies pivoted into the viewing plane.

FIG. 12B is a plan view of a transducer of the ultrasonic inspection assembly.

FIG. 12C is a cross section view of a tandem-duplex transducer.

FIG. 13 is a cross section view of a part of an ultrasonic inspection assembly.

FIG. 14 is a schematic drawing showing use of transducers on individual ultrasonic inspection assemblies for detecting and sizing flaws in a tension leg.

FIG. 15 is a cut-away view of a dual-duplex transducer.

FIG. 16 is a plan view of the wedge and of the transmitter and receiver elements of the transducer of FIG.

15.

The invention provides an apparatus for inspection of an interior of a vertically oriented tubular structure which can be a pipeline, flowline, tubular structural member or a tension leg. The apparatus includes features such as an integral clamping assembly which can also be referred to as a clamping assembly capable of dual mode remotely controlled deployment. The tool body is divided into modules to shield electronics from interference. An integral umbilical cable and miniaturized electronics to accommodate the small tooling envelope and amplify signals enable operation with a greater than 500 foot separation between a remote control station and the tool body and an inspection assembly equipped with video and ultrasonic inspection equipment. A method is provided for inspecting a tension leg while in service in a body of water utilizing a collapsible inspection tool.

The invention also provides an apparatus for ultrasonic inspection of the interior of a tubular structure which can be a pipeline, flowline, tubular structural member or a tension leg. The apparatus includes articulated arms for bringing transducers in contact with the tubular structure interior surface. Transducers have spring-loaded housings equipped with housing faces shaped to follow the curvature of the tubular structure and allow a thin film of couplant medium to intervene between the housing face and the tubular structure interior surface. Transducers used for the ultrasonic inspection can include tandem-duplex, 450 single and 0o dual transducers as well as a dual-duplex transducer having a wedge cut in a dual-duplex design.A nondestructive inspection method for detecting and sizing flaws in a tension leg while in service in water is provided.

FIGS. 2A, 2B, 2C, 3A, 3B, 3C, 4A, 4B and 4C show a tool body 30 for inspecting the interior of a tubular structure, in this embodiment a tension leg having a larger diameter section 31 with a 22 inch diameter and a constriction or smaller diameter section 33 with a 9 inch diameter. Tool body 30 is divided into modules designed to fit in the tooling envelope (8.5" in diameter X 28 feet in length) required by the diameter of the tension leg.

Umbilical connector module 34 connects tool body 30 with integral umbilical cable 35 which is controlled by winch 28 to raise and lower tool body 30 in the tension leg as well as to communicate power, control and data signals as well as fluid pressure between a remote control station 25 and tool body 30. Remote control station 25 includes an ultrasonic primary pulser trigger 26, video control 27 and fluid pressure control 29. Umbilical connector module 34 is joined with electronics module 36 which encloses miniaturized electronics used to make possible communication of power and control signals from a remote control station located in excess of at least 500 feet, and preferably in excess of 2000 feet from the tool body.

A guide shaft 40 connects electronics module 36 with upper centering mechanism and lower centering mechanism 37, 38 respectively, fluid pressure control module 41 and drive module 42. Upper and lower centering mechanisms 37, 38 include an integral clamping assembly 39 shown deployed in a collapsed mode of operation used to center tool body 30 and allow relatively free rolling motion as the tool is raised or lowered through constriction 33.

Drive module 42 includes an axial drive motor 43 and a rotary drive motor 44 which effect axial and rotary motion of inspection assembly 45. Inspection assemblies 45 include ultrasound inspection assemblies 46, more specifically 46A and 46B, consisting of ultrasonic transducers 47 mounted on articulated arms 48 shown in a collapsed mode to allow passage of the tool through constriction 33. Inspection assembly 45 is also equipped with video inspection apparatus 49 and 51 which can be cameras. Axial view camera 49 transmits images in the direction of tool motion and indicates the presence of any obstructions in the tool path.

Cameras 51 view the transducers and the tension leg interior surface scanned by the transducers.

Tool body 30 is raised or lowered through a tension leg with integral clamping and centering assemblies 39 deployed at low pressure rolling on wheels 50. Inspection assembly 46 is collapsed so that it can move through smaller diameter sections such as constriction 33.

When tool body 30 is brought into position at a weld between tension leg sections or another area requiring inspection, integral clamping assemblies 39 in upper centering mechanism 37 and lower centering mechanism 38 and articulated arms of ultrasonic inspection assemblies 46 are energized by fluid pressure, which can be pneumatic pressure. Upper centering mechanism 37 and lower centering mechanism 39 hold the tool stationary near an inspection location when energized at high pressure, while articulated arms 48 hold transducers 47 in close proximity to tension leg interior surface 52 to perform an ultrasonic inspection.

