GB2503561A

GB2503561A - Plug for subsea components

Info

Publication number: GB2503561A
Application number: GB201308277A
Authority: GB
Inventors: John White
Original assignee: Verderg Connectors Ltd
Current assignee: Deepsea Technologies UK Ltd
Priority date: 2013-05-08
Filing date: 2013-05-08
Publication date: 2014-01-01
Anticipated expiration: 2033-05-08
Also published as: GB201308277D0; GB2503561A9; GB2503561B

Abstract

A plug assembly for sealing an opening formed to access an internal gallery inside a subsea component is described. The plug assembly comprises a threaded body and a sealing body. The threaded body comprises a top surface; a bottom surface; a threaded portion for engaging with the walls of the opening between the top and bottom surface; and a head connected to the top surface for engaging with a tool for screwing the plug assembly into the opening; The sealing body comprises a cylindrical section having a flange extending from the top of the cylindrical section; and a frusta-conical section extending from the base of the cylindrical section and having an recess extending within the base of the frusto-conical section. The cylindrical section comprises an annular groove for receiving at least one sealing member; and the frusto-conical section has a first taper section and a second taper section, wherein the first taper section defines an undercut between the cylindrical section and the second taper section.

Description

PLUG FOR SUBSEA COMPONENTS

Technical field

This invention relates to a sealing plug. In particular sealing plugs for access holes and the likes formed in the surface of metallic components, particularly those exposed to high pressures. In particular, the invention relates to plugs for sealing access holes created in subsea components used in the oil and gas industry.

Background art

Subsea components used in the oil and gas industry often have an internal gallery.

This internal gallery is a conduit contained inside the body of the component.

Typically, there are two parallel conduits drilled in the parent material of the component which have to be interconnected at one end. The only way to connect the two conduits is to drill a third conduit through the external surface of the component into the first two conduits, such that the third conduit intersects both of the existing conduits.

Once this third access interconnecting conduit is complete it can be necessary to block up the external access hole through which the third interconnecting conduit was drilled. This is done by permanently blanking off the access hole with a plug.

A typical procedure to seal the access hole comprises machining out the access hole to about 1 cm below the surface to create a cavity having an enlarged opening compared to the original access hole. The cavity created having a diameter greater than the ot-iginal access hole, which is now centred at the base of the cavity. The walls of the enlarged opening are threaded at an angle so that a plug in the form of a tapered bolt can be screwed into the cavity. As the plug is screwed in, a seal is created by the mating of the screw threads in the interior of the cavity with the screw threads on the exterior of the bolt. The engagement of the threads causes large local surface stress. It is this contact load within the thread that stops anything getting into or out of the access cross-drilled conduit. In some variations the plug can have a small machined hexagonal head which can be filed off after insertion in the cavity to make removal of the plug more difficult.

However, such traditional plugs used in sealing access holes in subsea components have problems, in particular, with the integrity of the seal created and ease of removal of the plug. Correct engagement depth of these plugs can be hard to verify, without gauges. This can cause a plug to be incorrectly installed and then to subsequently leak or fail under pressure.

The present invention seeks to provide a plug which provides a seal in underwater conditions, which can be easily manufactured and reduces installation error.

Disclosure of the invention

This invention provides a plug assembly for sealing an opening formed in components in high pressure applications. In particularly the invention provides a plug assembly for sealing an opening formed to access an internal gallery inside a subsea component. One such type of component is a subsea connector.

In a first aspect, the invention provides a plug assembly for sealing an opening to a passage formed in a metal component, the plug assembly comprising a sealing body and a threaded body: the threaded body having: a top surface; a bottom surface; and a threaded portion engageable with the threaded walls of the opening; and a head connected to the top surface for engaging with a tool for screwing the plug assembly into the opening; and the sealing body comprising: a cylindrical section having a flange extending from the top of the cylindrical section and an annular groove for receiving at least one sealing member; and a frusto-conical section extending from the base of the cylindrical section and having an recess extending within the base of the frusto-conical section; wherein the frusto-conical section comprises a first taper section and a second taper region, wherein the first taper section defines an undercut between the cylindrical section and the second taper section.

The sealing body being configured such that in use a clearance is maintained between the surface defining the undercut and the walls of the component in which the plug assembly is inserted.

