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 prediction of ship motions in extreme seastates is very complex as it involves strong nonlinearities. It deals with high motions of the ship and implies strong mooring system loads. These seastates are usually modeled in tank tests but an alternative in the near future could be CFD computations.In this article, all required steps to setup and verify the hydrodynamic and numerical model are performed. The setup of the hydrodynamic and numerical model enable us to show that CFD computations of motion RAOS and pitch decay tests provide results in agreement with diffraction-radiation results.Wave only simulations enable us to verify that irregular waves are accurately modelled in the CFD domain. Since the wavemaker motion used in tank tests to generate irregular waves is not available, a process of linear back propagation is set up from the wave elevation on a wave probe in tank tests. High Order Spectral (HOS) simulations are performed to reproduce the seastate measured in tank tests.Finally, a test was performed to model the ship motions in irregular extreme waves with ICARE solver coupled to the computed HOS wave field through Spectral Wave Explicit Navier Stokes Equations (SWENSE).
The maximum displacement of turret moored floating production and storage units (FPSO, FLNG) has been steadily increasing over the past decades and is likely to increase even further in the future. Furthermore, some of these turret moored units are designed to keep position in even the harshest of environments (e.g. 10,000-year storms) and very deep water (e.g. more than 2000m depth). As a consequence, the demands on the capacity of mooring systems are also increasing and larger turret sizes with relatively smaller chaintable openings result. The turret cylinder is neither completely open (due to strength considerations) nor completely closed (due to installation and inspection requirements). Therefore, the mass of water inside the turret will to a degree be forced to move with the turret, but also to a degree be able to enter and exit the turret. As a result, a combination of inertial, hydrostatic, and piston mode effects similar to those found in open moonpools can occur. An additional complicating factor is that the turret is surrounded by an open annular space which influences the flow of water around the turret bottom. Both piston mode and inertia of water inside the turret will impose additional loads on the turret and its support system. It is therefore imperative that these loads be taken into account in the design phase. A number of tools, both experimental and computational may be used to obtain estimates of loads. However, whichever method is chosen, a necessary step is to identify the natural periods of the piston mode taking place in the turret. Investigations, whether numerical or experimental, must obviously focus on these periods. In this paper, analytical, computational (CFD) and experimental methods to obtain natural piston mode periods of turret moonpools are described. The methods are applied to simplified, yet realistic, turret models. Results from these different methods are compared. Respective figures of merit are discussed.
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