Video inspection apparatus, which can include a camera, transmits images of tension leg inner surface 52 in the area being inspected by transducers 47.

FIG. 3D, which is a section view taken along line 3D-3D of FIG. 2C, shows integral clamping and centralizing assemblies 39 equiangularly positioned, approximately 1200 apart, between actuator carriages 78 and guide posts 40.

FIGS. 4A-4C shows tool body 30 engaged in a stationary position for inspection of weld 54 between bottom transition section 56 and bottom connector shaft 58. To perform inspection of this weld, integral clamping and centering assemblies 39 are fully extended to engage tension leg inner surface 52 while ultrasonic inspection assembly 46 is collapsed to fit in the small diameter of bottom connector shaft 54 to keep transducers 47 in close proximity to tension leg inner surface 60.

FIG. 5 is a detail elevation, partially cross section view of an integral clamping and centralizing assembly 39.

Upper integral clamping and centralizing assembly 37 and lower integral clamping and centralizing assembly 38 are positioned about a center of gravity of tool body 30.

Articulated arms 70 protrude radially from a longitudinal axis 300 parallel to the long axis of the cylindrically shaped tool body. Articulated arms 70 are connected to a sole plate 72 on which is mounted shoe 76 which can be formed from hard polyurethane. Wheels 50 which can also be formed from a non-metallic material such as polyurethane are connected to sole plate 72 by flexible members 73 which can be steel. Equiangularly positioned sole plate 72 and wheel 50 engagement assemblies function to centralize the tool as it is raised and lowered within the tension leg 10.

When pneumatic cylinder 74 is energized by pressure application, it pulls moving spider 76 and actuator carriage 78 towards fixed spider 80 and bulkhead plate 82, pushing articulated arms 70 outward toward the tension leg interior surface 52. The degree of extension of articulated arms 70 can be adjusted by controlling pneumatic cylinder 74 to either maintain wheels 50 in rolling contact with the tension leg interior surface 52 or to extend articulated arms 70 further outward so that shoe 76 engages the tension leg interior surface 52. Flexible members 73 bend inward thus allowing the shoe 76 to firmly engage the tension leg interior surface 52 as shown in FIGS. 3A-3C and 4A-4C.

Spring 84, positioned äround piston shaft 86 limits the force applied to articulated arms 70 to limit the clamping load on a larger inner diameter tubular structure section, the 22 inch inner diameter tension leg in a preferred embodiment. When integral clamping and centering assembly 39 is fully extended, spring 84 engages fixed spider 80.

In a preferred embodiment, integral clamping and centralizing assembly 39 is designed to operate over a dynamic range between 9 and 22 inches, corresponding to the smallest and largest interior diameters of tension leg 10.

However, the integral clamping and centralizing assembly 39 design can be scaled to span other operating ranges. For example, a wider dynamic range requires a longer pneumatic cylinder 74 and hence, a longer tool body.

A single pneumatic cylinder 74 provides sufficient force to engage the tension leg interior surface at the larger 22 inch interior diameter. However, additional force is required at the smaller 9 inch tension leg interior diameter. The force generated by any kinematic linkage such as that shown in FIG. 5 varies with the included angle of the links so that a significantly larger applied force is required for operation at a smaller interior diameter.

Since the limited 8.5 inch diameter tooling envelope for tool body 30 can't accommodate a larger pneumatic cylinder or additional controls/hardware for higher pneumatic pressure, an additional booster cylinder 79 is activated at the small diameter end of the dynamic range of integral clamping and centralizing assembly 39. Booster cylinder 79 provides the increased applied force.

Furthermore, integral clamping assembly 39 includes "failsafe" operational features to protect the tool body 30 and the tension leg 10 in various failure modes. In the event of a loss of pressure, integral clamping and centralizing assembly 39 is designed to collapse so that the tool body 30 can pass through smaller diameter sections of the tension leg 10 such as constriction 33 allowing tool body 30 to be withdrawn from the tension leg 10 without damage to the leg or tool body. In the event of application of excessive pressure, articulated arms 70 have fracture points where the articulated arms 70 will break before any damage occurs to the tension leg 10. If the booster cylinder 79 is inadvertently activated during larger diameter operation, no damage can occur because it is positioned where it can not further extend already extended articulated arms 70.

Booster cylinder 79 is mounted on booster cylinder mounting plate 81 which is connected to guide shaft 40.

When integral clamping and centralizing assembly 39 is operated at the larger diameter end of its dynamic range, in a 22 inch inner diameter tension leg section in a preferred embodiment, actuator carriage 78 is sufficiently clear of short stroke booster cylinder 79 such that cylinder rod 83, even when fully extended does not contact and hence does not transfer force to actuator carriage 78.

