COUPLINGS PROVIDE ALTERNATIVE IN TLP TENDON DESIGN

A three-piece coupling design has been developed to suit the extreme demands of tension leg platform (TLP) tendon service. Using couplings is an alternative to one-piece tendons welded together onshore.

The prototype model of Hydril Co.'s XST coupling (Fig. 1), the XST-26S, has been designed under the auspices of a major oil company and is currently being tested, Hydril says.

The concept of using a TLP for development and production of large petroleum reservoirs in deep water is gaining popularity. The huge size of these buoyant platforms has fostered the development of new technology for mooring systems.

Hydril believes strategic planning at the conceptual phase of a TLP construction project will weigh the economic trade-offs of unitized (one piece) tendons against segmented tendons.

Unitized tendons are fully fabricated at a coastal site by welding pipe sections together. While this configuration saves the costs of machined couplings and offshore coupling assembly, the necessity to provide flotation modules and support vessels to tow the lengthy tendons to location adds expense and risk of dropping the tendons while in transit (OGJ, July 3, 1989, p. 24).

Segmented tendons require a coupling design that can be made up easily on site with accurate preload. The coupling must then exhibit an acceptable range of alternating stress as the coupling is subjected to cyclic service loads.

The XST coupling has several important features.

THREADED CONNECTION

First, the XST coupling employs the engagement of helical threads for forming the joint between two tendon segments. The threads are also the mechanism for producing the correct preload that is so important to extended fatigue performance.

Other coupling types, such as bolted flanges, breech blocks, segmented dogs, and interlocking shoulders, typically do not afford the control and accuracy of preloading that a helical thread provides.

Normally, a helical threaded connection consists of multiple wraps of interengaged male/female threads affording uninterrupted (axisymmetrical) load transfer capability with a comparatively high strength-to-weight ratio. Because preload value is directly related to box/pin angular relationship, a helical thread can induce a finely tuned preload.

The continuous nature of a helical thread provides freedom from the stress risers, nonaxisymmetrical preloading and load carrying, and cumbersome makeup procedures inherent to other coupling types.

VARIABLE PITCH THREAD

Most threads are machined so that the diametral pitch (the distance between thread forms measured parallel to the thread axis) is constant. While this gives satisfactory results in most applications, the special demands of a TLP tendon coupling warrant a better distribution of loads (among thread wraps) than this affords.

By a proprietary engineering method, the XST threads have a variation in pitch designed by detailed stress analyses to optimize load sharing among the thread wraps and attenuate both mean stresses and alternating stresses. This design feature significantly improves the long-term load carrying potential of the XST coupling.

SIMPLE MAKEUP

The pin end of the coupling consists of two pieces. The mandrel has a tapered profile looking down to facilitate easy stabbing. It also has an integral torroidal section that captures the power ring and serves as the primary compressive member when preloaded.

The power ring is fitted on the mandrel. It has a two-step design power thread that forms the connection with the box and effects the exact required preload.

The box end (looking up) of the coupling has the female two-step threads for joining tendon segments and applying preload to the coupling.

The XST is easily made up by stabbing the pin into the box, spinning the power ring (three to four turns), and then using a specially designed hydraulic tool (Fig. 2) to apply measured preload.

ACCURATE PRELOAD

Correct preloading of the coupling is ensured by exact dimensional control during machining of the box, power ring, and mandrel. By the use of computer-aided manufacturing, these large parts are made to precise geometric standards.

After snuggling-up the coupling, the preload is geometrically controlled by the fixed stroke of the preload tool. This allows free interchangeability and accurate preload repeatability. Thus, the skill and care of the roughnecks are not factors in makeup integrity.

Very significantly, the level of applied preload is not based on measurement of applied torque.

LONG FATIGUE LIFE

When a TLP is put in service it is usually planned that the tendons will see 30-50 years of uninterrupted subsea service. Aside from the nominal tension imposed by the buoyant lift of the platform, a variable cyclic load resulting from the wind, wave, current, and tide-induced platform motions is superimposed. This cyclic loading creates alternating stresses within the coupling members which tend to cause fatigue cracks in the material. That is, the greater the range of alternating tensile stress, the shorter the fatigue life of the coupling.

By conducting numerous iterations of finite element analysis, which were validated by strain-gauge testing of prototype couplings, the preload stress pattern for the coupling configuration was optimized through design.

Once this optimum stress pattern is established, the XST design permits the computer hypothesis to become an in-the-field reality by virtue of the correct preloading described above.

MAKEUP ASSURANCE

An inherent feature of the XST design is that the box and power ring are precisely oriented when the coupling is in the made-up mode. This affords still another benefit. A visible index on the power ring flange, when aligned with the corresponding index on the box, can provide clearly visible surety that correct preloading has been achieved.