Showing papers in "Journal of Offshore Mechanics and Arctic Engineering-transactions of The Asme in 2011"

Constrained Optimal Control of a Heaving Buoy Wave-Energy Converter

Abstract: Current prognoses are that, unless counteracted by very strong political measures, the world will meet both energy shortage and climate crisis within a horizon of a few decades, both of which are strongly related to our dependence on fossil fuels. Renewable energy sources may be harvested sustainably, and developing technology for their exploitation therefore forms an obvious part of strategies to reduce emissions and secure energy supply. Wave energy is a resource with relatively high power density, readily available along the coasts, and thus coinciding with the areas where industry and people tend to be accumulated. In some regions this resource is large enough to form a significant part of the energy mix. The technology for harnessing the power of ocean waves is today still on the research and development stage. The challenge is to make a design where the costs of investment, operation and maintenance (in terms of money, resources and energy) can be justified by the availability and potential earnings. This thesis focuses on two aspects of systems for wave energy conversion: How to model such systems, which is important for understanding and design, and how to control their motion, which is crucial for the primary power conversion – the inevitable step that forms the basis for revenues and energy output from such a device. The dissertation is based on articles published in scientific conferences and journals, as well as an account for background of the undertaken research and the methods used. The bond graph modelling language has been chosen as a promising aid for the modelling of the power converter dynamics. It enables a systematic and transparent approach to the path from drawing board to mathematical equations. Examples show how energy conversion systems may be modelled and simulated within this framework. These include heave-motion models for a semi-submerged sphere, a platform/buoy two-body system and a smallscale oscillating water column (OWC), as well as wave-to-wire models of two made-up systems. The OWC model was also studied by laboratory experiments. A range of control strategies has been studied and compared by numerical simulation, and in one case also by laboratory experiments. These strategies include phase control by latching and by clutching, approximations to complex-conjugate control, and model predictive control (MPC). Constraint handling and real-time parameter tuning are discussed, too. The constrained optimal power absorption is investigated, and for the example of a semi-submerged heaving sphere in irregular waves it is found that MPC in combination with a Kalman filter predictor is able to provide an absorbed power in excess of 90% as compared to the non-causal (and hence not completely realisable) constrained optimum. Other causal controller implementations gives an absorbed power ranging from 10 to 90% of that achieved with MPC. The best performing control strategies, however, involve a large flow of reactive power through the machinery, which in normal irregular-wave operation may give peak-to-average power ratio as high as 25 and above. This represents a challenge to the design of machinery and controller. An interesting observation from the numerical simulations is the possibility of increased absorbed power in irregular waves as compared to regular waves having about the same wavelength characteristics and the same wave power level. An explanation is suggested for this phenomenon.

A Comparison of Selected Strategies for Adaptive Control of Wave Energy Converters

Abstract: Current prognoses are that, unless counteracted by very strong political measures, the world will meet both energy shortage and climate crisis within a horizon of a few decades, both of which are strongly related to our dependence on fossil fuels. Renewable energy sources may be harvested sustainably, and developing technology for their exploitation therefore forms an obvious part of strategies to reduce emissions and secure energy supply. Wave energy is a resource with relatively high power density, readily available along the coasts, and thus coinciding with the areas where industry and people tend to be accumulated. In some regions this resource is large enough to form a significant part of the energy mix. The technology for harnessing the power of ocean waves is today still on the research and development stage. The challenge is to make a design where the costs of investment, operation and maintenance (in terms of money, resources and energy) can be justified by the availability and potential earnings. This thesis focuses on two aspects of systems for wave energy conversion: How to model such systems, which is important for understanding and design, and how to control their motion, which is crucial for the primary power conversion – the inevitable step that forms the basis for revenues and energy output from such a device. The dissertation is based on articles published in scientific conferences and journals, as well as an account for background of the undertaken research and the methods used. The bond graph modelling language has been chosen as a promising aid for the modelling of the power converter dynamics. It enables a systematic and transparent approach to the path from drawing board to mathematical equations. Examples show how energy conversion systems may be modelled and simulated within this framework. These include heave-motion models for a semi-submerged sphere, a platform/buoy two-body system and a smallscale oscillating water column (OWC), as well as wave-to-wire models of two made-up systems. The OWC model was also studied by laboratory experiments. A range of control strategies has been studied and compared by numerical simulation, and in one case also by laboratory experiments. These strategies include phase control by latching and by clutching, approximations to complex-conjugate control, and model predictive control (MPC). Constraint handling and real-time parameter tuning are discussed, too. The constrained optimal power absorption is investigated, and for the example of a semi-submerged heaving sphere in irregular waves it is found that MPC in combination with a Kalman filter predictor is able to provide an absorbed power in excess of 90% as compared to the non-causal (and hence not completely realisable) constrained optimum. Other causal controller implementations gives an absorbed power ranging from 10 to 90% of that achieved with MPC. The best performing control strategies, however, involve a large flow of reactive power through the machinery, which in normal irregular-wave operation may give peak-to-average power ratio as high as 25 and above. This represents a challenge to the design of machinery and controller. An interesting observation from the numerical simulations is the possibility of increased absorbed power in irregular waves as compared to regular waves having about the same wavelength characteristics and the same wave power level. An explanation is suggested for this phenomenon.

