The Optimal Design of TMD for Offshore Structures

This paper presents the optimal design procedure of Tuned Mass Damper (TMD) for reducing vibration of an actual steel jacket offshore platform excited by random wave loading. In this study, a frequency domain is taken. The force on the structure is determined by use of the linearized Morison equation for an input Power Spectral Density (PSD) of wave elevation. The sensitivity of optimum values of TMD to characteristic parameters of random wave spectrum is analyzed. An optimized TMD design for the modeled platform is given based on design conditions and the findings of the study.

Multiple-Step Predictive Control for Offshore Structures

Ocean wave propagation is slow, visible and measurable, so a wave theory can be used to approximately predict the imminnent wave force on an offshore structure based on measured, real-time wave elevation near the structure. This predictability suggests the development of a more efficient algorithm, than those that have been developed for structures under wind and seismic loads, for the active vibration control of offshore structures. The present study delveops a mutiple-step predictive optimal control (MPOC) algorithm that accounts for multiple step external loading in the determination of optimal control forces. The control efficiency of the newly developed MPOC algorithm has been Investigated under both regular (single-frequency) and irregular (multiple-frequency) wave loads, and compared with that of two other well-known optimal control algorithms: classical linear optimal control(CLOC) and instantaneous optimal control(IOC).

Analysis of Collision Protection for Ocean and Offshore Structures

An elasto-plastie impact model based on the p-version finite element method is presented for the collision protection of ocean and offshore structures. The impact force and responses of the impactor-absorber-structure system can be predicted efficiently and automatically. A cost-effective Cellular Reinforced Concrete Absorber （CRCA） is designed to smooth the impact force and absorb the impact energy. Quasi-static tests show that the concrete absorber has an excellent energy absorbing characteristic. The impact experiment of a scaled offshore oil-piping frame with the proposed concrete absorber is carried out. The simulation results of the elasto-plastie model and the p-version finite element method are in good agreement with the experimental ones. Owing to the plastic deformation of the absorber, the impact force during the impact and responses of the structure are considerably reduced. Further, the proposed impact model and the concrete absorber are applied to the design of collision protection of the sheet-pile groin on the Qiantang River used to weaken the famous Qiantang bore.

Dynamic Analysis of Tension Leg Platform for Offshore Wind Turbine Support as Fluid-Structure Interaction

Tension leg platform （TLP） for offshore wind turbine support is a new type structure in wind energy utilization. The strong-interaction method is used in analyzing the coupled model, and the dynamic characteristics of the TLP for offshore wind turbine support are recognized. As shown by the calculated results： for the lower modes, the shapes are water＇s vibration, and the vibration of water induces the structure＇s swing; the mode shapes of the structure are complex, and can largely change among different members; the mode shapes of the platform are related to the tower＇s. The frequencies of the structure do not change much after adjusting the length of the tension cables and the depth of the platform; the TLP has good adaptability for the water depths and the environment loads. The change of the size and parameters of TLP can improve the dynamic characteristics, which can reduce the vibration of the TLP caused by the loads. Through the vibration analysis, the natural vibration frequencies of TLP can be distinguished from the frequencies of condition loads, and thus the resonance vibration can be avoided, therefore the offshore wind turbine can work normally in the complex conditions.

Fatigue reliability analysis of fixed offshore structures:A first passage problem approach

This paper describes a methodology for computation of reliability of members of fixed offshore platform structures, with respect to fatigue. Failure criteria were formulated using fracture mechanics principle. The problem is coined as a “first passage problem”. The method was illustrated through application to a typical plane frame structure. The fatigue reliability degradation curve established can be used for planning in-service inspection of offshore platforms. A very limited parametric study was carried out to obtain insight into the effect of important variables on the fatigue reliability.

Surge Response of a Compliant Offshore Structure by Time Domain Simulation

A single-degree-of-freedom equation of motion was used for modeling a compliant offshore structure exposed to viscous hydrodynamic loads. The equation of motion contains nonlinearities in the forms of both Duffing stiffness and Morison drag force with current. The water particle velocity and acceleration for calculating the Morison inertia and drag forces are modeled as Gaussian processes based on a Pierson-Moskowitz (P-M) elevation spectrum. The single-degree-of-freedom equation of motion containing different ocean current values are then numerically integrated via a fourth-order Runge-Kutta scheme. Time trajectories of the surge response displacements of the offshore structure and the response probability density curves are obtained. Furthermore, the ocean current influences on the response central moments up to the fourth order are studied. A literature review reveals that this is the first treatment of such a pair of nonlinearities in time domain. The simulation results are analyzed, and some conclusions valuable for engineering design are pointed out.