Tool body 30 is divided into modules as indicated in FIGS. 2A, 2B and 2C. Electronics are contained in electronics module 36 shown in detail in the cross section view of FIG. 6. Module 36 isolates electronics from other tool components such as axial drive motor 43 and rotary drive motor 44 contained within drive assembly 42 shown in detailed cross section in FIG. 7.

Electronics are isolated in electronics module 36 to protect them from sources of interference internal to tool body 38 including relays, transformers and any type of electro-magnetic radiation including motors and external sources such as motors in machinery and RF from radios.

Electronic components have been miniaturized to fit within the limited tooling envelope made necessary by the tension leg 10 dimensions and are accommodated within electronics module 36 which is 48 inches long by 8.5 inches in diameter, allowing it to fit through small 9 inch interior diameter tension leg sections. Power supply 94 includes -12V DC and +5 V DC and +12 V DC power supplies.

Video power supply 100 includes a black and white power supply and a color power supply. Video data signals going to the remote control station are amplified by video amplifiers in assembly 106. Assembly 106 also includes a position encoder amplifier for axial and rotary drive motors and an audio amplifier for amplifying audio signals from a microphone positioned near drive assembly 42. Additionally, there is a gross position encoder on winch 28 which raises and lowers tool body 30 in the tension leg. Remote pulser assembly 108 is located within the module 36. BNC connectors 110 and connector terminal strips 112 connect electronics module components with umbilical cable 35.

Miniaturized electronics contained within electronics module 36 are designed to reduce the number of electronic components carried on the tool itself and allow placement of some of those components in the remote control station 25.

Controls are conventionally positioned close to or directly on an inspection tool. Since controls can be bulky or heavy, it would be impossible for them to be accommodated within the tooling envelope dictated by the tension leg dimensions. For example, the primary pulser trigger 26 is computer controlled and its placement in tool body 30 would have required a large number of signal lines in umbilical 35, resulting in the umbilical becoming larger and further requiring a larger winch, drum and motor for raising and lowering tool body 30. As additional components are added to tool body 30, modifications or increases in the sizes of other components are required, quickly exceeding the available tooling envelope.

According to this system design, conventionally integral system components are divided between a remote control station and a tool body 30. Any separation distance between the remote control station and tool body can be accommodated by this design including separations exceeding 500 feet and including the greater than 2000 foot separation of this preferred embodiment. Electronic components contained within electronics module 36 are designed to compensate for electrical losses incurred in transmitting a signal over a distance from the remote control station to inspection assembly 45. Electronics can be designed to allow any separation between a remote control station and tool body 30, including distances exceeding 500 feet. They provide operators at the control station with the same control that normal local operation would allow.

Ultrasonic transducers 47 are controlled by a primary pulser trigger located in the remote control station and a remote pulser located in the electronics module. The primary pulser trigger in the remote control station generates a square wave pulse having an amplitude in the range of about 50V to 300V, more preferably about 180V to 220V with a pulse width in the range of about 50ns to 500ns, preferably about 200ns to 225ns. By the time this signal from the primary pulser trigger travels over 2000 feet to the remote pulser, in a preferred embodiment, it is reduced in amplitude to around 100V to 150V.

In response to this signal, the remote pulser generates a spike having an amplitude in the range of about 450V to 500V, to activate transducers 47.

Since a limited number of signal conductors are available in the umbilical cable which is also designed with a minimized diameter to be compatible with tension leg interior dimensions, the ultrasonic transducers on the ultrasonic inspection assembly 46 are multiplexed. RF signals and power are all transmitted over shared conductors. Conductors carry 15V power from the remote control station to drive components located in electronics assembly 36, a trigger signal from the primary pulser trigger and a spike from the remote pulser. Once the remote pulser fires ultrasonic transducers 47, the data is transmitted back up to the remote control station over a shared conductor. Electronics are designed to separate the RF data from the DC power.

Axial drive motor 43 and rotary drive motor 44 shown in the cross section of drive module 42 in FIG. 7 are used to move the inspection assembly 45 to examine a particular weld or other area of tension leg 10 after tool body 30 has been positioned in tension leg 10 to within two inches of this desired location by raising or lowering tool body 30 with the winch. DC servo drive power supply to power axial drive motor 43 and rotary drive motor 44 are located in the remote control station. The winch 28 includes a closed loop hydraulic pump which makes possible precise (within two inch) positioning of tool body 30 unlike conventional open loop pumps which have far less accurate positioning capability, usually within twelve inches of a desired location.

