WO2011115601A1

WO2011115601A1 - Optical scanning tool for wellheads

Info

Publication number: WO2011115601A1
Application number: PCT/US2010/000790
Authority: WO
Inventors: Alan Graham; Daniel Mcstay
Original assignee: Fmc Technologies, Inc.
Priority date: 2010-03-15
Filing date: 2010-03-15
Publication date: 2011-09-22

Abstract

An optical scanning tool for use in imaging the surface of a bore of a wellhead comprises a base structure which is removably connectable to the wellhead and at least one scanner head which is mounted on the base structure. After the base structure is connected to the wellhead, the scanner head scans the surface of the bore and generates image data which is representative of the surface.

Description

OPTICAL SCANNING TOOL FOR WELLHEADS

BACKGROUND OF THE INVENTION

The present invention relates to a profiling tool for measuring the interior surface of an installed wellhead. More particularly, the invention relates to an optical scanning tool which scans the interior surface of the wellhead and uses optical triangulation to generate data from which a map or three dimensional model of the surface may be constructed.

Oil and gas production systems typically include a wellhead which is located at the upper end of a well bore. The wellhead comprises a central bore within which a number of casing hangers are usually landed. Each casing hanger is connected to the top of a corresponding one of a number of concentric, successively smaller casing strings which extend into the well bore, with the innermost casing string being connected to an uppermost casing hanger. In certain types of production systems, after the casing strings are installed a tubing string run into the well bore through the innermost casing string. The top of the tubing string is connected to a tubing hanger which includes a circumferential shoulder that lands on a seat that is formed at the top of the uppermost casing hanger.

The tubing hanger is often secured to the wellhead with a number of locking dogs which are positioned circumferentially around the tubing hanger and are expandable radially outwardly into a locking profile in the bore of the wellhead. The locking dogs are typically supported on a ring which is adjustably connected to the tubing hanger to allow the vertical distance between the shoulder and the locking dogs to be adjusted. In order to ensure that the tubing hanger is properly locked to the wellhead, the vertical distance between the shoulder and the locking dogs, which is commonly referred to as the wellhead space-out, must be the same as the vertical distance between the seat and the locking profile.

In the prior art, a lead impression tool (LIT) is sometimes used to measure the wellhead space-out. In subsea wellheads, the LIT is lowered on a drill string and landed on the casing hanger shoulder. The LIT is then hydraulically actuated to press typically three circumferentially spaced lead impression pads into the wellhead locking profile. After the impressions are taken, the LIT is retrieved to the surface and mounted on a storage/test stand, which is then manually adjusted to match the lead impression tool. The tubing hanger is then mounted on the storage/test stand and the vertical position of the locking dogs is adjusted to match the wellhead space-out.

Although the LIT provides a useful measurement of the wellhead space- out, the time required to run and retrieve the LIT can be relatively long, especially in deep water. Setting the tubing hanger on the storage/test stand and adjusting the vertical position of the locking dogs can also be a time consuming process.

SUMMARY OF THE INVENTION

In accordance with the present invention, these and other limitations in the prior art are addressed by providing an optical scanning tool for use in imaging the surface of a bore of a wellhead. The scanning tool comprises a base structure which is removably connectable to the wellhead; and at least one scanner head which is mounted on the base structure. After the base structure is connected to the wellhead, the scanner head scans the surface of the bore and generates image data which is representative of the surface.

In accordance with one embodiment of the invention, the scanning tool further comprises means for displacing the scanner head rotationally relative to the axial centerline of the bore. The rotational displacement means may comprise a rotary drive unit. The rotary drive unit may be connected to the base structure and the scanner head may be connected to the rotary drive unit.

Alternatively, the rotary drive unit may be connected to the base structure and the scanner head may be connected to a portion of the base structure which is rotated by the rotary drive unit.

In accordance with another embodiment of the invention, the scanning tool further comprises means for measuring the rotational displacement of the scanner head relative to the axial centerline of the bore and for generating rotational displacement data. The scanning tool may also include means for processing the image data and the rotational displacement data to provide an image map which is representative of bore.

In accordance with yet another embodiment of the invention, the scanning tool comprises means for displacing the scanner head axially relative to the bore. The axial displacement means may comprise a linear drive unit. The linear drive unit may be connected to the base structure and the scanner head may be connected to the linear drive unit. Alternatively, the linear drive unit may be connected to the base structure and the scanner head may be connected to a portion of the base structure which is displaced by the linear drive unit.