Extreme Dynamic Structural Response Analysis of Catenary Moored Spar Wind Turbine in Harsh Environmental Conditions

Abstract: Floating wind turbines can be the most practical and economical way to extract the vast offshore wind energy resources at deep and intermediate water depths. The Norwegian Ministry of Petroleum and Energy is strongly committed to developing offshore wind technology that utilises available renewable energy sources. As the wind is steadier and stronger over the sea than over land, the wind industry recently moved to offshore areas. Analysis of the structural dynamic response of offshore wind turbines subjected to stochastic wave and wind loads is an important aspect of the assessment of their potential for power production and of their structural integrity. Of the concepts that have been proposed for floating wind turbines, spar-types such as the catenary moored spar (CMS) and tension leg spar (TLS) wind turbines seem to be well-suited to the harsh environmental conditions that exist in the North Sea. Hywind and Sway are two examples of such Norwegian concepts; they are based on the CMS and TLS, respectively. Floating wind turbines are sophisticated structures that are subjected to simultaneous wind and wave actions. The coupled nonlinear structural dynamics and motion response equations of these turbines introduce geometrical nonlinearities through the relative motions and velocities. Moreover, the hydrodynamic and aerodynamic loading of this type of structure is nonlinear. A floating wind turbine is a multibody aero-hydro-servo-elastic structural system; for such structures, the coupled nonlinear equations of motion considering nonlinear excitation and damping forces, including all wave- and wind-induced features, should be solved in the time domain. In this thesis, the motion and structural responses for operational and extreme environmental conditions were considered to investigate the performance and the structural integrity of spar-type floating wind turbines. The power production and the effects of aerodynamic and hydrodynamic damping, including wind-induced hydrodynamic and wave-induced aerodynamic damping, were investigated. Negative damping adversely affects the power performance and structural integrity. In this thesis, the controller gains were tuned to remove servo-induced instabilities. The rotor configuration effect on the responses and power production was investigated by comparing the upwind and downwind turbines. To develop robust design tools for offshore wind power, the competencies of the offshore technology and wind technology must be combined. Both the offshore and wind energy industries have begun to extend their existing numerical codes to account for the combined aerodynamic and hydrodynamic effects on the structure. As a result verifications of extended codes by doing experiments and code-to-code comparisons are needed. One of the aspects of the present research was to fill this gap by performing hydrodynamic and hydro-elastic comparison between commercial codes. For both CMS and TLS concepts, the comparisons were carried out prior to using the tools to study the behaviour of the CMS and TLS under wave- and wind-induced loads. Offshore structures encounter a variety of operational and harsh environmental conditions. Limit states such as ultimate, fatigue, accidental collapse and serviceability limit states (ULS, FLS, ALS and SLS) are defined as the design criteria for offshore structures. In performing realistic ultimate limit state analysis, the extreme responses of a floating wind turbine over its life should be estimated. This estimation requires detailed analysis of the extreme response. In the present thesis, extreme value analysis for spar-type wind turbines subjected to simultaneous wave and wind actions was preformed. The structural responses and the effect of modelled forces such as turbulence on these responses were investigated. The joint distribution of the environmental characteristics of the wave and wind was applied through the contour surface method. Stochastic wave and wind analysis showed that, while rigid body modelling was sufficient for obtaining accurate motions, consideration of the elastic behaviour of the tower/support structure was necessary to predict structural responses. The blades structural responses were found to be significantly affected by the turbulent wind. However, the mean and standard deviation of global motion and structural responses were not affected by the turbulence. Thus, to reduce the simulation time in fatigue analysis, a constant wind speed model can be applied. The CMS and TLS wind turbines are inertia-dominated structures, and the hydrodynamic viscous drag did not affect their wave-induced responses, while an increase in viscous drag could effectively reduce the resonant responses of such turbines. Under operational conditions, aerodynamic damping was found to be active in reducing both wave frequency and resonant responses. The results showed that, for a floating wind turbine, extreme response could occur in survival conditions, while for a fixed wind turbine, the extreme response occurs in operational cases related to the rated wind speed. To estimate the extreme value responses, extrapolation methods were used to reduce the sample size in Monte Carlo simulations. The accuracy of methods to estimate the extreme responses as a function of sample size and methods applied was investigated. The normalized responses for both CMS and TLS offshore wind turbines were presented to draw more generalized conclusions.

Undrained Load Capacity of Torpedo Anchors Embedded in Cohesive Soils

Abstract: This paper presents a numerical based study on the undrained load capacity of a typical torpedo anchor embedded in a purely cohesive isotropic soil using a three-dimensional nonlinear finite element model. In this model, the soil is simulated with solid elements capable of representing its nonlinear physical behavior and the large deformations involved. The torpedo anchor is also modeled with solid elements, and its geometry is represented in detail. Moreover, the anchor-soil interaction is addressed with contact finite elements that allow relative sliding with friction between the surfaces in contact. A number of analyses are conducted in order to understand the response of this type of anchor when different soil undrained shear strengths, load directions, and number and width of flukes are considered. The results obtained indicate two different failure mechanisms: The first one involves significant plastic deformation before collapse and, consequently, mobilizes a great amount of soil; the second is associated with the development of a limited shear zone near the edge of the anchor and mobilizes a small amount of soil. The total contact area of the anchor seems to be an important parameter in the determination of its load capacity, and, consequently, the increase in the undrained shear strength and the number of flukes and/or their width significantly increases the load capacity of the anchor.

