This paper presents a comparative numerical study on liquefied natural gas (LNG) pump tower loads, while focusing on two aspects. First, are impact loads relevant for the structural design of LNG pump towers and, second, in which way does fluid–structure interaction influence these loads? Numerical simulations of the multiphase problem were conducted using viscous field methods. First, unsteady Reynolds-averaged Navier–Stokes (URANS) equations, extended by the volume of fluid (VoF) approach, were used to simulate, at model scale, the flow inside a three-dimensional LNG tank without the tower structure. Then, these results were used to validate the numerical method against model test measurements. Afterward, motion periods and amplitudes were systematically varied in the flow simulations. Flow velocities and accelerations along the positions of the main structural members of the pump tower were extracted and used as input data for load approximations based on the Morison equation. Finally, these load approximations were compared with loads determined from solving the delayed detached eddy simulation (DDES). Time histories as well as statistical measures of global loads acting on the entire and the simplified tower structure were of the same order of magnitude. However, their time evolution differed, especially at their peaks, and this was considered significant for structural design.

This paper proposes several schemes for optimal scheduling of power producers in a shipboard power system for a typical offshore supply vessel, having a multiple number of possibly varying capacity gensets. The proposed scheduling methods are illustrated using four alternative power system configurations, ranging from a few large gensets to many gensets of smaller ratings. Mixed integer linear programming is used to formulate the optimization problems. Three formulations are presented: one for minimizing online capacity without further objectives, one that includes a redundancy constraint for loosing a group of gensets (to account for a worst-case failure scenario), and one to also balance running hours and minimizing connect/disconnect of the gensets. These different objectives can be combined and weighted based on importance, with or without redundancy constraints. Simulations are carried out to demonstrate the properties of the three different scheduling methods. The first method, minimizing online capacity only, is also used to illustrate the differences between the four genset configurations. This shows, for instance, that using more small gensets ensures generally a lower online available power (the connected capacity matches better the prevailing load) and near optimal loading of each genset around 80% (assuming equal loadsharing), at the same time as resilience to genset failures is preserved.

Roll motion of a planing hull can be easily triggered at high speeds, causing a significant change in hydrodynamic pressure pattern, which can threaten the stability of the vessel. Modeling and investigating roll motion of a planing vessel may require a strong coupling between motions in vertical and transverse planes. In the present paper, we have used a mathematical model to analyze the roll of a planing hull by coupling surge, heave, pitch, and roll motions using 2D + T theory to study the effects of roll-induced vertical motions on roll coefficients and response. Mathematically computed forces and moments as well as roll dynamic response of the vessel are seen to be in fair quantitative agreement with experimentally measured values of previously published data. Using the 2D + T method, it has been shown that to model the roll of a planing hull at high speeds, we need to consider the effects of heave, pitch, and surge motions. Through our mathematical modeling, it is found that freedom in vertical motions increases time-dependent roll damping and added mass coefficients, especially at early planing speeds. The results of dynamic response simulations suggest that freedom in the vertical plane can decrease the roll response.

In this article, a coupled model is proposed for wave interaction with a pair of submerged floating tunnels in the presence of an array of bottom-standing trapezoidal porous breakwaters. The theory of Sollitt and Cross is adopted to govern the fluid flow inside the porous medium. For constant water-depth, the eigenfunction expansion method is employed, whereas for varying water-depth, the eigenfunction expansion method along with the mild-slope approximation is employed. The solutions, thus derived, are matched at the shared boundaries under defined physical conditions. First, the performance of a single breakwater of impermeable and permeable type in reducing wave forces on tunnels is analyzed. Next, the performance of two and three submerged breakwaters is studied. The reflection and transmission coefficients of waves are high in the absence of the submerged breakwater and in the presence of an impermeable breakwater. These coefficients significantly reduce in the presence of the submerged porous breakwater. As a result, the horizontal and vertical forces acting on bridges and tunnels are substantially subsided. Wave forces on tunnels reduce with an increase in the angle of incidence. Multiple porous breakwaters show better performance in mitigating wave forces on tunnels. Higher wave force on tunnels is noticed in intermediate water-depth. The findings can enhance the knowledge of submerged porous breakwaters’ performance in reducing wave loads on bridges and tunnels.

