CFD Simulation and Engineering Mitigation of Liquid Sloshing in Topside Process Vessel on Floating Production Platform
Authors: Lu Yaojun , Chai Shuhong 
Published: September 2008

Floating Production Systems (FPS) and Floating Production Storage and Offloading (FPSO) units are normally subjected to complex wave and current under varying sea conditions. As a consequence, performance of the topside oil & gas processing train is adversely affected by liquid sloshing as a result of the wave, especially for parcially liquid filled vessels based on gravity separation. In order to achieve the intended operating performance, it is essential to effectively control or mitigate the sloshing response in each process vessel. The current works on liquid sloshing in a moving vessel are mainly focused on suppressing the sloshing behavior using various perforated baffles, which is actually a passive control approach based on viscous damping. This paper, however, extends the scope of liquid sloshing and proposes an active control option, which mitigates the sloshing response by reducing the excitation source. To verify the active control concept, liquid sloshing in partially liquid filled vessels were numerically investigated under various wave and vessel conditions.

The CFD study has confirmed that vessel location on the top deck shows a weak correlation with the sloshing response under the roll and pitch excitations. Although the sloshing response is not very sensitive to the vessel location, it is beneficialable to locate the separation module or critical oil/gas process vessels as close as possible to the gravity center of the FPS/FPSO; it is evident that liquid sloshing in a topside vessel is closely related to the incident direction of the wave. The perpendicular incidence creates a minimum sloshing response; the parallel incidence generates a maximum response; while the angle incidence results in a sloshing response somewhere between the perpendicular and parallel incidences. Considering that the angle incidence is the most frequently encountered case in practical operation, it is essential to orientate the vessel to establish a perpendicular incidence according to the wave encountered. It is also verified that sloshing response is significantly increased with a vessel length-to-diameter ratio (L/D). The longer horizontal vessels experience more liquid sloshing related problems. Since vertical vessels can be considered as vessels with minimum L/D=1.0, they always exhibit better anti-sloshing behavior than the horizontal vessels. It is also important to prevent a vessel from operating near the resonance wave condition.

Although the active sloshing control approach shows attractive advantages over the convential approach, it is more applicable for layout design of the newly built topside processing vessels. Considering that most of the floating production systems are not built from scratch where the topside processing equipment can be placed in the optimal position and orientation but are rather retrofitted for the new equipment to fit onto an already limited deck space, the passive sloshing control approach may be more feasible in this application. Generally, a combination of the active and passive approaches should be considered to achieve an optimal control of liquid sloshing in a topside process vessel.