When a weld or inspection site is located, integral clamping assemblies 39 are energized such that shoe 76 engages tension leg interior walls stabilizing tool body 30 in the tension leg at the inspection point. However, an axial drive screw connects the fixed cylindrical tool body with the inspection assembly and allows axial motion of inspection assembly 45 so that it can move freely over the tension leg interior surface.

Axial drive motor 43 in this preferred embodiment can effect a total axial displacement of 12 inches; however, depending on tool body 30 and tension leg 10 or tubular structure dimensions, the design can be scaled to yield a smaller or larger maximum displacement. Drive screw 101 is fixed and rotating nut 103 in drive module 42 produces axial motion.

Typically, the winch 28 is used to grossly position tool body 30 at a weld or other desired inspection location and axial drive motor 43 moves the inspection assembly 45 five inches above and below the inspection site. Rotary drive 44 rotates inspection assembly 45 3750 to provide some overlap between rotary scans to insure that ultrasonic transducers 47, which are themselves only 1 inch square in a preferred embodiment, have fully examined the entire circumference of the tension leg inner surface.

Axial drive motor 43 and rotary drive motor 44 are computer controlled from the remote control station such that inspection assembly 45 moves along the tension leg axis and circumference in a boustrophedonic pattern. A boustrophedonic pattern is obtained by first executing a motion from right to left then from left to right along alternate lines.

Position encoders not shown are located on the drive motors and function as "digital" tachometers. Motors operate at power levels in the range 24-50 volts DC.

An audio microphone 120 is located in drive assembly module 42 to transmit sounds of axial drive motor 43 and rotary drive motor 44 to an operator in the remote control station 25. The sounds of drive motors 43 and 44 operation can assist an operator in identifying whether the motors are straining against some obstruction or otherwise jammed or malfunctioning.

Tool body 30 is raised and lowered in tension leg 10 by integral umbilical cable 35, shown in FIG. 2 and in cross section in FIG. 8. Integral umbilical cable 35 is a multifunctional component. In addition to being the primary and redundant load carrying member, it provides electrical, pneumatic and hydraulic couplant medium interconnection between tool body 30 and the remote control station.

Referring to FIG. 8, integral umbilical cable 35 contains conduits for power, control and data signals, as well as pneumatic and hydraulic conduits. Hoses 132 carry air or water from the remote control station to tool body 30 and inspection assembly 45 while shielded electrical conductors 134 carry control signals, power, and data to and -from tool body 30. Coaxial cables 136 are also provided.

An inner armor bed 138 which can be polyurethane approximately 0.1 inches thick surrounds electrical, air and water conduits 134, 136 and 132 respectively.

Around inner armor bed 138 is armor package 140 composed of two layers of steel wire which can be 0.092 inch galvanized improved plow steel wire which protect inner wires and hoses and function as the load carrying members of the cable. Armor package 140 is composed of corrosionresistant wire. The steel wire of armor package 140 is wound in a counter-helical fashion and torque balanced so that the overall cable is rotationally balanced to minimize twisting when integral umbilical cable 35 is reeled to raise the tool in tension leg 10. This counter helical winding also serves to suppress axial bounce or circumferential spin during raising and lowering of the tool. Thus, the integral umbilical cable works together with integral clamping and centralizing assemblies 39 to stabilize tool body 30 in tension leg 31, particularly during raising or lowering operations.

Armor package 140 is covered by outer jacket 142 which can be 0.125 inch thick medium density polyethylene. Outer jacket 142 protects armor package 140 and interior wires and hoses and improves overall corrosion resistance in the salt spray operating environment of integral umbilical cable 35.

Integral umbilical cable 35 is connected to tool body 30 by umbilical connector module 34 shown in a detailed cross section FIG. 9. Umbilical connector module 34 provides mechanical protection as well as a sealed environment for umbilical connections.

Armor package 140 is anchored in umbilical connector module 34 by armor anchorage 150 which can be made of cast aluminum oxide filled epoxy, armor pot 152, retainer ring 154, and load ring 156 which can all be fabricated from stainless steel. O-ring seals 157 are also provided. Nuts 155 can be removed to allow umbilical connector module 34 to be slipped up over integral umbilical cable 35 to expose connectors during making/breaking of integral umbilical cable 35 connection to tool body 30. A transition stiffener 158 which can be made of cast polyurethane is also provided for strain relief and to form a tapered, rather than stepped transition.

Inspection assembly 45 as shown in FIGS. 10 and 11 is used to examine an area of interest in a tubular structure, a tension leg in a preferred embodiment. The assemblies 170 and transducers 47 are deleted in FIG. 10 for ease of illustration. During an inspection, axial drive screw 101 remains fixed and rotating nut 103 in the drive assembly imparts axial motion to inspection assembly 45 independent of the tool body 30 which is held stationary by integral clamping assemblies 39.

