Optimization Procedure of Steel Lazy Wave Riser Configuration for Spread Moored FPSOs in Deepwater Offshore Brazil

Abstract

Abstract The interest in the application of a SCR connected to a FPSO for exploration and production in deep water, has motivated the need to carefully study this concept due to the high offsets and vertical motions imposed by the vessel at the top of the riser. Petrobras has developed through its Research Center the study of different steel riser configurations. For bow turret-moored and spread-moored FPSOs based on VLCC converted hulls, the steel lazy-wave riser (SLWR) has been considered as an adequate solution due to its structural dynamic behavior and costs when compared to other configurations. Although the SLWR furnishes acceptable results for fatigue and extreme environmental conditions, the search for the best configuration is very demanding as any changes to a geometric parameter affect its whole structural dynamic behaviour. The search for configurations that meet all the code criteria for the riser project required meticulous detail that has not always lead to the best results because the number of variables involved is quite significant. Another important aspect is the installation procedure that can also influence the final configuration. In order to reduce the engineering time in generating and analyzing several configurations, optimization tools were studied and used in association with Petrobras in-house software to help define a model that could achieve all design verification phases more easily. This paper presents the experience with the use of an optimization procedure applied to facilitate the design of a SLWR connected to a FPSO unit offshore Brazil. The process of optimization begins with a set of preliminary geometric variables and constraints that are associated with multiple objectives related to economic, construction and safety factors. The result of the optimization process is a set of feasible configurations from which, through careful selection, the "one of the best" configuration is chosen. Introduction At the end of the nineties, Petrobras became interested in the application of steel catenary risers connected to a FPSO for exploration and production in deep water which in turn has motivated the careful investigation of this concept. The Campos Basin environmental characteristics and the dynamic behavior of a FPSO based on VLCC converted hulls led to high offsets and vertical motions imposed by the vessel at the top of the riser. For a free-hanging configuration these motions cause high stress range levels leading to high fatigue damage, mainly in the touch down zone (TDZ). Flexibilization elements, such as buoyancy modules, allow a reduction of motions arriving at the TDP making the use of the steel catenary riser feasible. The studies concluded that the lazy-wave configuration was adequate and attractive due to its structural dynamic behavior and costs when compared to other configurations [1] [2] [3] [4] [5]. The project of a steel lazy wave riser (SLWR) needs to meet the same design criteria used for the free-hanging configuration design. Basically, the DNV-OS-F201 code [6] was used in this work.