Sealing Tubular Connections
Authors: Bollfrass C.A. 
Published: December 2007

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleum engineering.

Introduction

Perhaps the most efficient method for connecting Perhaps the most efficientmethod for connecting tubes that require subsequent disconnection is helicalthreads. In fact. the Standardization Committee of the American Petroleum Inst. was formed to develop interchangeability standards for pipe connections between manufacturers. Oil Country Tubular Goods (OCTG) are intended to withstand highexternal loads, including fluids under pressure. Therefore, connections useseals that contain or exclude pressurized fluids. pressurized fluids. OCTGconnections are essentially pressure vessels that comprise threads, seals, andstop shoulders. Low design factors and external and internal clearance are theunique application criteria. Seals for these connections take two general forms-those affected by the threads and those established separate from thethreads. The tribological distinction is whether the seal is initiated by plugging the clearances manufactured between mating threads or by intimatelyfitting metal or plastic seals and seats together to form an interface. As anexample, both conditions are found in 8-Round threads, where the root/crestclearances are manufactured leak passages and the flanks are intimately fitted metal seals.

Sealability

Any discussion of tribological leak passages should begin with the fluid tobe sealed and the allowable leakage. "Sealability" usually refers to the ratingof a sealing system, such as the fluids that can be contained or excluded without significant leakage. "Leak-tightness" indicates the performance limitof a seal, such as "no visible water for 15 minutes," "no visible nitrogen bubbles for 3 minutes," or "10 atmospheric cubic centimeters per second (atm cm/s) helium. "

Tubulars have been produced with threaded connections for more than acentury. Traditionally, a lubricant has been used to resist the gallingtendency of closely fitted, freshly machined surfaces and to seal or to plugthe leak passages. For many years thread dope for pipe threads was concocted ofan organic oil and "red" or "white" lead and sometimes graphite. As oilfieldservice grew more severe (leakage became more severe), thread lubricants changed, but their basic mission-to resist galling and to provide a seal-hasnot.

Thread lubricant for rotary shouldered connections can be replenished duringtrips, but casing and tubing service exposes thread lubricants to two new elements-time at temperature and reactive fluids. Thread lubricants tend to deteriorate with temperature over time. In addition, casing and tubingconnections are expected to seal more than drilling fluids. Some of the fluidssealed are chemically reactive with the organic base of thread lubricants. Thus, threaded connections are faced with damming up the leak passages against fluids that can deteriorate the thread passages against fluids that candeteriorate the thread lubricant.

API pipe threads that are expected to seal are the 8-Round (and 10-Round)and the Buttress, depicted in Fig. 1 (line pipe vee threads will not beaddressed, except to caution that they are relegated to service at less than5,000 psig [35 MPa] by API Specification 6A). The leak passages are helical capillaries that can be sealed if they are dammed at a single location alongthe passage. The 8-Round thread has two such leakage passages that are formedby the root-crest clearances. The Buttress leakage passage is a single, trapezoidal, helical passage at the stab flanks that can have about two tothree times greater cross-sectional area than both 8-Round leak passages. TheButtress thread interface can change under severe axial compression to create anew helical leakage passage at the load flanks as well (Fig. 2).

Metal and plastic seals have long been used for sealing fluids, but onlysparingly on pipe connections.

JPT

P. 955