Structural frequency monitoring for multi-phase flow estimation in vertical wells and risers

A lot of effort is dedicated to monitoring the flow properties in offshore pipelines through measurements at the riser section above sea level, where equipment installation and maintenance are feasible. However, the multi-phase flow in the pipelines is hard to monitor due to the rapid fluctuation in gas volume fraction (GVF) and varying water cut. This study presents a new, simple approach that offers flow monitoring through the frequency content in vibration measurements captured by installed accelerometers. Particularly, the methodological premise is to extract flow-induced vibrations from the total vibration response, which, besides the flow, is induced by external excitation from wind, waves, and current. The distinguishment between the vibration sources is conducted by use of continues wavelet transformation (CWT), which provides information about the frequency shifts and when these occur. The study demonstrates how the approach succeeds in detecting slug in a numerical model of a riser system located in the Danish North Sea. The successful application is obtained as the slug flow within the riser adds additional structural vibrations at a different frequency band than the vibrations induced from the wind, waves, and current.