In a further embodiment of the invention the threaded body and the sealing body are configured such that during installation, the sealing body does not rotate when driven axially by the rotation of the threaded body into the opening. The outer surface of the cylindrical section and the outer surface of the second taper section provide sealing surfaces which engage with the surface of the passage being sealed. Rotation of the sealing body during installation could create scores on the sealing surfaces if any foreign particles are present in the area where the plug assembly is to be filled. Scouring on the sealing surfaces may compromise the seal. Therefore reducing the rotation of the sealing body assists in enhancing the integrity of the seal formed between the component and the sealing body.

Rotation can be transmitted through to the sealing body as a side effect of friction during installation. To counter unwanted rotation the effects of friction induced rotation are controlled.

In order to prevent the rotation of the sealing body, the surface friction on the interface surfaces between the component and sealing body, and/or between the sealing body and the threaded body is controlled. Surface friction can be controlled by for example the provisions of suitable coatings on at least some of the interface surfaces of the threaded body and sealing body.

In a further embodiment of the invention the threaded body and the sealing body are configured such that during installation the gripping force between the sealing body and the component is greater than the gripping force between the sealing body and the threaded body.

Preferably the gripping force is at least two times, more preferably at least three times greater between the sealing body and the component than between the sealing body and the threaded body. Preferably the gripping force is measured as the gripping force between the interface between the frusto-conical section and the component and the gripping force between the bottom surface of the threaded body and the top surface of the flange.

Having a greater gripping force between the sealing body and the component, than between the sealing body and the threaded body, assists in preventing rotation being transmitted to the sealing body when the threaded body is rotated during installation of the plug assembly.

A recess extends into the base of the sealing body. Typically the term "recess" means a cavity, slot, blind hole, internal bore and/or the like that creates a depression in the bottom surface of the sealing body and extends into the interior of the sealing body.

The recess permits pressure energising of the plug assembly. This assists in obtaining a tight fit between the sealing surfaces of the sealing body and the surfaces of the passage of the component in which the plug assembly is installed.

The base of the sealing body can comprise a generally circular rim defining the opening of the recess. The walls of the recess extend axially within the interior of the frusto-conical section. The recess can be an inner cavity which extends from the inner edge of the rim into the frusto-conical section towards a closed end, defining the roof of the recess. The recess extends partially along the length of the frusto-conical section. The roof of the recess can be flat or curved.

In a further embodiment of the invention a protrusion extends down from the roof into the recess towards the opening.

A substantially frusto-conical portion can extend down from the central of the roof so as to define a substantially M shaped recess. The recess does not need to be Ni-shaped any suitably shaped protrusion from the roof of the recess can also be used, i.e. a frusto-conical or hemispherical protrusions.

The flange provides a flat planar surface at the top of the cylindrical section having an increased diameter compared to the diameter of the bottom section of the cylindrical section. The flange provides a stop surface to the plug assembly and assists in the correct positioning of the plug assembly in the access hole. In use, the flange provides a displacement stop against axial displacement provided by the threaded body during installation and prevents the sealing body entering further into the conduit.

The flange has a diameter greater than the diameter of the passage being sealed and in use sits against the bottom surface of the enlarged cavity.

An annular groove is provided in the surface of the cylindrical section. The term "annular groove" typically means an elongated recessed area that extends circumferentially around the outer surface of the cylindrical section. The groove is sized to accommodate at least one sealing member.

In a further embodiment of the invention the annular groove is adapted for receiving at least two sealing members.

The groove can have straight and/or angled side walls extending towards the base of the groove. Preferably the base of the groove has a greater width than the opening of the groove.

In one embodiment the top wall of the groove is substantially perpendicular to the central axis of the plug assembly and the bottom wall extends at an angle down from the opening, such that the opening of the groove is narrower than the base of the groove.

In a further embodiment both the top and bottom walls extend at angle away from the groove opening. The top wall extends at an angle up from opening and the bottom wall extends at an angle down from the opening such that the base of the grooves is wider than the opening of the groove. The angle of deflection of the walls can be the same or different.

In a further embodiment of the invention the cylindrical section can comprise two substantially circular collars. The groove can be interdisposed between the two substantially circular collars. The first circular collar is adjacent the flange. The second collar is located between the groove and the frusto-conical section. The collars define the outer surface of the cylindrical section.

