Estimating Design Loads with Environmental Contour Approach Using Copulas for an Offshore Jacket Platform

Jacket-type offshore platforms are widely used for oil, gas field, and energy development in shallow water. The design of a jacket structure is highly dependent on target environmental variables. This study focuses on a strategy to estimate design loads for offshore jacket structures based on an environmental contour approach. In addition to the popular conditional distribution model, various classes of bivariate copulas are adopted to construct joint distributions of environmental variables. Analytical formulations of environmental contours based on various models are presented and discussed in this study. The design loads are examined by dynamic response analysis of jacket platform. Results suggest that the conditional model is not recommended for use in estimating design loads in sampling locations due to poor fitting results. Independent copula produces conservative design loads and the extreme response obtained using the conditional model are smaller than those determined by copulas. The suitability of a model for contour construction varies with the origin of wave data. This study provides a reference for the design load estimation of jacket structures and offers an alternative procedure to determine the design criteria for offshore structures.     

Modal strain energy based structural damage localization for offshore platform using simulated and measured data

Modal strain energy based methods for damage detection have received much attention. However, most of published articles use numerical methods and some studies conduct modal tests with simple 1D or 2D structures to verify the damage detection algorithms. Only a few studies utilize modal testing data from 3D frame structures. Few studies conduct performance comparisons between two different modal strain energy based methods. The objective of this paper is to investigate and compare the effectiveness of a traditional modal strain energy method （Stubbs index） and a recently developed modal strain energy decomposition （MSED） method for damage localization, for such a purpose both simulated and measured data from an offshore platform model being used. Particularly, the mode shapes used in the damage localization are identified and synthesized from only two measurements of one damage scenario because of the limited number of sensors. The two methods were first briefly reviewed. Next, using a 3D offshore platform model, the damage detection algorithms were implemented with different levels of damage severities for both single damage and multiple damage cases. Finally, a physical model of an offshore steel platform was constructed for modal testing and for validat-ing the applicability. Results indicate that the MSED method outperforms the Stubbs index method for structural damage detection.     

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.     

Damage localization of marine risers using time series of vibration signals

Based on dynamic response signals a damage detection algorithm is developed for marine risers. Damage detection methods based on numerous modal properties have encountered issues in the researches in offshore oil community. For example, significant increase in structure mass due to marine plant/animal growth and changes in modal properties by equipment noise are not the result of damage for riser structures. In an attempt to eliminate the need to determine modal parameters, a data-based method is developed. The implementation of the method requires that vibration data are first standardized to remove the influence of different loading conditions and the autoregressive moving average (ARMA) model is used to fit vibration response signals. In addition, a damage feature factor is introduced based on the autoregressive (AR) parameters. After that, the Euclidean distance between ARMA models is subtracted as a damage indicator for damage detection and localization and a top tensioned riser simulation model with different damage scenarios is analyzed using the proposed method with dynamic acceleration responses of a marine riser as sensor data. Finally, the influence of measured noise is analyzed. According to the damage localization results, the proposed method provides accurate damage locations of risers and is robust to overcome noise effect.     

Safety assessment of platform loadout procedures based on unascertained measures

Safety assessment of offshore platforms is an urgent task. Such assessments are now focusing on the structure, maintenance, and retirement of a platform. Some methods employed have many shortcomings. For example, they cannot make the reliability adequately explicable. Therefore, a mathematical tool, the unascertained measure, was introduced. First, the basic knowledge of the unascertained sets was introduced briefly. Second, the unascertained measure was defined and credible identification was set up. The method has been introduced into the fields for safety assessment of a jacket loadout procedure. Engineering practices showed that it can complete the safety assessment systematically and scientifically without any assumption. The work should have significance in theory and practice for offshore engineering.     

System Reliability Analysis of an Offshore Jacket Platform

This study investigates strategies for solving the system reliability of large three-dimensional jacket structures.These structural systems normally fail as a result of a series of different components failures.The failure characteristics are investigated under various environmental conditions and direction combinations.Theβ-unzipping technique is adopted to determine critical failure components,and the entire system is simplified as a series-parallel system to approximately evaluate the structural system reliability.However,this approach needs excessive computational effort for searching failure components and failure paths.Based on a trained artificial neural network(ANN),which can be used to approximate the implicit limit-state function of a complicated structure,a new alternative procedure is proposed to improve the efficiency of the system reliability analysis method.The failure probability is calculated through Monte Carlo simulation(MCS)with Latin hypercube sampling(LHS).The features and applicability of the above procedure are discussed and compared using an example jacket platform located in Chengdao Oilfield,Bohai Sea,China.This study provides a reference for the evaluation of the system reliability of jacket structures.     

