Comparison of Simulated and Full Scale Disconnection of Turret Buoy

Abstract

This paper addresses mooring design methods, reveals records of site tests, and also compares the conventional theoretical numerical modeling approach with a real full-scale disconnection of this challenging scenario. The FPSO BW Pioneer set a new world record for deepest mooring system on a production unit at 2,500m (8,200ft) of water depth in the US Gulf of Mexico. The unit also pioneered this type of facility (FPSO) in the US Gulf of Mexico. This area of the globe is prone to hurricanes. The decision was made to proceed with a concept that would allow the vessel to detach from its mooring and risers system and sail away to sheltered waters when necessary. The mooring as well as all risers and umbilicals sink to a depth that is safely below the waves in case of a major storm. This system installed is the first production module for two deepwater fields, Cascade and Chinook, both operated by Petrobras America Inc. with the FPSO BW Pioneer. Various connect - disconnect scenarios were identified during the design phase. Since the system is detachable, the disconnection and reconnection dynamics were analyzed in detail. Due to the complexity of the interaction between the STP™ turret buoy and the mating cone, a model was introduced to better evaluate the local effect of dragging and hydraulic suction combined with the vessel motion. This effect is paramount to the integrity of the riser and umbilical structure, as well as to prevent collision between the vessel and the buoy. The model is described and simulations emphasize vertical velocities and accelerations, as compared to traditional methods of computation. At the time, it was unknown whether any previous projects had ever compared the theoretical numerical model with a real full-scale disconnection. The decision was made to proceed with an instrumented site free-drop test, using customized equipment similar to what Petrobsas uses to monitor the trajectory of the Petrobras' torpedo piles during installation. This paper presents to the Industry these site test results and draws conclusions about how accurate or robust design premises are when performed by state of the art design tools used on this project. Introduction The Cascade and Chinook fields are located in the Walker Ridge area of the Gulf of Meixco and are being produced from the FPSO BW Pioneer that is located in WR/Block 249. The Cascade field was discovered in 2002 and Chinook field was discovered in 2003. The mooring with turret buoy was installed in late 2009 and the FPSO BW Pioneer was first connected to the mooring late 2010. Installation of the mooring and connection of the FPSO set a new world record for deepest mooring system on a production unit at 2,500m (8,200ft) of water depth.