Theoretical and Experimental Study on a Porous Cylinder Floating in Waves

Abstract: In the present work, theoretical and experimental studies on the interaction of water waves with a truncated circular cylinder were performed The cylinder, which is partly made of porous materials, possesses a porous sidewall and an impermeable bottom A nondimensional parameter b is adopted in the theoretic formulation to describe the porosity, which is not directly related to the opening ratio τ of the porous materials To validate the theoretical work and computed results, a series of model tests are carried out in a wave basin Effort is made to establish an empirical relation between b and τ based on the comparison of the calculation and experimental data The phenomenon of the sloshing mode that occurred at a certain wave number is observed, which might have an application in breakwaters The validation of the Haskind relations is examined for the porous body It is found that the damping coefficient consists of two parts In addition to the component of conventional wave-radiating damping, there exists a second component caused by the porous effects

Modeling the Seasonality of Extreme Waves in the Gulf of Mexico

Abstract: Statistics of storm peaks over threshold depend typically on a number of covariates including location, season, and storm direction. Here, a nonhomogeneous Poisson model is adopted to characterize storm peak events with respect to season for two Gulf of Mexico locations. The behavior of storm peak significant wave height over threshold is characterized using a generalized Pareto model, the parameters of which vary smoothly with season using a Fourier form. The rate of occurrence of storm peaks is also modeled using a Poisson model with rate varying with season. A seasonally varying extreme value threshold is estimated independently. The degree of smoothness of extreme value shape and scale and the Poisson rate with season are regulated by roughness-penalized maximum likelihood; the optimal value of roughness is selected by cross validation. Despite the fact that only the peak significant wave height event for each storm is used for modeling, the influence of the whole period of a storm on design extremes for any seasonal interval is modeled using the concept of storm dissipation, providing a consistent means to estimate design criteria for arbitrary seasonal intervals. The characteristics of the 100 year storm peak significant wave height, estimated using the seasonal model, are examined and compared with those estimated ignoring seasonality.

A Spatiodirectional Model for Extreme Waves in the Gulf of Mexico

Abstract: The characteristics of extreme waves in hurricane dominated regions vary systematically with a number of covariates, including location and storm direction. Reliable estimation of design criteria requires incorporation of covariate effects within extreme value models. We present a spatiodirectional model for extreme waves in the Gulf of Mexico motivated by the nonhomogeneous Poisson model for peaks over threshold. The model is applied to storm peak significant wave height HS for arbitrary geographic areas from the proprietary Gulf of Mexico Oceanographic Study (GOMOS) hindcast for the US region of the Gulf of Mexico for the period of 1900-2005. At each location, directional variability is modeled using a nonparametric directional location and scale; data are standardized (or "whitened") with respect to local directional location and scale to remove directional effects. For a suitable choice of threshold, the rate of occurrence of threshold exceedences of whitened storm peak HS with direction per location is modeled as a Poisson process. The size of threshold exceedences is modeled using a generalized Pareto form, the parameters of which vary smoothly in space, and are estimated within a roughness-penalized likelihood framework using natural thin plate spline forms in two spatial dimensions. By reparameterizing the generalized Pareto model in terms of asymptotically independent parameters, an efficient back-fitting algorithm to estimate the natural thin plate spline model is achieved. The algorithm is motivated in an appendix. Design criteria, estimated by simulation, are illustrated for a typical neighborhood of 17×17 grid locations. Applications to large areas consisting of more than 2500 grid locations are outlined. © 2011 American Society of Mechanical Engineers.

Hydrodynamic Analysis of a Horizontal Axis Marine Current Turbine With a Boundary Element Method

Abstract: A low order potential based panel code is used to analyse the flow around the blades of a horizontal axis marine current turbine An empirical vortex model is assumed for the turbine wake which includes the variation of pitch of the helicoidal vortices trailing behind the blades The analysis is carried out for uniform inflow conditions in steady flow for a turbine with controllable pitch for two different pitch settings in a wide range of tip-speed-ratios Grid convergence studies carried out to verify the accuracy of predicted pressure distributions and integrated forces show a fast convergence with grid refinement for this geometry The effect of the helicoidal wake model parameters used in the analysis is found to have a strong influence in the performance curves The results are compared with experimental data from the literature and with the lifting line theory A discussion of viscous effects is also provided to help explaining the main discrepancies with the data© 2008 ASME

The Transient and Progressive Flooding Stages of Damaged Ro-Ro Vessels: A Systematic Review of Entailed Factors