Autonomous docking guidance is one of the key technologies to achieve the autonomous underwater vehicle (AUV) docking with the sub-sea docking station (DS) to realize long-term resident operation. In the process of AUV docking, the combination of long-distance acoustic guidance based on acoustic sensor and terminal visual guidance based on camera is often adopted. However, affected by the accuracy of the navigation sensor and acoustic positioning sensor carried by AUV, as well as the ocean current, AUV cannot accurately know its own position and the position of the DS, resulting in a large acoustic guidance error and the inability to enter the visual guidance stage with a reasonable deviation, thus leading to the docking failure. In this article, an improved FastSLAM algorithm is proposed to estimate the position of AUV and DS simultaneously. The positioning accuracy of traditional FastSLAM algorithm is affected by such factors as the estimation accuracy of the statistical characteristics of process noise. An improved algorithm for FastSLAM based on fuzzy Q-learning is proposed. The homing path is planned based on the Dubins theory. The path is tracked by line-of-sight guidance. The results of matlab simulation and experimental data analyzing of the portable AUV are applied to verify the effectiveness of the proposed algorithm.

This paper presents a state-of-the-art digital twin of a hydraulic actuated winch that is used for heave compensation in offshore applications. The digital twin is used as part of a larger simulation model that involves all necessary components to perform lift planning and, subsequently, determine the corresponding weather window. The winch simulation model is described and verified by means of full-scale measurements. In addition, a set of acceptance criteria are presented that should be used whenever verifying digital twins of heave compensating winches that are to be used for lift planning.

Hovering control of autonomous underwater vehicles (AUVs) via a variable ballast system (VBS) is challenging owing to the difficulty in precisely estimating related hydrodynamic coefficients and vertical disturbance. In this work, a hierarchical control strategy is proposed which comprises an upper layer—the proportional-integral-derivative (PID) type ballast water mass planner generating the desired ballast mass, and a lower layer—the continuous mass flowrate controller adjusting the actual ballast mass. The resulting flowrate algorithm endows the system with local uniform asymptotic stability and robustness to both modeling errors and vertical disturbance. Numerical results verify the feasibility and effectiveness of the proposed hierarchical hovering control strategy.

In this present work, effects of three Euler angles (angle of attack (AOA), angle of trim (AOT), and angle of pitch (AOP)) of vertical cambered otter board on hydrodynamic characteristics (drag coefficient (C d ), lift coefficient (C l ), center-of-pressure coefficients (C p )) were studied based on numerical simulation combined with Kriging response surface methodology (KRSM) and multi-objective genetic algorithm (MOGA). Wind tunnel experiments were carried out to validate the accuracy of the response surface based on numerical simulation. It was demonstrated that AOA had noticeable effects on C d and C l , while AOT and AOP had fewer effects. The working posture of the otter board was recommended to lean inward (0 deg–6 deg) and forward (−10 deg–0 deg) to improve the lift-drag ratio without sacrificing C l . The influences of AOT and AOP on positions of center-of-pressure points were less significant than that of AOA and decreasing with the increase of AOA. Besides, the response surface of hydrodynamic coefficients around the critical AOA was a decent indicator of the occurrence of stall. Finally, three candidate cases were selected to satisfy the high working efficiency by MOGA, which was consistent with the above recommendations. This study provided a scientific reference of response surface experimental investigations methodology in the fishery engineering and the configuration of Euler angles of otter board.

An liquefied natural gas (LNG) FPSO is a floating, production, storage, and offloading facility with a LNG plant, including all ancillary facilities. In contrast to commercial vessels, the various parts of the process are located topside and are distributed as modules that are installed on the deck. The design of an LNG FPSO requires many considerations such as international codes and standards, the owner's requirements, and the operation and maintenance philosophy. To resolve the many considerations in FPSO design, we proposed an optimization method for the topside arrangement in the previous study. Based on the previous study, an integrated optimization technique for an LNG FPSO is proposed to obtain the optimal principle dimensions, the arrangement of the hull tanks, and the layout of the topside modules and equipment that satisfies many requirements for not only the topside but also the hull. As a case study, the topside and hull layout of an LNG FPSO is optimized in terms of safety, economics, and stability. The case study shows that the proposed method can develop the optimal layout design of an LNG FPSO.

Despite the common presence of water in oil production, just recently, the scientific community has devoted efforts to studying the influence of emulsion phenomena effects related to oil production using pumps. In the context of this study of phase inversion phenomena, the influence of viscosities and rotational speeds in electrical submersible pumps (ESPs) are evaluated as part of this effort. This study is aimed at investigating the influence of viscosity in phase inversion phenomena. An eight-stage ESP was tested with three different rotational speeds and two different oil viscosities for the best efficiency point (BEP) flow rates. Initially, the total flow rate was obtained in relation to BEP using ESP performance curves for pure oil at 52 cP and 298 cP and rotational speeds of 800 rpm, 1200 rpm, and 2400 rpm. The total flow rate was kept constant and the water cut was increased from 0 to 100%. The inversion phase phenomenon was detected by performance improvement when the water cut increased. The factors analyzed were the head and efficiency of the ESP as a function of the water cut. The phase inversion experimental data obtained in this study were compared with literature models for horizontal pipes. The results of this comparison presented satisfactory agreement. The phase inversion phenomena occur in all eight stages at the same time. Hysteresis was observed in ESPs for oil viscosity of 52 cP and rotating speed of 800 rpm and 1200 rpm.