In a further embodiment of the invention a first sealing member can be retained in the groove. The first sealing member can be an elastomeric seal, such as an 0-ring.

However the seal can be formed fiom any suitable material or combination of materials.

Other seals that are capable of providing a good seal with the surfaces of the component can also be used.

A second sealing member can also be retained in the groove. The second sealing member can be a back-up ring. The backup rings may be composed of high-strength thermoplastic materials such as a polyether ether ketone (PEEK) based material or elastomeric materials such as nitrile butadiene rubber (NBR). However rings made from other suitable materials can also be used. The second sealing member acts as a backup ring and helps retain the 0-ring within the recess when the plug assembly is under high pressure.

The sealing body comprise a frusto-conical section. In other words a section that axially conically tapers to form a generally frusto-conical shape, having a generally parallel top and bottom plane. The larger diameter end of which is adjacent the base of the cylindrical section. The smaller diameter end forming the base of the sealing body.

The frusto-conical section can comprise a first taper section and second taper section. The larger diameter end of the first taper section is adjacent the base of the cylindrical section and the smaller diameter end of the first taper section ends at a shoulder formed by the base of the larger diameter end of the second taper section.

The smaller diameter end of the second taper section ends at the base of the frusto-conical section.

The frusto-conical section comprises an undercut region adjacent the cylindrical section. By undercut" it typically means that an annular groove or recess in the surface of the frusto-conical section that extends circumferentially around the wall of the frusto-conical section.

In one embodiment of the invention the undercut is defined by the first taper section.

The first taper section defines an undercut region between the cylindrical section and the second taper section. The presence of an undercut reduces the surface to surface contact between the plug assembly and the component to be sealed.

The angle of the first and second taper sections is preferably in the range of 20-35° with respect to the central vertical axis of the plug assembly. Preferably, the angle is about 23°. Preferably the angles of the first and second taper sections are substantially the same.

The threaded body has a screw thread provided on the outer surface between the top surface and the bottom surface of the body. The thread is configured to couple with a matching thread made in the cavity which is to receive the plug assembly.

In one embodiment the thread is disposed about the substantially circular body such that a bevelled edged is provided towards the bottom surface of the threaded body.

A shearable head extends from the top surface of the threaded body. In an embodiment of the invention the head is shearable from the threaded body at a predetermined torque, such that when torque is applied the threaded body can be screwed into the opening and wherein the torque applied exceeds the predetermined torque level the head of the plug shears off at a pre-determined breakage point.

The head can be connected to the threaded body by a portion weaker than the body or the head such that on screwing the plug assembly into the opening the head will shear off the top surface at the predetermined breakage point once a per-determined level of torque is exceeded. The pre-determined torque level is selected to be sufficient to ensure correct sealing whilst minimising any damage to the plug assembly.

The head can be integrally connected to the body via a shank. In one embodiment the shank can be provided with at least one weakened point to form the predetermined breakage point to encourage the shank to shear at those points. Typically those points are at the base of the shank, such that when the head is sheared the threaded body of the plug assembly is left with a substantially flat top surface.

In one embodiment the shank tapers, such that the base of the shank has a smaller diameter than the top of the shank. Shearing of the head off the top surface of the threaded body after installation helps inhibit subsequent removal or loosening of the plug.

The head is shaped to engage with conventional torque applying tools to screw the plug assembly into the opening. Preferably a non-circular shaped head is used, such as a hexagonal shaped head. Other polygonal shaped heads can also be used.

The shear torque at which the shank is designed to shear is defined by the outside diameter of the shank, the height of the shank and the material from which the shank is manufactured.

The plug can be screwed in with a torque wrench set at a predetermined torque value which is sufficient to ensure the integrity of the seal and also shear off the head without damage to the plug. The torque required to shear off the head is preferably greater than llONm. Preferably the torque required to shear off the head is about ll7Nm. The torque level is selected to ensure that the plug assembly's preload is greater that the axial force generated by the internal pressure in the conduit to be sealed. In one embodiment the head and shank are configured to shear off the top surface of the threaded portion ata torque greater than llONm The threaded body and the sealing section can be manufactured from the same or different materials. Preferably the threaded body and the sealing body are manufactured from different metallic materials.

