Mechanical Connectors A. Gledhill and B. Hart,

Cameron Iron Works

INTRODUCTION

Mechanical connectors are present in many different areas of subsea oilfield equipment ranging from drilling BOP stacks through Christ- mas trees and flowlines to the latest concepts in modular comple- tions.

They perform a very important interface and absolute integrity is required to prevent pollution. This is particularly so with their extensive use in modular template completions when many more such connectors are used than would be the case with more traditional methods.

PURELY DIVER-INSTALLED CONNECTOR

In the case where diver involvement is acceptable, a very cost-effective and reliable type of connector is that of the type shown in Fig. 1. This uses individual tie-down screws which force

Advances in Underwater Technology, Ocean Science and Offshore Engineering, Volume 10: Modular Subsea Production Systems 0 Society for Underwater Technology (Graham & Trotman, 1987)

ADVANCES IN UNDERWATER TECHNOLOGY

Fig. 1 Mechan~cal wellhead connector

wedge-shaped segments radially inwards against the tapered shoul- der of the wellhead housing. Its mam advantages are low initial cost and reliability with no hydraulic components to denature after prolonged installation. However, it is labour intensive and obviously not suited to use m deep water and requlres longer to make up tight than hydrauhcally actuated types.

WELLHEAD HYDRAULIC CONNECTORS

A type of connector using hydraulically actuated collet fingers is shown in Fig. 2. In this design, a number of hydraulic cylinders dnve a tapered sleeve which acts against the collet fingers, dnving them onto the matmg hubs.

T h s design is very versatile and is equally suited for use as a dnlling connector or a Christmas tree connector.

In the unlocked position the clamp fingers open out at the bottom to provide a funnel whch Improves guidance when used as a wellhead connector. This feature is particularly useful during guidelineless drilling.

When greater imtial alignment is requlred as in the case of a tree connector, where internal stab mandrels are used, a lower skirt nng is employed to provide initial lateral clearance to prevent damage.

As the connector mechamsm relies on sliding tapered sleeves to denve its mechanical advantage, the effects of fnction have an unportant beanng on overall efficiency.

MECHANICAL CONNECTORS

Fig. 2 Model 70 collet connector

For the connector shown In Flg. 1, the mechanical advantage IS gven as

where a = angle of outer tapered sleeve b = angle of wellhead housing and connector body fl = fnctlon angle for slldlng sleeve/collet finger interface f2 = frlctlon angle for collet fingerlwellhead houslng mter-

face.

A typlcal value of a IS 4" and of b IS 25". Consequently, ~f fnctlon IS ~gnored, the mechamcal advantage IS 14.83: 1.

However, the effects of fnctlon greatly reduce t h s figure such that a practically achievable mechanical advantage is of the order of 3: - 1. T h s figure IS obtalned uslng an angle of frlctlon of about 8" for both fl and fi, whlch IS equivalent to a fnctlon coefficient of 0.14.

Slnce frlctlon has such a dramatlc effect on efficiency, particular attention IS gven to the use of low-fr~ction coatlngs and speclal lubricants. After a considerable amount of use or after a long perlod subsea ~t IS unllkely that a connector w11 perform as efficiently as when new.

Gwen the practical llmltatlons of the current deslgn an Improved way of lncreaslng connector preload IS to lncrease the hydraullc force on the tapered slldlng sleeve as on the annular plston hydraullc

50 ADVANCES IN UNDERWATER TECHNOLOGY

connector shown in Fig. 3. The annular piston gves a larger surface area than the total for the individual cylinders of the non-integral design of a similar size and operating pressure Mechan~cally, this connector is sunilar to that shown in Fig. 2, but it is more suitable for use when high cyclic bending loads are encountered such as when dnlllng in very deep water or during wellhead tieback operations. Increased preload prevents joint separation under load and reduces the nsk of fatigue failure of the clamp fingers and fretting of the metal gasket.

Fig. 3 HC collet connector

These types of connectors are considered to be self-lockmg, smce the angle of the outer tapered sleeve at 4" is less than the assumed fnction angle of about 8".

External mecharucal locking devices may be provided, using the standard overnde rod feature if any vibration is expected whch may cause the connector to loosen during installation.

The collet connectors descnbed previously are integral w t h their hydraulic actuators. T h s can be a disadvantage where such connectors are unmersed subsea for extended periods w t h the nsk of corrosion and damage to the hydraulic components.

A tree connector whch works in the same principle as those previously described, although with remote actuators, is shown m

MECHANICAL CONNECTORS 51

Fig. 4. The hydraulic actuating cylinders are incorporated mto the tree runrung tool shown m Fig. 5. The runnlng tool is shown attached to the tree m Fig. 6 and close-up photographs of the connection between the hydraulic cylinders on the rumng tool and the extended operating rods of the wellhead connector are shown m Figs 7 and 8.

-. - .. Fig. 4 Connector w ~ t h remote operators

A special lockdown mechanism is promded at the top of each operating rod (Flg. 9) to prevent accidental unlocking dunng servlce.

After the operating rods have been pushed down by the actuators to lock the connector, on removing the actuation tool, four serrated fingers spring out and engage wth a mating fixed collar.