Abstract: Roll-on/roll-off vessels appear to be sensitive to rapid capsizing due to an abrupt ingress of water caused by maritime accidents As a result of the damage creation, the flooded ship can experience intermediate stages, which might be more devastating than the final condition, as the sudden loading could significantly alter the ship stability characteristics Far from a probabilistic analysis, the paper under study presents the state-of-the-art in regards to flooding physics by treating some relevant important topics It sheds light on the transient and progressive flooding stages, focuses on relevant factors, and suggests combinations between factors that strongly affect the flooding before the steady state is reached Furthermore, the authors comment on some points, which remain difficult to take into consideration either numerically or experimentally, and propose, where found necessary, recommendations for a more reliable assessment of the flooding process This review shows that the intermediate flooding phase depends upon many factors, and its assessment could be adequate in calm water condition The effects and interdependency between these factors still require further investigation Therefore, we recommend carrying out a wide range parametric investigation into these factors, which consider their interdependency and encourage the application of the design of experiments methodology

A Joint Probability Model for Environmental Parameters

Abstract: The joint probabilistic models (JPM) of the environmental parameters of wave, wind and current are nowadays extremely needed in order to perform reliability analyses of offshore structures. These JPM are also essential steps for the design of offshore structures based on long-term statistics and to perform dynamic response analysis of floating units that are strongly dependent on the directionality of the environmental actions, such as turret-moored FPSOs. Recently, some JPM have been proposed in the literature to represent the joint statistics of a reduced number of environmental parameters. However, it is difficult to find a practical and fully operational model taking into account the complete statistical dependence among all the environmental parameters intensities and their correspondent directions. In this paper, it is presented a straightforward methodology, based on the Nataf transformation, to create a JPM of the environmental parameters taking into account the dependence between the intensity and direction of all variables. The proposed model considers the statistical dependence of ten short-term variables: the significant wave height, peak period and direction of the sea waves, the significant wave height, peak period and direction of the swell waves, the amplitude and direction of the 1-h wind velocity and, finally, the amplitude and direction of the surface current velocity. The statistical dependence between them is evaluated using concepts of linear-linear, linear-circular and circular-circular variables correlation. Some results of the proposed JPM methodology are presented based on simultaneous environmental data gathered in a location offshore Brazil.Copyright © 2008 by ASME

Friction Factor Estimation for Turbulent Flows in Corrugated Pipes with Rough Walls

Abstract: The motivation of the investigation is the critical pressure loss in cryogenic flexible hoses used for LNG transport in offshore installations. Our main goal is to estimate the friction factor for the turbulent flow in this type of pipes. For this purpose, two-equation turbulence models (k-epsilon and k-omega) are used in the computations. First, the fully developed turbulent flow in a conventional pipe is considered. Simulations are performed to validate the chosen models, boundary conditions, and computational grids. Then a new boundary condition is implemented based on the "combined" law of the wall. It enables us to model the effects of roughness (and maintain the right flow behavior for moderate Reynolds numbers). The implemented boundary condition is validated by comparison with experimental data. Next, the turbulent flow in periodically corrugated (flexible) pipes is considered. New flow phenomena (such as flow separation) caused by the corrugation are pointed out and the essence of periodically fully developed flow is explained. The friction factor for different values of relative roughness of the fabric is estimated by performing a set of simulations. Finally, the main conclusion is presented: The friction factor in a flexible corrugated pipe is mostly determined by the shape and size of the steel spiral, and not by the type of the fabric, which is wrapped around the spiral.

Parallel Dynamic Optimization of Steel Risers

Abstract: The design of an ultradeepwater steel riser is known to be a challenging task, considering the several environmental load conditions to be simulated and the long time took by each simulation To aid in this task, optimization algorithms can be used to automate the search for the best design The problem, however, is very expensive computationally, motivating the implementation and use of a parallel optimization software coupled with a frequency domain dynamic model This paper deals with the use of two algorithms to optimize a steel riser of given inner diameter and material for both pipe and floater: the genetic algorithm and the simulated annealing The optimizer is left free to vary the length of the segments and the floater diameter, inside user-defined ranges Each obtained configuration is tested in a set of load cases with different currents, offsets and waves Optimization times and performance gain with parallelization is addressed for both algorithms© 2008 ASME

Structural Response to Sloshing Excitation in Membrane LNG Tank.

Abstract: Sloshing in LNG membrane tanks may cause large pressures on the tank structure. To keep the cargo at the required low temperature, the tank structure is covered with an insulation, which has a much less strength than steel. The containment system is a very complex structure, which consists of different materials and requires a careful analysis with due consideration of the load process and dynamic effects in the response. The structural response of the membrane tank wall is investigated in this paper by finite element analyses. First, a modal composition of the structural response is studied. It is shown that many modes contribute to the response, which makes it difficult to establish the simplified DLF approach. The dynamic structural response to a typical sloshing impact is investigated in detail. An important observation is that, although the containment system has traditionally been modeled with a rigid support, the steel plate that supports the insulation may be flexible under the relevant load conditions. It is shown that the flexibility of the steel plate causes significant stress variation in the insulation. Different response patterns of the Mark III containment system are presented, and mechanisms that cause large stress concentrations and different response patterns in the static and dynamic cases are discussed. The scaling issue in view of the response is also investigated. Various scaling formulations may apply in post-processing sloshing experiments. While the Froude law yields conservative scaling for pressure magnitude, its conservatism for scaling the time needs to be investigated in view of the relevant dynamic response. By analyzing the structural response to the differently scaled loads, it is found that the Froude approach is conservative, but the scatter of results may be very large.