In a further embodiment the sealing body is formed from a material softer than the material of the component being sealed. Preferably the sealing body is manufactured from a softer material than the material from which the threaded body is formed from.

Using a softer material for the sealing body reduces the risk of damage to the component being sealed.

A preferred material for the sealing body is a nickel-chromium alloy. The threaded body can be manufactured from the same material as the component which is being sealed. Typically, this material would be steel. In one embodiment the threaded body can be made from steel and the sealing body can be made from a nickel-chromium alloy. Other suitable combinations of materials can also be used.

The threaded body and sealing body of the plug assembly can be manufactured from solid circular blanks. The head and thread being machined into a blank and an undercut shank machined away to a predetermined size and shape to form the threaded body. The plug assembly is dimensioned to fit within the formed cavity and access hole in the component.

The sealing body is dimensioned such that that undercut region can not engage with the tapered walls of the passage in which the sealing body is inserted. In other words a clearance is maintained between the wall defining the first taper section of the frusto-conical section of the sealing body and the walls of the tapered hole of the subsea component in which the plug assembly is fitted.

In a further embodiment of the invention the plug assembly comprise a low coefficient of friction coating. The threaded body and the sealing body can both be coated with a low coefficient of friction coating. Providing the threaded body with a low friction coating helps reduce the maximum torque required to generate the required seal and reduces the rotational drive transmitted to the sealing body during installation.

Any suitable low coefficient coating can be used for example a PTFE based coating, such as Xylan, or a graphite coating. Other suitable coatings can also be used.

Preferably friction co-efficients of less than 0.18 are obtained with the coating used.

More preferably a co-efficient of 0.04 is obtained.

In a furthei embodiment different low coefficient of friction coatings can be used on the sealing body and on the threaded body. Preferably one coating is an electrically conductive coating and the other coating is an electrically non-conductive coating.

A low co-efficient conductive coating can be used on the threaded body. The coating can be applied to a portion of the threaded body. Preferably the conductive coating is applied to at least the surface of the screw threads. Preferably the conductive coating is a graphite based coating, however other suitable low coefficient coatings can also be used.

Using a conductive coating helps maintain electrical continuity through the plug assembly and subsea component. Maintaining electrical continuity through the assembly can be helpful when the plug assembly is used on subsea components with a cathodic protection system. The use of a conductive coating assists in limiting the corrosion of the plug assembly.

A low co-efficient non-conductive coating can be used on the sealing body. The coating is applied to a portion of the sealing body. The coating on the sealing body can be applied to at least the outer cylindrical surfaces of the cylindrical section and the frusto-conical section. In a further embodiment of the invention the top surface of the flange is uncoated. Optionally the lower surface of the flange is also uncoated.

Leaving the top surface of the flange uncoated allows electrical continuity from the threaded body to the plug. Leaving the lower surface uncoated further helps maintain electrical continuity to the subsea component.

By "uncoated" it is mean a region of the plug assembly, where the low coefficient coating has not been applied to the surface of the plug assembly.

The elastorneric seal and back-up ring are to be fitted to the sealing body of the machined plug assembly, to form a complete plug assembly for installing into an access opening formed in a subsea component to seal a conduit.

To insert the plug assembly into the opening, the plug assembly is inserted into the prepared opening. A section of the component is machined out from the surface of the component to form a larger diameter section compared to the diameter of the passage being sealed. This provides a cavity in which the threaded body can be inserted and forms an outwardly facing annular shoulder upon which the flange of the sealing body can sit. The walls of the cavity are tapped to form screw threads corresponding to the screw threads of the threaded body.

A further section is formed down from the base of the cavity, having a diameter smaller than the diameter of the cavity and larger than the diameter of the passage, such that the section has a diameter which enables the sealing surfaces and sealing members of the sealing body to engage the surface of the component. This further section can taper at its lower end. The taper corresponds to the taper on the second taper section of the sealing body, to provide a surface against which the second taper section can engage and form a seal.

The sealing body slides axially into opening by axial displacement provided by the rotation of the threaded body, until the threaded body is stopped by the flange of the sealing body which rests on the base of the cavity. In other words the sealing body is driven axially by rotation of the threaded body in contact with the sealing body. As the gripping force between the base of the threaded body and the top surface of the sealing body is less than the gripping force between the sealing surfaces of the sealing body and the walls of the opening the sealing plug does not rotate as it is driven axially into the opening by the rotating threaded body.

