Moon Pool Research Articles

Hydrodynamic and energy-harvesting performances of a compact-array OWC device: An experimental study

This study proposed and evaluated a compact-array oscillating water column device called “Rainbow”, which is expected to adapt to the nearshore wave climate characteristics in China, and satisfy the quick response requirements to coastal and offshore electricity. The proposed device employs a bent-duct buoy concept, with an open mouth in two opposite directions, and embraced a moon pool in a ring shape. Moreover, models under a scale ratio of 1:10 are designed and manufactured, which are fixed on the bottom and tested in regular and irregular wave scenarios in a wave tank. The time histories of critical performance parameters, hydrodynamic and aerodynamic performances, and capture width ratios in different modules were recorded and analyzed to reveal the operating mechanisms of the model. The various module arrangement topologies were found to cause significant differences in the value distribution characteristics of the capture width ratio in different modules, peaking at 0.26. The experimental results can be employed as critical benchmark cases for further designs and optimizations.

Simulation of sloshing in the moon pool with Smoothed Particle Hydrodynamics

In maritime heavy industry, the moon pool is an opening through the hull of some vessels. We replicate a generic design of the pool with the Lagrangian approach Smoothed Particle Hydrodynamics (SPH) whose mathematical model efficiently simulates the interphasial surface. In this study we present the capability of our in-house SPH code to handle the sloshing in the moon pool. We validate the SPH approach to flows for the sloshing tank benchmark and show the stability of this approach for the non-trivial case of the moon pool.

Numerical investigation on the hydrodynamic behavior of a floating breakwater with moon pool through a coupling SPH model

The moon pool opened in a floating structure significantly affects its wave attenuating performance. In this paper, the hydrodynamic behaviors of several floating breakwaters with moon pool were investigated preliminarily by the Smoothed Particle Hydrodynamics (SPH) method. For a comparative study, wave interactions with a rectangular floating breakwater and a cylindrical dual pontoon floating breakwater were also simulated. The wave was generated with a relaxation zone method which had displayed its accuracy as well as applicability. In addition, the lumped mass method was adopted to provide mooring forces for the floating breakwater. The coupling SPH model was validated by a physical model test and the numerical results were in good agreement with the experimental data. After that, the wave coefficients, such as transmitted wave coefficient, reflected wave coefficient and dissipated wave coefficient, were computed to evaluate the wave attenuating performance of floating body models. Meanwhile, for each model, the motion responses, including sway, heave and pitch, were analyzed to investigate their hydrodynamic performance. The numerical results and analysis are useful for the design of floating breakwaters, and the conclusions could be further applied in the design of wave energy converters.

Design and application of moonpool system in polar icebreaker Xuelong 2

Objective In polar regions, the operation mode of launching and recovering detection and sampling devices for scientific research by shipside is limited due to dense ice floes around ship. For achieving the smooth acquisition of data records and samples in the densely packed ice floes regions, a square shape moonpool system was designed in the mid-ship of Xuelong 2 icebreaker. Methods Benefiting from the accumulated experiences from the polar surveys for decades, and taking into full account of operation characteristics in the ice zone, the design option of reasonable setting of switching process between shipside and moonpool operation was put forward. Specifically, innovative use of techniques such as water-tight bottom cover of moon pool, special track docking plan, de-icing of movable joints in low temperature environment was implemented. Result The field tests in Antarctic area demonstrated that the proposed scheme can effectively deal with the problem of launching and recovering of apparatus and devices for scientific survey in the densely packed ice floe region. Conclusions The proposed moonpool scheme fullfil the actual need of Chinese polar expedition, thereby increased the sampling capability of Xuelong 2 in the densely packed ice floes regions and match the capacity of icebreaking of the ship.