Simultaneous video and ultrasonic inspections can be conducted using camera 49 shown in FIG. 10 and transducers 47 shown in FIG. 11. Three cameras each having a fixed field of view perform the video inspection. One camera with lens 160 has an axial field of view and provides color images to an operator in the remote control station of the area directly in the tool path as it moves axially within tension leg 10. The two black and white other cameras are positioned such that the tension leg interior surface being examined by ultrasonic transducers 47 is within their field of view. The axial camera locates any obstructions in the path of tool body 30, while the cameras examining the tension leg interior surface locate any debris or surface irregularities which could interfere with the accuracy of ultrasonic test measurements.Cameras are encased in water tight, submersible impact resistant housings.

Light source 162 which can be a 120 V AC halogen lamp provides a high intensity variable light source. Intensity is regulated from the remote control station 25. One of the two such light sources 162 used in inspection assembly 45 is visible in FIG. 10. Light source 162 is enclosed in a glass housing to provide for submerged operation. However, the interior of tension leg 10 is generally dry presenting a problem for heat dissipation by such a high intensity light source. Heat sinking is provided by a metal heat sink 164 which can be aluminum at the base of light source 162 and a slotted metallic shroud 166 which surrounds light source 162. Arms 168 are part of transducer deployment assembly 170 shown in FIG. 11.

Referring to FIG. 11, arms 168 and deployment assemblies 170 allow deployment of transducer assemblies 46 independently of integral clamping and centralizing assemblies 39 leaving the inspection assembly 45 free to move over the tension leg interior surface when the tool is locked in a stationary position in the tension leg 31. Such independent deployment of transducer assemblies 46 is illustrated in FIGS. 3A-3C and 4A-4C. In FIGS. 3A-3C, integral clamping and centralizing assemblies 39 are nearly fully deployed, engaging the tension leg interior surface while transducer deployment assemblies 170 are nearly fully extended.In FIGS. 4A-4C, integral clamping assemblies 39 are nearly fully deployed; however, the inspection assembly 45 is positioned in a tension leg constriction 33 and transducer deployment assemblies 170 are compressed conforming to the smaller tension leg interior diameter of the constriction 33.

Transducer deployment assemblies 170 are independently energized via a mechanical linkage from the circumferential drive assembly and are also pneumatically actuated, allowing for compliance of the inspection assembly to the tension leg interior surface. Inspection assembly 45 is connected to rotary drive 43 at attachment point 171. Air cylinders 173 with attached air lines 175 have cylinder rods not shown which attach to actuator blocks 177A and 177B, respectively, which move arms 168 and deployment assemblies 170. Thus the right hand deployment assembly 170A can deploy inspection assembly 46A independently of the left hand deployment assembly 170B which deploys inspection assembly 46B. Camera 51 has its lens 179 positioned to view deployment assembly 170. A second camera 51 shown in FIGS.

2C, 3C and 4C but not shown in the view of FIG. 11 is positioned 180C opposite to camera 51 shown in FIG. 11 to view the opposite deployment assembly 170. Transducer deployment assemblies 170 collapse if pneumatic pressure or electrical power is lost, preventing damage to the deployment assemblies 170 and the tension leg 10.

Ultrasonic transducers 47 require a medium to couple ultrasonic waves from transducer faces to the tension leg surface. In the preferred embodiment, water serves as the couplant medium. Integral couplant feed ports 176 and distribution channel 178 supply a couplant medium, water in this embodiment, as best shown in FIG. 12B. Couplant medium such as water is delivered to the inspection assembly 45 through integral umbilical cable 35 and regulated to a desired working pressure and flow rate in hydraulic and pneumatic control module 41 obviating any need for an on board couplant supply tank and pumps and allowing a continuous scan time for the inspection assembly. Given the rigid tooling envelope constraints for tool body 30, such equipment would have been able to carry only small quantities of couplant medium, thus limiting scan time.

Since a continuous water supply is available from the remote control station 25 the current design permits the tool to remain in the tension leg and perform inspections indefinitely.

A preferred method for inspecting a variable diameter tension leg such as tension leg 10 in service in a body of water includes steps of packaging inspection tool electronics, fluid pressure controls and inspection drive assemblies within a tooling envelope able to pass through a smallest interior diameter of the tension leg like the modules already described and illustrated in FIGS. 2A, 2B and 20.

Such a modular inspection tool also including collapsible articulated centering mechanisms and ultrasonic inspection assemblies such as already described is lowered into the tension leg using an integral umbilical cable such as that already described. Using the integral umbilical cable, the tension leg inspection apparatus is lowered into the tension leg until a desired inspection point is located whereupon the dual mode integral clamping assemblies 39 are activated to engage the interior of the tension leg and hold the inspection tool stationary in the tension leg while the tension leg interior is inspected in a boustrophedonic pattern by any combination of video, ultrasonic and audio inspection assemblies.

