Fundamentals of Buoyancy Can Riser Tensioner Systems: Performance Characterization and Redundancy Philosophy Development

Abstract Buoyancy Can Riser Tensioner Systems (BCRT) are designed to provide tension to Top Tension Risers (TTRs). BCRT systems do not transfer the riser weight to the floater and minimize the interaction between the floating platforms and the riser systems. For deepwater field developments, this attractive feature allows efficient design of the floaters as well as the riser systems. A number of deepwater fields have been developed with Classic and Truss Spar floaters utilizing this class of riser tensioning systems. The design philosophy and performance characterization of BCRTs are rapidly maturing. Recent technology advances pushed the net buoyancy capacity of BCRTs up to 1,550 kips for dual casing riser applications at 5,400 ft water depth (Mad Dog and Horn Mountain Truss Spars). This paper presents an overview of the current state-of-theart in BCRT design and addresses some of the challenges in development of larger tensioner systems for deepwater applications. Buoyancy Can Riser Tensioner System Components A BCRT is primarily a passive tensioning method for TTRs. Figure 1 illustrates a TTR system tensioned by a BCRT. The BCRT is designed to have relative vertical movement with respect to the hull while transferring horizontal loads at hull contact locations. The BCRT system consists of three main segments: upper stem, buoyancy can, and lower stem. The main functions of the BCRT components are as follows: Upper Stem. The upper stem transfers the tension provided by the BCRT to the riser. The surface wellhead and work platform are connected to the top of the upper stem. The elevation of the top of the upper stem is selected to locate the well equipment at a favorable elevation for easy access under normal operating conditions. The stem-to-wellhead interface and all other stem-to-stem interfaces can be sealed off to provide additional buoyancy. Figure 1 - An illustration of the BCRT components and critical elevations of a Truss Spar floater. (Available in full paper) The upper stem is equipped with up and down stops. The stops are designed to make contact with the deck stopper and have three main functions:provide a support location to rest buoyancy can during installation operations,provide a potential barrier for riser parting, andlimit total stroke to prevent damage to the production trees and riser system. The deck stopper is secured at the Spar deck level and restricts the lateral and rotational motions of the BCRT relative to the hull. Buoyancy Can. The buoyancy can provides the tension required by the riser system. It is composed of individual chambers to ensure that the failure of any one of the chamber will not compromise the riser operations. The buoyancy can chambers are filled with air or nitrogen gas.