An example for transatlantic hitchhiking by macrozoobenthic organisms with a research vessel

In 2019 the RV Meteor cruised from Guadalupe in April/May to Cape Verde in June/July and to Namibia in August/September. The distance is about 10,000 km. The ship has a moon pool for installation of instruments. In Cape Verde we had a first glimpse of the already sparsely populated moon pool. We reached Namibian waters in mid-August. In mid-September, 47 days later and 6000 km south, the ship's moon pool was sampled in the port of Walvis Bay. 13 different taxa could be identified belonging to two phyla, four classes, six orders and 10 families. Most of these species have not yet been observed in the port or in the adjacent areas and are new records for the entire Namibian coast. The goose barnacles Conchoderma auritum (Linnaeus, 1767), Conchoderma virgatum Spengler, 1789 and Lepas anatifera Linnaeus, 1758 were particularly noticeable. They were only surpassed by the large number of amphipods. The species Ericthonius brasiliensis (Dana, 1853), Jassa marmorata Holmes, 1905, Stenothoe senegalensis Krapp-Schickel, 2015 and Paracaprella pusilla Mayer, 1890 are particularly noteworthy here. In addition, the pycnogonid species Endeis straughani Clark, 1970 and the titan acorn barnacle Megabalanus coccopoma (Darwin, 1854) should be mentioned, which occurred very frequently as well. The present study shows, on the one hand, an example of the transatlantic spread of bioinvasive species by ships as vectors and, on the other hand, a convenient method for sampling ship hulls.

Experimental and numerical study of a barge-type FOWT platform under wind and wave load

In this study, a floating barge moon pool platform model equipped with an NREL 5 MW wind turbine and mooring systems was numerically and experimentally tested under wave and wind loads to analyze the hydrodynamic performance and stability of the barge platform and the mooring system in a 50 m water depth condition. The numerical model was based on ANSYS AQWA, and this hydrodynamic model included nonlinear hydrostatic and Froude–Krylov forces, diffraction/radiation forces obtained from the linear potential theory, and Morison forces to consider viscous effects on the heave plates. The experiment model was a 1:64 scaled barge platform with an NREL 5 MW wind turbine scaled in the same ratio. Both full coupling platform were simulated under three conditions, namely free decay test, regular wave test, and irregular wave test, including a typhoon extreme case, with wind operation and parking.First, the hydrodynamic model was calibrated against the free decay test results. Consequently, a good agreement was achieved by calibrating the mooring properties of the experiment model. A comparison of the regular wave cases was then performed, with the same trend of results observed for the surge, heave, and pitch motions of the floating barge system. Finally, irregular model tests, which aimed to measure the dynamics of the barge platform and the mooring system in a 50-year sea state in Taiwan offshore, were conducted. The motion responses of the barge platform and the mooring tensions were recorded and compared with full coupled dynamic analysis results. The DNV GL standards were used to check the reliability of the mooring system and the stability of the barge platform.

Safety analysis of deep-sea mining pipeline deployment operations considering internal solitary waves

Internal solitary waves bring great challenges to the safety of deep-sea mining pipeline deployment operations. The three-dimensional finite element model of deep-sea mining pipeline system under the action of internal solitary waves is established and solved. The shape, tension, stress, upper rotation, offset, and transient response of the pipe string during the deployment process of the deep-sea mining system under the internal solitary wave environment are analysed, and dangerous stages and risk factors are obtained. The envelopes of deployment operations under different internal solitary waves are calculated, and the technical measures to enlarge the operation envelope are discussed. The results show that the upper rotation and offset at moon pool position are more easily affected by internal wave energy relative to stress and tension, and they are the main factors affecting the safety of operation. The stage that the mining vehicle passes through the high-velocity zone of internal solitary waves is the high-risk stage. The operation envelope is the smallest when the IW-OC direction angle is 0°, and is the largest when the angle is 180°. Strengthening the upper restraint has limited effect on improving the operation envelope, in contrast, turning on the propellers of mining vehicle to resist the internal wave when necessary can effectively expand the operation envelope.