Estimation of wave forces on large compliant platforms

Compliant offshore structures such as spars, tension leg platforms (TLPs) and semi-submersibles have been dramatically improved in recent years due to their capability for deep water operation. Waves are the most important environmental phenomenon affecting these offshore structures. Estimation of wave forces is vital in offshore structure design. For large compliant offshore plat-forms, Morrison's equation is not valid anymore and usually diffraction theory is used. In this research, by using the finite difference method, a detailed analysis of the first-order diffraction of monochromatic waves on a large cylinder as a structural element is per-formed to solve the radiation and diffraction potentials. The results showed that the developed model is a reliable tool to estimate the wave forces and hydrodynamic coefficients on large structure elements when wave diffraction and radiation are considered.     

i4OilSpill,an operational marine oil spill forecasting model for Bohai Sea

Oil spill models can effectively simulate the trajectories and fate of oil slicks, which is an essential element in contingency planning and effective response strategies prepared for oil spill accidents. However, when applied to offshore areas such as the Bohai Sea, the trajectories and fate of oil slicks would be affected by time-varying factors in a regional scale, which are assumed to be constant in most of the present models. In fact, these factors in offshore regions show much more variation over time than in the deep sea, due to offshore bathymetric and climatic characteristics. In this paper, the challenge of parameterizing these offshore factors is tackled. The remote sensing data of the region are used to analyze the modification of wind-induced drift factors, and a well-suited solution is established in parameter correction mechanism for oil spill models. The novelty of the algorithm is the self-adaptive modification mechanism of the drift factors derived from the remote sensing data for the targeted sea region, in respect to empirical constants in the present models. Considering this situation, a new regional oil spill model (i4OilSpill) for the Bohai Sea is developed, which can simulate oil transformation and fate processes by Eulerian-Lagrangian methodology. The forecasting accuracy of the proposed model is proven by the validation results in the comparison between model simulation and subsequent satellite observations on the Penglai 19-3 oil spill accident. The performance of the model parameter correction mechanism is evaluated by comparing with the real spilled oil position extracted from ASAR images.     

An improved CHC algorithm for damage diagnosis of offshore platforms

1 Introduction Health monitoring of structures, especially large spacestructures intended for long-term operation, is essential indeveloping short-term or long-term repair plans. Becausethe maintenance, replacement, and time out of service arecostly, it i…     

Parallel hydrodynamic finite element model with an N-Best refining partition scheme

We enhance a robust parallel finite element model for coasts and estuaries cases with the use of N-Best refinement algorithms, in multilevel partitioning scheme. Graph partitioning is an important step to construct the parallel model, in which computation speed is a big concern. The partitioning strategy includes the division of the research domain into several semi-equal-sized sub-domains, minimizing the sum weight of edges between different sub-domains. Multilevel schemes for graph partitioning are divided into three phases: coarsening, partitioning, and uncoarsening. In the uncoarsening phase, many refinement algorithms have been proposed previously, such as KL, Greedy, and Boundary refinements. In this study, we propose an N-Best refinement algorithm and show its advantages in our case study of Xiamen Bay. Compared with original partitioning algorithm in previous models, the N-Best algorithm can speed up the computation by 1.9 times, and the simulation results are in a good match with the in-situ data.     

Experimental study on the effectiveness of TLDs under wave loading

The objective of this study is to experimentally investigate the effectiveness of Tuned Liquid Dampers (TLDs) for suppressing the dynamic response of a platform structure subjected to wave loading and to explore the applicability of TLDs for suppressing the structural vibration of fixed offshore platforms. The experimental model is scaled according to a full size platform by matching its dynamic properties. Rectangular and circular TLDs of various sizes and water depths are examined.The experiments were performed in a 2-D wave flume. The effectiveness of TLDs is evaluated based on their response reduction. By observing the performance and the behavior of TLDs through laboratory experiments, the effects of a number of parameters including container shape, container size, number of dampers, frequency ratio, mass ratio, and incident wave characteristics are investigated.     

