Prior to subsea installation, a subsea system has to be tested to verify whether it performs in accordance with specifications and component specific performance evaluation criteria. It is important to verify that the assembled components work in accordance with the assumptions and design criteria used in the detailed engineering. These criteria also cover the vibration performance. In the current study, the pump module within the Åsgard subsea compression station has been subjected to such system evaluation test, including its vibration performance. Vibrations may be caused by internal and external flow through a complex process that is affected by numerous factors such as the piping geometry, flow and operating conditions and also the fluid properties. When severe, mechanical vibrations can lead to fatigue failure of the equipment components.
One of the major parameters that affects the vibration response of the subsea piping is the surrounding water. It is generally known that surrounding water does participate in some vibration modes by adding mass to the total, dynamic mass participating in the vibration. Therefore, resonant frequencies of a piping system will have different values for non-submerged and submerged cases. In addition, the surrounding water can also lead to higher damping of the vibration modes.
In this paper the effect of submerging a pipe system in water is quantified, by analyzing the changes in damping coefficient and the characteristics of measured pipe vibration in-situ. This is achieved by analysis of full-scale frequency response tests performed on a subsea pipe system within the pump module in both non-submerged and submerged conditions. The results are used for validation of numerical techniques that are used to quantify pipe vibration in submerged conditions. Different modeling techniques for the submerged case are investigated.
It is shown that the effects from the surrounding water on pipe vibrations are different for small-bore piping than that for main piping. Furthermore the different modeling approaches and general observations and trends in damping coefficients are discussed and compared with the measurements.