Theoretical Study for Predicting Response Water Column in a Moonpool

Moonpool commonly installed in production barge, drilling vessel, pipe laying vessel, survey vessel and offshore diving vessel. Moonpool is an opening vertical wall through the deck from top to bottom of the vessel. The main purpose of using moonpool is to perform lifting/lowering of subsea equipment. It considers as the most effective subsea handling method, but it has disadvantages with oscillation of water column inside moon pool. The purpose of this study is to understand the correlation between response water column inside the moonpool with environmental load and the motion of the floating structure during operational time or transit condition. During transit condition, the forward speed of the barge is zero, the excitation force only caused by incoming waves and barge heave motion. The study started by defining the barge vertical movement and all the forces acting in the body of moonpool to obtain simplified harmonic equation for water column motion and plotting the result using MATLAB software. The amplitude of heave barge and water column piston mode is strongly influenced by environmental conditions, in this case the significant wave height and also the dimension of the moonpool area since it is giving additional added mass into the motion. The large amplitude of the water column likely to occur if critical frequency is in the range of wave excitation. Thus, to prevent any over flowing condition during lowering/lifting operation via moonpool, the limiting sea-state condition should be identified for the designated location.

Validation of simulated wave energy converter responses to focused waves

Focused waves are increasingly used as a design method to predict extreme loads for offshore structures, such as wave energy converters (WECs). However, the ability of computational methods to accurately simulate the prescribed focused waves and resulting wave structure interactions is not well validated. This study uses two computational fluid dynamic methods, WEC-Sim and STAR-CCM+, to evaluate the responses of two taut-moored WEC-like buoys to three different focused waves. WEC-Sim is a computationally efficient, mid-fidelity model, based on linearised potential flow theory. In contrast, STAR-CCM+ is a high-fidelity, three-dimensional, unsteady, Reynolds-averaged Navier–Stokes-based model. The two buoy geometries considered are a hemispherical-bottom cylinder and a truncated cylinder with a cylindrical moon pool. Three prescribed focused waves are generated in both codes, and the simulated displacements and mooring loads are obtained for each of the hydrodynamic bodies. The resulting STAR-CCM+ generated focused waves have approximately the same accuracy, in comparison with the analytic solution, as experimentally generated focused waves. The WEC-Sim simulated responses, when using viscous coefficients tuned to the measured displacements, are, on average, within 11.5% of experimentally measured values, whereas the STAR-CCM+ simulated responses are, on average, within 14.2% of the experimental values.

The appearance and disappearance of Birket El-Kamar, Jaffa, Israel

ABSTRACTThe harbor of Jaffa is one of the oldest in the land of Israel and was a significant entry from the sea for the last 4000 years, from the Middle Bronze Age to the twentieth century. Birket el-Kamar was a sandy bay in the coastal strip south of modern Jaffa harbor that had been part of the harbor during various past periods, but no longer exists. Its name, meaning ‘the Moon Pool’, occurs in historical archival documents, mainly maps, from the nineteenth century to the first half of the twentieth century. This study focuses on the changing outlines of Birket el-Kamar during the last 800 years, based on recently revealed archaeological remains, and documentary evidence. It seems that in addition to the natural processes, the Ottoman fortress that was built on the natural sand strip connecting the rocks in the sea with the coast, also affected the on-going processes, through which the inner port, the central port and the north anchorage suffered higher erosion rates and the shoreline retreated eastward.

Evaluation of Structural Integrity of the ISO-based Moon Pool Type Diver Boats

Evaluation of Structural Integrity of the ISO-based Moon Pool Type Diver Boats

Preliminary chart of drill ship operability considering moon pool sloshing effects

The safety and feasibility of a drill ship operation can be synthesized in a chart of safety zones, which can ease taking fast decisions given a sea state. We propose a preliminary chart, based on a collection of parameters, including RAO, structure loads, and internal characteristics as DPS and moon pool specifications. The calculations were made for a Series 60 model modified to attend the geometric characteristics of real drill ships. The influence of moon pool’s sloshing effects and hole’s shape and location are analyzed through potential velocity model with free surface damping, and varying the incident wave’s encounter angle.

