ABSTRACT

Elastomers are employed within critical components of deep water oil production systems designed for twenty to fifty years’ service. Elastomer properties are influenced by chemical changes, collectively identified as ageing processes, that occur over time. Time / temperature reaction rate (Arrhenius) transformation applied to controlled ageing experiments provides a conservative means of characterizing the long-term ageing effects. In this study, several different experimental techniques for obtaining the necessary measurements of accelerated material behavior are employed and compared. Various analytical techniques for characterizing rates of change over time and across temperatures are applied to the measured data. Significantly different results are obtained depending upon the choice of assumptions. Variation in results depend upon accelerated ageing temperatures, the relative availability of oxygen to the test specimen during ageing, and the size of the test specimen.

INTRODUCTION

Flex elements have been used in offshore service for approximately twenty years. In addition to use as tether bearings for tension leg platforms (TLPs), applications for elastomeric flex elements have included flexible sealing couplings for drilling risers, import and export production risers, thrust diverters for solid fuel rocket motors, and hydraulic and feedwater lines for nuclear submarines.

Rotational stiffness of the flex element in service is of interest because the bending moment transferred through the coupled structure or pipe is determined by this stiffness. If rotational stiffness increases, a corresponding increase occurs in the bending moment, reducing the fatigue life of the coupled structure or pipe.

The rotational stiffness of elastomeric flex elements is inversely proportional to the total thickness of the elastomer layers and is directly proportional to the shear modulus of the elastomer.

Shear modulus can be readily measured and characterized. Flex element geometry and linear shear modulus have been successfully employed to predict rotational stiffness of tether bearings. However, the shear stress response of an elastomer is non linear with shear strain, varies with temperature, and is subject to change due to chemical ageing, strain history, and environmental effects on composition.

Although strain history and compositional changes over time must be considered, ageing effects on shear modulus within molded parts with significant cross-sectional thickness are largely due the progressive chemical reactions that occur within the elastomer. The rate of these reactions at any particular stage of their progress depends upon the temperature of the elastomer. It is the purpose of this study to provide a practical basis for estimating the stiffening that would occur over a thirty year period due to chemical ageing of the elastomer at a sea water temperature of 20ºC (68ºF).

For the Ram-Powell TLP, two elastomer compounds were considered for potential use in the tendon flex elements (Figures 1 and 2): an acrylonitrile-butadiene (NBR) compound and a natural rubber (NR) compound. Both compounds are semi-efficient vulcanization systems and are derived from compounds that have been successfully used with tendon bearings for previous TLPs. As the effects of ageing on tie NBR compound had not previously been studied in depth.

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