Optimum Design of Structure Shape for Offshore Jacket Platforms

With the introduction of the design variables of nodal coordinates, which reflect the embedded depth of the pile and the jacket bed height, a shape optimum design model for offshore jacket platforms is established. A sequential two-level optimum algorithm is developed based on the design variable gradation. On the basis of the finite element method, the sensitivity of the objective function and nodal displacement is analyzed. As an example, the BZ281 oil storage offshore platform, which ties in the Bohai oil field, is designed with the shape optimum model. The results are compared with the cross-section optimum design. The tendency of design variables and its reasons in the two methods are analyzed. In the shape optimum design, the value of objective function is obviously smaller than that of the initial design and the cross-section optimum design. Therefore, the advantage of structure shape optimum design for jacket platforms is remarkable.

Scour and liquefaction issues for anchors and other subsea structures in floating offshore wind farms: A review

This article reviews scouring and liquefaction issues for anchor foundations of floating offshore wind farms.The review is organized in two sections:(1)the scouring issues for drag-embedment anchors(DEAs)and other subsea structures associated with DEAs such as tensioners,clump weights,and chains in floating offshore wind farms;and(2)the liquefaction issues for the same types of structures,particularly for DEAs.The scouring processes are described in detail,and the formulae and design guidelines for engineering predictions are included for quantities like scour depth,time scale,and sinking due to general shear failure of the bed soil caused by scoui\The latter is furnished with numerical examples.Likewise,in the second section,the liquefaction processes are described with special reference to residual liquefaction where pore-water pressure builds up in undrained soils(such as fine sand and silt)under waves,leading to liquefaction of the bed soil and precipitating failure of DEAs and their associated subsea structures.An integrated mathematical model to deal with liquefaction around and the resulted sinking failure of DEAs,introduced in a recent study,has been revisited.Implementation of the model is illustrated with a numerical example.It is believed that the present review and the existing literatures from the"neighboring"fields form a complementary source of information on scour and liquefaction around foundations of floating offshore wind farms.

Torsionally Coupled Response Control of Offshore Platform Structures Using Circular Tuned Liquid Column Dampers

In this paper, the control performance is investigated of Circular Tuned Liquid Column Dampers (CTLCD) over torsional response of offshore platform structures excited by ground motions. Based on the equation of motion for the CTLCD-structure system, the optimal control parameters of CTLCD are given through some derivations on the supposition that the ground motion is a stochastic process. The influence of systematic parameters on the equivalent damping ratio of the structures is analyzed with purely torsional vibration and translational-torsional coupled vibration, respectively. The results show that the Circular Tuned Liquid Column Damper (CTLCD) is an effective torsional response control device.

Analysis and Research of Ice Loads Acting on Offshore Structures

Usually, the action of sea ice on offshore engineering structures is one of the controlling loads in cold waters engineering structure design. The reasonable selection of environmental condition and the physical mechanical properties of ice in the region are directly related to the structure design, operation and safety. In this paper, the sea ice force acting on the structure, the physical mechanical properties of ice and the selection of parameters in calculation are discussed. Some suggestions are proposed as to the calculation of various kinds of ice loads acting on the structure.

Review on Artificial Intelligence-aided Life Extension Assessment of Offshore Wind Support Structures

The primary objective of the present literature review is to provide a constructive and systematical discussion based on the relevant development,unsolved issues,gaps,and misconceptions in the literature regarding the fields of study that are building blocks of artificial intelligence-aided life extension assessment for offshore wind turbine support structures.The present review aims to set up the needed guidelines to develop a multi-disciplinary framework for life extension management and certification of the support structures for offshore wind turbines using artificial intelligence.The main focus of the literature review centres around the intelligent risk-based life extension management of offshore wind turbine support structures.In this regard,big data analytics,advanced signal processing techniques,supervised and unsupervised machine learning methods are discussed within the structural health monitoring and condition-based maintenance planning,the development of digital twins.Furthermore,the present review discusses the critical failure mechanisms affecting the structural condition,such as high-cycle fatigue,low-cycle fatigue,fracture,ultimate strength,and corrosion,considering deterministic and probabilistic approaches.

