Reliability Analysis Of Subsea Control Pod

ABSTRACT A subsea production control pod was withdrawn from service, disassembled and analyzed to evaluate pod design. This paper examines the results and compares information derived with results of other work toward improved pod reliability. Photographs show condition of the pod, affect of marine growth and condition of internal components after an extended period of subsea operation. Information presented may be useful in evaluating future design, materials and component development, and may affect pod configuration and operating/service criteria for future subsea control systems. INTRODUCTION The evolution of subsea production control systems has only in the past two or three years reached the stage at which autonomous, retrievable pods are used. These equipment packages provide in one module all functional hardware needed beneath the surface to ensure reliable, long term control of the wellhead production, safety and servicing valves. Although theoretically proven by design and laboratory testing, the subsea production control pod's reliability and serviceability is still in a state of development. Pod configuration, component reliability, materials selection and other factors can benefit significantly from analysis of pods which have completed extensive service. In July, 1976, a prototype Koomey subsea control pod was removed from service and returned to the factory for inspection, analysis and refurbishment. The pod, incorporating the Stewart & Stevenson patented Hydraplex sequence module, had seen successful service on the Tenneco Ship Shoal Block 176, Well 2, since May, 1975. This was a wet installation in about 100 feet of water. The pod had failed subsequent to an incident in which a boat anchor dragged across the sea floor and broke the hydraulic control line leading from the surface to the pod. The line had been repaired and the pod continued to function until it developed uncontrolled venting of hydraulic fluid and was removed from service. POD CHARACTERISTICS Pod design employed a single-line hydraulic supply pipe from the surface. The supply line provided both signal and source pressures used by the pod to control three valve operator functions on the tree. (The design is not inherently limited in the number of functions or any particular operating sequence.) In the prototype pod, Figure 1, normal hydraulic operating pressure is supplied by the surface unit through surface and subsea filter assemblies and adjustable orifices to a subsea accumulator. Supply accumulators provide reserves of hydraulic fluid regulated at 1500 psi for tree valve actuation. A separate reference accumulator is also regulated at 1500 psi. A signal accumulator, charged from the hydraulic source to pressures ranging from 0 to 2700 psi, is controlled from the surface. A flow control consisting of a variable orifice and check valve is connected and positioned in the signal manifold with the check valve directed to flow liquid freely out of the signal accumulator and manifold. Another flow control consisting of a second variable orifice and check valve is positioned in the reference manifold with the check valve directed to flow fluid freely into the reference accumulator.