Improved Concentric Thread Connectors for SCRs and Pipelines
Authors: Pollack Jack , Riggs David C. 
Published: May 2011

Concentric thread mechanical connectors have been developed and qualified to ISO 21329 for offshore pipelines and risers. The connector design consists of a conical box and pin with interlocking concentric grooves and redundant metal-to-metal seals. Instead of make-up by screwing, it is designed to " snap?? together under simultaneous axial clamping and hydraulic pressure in the thread annulus. After make-up, the connector cannot unscrew, back-off or loosen under dynamic loading and/or thermal cycling. Concentric thread connectors have been used in various tubular applications for more than 20 years.

However, recent ISO qualification tests and modern FE analyses have been used to identify and design significant improvements. These improvements include: preloading of ID and OD abutment shoulders; elimination of a small gap at the connector ID; elimination of hot spot stresses; and lockout of any box-pin slippage due to extreme torque loading. In addition, smaller (6-5/8-in) concentric thread designs that are integral to high strength pipe are available. The improved concentric thread design has approximately 10x more fatigue life than a field-welded pipeline. It is also leak-tight and stronger than the pipe body in destructive testing, i.e., bending, tension and hydrostatic pressure to failure. These improved connectors enable pipeline construction with lighter, high-strength steel, or with corrosion resistant cladding. Since connected pipe joints are completely fabricated onshore, special materials and fabrication processes have no impact on pipelay rates.

The improved concentric thread connector is now qualified and project ready for deepwater Steel Catenary Risers, Oil Offloading Lines and subsea pipelines in locations such as the Gulf of Mexico, West Africa, Brazil and the North Sea. Also, pipelay with concentric thread connectors may be done faster with lower cost modular systems in lieu of mobilizing welding pipelay vessels with high cost day rates.

Introduction

A mechanical connector with concentric threads has been designed and qualified for use in SCRs, flowlines and other subsea pipeline applications. The concentric thread design consists of an internally tapered box which mates with an externally threaded and tapered pin. Instead of screwing together like helically threaded connectors, the connector is snapped together under simultaneous axial clamping and pressurization of the threaded annulus. Disconnection is performed by simply reversing the connection process of clamping and pressurizing the annulus. The connector is ready for use on deepwater FPS risers in locations such as the GOM, West Africa, Brazil and the North Sea.

Mechanically Connected Pipelines and Risers

Deepwater pipelines and risers are usually welded on location and deployed from J-lay, S-lay or Reel Lay vessels. These vessels typically have day rates in the hundreds of thousands of dollars and can cost millions of dollars to mobilize. Installation with mechanical connectors would simplify the construction process and significantly speed-up the overall operation. Also, since offshore welding and NDT would be eliminated, the overall spread costs would be reduced.

The onshore fabrication of connected double, triple or quad joints can be done ahead of time and out of the critical path. Pipe joints which require high fatigue resistance can be made with double-sided welds which are ground flush and inspected to tighter NDT criteria. This would raise the fatigue rating from DNV E to DNV C classification, resulting in a fatigue life which is 10x more than field welded joints.