Design Of Offshore Structures Research Articles

Evaluation of Hydrodynamic Force Coefficients in Presence of Biofouling on Marine/Offshore Structures, a Review and New Approach

Novel attempts to optimize the design and requalification of offshore structures draws attention to the importance of updating information about the environmental forces. One of the important steps to design or re-assess offshore structures is the re-evaluation/evaluation of bio-colonization’s effects. This paper presents a review of studies that considered biofouling in marine/offshore structures. Most of the previous researchers conducted the effects of biofouling as a surface roughness; however, some others proved that despite the surface roughness, other marine fouling components such as surface coverage ratio, biofouling species, and aggregation, may significantly influence hydrodynamic force coefficients, particularly at higher Reynolds numbers (Re). In addition, a new approach is proposed in this paper to estimate the drag coefficient of circular members covered by biofouling. The new approach relies on a multiple parameter equation and builds on the existing measurement of the drag force coefficient. Two relationships between biofouling parameters and drag coefficient are given for hard biofouling at the post-critical Re regime.

A Fourier Series Approximation for Deep-water Waves

Dean (1965) proposed the use of the root mean square error (RMSE) in the dynamic free surface boundary condition (DFSBC) and kinematic free-surface boundary condition (KFSBC) as an error evaluation criterion for wave theories. There are well known wave theories with RMSE more than 1%, such as Airy theory, Stokes theory, Dean’s stream function theory, Fenton’s theory, and trochodial theory for deep-water waves. However, none of them can be applied for deep-water breaking waves. The purpose of this study is to provide a closed-form solution for deep-water waves with RMSE less than 1% even for breaking waves. This study is based on a previous study (Shin, 2016), and all flow fields were simplified for deep-water waves. For a closed-form solution, all Fourier series coefficients and all related parameters are presented with Newton’s polynomials, which were determined by curve fitting data (Shin, 2016). For verification, a wave in Miche’s limit was calculated, and, the profiles, velocities, and the accelerations were compared with those of 5th-order Stokes theory. The results give greater velocities and acceleration than 5th-order Stokes theory, and the wavelength depends on the wave height. The results satisfy the Laplace equation, bottom boundary condition (BBC), and KFSBC, while Stokes theory satisfies only the Laplace equation and BBC. RMSE in DFSBC less than 7.25×10-2% was obtained. The series order of the proposed method is three, but the series order of 5th-order Stokes theory is five. Nevertheless, this study provides less RMSE than 5th-order Stokes theory. As a result, the method is suitable for offshore structural design.

Computer-Aided System for Parametric Design of Ship Hull Structures—CADS-Hull

The problem of the parametric design of ship hull structures is considered here. The parametric design of ship hull structures is intended to determine the dimensions of structural elements that meet the requirements of regulatory documents (Rules of Classification Societies/Strength Standards). It is shown that, in all existing shipbuilding CAD-CAM systems, the problems of the parametric design of ship hull structures are essentially not affected. A modern methodology for the parametric design of hull structures is proposed, which defines a set of principles for the structural and logical organization of the design process, methods, and technical means for implementing design procedures. The most characteristic methodological principles of the parametric design of ship hull structures and other offshore engineering structures are identified: A systematic approach; modeling as one of the main methods for solving design problems; and a rational design strategy based on iterative search procedures that implement the methodological principle of successive complication of models and procedures for designing structures. The article considers the architecture and methodological foundations of the system for automated parametric design of ship hull structures (CADS-Hull), developed at the Saint Petersburg State Marine Technical University, Russia. The system is intended for use in design bureaus when designing civil ship hull structures on the basis of regulatory requirements of the Russian Maritime Register of Shipping and IACS General Rules in terms of large-capacity tankers, bulk carriers, and container ships.