Wave-in-Deck Load Analysis for a Jack-Up Platform

Abstract: This paper describes the prediction of environmental loads on a typical three-leg jack-up platform under freak wave conditions. Considered were cases where the air gap is small and the hull is subject to impact-related wave-in-deck loads. The technique to predict wave loads was based on the use of a validated CFD code that solves the Reynolds-averaged Navier―Stokes equations. This code relies on the interface―capturing technique of the volume-of-fluid type to account for highly nonlinear wave effects. It computes the two-phase flow of water and air to describe the physics associated with complex free-surface shapes with breaking waves and air trapping, hydrodynamic phenomena that had to be considered to yield reliable predictions. The Stokes fifth-order wave theory initialized volume fractions of water, velocity distributions in the solution domain, and time-dependent boundary conditions at inlet and outlet boundaries. This paper demonstrates that this technique can be a valuable numerical tool for preliminary designs as well as subsequent safety assessments. In particular, it shows that effects of different operating and design parameters on wave-in-deck loads, such as wave direction, wave height, wave period, and wind speed, can be evaluated with an affordable computing effort.

Numerical Investigation of 2D Optimal Profile of Backward-Bent Duct Type Wave Energy Converter

Abstract: A floating type backward-bent duct buoy (BBDB) is a wave energy conversion device with an oscillating water column (OWC) at the front side. The device captures the wave energy using the heaving, the pitching, the surging motion of BBDB, and the heaving motion of OWC. Investigations are carried out to find more reasonable devices than the traditional OWC type floating device. An eigenfunction expansion method is introduced for analyzing the BBDB with OWC. It is confirmed that these solutions give good agreement with several experimental results in this paper. It is shown in a design method how to make BBDB match the turbine characteristics. This feature is being able to select the optimum profile of the turbine and the BBDB individually from each characteristic before comprehensive evaluation of the BBDB and the turbine in the design. After grasping the element characteristics, which are appropriate for the wave energy conversion system, the synthetic design method is built. The BBDB size and the turbine diameter are determined by considering the cost corresponding to the smallest size under the same output. In this way, we can obtain the optimal profiles considering the construction cost including the turbines.

A Numerical Study of Liquefaction Induced Deformation on Caisson-Type Quay Wall Using a Partially Coupled Solution

Abstract: A partially coupled effective stress analysis is assessed in predicting the seismic performance of a caisson-type quay wall. Using a nonlinear finite difference program, a numerical study of shaking table tests, carried out at Tokyo University, is performed. In this paper, the formulation of an employed computational code is described, and the results of numerical simulation are compared with the measured records. The results demonstrate that the trend and magnitude of vertical and horizontal displacements of quay wall appear to be predicted reasonably well. The overall tendency is to overpredict the horizontal movement, but the vertical movement is underpredicted.

Coupled Surge-Heave-Pitch Dynamic Modeling of Spar-Moonpool-Riser Interaction

Abstract: Due to the rather intense ongoing development of deep water gas and oil fields, the technical community has devoted considerable attention to the dynamic behavior of Spar floating systems. Spar dynamics exhibits a highly nonlinear behavior due to the presence of various components such as mooring lines, moonpool, and risers. Certain studies have focused on the reduction of the heave response of single-degree-of-freedom Spar models due to the oscillations of water entrapped in the moonpool through the partially closed bottom plates. In this paper, a novel coupled six-degree-of-freedom analytical model of a Spar system comprising top tensioned risers is proposed. The model accounts for the interactions among the Spar hull kinematics (heave, surge, and pitch), the riser kinematics (heave and surge), and the moonpool. This model involves six coupled differential equations comprising nonlinearities associated not only with stiffness and damping but also with inertia terms. A dynamic analysis is performed by subjecting the model to JONSWAP ocean wave spectrum compatible extreme forces (corresponding to the 100 year wave) and to moments applied to the center of gravity computed by means of a standard motion simulation program. Both numerical and analytical techniques (statistical linearization including inertia terms) are used for the determination of the response of the proposed dynamic model, both in the time and the frequency domains. Related parameter study results are reported, including ones pertaining to the dependence of the Spar system motion on the degree of opening of the bottom plates.

Investigation of Restoring Stiffness in the Hydroelastic Analysis of Slender Marine Structures

Abstract: The restoring stiffness, which couples displacements and deformations, plays a very important role in hydroelastic analysis of marine structures. The problem of its formulation is quite complex and is still discussed in relevant literature. In this paper, the recent formulations of restoring stiffness are correlated and analyzed. Due to some common terms of the restoring and geometric stiffness, the unified stiffness is established and compared with the complete restoring stiffness known in relevant literature. It is found out that the new formula deals with more terms and that under some assumptions, it is reduced to the known complete restoring stiffness. The unified stiffness constitution is analyzed through derived analytical formulae for prismatic pontoon. Its consistency is checked for the rigid body displacements. Also, numerical results of the hydroelastic response of segmented barge are correlated with available model test results. Some issues, that are important for practical implementation in the hydroelastic code for flexible structures, are described.

