ABSTRACT

The paper seeks to examine and discuss the safety implications of current developments In Subsea Wellhead Control Systems employed both in production and development Drilling

Such developments include the use of power line signaling, subsea power generation with acoustics signaling, and the incorporation of both surface and subsea programmable electronic systems (PES's) to control and monitor wellhead valves and blowout preventer stacks (BOP'S) Such systems are said by then proponents to offer significant operational and cost advantages over the conventional remotely piloted hydraulic systems, mainly by the elimination of costly umbilicals

The various safety related and safety critical functions demanded of such systems are defined, and the ways in which they are Implemented In both present and proposed systems examined There follows a comparison of then merits from a safety view point, and a discussion of the design principles needed to ensure that risk has been reduced to a level which is as low as reasonably practicable (ALARP), and that the remaining risks are tolerable To meet these criteria, a clear distinction is required between safety critical and safety related functions in the design specification, this implicitly a major requirement for safety critical items

Reference 1s made to the current regulations and guidance including Offshore Installations (Safety Case) Regulations 1992, and Programmable Electronic Systems (PES) in Safety related Applications, General Technical Guidelines

The paper concludes that by following the stated design principles and the proper application of quality systems, it is possible to produce equipment that can be shown to be acceptable as part of a Safety Case submitted under the requirements of the Offshore Installations (Safety Case) Regulations 1992

INTRODUCTION

There is a steadily growing requirement for the use of subsea well completions in the UKCS This is driven by the need to develop the smaller fields in a commercially stringent environment, and IS particularly appropriate when a new field can be tied back to an existing offshore process facility

Production Systems

The main system parameters that require enhancement to achieve commercial viability are

  • Increased distance from wellheads to production facility

  • Simplification of Umbilical requirements

  • Improved information retrieval from Wellheads

  • Minimize capital and operating costs

  • Improve system safety levels

There are a number of methods currently proposed to achieve these objectives, all have advantages and some disadvantage from a commercially viewpoint This paper will outline them, and compare and contrast their safety implications Development Drilling

Whilst there IS not the pressure to provide lateral distance between drilling rigs and wellheads, the increasing water depths in which wells are being drilled has provided a demand for more sophisticated control systems for Blowout Preventer Stacks (BOP'S) This has produced designs which include subsea electronic processing power within BOP systems The safety implications of this development will be considered

SYSTEM DESIGNS
Subsea Production Wellhead Controls

The basic requirements for these systems are to provide reliable operation of the well shutdown valves and flow control chokes, and to feed back information regarding the flowing pressure and temperature of the well fluids

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