Study of the natural vibration characteristics of water motion in the moon pool by the semi-analytical method

The three-dimensional natural vibration characteristics of water inside a moon pool of an ocean structures are studied. The governing equations are derived based on the linear potential flow theory, and the boundary condition of the total opening bottom suggested by Molin is adopted. A semi-analytical method is used to solve the governing equations, and the natural frequencies and the motion modes are obtained. Two types of motions are studied: (1) the piston motion in the vertical direction, and (2) the sloshing motion of the free surface. The influences of moon pool's structural parameters on the natural frequencies, and the modal shapes are analyzed.

LH-moment estimation for statistical analysis on the wave crest distributions of a deepwater spar platform model test

The design of fixed and compliant offshore platforms requires the reliable estimation of extreme values with small probabilities of exceedance based on an appropriate probability distribution. The Weibull distribution is commonly utilised for the statistical analysis of wave crests, including near-field wave run-ups. The parameters are estimated empirically from experimental or onsite measurements. In this paper, the data set of wave crests from a Spar model test was statistically analysed by using the method of LH-moments for parameter estimation of the Weibull distribution. The root-mean-square errors (RMSEs) and the error of LH-kurtosis were used to examine the goodness-of-fit. The results for the first four LH-moments, the estimated parameters, and the probability distributions showed that the level of the LH-moments has a significant influence. At higher levels, the estimation results gave a more focused representation of the upper part of the wave crest distributions, which indicates consistency with the intention of the method of LH-moments. The low tail RMSE values of less than 2.5% demonstrated that a Weibull distribution model estimated by using high-level LH-moments can accurately represent the probability distribution of large extreme wave crests for incident waves, wave run-ups, and moon pool waves. Goodness-of-fit test on the basis of comparison of sampling LH-kurtosis and theoretical LH-kurtosis was recommended as a procedure for selecting an optimum level.

Semi-Analytical Solution of Optimization on Moon-Pool Shaped WEC

Abstract In order to effectively extract and maximize the energy from ocean waves, a new kind of oscillating-body WEC (wave energy converter) with moon pool has been put forward. The main emphasis in this paper is placed on inserting the damping into the equation of heaving motion applied for a complex wave energy converter and expressions for velocity potential added mass, damping coefficients associated with exciting forces were derived by using eigenfunction expansion matching method. By using surface-wave hydrodynamics, the exact theoretical conditions were solved to allow the maximum energy to be absorbed from regular waves. To optimize the ability of the wave energy conversion, oscillating system models under different radius-ratios are calculated and comparatively analyzed. Numerical calculations indicated that the capture width reaches the maximum in the vicinity of the natural frequency and the new kind of oscillating-body WEC has a positive ability of wave energy conversion.

Effects of Water Inside Semiclosed Moon Pool on the Hydrodynamic Coefficients and Heaving Damping of a Truss Spar Platform

The hydrodynamic coefficients and heaving damping of a truss spar platform were studied by the computational fluid dynamics (CFD) method with considering the semiclosed moon pool. A partial model was established to investigate the added mass and viscous damping of platform heaving; a full model and a simplified model were established to study the nonlinear damping of platform heaving. The influences of opening ratios of moon pool on the added mass and viscous damping of platform heaving were analyzed. It is found that the natural period and the heaving added mass of the platform increase if the semiclosed moon pool is considered. Different opening ratios of the moon pool will lead to different heaving damping of the platform and dynamic pressure acting on the guide plate of the moon pool. In practice, an optimal opening ratio of the moon pool can be designed. The numerical results were verified by comparing with the results of model experiments.