Results of Field Monitoring on Ice Actions on Conical Structures

Abstract: Ice-structure interaction plays a central part in determining ice loads and ice-induced vibrations. This is a controversial research issue, and many factors make the problem more complicated. The authors have been monitoring several ice resistant structures in the Bohai Sea for 20 years and have measured ice forces and simultaneously observed ice-structure interaction processes. This paper describes typical physical ice sheet–conical structure interaction processes, field data, and theoretical explanations for different ice conditions and structure dimensions. The conclusions are more widely applicable, and we relate them to field work on ice resistant conical structures in other ice-covered regions. Further work will quantify ice loads on conical structures once the interaction process is understood.

Design of a test rig for vibration control of oil platforms using magneto-rheological dampers

Abstract: Offshore steel structures are subjected to hydrodynamic forces. These forces induce vibration in the structure, which in turn has a great effect on the safety and durability of the structure and the manpower operating it. Magnetorheological (MR) dampers are proven as a feasible alternative to reducing structural vibrations. This paper presents the initial investigation of integrating MR dampers in Offshore Steel Jacket Platforms to decrease the vibrations discussed. A Steel Jacket Structure with MR damper is simulated using Simulink and the results are presented. In addition, an experimental model is designed to verify the simulation responses. 1. Introduction Offshore platforms have been developed over the years from traditionally shallow water structures to modern deep sea structures. They are commonly used for oil and gas extraction. Although there are many different types of offshore structures the steel jacket type platform on a pile foundation is the most commonly used [1]. These structures, normally anchored to the sea bed, are affected by wave forces, unlike onshore structures which incur vibrations due to seismic excitations and excessive random shocks [1, 2]. The flexibility of the steel jacket platforms generates self-excited non-linear hydro dynamic forces in addition to their non-linear response to large deformations [3]. Although the risk of failure or damage in such structures is higher than experienced in other structures the safety of these can usually be insured by increasing their stiffness so as to shift the natural frequencies away from the resonant range of frequencies. Such technique is costly and requires high maintenance for large offshore structures [4]. Several models for offshore structures are available in the literature; the majority of which are derived using finite elements methods. The mass matrix in finite element analysis is usually calculated using the consistent mass method or the lumped mass method. In either case, calculating the mass matrix is time consuming and the resultant matrix tends to be very large for control purposes. Another technique developed by Horr et al. [5] makes use of the spectral element method to derive the element matrices and uses the Timoshenko beam theory. This method calculates the distributed mass exactly and only one element needs to be placed between two nodes. The Euler-Bernoulli theory [6] was shown to be inadequate for the prediction of higher modes of vibration and also with beams where the effect of crosssectional dimensions is not negligible; therefore, an alternate technique should be used to derive a more accurate model of the structure. Timoshenko beam theory [5] takes into account the effects of rotary inertia and shear deformations. If shear deformation is significant, the assumption that plane sections remain plane is no longer valid. Therefore, Timoshenko corrections are necessary to make more accurate predictions of high modes of vibration. In this work, the calibrated spectral Timoshenko pipe element [5] is used to model an offshore oil platform. The goal of this research is to develop an accurate model of the platform and to reduce its vibrations using MR dampers. Traditionally, over the last several decades, MR dampers have been used extensively in reducing the vibrations of civil engineering structures [7, 8, 9, 10, 11], automobiles, and household appliances. MR dampers have a very important advantage [7]; they can provide large force outputs for a small change in current input. Moreover, these dampers can be actively used to systematically reduce or eliminate the vibrations of structures. Offshore oil platforms are known to undergo a lot of vibration during extreme weather conditions and when strong waves and currents hit the platform [8]. MR dampers are capable of providing the large damping forces [9] necessary to absorb the effect of the hydro-dynamic forces on the platform. To validate the theoretical findings, an experimental setup is built and tested in our laboratory. 2. Dynamic Models In order to simulate the system understudy, the theoretical models of both devices – the MR damper and the Offshore Steel Jacket Platform are presented and discussed in the next section.

Ultimate Strength Analysis of Ship Hull Girder Under Random Material and Geometric Properties

Abstract: The ultimate strength of a ship’s hull depends on its material and geometric properties, some or all of which may be random in nature. In addition, initial imperfections in the form of initial deflection and residual welding stresses in plating between stiffeners can significantly affect the hull ultimate strength. In this paper, the effect of randomness in yield strength and in the initial imperfections on ultimate hull girder strength is determined. Different levels of statistical dependence between yield strength and initial imperfection of stiffeners and plating between stiffeners have been considered. The methodology is applied on a bulk carrier and a VLCC tanker. DOI: 10.1115/1.4002738

Extreme response analysis of floating structures using coupled frequency domain analysis

