High-Temperature Variable Bore Ram Blowout Preventer Sealing
Authors: McWhorter D.J. 
Published: October 2008

The enhancement of subsea well control programs through the utilization of high temperature variable bore ram packers in subsea BOP stacks will be analyzed through the discussion of four critically relevant topics. These topics are: The definition of the high temperature wellbore environment. Elastomer selection criteria for the high temperature wellbore environment. Geometric design considerations. Testing criteria and minimum product performance Requirements.

By discussing the advantages, design considerations, and required product performance of a high temperature VBR, realistic expectations can be established and an informed dialogue can begin as these products move from the test lab to the field. Once a high temperature weIlbore environment is defined, and realistic product performance is understood, a decision can be made by the operator regarding the advantages of high temperature VBRs in his particular application.

INTRODUCTION

The high temperature wellbore environment of the BOP cavity will be defined using data compiled from empirical sources and finite element analysis. High temperature elastomer selection criteria and physical property requirements, based on laboratory testing and field evaluation, will be defined. Variable seal geometry that has been found to be successful during high temperature lab testing will be described. The current Cooper oil Tool (COT) test setup and test criteria will be described.

DISCUSSION

I. THE HIGH TEMPERATURE WELLBORE ENVIRONMENT

To declare all the BOP seals in a 350°F well to be at 350°F would be overly conservative and unrealistic. Finite element analysis (FEA) conducted by COT, along with collected empirical data, indicates that the ram packer is the only elastomer seal to exceed 250°F in a flowing HT well.1Top seals, bonnet seals and connecting rod seals, are all well below 250°F while the packer is at 350°F.

COT's FEA study assumed a subsea ambient and BOP skin temperature of 40°F. To achieve a wellbore temperature of 350°F in the test lab, a skin temperature of over 500°F had to be maintained while flowing 450°F oil through a wellbore circulating coil. For these reasons, it is inherent that any laboratory test that heats the wellbore fluid to 350°F is conservative when compared to field conditions. Heating from the outside in and without the ocean as an infinite heat sink is unusually harsh on top seals, bonnet seals, and connecting rod seals.

II. ELASTOMER SELECTION

There is extensive history and experience in BOP sealing and associated elastomers. While ram packers are not generally considered to be delicate pieces of equipment, they can be rendered useless with a poor choice of elastomer. Selecting a high temperature (HT) elastomer requires consideration of the following two characteristics:mechanical properties andstability through temperature fluctuations.

The mechanical properties of acryla nitrile based elastomers used in ram packers determines the complexity of the seal design and application. The key to any elastomer seal design is to prevent the elastomer from extruding or abrading away.