In accordance with another embodiment of the invention, the scanning tool comprises means for measuring the axial displacement of the scanner head relative to the axial centerline of the bore and for generating axial displacement data. The scanning tool may also comprise means for processing the image data and the axial displacement data to provide an image map which is representative of bore.

In accordance with yet another embodiment of the invention, the base structure comprises a cap for the wellhead. In this embodiment, the wellhead may be located subsea and the cap may be wireline deployable from a surface vessel.

In accordance with still another embodiment of the invention, the cap comprises a generally cylindrical housing which is configured to fit over the top of the wellhead and a shaft which extends axially relative to the housing. In this embodiment, the scanner head may be mounted on the shaft.

In various other embodiments of the invention, the optical scanning tool may comprise means for cleaning the bore, such as a flushing system, and/or means for injecting a dye into the bore. In the latter case, the scanner head may emit light at a particular wavelength which causes the dye to fluoresce, and the scanner head may detect the particular wavelength.

In yet another embodiment of the invention, the scanner head may emit light at a plurality of wavelengths, and the scanner head may detect each of the plurality of wavelengths.

Thus, the optical scanning tool of the present invention provides a quick and convenient means for mapping the wellhead bore. The image data provided by the tool can be used to obtain dimensional information, such as the wellhead space-out. The image data can also be used to detect any irregularities, such as cracks, that may exist in the surface of the bore.

These and other objects and advantages of the present invention will be made apparent from the following detailed description, with reference to the accompanying drawings. In the drawings, the same reference numbers are used to denote similar components in the various embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is cross sectional representation of one embodiment of the optical scanning tool of the present invention shown mounted on an exemplary wellhead;

Figure 2 is a cross sectional representation of a second embodiment of the optical scanning tool of the present invention;

Figures 3 through 6 are schematic representations of additional

embodiments of the optical scanning tool of the present invention;

Figures 7A and 7B are schematic representations of certain multiple wavelength imaging optics which may be used in the various embodiments of the optical scanning tool of the present invention;

Figure 8 is a cross sectional representation of another embodiment of the optical scanning tool of the present invention; and

Figure 9 is a cross sectional representation of a further embodiment of the optical scanning tool of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1 , the optical scanning tool of the present invention, generally 10, is shown in conjunction with a representative wellhead 12 which is located at the upper end of a well bore (not shown). The wellhead 12, only the upper portion of which is shown, includes a central bore 14 within which a number of casing hangers are landed. Each casing hanger is connected to the top of a corresponding one of a number of concentric, successively smaller casing strings which extend into the well bore, with the innermost casing string being connected to an uppermost casing hanger 16.

After the casing strings are installed, a tubing string is run into the well bore through the innermost casing string. The top of the tubing string is connected to a tubing hanger (not shown) which includes a circumferential . shoulder that lands on a seat 18 which is formed at the top of the uppermost casing hanger 16. The tubing hanger is secured to the wellhead 12 by a number of locking dogs which are positioned circumferentially around the tubing hanger and are expandable radially outwardly into a locking profile 20 that is formed in the bore 14 of the wellhead. The locking dogs are typically supported on a ring which is adjustably connected to the tubing hanger to allow the vertical distance between the shoulder and the locking dogs to be adjusted. In order to ensure that the tubing hanger is properly locked to the wellhead 12, the vertical distance between the shoulder and the locking dogs must be the same as the vertical distance between the seat 18 and the locking profile.

In accordance with the present invention, the optical scanning tool 10 is used to, among other things, provide an accurate measurement of the distance between the seat 18 and the locking profile 20 so that the vertical distance between the shoulder and the locking dogs of the tubing hanger can be set to this distance.

As shown in Figure 1 , the optical scanning tool 10 of one embodiment of the invention includes a number of scanner heads 22 which are supported on a base structure that in turn is removably connectable to the wellhead 12. In this illustrative embodiment of the invention, the base structure is configured as a cap 24, such as a temporary abandonment cap, which is connected to the top of the wellhead 12. Thus, if the wellhead 12 is located subsea, the optical scanning tool 10 may be deployed from a surface vessel on a wireline and connected to the wellhead in the same manner as a conventional temporary abandonment cap.

