Hybrid Riser Base Jumper Design Methods, Challenges and Solutions

Abstract The increased number of hybrid riser systems, either installed or planned, in deepwater regions across the world has driven a corresponding growth in the use of dynamic riser base steel jumper jumpers to connect these towers to subsea pipelines. The experience of the authors across several West of Africa and Gulf of Mexico hybrid riser projects has identified several key design challenges and highlighted the importance of robust design analysis methodologies. This experience has also demonstrated the key strength and fatigue response issues associated with the design of these jumpers. The analysis techniques and design solutions presented in this paper are applicable to dynamic steel jumpers/spools and are therefore also applicable to static jumpers. These techniques encompass the strength, wave-induced fatigue, modal response and VIV fatigue, riser VIV induced fatigue, slugging-induced fatigue and thermal fatigue analyses, some of which have proved particularly challenging for jumper design. The paper also reviews key input parameters such as flowline expansion, slug profiles and metrology and their impact on the design. The use of finite element riser design software for this design problem has also been evaluated and validated against other general purpose finite element analysis packages. A coupled analysis model of the hybrid riser and base jumpers has proven to be an effective tool for the design and analysis of dynamic riser base jumpers, by implicitly considering the effect of riser dynamic response on the jumpers. Among these responses, the effect of riser VIV on the jumpers has been found to influence jumper feasibility and configuration. One of the key aspects identified was the importance of considering the effects of all fatigue sources at an early stage of the project, and the impact on the extreme response of changes to the configuration to improve fatigue life. The paper will highlight the alternative fatigue and extreme response optimisation solutions that have been developed for efficient jumper design. Moreover, the knowledge of the challenges associated with the design of dynamic jumpers and the accurate definition of input parameters will be shown to be key to an efficient system definition. Introduction Hybrid risers (Single Hybrid Riser and Riser Bundle Towers) have been designed and installed on a number of fields and are mainly used in combination with FPSO type floating structures, either spread moored or turret moored. The majority have been designed for application in the West of Africa (Angola and Nigeria), but have been used more recently in Brazilian waters as well as in the Gulf of Mexico. Rigid Riser Base Jumpers (RBJ) (or riser base spools) are used to connect the bottom of a hybrid riser to the flowline end termination (FLET). Figure 1 shows a typical hybrid riser for West of Africa application [1] while Figure 2 shows typical M-shape and 3-D RBJ designs. The dimensions of the RBJs can vary significantly depending on their shapes. The height of an M-shape RBJ can be between 15 and 25 meters while its length can vary between 25 and 60 meters, depending on the application and limitations that are discussed further in this paper. Riser base jumpers are significantly more challenging to design than static flowline jumpers as they experience dynamic loading as a result of the motions of the hybrid riser subjected to wave and current loading. More specifically, the rigid riser base jumper experiences wave, VIV and hybrid riser VIV induced fatigue from the motions of the riser at the connection point with the RBJ.