Moonpools on drilling vessels are structures located at the mid-ship position to facilitate drilling and other marine operations. The existence of the moonpools is prone to result in intense resonant fluid behaviors even other unexpected violent nonlinear impact on the flow system under external excitations, especially for those types with one or more recesses. The present study focuses on the resonant fluid behaviors and the induced violent nonlinear effects so-called slamming in a moonpool with a recess within the frameworks of both theoretical and numerical investigations. The natural frequencies of the piston mode and the first-three modes of the studied moonpool are approximated based on the theoretical formulas extracted from the linear potential flow theory. Furthermore, the calculated frequency of piston mode are carried out as the external wave excitations combined with varying current velocity to investigate the slamming effects at the typical locations such as the wall of the moonpool and the bottom of the recess. Finally, the characteristics of the slamming pressure both in space and time are discussed in detail based on the RANS-based numerical simulations. The result shows that the resonant fluid behaviors under the wave-current interactions are greatly different from the wave-only excitations. Moreover, intense slamming occur in a moon-pool with a recess under some particular wave-current interactions due to the shallow water effects in the recess region and the phenomenon of energy transfer. The phenomenon of slamming must be treated with special cautions in practical engineering because of the fact that some induced adverse effects could weaken the hydrodynamic performance of the drilling vessel, as well as the structural strength of the moonpool.

Installation of spar floating wind turbine offshore is a challenging task. Usually, the spar platform is upended first, and mating of the tower assembly with the spar platform is assisted by a crane vessel. Due to motions of the spar platforms and of the crane vessel, such an operation often takes place in shielded areas with relatively small wave heights and wind speed. The floating dock concept has been recently proposed to expand the weather window for installing spar floating wind turbines. The idea is to use a cylindrical dock to shield the spar platform from wave excitations. However, because of the trapped internal fluid, the cylindrical geometry is subjected to piston mode and sloshing mode excitations, and these modes may fall in the wave periods and cause unfavourable response characteristics. This paper investigates the influence of floating dock geometries on the piston and sloshing modes. We assumed a homogeneous mass distribution of the floating dock and changed the geometry of the cylindrical dock by expanding or reducing the neck area. Then, hydrodynamic analysis of the alternative geometries was carried out using a potential flow code. By comparing the system’s piston modes and sloshing modes, we identified the trend of variation and found that an expanded neck area can lead to increased sloshing period and piston mode. This indicates a potential improvement of the dock responses in operating sea states. The results of this analysis can be used in the shape design optimisation in a future work.

Hydrodynamic behavior of wave responses and wave forces induced by piston-modal resonance in the narrow gap formed by a ship section in front of a bottom mounted terminal is investigated based on the OpenFOAM ® package. Numerical simulations suggested that the free surface piston-modal oscillations in the narrow gap have closely relationship with the vertical velocity along the gap bottom, implying the velocity flow around the gap bottom is able to affect the wave amplitude in the narrow gap, significantly. The horizontal wave forces can be decomposed into static water forces due to the difference of water level besides the box and the dynamic water forces due to the velocity flow. The amplitudes of static water forces are larger than that of horizontal wave forces, implying that the dynamic water forces can counteract part action of the static water forces. The dynamic wave forces approach to the maximal values at resonant frequencies, indicating that the extreme flow velocity around resonant frequency is useful for reducing the horizontal wave forces.

Coupling between moonpool resonance and vessel motion is investigated in two-dimensional and quasi three-dimensional settings, where the models are studied in forced heave and in freely floating conditions. The two-dimensional setups are with a recess, while the quasi three-dimensional setups are without recess. One configuration with recess is presented for the two-dimensional case, while three different moonpool sizes (without recess) are tested for the quasi three-dimensional setup. A large number of forcing periods, and three wave steepnesses are tested. Boundary Element Method (BEM) and Viscous BEM (VBEM) time-domain codes based on linear potential flow theory, and a Navier–Stokes solver with linear free-surface and body-boundary conditions, are implemented to investigate resonant motion of the free-surface and the model. Damping due to flow separation from the sharp corners of the moonpool inlets is shown to matter for both vessel motions and moonpool response around the piston mode. In general, the CFD simulations compare well with the experimental results. BEM over-predicts the response significantly at resonance. VBEM provides improved results compared to the BEM, but still over-predicts the response. In the two-dimensional study there are significant coupling effects between heave, pitch and moonpool responses. In the quasi three-dimensional tests, the coupling effect is reduced significantly as the moonpool dimensions relative to the displaced volume of the ship is reduced. The first sloshing mode is investigated in the two-dimensional case. The studies show that damping due to flow separation is dominant. The vessel motions are unaffected by the moonpool response around the first sloshing mode.