Referring also to Figure 2, the cap 24 includes a housing 26 which is configured to fit over the top of the wellhead 12. The housing 26 comprises a cylindrical side wall 28 and a top plate 30 which is connected to the upper end of the side wall. An axial stem 32 extends upwardly from the housing 26 and comprises a lower end 34 which is secured in a corresponding hole in the top plate 30 and an upper end 36 which is connectable to a running and retrieval tool (not shown). A number of brace plates 38 may be connected between the top plate 30 and the stem 32 to provide structural support for the stem during running and retrieval of the optical scanning tool 10. An elongated tube or shaft 40 is positioned coaxially with the housing 26 and includes an upper end 42 which is connected to the stem 32 and a lower end 44 which extends into the wellhead bore 14. A protector sleeve 46 may be connected to the lower end of the shaft 40 to protect the shaft as the cap 24 is lowered onto the wellhead 12.

The cap 24 is secured to the wellhead 12 by a number of spring-loaded retainer pins 48 which are slidably mounted in corresponding holes in the side wall 28. As the cap 24 is initially lowered onto the wellhead 12, the pins 48 engage the top outer surface of the wellhead and are forced thereby into their corresponding holes. Then, as the cap 24 is further lowered onto the wellhead 12, the pins 48 expand into a circumferential groove 50 formed on the outer surface of the wellhead to thereby secure the cap to the wellhead. Thus, once the cap 24 is fully lowered onto the wellhead 12, the cap is fixed in position relative to the wellhead and provides a secure base for the scanner heads 22. In order to remove the cap 24 from the wellhead 2, a sufficient upward force is applied to the stem 32 to cause the pins 48 to shear and thereby disconnect the cap from the wellhead.

It should be understood that the base structure need not be limited to the configuration shown in the drawings. For example, an alternative base structure may be similar to the cap 24 without the stem 32, the brace plates 38 and/or the shaft 40. Alternatively, the base structure may comprise a plug member which is secured in the wellhead 12 or the casing hanger 16. The only requirement for the base structure is that it provide a secure, fixed base for the scanner heads 22.

In an exemplary and non-limiting embodiment of the invention, each scanner head 22 comprises a laser-based profilometery (LP) sensor which operates on the principle of optical triangulation to measure certain features of the wellhead bore 14, such as the vertical distance between the seat 18 and the locking profile 20 at various circumferential locations around the bore. As is understood by persons of ordinary skill in the art, the optical triangulation method uses a light source and focusing optics to project a focused spot or line of light onto a target surface. An imaging lens focuses the reflected light onto a photodetector, which generates a signal representative of the position of the spot or line in the image plane of the photodetector. As the surface being scanned rises and falls, the spot or line is displaced and this displacement is detected by the LP sensor to provide an accurate measurement of the surface.

Referring again to Figure 1 , each scanner head 22 includes an optical source 52 and an optical receiver 54 which is offset from the optical source by a known distance. The optical source 52, which may comprise, e.g., one or more diode lasers, projects a spot or fan-shaped beam of light onto the surface of the bore 14. The optical receiver 54, which may comprise, e.g., a single-element position-sensing device (PSD), a video camera or a charge-coupled device

(CCD), captures the image reflected by the surface and generates data which is representative of the image and thus the surface. One embodiment of an LP sensor which is suitable for use in the present invention is the 3DL3 laser imaging system sold by Smart Light Devices Limited of Aberdeen, Scotland. The image data is transmitted via suitable communications means, such as a signal cable or an acoustic transmitter, to a central processor 56 which is located, for example, on a surface vessel (not shown). The processor 56 processes the image data using appropriate imaging software to provide the desired information about the bore 14. For example, the processor 56 may calculate the vertical distance between the seat 18 and the locking profile 20 at a number of circumferential locations around the bore 14. The processor 56 may also generate a map of the bore 14 from which dimensions may be taken or a three dimensional model of the bore which can be compared to the original CAD model of the wellhead 12 to obtain a detailed analysis of the surface of the bore.

In the embodiment of the invention shown in the drawings, the scanner heads 22 are mounted on the shaft 40, which as discussed above is positioned coaxially with the housing 26 and, therefore, the bore 14. Consequently, each scanner head 22 is positioned near the axial centerline of the bore 14. However, this need not be the case.

In order to scan the bore 14, the spot or line which is projected by the optical source 52 must be moved across the surface of the bore. Thus, if the scanner heads 22 are configured to project a vertical line, the optical scanning tool 10 ideally includes means for rotating the scanner heads around the vertical centerline of the bore 14. Such rotational displacement means may comprise, for example, a device similar to a conventional rotary valve actuator (not shown) which is mounted on the cap 24 and which when actuated rotates the shaft 40, and thus the scanner heads 22, about its axis. Alternatively, as shown in Figure 1 the scanner heads 22 may be mounted on a rotary drive unit 58 which is connected to the shaft 40.