Two phenomena: Honji instability, and ringing of offshore structures

Honji instability and ringing of offshore structrures are two different phenomena. Honji instability occurs at a circular cylinder in transverse periodic finite motion in a water tank. It is superposed on the streaming flow induced by the cylinder’s boundary layer. Its oscillation period is half of the period of the cylinder oscillation. Finite volume calculations of the filtered Navier-Stokes equations visualize the three-dimensional instability, where fluid particles transported by the circumferencial roll pairs exhibit a periodic mushroom-like pattern. Force is the same with and without the Honji instability. The large eddy simulation calculations for high Reynolds number support a drag coefficient in accordance with the Stokes-Wang solution below separation and conform with experimental measurements of the damping force on a harmonically oscillating cylinder. Ringing of offshore structures are vibrations which appear at natural frequencies and concern fatigue. It is generated by a higher harmonic force oscillating with frequency being 3-4 times the fundamental wave frequency. Together with a strong inertia load in phase with the incoming wave’s acceleration, a secondary load cycle appears in strong seas when the wave crest leaves the structure; this occurs about 1/4 wave period after the main force peak, it starts when the wave crest is about one cylinder radius behind the cylinder, lasts for about 15-20 percent of the wave period and has a magnitude up to 11 % of the peak-to-peak total force. It is a gravity effect and appears in strong irregular seas when kA > 0.18 and um/√gD > 0.4 (k wavenumber, A amplitude, um maximal wave induced velocity, g acceleration of gravity, D cylinder diameter).

On the Exceedance Probabilities of Extreme Drift Motions of An Offshore Structure

The response statistics of a compliant offshore structure excited by slowly varying wave drift forces is calculated by use of a numerical path integral solution method. The path integral solution is based on the Ganss-Legendre interpolation scheme, and the values of the response probability density are obtained at the Gauss quadrature points in sub-intervals. It is demonstrated that a distinct advantage of the path integral solution is that the joint probability density of the response displacement and velocity is one of the by products of the calculations. This makes it possible to calculate the mean level up-crossing rates, which provides estimates of the exceedance probabilities of specified response levels for given time periods.

Recent Developments in Fatigue Assessment of Ships and Offshore Structures

A review is provided of various approaches that have been adopted recently to assess the fatigue of ships and offshore structures.The relevant fatigue loading is reviewed first,focusing on the successive loading and unloading of the cargo and the transient loadings.The factors influencing fatigue strength are discussed,including the geometrical parameters,material,residual stress,and ones related to the environment.Different approaches for fatigue analyses of seam-welded joints are covered,i.e.,the structural stress or strain approach,the notch stress or strain approach,notch intensity approach,and the crack propagation approach.

Theoretical Analysis of Mechanical Vibration for Offshore Platform Structures

A new class of support structures, called Periodic Structures, is introduced in this paper as a viable means for isolating the vibration transmitted from the sea waves to offshore platform structures through its legs. A passive approach to reduce transmitted vibration generated by waves is presented. The approach utilizes the property of periodic structural components that create stop and pass bands. The stop band regions can be tailored to correspond to regions of the frequency spectra that contain harmonics of the wave frequency, attenuating the response in those regions. A periodic structural component is comprised of a repeating array of cells, which are themselves an assembly of elements. The elements may have differing material properties as well as geometric variations. For the purpose of this research, only geometric and material variations are considered and each cell is assumed to be identical. A periodic leg is designed in order to reduce transmitted vibration of sea waves. The effectiveness of the periodicity on the vibration levels of platform will be demonstrated theoretically. The theory governing the operation of this class of periodic structures is introduced using the transfer matrix method. The unique filtering characteristics of periodic structures are demonstrated as functions of their design parameters for structures with geometrical and material discontinuities, and determine the propagation factor by using the spectral finite element analysis and the effectiveness of design on the leg structure by changing the ratio of step length and area interface between the materials is demonstrated in order to find the propagation factor and frequency response.