As the demand for offshore wind energy continues to grow, the strive to understand the wave forces acting on the substructure of the wind turbines continues. In regard to wind turbine design, it is vital to consider not only the total wave force, but also the local wave forces. Local forces are particularly important for the design of secondary structures as e.g. mooring platforms. Typically, however, experimental studies mainly concern total forces or idealized local forces. We present here a rather simple way to measure local forces along a model monopile. The study is conducted in a wave flume of 28 m in length, in which waves are generated by a piston-type wave maker at a water depth of 0.515 m and shoal onto a bed of slope 1:25. A model monopile is installed and subjected to forcing from a series of both regular and irregular waves. In the experimental set-up, the model monopile is fixed at the bottom and the top and consists of seven independent cylindrical sections. The cylindrical sections are connected by force transducers which measure local shear, and so the associated local forces may be determined. The measured local forces are compared to the force distribution given by Morisons equation combined with linear theory and Wheeler stretching, which is a force estimate commonly used in the industry. This study shows that the total force is rather well captured by Morison’s equation. The force distribution estimated from Morison’s equation, however, shows larger discrepancies from the measured forces. This encourages for further measurements. In this study, we show that it is possible to measure force distribution on a model monopile in a simple and cost-effective manner. The aim is here to demonstrate the method and we will later present a larger body of work associated with the outcome of the measurements.

When conducting model tests in a water tank, available model sizes and wave conditions are determined for each tank, depending on measurement accuracy and tank specifications. For deep-water mooring of a floater, a mooring extent in model scale is presumably over 10 meters in depth, making it difficult to be conducted in small-sized tanks without mooring line truncation. The purpose of the research is to develop a device, which could be used as deep-water mooring system in small-sized tanks. Although the law of geometrical similarity is compelled to quit because of the line truncation, the law of mechanical similarity can be maintained by keeping the same restoring, damping and inertia characteristics as those of the full-scale mooring system obtained by numerical simulations. The mooring device consists of a cylinder, a piston, an orifice, springs, pulleys and weights. A spring attached to the mooring line is to generate required restoring force. The orifice, together with the piston, is to generate required damping forces. Inertia forces are generated by the motions of hanged weights, also by the motion of the fluid inside the cylinder. Even negative inertia forces can be given by adjusting natural frequencies of the weight-spring system. With all these examined elements, the mooring device works like the full-depth mooring system. Particulars of the elements of the device have been determined by numerical simulations of the floater moored in the full-depth condition. It has been confirmed that the mooring device behaves as expected in comparison with forced oscillation tests, where prescribed motions were given to the floater-side end point of the mooring line. A tank test has been conducted of a floater with a turret multipoint-moored with the devices and has been satisfactorily compared with numerical simulations of the full-depth system. With the present research it is verified that the mooring device can well simulate actual deep-water mooring system, which makes it possible for small water tanks to deal with deep water mooring experiments.

Estimating moonpool piston mode response at resonance is important for operation safety. This is a difficult task, in particular, due to nonlinear nature of the moonpool response connected to the damping imposed by the flow separation at the moonpool’s inlet. In the present work, the applicability of a simplified model, based on decomposing the problem into potential and viscous components, is investigated. The moonpool piston mode response is modeled as an additional degree of freedom. The coupling terms between this new degree of freedom and other vessel’s modes of motions are calculated based on potential flow calculations. Radiation and diffraction problems are considered separately. A finite volume solver with linearized boundary conditions is used to obtain the moonpool response under forced vertical motions. A quadratic damping model is fitted to the obtained responses and added to the free-surface condition of the potential flow formulation. The problem is solved both in frequency and time-domain. The validity of the obtained model is investigated by model test comparison for a dummy vessel with moonpool undergoing regular and irregular forced oscillations, as well as an offshore operation vessel with moonpool exposed to irregular waves. The benefits and shortcomings of the model are discussed. It is suggested that this method can be used as a practical tool to address moonpool piston mode response in irregular waves.