The optical scanning tool 10 may also include means for measuring the rotational displacement of the scanner heads 22 relative to axial centerline of the bore 14. Such means may comprise, for example, a conventional encoder (not shown) for generating data representative of the rotational displacement of the shaft 40 relative to the cap 24 or the rotational displacement of the rotary drive unit 58 relative to the shaft. This rotational displacement data, which is

determinative of the angular positions of the vertical scan line on the surface of the bore 14, may be combined with the image data generated by the scanner heads 22 to provide a complete mapping of the bore 14 within the vertical range of the optical source 52.

If the vertical range of the optical source 52 is insufficient to measure the desired vertical extent of the bore 14, the optical scanning tool 10 may also be provided with means for displacing the scanner heads 22 axially relative to the axial centerline of the bore. Such means may include, for example, a device similar to a conventional linear valve actuator (not shown) which is mounted on the cap 24 and which when actuated moves the shaft 40, and thus the scanner heads 22, longitudinally relative to its axis. Alternatively, the scanner heads may be mounted on a linear drive unit which is connected to the shaft 40. As shown in Figure 2, for example, the scanner heads 22 are mounted on a rotary drive unit 60 which is coupled to a linear drive unit 62 that is connected to the shaft 40. As illustrated schematically in Figure 3, this arrangement moves the scanner heads 22 axially to defined vertical positions on the shaft 40 and rotates the scanner heads at each vertical position to obtain a complete scan of a corresponding vertical section of the bore 14.

The optical scanning tool 10 ideally also includes means for measuring the axial displacement of the scanner heads 22 relative to the bore 14. Such means may comprise, for example, a conventional encoder (not shown) for generating data representative of the axial displacement of the shaft 40 relative to the cap 24 or the axial displacement of the linear drive unit 62 relative to the shaft. This axial displacement data, which is determinative of the vertical positions of the vertical scan line on the surface of the bore 14, together with any rotational displacement data (obtained as described above), may be combined with the image data generated by the scanner heads 22 to provide a complete mapping of the bore 14 within any vertical range desired.

In an another embodiment of the invention which is shown schematically in Figure 4, the optical scanning tool 10 comprises multiple arrays 64 of one or more scanner heads 22 each. Each array 64 is mounted at a fixed axial position along the shaft and is rotated by means such as described above to provide a complete scan of a corresponding vertical section of the bore 14.

In the embodiments of the invention which employ multiple vertical scans of the bore 14, overlaps in successive vertical scans may be used to determine the vertical displacement or distance between scans or scanner heads 22. This may be achieved, for example, by identifying common features in the sequential scans, such as fixings, test ports, welds and flaws, and then using these common features in conjunction with the know resolution and scale of the scanner heads 22 to determine the vertical displacement of the scanner heads. This information may also be used to calibrate or cross reference the axial displacement data obtained as described above to determine the vertical positions of the scanner heads 22 within the wellhead 12.

In another embodiment of the invention which is shown schematically in Figure 5, the scanner heads 22 are configured to project a horizontal line onto the surface of the bore 14. In this embodiment, one or more scanner heads 22 are used to project a horizontal line around the entire circumference of the bore 14 or a particular section of interest. The scanner heads 22 are then moved vertically within the wellhead 12 by the axial displacement means described above to produce a complete scan of the bore 14.

This embodiment of the optical scanning tool 10 may also include means such as described above for measuring the axial displacement of the scanner heads 22 relative to the bore 14. Such means will provide axial displacement data which is determinative of the vertical positions of the horizontal scan lines on the surface of the bore 14, and this data may be combined with the image data generated by the scanner heads 22 to provide a complete mapping of the bore 14.

A further embodiment of the invention is shown schematically in Figure 6. In this embodiment, the optical scanning tool 10 comprises a plurality of scanner heads 22 which are aligned vertically but are oriented to project horizontal scan lines over different segments of the bore 14. With this arrangement, the^' entire circumference of the bore 14 at a particular vertical location may be scanned at once. Consequently, the entire bore 14 may be scanned by with a single vertical pass of the scanner heads 22.

As an alternative to the embodiments described above, the scanner heads 22 could also be configured to project one or more spots of light on the surface of the bore 14. In such an embodiment, the rotational and axial displacement means described above would move the scanner heads 22, and thus the spots, over the entire surface of the bore 14. Together with the means described above for measuring the rotational and axial displacement of the scanner heads 22, this arrangement would provide a complete mapping of the surface of the bore 14.