GMPEs for elastic input energy spectra of horizontal and vertical offshore ground motions based on the ETMC database in Japan

In this study, the characteristics of offshore ground motions related to elastic relative and absolute input energy are investigated, and ground-motion prediction equations (GMPEs) for elastic input energy of offshore ground motions are proposed. The GMPEs are developed using mixed-effects models calibrated through regression analysis of elastic responses which are derived from dynamic analyses of single-degree-of-freedom (SDOF) systems subjected to offshore ground motions. To compare the differences between offshore and onshore ground motions with regard to input energy, an onshore GMPE is derived from the onshore database. The ground motion database includes 892 offshore records and 4033 onshore records; the offshore and onshore records are collected from the same set of 273 earthquakes. To quantitatively analyze the elastic input energy of offshore ground motions, the moment magnitude, tectonic source type, focal depth, hypocentral distance and site condition are considered in the proposed equations. A comparison and analysis of the relative and absolute input energy of offshore and onshore ground motions manifest that the horizontal input energy spectra of the entire period band and the vertical input energy spectra of moderate and long period band of offshore ground motions are larger than those of onshore motions of all site classes, except that the vertical input energy of offshore motions is slightly smaller than that of onshore motions for short periods due to the combined effects of water layer and soft sediments under seafloor. The differences between the relative and absolute input energy of offshore ground motions and comparisons of the proposed GMPEs with others from the literature are also discussed in this study. The comparisons show that the proposal of the GMPE for elastic input energy of the offshore ground motion is necessary due to the offshore ground motion being considerably different from the onshore motion and the proposed onshore GMPEs. • The prediction equations for elastic input energy of offshore ground motions are proposed, providing the seismic demand for the energy-based seismic design of offshore structures. • The site condition of offshore ground motions is significantly different from those of onshore motions. • The offshore ground motions are larger than those of onshore ones for long period due to the effect of thick soft sediments.

Application of Arbitrary Lagrangian–Eulerian strips with fully nonlinear wave kinematics for force estimation

The initial design of offshore constructions generally uses a combination of model tests and second-order wave theory to reconstruct the wave kinematics to determine wave forces using the Morison equation. This routine becomes difficult to apply to steep waves without model test data where the second-order theory is not valid. In this article, a novel approach combining a fully nonlinear numerical wave tank (NWT) using the σ-grid with strip theory and the Morison equation in an Arbitrary Lagrangian–Eulerian (ALE) framework is presented. This provides improved wave kinematics from the nonlinear numerical wave tank, an improved representation of the instantaneous velocity field associated with the instantaneous location of the free surface, making an accurate estimation of the wave hydrodynamics of highly nonlinear and breaking waves possible at a reasonable computational cost.The method is verified by comparing the calculated wave kinematics for a fifth-order Stokes wave with the analytical solution. The procedure for the estimation of the hydrodynamic loads on a cylinder is validated using comparisons with model test data for wave forces on a cylinder due to regular, irregular and focussed waves. A good agreement is seen for the estimated wave forces, including for the breaking waves in the irregular wave train. Some overestimation of the wave forces due to irregular waves is seen due to the frequency-independent force coefficient used in the Morison equation. The results demonstrate the potential of the ALE method combined with a nonlinear NWT to evaluate a three-hour irregular sea state simulation and generate wave loading statistics for the initial stochastic design of offshore structures without model test data.

Study on Dynamic Response of Offshore Wind Turbine Structure Under Typhoon

Abstract Floating offshore wind turbines are easily affected by typhoons in the deep sea, which may cause serious damage to their structure. Therefore, it is necessary to study further the dynamic response of wind turbine structures under typhoons. This paper took the 5MW floating offshore wind turbine developed by the National Renewable Energy Laboratory (NREL) as the research object. Based on the motion theory of platforms in waves, a physical model with a scale ratio of 1:120 was established, and a hydraulic cradle was used to simulate the effect of waves on the turbines. The dynamic response characteristics of offshore wind turbines under typhoons are systematically studied. The research results clarified that the turbine structure is mainly affected by wave loads under typhoons, and its motion response reaches its maximum value under the action of extreme wave loads. The research results of this paper can provide reference value for the design of offshore wind turbine structures under typhoons.