Experimental and numerical studies are carried out to examine the moonpool gap resonance for a drillship at both stationary position and forward speed conditions. The moonpool size and draft are also changed to study their effects for the gap resonance phenomenon. An OpenFOAM based CFD model is developed and the numerical results show good agreement with model tests. Both piston and sloshing modes gap resonances are clearly observed. The study shows that the resonance frequency and RAO of the wave elevation inside the moonpool are subject to the effects of moonpool length, drill ship draft and ship forward speed. The model test shows that moonpool elevation RAO generally significantly increases in head seas and noticeably decreases in following seas condition. It is interesting to notice that the wave flume sidewall significantly depresses the moonpool elevation RAO at a certain frequency regardless of moonpool length and draft. Further study shows that the presence of the flume sidewall results in a trapped mode that coincides with the moonpool piston mode resonance at zero speed. This depresses the peak of the moonpool resonance, which occurs at the same frequency.

This paper focuses on the implementation of a nonlinear Lid Technique, as done in the software Diodore™. Several dummy plates are used and modelled as mechanical structures with their own degrees of freedom, capable to account for nonlinear damping. The paper describes first the general equations governing the multi-body sea-keeping model including dummy plate structures, and the expression of the relative quadratic damping. In a second step, examples of applications with gap and moonpool are exposed, demonstrating the capability of the nonlinear Lid Technique with multiple plates to render not only the piston mode, but also the other modes of deformation of the free surface.

This paper studies the wave diffraction of a two-dimensional moonpool in a two-layer fluid in finite water depth by using a domain decomposition scheme and an eigenfunction matching method. The formulae of the wave exciting forces, the free surface and internal wave elevations at zero-frequency are derived. Numerical convergence has been assessed by repeating the computations for increasing values of the truncation orders. The present model has been validated by comparing a limiting case with a single-layer fluid case and the comparisons are in general satisfactory. Although the wave exciting forces and free surface wave elevations around resonance frequency are overestimated, the piston mode resonance frequency is well predicted. Two typical configurations with different moonpool widths are selected for computations in both free surface and internal wave modes. It is found that, the wave exciting forces, free surface and internal wave elevations in internal wave mode are much smaller than those in free surface wave mode. In addition, the wave exciting forces in internal wave mode attenuate to zero quickly as incident wave frequency increases. For moonpool with small width, only piston mode resonance can be observed. The piston mode resonance frequencies identified in free surface and internal wave modes are the same. The characteristics of piston mode resonance can also be observed in the horizontal and vertical wave exciting forces. Around the piston mode resonance frequency, the wave exciting forces reach their local maximums. It is revealed that, as moonpool width increases, the piston mode resonance frequency decreases. Meanwhile, it shows that more asymmetric and symmetric sloshing mode resonances appear alternately and occur at higher frequencies than the piston mode resonance. Moreover, the predicted sloshing mode resonance frequencies are compared with those estimated by a simple approximate formula.

Moonpool resonance is investigated in a two-dimensional setting in terms of regular, forced heave motions of a model with moonpool with different rectangular-shaped recess configurations. A recess is a reduced draft zone in the moonpool. Dedicated experiments were carried out. The model consisted of two boxes of 40 cm width each, with a distance of 20 cm between them. Recess configurations varying between 5 cm to 10 cm in length and 5 cm in height were tested. Different drafts were also tested. The free-surface elevation inside the moonpool was measured at eight locations. A large number of forcing periods, and five forcing amplitudes were tested. A time-domain Boundary Element Method (BEM) code based on linear potential flow theory was implemented to investigate the resonance periods, mode shapes as well as the moonpool response as predicted by (linear) potential flow theory. Dominant physical effects were discussed, in particular damping due to flow separation from the sharp corners of the moonpool inlet and recess. The effect of the recess on the piston-mode behavior is discussed. BEM simulations where the effect of flow separation is empirically modelled were also conducted. The non-dimensional moonpool response suggests strong viscous damping at piston-mode resonance. The viscous BEM simulations demonstrate improvement over inviscid BEM, although further improvement of the method is needed. The piston mode shapes are clearly different from the near flat free-surface elevation for a moonpool without recess, consistent with recently published theory.

In this work, a frequency-domain method for the numerical solution of the nonlinear dynamics of a wave energy converter with a pumping system is presented. To this end, a finite Fourier series is used to describe the nonlinear force components, i.e., the pumping force. The dynamics of the buoy and the piston are obtained by solving a set of linear motion equations. The numerical model is validated by comparing it with experimental results. The dynamic characteristics of the pumping system are investigated by performing a series of numerical simulations for several approximation orders under various wave conditions. The advantages and limitations of this method are also discussed in the paper. This work provides great insight into the mechanism of a WEC with a pumping unit under various sea states, which can guide the design of a multi-piston pump (MPP) system. This model will be applied to investigate the optimal configuration for the MPP system in the future.

