Deepwater Rigid Spools Slugging Flow Fatigue Design

Tie-in spools form an important part of any deepwater flowline system. Successful designs have the potential to deliver significant savings in fabrication, transportation and installation costs, whilst protecting project schedules. For multiphase flowlines, slug-induced fatigue damage has emerged as a governing design criterion in recent projects. Spans have become the focus of attention for slug-induced fatigue damage. These may occur at tie-in spools used to connect flowlines in deepwater developments. Conventional piping design software tools are commonly used for rigid spools design. Limitations/disadvantages of conventional tools were identified by comparison with detailed simulation of critical aspects of the design using more advanced numerical simulation tools. Rigorous 3D numerical dynamic analysis was used to simulate gravity variation of the slugs and bubbles, and the dynamic impact effect due to the passage of slugs through bends. Resonance effects of spools exposed to slugging flow were inspected and the cause of spools resonance was investigated. The consequential fatigue damage was computed using a time-domain FEA and rainflow counting algorithm. While conducting slugging flow fatigue FEA, bespoke pipe-soil interaction models were developed to simulate cyclic lateral and vertical resistances of the very soft seabed soils typically found in deepwater fields. A contact technique with nonlinear normal and decoupled bi-axial tangential interactions was implemented using FORTRAN subroutines. The analysis procedures developed are outlined, and typical spool designs are presented. The paper seeks to understand the slugging flow effects to the deepwater spools fatigue design, fatigue design, especially while the spools resonance can not be mitigated, and provide the optimised spool configuration in which the slugging effects are minimised, taking due account of the complexities outlined above.