Traditionally, oil rigs and workover rigs (which are used to increase production from established rigs) employ a vertical hoisting mechanism, or draw-works. This takes in or spools out a drilling line to raise or lower a travelling block – a system carrying the jointed drill-pipe assembly. The draw-works is connected to a derrick, which is connected to the ground. In this system, the weight of the tubular components is not transferred to the wellhead, and therefore stress on the flanges is minimised.

With the depletion of oil reserves new drilling methods, such as directional (non-vertical) drilling have become common. New technology allows the angle of bore to extend 90 degrees and beyond. For rigs like these, coiled tubing, rather than jointed pipe, is used.

While this offers numerous advantages compared to conventional drilling operations, there are drawbacks. The main one being that instead of a draw-works, an injector head is used. This is connected directly to the wellhead, via flanges or couplings. The wellhead is therefore subjected to considerable loads. As well as the weight of the tubing in the well, there are additional stresses generated by the twisting movements of the coil on the surface. The components affected include the Christmas tree joints, tee-joint pipe fittings, blow-out preventers, risers and casing flanges.

The method used to transmit the load back to the ground, rather than the wellhead, is a system of simple chains, which are ineffective. Now, rigid framework rigs are being tested. In the meantime, we at Chemipetro recommend rig owners buy the strongest flanges and pipe fittings they can.