Abstract

A Joint Industry Project, involving a number of operating companies and allied organisations (including ExxonMobil, BP, Statoil, the UK Health & Safety Executive, TNO and Bureau Veritas), is currently addressing the technical issues associated with high amplitude pressure pulsations generated by gas flow through flexible risers. Combining actual offshore measurement data; part and full scale test results at low, medium and high pressures; and both theoretical acoustic and flow simulations, a good understanding of the phenomenon has been achieved. Guidelines for existing and planned developments have been developed, based on precautionary measures, i.e. how to minimise the risk of this phenomenon occurring at the design stage, and practical assessment and mitigation measures for existing assets. This paper will describe the history of the concern and actual offshore experiences, review the work undertaken by the JIP and provide initial guidelines to address the issue.

Introduction

In the last 5 years, a number of offshore installations that utilise flexible risers for single phase gas service have experienced high levels of piping noise and vibration. This has resulted in at least two piping failures. The immediate means of minimising the risk of additional failures was to significantly reduce the gas export rate. The problem has been attributed to flow induced pressure pulsations from the flexible risers.

The pressure pulsations are generated due to vortex shedding on corrugations of the innermost layer of the flexible riser carcass. These pressure pulsations can be sufficiently extreme to cause vibration induced piping fatigue failures, for example to small-bore connections on both topside and subsea piping. The problem has been seen to occur for gas flow velocities as low as 1.5 m/s, which is potentially a severe limiting factor to an installation's production capabilities.

Fig 1 Typical flexible riser showing corrugated carcass (available in full paper)

As an example, one installation experienced severe noise and high frequency pipework vibration on the topsides gas export system once gas export commenced. A comprehensive measurement programme was undertaken which identified the cause as vortex shedding from the internal corrugated carcass of the riser locking on to discrete acoustic resonances of the gas within the export system pipework. The noise, vibration and dynamic pressure within the gas displayed clear tonal characteristics with the fundamental frequency varying with gas velocity (figure 2). The fundamental frequency varied from 84Hz to 208Hz over the range of gas velocities encountered; harmonics of the fundamental frequency were also seen.

Fig 2 Step change in pressure pulsation frequency with flow velocity increase (available in full paper)

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