Displacement of the sealing body before the stop point provides the required level of preload to the sealing body to provide adequate contact stress on the sealing surface of the subsea component and the plug assembly. Once the flange reaches the base of the cavity any torque applied to the plug assembly after this point provides a level of preload. The torque applied to the plug assembly is increased causing the head and shank to shear off the top of the threaded body.

The sealing body provides a floating component to the plug assembly such that in use the sealing body can find the best position to achieve a good seal with the subsea component. By having the plug assembly comprise two separate components the tolerances critical for sealing are included in the sealing body. This means that when manufacturing the plug assembly it is not necessary to hold very tight tolerances in the threaded body. The plastic deformity of the sealing members of the sealing body allows reasonable tolerances when creating the enlarged opening in which the plug assembly is fitted. This reduces the likelihood of the structure of the component being compromised when producing the enlarged opening to insert the plug assembly.

The plug assembly is particularly suitable for use in components used in the oil and gas industry. More preferably those components used in a subsea environment, in water depths between Om to 5000rn.

The plug assembly is also suitable for use in sealing openings under high pressure, for example where the internal pressure in the passage being sealed is up to at least 1500 bar (150,000kPa).

Brief description of the drawings

The invention will now be described by way of example with reference to the accompanying drawings: Figure 1 shows a cross sectional side view of an embodiment of the plug assembly in accordance with the invention inserted into an opening formed in a subsea component; Figure 2 shows a cross sectional side view of an embodiment of the plug assembly in accordance with the invention after the plug has been inserted into an opening and the head has been sheared from the plug assembly; Figure 3 is a partially enlarged view of part of the sealing body of the plug assembly; Figure 4 shows a cross sectional view of the plug assembly in accordance with the invention; Figure 5 shows a cross sectional view of an embodiment of the plug assembly in accordance with the invention; Figure 6 is a side view of an embodiment of the plug assembly in accordance with the invention; Figure 7 is a top view of the plug assembly of figure 6; and Figure 8 is a bottom view of the plug assembly of figure 6.

Detailed Description of the invention

Referring to the drawings, Figure 1 shows a plug assembly 10 inserted into an enlarged opening. The plug assembly plugs the access opening of a passage 12 formed, for example, in a subsea component 100 to access an internal gallery (not shown). The plug assembly prevents fluids from inside the internal galleries reaching the external environment and prevents external environmental fluids from entering the cross-drilled passage.

For the purpose of clarity the plug assembly will be described with reference to its normal installation in an access hole with the portion of the plug assembly first entering the hole considered the bottom or lower end and the opposite end considered the top or higher end.

With reference to Figures 1, 2, 3 and 4 the plug assembly 10 comprises two main components a threaded body section 14 and a sealing body section 16.

The threaded body section 14 comprises a substantially cylindrical body having a head 18 integrally connected to the top surface 26 of the threaded body section 14 via a shank 20. The body is a solid body with the outer surface provided with an external helical screw thread 22 between the top surface 26 and the bottom surface 62. The screw thread 22 is formed to engage with a corresponding thread located on the walls of the cavity 24. The cavity is created to enlarge the opening in the subsea component.

The threaded body 14 comprises a hexagonal head 18 connected integrally to the body 14 via the shank 20. The head 18 is shearable from the body 14 when torque is applied to the head and a predetermined level of torque is exceeded. The head 18 is configured such that a tool, such as a torque wrench, not shown, can engage the head rotating the threaded body to screw the threaded body into the formed cavity.

The shank 20 comprises a weakened portion to encourage the head to shear at this particular location. The shank 20 is tapered from the base of the head 18 towards the top surface 26 of the threaded body 14, such that the diameter of the top of the shank is greater than the diameter of the bottom of the shank.

As illustrated in Figure 2 the head is shearable from the body 14 at the base of the shank such that a substantially planar top surface of the threaded body 14, perpendicular to the longitudinal axis of the plug assembly, remains when the shank and head 18 has been removed from the plug assembly. This helps prevents subsequent removal or loosening of the plug assembly from the cavity and thereby assists in maintaining the integrity of the seal.