An operator in a remote control station controls the tool body -30 by adjusting integral clamping assembly deployment pressure including booster cylinder engagement at smaller tension leg interior diameters, as well as independent deployment of the ultrasonic inspection assemblies. An operator can also control couplant medium flow and inspection assembly rotary and axial motion.

As shown in FIG. 12A, ultrasonic inspection assemblies 46A and 46B can include more than one type of transducer 47 to allow for locating and sizing flaws on both the tension leg outer and inner diameters. In a preferred embodiment, inspection assembly 46A includes a dual-duplex transducer 180 for sizing flaws on a tension leg outer diameter surface and tandem duplex transducer 182 for sizing flaws on a tension leg interior diameter surface. Ultrasonic inspection assembly 46B further includes combined 450 single element shear wave and dual OC transducers 184, 185 for detection of flaws. The ultrasonic inspection assembly can include any single element shear wave transducer. A 300, 450, 600 or 700 shear wave transducer can be selected depending on the dimensions of the tubular structure being examined.

A tandem-duplex transducer 182 which can also be referred to as a tandem duplex transducer assembly consists of a receiver element 245 and transmitter element 247 as shown in FIG. 12C. Receiver element 245 includes receiver crystal assembly 250 and receiver wedge 252. Transmitter element 247 is separated from receiver element 245 by insulator 249 and includes transmitter crystal assembly 254 and transmitter wedge 256. Crystals 250 and 254 are oriented in nearly the same direction by contrast with crystals 212 of a dual-duplex transducer shown in FIG. 15 which are oriented 1800 apart. The details of construction of the tandem duplex transducer 182 are shown in U.S. Patent No. 4,658,649 to Brook, which is incorporated herein by reference.

A 0 dual transducer such as used for transducer 185 consists of a transmitter and a receiver and projects ultrasound normal to the surface of the object undergoing inspection.

Inspection assemblies 46A and 46B are equipped with wheels 186 to allow the ultrasonic inspection assemblies to roll freely along the tension leg interior surface and allow the multiple transducers to come into close contact with the tension leg interior surface.

To minimize the amount of couplant medium needed to couple ultrasound waves from a transducer to the tension leg surface, couplant medium is fed from internally fed couplant weeping ports 176 and combined internally fed couplant weeping ports 176 and distribution channels 178 which surround each transducer as seen in FIG. 12B for transducer 184. Contoured transducer housing faces 188 and an additional spring-loaded outer sleeve 190 as shown in FIG.

13 allow transducer assembly 187 to float on couplant medium such as water intervening between transducer face 188 and the tension leg interior surface 52 while maintaining a constant force surface contact. The shape of transducer housing face 188 is designed to match tension leg interior surface curvature and thus allow a thin film of water in a range of about 0.001 inch to .005 inch to intervene between transducer face 188 and the tension leg interior surface 52.

The spring-loaded design of outer sleeve 190 is shown in greater detail in the cut away view of FIG. 13. Springs 192 allow transducer assembly 187 to maintain a constant force surface contact. Although a coil spring 192 is illustrated, other types of springs such as wave spring washers can be used. This transducer design produces transducers housed in relatively compact housings operating with small amounts of couplant medium as compared with conventional "squirter-type systems" which use a large volume of water, approximately several gallons per minute, continuously flowing between the transducer and the surface of the object under inspection.

Retaining ring 193 holds transducer assembly 187 in springloaded outer sleeve 190. Transducer assembly 187 is closely received in sleeve bore i90B. Gimbal pivots 194 are aligned with the direction of scanning motion, allowing transducer assembly 187 to pivot to accommodate surface irregularities without tending to tip over as it moves along the tendon leg wall.

It will be understood that each of the previously described transducers 180, 182, 184 and 185 are contained in an assembly like 187. As best seen in FIG. 15, each assembly 187 includes a housing 187A having the piezoelectric transducer elements contained therein and having housing face 188 defined thereon.

The housing face 188 is curved to match the radius of curvature of the smaller I.D. of the tension leg, which in the example described is a 9" I.D. The sleeve 190 has a sleeve face 190A which has a second radius of curvature to match the larger I.D. of the tension leg, which in the example described is a 22" I.D.

Thus when examining the smaller 9" I.D. of the tension leg the housing face 188 will closely engage the inner curved surface of the tension leg with a thin film of couplant fluid therebetween. When examining the larger 22" I.D. of the tension leg the sleeve face 190A will closely engage the inner curved surface of the tension leg and will contain couplant fluid across the housing face 188.

