Getting to grips with subsea, drilling connections

Ball and taper connection technology.

Ten years on, the progression of these devices was highlighted in the OTC Technology Awards. Among the recognized achievements was Canrig Drilling Technology's SureGrip automated casing running tool, which incorporates the ball and taper gripping mechanism for handling drill pipe.

The ball and taper connection works on the principle of balls engaged in tapers in the male mandrel part of the connector; which is inserted into a closely fitting tube or female connector. It is self-aligning and has a simple self-activating mechanism.

As the male connector is inserted into the female, the balls roll up the tapers. Attempting to remove the tube from the male connector causes the balls to try to roll down the taper, pressing against the female connector tube, resulting in a multi-point grip in which all grip elements are in compression, and where the grip is directly proportional to the load applied. The ball and taper connection can take loads up to 4,000 metric ton minimum break load (MBL).

Fatigue-free handling

The ease of ball and taper connection makes it suitable for applications involving gripping, pulling, and holding connections under load. The primary difference between a ball and taper connection and other connection methods is the way in which it minimizes gripping surface deformation. Many gripping techniques create sharp indents in the surface and introduce potential fatigue failure points. The balls' tear-drop shaped indents on the surface have minimal long-term fatigue effects.

Two types of ball and taper connector are available. The male ball and taper connector is commonly used for mooring systems including spars, FPSOs, floating production units, and submerged turret buoys; diverless and ROV-less bend stiffener connections for risers and umbilicals; hose connectors, pipeline recovery, repair and decommissioning; and heavy lift and buoy retrieval.

The alternative female ball and taper connector is one in which the balls are inside the female tube, and the male is passive. Two examples of this connection mechanism are the Canrig SureGrip tool for handling drill pipe, and a pipeline recovery tool for small diameter pipe – the ball and taper grip is so strong that pipe can be successfully pulled from under rock berms during pipeline decommissioning.

Engineering integrity

The simplicity of the connector design belies the sophistication of the engineering in the ball and taper connection. All ball and taper mooring connectors are made from large-scale metals forgings, and are designed to meet project-specific criteria for dynamic load and design life, and tested in accordance with certifying authority standards.

For a subsea mooring project which may involve 16 male connectors, a test connector is also produced during the forging process and tested to the full MBL, while the remaining 16 male connectors are subjected to a proof load 78.83% of MBL. A Series III (16,126 kN). A Ballgrab subsea mooring connector tested to more than 3,000,000 fatigue cycles was calculated to have a fatigue life of 9,735 years.

Questions about the integrity of forged steel mooring components were raised in 2007 following the failure of a mooring shackle in the Far East. This highlighted a lack of knowledge within the oil and gas industry about high-integrity, large-scale forgings, specifically strength and toughness properties. Subsequent research by First Subsea and the University of Sheffield has made it possible to determine the optimum forging and heat treatment process needed to achieve consistent mechanical properties during forging, significantly increasing the engineering integrity of the ball and taper connector.