In another embodiment of the invention a structured line pattern may be projected onto sections of the bore 14. In such an embodiment, the rotational and axial displacement means described above would move the scanner heads 22, and thus the structure light pattern, over the entire surface of the bore 14. Together with the means described above for measuring the rotational and axial displacement of the scanner heads 22, this arrangement would provide a complete mapping of the surface of the bore.

In accordance with one embodiment of the present invention, the scanner heads 22 are designed to generate light at two or more optical wavelengths. This may be accomplished by providing the scanner heads 22 with an optical source 52 that is able to emit multiple wavelengths, such as a tunable laser, or by providing the scanner heads 22 with multiple optical sources that each emit different wavelengths. In this embodiment, the scanner heads 22 are operated to selectively project one wavelength during a given scan. The resulting image data from successive scans of the same area at different wavelengths may then be used to provide useful information regarding the scanned surface. For example, an analysis of the intensities detected at the different wavelengths maybe used to assist in identifying areas of corrosion or other surface irregularities.

In accordance with another embodiment of the invention, the scanner heads 22 may be designed to emit multiple wavelengths of light simultaneously during a single scan. This may be achieved by providing the scanner heads 22 with an optical source 52 which emits multiple wavelengths and projects these wavelengths through an optical system that generates a single spot or line scan.

Alternatively, the scanner heads 22 may be provided with multiple optical sources 52, each of which emits at a different wavelength. In this embodiment, the beams from the multiple optic sources 52 are coupled into the spot or line scanning optics using, e.g., a partially reflecting wavelength-selective mirror arrangement. As an alternative, separate spot or line scan optics may be provided for each beam, and these optics may be arranged so that the spot or line scans from each optical source are coincident on the surface to be scanned.

Examples of the imaging optics which may be used to detect the multiple wavelengths of light are shown schematically in Figure 7A for two wavelengths and in Figure 7B for three wavelengths. In these multi-wavelength embodiments, the wavelengths λι , λ2, λ₃ are separated using wavelength selective filters 66, such as dielectric multilayer filters and/or a beam splitters, and each wavelength is detected by a separate optical receiver 54.

In order to obtain accurate and meaningful profiles of the surface of the bore 14, the surface should be as clean as possible. In accordance with the present invention, therefore, the optical scanning tool 10 may include means for cleaning the surface of the bore 14. Referring to Figure 8, such means may comprise a system, generally 68, for flushing the bore 14 with a cleaning solution. The solution, which may be stored in a tank 70 mounted on the cap 24 or supplied to the cap via an umbilical (not shown), may be injected into the bore 14 through a suitable valve 72 and flushed through the wellhead 12 in a

conventional manner. As an alternative or in addition to the flushing system 68, the cleaning means may include mechanical means for cleaning the bore 14. As shown in Figure 8, for example, the optical scanning tool 10 may include a number of water jets 74 which are mounted on the shaft 40 and are supplied with pressurized water or a cleaning solution through conventional means. The cleaning means may also or alternatively include other mechanical cleaning devices, such as brushes.

In this embodiment, the scanning heads 22 may include retractable covers over their optical windows to prevent the windows from becoming fouled during the cleaning process.

In another embodiment of the invention, the optical scanning tool 10 may include a broadband or white light source to provide illumination for generating full color video images using either the optical receiver 54 or a separate video camera. The illumination source and the video camera may be built into the scanner head 22 or provided as separate units which are mounted on the shaft 40 and are moved in sequence with the scanner head.

In a further embodiment of the invention, the optical scanning tool 10 includes means for injecting a dye penetrant into the bore 14. Referring to Figure 9, the dye injecting means may comprise one or more injector units 76 which are mounted on, e.g., the shaft 40 and are supplied by an external source of dye that is connected to the injector units by, for example, an umbilical extending from the surface to the cap running and retrieval tool. Alternatively, the injecting means may comprise a dye reservoir 78 which is mounted on the cap 24 and is connected to the bore 14 through a suitable valve 80.

After the dye is injected into the bore 14, the dye will penetrate any imperfections in the surface of the bore. After waiting for a fixed period of time, the excess dye may be removed from the bore 14 by flushing the bore using, for example, the flushing system 68 described above, preferably with clean water.

The surface of the bore 14 may then be scanned at a wavelength corresponding to the absorption band of the dye which is required to induce fluorescence. The resultant fluorescence is detected using the optical receiver. 54. In this regard, the scanner head 22 may include an appropriate optical filter in front of the optical receiver 54 to reject any non-fluorescent light. The filter may be mounted, for example, on a filter wheel which is positioned in front of the optical receiver 54.