The hydrodynamic interaction between floating bridge pontoons and its impact on the bridge global responses are investigated in the current study. The global model for end-anchored floating bridge for Bjørnefjord crossing is modelled in ORCINA-OrcaFlex. Forty-six pontoons are used to support the bridge, with a centerline distance of 100m between two pontoons. Two models are setup for comparison: (1) The OrcaFlex model with hydrodynamic coefficients of pontoons without hydrodynamic interaction; (2) The OrcaFlex model with hydrodynamic interaction coefficients, which were calculated by ANSYS-AQWA. Firstly, a case study of hydrodynamic interaction effects on added mass, potential damping and diffraction force is given, showing that the piston and sloshing modes have strong correlation with the resonances. Then two sea states were run on the two models with and without hydrodynamic interaction effects. The first order wave effects are included in the analysis. The observed extremes of the time-domain bridge girder bending moments and motions were compared between two models. The comparison shows both reduction and increase of bridge responses depending on the wave directions. A sensitivity test of drag coefficients applied on the pontoon vertical motion is carried out as a rough examination of the neglected viscous damping on the hydrodynamic resonances. The viscous damping effects on the resonances should be further quantified.

Model tests of a drillship with a rectangular moonpool opening were conducted in regular and irregular waves from the bow and bow-quarter. Most tests were conducted at zero speed, the rest was performed with the model towed to a speed of 10 kn. From the video-recordings and transfer functions of the measured relative water elevation inside the moonpool, the typical piston and first sloshing modes are well captured, for wave frequencies that agree relatively well with relevant formulations. A few tests conducted at varying wave amplitudes show that the water elevation is non-linear by nature, while repeat tests conducted with the moonpool fitted with two layers of side wall flanges shows that these can reduce the water motions by nearly 40 %.

In this work, the solitary wave loads on a submerged horizontal circular cylinder are studied by comparing new analytical results with ad-hoc experimental data. The proposed analytical solution has been recently proposed by Gurnari and Filianoti (2017) and represents an extension of the formulation developed by Filianoti and Piscopo (2008) to assess the solitary wave loads acting on a submerged breakwater and tested in a wave flume on a small-scale model (Filianoti and Di Risio, 2012). Here we deal with a submerged horizontal cylinder following the concept that a solitary wave is subjected to a slowdown passing over a submerged cylinder. A laboratory investigation was addressed to calibrate the adopted semi-analytical method. The speed of the solitary wave celerity crossing the solid submerged cylinder was measured for several wave amplitudes. In the adopted wave flume at the University of Calabria equipped by a piston-type wavemaker, an horizontal cylinder with diameter D = 0.127 m was posed with its center at a depth of 0.2 m. Twelve transducers measured the instantaneous pressures along the external contour of the body. A battery of wave gauges measured the free surface elevation to evaluate the celerity crossing the equivalent water cylinder. Tests confirmed the existence of the slowing down of the celerity of the wave pressure. In other words, we found that the pressure wave is nearly double the time necessary to cross the cylinder with respect to the time necessary to cover the same distance in the undisturbed field, for solitary waves amplitudes ranging from about 0.08 and 0.19 times the water depth. The slowing down increases the horizontal wave force on the solid body with respect to the Froude-Krylov one. Moreover, it appears that in the adopted experimental range the wave force is essentially inertial if compared to the drag one, enabling us to rely upon a simplified analytical model to obtain an effective estimate of the horizontal force produced by a solitary wave on a cylinder.

