Improved Concentric Thread Connectors for SCRs and Pipelines

Abstract

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.

Threaded connectors for sandwich pipes – Part 1: Parametric & comparative studies

“…A critical zone in the snap-fit connector is the nib groove, which acts as a redundant metal-to-metal seal. As can be seen in Figure 4a, the groove reduces the wall thickness at this region and hence (Pollack and Riggs, 2011).…”

“…In view of this, research into these joint types would be expedited by gaining full understanding into the joint setup, geometrical improvements and , its limitations. So far with recent developments, the most publicly utilized sub-categories for the hydraulically assisted clamped joints remain the push-fit and snap-fit connectors (Verbeck and Holland, 2010, Pollack and Riggs, 2011, OSI, 2013. Both sub-categories achieve the joining of pipelines by some means of radial-interference and surface friction and as such its integrity is largely dependent on surface preparation.…”

“…Disconnection can be achieved by simply depressurizing the annulus and with its fast make-up, considerable cost savings can be accrued by using this type of connector (Pollack and Riggs, 2011).…”

Threaded connectors for sandwich pipes – Part 2: Optimisation of stress relief groove

“…revealed that the use of SRG and BB in a standard NC46 and NC50 connector indeed had advantages in reducing the stress concentration in T&C connector under axial, bending and torsion loads. No doubt the adaptation of stress relief grooves into the design of T&C connectors has been welcomed and its effectiveness in prolonging connector life established (Pallini et al 2002) (Pollack and Riggs 2011). (API-RP-7G 1998) draws conclusions via experimental fatigue testing under service conditions that an optimal stress reduction can be achieved in an NC50 connection with a relief groove width of approximately 1-inch.…”

Technical Challenges and Impacts of Extending Spar and Semisubmersible Platform Capabilities to 4,500 m Water Depth

“…The use of high strength SCR sections mechanically connected (concept similar to TLP tendons Merlin connector) should be looked into to reduce installation time required by welded connections [18]. An assessment of key riser components: flex joints, stress joints, VIV strakes and buoyancy modules, is recommended to identify qualification required to ensure they are project ready.…”