Casing, tubing strings locked into wellhead with friction grip device

The system is applicable in surface adjustable wellheads and deepwater dry-tree systems, and in deepwater connection of BOP's and christmas trees to subsea structures

A method of supporting and connecting casing and tubing hangers within wellhead housings using a friction holding system known as POS-GRIP has been developed by UK based Plexus Ocean Systems Limited, as an alternative to the present means of landing strings.

Also, the method can be used in the design of high capacity, low stress hydraulic connectors for use on subsea and to connect deepwater riser joints. The system uses the tried and tested principal of elastically deforming a tubular body onto an inner member with external force, so as to create a reversible friction-hold. In wellhead applications the system is used to:

• Support casing or tubing load
• Lock casing or tubing hangers down
• Remove eccentricity between the wellhead bore and internal hangers
• Isolate dynamic loads from seal areas
• Connect well control components to subsea structures
• Activate high quality seals on the extremities of concentric components.

The use of wedge and taper interface techniques to provide a friction lock is not a new concept. Ship and airplane propellers, and heavy industrial gears have been connected to solid and hollow shafts in this manner for years. Casing slips used in conventional wellhead technology make use of similar principles, although the POS-GRIP system is externally activated and precisely controllable. Where this system really differs from casing slips is that no components need to be disassembled during the drilling of a well.

Activation of the system is achieved by the hydraulic, or mechanical movement of flanges external to the wellhead, resulting in a wellhead concept which is inherently quicker to install and safer to operate. After some years of development by Plexus Ocean Systems Ltd., this concept is now a field proven product with a wide ranging advantages and applications.

Advantages and Capabilities

This friction-grip concept offers the flexibility of slip and seal systems, and the dependability, time-saving and safety features of mandrel hanger technology.

In a surface wellhead, the mechanism's design enables a casing hanger to be landed within a space-out range, rather than at an exact point on a landing shoulder. This also means that a casing or tubing hanger can be passed through the wellhead and then pulled back up into the hang-off range under precise tension. When the mechanism is activated, the hanger is firmly locked in position. This action not only lands and locks down the hanger, but also forces it exactly parallel and concentric within the wellhead. The pack-off can then be set in a perfect annulus, which is isolated from movement throughout field life.

The unique capabilities of a friction-hold support method for casing and tubing hangers enable:

• Casing hangers to be affixed within the wellhead bore over a pre-determinable space-out range
• Casing hangers and tubing hangers to pass their respective landing locations in the wellhead, thereby facilitating accurate and unlimited control over casing tension
• External activation of the friction grip mechanism using either mechanics or hydraulic power for convenience and to eliminate the need to lift the blowout preventers (BOPs)
• Loading of casing and tubing hangers beyond casing weight, and in excess of the ultimate capacity of conventional shoulder technology
• Wellheads to be designed without landing shoulders for slimhole drilling applications and unlimited tensioning of casing
• Installation of annular seals in perfect seal pockets, facilitating the use of "metal-to-metal" seal technology
• Isolation of the hanger seal area from the dynamic forces imparted on a surface wellhead and casing strings.

The method can also has ad vantages in:

• Remote connection of deepwater blowout preventers and christmas trees to subsea structures, providing extreme capacity in tensile and bending strengths, while stress concentrations kept lower than any alternative method
• Design of a low stress riser connector for deepwater applications.

Applications

A number of applications have been identified where a friction-grip mechanism can offer significant advantages over current systems:

• Adjustable jack-up drilling wellheads
• Adjustable wellheads for tie-back to mudline hangers
• Adjustable wellheads for tie-back to subsea systems
• Platform wellheads for lightweight structures
• Tensioned casing hanger systems
• Tensioned tubing hanger systems
• Slim-hole drilling wellhead systems
• Specialized tubing hanger seal systems
• Deepwater dry-tree drilling and completion systems
• TLP tensioner systems
• Deepwater workover wellheads
• Low stress riser connectors
• Jackup drilling: In this application, a friction-grip mechanism casing hanger system is used to conveniently space out the surface casing hanger over a predetermined range in the surface wellhead after casing is landed at the mudline. Once the surface hanger is located in the wellhead, hydraulic bolt tensioners on the wellhead flange are used to activate friction-grip element. In the field, this procedure is achieved in under 20 minutes. The system ensures that the surface hanger is supported, locked down, held in torsion and be truly centralized in the wellhead bore. The annular seal can be readily and reliably installed at low torque. The wellhead application achieves time savings since the entire procedure is carried out without lifting the blowout preventer. The system, which is fully re-usable, has an estimated life cycle of 20 wells, resulting in lower rental costs. During May 1999 and February 2000, Talisman drilled two wells in the North Sea using these systems prepared for subsea crossover equipment. The POS-GRIP wellhead system was selected for use at the surface. Full stack-up tests were independently witnessed and the wellhead systems functioned as intended.
• Tieback production: In tieback applications, either to mudline systems or subsea wellheads, the friction system provides space-out capability for the surface hanger of the inner riser strings. The friction hold method allows precise tensioning of the tieback risers and uniquely eliminates eccentricity from the annulus, thus providing the perfect environment for the installation of long life seals, either metal to metal, or resilient.
• Deepwater dry-tree drilling, completion, and production systems: One of the major difficulties in designing wellhead equipment for deepwater dry-tree applications, such as those used on TLPs, spars, and FDPSOs, is to isolate the seals from the perpetual motion of the rig. Additionally, wellheads need a large range of adjustability to accommodate the space-out error between the landing point of a riser at the ocean floor and the surface wellhead. The unique shrink-to-fit aspect of the concept solves both of these issues in a very neat and unique way. A long surface hanger is used and can be gripped anywhere along its length to provide the large space-out range. Once activated, the contact forces between the production casing hanger and wellhead housing are such that the hanger is effectively welded inside the wellbore. Dynamics and thermal effects in the casing strings are thus completely isolated from the seal areas in the wellhead, resulting in a very reliable metal-to-metal annular sealing. The BOPs remain in-situ throughout the installation process.
• Tubing head for deepwater workover risers: Workover risers need space-out and tensioning capability between the outer environmental riser and the inner tieback tubing. This tubing hanger design allows the hanger body to pass the landing location during tieback, to be tensioned subsequently prior to hydraulic activation of the grip force.

This procedure can be effected through the BOPs.

• Tensioned casing hangers: The friction-grip casing hanger support method allows casing to be cemented, after which tension, beyond casing weight, can be applied, without having to lift the BOPs.
• Tensioned tubing hangers: The friction-hold support method allows tubing hangers on production wells to be set under accurate tension, without lifting the BOPs.
• Externally activated metal-to-metal bore seals: In tubing hanger applications, it is desirable to effect metal sealing in the wellhead bore. The friction-hold method can effectively bring the wellhead bore to the hanger and so provide a unique and superior method of loading replaceable metal-to-metal seals in inaccessible locations, such as the lower seals on cantilever tubing hangers.
• High bending moment resistant wellhead connectors: A low stress wellhead connector using the friction-grip principle has been developed for subsea and HP/HT applications.

Design principle

The system load bearing capability is calculated by multiplying the friction interface area, A, with the contact stress, s, and the friction coefficient, m. The interface area is a function of the wellhead internal diameter, which is limited by operational constraints, and the height of the friction element.

The potential of this friction-hold system is in reducing the complexity of "through-the-BOP" wellhead systems and increasing seal reliability.

The contact stress allowed is limited to the collapse pressure rating of the inner hanger body. A safety factor of two was applied for the purpose of calculation of the capacity ratings as listed in the table below.

The friction coefficient is dependent on surface finish and preparation of the hanger body and the friction element. Standard finishes usually suffice to provide the required capacity, however, proprietary coatings can be used to further increase the rating if necessary.

The friction value used to calculate the capacities in the table below is m = 1.2. Laboratory tests have confirmed this value to be the best representative value. For comparison purposes, other friction coefficient values are:

(equivalent coefficient)

The POS-GRIP system relies on the dimensional stability of external components, which induce an interface friction grip used to support casing loads. It would appear that under extreme conditions, a state can be reached at which slippage of the gripped hanger might occur. This issue is addressed at the detailed design stage of each product. However, the general principle used to address these issues is to design a system where the load capability remains above the required capacity even in the most extreme circumstances. As the capacity table shows, only moderate interface stresses need to be generated to provide the system with load capacities far in excess of those required.