Dynamics of Entrapped Water in Large Diameter Turret Moonpools

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.