In order to unlock the connector, the actuation tool attaches to the top of the connector operating rod and compresses the serrated fingers, remomng them from mesh wth the collar. (Fig. 10). The operating rod may then be drawn upwards to unlock the connector.

MECHANICAL CONNECTORS

Fig. 9 Cross sectlon of locking Fig. 10 Cross section of mecharusm operating rod wth-actuator-

attached

CONNECTORS FOR MODULAR SUBSEA COMPLETIONS

The latest developments in subsea completions employing the modular concept, where subsea production systems are remote from wellheads, require connectors whch have multiple bores and are capable of bemg remotely mstalled and tested w t h a h g h degree of reliabihty.

A typical small process line connector is shown m Fig. 11. This works on the annular piston concept as descnbed earher for a wellhead connector, but mcorporates two non-integral hydraulic pistons whch act as an emergency secondary unlock by means of two rods threaded into the annular piston.

Metal-to-metal seals for each bore are obtained by using nng gaskets squeezed into tapered seats. As the gaskets are flexlble m a radial direction when compared w t h the hubs, they are pressure- energzed but they are nevertheless gven a substantial lnltial squeeze on make-up of the connector m order to prevent leakage at low pressure.

I~utial alignment on bnngmg the connector into proxlrnity w t h its

Fig. 11 Annular p~ston process line connector

matmg hub is gtven by the lower slurt nng which incorporates a guide funnel. After the funnel has entered over the neck of the hub, two alignment pins are provided whch extend beyond the gaskets and enter mto pockets in the hub to gtve correct angular orientation before the gaskets enter their seats.

Most collet connectors were ongtnally mtended for use m a vertical mode and m thls situation no special consideration need be given to circumferential location of the collet fingers. However, m the honzontal mode, gravlty would cause the fingers to bunch together on the lower side. Although the connector would still function, since the fingers have a lateral relief provided at the top and bottom, mcreased fnction ansing between each finger would Impair the connector efficiency.

To overcome t h s problem, a number of small guide blocks are provided to locate the fingers circumferentially. They are screwed to the connector body and fit into grooves machned m the clamp finger.

Alternatively, a small grub screw may be provlded in the side of each clamp finger, protruding the same amount as the normal gap to provlde equal finger spacmg.

The annular piston is provlded mth lip seals and beanng nngs to prevent galling. The bore of the cylinder is hard surfaced mth either an electroless mckel fimsh or a corrosion and wear-resistant weld cladding.

MECHANICAL CONNECTORS 59

SPECIAL FLOWLINE CONNECTORS FOR HARD PIPE

Although flexlble pipe conveys certain mstallation advantages, it has drawbacks dunng service. It lacks the reliable durabhty of hard pipe and thus requlres more frequent statutory inspechon. Once hard pipe is mstalled, it can be reasonably assumed to have a service life of up to 30 years.

In order to facilitate installation of hard pipe, some means of increasing flexibility is required. This can always be acheved by usmg either long pipe runs or loops or colls, but the use of pipelme swvels is usually most efficient.

Pipelme swivels require a fine surface finish m order to prevent leakage dunng service since they are not preloaded. However, they do permit subsequent movement and are thus able to contend w t h rmsalignment occumng due to thermal growth or surface movement.

The alignment swvel flange (Fig. 19) allows angular rmsalignment of up to 10" in any direction and also permits disconnection, albeit w t h diver involvement but this is acceptable for pipelmes laid in shallow waters. The connection uses a standard API nng gasket and when the bolts are correctly torque tightened, the preload in the joint g v e s a full metal-to-metal seal in the swvel mechanism. However, once the jomt is made up, no subsequent movement is possible unless the studs are slackened first.

Where diver mvolvement is not desirable, an alternative solution is to use the self-aligmng collet connector (Fig. 20), whch contams all of the beneficial features of the alignment swvel flange combmed w t h remote operation.

Fig. 19 Alignment swvel flange Fig. 20 Self allgnlng collet connector

60 ADVANCES IN UNDERWATER TECHNOLOGY

CONCLUSIONS

Subsea completions are formed of discrete packages of hardware, each dedicated to a particular function, whlch must be llnked together so as to operate as an Integral unit.

The interface, whether it be for the passage of production fluids, hydraulics actuation fluids or electncal signals, is provided by the mechanical connector.

The type of connector selected depends largely upon its intended purpose, whether it be made up by divers or operated and mstalled using remote means. Cntena such as initial cost notwithstanding, the current trend favours fully remote operation as a means of ensurmg reliability irrespective of envlronrnental conditions.

Apart from the use of connectors dunng drilling operations, most other connector installations remain subsea for extended penods. It is essential that they should disconnect reliably when required, such as when equipment is recovered for overhaul. Corrosion in salt water is a senous problem and, although care is taken that the connector mechanical components are adequately protected with special coatings, the use of corrosion inhibitors which displace sea water from the working parts greatly improves reliability. Some damage to protective coatmgs is usually unavoidable and cathodic protection devices are just as essential overall as are inhibitors.

Special stainless steel welded inlays are usually provided in the metal gasket seats to prevent corrosion. Even a small amount of corrosion occurring at the gasketlseat interface would cause leakage to occur.