SLWR Design and Configuration Optimization Considering Strength, Interference, Wave and Slugging Fatigue Responses

Abstract Steel catenary risers (SCRs) have traditionally been the preferred choice in offshore applications due to their simplicity and cost-effectiveness. However, in deep water with more challenging production conditions, SCRs present unique design challenges related to riser payloads and touch-down point (TDP) strength and fatigue responses. In contrast, steel lazy wave risers (SLWRs) have been gaining popularity as an alternative solution. This is primarily attributed to their ability to reduce the payload requirements on the host vessel and their capacity to effectively manage strength and dynamic response at the riser TDP. Nonetheless, the adoption of SLWRs introduces its own set of design complexities, including in mitigating slugging fatigue damage when transporting multiphase oil and gas through the SLWR system and interference with adjacent structure. This paper focuses on the optimization design and analysis for a SLWR to meet strength, interference, fatigue design criteria considering extreme environmental loads and all fatigue sources while maintaining economic feasibility. OrcaFlex (Orcina 2022) is utilized to conduct strength, interference, and fatigue analyses for a series of riser configurations. To enhance analysis efficiency and reduce computational time, wave fatigue analyses are performed in the frequency domain. Strength analysis results, obtained through nonlinear finite element simulations, are crucial for assessing the feasibility of proposed configurations. These results offer valuable insights into key parameters, such as the riser pipe size, hang-off angle, hang-off location on the vessel, sag bend and hog bend elevations, and azimuth angle. Among these parameters, sag bend height and arch height are identified as pivotal in defining the SLWR configuration. Total fatigue damages, encompassing installation fatigue damage, vortex induced vibration (VIV) and heave induced vibration (HVIV) fatigue damage, wave-induced vessel motion fatigue (WIF) damage, and slugging fatigue damage, are calculated. Mitigation methods based on the fatigue damage results are discussed. SLWRs offer the advantage of reducing the payloads to the vessel while effectively mitigating critical stress and potential compressive forces at TDP. The slugging fatigue damage can be of significant concern when dealing with slug flow conditions. This paper introduces an enhanced and more efficient method for the development of SLWR configurations. Implementing this method, the buoyancy section length can be optimized. This, in return, leads to substantial reductions in project costs, making SLWRs a more economically viable choice.