The Reliability Analysis of a Jacket Platform under Ice Loads Based on Respond Surface Method

When ships navigate in ice-covered waters, their propellers inevitably collide with sea ice, causing safety problems. Understanding the ice loads and structural responses during propeller–ice interactions is crucial to optimizing propeller design and ensuring operational safety. Unlike conventional hydrodynamic analysis of propellers, the structural response analysis of ice-strengthened propellers operating in polar regions involves a complex nonlinear fluid–structure interaction among ice, water, and the propeller, which remains a challenging research topic in the current academic field. This study establishes a multiphase ice–water–propeller coupling model using the nonlinear finite element method to investigate the dynamic response of ice class propeller under various ice conditions. With the nonlinear finite element analysis software LS-DYNA, a sea ice model is developed by combining the cohesive element method with elastic-brittle material failure model. The arbitrary Lagrangian–Eulerian method is adopted for multiphase coupling, aimed to analyze the propeller response under combined influence of seawater and sea ice. Numerical simulation results are compared with experimental results, indicating that peak load and average errors are within 15%, and load duration errors within 5% by considering water effect. Neglecting water effect, the deviation between simulated and experimental resultant moment peak is up to 43%. An ice–propeller milling model was proposed to examine dynamic impact processes, nonlinear ice loads, and the stress and deformation distribution on propeller blades during milling. The results reveal that under direct contact, ice loads exhibit low-frequency discrete characteristics, with significant stress concentrations at blade edges and roots, and large deformations at blade tips and leading edges. The findings show that milling depth and propeller speed significantly affected load duration, with greater milling depth and ice floe velocity resulting in increased ice loads and deformation areas. The multiphase coupling model effectively simulates different ice–propeller interaction processes, providing insights into ice loads and structural responses and offering important guidance for propeller design and operational strategies in polar environments.

In this work, the icebreaking performance of the cone structure was investigated using a new numerical model called the common-node DEM developed within LS DYNA. The icebreaking characteristics of a typical conical jacket platform in the Bohai Sea focusing on the JZ20-2NW single-pile-leg platform was studied and the ice load characteristics of the cone structure and the dynamic response of the jacket platform under various ice conditions was investigated. The findings indicate that ice thickness significantly impacts the icebreaking mechanism of the cone structure. Specifically, both the peak ice load and the peak acceleration of ice-induced vibrations are proportional to the square of the ice thickness. Additionally, the upward trend in positive vibration displacement of the jacket platform becomes more pronounced with increasing ice thickness. While both the acceleration and displacement caused by ice-induced vibrations on the jacket increase with rising ice velocity, this effect is less significant compared to the influence of ice thickness. Importantly, the ice load remains below the yield strength of the conical shell plate, demonstrating that traditional conical shell plate structures possess a margin of strength redundancy.

Objectives/Scope In order to explore the benefits and potentials of digital transformation in China's offshore oilfields and improve the safety and efficiency of offshore oilfield's operations, China National Offshore Oil Corporation (CNOOC) has established a digital transformation pilot project in the Bohai Oilfield, which is not only the very first offshore digital transformation project but also the largest offshore oilfield in China. Integrating all aspects into one highly interactive system and utilizing the most advanced computational technologies, the pilot implementation provides seven modules to cover all aspects of field operations. Design, Technology and Implementation To meet the needs for less human operation, reservoir visualization, collaborative operation and robust decision-making for offshore field development and production, the pilot implementation of digital transformation at Bohai Oilfield includes three parts, 1) information system construction; 2) intelligent transformation of offshore platform equipment and facilities; and 3) the construction of a control center on shore. All the parts above are carried out through the V&V (verification and validation) method, get good controls of data, technology, process and organization. The first part is an information system construction, which applied new technology in big data, artificial intelligence, cloud computing, internet of things and micro service architecture. It provides improved functions of safety management, reservoir management, production optimization and mobile inspection. The second part is intelligent transformation of offshore platform equipment and facilities, such as utilization of drones, robotics, personal digital assistant, intelligent water injection pump and so on. The application of these intelligent equipment and facilities not only can collect much more data about for production and systems optimization, but also can emancipate the labor force. The third part is the construction of a control center on shore, which is the decision center of the whole oilfield. Linked with data lake and optical fiber cable, the control center can achieve remote control for the works on the platform. Expected Results and Benefits The whole pilot project has been executed from scratch and started operation in July 2021. With the digital transformation, the digital coverage rate for core business is 100%; Once the system runs smoothly per design, the offshore operators are expected to be reduced by 50% due to high level of automation; the equipment failure rate is expected to decrease by 20% due to more intelligent and thorough inspection and maintenance; the incident rate is expected to lower by 20% due to more intelligent QHSE management and high-level of automation; and the reservoir recovery rate is expected to increase by 5% due to optimized reservoir management and well operations. Novelty/Additional Information This pilot is the very first digital transformation trial in offshore China. The project team studied examples from many national and international companies and could not find anything that did what we wanted to do, which was to highly integrate every aspect of field operations into one intelligent system. The project team summoned resources from more than 30 companies, including leading service companies in the petroleum industry and IT and communication companies, 300 people, and started from the scratch to design a system that serves our purpose. Then the team implemented and tested all the modules, purchased, or developed all the hardware and devices, and started pilot operation of the system within 9 months from the project start. To the project team's knowledge, this highly integrated system might be a novel one in the industry. This will provide good experiences and lessons learned for the other 80 offshore oilfields in China. The design, implementation, proved technology, operation experience, etc., can be readily expanded to other fields shortly after the successful deployment of this pilot.