An Extended Boundary Integral Equation for Structures with Oscillatory Free-Surface Pressure

The conventional boundary integral equation for the solutions of linear wave–body interactions is extended to include the radiation solutions because of oscillatory pressures applied on the free surface. Coupled with the constraints that determine the pressure, the extended integral equation is applicable to various problems where a part of the bodies contains the free surface with oscillatory pressures. As illustrative examples, the analyses of wave interactions with an air-cushion vehicle and an oscillating water column device are made. As an additional application, we consider moon pool and gap resonances where the use of oscillatory pressures is an efficient way to introduce artificial damping to suppress excessive free-surface elevation of the potential flow.

A novel method of measuring upwelling radiance in the hydrographic sub-hull

In this study we present a new method useful in collecting upwelling radiance (Lu) from a platform submerged in a hydrographic sub-hull or moon pool of a research vessel. The information analyzed here was obtained during a field campaign in the Northwestern European shelf seas aboard the new research vessel SONNE. As the platform was located at the center of the ship, there is minimal effect from pitch and roll which is known to influence upwelling radiance observations. A comparison of the measurements from this platform with a free falling hyperspectral profiler was performed to determine the degree of uncertainty that results from ship shadow. For given Lu(λ) in situ data we observed ±33% intensity deviations compared to profiling measurements that can be attributed to instrument shading during moon pool installation and environmental perturbations. Furthermore Lu(λ) in situ spectra variations were observed at lower wavelengths, therefore a form fitting algorithm was adapted to receive corresponding depths with identical spectral form from Lu(z, λ) profiler casts. During an east to west transect in North Sea with a schedule speed up to 12 knots in situ radiance reflectance rrs(7, λ) measurements at 7 meter depth were performed with this novel radiometer setup. In spite of any restrictions originating from the sub-hull installation, water masses mixing zone from CDOM dominated coastal waters in the Skagerrak Strait towards the open North Sea were successfully derived thus offering an underway applicable upwelling radiance sensing not suffering from sun glint or other typical restrictions of above water radiometer installations.

Experimental and numerical investigation on hydrodynamic response of buoy form spar under regular waves

Spar platforms are used for drilling, production, storage and offloading of oil in deep water. Spar with its deep draft and large inertia experience low heave and pitch motions in operating conditions. However, the heave motions can be large when encountered by long-period swells or near resonant period. The heave motion of the Spar platforms can be reduced by decreasing the water plane area, increasing the draft, the added mass and/or damping. Various alternatives to reduce the heave motion in long-period swells have been in the fore front of research for the last two decades. An alternate hull form of a shape similar to a buoy with deep draft has been proposed in this study. The buoy form Spar is a cylindrical floating vessel with curved surface near the water plane. The present study focus on the efficiency of the buoy form Spar in reducing the heave motion and increasing the heave natural period. A classic Spar of 31 m diameter and deep draft buoy form Spars with 25 and 20 m diameter at the water plane area has been considered. The moon pool diameter of 12.5 m and the displacement of 63,205 tonnes are maintained for all Spars. The experimental investigations are conducted using 1:100 scale models in the wave flume. The natural period and the damping ratio for the heave and pitch motions were obtained by conducting free decay tests. Numerical simulations have been carried out using panel method. Based on the study, it is concluded that the reduction in water plane area is effective in reducing the hydrodynamic response of the buoy form Spar and increase in the heave natural period is noted.

해양용 UNS S32654강의 기술 개발 현황

Nickel alloy, especially Alloy 625, is used widely in industrial applications such as aerospace, chemical, petrochemical and marine applications due to their good mechanical properties, excellent fabricability, weldability and good corrosion resistance. Especially, it is obviously applied in FPSO's moon pool structure and caisson tubular of CPF under seawater environment. Recently, UNS S32654 was developed to substitute alloy 625 because of high cost price. In this study, mechanical properties and corrossive properties of UNS S32654 are reviewed and proposed its applications.