Abstract
This study presents the optimization of the hybrid risers in deepwater applications by the integration of a tether. It analyses the feasibility, impact and essence of attaching Polyester tether to the buoy of a conventional hybrid system using the Gulf of Guinea (GoG) wave conditions, and adopting a water depth of 1800m. The sensitivity of the convectional hybrid riser (CHR) and Tethered-Hybrid Riser system (THRS) were also analyzed and compared to observe its response with respect to effective tension and maximum von Mises stress. The findings reveals that the effective tension and stress experienced a 2.5% and 6.4% reduction for the fixed standing hybrid riser (FSHR) and flexible jumper, respectively; for the THRS when compared to the CHR configuration (with 10-years wave return period). The results also show that there should be appropriate vessel control in order to maintain the best response when in service over 10-year and 100-years extreme conditions. Hence, the Tethered Hybrid Riser System (THRS) yields a significant improvement when compared to the CHR.
Highlights
Vessel Properties A turret moored Floating Production Storage and Offloading (FPSO) unit of 241.78m in length was adopted in this analysis, the vessel motion characteristics and response is very important in this design as the Hybrid riser’s integral parts comprising of the flexible jumper, standing riser, buoyancy can and tether are linked to the vessel
We show the effective tension and maximum von Mises stress occurring at the top end of the Fixed Standing Hybrid Riser (FSHR) and flexible jumper of a convectional hybrid riser (CHR) and a Tethered Hybrid riser (THR) while varying wave direction and vessel heading; it is aimed at displaying the vessel headings to be avoided during a particular wave direction
This section further shows a comparison between a CHR and a THR with respect to the maximum von Mises stress and effective tension
Summary
In the last 10 years, the exploration, production and development in the deep water (> 500m depth) has expanded, this has accounted for more than half of the global oil and gas reserves. The statistical evaluation from 2005 to 2009 by Information History Service (IHS) on discovery recorded that ultra-deepwater (> 1500m depth) accounted for 41% of the total newly discovered reserves Despite these huge advances, setbacks such as technological challenges, harsh environmental conditions, production cost, etc; limits the overall offshore operations. Riser systems are used to transport fluids (or serve as service line as in the case of umbilical (UFL)) between a well head or subsea structure(s) on the seabed and a fixed platform (or a floating vessel) on the free surface. Challenging factors such as the hydrostatic pressure, current, ocean waves and vessel’s motion pose a difficult challenge for riser design engineers
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