An Improved Lower Order Method of Modal Parameter Estimation for Offshore Structures Using Reconstructed Signals

For modal parameter estimation of offshore structures, one has to deal with two challenges: 1) identify the interested frequencies, and 2) reduce the number of false modes. In this article, we propose an improved method of modal parameter estimation by reconstructing a new signal only with interested frequencies. The approach consists of three steps: 1) isolation and reconstruction of interested frequencies using FFT filtering, 2) smoothness of reconstructed signals, and 3) extraction of interested modal parameters in time domain. The theoretical improvement is that the frequency response function(FRF) of filtered signals is smoothed based on singular value decomposition technique. The elimination of false modes is realized by reconstructing a block data matrix of the eigensystem realization algorithm(ERA) using the filtered and smoothed signals. The advantage is that the efficiency of the identification process of modal parameters will be improved greatly without introducing any false modes. A five-DOF mass-spring system is chosen to illustrate the procedure and demonstrate the performance of the proposed scheme. Numerical results indicate that interested frequencies can be isolated successfully using FFT filtering, and unexpected peaks in auto spectral density can be removed effectively. In addition, interested modal parameters, such as frequencies and damping ratios, can be identified properly by reconstructing the Hankel matrix with a small dimension of ERA, even the original signal has measurement noises.     

Calculation of significant wave height using the linear mean square estimation method

Significant wave height is an important criterion in designing coastal and offshore structures. Based on the orthogonality principle, the linear mean square estimation method is applied to calculate significant wave height in this paper. Twenty-eight-year time series of wave data collected from three ocean buoys near San Francisco along the California coast are analyzed. It is proved theoretically that the computation error will be reduced by using as many measured data as possible for the calculation of significant wave height. Measured significant wave height at one buoy location is compared with the calculated value based on the data from two other adjacent buoys. The results indicate that the linear mean square estimation method can be well applied to the calculation and prediction of significant wave height in coastal regions.     

Dynamic performance of a semi-submersible platform subject to wind and waves

By applying experimental and numerical simulations, the motion performance of a semi-submersible platform with mooring positoning system under combined actions of wind and waves is studied. The numerical simulation is conducted by the method of nonlinear time domain coupled analysis, and the mooring forces are calculated by the piecewise extrapolating method. The scale in the model experiment is 1:100, and the mooring system of the model is designed with the method of equivalent water-depth truncation by comparing the numerical and the experimental results, the platform motion and mooring forces subject to wind and waves are investigated. The results indicate that the numerically simulated mooring forces agree well with the experimental results in static equivalent field, but show some difference in dynamic equivalent field; the numerically simulated platform motions coincide well with the experimental results. The maximum motion of the platform under operating conditions is 20.5 m. It means that the horizontal displacement is 2% less than the water depth, which satisfies the operating requirements.     

Experimental Study on Interaction Between Degrees of Freedom in a Wave Buoy

Increasing degrees of freedom(DOFs) is a useful way to raise the power capture efficiency of oscillating wave energy converters. Thus, this study proposes a buoy with three DOFs, which are surge, heave, and pitch. The hydrodynamic performance and power capture efficiency of the buoy is physically modeled. Amplitudes of unidirectional and coupled motions are compared to analyze the interaction effect between freedoms under conditions with and without power take-off damping. The capture width ratio and corresponding growth rates are also calculated. Results show that the buoy makes a periodic sinusoidal(or approximate) movement in every DOF. Coupling effect can cause an increase in the amplitude in one DOF and a decrease in the amplitudes of the others. This phenomenon shows that the kinematic energy of the buoy redistributes to all DOFs compared with the unidirectional conditions. Adding DOFs can improve the power absorption of the buoy in most cases, but the number of DOFs is not the more the better.     

Shear effects in shakedown analysis of offshore structures

In this paper, a formulation for shakedown analysis of elastic-plastic offshore structures under cyclic wave loading is presented. In this formulation, a fast numerical solution method is used, suitable for the Finite Element Method (FEM) analysis of large offshore structures on which shear effects in addition to bending and axial effects are taken into account. The Morison equation is adopted for converting the velocity and acceleration terms into resultant forces and it is extended to consider arbitrary orientations of the structural members. The theoretical methods of the shakedown analysis are discussed in detail and the formulation is applied to an offshore structure to verify the concept employed and its analytical capabilities.     