Referring ag#ain to FIG. 12A, dual-duplex transducer 180 and tandem-duplex transducer 182 are a set of transducers used for sizing flaws and transducers 184 and 185 are a set of transducers for detection of flaws. Dual-duplex transducer 180 sizes cracks on a tension leg outer diameter surface while tandem-duplex transducer 182 sizes cracks on a tension leg inner diameter surface. Transducers 184, 185 are identical 450 single combined with dual 0o elements in single transducer assembly 187, packaged together in ultrasonic inspection assembly 46 so that they emit sound 1800 opposite each other in an axial direction (i.e., both up and down the tension leg) and detect flaws in tension leg 10 wall volume.Sizing transducers 180, 182 can be combined in a single ultrasonic inspection assembly 46A, and detection transducers 184 and 185 can be combined in a single assembly 46B as shown in FIG. 12A or the four transducers can be deployed individually as seen in FIG. 14.

FIG. 14 shows an apparatus for ultrasonic inspection of a tubular structure 202 including tandem duplex transducer 182 and dual-duplex transducer 180 deployed individually in transducer assemblies 187 for flaw sizing on a tubular structure inner diameter surface 203 and outer diameter surface 204 respectively. On the opposite wall of tubular structure 202, a set of combined 450 single and 0 dual transducers 184 and 185 direct sound waves 1800 apart in an axial direction (i.e., both up and down the tension leg) into a wall 200 of tubular structure 202 for detecting flaws within the bulk of wall 200.

The design of dual-duplex transducer 180 is novel and is shown in greater detail in the cross section view of FIG.

15. The dual-duplex transducer includes wedge 210 (best seen in plan in FIG. 16) to direct sound beams generated by dual sound wave transmitters 212 which can be piezoelectric crystals into a material in two directions 1800 apart in an axial direction (i.e., both up and down the tension leg) from each other. Receiver elements 316 and 318 are also mounted on wedge 210 behind the transmitter elements 212 in the view of FIG. 15. The roof has two roof sides 312 and 314. One transmitter 212 and its associated receiver 316 are mounted side by side on first roof side 312. The other transmitter 212 and its associated receiver 318 are mounted side by side on second roof side 314.A roof angle 213 is chosen such that an incident angle (the angle between the ultrasonic beam and a line normal to the tension leg inner surface into which the bottom 310 of wedge 210 projects the beam) is slightly (10 to 30) less than the first critical angle. The first critical angle is the incident angle beyond which a refracted longitudinal wave no longer exists in the structure being tested, but instead propates as a surface wave. This causes the ultrasonic beam from each transmitter 212 to be refracted into a direct and an indirect shear wave. The waves from one transmitter 212 propagate in one axial direction along the tension leg and the waves from the other transmitter 212 propagate in the opposite axial direction along the tension leg. The direct shear wave will give a reflected signal from the "trap" of a crack, i.e. the corner defined by the tension leg surface and the open end of a surface crack. The indirect shear wave will give a reflected signal from the "tip" of the crack, often referred to as the "tip diffraction signal".

Piezoelectric crystals 212 are embedded in epoxy 214. A dual-duplex transducer design results in a significant reduction in the size of the transducer footprint by comparison with standard transducer wedge designs. A 50% reduction in transducer length was achieved by the design shown in FIG. 15.

Transducer wedge 210 can be manufactured from lucite material and scaled according to the dimensions of the tubular structure to be non-destructively evaluated. Dualduplex wedge design 210 generates direct and indirect shear waves in the tension leg walls.

A preferred method for nondestructive inspection of a tension leg while in service in a body of water includes the steps of providing a tension leg to be examined for flaws, moving the flaw locating and sizing means into contact with the inner surface of the tension leg 31 and coupling locating and sizing means with an inner surface of the tension leg 31 locating the flaws on the tension leg, and sizing flaws on the tension leg inner and outer surfaces.

The steps of the preferred method can be performed using any of the transducers 180, 182, 184 and 185 or other components of ultrasound inspection assembly 46.

Thus it is seen that the apparatus and methods of the present invention readily achieve the ends and advantages mentioned as well as those inherent therein. While certain preferred embodiments have been illustrated and described for purposes of the present disclosure, numerous changes in the arrangement and construction of the invention may be made by those skilled in the art, which changes are encompassed within the scope of the present invention as defined by the appended claims.

Claims (18)

CLAIMS:

1. An apparatus for inspecting the interior of a tubular structure comprising: a tool body; an inspection assembly connected to the tool body and including an ultrasonic inspection means deployed on more than one articulated arm, and means for activating said articulated arms by fluid pressure; a control station at a remote location from the tool body and inspection assembly; communicating means connected to the tool body for communication with the control station; and raising means for raising and lowering the tool body and inspection assembly in the structure.