Damage Detection of Offshore Jacket Structures Using Frequency Domain Selective Measurements

The development of damage detection techniques for offshore jacket structures is vital to prevent catastrophic events. This paper applies a frequency response based method for the purpose of structural health monitoring. In efforts to fulfill this task, concept of the minimum rank perturbation theory has been utilized. The present article introduces a promising methodology to select frequency points effectively. To achieve this goal, modal strain energy ratio of each member was evaluated at different natural frequencies of structure in order to identify the sensitive frequency domain for damage detection. The proposed methodology opens up the possibility of much greater detection efficiency. In addition, the performance of the proposed method was evaluated in relation to multiple damages. The aforementioned points are illustrated using the numerical study of a two dimensional jacket platform, and the results proved to be satisfactory utilizing the proposed methodology.

Multiple Tuned Mass Damper Based Vibration Mitigation of Offshore Wind Turbine Considering Soil–Structure Interaction

The dynamics of jacket supported offshore wind turbine （OWT） in earthquake environment is one of the progressing focuses in the renewable energy field. Soil-structure interaction （SSI） is a fundamental principle to analyze stability and safety of the structure. This study focuses on the performance of the multiple tuned mass damper （MTMD） in minimizing the dynamic responses of the structures objected to seismic loads combined with static wind and wave loads. Response surface methodology （RSM） has been applied to design the MTMD parameters. The analyses have been performed under two different boundary conditions： fixed base （without SSI） and flexible base （with SSI）. Two vibration modes of the structure have been suppressed by multi-mode vibration control principle in both cases. The effectiveness of the MTMD in reducing the dynamic response of the structure is presented. The dynamic SSI plays an important role in the seismic behavior of the jacket supported OWT, especially resting on the soft soil deposit. Finally, it shows that excluding the SSI effect could be the reason of overestimating the MTMD performance.

Mode Shape Identification Using Residues of Measured Offshore Structure Data

Compared to traditional mode shape identification methods such as eigensystem realization algorithm(ERA),this article proposes a mode shape identification method based on estimated residues of measured data and the theoretical relationship between the estimated residues and the mode shapes from the state space model is obtained by defining a coefficient matrix.A mass-spring model with five degrees of freedom(DOFs) is utilized to demonstrate the approach.The numerical results indicate that the estimated residues are the mode shapes of structures,but with a coefficient matrix to maintain consistency with the mode shapes from the ERA.Using MATLAB a complicated numerical jacket platform is built to further study the proposed method.The results show that mode shapes consistent with those from the ERA could be obtained by taking the defined coefficient matrix into account,which is also demonstrated by a physical beam model that was built at Ocean University of China.

Dynamic Analysis of A 5-MW Tripod Offshore Wind Turbine by Considering Fluid–Structure Interaction

Fixed offshore wind turbines usually have large underwater supporting structures. The fluid influences the dynamic characteristics of the structure system. The dynamic model of a 5-MW tripod offshore wind turbine considering the pile-soil system and fluid structure interaction （FSI） is established, and the structural modes in air and in water are obtained by use of ANSYS. By comparing low-order natural frequencies and mode shapes, the influence of sea water on the free vibration characteristics of offshore wind turbine is analyzed. On basis of the above work, seismic responses under excitation by E1-Centro waves are calculated by the time-history analysis method. The results reveal that the dynamic responses such as the lateral displacement of the foundation and the section bending moment of the tubular piles increase substantially under the influence of the added-mass and hydrodynamic pressure of sea water. The method and conclusions presented in this paper can provide a theoretical reference for structure design and analysis of offshore wind turbines fixed in deep seawater.

STRESS ANALYSIS OF RING STIFFENED TUBULAR JOINTS FOR OFFSHORE STRUCTURES——REVIEW OF STUDIES IN CHINA

In China, the stress analysis of tubular joints for offshore structures was started at the end of 1970's. In addition to simple joints, the ring stiffened tubular joint have been analyzed recently. In this paper, the author reviews the research work of stress analysis of ring stiffened joints, including brief introduction to the methods used, stress concentration behaviour, effects of ring stiffeners' number, location, dimension, type and stress distribution of ring stiffeners. Emperical formulae for predicting SCF of ring stiffened joints are also presented.