Emergency Shutdown Systems : Requirements Improved Understanding of Design
Authors: Bratland Ove 
Published: October 2008

Wellhead- and process emergency shutdown systems are complex in the sense that their design has to rely on knowledge from many different disciplines. This paper discusses the most common weaknesses in traditional ESD system design and proposes some modifications with emphasis on the hydraulic part of topside ESD systems. The information is based on inspections carried out on most installations in the Norwegian sector of the North Sea, countless interviews of offshore personnel and design experience from some of the largest platforms in the world. It also outlines possible simplifications and cost savin9s during upgrading of old systems.

A newly developed valve that has the potential to reduce complexity and save costs in ESD systems is presented. In subsea control systems, the new technology can reduce installation costs considerably by reducing the need for electrical cables.

The paper is relevant to designers of wellhead- and process emergency shutdown systems, hydraulic power units, and accumulator banks.

INTRODUCTION

Norway is now one of the world's largest oil exporters, and the country also exports natural gas. All the Norwegian hydrocarbon reserves are located offshore and it is complicated and expensive to exploit them. For this reason, safety considerations have always had high priority since the offshore activities began in 1967.

The reliability of an emergency shut-down system is crucial to total safety and operation of the offshore installation. Therefore, most safety systems are designed fail-safe. This reduces the risk of fail-to-dangermodes of failure but it leads to frequent unintended shutdowns. Such events are highly undesirable due to the production losses involved. They also represent acertain threat to safety because they impose additional loads on some components and can result in faults due to human interaction.

Human factors are an important aspect of safety system reliability and availability. Some of the nuisance shutdowns are caused by humans, generally while doing maintenance or testing of a system. Such activities are of great importance and accordingly more effort should be put into the design of equipment for easy installation, maintenance, and testing.

Lack of basic skills in hydraulics and pneumatics, bothb for engineers, installation contractor, and servicepersonnel seems to be one of the most common problems associated with already installed systems. This has generally lead to more complicated systems and less efficient maintenance than required. Theproblem, is to some extent, present in all offshore hydraulic systems, such as cranes, winches, and drilling ystems. Producing a good design of a well control system means combining detailed knowledge about valve technology, accumulator banks, Hydraulic Power Units, well operations, well testing procedures, christmas tree testing procedures, wireline operations, snubbing and drilling operations, regulations and different kinds of emergency situations. It is a complex task.

THE OFFSHORE EMERGENCY SHUTDOWN SYSTEM

The Norwegian Petroleum Directorate's regulations, reference 1, section 18 relating to safety andcommunication systems, stare: installations with equipment containing hydrocarbons shall have an emergency shutdown system,... The system shall prevent or limit the consequences of leakage and shalleliminate potential ignition sources.