Study on the Mechanical Characteristics and Operating Envelope of Freestanding Drilling Riser in Deepwater Drilling

Abstract

Freestanding drilling riser (FSDR), a new type of riser in deepwater drilling, though not in commercial use, can significantly reduce the engineering sensitivity to severe weather compared to the conventional risers. The optimal installation depth of the near surface disconnection package (NSDP) and the optimal number of buoyancy cans are two important parameters in the FSDR system. In this paper, the key mechanical problems of the FSDR system have been proposed and the mechanics of the FSDR system in normal drilling mode and freestanding mode have been studied. The above two optimal parameters have been calculated on the basis of a specific marine environment and system configuration. The operating envelope of the FSDR system has been figured out through parameter sensitivity analysis. Analysis results show that the NSDP should be installed 200 m below the sea surface to avoid strong wave-current profile and enhance the performance of the FSDR in freestanding mode. From the mechanical point of view, the weakest section of the FSDR is located at the junction between the buoyancy cans and the riser joints, where stress joint should be equipped to improve the stress condition of the system. Further, the maximum von Mises stress of the FSDR in normal drilling mode is the dominant factor restricting the increase of the number of buoyancy cans. The normal operating envelope of the FSDR is mainly limited by the deflection angle of the upper flexible joint (UFJ) and the von Mises stress. On the basis of the mechanics and operating criteria of the FSDR, the optimal number of buoyancy cans is six and the vessel offset should be less than 2% of the water depth to ensure the safety of the FSDR in normal drilling mode. Finally, suggestions on the future study on the FSDR system have been proposed.