Representation of 3-h Offshore Short-Crested Wave Field in the Fully Nonlinear Potential Flow Model REEF3D::FNPF

Abstract Stochastic wave properties are crucial for the design of offshore structures. Short-crested seas are commonly seen at the sites of offshore structures, especially during storm events. A long time duration is required in order to obtain the statistical properties, which is challenging for numerical simulations. In this scenario, a potential flow solver is ideal due to its computational efficiency. A procedure of reproducing accurate short-crested sea states using the open-source fully nonlinear potential flow model REEF3D::FNPF is presented in the paper. The procedure examines the sensitivity of the resolutions in space and time as well as the arrangements of wave gauge arrays. A narrow band power spectrum and a mildly spreading directional spreading function are simulated, and an equal energy method is used to generate input waves and avoid phase-locking. REEF3D::FNPF solves the Laplace equation together with the boundary conditions using a finite difference method. A sigma grid is used in the vertical direction and the vertical grid clustering follows the principle of constant truncation error. High-order discretization methods are implemented in space and time. Message passing interface is used for high performance computation using multiple processors. Three-hour simulations are performed in full-scale at a hypothetic offshore site with constant water depth. The significant wave height, peak period, kurtosis, skewness and ergodicity are examined in the numerically generated wave field. The stochastic wave properties in the numerical wave tank (NWT) using REEF3D::FNPF match the input wave conditions with high fidelity.

The effects of smoothing length on the onset of wave breaking in smoothed particle hydrodynamics (SPH) simulations of highly directionally spread waves

Ocean wave breaking is a difficult-to-model oceanographic process, which has implications for extreme wave statistics, the dissipation of wave energy, and air–sea interaction. Numerical methods capable of reliably simulating real-world directionally spread breaking waves are useful for investigating the physics of wave breaking and for the design of offshore structures and floating bodies. Smoothed particle hydrodynamics is capable of modelling highly steep and overturning free surfaces, which makes it a promising method for simulating breaking waves. This paper investigates the effect of smoothing length on simulated wave breaking in both following and crossing seas. To do so, we reproduce numerically the experiments of highly directionally spread breaking waves in McAllister et al. (J Fluid Mech 860:767–786, 2019. https://doi.org/10.1017/jfm.2018.886) using a range of normalised smoothing lengths: \(h/d_p=1.4\), 1.7, 2.0, 2.3, with h smoothing length and \(d_p\) particle spacing. The smallest smoothing length we use appears to adversely affect the fidelity of the simulated surface elevation, so that the tallest wave crest observed in experiments is not fully reproduced (coefficient of determination \(r^2\approx 0.7\)). For smoothing lengths \(h/d_p=1.7\), 2.0, and 2.3, the experiments are well reproduced (\(r^2\ge 0.88\)); in these simulations smoothing length predominantly affects the spatial extent and duration of breaking. Qualitative and quantitative comparison of our simulations shows that values of \(h/d_p\) in the range \(1.7{-}2\) best reproduce the wave breaking phenomena observed in experiments.

Analysis, design and construction of gravity offshore structure; state-of-the-art

Abstract: Offshore platforms are divided into many types which are mainly categorized according to waterdepth in the installation location. However, the design differs for each type to accomplish the target of the operation. But for some case of sea waterdepths and an aggressive environment such as the North Sea, steel ones are not suitable, so the heaviest type called gravity platform having enormous mass is used. This type of platform has its special requirements and procedures for construction and needs special types of construction materials in order to resist the climate factors applied due to the aggressive environment. The paper carefully illustrates how the principal Environmental loads (wind and wave), current forces, loads from ice and loads from earth-quake for (earth-quake prone zones) are deployed to archive the design and construction of offshore concrete gravity platforms. Two design methods (Analysis and Design of Shell structures) and the Tangent Modulus Methods of design of Offshore Concrete Gravity platforms are discussed

Varying ocean wave statistics emerging from a single energy spectrum in an experimental wave tank

