Experimental Study of Hydraulic Accumulator Discharge
Master thesis
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http://hdl.handle.net/11250/2561552Utgivelsesdato
2018Metadata
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Sammendrag
This thesis aims to identify the behaviour of a 4 l hydraulic bladder accumulator undergoingdischarge under various conditions and ultimately shed some light on irregularitiesthat OneSubsea Processing is experiencing regarding rapid initial pressure reduction inone of their off-shore accumulator systems connected to a subsea multiphase pump witha hydraulic umbilical.
An analytical solution for P(t) in the accumulator has been developed and has shownhigh levels of accuracy provided that the discharge coefficient of the orifice that the accumulatordischarges through is calculated properly. The means of identifying and using thedischarge coefficient has been thoroughly documented.
The impact of changing the working conditions for the accumulator such as alteringglycol concentrations in the hydraulic fluid, varying its surrounding temperature, changingpre-charge pressure in the accumulator and changing the valve orifice of which the accumulatordischarges through have all been researched. The change of glycol concentrationin the hydraulic fluid also alters viscosity. The fluid viscosity increases exponentially withthe glycol concentration, which is also the case regarding the time it takes to dischargethe accumulator. Altering the temperature of the accumulator impacts fluid viscosity aswell. However, the most substantial impact of temperature alteration has proven to be thechange in internal energy of the gas in the accumulator and how it affects the final chargepressure.
Tests where an accumulator is installed at an elevation and connected to a long tube ineffort to replicate an umbilical system has given insight into how the discharge is affectedby gravitational and frictional force. Multiple tests at different elevations and tube lengthshave shown how the tube friction and gravitational force work against each other whendischarge commences. In this specific study, a tube height to length travelled ratio of 1:1.9resulted in the two forces cancelling each other out.
An isothermal experiment where the accumulator was discharged at a rate of 1.08 l/hshows similar results to those documented by OneSubsea regarding pressure reduction,and indicates thermal losses in the gas as being partly the cause of the accelerated pressurereduction in the start of the discharge cycle.