Abstract: In the dynamic analysis of a floating structure, coupled analysis refers to a procedure in which the vessel, moorings and risers are modeled as a whole system, thus allowing for the interactions between the various system components. Because coupled analysis in the time domain is impractical owing to prohibitive computational costs, a highly efficient frequency domain approach was developed in a previous work, wherein the drag forces are linearized. The study showed that provided the geometric nonlinearity of the moorings/risers is insignificant, which often holds for ultra-deepwater systems, the mean-squared responses yielded by the time and frequency domain methods are in close agreement. Practical design is concerned with the extreme response, for which the mean upcrossing rate is a key parameter. Crossing rate analysis based on statistical techniques is complicated as the total response occurs at two timescales, with the low frequency contribution being notably non-Gaussian. Many studies have been devoted to this problem, mainly relying on a technique originating from Kac and Siegert; however, these studies have mostly been confined to a single-degree-of-freedom system. The aim of this work is to apply statistical techniques in conjunction with frequency domain analysis to predict the extreme responses of the coupled system, in particular the modes with a prominent low frequency component. It is found that the crossing rates for surge, sway and yaw thus obtained agree well with those extracted from time domain simulation, whereas the result for roll is less favorable, and the reasons are discussed.Copyright © 2010 by ASME

Distribution of Wave Height Maxima in Storm Sea States

Abstract: The effect of the coefficient of kurtosis, as a measure of third order nonlinearity, on the distribution of wave height maxima has been investigated. Measurements of the surface elevation during a storm at the North Alwyn platform in the North Sea have been used. The mean number of waves in the series is around 100. The maximum wave statistics have been compared with nonlinear theoretical distributions. It was found that the empirical probability densities of the maximum wave heights describe qualitatively the shift of the distribution modes toward higher values. The tendency for the peak of distribution to diminish with an increase in the coefficient of kurtosis up to 0.6 is also clearly seen. However, the empirical peak remains higher than the theoretically predicted one. The exceedance probability of the maximum wave heights was also estimated from the data and was compared with the theory. For the highest coefficients of kurtosis, estimated at nearly 0.6, the theoretical distribution approximates very well the empirical data. For lower coefficients of kurtosis, the theory tends to overestimate the exceedance probability of the maximum wave heights.

Horizontal Capacity of Embedded Suction Anchors in Clay

Abstract: The embedded suction anchor (ESA) is a type of permanent offshore foundation that is installed by a suction pile. The primary factors influencing the horizontal pullout capacity of an ESA include the loading point, the soil type, the embedment depth, and the addition of flanges. The main purpose of this study is to develop an analytical solution that is capable of estimating the horizontal pullout capacity of ESAs with the loading point being anywhere along its length with or without flanges. An analytical solution has been developed to estimate the horizontal pullout capacity of embedded suction anchors in clay seafloor. Validation has been made through comparisons with the centrifuge model test results. Results indicate that the horizontal pullout capacity of the embedded suction anchor in clay increases, reaches its peak, and then starts to decrease as the point of the load application moves downward. The effect of flanges on the horizontal pullout capacity is also found to be significant. The horizontal pullout capacity is a direct function of the loading point. The horizontal pullout capacity increases as the loading point moves downward and the maximum pullout capacity is obtained when the loading point is approximately at the mid-depth. The increase in horizontal pullout capacity can be significant, i.e., more than twice in magnitude when the maximum pullout capacity is compared with that associated with the loading point near the top or tip.

Experimental Investigations of the Efficiency of Round-Sectioned Helical Strakes in Suppressing Vortex Induced Vibrations

Abstract: The present work highlights some aspects related to the analyses of Arctic offshore floating structures. This thesis consists of five papers, which can be divided into two main categories. One category deals with the dynamics of slender structures with an emphasis on the prediction and suppression of vortex induced vibrations (VIV), and the other category examines the process of interaction between sloping structures and sea ice with focus on developing a numerical model to simulate this process in real time. Slender structures, such as mooring lines and marine risers, are very important for the offshore petroleum industry, which is currently approaching deeper waters. Increasingly, attention has been focused on predicting the susceptibility of these structures to VIV. In this thesis, two asymptotic techniques namely, the local analysis and the WKB methods, were used to derive closed-form solutions for the natural frequencies and mode shapes of slender line-like structures. Both the top-tensioned nearly-vertical configuration and the catenary configuration were considered. The accuracy of the solutions derived was established through comparison with other analytic solution techniques and with results of numerical finite element solutions. The effects of the bending stiffness and the effects of approximating the tension variation as a linear function were discussed. Experimental data on the multi-modal in-line and cross-flow response behaviour of a towed catenary model were analysed to examine the usefulness of the solutions for predicting the response frequencies and envelopes due to VIV. Helical strakes are often used as a mitigating measure to suppress the VIV of slender structures. This thesis presented an innovative method to fit ropes helically to a riser in the installation phase. Such a procedure will help to overcome the handling problem associated with the use of conventional sharp-edged strakes. Experimental investigations were then performed to verify the efficiency of these ropes (round-sectioned helical strakes) in suppressing VIV. Systematic experimental investigations including twenty-eight configurations of round-sectioned helical strakes were tested in an attempt to find the most suitable strake configuration. The effects of varying pitch, the surface roughness and the ratio between the cross-flow and in-line natural frequencies on the efficiency of the proposed configuration of round-sectioned helical strakes were also investigated. The process of interaction between sea ice and offshore sloping structures (e.g., conical structures and ship-shaped structures) is quite complex. Modelling this process is very demanding and often computationally expensive, which typically hinders the chances for realtime simulations. This kind of simulation can be very useful for training personnel for Arctic offshore operations and procedures, for analysing the efficiency of various ice management concepts and as a part of the onboard support systems for station keeping. The challenge of meeting the real-time criterion was overcome in the present work. This thesis developed a numerical model to simulate the process of interaction between sea ice and sloping structures in real time. In this model, only level- and broken-ice features were studied. New analytical closed-form solutions were established and used to represent the ice breaking process. PhysX was used for the first time to solve the equations of rigid body motions with six degrees of freedom for all ice floes in the calculation domain. The results of the simulator were validated against experimental data from model-scale and full-scale tests. Accurate predictions of ice actions are also vital to optimise the design of the structures in the Arctic regions. A good understanding of the role of seawater in the process of interaction between the sloping structures and level ice will help to establish reliable models to estimate the ice forces. This work formulated both the static and dynamic bending problems for a floating wedge-shaped ice beam interacting with an offshore sloping structure. For the dynamic interaction, the effects of the water foundation on the bending failure of the ice were studied by comparing the results of an elastohydrodynamic approach with a model of a Winkler foundation. The thesis also investigated the breaking lengths of the ice wedges (i.e., the frequency of the ice loads) as a function of the ice thickness, the compression in the ice and the acceleration of the interaction.

