Device(s) installed between the bottom of the diverter and LMRP joint, to permit relative angular movement of the riser, to reduce stresses due to vessel motion and environmental forces.
Source: API STD 53, Blowout Prevention Equipment Systems for Drilling Wells, Upstream Segment, Fourth Edition, November 2012. Global Standards
A device installed directly above the subsea BOP stack and at the top of the telescopic riser joint to permit relative angular movement of the riser to reduce stresses due to vessel motions and environmental forces.
Source: API RP 64, Recommended Practice for Diverter Systems Equipment and Operations, Second Edition, November 2001 (March 1, 2007). Global Standards