Prevention of Pulsations Caused by Flexible Risers

Abstract

Abstract In the last few decades, flexible risers have increasingly been used in the offshore oil and gas industry. In gas applications these risers can generate high amplitude tonal pressure fluctuations when the gas velocity reaches a threshold value. The resulting pressure fluctuations can then cause high vibration and cyclic stress levels in the associated topside and subsea pipe work. On a number of platforms, riser pulsations have led to serious limitations of the production capability and in at least one case to the fatigue failure of two small bore side branches, resulting into a shutdown of the platform for five months. An acoustic assessment is performed to determine the risks of a flexible riser to be a source of Flow-Induced Pulsations (FIP) within its intended operating envelope. This assessment is based on models and methods to predict the onset velocity, pulsation frequency and amplitude and is based on actual field measurements on five different platforms and on research performed in collaboration with multiple operators and universities. Relevant inputs include the riser carcass geometry, operating conditions and the reflective characteristics of the topside and subsea piping. The outcome of the assessment allows for the implementation of corrective measures during the design phase of a project and mitigation strategies for already existing risers.