Improvements to Ultra-deepwater Controls Reliability through State-of-the-Art Analytical Techniques
Authors: Miller Jack E. , Stidston Edward 
Published: October 2008

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 hosesand 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.

Introduction

Regulatory authorities in some parts of the world have bstipulated 30-second closure times for BOP's. The greater hydrostatic pressures of ultra-deepwater mud columns create greater resistance to BOP ram closure. Moreover, the requirements to shear heavier pipe has led to greater size and fluid volumes for the BOP ram operators. Further, rapidly sequenced BOP closures are now required to achieve stipulated emergency disconnect times. All of these factors result in the need to create higher flow rates of BOP control fluid than was required in earlier generations of equipment.

Many of the components used in the BOP controls have been simply adapted to work with the newer, higher capacity, systems without adequate analysis of their suitability for the service. The environment is especially challenging. Drilling operations are now being conducted in 10,000 ft. water depths where seawater pressure at the sea floor is approximately 4,500 lbs/in2. This pressure exceeds the internal pressure ratings of much of the equipment, meaning that evacuation of the internal pressure in any such component will expose it to external pressures sufficient to exceed equipment ratings.

We have described the need to move more fluid faster, and the resulting potential for damaging waterhammer events at the greater water depths. We thus have concluded that there is a need for higher technical scrutiny of these systems and that the system designer / integrator must be able to analyze the equipment as an integrated system if the best reliability is to be achieved. The major components to be integrated into a control system are the accumulators, directional control valves, pressure regulators, shuttle valves, operating valves, piping, hoses and BOP's.