Session: 04-02-03 Rigid Risers III

Paper Number: 85000

85000 - A Novel Free-Standing Riser Concept for Deepwater Developments 

Free-standing hybrid risers (FSHR) are well established in deepwater field developments in the oil & gas industry.  The major advantages foreseen for the FSHR are the possibility of safely anticipating production and requiring significantly reduced payloads at the Floating Production Unit (FPU). This work addresses the feasibility of a novel proposed optimized concept, the Free-Standing Integrated Riser (FSIR), where the pipeline is integrated to the riser by means of a transition section, which is kept in place by a multiple line mooring system, developed to keep the functionality and integrity of the entire riser along its lifetime.  The work provides the findings from numerical analysis simulated in Orcaflex, a finite element simulation software dedicated to global analysis of risers, considering the design criteria provided in API 2RD Standard. As part of the FSIR evaluation, screening static analysis was performed to identify the most promises configurations for the multiple line mooring system, followed by a series of dynamic analyzes to evaluate the behavior of the riser in extreme conditions.   In addition, several investigations were performed to assess the functionality and the robustness of the system, including parametric studies to check for installation and dimensional tolerances, assessment of fatigue damage, and checking how the pipe sizes would impact the design.

The major results obtained from the work can be summarized as follows:

Multiple line mooring system should be designed with Circumscriptive Lines. Since the Circumscriptive Lines are tangent to the riser at the connecting points, the system is envisaged to consider two upper and two lower parallel circumscriptive lines, which are connected to a lifting beam to distribute the loads to the main line.  A single main line working as a tether should be sufficient to connect the lifting beam to the suction pile;

The materials for the mooring lines were selected considering the permanent long-life purpose of the system, also taking into consideration the weight and the elongation of the materials.  It was found that the most suitable material for the Circumscriptive Lines is the spiral strand steel wire, provided with a polymeric material sheathing, where for the main line, the proposed material is the polyester ropes having parallel strand construction with added layer for sand and mud barrier underneath the cover;

The system demonstrated that it can withstand extreme loads consistently, because the FPU movements do not affect considerably the curvature of the riser along the transition section;

The proposed riser system can accept variation in design parameters and fabrication/ installation tolerances reasonably well, such as the case of pipeline end expansion, changes in fluid density, and variations in the length of the lines and in the position of the suction pile;

It can be foreseen that the riser is not likely to suffer from wave fatigue due to the relatively low stress variations observed even in extreme conditions, although it may require the application of VIV suppressors in sections of the riser to mitigate the potential for riser fatigue due to VIV;

Larger pipe sizes require higher uplift thrust from the buoyancy tank, since the buoyancy tank is required to withstand the riser and the jumper in case of flooded pipes. The uplift thrust, however, is within the same range as for the most conventional free-standing riser system;

Finally, the FSIR shown to be a potential solution for deepwater developments, demonstrating significant benefits against the most conventional FSHR, due to its simplicity, eliminating the need for the rigid spool, the flex-joint at the foundation, the Pipe Line End Termination (PLET), the riser lower assembly and the stress joint at the bottom of the riser. 

Presenting Author: Carlos Bomfimsilva Pré-Sal Petróleo

Authors:

Carlos Bomfimsilva Pré-Sal Petróleo
Theodoro Netto Universidade Federal do Rio de Janeiro