The initial treatment of heave-induced dynamic loading(SPE/IADC 87152) introduced a mathematical model forestimating the dynamic axial loads imposed on a deepwaterlanding string by a drilling vesselís response to ocean waves.The landing string was treated as an open-ended elastic steelcolumn with allowable variations in cross-section as afunction of depth inside of a vertical wellbore.This paper extends the investigation of heave-induceddynamic loading on deepwater landing strings to account forthe mass inertia of fluid columns in the wellbore. The resultspresented suggest that the effect of the fluid column mass isnegligible only for cases when the casing is run open-ended.For closed-end casing (especially with low clearance annuli),the resonance frequency of the landing string is loweredsignificantly by the fluid column, causing it to more closelymatch the heave excitation frequency observed for most drillships.Evidence is also presented to validate the originalassumption that drilling vessel resonance behavior exhibits anarrowband response to ocean waves with a fundamentalresonance frequency between 0.1 and 0.14 Hz (periodsbetween 7 and 10 sec). This important assumption was basedon a single drillshipís MRU data and allowed reasonablyaccurate dynamic load approximation without having toconsider high order harmonics. To validate this assumption, aspectral analysis was performed on additional heave data fromseveral floating drilling vessels. The results are presented andsuggest that the original assumption is well-founded.Finally, the original model was calibrated with an actuallanding string load case. This allowed for range specificationof the modelís damping coefficient, which improves theestimates of dynamic load magnitude, much like calibratingfriction factors in a torque & drag model.
展开▼