With reference to Figures 1, 2 and 4 the sealing body 16 comprises a substantially cylindrical section 30 having a flange 28 and a frusto-conical section 32. A substantially circular flange part 28 extends radially from the top edge of the cylindrical section 30. The diameter of the flange 28 is greater than the diameter of the substantially circular collars 30a, 30b defining the upper and lower regions of the cylindrical section 30, but the diameter of the flange 28 is smaller than the diameter of the threaded body 14. The diameter of the flange will be greater than the access hole to the conduit being plugged such that in use the flange will sit on the base of the cavity 24 formed in the connecter. The flange provides a means of displacement control for the threaded body, to limit the depth that threaded body can be screwed into the cavity.

Having the diameter of the flange smaller than the diameter of the threaded body results in a space in the cavity 24 in which oil and/or other residues, which are swept down towards the base of the cavity, can be accommodated when the plug assembly is inserted. By accommodating debris in the space, the debris is kept from interfering with the seal formed between the sealing surfaces of the plug assembly and the walls of the subsea component. This assists in a fluid tight seal being formed between the plug assembly and the subsea component.

A groove 34 is provided in the cylindrical section 30 of the sealing body 16. The annular groove 34 extends around the circumference of the cylindrical section for retaining at least one sealing member. The groove 34 is interdisposed between two substantially circular collars 30a, 30b, one of which is adjacent the flange 28, and the other defines the lower region of the cylindrical section 30 adjacent the frusto-conical section 32. The presence of the top collar 30a provides a stepped region between the groove 34 and the flange 28.

With reference to Figure 4 the opening of the groove 34 is narrower than the base of the groove. The top side wall of the groove extends substantially perpendicular to the longitudinal axis of the plug assembly and the bottom side wall extends at an angle down from the opening.

With reference to Figures 1 and 2 the groove 34 retains a first sealing member 36 and a second sealing member 38. The first sealing member 36 is an elastomeric 0-ring.

The 0-ring assists in resisting any external hydrostatic pressure. The second sealing member 38 is a backup ring. The backup ring helps retain the 0-ring within the groove 34 when the plug assembly is under high pressure.

The frusto-conical section 32 comprises a first taper section 40 and second taper section 42 such that the first taper section 40 defines an undercut region 44 between the cylindrical section 30 and the second taper section 42. The undercut 44 reduces the contact profile between the plug assembly and the subsea component.

With reference to Figure 3, in use the undercut region 44 provides a gap between the sealing body and the wall of the component 100, the plug assembly has been inserted into.

The walls of the first conical taper section 40 extend at an inward angle down from the base of the cylindrical section 30 towards the central axis of the plug assembly 10.

The larger diameter end of the first taper section 40 is adjacent the lower section of the cylindrical section 30, the smaller end of which ends at a shoulder formed by the top surface of the second taper section. The top surface of the second taper section 42 extends out from the base of the first conical taper section 40 at a distance such that base of the first conical taper section has a smaller diameter than the top of the second conical taper section 42. The top surface of the second taper section 42 defines the base of the undercut region. The walls of the second taper section 42 extend at an inward angle down from the edge of its top surface towards the central axis of the plug assembly to a base surface of the frusto-conical section 32. The larger diameter end of the second taper section 42 being adjacent the smaller diameter end of the first taper section and the smaller diameter end forms the base surface of the frusto-conical section 32. The base surface of the frusto-conical portion comprises a substantially circular rim 46 surrounding a recess 48 extending into the frusto-conical section 32.

A substantially "M" shaped recess 48 with a substantially frusto-conical protrusion 50 extending out from the roof 52 of the recess 48 is provided in the sealing body 16.

The walls of the recess 48 extend upwardly from the inward edge region of the rim 46 substantially parallel to the central axis of the plug assembly to a roof surface. Inward from the walls of the recess the surface of the roof 52 extends downwards, at an angle to the central axis, to a central planar section, such that a central frusto-conical protrusion 50 is defined. The protrusion 50 extends partially into the recess with the height (Xl) of the protrusion 50 being shorter, than the depth (Yl) of the recess 48.

