Free-Hanging Rigid Riser: Innovation X Market Solutions

Abstract

Abstract The exploration of oil and gas in ultra-deep waters is characterised by a series of technical challenges that have been gradually overcome with innovations applied to risers. When it comes to free-hanging rigid risers, the most common solution is the Steel Lazy Wave Riser (SLWR), which relies on installing buoyancy modules along an additional section of linepipe. Among the main challenges of the SLWR system are the additional cost and time for installation of the buoyancy modules, besides the risks related to installation and the need for additional riser length to accommodate the buoys. Alternative solutions have been conceived, such as the Gimbal Joint Riser (GJR), an articulated joint in-line with the riser that reduces the dynamic effect at the Touch Down Zone (TDZ), or the Residual Curvature Riser (RCR), which consists of deformed rigid riser sections that dissipates the energy transmitted from the hang-off to the Touch Down Point (TDP). All solutions have their own sweet spot depending on the harsh environmental conditions and on the floating unit type. This article aims to present the concept of the three solutions, comparing them in terms of cost and technical feasibility assuming a reference case. The three mentioned technologies address the problem of energy dissipation from the hang-off to the TDP in different ways, being independent solutions that use different resources and work based on distinct concepts. Assuming that installation time and total cost are two of the most relevant aspects of free-hanging rigid riser systems, these were the main criteria used for a comparison between the solutions, in addition to a line tension criterion, especially at the hang-off. The SLWR solution is established, and several results were published in literature. Such results are used as a reference for the present work. RCR is currently at Technology Readiness Level 2 (API/TRL-2), according to the API scale, with estimates based on numerical models and analytical calculations. The GJR is a solution at Technology Readiness Level 2 (API/TRL-3) and is currently undergoing the conclusion of technological qualification process. Therefore, for evaluation of GJR, calibrated numerical models are used according to the experimental tests already carried out in the qualification program. Results indicate that RCR and GJR present better performance in comparison with SLWR for operational and extreme cases with lower costs. SLWR is slightly better than competing technologies in terms of fatigue life, although, fatigue results of GJR and RCR also comply with operational requirement.