Evolution of Metal Seal Principles and Their Application in Subsea Drilling and Production
Authors: Milberger L.J. , Radi A. 
Published: October 2008

ABSTRACT
This paper presents a categorized grouping, by function, of metal seals used in oil field drilling and production equipment. A set of inter-related design principles and a design methodology are presented to aid in the design process.

INTRODUCTION
The use of metal seals in oil field equipment has steadily increased over the years. One impetus for a metal seal is the ability to perform in high pressure environments. An example of elastomeric seal problems is shown in Figure 1, an exploding resilient seal exposed to high pressure gas. Another driving force for metal seal development has been the need for fire-safe oil field equipment where seals must operate when the equipment is exposed to ambient flame temperatures up to 20000 F.[1] A third impetus has been the increased number of requirements specified by operators in their search for improved reliability of performance.
To trace this evolutionary process, an extensive literature and patent search was conducted. The results showed an early metal gasket for use with a water, steam, or gas pipe[2] patented as early as 1868. A clear reference to the first use of a metal gasket or seal was not found, but probably predates, the above reference and may be associated with lead seals on water or sewer pipes.
Likewise, the early use of metal seals or gaskets in the oil field is not well documented in technical literature. Better documentation is found in old product information and in the patent art. For instance, Callahan [3] patented a soft metal well packer in 1906 which included a lead element as a well packer. The use of lead and other soft metal for seals has continued in the oil field. The ability to fill machining asperities in the base metal by plastically deforming the seal material has long been used as a sealing Principle in oil field equipment
In the 1920's and early 1930's, as oil well drilling progressed in America, adaptions of the early NPT pipe threads andANSI flanges were incorporated into oil field wellhead equipment. The use of crush-type ring gaskets proliferated and by 1937, the American Petroleum Institute (API) had issued a standard for ring joints and flanges.[4] Development continued and resulted in a few additional, but widely accepted and largely proprietary designs, such as the AX and Grayloc®seals.
Detailed technical information about what actually makes the seals work was either nonexistent or based on experience and testing. The information available was treated as trade secret by the various equipment manufacturers. The early patent art attests not so much to development oftechnical data, but rather to mankind's ingenuity to dream up countless mechanisms and contraptions for the solepurpose of energizing the metal seal.
Not until the onset of the U.S. space program did real metalseal technical design information and data begin to appear publicly, when two important conferences were held in 1964 [5] and 1965 [6]. For example, Rathbun, reports on five regimes of metal-to-metal sealing [7], and establishes relationships between metal contact stress and sealing.