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Some drilling operators have experienced a loss of pressure from drilling riser auxiliary lines during operations. In some instances the loss of pressure has been as a result of auxiliary line seals experiencing excessive wear at the pin-box connection. In the case described within this paper, the failure was investigated and attributed to Vortex Induced Vibration of the main drilling riser. The rigid interface between the main riser and the outer auxiliary lines results in transfer of main drilling riser dynamic curvature motions directly into vertical axial motions at the pin-box connection. In the case considered, a high degree of VIV excitation was experienced through the drilling campaign which resulted in excessive wearing and ultimately failure of the pin-box seal. This paper presents a methodology to compute the auxiliary line axial displacement resulting from the drilling riser displacement and curvature. This methodology provides a means of estimating the relative axial motion of the pin-box connection when the main drilling riser is under VIV lock-in. It will also assist in identifying if excessive seal wear will be a concern during a drilling campaign. In this case it suggested that the limiting condition for the drilling riser was not fatigue of the main drilling riser, but wear of the pin-box seal. As a consequence of this investigation, it is recommended to include checks for excessive relative axial motion in general analysis methodology for deepwater drilling risers. As with VIV fatigue issues, the issue may be mitigated through VIV suppression. |