基于VIIRS数据的油气平台提取技术研究

油气平台作为海上油气资源勘探开发的主要设备之一,其数量与空间分布反映了一个区域油气资源的开发状况。普通光学影像易受天气状况影响,而雷达数据成本较高,这给油气平台的检测带来了一定的困难。由于油气平台作业过程中,需要灯光照明,同时伴气的燃烧也产生很强的灯光,因此可以通过检测灯光来实现油气平台的提取。针对VIIRS数据具有强夜间光探测能力,本文提出了一种卷积运算临界值法对海上油气平台进行提取。首先对2期不同时相的VIIRS数据进行卷积运算,对亮像元进行了增强处理,对背景像元进行弱化处理,从而明确区分疑似目标与背景,然后以0值为分界点,对疑似目标进行提取,最后利用油气平台相对静止的特性,通过邻域分析,实现油气平台的提取。结果表明,本文提出的卷积运算临界值法可以有效地提取油气平台,提取准确率约为85.4%,同时可以有效地减少由于经验阈值对提取结果造成的误差。     

Numerical evaluation of longitudinal motions of Wigley hulls advancing in waves by using Bessho form translating-pulsating source Green’S function

In the framework of 3D potential flow theory, Bessho form translating-pulsating source Green’s function in frequency domain is chosen as the integral kernel in this study and hybrid source-and-dipole distribution model of the boundary element method is applied to directly solve the velocity potential for advancing ship in regular waves. Numerical characteristics of the Green function show that the contribution of local-flow components to velocity potential is concentrated at the nearby source point area and the wave component dominates the magnitude of velocity potential in the far field. Two kinds of mathematical models, with or without local-flow components taken into account, are adopted to numerically calculate the longitudinal motions of Wigley hulls, which demonstrates the applicability of translating-pulsating source Green’s function method for various ship forms. In addition, the mesh analysis of discrete surface is carried out from the perspective of ship-form characteristics. The study shows that the longitudinal motion results by the simplified model are somewhat greater than the experimental data in the resonant zone, and the model can be used as an effective tool to predict ship seakeeping properties. However, translating-pulsating source Green function method is only appropriate for the qualitative analysis of motion response in waves if the ship geometrical shape fails to satisfy the slender-body assumption.     

海洋信息网格服务平台的设计与实现

本文针对海洋信息的分布性、信息类型及格式的多样性、数据的海量性,以及数据处理的计算量大等特性,对地理信息系统、网格技术、Web服务技术,以及远程可视化技术等进行了综合研究。首先,探讨了海洋信息网格服务平台构建的关键技术,包括海洋信息网格技术、网格环境下海洋信息的集成方法,以及远程可视化技术,并提出了海洋信息网格服务平台的系统架构;其次,选定南海海域为实验区,以Argo数据、海温和海流数据为实验数据,实现了海洋信息网格服务平台原型系统。该平台可以集成异构、分布式环境中的数据服务和应用服务,为用户提供透明的、"一站式"的海洋环境信息Web应用。该系统为海洋用户充分利用各种海洋环境信息,提高海洋工作的效率,减少重复性投资、充分合理利用现有设备和资源提供了借鉴。同时实验表明,本文所提出的技术路线是可行和正确的。     

Using geochemistry of rare earth elements to indicate sediment provenance of sand ridges in southwestern Yellow Sea

The Jianggang Harbour-centered radial sand ridge (RSR) is the largest sand body in the Yellow Sea. Its formation and evolution are of interest for scientists of various fields; however, the sediment provenance is uncertain. In this study, rare earth element (REE) geochemical compositions of the RSR sediments together with their potential sources are investigated to identify the provenance of the RSR sediments. The typical parameters ((La/Yb)N, (La/Sm)N and (Gd/Yb)N) as well as the upper continental crust-normalized patterns of REEs can only be associated with source rocks, and thus can be used as effective tracers for the origin and sources of sediments. However, the REE contents of sediments are affected by many factors, such as particle sorting and chemical weathering. Onshore RSR sediments are different in REE geochemical composition from offshore RSR sediments to some extent, suggesting that not all of the offshore RSR sediments have the same sources as the onshore RSR sediments. Meanwhile, the sediments adjacent to the northeast of Cheju Island and at Lian Island near the Lianyun Harbour were not the source of the RSR sediments due to their distinctive REE patterns, δEu, (La/Yb)N, (Gd/Yb)N and (La/Sm)N. The Korean river sediments could be dispersed to the Jiangsu Coast slightly impacting the fine fractions of the RSR sediments, particularly the offshore RSR sediments. Additionally, geochemical comparisons show that the modern Yellow River was responsible for the onshore RSR sediments, whereas the sediment loads from the Yangtze River could serve as a major contributor to the RSR, particularly the offshore RSR. In addition, the offshore RSR could also be partly fed by an unknown source due to some high values of (La/Yb)N, (La/Sm)N and La contents differing from those of the Chinese and Korean river sediments.