2. The apparatus of claim 1 wherein the ultrasonic inspection means further includes an ultrasonic transducer assembly having a housing having a face contoured to complement a curvature of an interior surface of said tubular structure and allow a thin film of a couplant medium to intervene between the housing face and interior surface of the tubular structure.

3. The apparatus of claim 2 wherein the ultrasonic the ultrasonic transducer assembly further includes internally fed couplant medium weeping ports and wherein said transducer assembly is a tandem-duplex, dual-duplex, single element shear wave transducer assembly.

4. The apparatus of claim 1, 2 or 3 wherein the ultrasonic inspection means includes more than one transducer assembly and wherein: one of said transducer assemblies provides a means for locating flaws in said tubular structure; another of said transducer assemblies provides a means for sizing flaws in said tubular structure.

5. A dual-duplex transducer for non-destructive testing of a solid medium comprising: a wedge having a peaked roof having first and second roof sides each defining a roof angle; first and second transmitter elements mounted on said first and second roof sides, respectively, first and second receiver elements mounted on said first and second roof sides, respectively, beside said first and second transmitter elements, respectively, and wherein said roof angle is chosen so that an incident angle of an ultrasonic beam from each of said transmitter elements is sufficiently near a first critical angle of said medium that the ultrasonic beam from said first transmitter element generates both direct and indirect shear waves propagating in a first direction through said medium, and the ultrasonic beam from said second transmitter element generates both direct and indirect shear waves propagating though said medium in a second direction opposite said first direction.

6. An apparatus for inspecting an interior of a tubular structure comprising: a tool body; an inspection 'assembly including an ultrasonic transducer further including an ultrasound generator, a wedge means for generating direct and indirect shear waves in the structure and an ultrasound receiver means for receiving ultrasound which has interacted with the structure; a control assembly; means for communication between the control assembly and the inspection assembly; and means for positioning the inspection assembly in the tubular structure.

7. A method for non-destructive inspection of a tension leg while in service in a body of water comprising: (a) moving flaw locating and sizing means into contact with an interior surface of the tension leg; (b) coupling the flaw locating and sizing means with the interior surface of the tension leg; (c) locating flaws in the tension leg; (d) sizing flaws on the interior surface of: the tension leg; and (e) sizing flaws on an outer surface of the tension leg.

8. The method of claim 7 wherein the step of locating flaws is performed using a first transducer and a second transducer, wherein one of said first and second transducers is a single element shear wave transducer, and wherein the single element shear wave transducer is a 30, 45, 60 or 70 single element shear wave transducer.

The method of claim 7 or 8 wherein the step of locating flaws is performed using a first transducer and a second transducer including positioning the first and second transducers to emit sound 180@ axially opposite each other including mounting the first transducer in a first inspection assembly and the second transducer in a second inspection assembly.

10. The method of claim 9 further including housing each transducer in a housing having couplant feed means, said housing being shaped to conform to a tension leg inner curvature, thus reducing the amount of couplant medium needed.

11. An ultrasonic transducer assembly comprising: an ultrasonic transducer; a housing containing said transducer, said housing having a housing face; and biasing means for biasing said housing toward a structure surface to be inspected so that said housing face remains in contact with said structure surface.

12. The assembly of claim 11 wherein said housing face is contoured complementarily to said structure surface and further comprising: couplant medium weeping ports defined in said housing adjacent said housing face.

13. The assembly of claim 11 or 12 further comprising: gimballed mounting means for allowing said housing face to remain in contact with said structure surface as said assembly is moved across said structure surface.

14. The assembly of claim 11 further comprising: an outer sleeve having a bore therein with said housing slidably received in said bore; and said biasing means being a means for biasing said housing outward relative to said sleeve.

15. The assembly of claim 14 wherein: said housing face has a first radius of curvature to complement a first curved structure surface; said sleeve has an outer sleeve face surrounding said housing, said sleeve face having a second radius of curvature to complement a second curved structure surface; and wherein said biasing means is sufficiently compressible to allow said sleeve face to engage said second structure surface and said second radius of curvature is greater than said first radius of curvature, further comprising: couplant fluid weeping ports defined in said housing adjacent said housing face; and wherein said housing is closely received in said housing bore so that when said sleeve face engages said second structure surface couplant fluid is contained within said sleeve across said housing face.

16. Apparatus for inspecting the interior of a tubular structure substantially as herein described with reference to the drawings.

17. A method for inspecting the interior of a tubular structure substantially as herein described with reference to the drawings.

18. An ultrasonic transducer assembly, substantially as herein described with reference to the drawings