Plastic Collapse Loads of Cracked Square Hollow Section T-, Y-, and K-joints

Abstract: This paper concerns on the development of numerical models for cracked square hollow sections (SHSs) T-, Y-, and K-joints. Based on these numerical models, the plastic collapse loads P c are calculated using nonlinear finite element method and through twice-elastic compliance criterion. It is found that the numerical plastic collapse loads P c are slightly conservative compared with the ones calculated using formulae proposed by BS7910 [British Standards, 2005, "Guide on Methods for Assessing the Acceptability of Flaws in Metallic Structures," BS 7910-Amendment 1] and are in close agreement with the experimental tests data. Therefore, the proposed numerical model is robust and it can be used to calculate the plastic collapse loads P c of the cracked (SHS) T-, Y-, and K-joints.

Multi-Objective Optimization Design of Tanker Ships via a Genetic Algorithm

Abstract: The cost of a new ship design heavily depends on the principal dimensions of the ship; however, dimensions minimization often conflicts with the minimum oil outflow (in the event of an accidental spill) This study demonstrates one rational methodology for selecting the optimal dimensions and coefficients of form of tankers via the use of a genetic algorithm Therein, a multi-objective optimization problem was formulated by using two objective attributes in the evaluation of each design, specifically, total cost and mean oil outflow In addition, a procedure that can be used to balance the designs in terms of weight and useful space is proposed A genetic algorithm was implemented to search for optimal design parameters and to identify the nondominated Pareto frontier At the end of this study, three real ships are used as case studies

Modal Analysis of the Ice-Structure Interaction Problem

Abstract: In this paper the authors build upon the single degree of freedom ice-structure interaction model initially proposed by Matlock, et al. (1969, 1971). The model created by Matlock, et al. (1969, 1971), assumed that the primary response of the structure would be in its fundamental mode of vibration. Modal analysis is used in this study, in which the response of each mode is superposed to find the complete modal response of the entire length of a pier subject to incremental ice loading. In Matlock, et al., the physical system is a bottom supported pier modeled as a cantilever beam. Realistic conditions such as ice accumulation on the pier modeled as a point mass and uncertainties in the ice characteristics are introduced in order to provide a stochastic response. The impact of number of modes in modeling is studied as well as dynamics due to fluctuations of ice impact height as a result of typical tidal fluctuations. A Poincare based analysis following on the research of Karr, et al. (1992) is employed to identify any periodic behavior of the system response. The intention of this work is to provide a foundation for future work coupling multiple piers and connecting structure for a comprehensive ice-wind-structural dynamics model.

Experimental Study on Dynamic Response and Shock Damage of Cylindrical Shell Structures Subjected to Underwater Explosion

Abstract: This study investigates the dynamic linear, nonlinear responses, and shock damage of two kinds of submerged cylindrical shell models exposed to underwater spherical trinitrotoluene (TNT) charge explosions in a circular lake. Two endplates and a middle plate are mounted on the cylindrical shells to provide support and create two enclosed spaces. The two kinds of cylindrical shell models are unfilled and main hull sand-filled, respectively. Fifteen different tests are carried out according to changing the TNT explosive weights of 1 kg and 2 kg, standoff distances ranging from 3 m to 0.3 m, and two explosion positions, and the measured experimental results are compared with each other. Detailed discussions on the experimental results show that the dynamic responses and damage modes are much different, and the main hull sand-filled cylindrical shell is more difficult to be damaged by the shock wave loading than the unfilled model. The edge cracks are mainly observed at the instrument hull of the main hull sand-filled model, but surface tearing and cracks take place both on the main and instrumental hulls of the unfilled model, respectively.