With reference to Figure 5, in one particular embodiment of the invention a low co-efficient of friction coating is provided on at least some of the surfaces of the sealing body and threaded body. A first coating 54 is provided on the surface of the threaded body. The first coating is a conductive coating and allows electrical continuity through the plug assembly and the subsea component. A second coating 56 is provided on at least some of the surfaces of the sealing body. The second coating is a nonconductive coating. The top surface 58 of the flange 28 is not coated with the non-conductive coating. The bottom surface 60 of the flange is also not coated with the non-conductive coating. Leaving these regions of the sealing body uncoated assists in maintaining electrical continuity with the threaded body and subsea component to the sealing body.

With reference to Figures 6 to 8, in one particular embodiment of the invention a plug assembly 110 has a threaded body 114 having a helical thread 122 with a diameter (Z1) of approximately 27mm. A head 118 extends from the top surface 126 of the body 114 via a shank 120. The shank can have a height (X2) of about 5mm.

The sealing body 116 of the plug assembly 110 comprises a frusto-conical section 132 wherein the outer side walls of the frusto-conical section have a tapering angle (a) of approximately 23° with respect to the central vertical axis (A) of the plug assembly 110.

The side wall of the frusto-conical section 132 includes an undercut region 144 to reduce the contact area between plug assembly 110 and the subsea component it is inserted in.

The undercut region 144 is provided by the frusto-conical section 132 having a first tapered section 140 and a second tapered section 142. The second taper section 142 extends towards the base surface 146 of the frusto-conical section 132. A recess 148 extends into the interior of the frusto-conical section from the base surface 146. A central protrusion 150 extends down from the roof of the recess. The cylindrical section 130 of the sealing body 116 has a diameter (72) of about 18mm and the sealing body 116 has a height (X3) of about 13mm. A flange 128 extends from the top surface of the cylindrical section 130 having a diameter (Z3) greater than the diameter of the central cylindrical section.

A groove 134 encircles the cylindrical section 130 of the plug and is dimensioned to retain an elastomeric 0-ring seal and a back up ring. The groove has a depth of about 1.2mm and a width (Wi) of about 3.5mm.

The threaded body can be formed from a high yield steel and the sealing body from a nickel chromium alloy, such as Alloy 625. The plug assembly 110 is provided with low coefficient coating (not shown) on at least some of the outer surfaces of the assembly.

The threaded body is provided with a low coefficient of friction coating, such as a electrically conductive graphite based coating. The sealing body is provided with a low co-efficient nonconductive PFTE coating on at least the outer surface of the cylindrical section and frusto-conical section. The top and bottom surfaces of the flange remain uncoated.

Whilst Figures 6 to 8 are described with reference to one particular plug suitable for insertion into an opening having 12mm diameter, plug assemblies dimensioned to fit other sized openings are also contemplated.

After the internal galleries or enclosed passages in a subsea component have been connected, the interconnecting passage formed in the component to provide the access to the internal galleries is required to be sealed. The access opening of the passage is further machined to enlarge the opening at the surface of the component.

The opening of the passage is machined out below the surface, to enlarge the opening of the passage. This creates an enlarged open cavity in the surface of the component and creates a cavity into which the plug assembly is to be installed. The base of the cavity forms an outwardly facing annular shoulder upon which the flange of the sealing body can sit.

At the base of the cavity a second enlarged section leading to the drilled interconnecting passage is provided. The second section has a first region having walls substantially parallel to the central axis of the passage with a diameter smaller than the threaded section and a second taper region extending to the drilled interconnecting passage. The walls of the taper region extend at an inward angle towards the central axis of the interconnecting passage. The original interconnecting passage drilled extends from the end of the taper region. The diameter of the second section is such to enable the sealing surfaces and sealing members of the sealing body to engage with the surface of the component. The taper of the section corresponds to the taper on the second taper section of the sealing body, to provide a surface against which the second taper section can engage and form a seal.

The internal walls of the cavity are threaded and the base of the cavity provides a planar stop surface to control the plug assembly displacement and to provide a means of applying a large preload. The walls of the second section define a sealing surface area against which the sealing members and sealing surface of the sealing body can engage, and in which the undercut region of the frusto-conical section does not engage. By providing an undercut region, such that there is a region on the outer surface of the sealing body which does not contact the sealing surface of the passage a good seal can be maintained, whilst still generating the required contact pressure. This can also result in the tolerances in the manufacture of the plug assembly being less critical.