The dynamic arc behaviors have a significant effect on the process stability and welded joint quality in underwater wet flux-cored arc welding (FCAW). In this study, clear underwater arc images are obtained by using a high-speed camera, which further reveal the complex physical process of underwater wet welding. The mechanism of underwater contact arc ignition is described in detail. Gas ionization and electron emission processes are carried out in bubbles generated by resistance heat. The dynamic arc behaviors are obviously different between bubble growth and separation stages. The high-speed gas flow in the bubble separation stage has a great disturbance to the arc. The arc shapes under water and in air under the same parameters are compared, and the mechanism of arc column shrinkage and arc length shortening is revealed from the aspects of water environment cooling, gas composition in bubbles, disturbance of gas flow, water pressure, and electric field strength.

An experimental campaign for the flow around a stationary array of three and four columns with low aspect ratio, H/L = 1.5, piercing the water free surface, was carried out in a towing tank. These numbers of columns correspond to typical multi-column offshore systems, such as semi-submersibles (SS), tension leg platforms (TLPs), and floating offshore wind turbines (FOWTs). Three parameters were investigated: the spacing ratio between column centers (from two up to four characteristic lengths), current incidence angles, and column section geometries (circular, square, and diamond). The Reynolds number of the experiments was 100,000. Forces were measured in each column using a three degrees-of-freedom load cell, and results of lift and drag forces were presented for each column separately and the whole system. The results of mean and standard deviation of forces were assessed using a statistical uncertainty analysis procedure for finite length measurements’ signals. This methodology not only assesses the quality of the experimental data but also facilitates validation of numerical tools. The objectives of the current work were therefore manifold: to better understand the influence of the relative position, shape, and incidence angle on multi-column offshore structures; to create a reliable database for computational fluid dynamics (CFD) validation; and to prepare the path to flow-induced motions (FIMs) experimental and numerical work of free-moving multi-column offshore systems.

This study suggests three different designs of the moonpool to reduce the internal flow of the moonpool. The experimental and numerical studies were performed to improve the dillship's operability by the flow reduction in the moonpool. Based on the previous result of model tests in two-dimensional (2D) wave flume, three different moonpool designs were proposed: design-1: combination of forward recess deck, vertical bulkhead and beach-type wave absorber; design-2: a step-type forward recess deck; and design-3: a forward recess deck with beach-type wave absorber. Experimental study was carried out in the Offshore Engineering Basin for 87 K compact drillship which was 87,000 Mt as design gross tonnage. To verify the experimental results and clearly understand the internal flow of the drillship's moonpool, numerical study was carried out using Reynolds-averaged Navier–Stokes based computational fluid dynamics (CFD) code. From these studies, it was found that all the proposed designs showed significantly reduced internal flow at resonance frequency compared to the original moonpool design of the target drillship, although the characteristic of internal flow reduction was different for each moonpool design. Additional studies were carried out to obtain the improved moonpool design that is expected to further reduce the internal flow of moonpool through CFD. Based on the results of three moonpool designs, the combined one with the step-type recess deck and the wave absorber presents best performance in the viewpoint of flow reduction in the moonpool.

Global warming is inducing sea ice retreat, which is opening new shipping routes and extending the accessible area for resource exploration. This encourages an increasing research interest in sea ice behavior. With the sea ice melting, level ice is broken up by waves propagated from the open ocean, resulting in an environment where both floating ice floes and waves exist. Such wave–ice interaction can bring significant influences on the potential human activities. This work presents a series of numerical simulations to predict the behavior of a circular ice floe forced by regular waves, with different wavelength and wave amplitude conditions being investigated. The numerical model was validated against experiments, and it revealed good accuracy in predicting the rigid body motion of an ice floe, including some extreme cases that are difficult to model by previous methods. Two specific behaviors were observed during the numerical simulations, namely overwash and scattering. Both behaviors are discussed in detail to analyze their linear/nonlinear effect on the ice floe motion. The applied model could be used to provide valuable estimations for arctic engineering purposes.

