Dynamics of Entrapped Water in Large Diameter Turret Moonpools

Abstract

Development of Turret Mooring Systems (TMS) for harsh environment and large number of risers has led to a drastic increase of the size of the chaintable and consequently of the turret cylinder diameter. Furthermore, harsh environments usually require relatively deep drafts. As a consequence, the volume of entrapped water in large turrets increases to levels never designed for before. In some cases, the mass of the entrapped water can be comparable to the turret mass. Whilst this entrapped water does not exert any weight on the weathervaning system, its acceleration due to ship motions induces inertia loads which could affect the balance of loads. Estimation of these inertia loads is easily carried out assuming the entrapped water as frozen. However, to what extent is this assumption valid in view of the large amount of entrapped water involved and of the extreme ship motions expected in harsh environments? Should we expect sloshing of the entrapped water? In this paper, insights will be drawn from numerical techniques of diverse complexity. This will be preceded by a brief literature review on sloshing in moonpools. Practical analysis and design recommendations will be proposed. Operational aspects related to installation will be covered as well.

The Dynamic Effects of Water Column Inside the Turret of an FPSO

“…In a continuation of his work [7], he has presented expressions for the sloshing modes also for the same barge. For a circular moonpool, the natural frequency of piston mode can be written as: (1) in which d is the moonpool draft and the added mass α shall be greater than 0 assuming no reaction of the fluid below the moonpool bottom onto the water column motion. For a circular moonpool, the natural frequency of piston mode can be written as: (1) in which d is the moonpool draft and the added mass α shall be greater than 0 assuming no reaction of the fluid below the moonpool bottom onto the water column motion.…”

“…In order to avoid the convergency problem obtained in [1], the annulus between the FPSO hull and the turret has not been considered and the shell of the turret and that of the hull have been merged for the sake of robustness of the results. In order to obtain an insight on how discrepant the responses should be from the case with a simple moonpool, a more detailed model of the turret has been performed in HydroStar.…”

“…In de Vries et al [1], the trend to increase turret dimensions is demonstrated based on a track record of 15 years of the turret designer, and it is observed that "latest turret designs are today 3 to 4 times more massive than the systems designed fifteen years ago". For that purpose, the state-of-the-art is to use the 3D potential theory which is demonstrated to provide reasonably accurate results in case of large bodies.…”

“…Even though the problem is under discussion for a long time, there are only few papers focused on FPSO turret moonpools, among which we have already cited Ref [1], in which the resonance effects are well discussed and results from potential theory as well as from CFD simulations are presented. It is well known that close to the resonance frequencies the potential theory, which is based on the assumption of perfect fluid, overpredicts the wave elevations while in reality it is expected that the energy dissipation by viscous effects would play a major role in limiting the elevations in the confined area.…”