We study the collapse of a transient cavity of air in water created by the impact of a solid body. Experimentally, we characterize the dynamics of the cavity from its creation (t = 0) until it collapses (t = τ) in the limit where inertia dominates viscous and capillary effects. Theoretically, we find in this regime an approximate analytical solution which describes the time evolution of the shape of the cavity. This theoretical solution predicts the existence of two different types of cavities that we also observe experimentally.

A novel concept of wind turbine floater based on tension-leg technology is introduced. This floater, dedicated to support wind turbines up to 10 MW, aims at minimizing weight and operations while decreasing the level of motions at the nacelle, hence reducing the loading on the turbine and the need for maintenance. The lightness and modularity of the concept allows for use of typical construction means, flexible procurement and building. The self-installability, meaning towing on site by means of standard anchor handling vessel (AHV) with the turbine already installed, facilitates and accelerates the towing and maintenance procedures. Regarding motions, especially tilt angle and nacelle accelerations, excellent floater performance in both operational and extreme conditions, is ensured through an innovative mooring system and reduced wave loads. The latter are kept low thanks to a high transparency of the floater to wave excitation.

Owing to friction in the weathervaning system and in the swivel stack of a turret-moored FPSO, the turret can be locked to the FPSO in day-to-day environments meaning that no relative yaw motions occur in such conditions. When the FPSO yaws sufficiently, the mooring yaw restoring moment builds up to a point where the friction torque is overcome freeing within seconds the turret from the FPSO which then yaws back towards its neutral yaw position. This phenomenon is referred to as a turret release event. Evidence of the existence of such a phenomenon has been confirmed for one North Sea FPSO. In this paper, the basic physics of turret release are investigated from first principles. Incorporation of a friction module in a readily available time domain simulation software is described. Application of this enhanced tool towards a better understanding of the FPSO/Turret relative yaw motions & velocities during the FPSO service life is then discussed in the context of existing SBM Offshore turret systems.

This paper reviews the issues associated with mooring a ship shaped Floating Production Unit (FPU) in arctic conditions, and presents the development of a novel disconnection and reconnection system for such conditions. The mooring systems of FPUs operating in arctic conditions must be disconnectable to allow the FPU to leave the station to avoid collision with icebergs, or to avoid overloading the mooring legs due to sea ice acting on the FPU hull. In the case of sea ice, the FPU may be required to disconnect under much higher loads than the non-arctic disconnectable systems in operation today. A recent study for the design of an arctic mooring system identified a number of key developments that are required before such systems could be deployed. The disconnection system is a safety critical element, and requires high reliability and redundancy to ensure the FPU can always rapidly disconnect from its mooring when required. In addition, the large number of risers that may be installed for these large field developments, combined with the significant suspended weight resulting from the high capacity mooring system, leads to large buoyancy requirement for the buoy which must support the risers and mooring system when disconnected from the vessel. As a consequence the analysis of the reconnection process must account for the coupled behaviour of the large buoy body, the mooring system, and the risers and umbilicals. Such analysis has shown that using conventional disconnectable turret technology, the large buoy size coupled with the requirement to reconnect in heavy sea states, can readily generate snatch loads that would break the pull-in winch wire. In response to the above, this paper presents two significant advances in disconnectable mooring technology. The first is the development of a new locking system to connect the buoy to the turret, which has been qualified at full scale. The second concerns the design of a new pull-in arrangement that eliminates the risk of snatch loads even in sea states in excess of 3m Hs. The system robustness has been demonstrated through model testing.

A small air bubble (radius $a$) is injected in water (kinematic viscosity $\nu$) in the vicinity (distance $r_0$) of a propeller (radius $r_p$, angular frequency $\omega$). We study experimentally and theoretically the conditions under which the bubble can be ‘captured’, i.e. deviated from its vertical trajectory (imposed by gravity $g$) and moved toward the centre of the propeller ($r\,{=}\,0$). We show that the capture frequency $\omega_{\hbox{\scriptsize\it capt}}$ follows the relationship \[\omega_{\hbox{\scriptsize\it capt}}=\left(\frac{2ga^2}{9\beta\nu r_p f(\hbox{\it Re}_b)}\right)\left(\frac{r_0}{r_p}\right)^2(1+\cos\varphi_0),\] where $\beta$ is a dimensionless parameter characterizing the propeller, $f(\hbox{\it Re}_b)$ is an empirical correction to Stokes' drag law which accounts for finite-Reynolds-number effects and $\pi/2-\varphi_0$ is the angle between the axis of the propeller and the line between the centre of the propeller and the point where the bubble is injected. This law is found to be valid as long as the distance $d$ between the propeller and the water surface is larger than $3r_0$. For smaller distances, the capture frequency increases; using an image technique, we show how the above expression is modified by the presence of the surface.

A new floating foundation for multi-MW wind turbine is being developed within a collaboration between SBM Offshore and IFP Energies nouvelles. This inclined leg TLP, is made up of four immersed buoys and a bracing structure, making the floater transparent to wave excitation. The particular mooring arrangement gives the floater interesting motion properties since it creates a fixed point close to the nacelle, strongly reducing the motion at this elevation. In order to validate the concept and the simulation strategy, a model test campaign has been carried out during three weeks in 2015 at MARIN’s offshore basin. The downscaling is performed according to the Froude law of similitude to maintain the hydrodynamic loadings and behavior. The tower bending natural period, the mooring stiffness, and the turbine rotation speed are also maintained in order to reproduce the relevant structural modes and check that no unexpected phenomena occur in the system during production or parked conditions. The scale 1/50 was initially selected so that the MARIN Stock Wind Turbine (MSWT) can be used. This model wind turbine was designed by MARIN with low Reynolds blade airfoils to mimic the NREL 5 MW wind turbine, especially the thrust force. However because of mass distribution issues, the scale has to be changed from 1/50 to 1/40, at this scale only the thrust force and the rotation speed can be replicated. First, a set of calibration tests are performed in the basin and simulated with Orcaflex™ and DeepLinesWind™ for a better understanding of the system and to validate independently the various components of the numerical models. Secondly, design parked and operational cases are conducted with wind, wave and current loadings for two floater orientations and two water depths. The objective of this campaign is to validate the concept behavior as well as the simulation tools and methodologies. Hydrodynamic and structural models are very similar in both software and are checked with the calibration tests from the basin, whereas two strategies are implemented to model the aerodynamic contribution. The Simplified Coupled Simulations (SCS), performed with Orcaflex, use the aerodynamic forces recorded during the model tests to be imposed at tower top; the Fully Coupled Simulations (FCS), run with DeepLinesWind, use the aerodynamic loading computed with the BEM theory from the measured wind.