Differential Thread Riser Coupling for TLP and Platform Tieback Applications
Authors: Williams M.R. , Chen L.Y. 
Published: October 2008

ABSTRACT
This paper presents the design concept and principle of operation for a new threaded riser coupling called a "Differential Thread Connector" (DTC). Connector designs are presented for several TLP (tension leg platform) and platform tieback riser applications.
The new connectors have several features which make them particularly well suited for subsea riser service:Mechanical interlock (prevents couplings from backing off under reverse torque loads)Non-rotating, gall-resistant, metal-to-metal sealsHigh strength and fatigue 1ifeSmall Outside DiameterFast running/retrieval time
The DTC's performance has been confirmed via extensive finite element analysis and full-scale prototype testing.
These new connectors offer several functional advantages and promise high performance/reliability for demanding subsea riser applications. They also promise significant cost benefits through reductions in rig time (faster running and retrieving), elimination of welding costs, and reductions in connector renovation costs.

INTRODUCTION AND BACKGROUND
In 1975, FMC designed and manufactured a 10 3/4" multi-bore subsea completion riser for use in the Gulf of Mexico (figure 1). The connector developed for the riser was based on the common "pipe union nut" principle. This connector re- qui red no relative rotation between adjacent riser joints during make-up, and its 0.0. was small enough to pass through a standard marine drilling riser. In 1976, this riser was successfully used to run and land a subsea tubing hanger through a drilling riser in approximately 450 feet of water.
In 1978, this completion riser design was evaluated for potential use in open-water subsea completions where no protective drilling riser would be employed. Although these risers would be used only intermittently, the risers' connectors would be subjected to dynamic loads of significant magnitude. Therefore, the "union nut" connector was subjected to a comprehensive finite element (F.E.A.) stress/fatigue analysis. The results revealed that the connector's fatigue life would be adequate for the "worst" anticipated completion conditions in the Gulf of Mexico (180 day life in 12' seas) but would not be acceptable for North Sea conditions (6 day life in 28' seas).
The weak point in the connector was the sharp corner in the union nut's flange (see arrow infigure 1). Attempts to correct the problem through redesign were unsuccessful, and the "union nut" design was eventually abandoned.
Other connector designs based on flanges and "dri11ing riser" connector concepts were considered but were rejected due to their inherently large O.D.'s. Conventional threaded-type tubular connectors and couplings were also eliminated since they required relative rotation between adjacent riser joints during make-up (impractical for multi-bore risers).
Further study of the "union nut" F.E.A. results provided the key to the solution. Although the stresses in the union nut's flange were quite high, the stresses in the threads were relatively low. Thus, if a "union nut" connector could be developed with threads on both ends, the problem might be solved. The "differential thread" connector concept eventually resulted from this line of reasoning.