This paper presents the efforts of experimental and numerical studies to reduce internal flow of moonpool. Drillships are equipped with a vertical opening at the center of the hull called ‘moonpool.’ Recently, the moonpools are getting longer and wider for the higher operability. With this trend, violent internal flows are getting more concerned in terms of the safety and operability, which have been reported during the operations even in mild seas. Also, it is well known that the internal flow gives higher resistance during the transit of drillship. Therefore, there have been a number of motivated researches on the reason and the source of excitation, the pattern of the internal flow and the way to suppress it. Typically an internal flow of the moonpool has two types of oscillation modes: piston- and sloshing-mode. The excited oscillations of water and resultant internal flows are highly dependent on the shape of the moonpool, partly due that the resonant periods are varied with the size and shape of the moonpool. However, since the shape and size of the drillships are quite standardized, there may be no room for the change of shape to reduce the inflow from the bottom of moonpool. Therefore, more efforts have been made to develop the damping devices such as splash plates, which can be easily installed inside of the moonpool. In this study, to see the effect of larger damping devices, a series of experimental and numerical study was carried out for the four moonpool designs; the ordinary plain moonpool, the moonpool with a recess deck, the moonpool with an isolated recess deck (island deck) and moonpool with a combination of island deck, splash plates and wave absorber. From the model tests, it was found that the internal flow of the moonpool was significantly reduced by the application of the wave absorber. In case of the moonpool with the island deck, the sloshing mode oscillations was not observed due to the gap flow between the inner wall of the moonpool and the recess, while the piston mode oscillations were remained unchanged. For the in-depth understanding of the flow behaviors and characteristics, the internal flow of the moonpool has been investigated using RANS based CFD code. The various moonpool designs were simulated to identify the effect of each device for the internal flow reduction of the moonpool. The CFD analysis results with regular waves, the water surface responses inside moonpool such as the flow pattern and resonance frequency were compared with model test results and showed reasonably good agreements.

Damping of waves in moonpools has been investigated. Several different geometries of the moonpool have been considered, which are being mainly characterised by a piston motion of the free surface in the moonpools, while some sloshing effects are visible. A numerical damping approach has been implemented into a viscous solver to simulate the effect of perforated bulkheads without the necessity to create a complex grid. The simulations have been compared with respective model tests. Two different porosities of the bulkhead have been experimentally investigated. The simulations have been conducted for a variety of Froude numbers, moonpool layouts and in calm water as well as waves. A dependency of the surface elevation natural frequency on the ship speed has been found. Furthermore, the numerically determined surface elevation inside the moonpool has been compared with respective experimental data. Based on the surface elevation, the resulting damping for the different layouts is determined and discussed.

A moonpool is meant for access to the underwater part of the ship from onboard. It is a vertical opening along the depth having an effect on the performance of the floating platform. Inside the moonpool, water motions in horizontal plane is called sloshing and in vertical planes it is called piston mode. Moonpool causes deck wetness and sometimes result in the downtime of the platform. It is the necessity of the operator to be at the safe conditions of platform facing varied environmental conditions. In the present study, vessel response in the region of moonpool resonance was investigated with different shapes of moonpool and comparison is made with Molin’s (2001) theoretical and Fukuda’s (1977) empirical formulas. It is seen that there is a shift in the frequency of resonance based on moonpool shapes. The effect of moonpool on the ship motion with forward speed is also attempted in this paper. Proven packages are used to calculate the calm water resistance of the ship with moonpool of various cross-sections. Wave making coefficient of the ship is modified due to opening to accommodate the moonpool. The openings to accommodate moonpool causes further entry of water both zero and non-zero Froude number especially in the presence of waves.

The efforts to discover the world’s oceans — even in extremely deep-sea environments — have grown more and more in the past years. In this context, unmanned underwater vehicles play a central role. Underwater systems that are not tethered need to provide an apparatus to ensure a safe return to the surface. Therefore, positive buoyancy is required and can be achieved by either losing weight or expanding volume. A conservative method is the dropping of ballast weight. However, nowadays this method is not appropriate due to the environmental impact. This paper presents a ballast system for an automated ascent of a deep-sea seabed station in up to 6000 m depth. The ballast system uses a DC motor driven modified hydraulic pump and a compressed air auxiliary system inside a pressure vessel. With regard to the environmental contamination in case of a leakage, only water is used as ballast fluid. The modification of an ordinary oil-hydraulic radial piston pump and the set-up of the ballast system is introduced. Results from sea trials in the Atlantic Ocean are presented to verify the functionality of the ballast system.

The forced heave motion of a dummy ship model with moonpool, including a fixed box-shaped object, was realized experimentally in the Towing tank at MARINTEK. The blockage effect caused by a large object was investigated. Regular and irregular forced heave motions were imposed. In the regular motion tests, four forcing amplitudes, and 11 forcing periods near the piston-mode resonance period were tested. PVC3D (Potential Viscous Code) was used to study the regular heave motion problem numerically. PVC3D is a code developed at MARINTEK, in collaboration with Statoil RDI, which couples a Naviér-Stokes solver with the linear potential flow theory for the free-surface waves. PVC3D has in previous studies proven to be fast, robust and accurate for marine resonance problems. It has not previously been validated for object in moonpool. Here, a validation study is presented. The moonpool response is well predicted by PVC3D both for the case of empty moonpool and moonpool with object. The studied object has a non-negligible blockage effect in resonant condition.