As it strongly impacts the design of offshore structures, the realistic reproduction of sea states in experimental and numerical wave tanks is of great interest to the ocean engineering community. The vast majority of wave qualification procedures rely on the accurate control of i) the wave energy spectrum and ii) the wave crest statistics at a target location in the domain. However the control of the wave field is strongly challenged by nonlinear phenomena such as breaking and high-order nonlinearities, which are at the origin of significant variations of the wave properties along the tank. Considering this issue, the most common industry methodologies focus on reproducing the wave energy spectrum at the target position. The wave crest statistics are compared to reference distributions. The present study aims to explore the limitations of such a practice, investigating in detail the wave field properties for various target locations over a long domain. We address the problem within the framework of deep water long-crested irregular waves. In this respect, using the Ecole Centrale de Nantes (ECN) experimental facility, a specific sea state is consecutively generated at three positions of the wave tank using a dedicated procedure based on wave maker motion iterative corrections. For such nonlinear wave conditions the wave crest statistics are known to be enhanced along the tank by high-order nonlinearities. As a result, configurations characterized by identical wave spectra lead to the generation of different wave crest distributions, revealing an increasing number of extreme events as the spectrum is generated farther from the wave maker. The data yielded by the study provide convincing evidence that controlling the wave field at a target location by correcting the sole energy spectrum is insufficient. Particular attention must be paid to the nonlinear spatial dynamics of the wave field in order to control the wave crest statistics.

Wave Analysis for Offshore Aquaculture Projects: A Case Study for the Eastern Mediterranean Sea

The investigation of wave climate is of primary concern for the successful implementation of offshore aquaculture systems as waves can cause significant loads on them. Up until now, site selection and design (or selection) of offshore cage system structures on extended sea areas do not seem to follow any specific guidelines. This paper presents a novel methodology for the identification of favorable sites for offshore aquaculture development in an extended sea area based on two important technical factors: (i) the detailed characterization of the wave climate, and (ii) the water depth. Long-term statistics of the significant wave height, peak wave period, and wave steepness are estimated on an annual and monthly temporal scale, along with variability measures. Extreme value analysis is applied to estimate the design values and associated return periods of the significant wave height; structures should be designed based on this data, to avoid partial or total failure. The Eastern Mediterranean Sea is selected as a case study, and long-term time series of wave spectral parameters from the ERA5 dataset are utilized. Based on the obtained results, the most favorable areas for offshore aquaculture installations have been identified.

Review of Recent Offshore Wind Turbine Research and Optimization Methodologies in Their Design

As international efforts to address climate change grow, an increasing number of countries and companies have put forward a clear “net zero” goal through accelerated renewable-energy development. As a renewable energy source, offshore wind energy has received particular attention from many countries and is a highly active research area. However, the design of offshore wind turbine structures faces challenges due to the large and complex design parameter space as well as different operational requirements and environmental conditions. Advanced optimization technology must be employed to address these challenges. Using an efficient optimization algorithm, it is possible to obtain optimized parameters for offshore wind turbine structures, balancing energy generation performance and the life of the floating wind turbine. This paper presents a review of the types and fundamental principles of several critical optimization technologies along with their application in the design process, with a focus on offshore wind turbine structures. It concludes with a discussion of the future prospects of optimization technology in offshore wind research.

Efficient derivation of extreme non-Gaussian stochastic structural response using the finite-memory nonlinear system (FMNS NL ). Part 1: model development

ABSTRACT For offshore structural design, the load due to wind-generated random waves is usually the most important source of loading. The most accurate method that is commonly used to evaluate the offshore structural response on the subjected load is the Monte Carlo time simulation which can account for all sorts of nonlinearities without introducing any approximations. However, it is computationally very demanding due to its complex procedure. A simple method using finite-memory nonlinear system (FMNS) has been introduced. The method is, however, only applicable based on the linear wave analysis. Hence, by taking advantage on the efficiency of FMNS method, a new model needs to be developed by apply the FMNS method with a nonlinear wave analysis for a more reliable result. The most appropriate model can then be used to determine the probability distribution of response extreme values with great efficiency and accuracy.

