ABSTRACT

This study focuses on examining the coefficients of friction in materials commonly used in subsea clamps. A test setup was designed and fabricated during the study to find the friction coefficients. The test setup utilizes bolts to generate normal force and a hydraulic press to generate movement between the materials. We tested materials S355 steel and polyoxymethylene (POM-C). These two materials were friction tested against different surface roughnesses of S355 steel. We performed the tests under similar contact conditions regarding contact area, surface stress, and surface characteristics. We have done wet and dry tests. Load cells were used to log the data, in combination with an analog-to-digital converter. We also used microscopes and microscope cameras to document any changes in the surface of the test materials. The results indicate that the designed test setup is helpful in evaluating and comparing the friction between steel plates and different materials. The results show little spread, indicating that the process is stable and repeatable. The test method allows comparing of varying surface roughnesses and -treatments regarding friction. Four different surface roughness were examined, which are 1.6, 3.2, 6.3, and 12.3 μm (Ra). As expected, we found that the surface roughness significantly affects the coefficient of friction. This study summarizes the test results for wet and dry tests. We hope that this study can provide insights into the design of subsea clamps.

INTRODUCTION

Subsea clamps are specialized equipment used in the whole life cycle of oil and gas field development, e.g., from initial installations to maintenance operations. They are mainly used to establish and secure reliable connections by creating high enough friction force on the attached component's surface. Typical components could be steel pipes/risers or flexible cables. Fig. 1 and Fig. 2 illustrate an example design of the subsea clamp and its usage in a subsea project. There is little data on friction coefficients between various materials used offshore and underwater. The current practice is to tighten the screw until the installer (diver or ROV pilot) is "sure" that the clamp is with a significant enough pressure. Consequently, there is a risk of damaging the object to be attached, especially for flexible risers or umbilicals. Another challenge is that the uncertainty of the friction coefficient makes the design not straightforward, which has been also discussed by Xing et al. (2019) in their prototype product for subsea pipelines.

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