Pipeline Pressure Loss in Deep-Sea Hydraulic Systems Considering Pressure-Dependent Viscosity Change of Hydraulic Oil

Abstract

The high ambient pressure in deep-sea conditions greatly increases the viscosity of hydraulic oil and then the pipeline pressure loss in deep-sea hydraulic systems. Large pipeline pressure loss can lead to a further change of viscosity on the basis of the viscosity increase caused by the ambient pressure when the hydraulic oil flows through the pipeline. Therefore, the classic Poiseuille’s law can no longer accurately calculate the pipeline pressure loss in deep-sea conditions since it treats the viscosity as a constant. In this paper, based on laminar flow theory and the viscosity-pressure characteristics of hydraulic oil, a novel equation for pipeline pressure loss is proposed, in which the viscosity change when flowing through the pipeline is taken into account. A CFD (Computational Fluid Dynamics) model of a pipeline in the deep-sea hydraulic system has been established, and CFD simulations have been conducted to verify the correctness of the proposed equation for pipeline pressure loss. Theoretical analysis shows that the proposed novel equation for pipeline pressure loss is equivalent to the classic Poiseuille’s law when the pipeline pressure loss or the viscosity change is low. The research results in this paper can provide theoretical support for work efficiency optimization, load capacity improvement, and precise control of deep-sea operation equipment or deep-sea hydraulic systems.

Working Performance of the Deep-Sea Valve-Controlled Hydraulic Cylinder System under Pressure-Dependent Viscosity Change and Hydrodynamic Effects

“…The novel pipeline pressure loss equation proposed in our previous work has taken the above issue into account and it is expressed as follows [28].…”

“…The viscosity-pressure characteristic of hydraulic oil utilized in the novel pipeline pressure loss equation is defined by Equation ( 1). The novel pipeline pressure loss equation is derived based on the laminar state and has been proved to be equivalent to the classic Poiseuille's law when the change in viscosity or the pressure loss is small [28].…”

“…Our previous work proposed a novel pipeline pressure loss equation that takes into account the pressure-dependent viscosity change of hydraulic oil when flowing through the pipeline [28]. Therefore, a more accurate deep-sea VCHCS model can be constructed with this novel equation.…”

“…• dl • dv n dt ,(28) where dF hd is the hydrodynamic force applied to the micro-body, ρ s is the density of the seawater, C D and C M are the drag coefficient and inertia coefficient respectively, dl, dA and dV are the length, projected area and volume of the micro-body respectively, D c is the cylinder diameter, and v n is the normal velocity of the relative motion between the micro-body and the seawater.…”