Three years ago, unexpected component failures were encountered in the blowout preventer (BOP) hydraulic systems of one of Transocean’s newly built ultra-deepwater drillships. Specifically, the problems were collapsed hydraulic hoses. Attempts to mitigate the problems by replacing hoses with hard pipe were futile. The hard pipe bolting and weld connections also failed.
The Authors and their colleagues recognized that reliable and effective long-term solutions would require a deeper understanding of the physics underlying the failures. We quickly realized that the deepwater pressure environment extends the concern for hydraulic waterhammer arising from sudden changes in flow. The initial failure theory was that the sudden arrest of the BOP operator at end-of-stroke created a “negative” pressure wave that propagated throughout the BOP exhaust circuits, thus creating collapse conditions in the hoses and piping. While this was proven to be true, subsequent hyperbaric chamber testing showed that the shuttle valve spool positions were unstable which resulted in high frequency, high amplitude, waterhammer waves and thus very rapid damage.
The observed phenomena became well documented through testing, but the underlying causes were not well understood, leading to little confidence that such problems could be anticipated and avoided in the future.
A computer simulation dynamic systems model (DSM) of the BOP operating circuit was created and the shuttle valves were analyzed using computational fluid dynamics (CFD) to develop algorithms of the flow properties and spool forces of the shuttle valves as a function of flow rate and spool position. The finished model recreated the instability observed during computer simulation and provided a tool to explore system configuration changes to avoid future problems.
Miller, J.E., Stidston, E., “Improvements to Ultra‐deepwater Controls Reliability through State‐of‐the‐Art Analytical Techniques”, OTC-15232-MS, Offshore Technology Conference, May 5-8, 2003, Houston, Texas.