Mechanically Energized Sealing Systems for Underwater Pipeline Applications
Authors: Decker L.A. 
Published: October 2008

ABSTRACT
Design criteria for mechanically energized elastomer seals are presented and discussed with respect to applications for underwater oil and natural gas pipelines. Significant design parameters are identified and evaluated through the use of a mathematical model which predicts the thermal-mechanical response of a sealing system. A closed form solution is presented for calculating the stress distribution in pipe caused by external seal pressure. Statistical data on underwater pipeline applications is also presented and past problems are evaluated with respect to capabilities of current seal technology.

INTRODUCTION
The two basic types of elastomer seals are pressure energized and mechanically energized seals. A common example of a pressure energized seal is an o-ring. The sealing principle of a pressure energized seal involves installing a seal ring which has dimensions that interfere with its respective seal surfaces when assembled. The interference creates a small mechanical loading which gives an initial seal to hold low pressure. As increasing pressure is applied to the seal, it deforms and continues to seal with higher internal stress and contact pressure against the seal surfaces. The stress in the seal is essentially equal to the pressure differential supported by the seal. The pressure energized seal is limited in several ways such as:dependence on interference creates sensitivity to cutting during assembly.dependence on interference makes the seal sensitive to dimensional variations.The low initial contact load make the seal sensitive to scratches
A mechanically energized elastomer seal is a relatively large volume, packer type elasomter seal which is compressed by external means to create a contact pressure between the seal and the pipe or tubular element to be sealed. Generally, the contact pressure is greater than the maximum hydrostatic pressure which the packer is expected to seal. Figure 1 is mechanically energized by tightening the packing flange fasteners. The same setting action can also be achieved by hydraulically activated methods. The mechanically energized elastomer seal is initially in a configuration with an inside diameter substantially greater than the outside diameter of the pipe to be sealed. The reaction between the seal and packing flange compresses the seals axially which causes a radial contraction of the inside diameter of the seal. This action will continue at a relatively low load until the seal inside diameter contacts the outside of the pipe. As the packing flange load is increased, the internal seal pressure will also increase and a pressure will be generated where the inside of the seal contacts the outside diameter of the pipe. A similar pressure will exist where the outside diameter of the seal contacts the inside diameter of the connector body. The pressure is transmitted throughout the seal much like a fluid. The magnitude of the contact pressure us usually considered as the primary criteria regarding the amount of hydrostatic pressure which the seal will hold. The contact pressure should be sufficiently higher than the hydrostatic pressure for the mechanically energized elastomer seal to function properly.