Thread Form for Connector Collar of Offshore Well Riser Pipe

A pipe connection includes a pin having circumferentially extending external grooves. A box has an annular base with deflectable fingers extending upward from the base. Each of the fingers has circumferentially extending internal grooves on an inner side and an external thread on an outer side. A collar has an internal thread on an inner side. A radial dimension from the axis to the internal thread crest decreases from turn to turn of the internal thread in a downward direction. The box and the pin are movable from a stab-in position to a locked position in response to rotation of the collar. In the locked position, the external thread crests are in engagement with the internal thread crests, and the internal grooves are in full engagement with the external grooves.