The image detected by optical receiver 54 will thus be of the imperfections in the surface of the bore 14 in which the dye penetrant is located. This information can be integrated with the image data to provide more surface detail of the bore 14 and to identify problem areas and features, such as cracks.

It should be recognized that, while the present invention has been described in relation to the preferred embodiments thereof, those skilled in the art may develop a wide variation of structural and operational details without departing from the principles of the invention. Therefore, the appended claims are to be construed to cover all equivalents falling within the true scope and spirit of the invention.

Claims

What is Claimed is:

1. An optical scanning tool for use in imaging the surface of a bore of a wellhead, the scanning tool comprising:

a base structure which is removably connectable to the wellhead; and

at least one scanner head which is mounted on the base structure; wherein after the base structure is connected to the wellhead, the scanner head scans the surface of the bore and generates image data which is representative of the surface.

2. The optical scanning tool of claim 1 , further comprising means for displacing the scanner head rotationally relative to the axial centerline of the bore.

3. The optical scanning tool of claim 2, wherein the rotational displacement means comprises a rotary drive unit.

4. The optical scanning tool of claim 3, wherein the rotary drive unit is connected to the base structure and the scanner head is connected to the rotary drive unit.

5. The optical scanning tool of claim 3, wherein the rotary drive unit is connected to the base structure and the scanner head is connected to a portion of the base structure which is rotated by the rotary drive unit.

6. The optical scanning tool of claim 2, further comprising means for measuring the rotational displacement of the scanner head relative to the axial centerline of the bore and for generating rotational displacement data.

7. The optical scanning tool of claim 6, further comprising means for processing the image data and the rotational displacement data to provide an image map which is representative of bore.

8. The optical scanning tool of claim 1 , further comprising means for displacing the scanner head axially relative to the bore.

9. The optical scanning tool of claim 8, wherein the axial displacement means comprises a linear drive unit.

10. The optical scanning tool of claim 9, wherein the linear drive unit is connected to the base structure and the scanner head is connected to the linear drive unit.

1 1 . The optical scanning tool of claim 9, wherein the linear drive unit is connected to the base structure and the scanner head is connected to a portion of the base structure which is displaced by the linear drive unit.

12. The optical scanning tool of claim 8, further comprising means for measuring the axial displacement of the scanner head relative to the axial centerline of the bore and for generating axial displacement data.

13. The optical scanning tool of claim 12, further comprising means for processing the image data and the axial displacement data to provide an image map which is representative of bore.

14. The optical scanning tool of claim 2, further comprising means for displacing the scanner head axially relative to the bore.

15. The optical scanning tool of claim 14, further comprising:

means for measuring the rotational displacement of the scanner head relative to the axial centerline of the bore and for generating rotational displacement data; and

means for measuring the axial displacement of the scanner head relative to the axial centerline of the bore and for generating axial displacement data.

16. The optical scanning tool of claim 15, further comprising means for processing the image data, the rotational displacement data and the axial displacement data to provide an image map which is representative of bore

17. The optical scanning tool of claim 1 , wherein the base structure comprises a cap for the wellhead.

18. The optical scanning tool of claim 17, wherein the wellhead is located subsea and the cap is wireline deployable from a surface vessel.

19. The optical scanning tool of claim 17, wherein the cap comprises a generally cylindrical housing which is configured to fit over the top of the wellhead and a shaft which extends axially relative to the housing.

20. The optical scanning tool of claim 19, wherein the scanner head is mounted on the shaft.

21 . The optical scanning tool of claim 20, further comprising means for displacing the scanner head rotationally relative to the axial centerline of the bore.

22. The optical scanning tool of claim 20, further comprising means for displacing the scanner head axially relative to the axial centerline of the bore.

23. The optical scanning tool of claim 1 , further comprising means for cleaning the bore.

24. The optical scanning tool of claim 23, wherein the cleaning means comprises a flushing system.

25. The optical scanning tool of claim 1 , further comprising means for injecting a dye into the bore.

26. The optical scanning tool of claim 25, wherein the scanner head emits light at a particular wavelength which causes the dye to fluoresce.

27. The optical scanning tool of claim 26, wherein the scanner head detects the particular wavelength.

28. The optical scanning tool of claim 1 , wherein the scanner head emits light at a plurality of wavelengths.

29. The optical scanning tool of claim 28, wherein the scanner head detects each of the plurality of wavelengths.