The plug assembly is installed into the enlarged opening by placing the sealing body into of the enlarged opening, followed by the threaded body. A wrench engages the head and the threaded body is rotated within the cavity. The rotation of the threaded body 14 drives the sealing body 16 axially into the opening of the passage 12 without rotation of the sealing body. The sealing body 16 is driven into the opening until the bottom surface of the flange 28 engages the base 64 of the cavity 24. This enables both the sealing body and the threaded body to form a fluid tight seal with the walls of the cavity and the opening. Once a fluid tight seal is formed, the torque applied to the head can be increased above a pre-determined level. This causes the head to shear from the top surface of the plug, and leaves a plug inserted into the subsea component which cannot be easily removed.

The sealing body floats relative to the threaded body and this enables the sealing body to find the best position when it is set. The sealing body plastically deforms in use, which allows for reasonable manufacturing tolerances when producing the plug assembly. This plastic deformation is beneficial in providing a robust metal seal.

The above embodiment is described by way of example only. Other variation can be made to the plug without departing from the scope of the invention. Whilst the plug assembly is described with reference for sealing an opening created in a subsea component, a plug assembly according to the invention can also be used for sealing other metallic components, in particular where the plug would be exposed to high internal pressure in the passage being sealed.

Claims

Claims 1. A plug assembly for sealing an opening formed in a metal component, the plug assembly comprising a threaded body and a sealing body: the threaded body comprising: a top surface; a bottom surface; a threaded portion for engageable with the threaded walls of the opening; and a head connected to the top surface for engaging with a tool for inserting the plug assembly into the opening; and the sealing body comprising: a cylindrical section having a flange extending from the top of the cylindrical section and an annular groove for receiving at least one sealing member and a frusto-conical section extending from the base of the cylindrical section and having an recess extending within the base of the frusto-conical section; wherein the frusto-conical section comprising a first taper section and a second taper section, wherein the first taper section defines an undercut between the cylindrical section and the second taper section.
2. A plug assembly according to claim 1 wherein the threaded body and the sealing body are configured such that during installation, the sealing body does not rotate when driven axially by the rotation of the threaded body into the opening.
3. A plug assembly according to claim 2 wherein the threaded body and the sealing body are configured such that in use the grip between the sealing body and the component is at least three times greater than the grip between the sealing body and the threaded body.
4. A plug assembly according to any one of claims I to 3 wherein the annular groove is adapted for receiving at least two sealing members.
5. A plug assembly according to any one of claims 1 to 4 further comprising a first sealing member retained in the groove, wherein the first outer sealing member is an elastomeric seal.
6. A plug assembly according to claim 5 wherein the first sealing member is an 0-ring.
7. A plug assembly according to any one of claims 1 to 6 wherein the cylindrical section comprises two cylindrical collars between which the groove is disposed.
8. A plug assembly according to anyone of claims I to 7 wherein the head is integrally connected to the body by a shank.
9. A plug assembly according to anyone of claims 1 to 8 wherein the head is shearable from the threaded body at a predetermined torque, such that when torque is applied the threaded body can be screwed into the opening and wherein the torque applied exceeds the predetermined torque level the head of the plug shears off.
10. A plug assembly according to anyone of claims ito 9 wherein the head is hexagonal shaped.
ii. A plug assembly according to any one of claims 1 to 10 comprising a protrusion extending down from the roof of the recess.
12. A plug assembly according to any one of claims Ito ii wherein the sealing body is formed from a material softer than the material of the component being sealed and the material of the threaded body.
13. A plug assembly according to any one of claims ito 12 wherein the threaded body is made from steel and the sealing body is made from a nickel-chromium alloy.
14. A plug assembly according to any one of claims ito 13 further comprising a low coefficient of friction coating.
15. A plug assembly according to claims 14 wherein the threaded body and the sealing body comprise different low coefficient of friction coatings.
16. A plug assembly according to claim 14 or 15 wherein the coating on the threaded body is a conductive coating.
17. A plug assembly according to any one of claim 14 to 15 wherein the coating on the sealing body is a non-conductive coating.
18. A plug assembly according to any one of claims 14 to 17 wherein the top surface of the flange is uncoated.
19. A plug assembly substantially as herein described with reference to the drawings.