The objective of this research is to investigate the velocity of oil drops within the impeller of an electrical submersible pump (ESP) working with oil-in-water dispersion flows at different operational conditions. An experimental study was conducted using an ESP prototype with a transparent shell designed to enable flow visualization within the impeller channels. The tests were performed at three rotational speeds, 600, 900, and 1200 rpm, for three water flow rates, 80%, 100%, and 120% of the best efficiency point (BEP). A high-speed camera (HSC) with a lighting set captured images of the oil-in-water dispersion at 1000 frames per second. The images observation suggests the presence of a turbulent flow in the impeller. The turbulence, associated with high rotation Reynolds numbers, causes the oil drops to become smaller as the impeller rotational speed and the water flow rate increase. Despite this rotating environment, the oil drops generally have a spherical shape. Regarding the kinematics, the images processing reveals that the velocity of oil drops has a magnitude around a unit of m/s. The velocity depends on the oil drop position in the channel: oil drops that stay close to a suction blade (SB) have significantly higher velocities than oil drops that stay close to a pressure blade (PB). Considering a complex flow with water velocity profiles and pressure gradients, the analysis of oil velocity curves indicates the occurrence of accelerations that may be caused by drag and pressure forces acting on the oil drops.

Fatigue design standards for offshore structures became needed with development of offshore structures in harsh environments like the North Sea during the 1970s. The need for fatigue design of ship structures became increased as more high strength steel was being used in these structures during the 1970s. New types of structures and structural components have been developed such as tension leg platforms and floating production platforms and support structures for wind turbines. These structures are subjected to significant dynamic loading such that fatigue design becomes the main issue and relevant fatigue design standards are needed. This paper gives an overview of the development of fatigue design standards for marine structures over the last 40 years.

This work is motivated by the need to identify the fire and explosion risk on liquefied natural gas carriers (LNGCs) developed by Daewoo Shipbuilding & Marine Engineering Co., Ltd., because the main engines are designed to use highly pressurized natural gas (about 300 bar), which has caused vague fears of fire and explosion risks. In this context, to identify the risk of fires and explosions quantitatively, a fire and explosion risk analysis (FERA) was carried out for the LNGCs. This paper, as a part of the FERA, presents the results of a preliminary investigation on the effect of introducing the highly pressured fuel gas system into LNGCs on the fire and explosion risk especially in the cargo compressor room. This study is conducted in a comparative way considering the risk contribution of each parameter that could impact on the fire and explosion risk. The effect of the highly pressured fuel gas is indirectly taken into account by the change of the initial leak rate in the system. To identify effects of the considered parameters quantitatively, dozens of simulations for the selected gas dispersion, explosion, and fire scenarios were carried out using FLACS and KFX. Based on the simulation results, it is concluded that, in case of the LNGCs, the effects of the initial large leak rate due to the high pressure in the fuel gas pipes on the fire and explosion risk are not significant compared with the effects of other parameters such as leak amount, leak location, and leak direction.

Here, we present the concept of an open virtual prototyping framework (VPF) for maritime systems and operations that enables its users to develop reusable component or subsystem models, and combine them in full-system simulations for prototyping, verification, training, and performance studies. This framework consists of a set of guidelines for model coupling, high-level and low-level coupling interfaces to guarantee interoperability, a full-system simulation software, and example models and demonstrators. We discuss the requirements for such a framework, address the challenges and the possibilities in fulfilling them, and aim to give a list of best practices for modular and efficient virtual prototyping and full-system simulation. The context of our work is within maritime systems and operations, but the issues and solutions we present here are general enough to be of interest to a much broader audience, both industrial and scientific.

Recently, drillship 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. 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. For the in-depth understanding of the flow behaviors and characteristics, the internal flow of the moonpool has been investigated using Reynolds-averaged Navier–Stokes based computational fluid dynamics (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.

Inflatable devices that provide reserve buoyancy to damaged ships, preventing capsizing and/or sinking, along with lifting wreckages from the seabed, were studied within the framework of the European funded project “SuSy” (Surfacing System for Ship Recovery). Part of the work involved material evaluation and testing as well as simulations of the structural response. This paper first describes an orthotropic hyperelastic constitutive model for a candidate material also used in the fabrication of prototype inflatable devices. A strain energy density function is proposed that is further used to derive the stress and elasticity tensors required for the numerical implementation of the model in the user-defined subroutine (UMAT) of abaqus / standard . The second part of the paper presents the finite element simulation of the latter stages of inflation of two salvage devices inside an actual double bottom structure. The numerical results are in good agreement with tests conducted in dry land and under water, with the structure raised following the inflation of the devices. The evolving stress state in both the devices and the double bottom structure under increased contact interaction leads to useful conclusions for future use in the development of this salvage system.