A new mathematical model for evaluating the safe margin of deepwater landing operation

Abstract

Landing large diameter conductor or surface casing strings in deepwater has become a great concern to offshore oil companies, since the dynamic tensile load induced by the heave motion of drillship approaches the capacity of today's landing system. For an accurate prediction of heave induced dynamic tensile load on landing strings and determining the safe margin of deepwater landing operation, a comprehensive mathematical model is proposed in this study. The novelty of this mathematical model lies in that it incorporates the effect of Bottom Hole Assembly (BHA) as well as Guide Base Plate (GBP) which are often neglected by other researchers. The mathematical model proposed in this study is solved with Galerkin's method and its validity is verified by comparing the corresponding results calculated by the numerical method established in a previous research work. The results derived from this model are used to compare with those from existing models to emphasize the important role that BHA and GBP act on heave-induced dynamic response of the landing string assembly. Meanwhile, safe margins of deepwater landing operation derived with/without considering the effect of BHA are presented and compared. At last, sensitivity analysis is carried out to investigate the effect that GBP area and hydrodynamic damping coefficients have on heave-induced dynamic response of the landing string assembly. This study shows that the effect of BHA is essential for an accurate evaluation of the safe margin for deepwater landing operation. By incorporating the effect of BHA, the resonant frequency and stress state of the landing string assembly both have changed; while, the GBP only affects the stress state. Besides, the dynamic tensile load decreases with the increase of GBP area and hydrodynamic damping coefficients in near resonant conditions. This work provides a theoretical guidance to the design of deepwater landing string, which is essential for deepwater drilling operation.