Development of a probability distribution model for the SCFs in tubular X-connections retrofitted with FRP

In this study, results of performing 324 FE analyses on 108 circular hollow section (CHS) X-connections with fiber reinforced polymer (FRP) have been used to derive a probability distribution model for SCFs in the retrofitted joints under axial, IPB, and OPB loads. As far as the authors are aware, no study has been conducted on the probability distribution of SCFs in X-connections with FRP. However, the probability distribution models are essential for the reliability-based analysis and design of offshore structures. Therefore, based on the retrofitted X-connection FE models which were verified with several experimental tests, a set of SCF sample databases was generated. In these SCF sample databases, different FRP properties (layer number and material) and connection geometry (β, τ, and γ) are included. 10 various probability density functions (PDFs) were fitted to the relative frequency histograms of the SCF samples. For obtaining the parameters of each distribution, the maximum likelihood method was used. Assessing the goodness-of-fit based on the Chi-squared and Kolmogorov-Smirnov tests shown that the Log-normal distribution is the best one for the maximum SCF values in CHS X-connections retrofitted with FRP subjected to axial load. In addition, the Log-logistic and Generalized Extreme Value distributions were set as the governing probability function for the retrofitted X-connections under IPB and OPB loads, respectively. In the last step, five fully defined PDFs were established for the maximum SCFs in CHS X-joints retrofitted with FRP under three various types of loadings.

Wave Climate Associated With Changing Water Level and Ice Cover in Lake Michigan

Detailed knowledge of wave climate change is essential for understanding coastal geomorphological processes, ecosystem resilience, the design of offshore and coastal engineering structures and aquaculture systems. In Lake Michigan, the in-situ wave observations suitable for long-term analysis are limited to two offshore MetOcean buoys. Since this distribution is inadequate to fully represent spatial patterns of wave climate across the lake, a series of high-resolution SWAN model simulations were performed for the analysis of long-term wave climate change for the entirety of Lake Michigan from 1979 to 2020. Model results were validated against observations from two offshore buoys and 16 coastal buoys. Linear regression analysis of significant wave height (Hs) (mean, 90th percentile, and 99th percentile) across the entire lake using this 42-year simulation suggests that there is no simple linear trend of long-term changes of Hs for the majority (>90%) of the lake. To address the inadequacy of linear trend analysis used in previous studies, a 10-year trailing moving mean was applied to the Hs statistics to remove seasonal and annual variability, focusing on identifying long-term wave climate change. Model results reveal the regime shifts of Hs that correspond to long-term lake water level changes. Specifically, downward trends of Hs were found in the decade of 1990–2000; low Hs during 2000–2010 coincident with low lake levels; and upward trends of Hs were found during 2010–2020 along with rising water levels. The coherent pattern between the wave climate and the water level was hypothesized to result from changing storm frequency and intensity crossing the lake basin, which influences both waves (instantly through increased wind stress on the surface) and water levels (following, with a lag through precipitation and runoff). Hence, recent water level increases and wave growth were likely associated with increased storminess observed in the Great Lakes. With regional warming, the decrease in ice cover in Lake Michigan (particularly in the northernmost region of the lake) favored the wave growth in the winter due to increased surface wind stress, wind fetch, and wave transmission. Model simulations suggest that the basin-wide Hs can increase significantly during the winter season with projected regional warming and associated decreases in winter ice cover. The recent increases in wave height and water level, along with warming climate and ice reduction, may yield increasing coastal damages such as accelerating coastal erosion.

Design of the submerged horizontal plate breakwater using a fully coupled hydroelastic approach

AbstractThis work provides a novel approach that combines computational fluid dynamics (CFD) with computational solid mechanics (CSM) to dynamically simulate the fully coupled hydroelastic interaction between nonlinear ocean waves and a submerged horizontal plate breakwater (SHPB). Based on a systematic series of simulations, it is shown that the wave damping performance of an SHPB can be evidently improved by an appropriate extent of deformation, which can be achieved through the design of its bending stiffness by varying elasticity and/or aspect ratio. The wave attenuation effect is found to be maximized when an SHPB has a deformation amplitude close to the incident wave amplitude. By accounting for the hydroelastic effect through the fully coupled CFD+CSM approach, this work presents a new computational technique that supports the design of deformable offshore and coastal structures.

