Safety by Down-Hole Well Control
Authors: Raulins G.M. 
Published: December 2007

With a full understanding of the capabilities and limitations of down-hole safety valves, and with an adequate testing and maintenance program, operators can provide offshore platforms with total safety program, operators can provide offshore platforms with total safety systems that will minimize danger to lives, risk of pollution, and possible loss of investment. possible loss of investment.

Introduction

The oil industry's continuing endeavors to provide maximum protection against offshore disasters has lately been brought into the public eye as a result of an unusually high interest in preventing pollution. The development of offshore oilfields has been paralleled by a corresponding development in down-hole paralleled by a corresponding development in down-hole safety equipment designed not only to protect the environment but also to insure against loss of lives and damage to property. It is important that all persons dealing with it understand thoroughly the persons dealing with it understand thoroughly the capabilities and limitations of that equipment.

Safety systems for offshore producing platforms can be divided into two general categories, surface safety systems and down-hole safety systems.

Surface safety systems are the first line of protection against all minor mishaps or failures in control protection against all minor mishaps or failures in control systems, treating facilities, and process lines. They generally consist of valves - normally closed - in the vertical run of the producing well, in the flow line, or in some other gathering or service line. They protect against all routine malfunctions of equipment protect against all routine malfunctions of equipment on the platform or downstream of the platform. They can shut the well in at the wellhead or at any point in the system where a valve is located, and are readily accessible for repair and maintenance.

Most surface safety valves in use today are of a type that does not depend on well pressure for closure. They are held open by low-pressure gas on one side of a piston that acts against a spring and line pressure acting on a smaller area. This low-pressure gas can be tied in to a network of control lines running to sensors installed at any point on the platform or beyond to detect abnormalities in platform conditions or malfunctions in equipment. Some of the many types in general use today are high- and low-pressure sensors, temperature sensors (fusible plugs), gas sniffers, and fluid-level sensors.

Down-hole safety systems are commonly referred to as "catastrophe" systems because normally they are actuated (except for testing) only if the surface safety equipment fails or if there is an actual or imminent disaster on the platform. It is the final point of closure of the well.

Down-Hole Safety Systems

The first down-hole safety valve widely used was a simple poppet valve whose stem was held in the down (open) position by its own weight. Production flowed around the poppet and up through the body of the valve. When the flow rate increased enough to create a relatively low pressure above the poppet, the higher static pressure below it lifted the poppet up into the flow stream, which slammed it to the closed position. This valve was highly successful; but to predict its behavior was almost impossible. Also, the valve was very sensitive to surges, which would cause it to close prematurely. prematurely. A natural evolution from the poppet-type velocity valve was the differential pressure valve, commonly referred to as the "velocity valve" or Storm Choke. This type of valve, which is in use today in more wells than all other types combined, offers advantages over the original velocity valve in that it allows accurate calculation of closure, a wide range of settings, and resistance to surge closing. A relatively recent innovation in well-controlled valves is the "ambient pressure" valve, which is discussed in detail later. pressure" valve, which is discussed in detail later.

JPT

P. 263