Deepwater Span Management on a Mobile Seabed

Abstract

This paper describes a multi-pronged continuous improvement effort to manage the spans of a deep-water pipeline, where even without wave action, the currents are strong enough to move sediments, leading to constantly changing spans. The effort includes: (1) using strakes rather than intermediate supports to suppress VIV, since the latter proved not to be a long-term solution, (2) using partial rather than full strake coverage of spans and developing tools to quantify the level of strake coverage that is required, (3) improving estimates of soil stiffness and damping, and tools to handle amplitude-dependent soil stiffness and damping, (4) performing pluck tests of the operating pipeline by pulling it laterally at midspan with the ROV until a weak link connecting the ROV to the pipe breaks, whereupon the resulting motion of the pipe is recorded with accelerometers, (5) developing tools that allow fatigue damage to be estimated and accumulated for every location along the pipeline (6) sponsoring a joint industry program by DNVGL to quantify the effect of seabed proximity and trench effects on VIV. This paper provides an overview of these activities, with emphasis on recent results and new assessment tools and methods developed and their validation covering items (2–4) above.