The influence of Stress Concentration Factors and SN curves in assessing the reliability of jacket marine platforms

The design of offshore structures, generally located in a hostile environment, with complex and repetitive loads, with low and difficult intervention possibilities in case of accidents, is a challenge for engineer teams, starting from the design phase and ending with that of decommissioning. In addition to the approximations generated by the structural calculation, the specification of the level of effort concentration and the relationship between effort and fatigue life are also sources of uncertainties in estimating fatigue degradation. Analysis of the sensitivity of fatigue degradation to stress concentration factors (SCF) and S-N curves is an important factor in obtaining realistic results. The paper seeks to establish the optimal yield theory that can be used to evaluate the structural response to cyclic loads associated with computational assumptions by comparing fatigue conditions due to fatigue at the nodes of a complex lattice-type marine drilling structure, to minimize assessment errors of structural reliability. A structural model of jacket marine platform with dimensional and material characteristics corresponding to Romanian marine platforms of “Gloria” type it is analysed, for which is evaluate and compared the fatigue life generated by hydrodynamic loads, using stress concentration factors given by Kellog and Kuang, SN curves: AWS-X, modified AWS-x and respectively BS-F. The results show that for the self-erecting platforms with lattice beam type, with small dimension elements, the stress concentration factors determined by Kellog are closer to reality while Kuang factors gives more covering results.

Thermal analysis of marine structural steel EH36 subject to non-spreading cryogenic spills. Part II: finite element analysis

ABSTRACT In Part I of this paper, six liquid nitrogen (LN2) pool boiling tests were carried out to locally cool down six different EH36 steel plates to cryogenic temperatures. Leindenfrost and Critical Heat Flux (CHF) points of LN2 boiling curve were estimated. These estimations together with the recorded temperature histories are used in this part for the development of a heat flux curve for EH36 steel-LN2 pool boiling through Finite Element Thermal Analysis (FETA) of the problem investigated experimentally in Part I. Thermal conductivity and heat capacity of EH36 are defined by two unprecedented temperature functions based upon experimental studies in cryogenic temperatures. A user subroutine written in FORTRAN defines the air convection coefficient as a function of surface-air temperature difference, which changes with time and location. Results of this study contribute to the Accidental Limit State (ALS) design of marine and offshore structures for liquified gas spill scenarios.

CFD study of wave interaction with single and two tandem circular cylinders

Wave-structure interaction is a key consideration for the safe design of offshore structures. This work investigates the non-linear wave interactions with a single cylinder and two tandem cylinders using a numerical wave tank. The numerical simulations are carried out by the in-house CFD solver naoe-FOAM-SJTU, which is developed on the open-source platform OpenFOAM. The numerical simulation is first validated by comparing numerical results and experimental data for regular wave interaction with a single cylinder. Wave interaction with tandem cylinders is then investigated for different center-to-center distances between the cylinders. The surface elevation around the cylinder is probed by a series of wave probes and analyzed using the Fast Fourier analysis. The response amplitude operates of surface elevation for single cylinder case agrees well with experimental data. The wave scattering around the cylinder is qualitatively compared with experimental observation and discussed in detail. The wave scattering fields and wave forces are systematically analyzed. Then the interaction between wave and two tandem cylinders is investigated. The distance between the cylinders highly influences the free surface elevation and wave forces experienced by the tandem cylinders. The local surface elevation and the scattered wave field around the cylinders are also discussed.

Penetration and Pullout Capacity of Low-Skirted Suction Caissons

The bearing capacity of suction caissons is the key to the design of offshore structures. A new type of cross-shaped low-skirted suction caisson is invented to effectively improve the bearing capacity, considering inevitable “soil plug” phenomenon. The behaviors of penetration and pullout for new low-skirted suction caisson are investigated by performing model tests. A new formula for calculating the penetration resistance is suggested based on the limit equilibrium theory and the test data, which can consider the change of the lateral area of the suction caisson during penetration. The behaviors of low-skirted suction caisson under inclined loading are analyzed by carrying out finite element simulation. The effects of loading angles and loading positions on the ultimate bearing capacity and failure mechanism of low-skirted suction caissons are discussed. The research results can provide a reference for the design of suction bucket foundation for offshore structures.