21 - Reliability quantification of the overhead line conductor

Reliability analysis of solar photovoltaic system using hourly mean solar radiation data

A probabilistic description of the seismic resistance of six containment vessels was developed. A random vibration approach and an advanced first order second moment reliability method were combined to predict the cumulative distribution of the resistance. Strain ductility was used as the failure criteria. BOSOR4 was used to determine linear vibration modes. Extreme Value Type I distributions were used to characterize the maximum responses (stress resultants). BOSOR5 was implemented to predict the proportional increase of these maximums which would cause buckling. Imperfections were included in the shell models. Only shell failure modes were considered. 74 references, 39 figures, 9 tables.

Although there is abundant heavy oil resource in Bohai oilfield, only limited oil production is obtained by non-thermal production technologies such as ESP in past. The practical challenging of thermal production technologies, for instance, CCS and SAGD is the difficulty to install the large conventional steam generator onto offshore production platform. In order to recover offshore heavy oil in Bohai Oilfield, the largest offshore oilfield in China, at large oil production rate in the limited production platform lifetime, a portable steam and gas generator is developed to produce high temperature and high pressure steam, CO2 and N2 on production platform using diesel oil or natural gas as fuel. The temperature of produced steam could be regulated, usually 150 to 300, and the pressure is up to 20MPa. The components such as fuel tank, water treatment, air injection unit could be easily resembled on the production platform. The steam generated and exhausted CO2and N2 could be injected into oil well to stimulate offshore heavy oil reservoir in huff and puff or SAGP processes. In past three years, there are 2 non-thermally completed heavy oil wells and 7 thermally completed heavy oil wells are successfully stimulated by co-stimulation with steam and flue gas generated by above generator. As a result, the gas generator is operated soundly, the engineering design and related implement techniques such as insulation, sand control and corrosion inhibition are feasible, and oil production rate increased obviously (more than 100 m3/day oil production rate obtained, it is up to 5 times than cold production), total increased heavy oil production compared to cold production per cycle is about 5000m3 to 10000m3. Primarily, it shows that co-stimulation is much effective and practical for offshore heavy oil production. In this paper, cases of co-stimulation with steam and flue gas are introduced in detail, and related techniques and practices discussed as well.

Oil and gas industry is one of the most important industries contributing to the Malaysian economy. To extract hydrocarbons, various types of production platforms have been developed. Fixed jacket platform is the earliest type of production structure, widely installed in Malaysia’s shallow and intermediate waters. To date, more than 60% of these jacket platforms have operated exceeding their initial design life, thus making the re-evaluation and reassessment necessary for these platforms to continue to be put in service. In normal engineering practice, system reliability of a structure is evaluated as its safety parameter. This method is however, much complicated and time consuming. Assessing component's reliability can be an alternative approach to provide assurance about a structure’s condition in an early stage. Design codes such as the Working Stress Design (WSD) and the Load and Resistance Factor Design (LRFD) are well established for the component-level assessment. In reliability analysis, failure function, which consists of strength and load, is used to define the failure event. If the load acting exceeds the capacity of a structure, the structure will fail. Calculation of stress utilization ratio as given in the design codes is able to predict the reliability of a member and to estimate the extent to which a member is being utilised. The basic idea of this ratio is that if it is more than one, the member has failed and vice versa. Stress utilization ratio is a ratio of applied stress, which is the output reaction of environmental loadings acting on the structural member, to the design strength that comes from the member’s geometric and material properties. Adopting this ratio as the failure event, the reliability of each component is found. This study reviews and discusses the reliability for selected members of three Malaysian offshore jacket platforms. First Order Reliability Method (FORM) was used to generate reliability index and the probability of failure of the members. It was found that probability of failure is inversely related to reliability index for component reliability and variation in metocean values does not have much effect on the component reliability. High reliability indices indicate that component level reliability analysis is sufficient.

As the world is progressing towards deeper water technology, jacket platform still remains as an important production structure for oil and gas in Malaysian shallow water regions. Besides the discovery of new oil and gas, enhanced oil recovery (EOR) allows more oil to be extracted from the old fields. As a result, service life of existing fixed jacket platforms need to be extended. The overall reliability of aged platform has to be reevaluated in order to monitor the integrity of the structure, and to prevent unexpected failures. Reserve Strength Ratio (RSR) is a widely adopted measure to quantify the ultimate strength of a structure. However, structural strength alone is inadequate to express a structure's reliability. Reliability is best defined as the probability that the extreme load will not exceed the ultimate strength of a structure. It can be expressed either in probability of failure or reliability index, which is the inverse normalized form of probability of failure. This paper presents the study of existing jacket platforms in Malaysia's three oil and gas operating regions. Relationship between probability of failure, reliability index and RSR has been formed to provide a simple reliability assessment model of jacket structure in Malaysia water regions.

A great deal of existing jacket platforms need to be assessed in order to determine whether they can be reused or not. In allusion to the differences between the existing service platforms and the drafting ones, a methodology of system reliability-based assessment of existing jacket platform is presented and is applied for the assessments of BZ28-1 three jacket platforms which have operated for sixteen years. In terms of nonlinear mechanical analysis, correcting the statistical parameters of the sea states and considering pile-soil-structure nonlinear interaction, the mechanics characteristics of three jacket platforms subjected to extreme loads were analyzed. Based on the FEM model, with Monte Carlo simulation, the reliability of the platforms was studied. From the practical application, the method can be used to estimate the service life of the existing platforms and ensure their secure operation.

Take a typical cross-section of rockfill embankment slope in Yaan-Luku highway as the research object, reliability analysis is studied under the condition of water table of 840.85m, 851.50m, and loading condition of natural state and horizontal seismic acceleration of 0.2g, respectively. Raw data use Kolmogorov-Smirnov test (K-S test) to determine the distribution type of parametric variation. And the parameters were sampling with Latin hypercube sampling (LHS) method and Monte Carlo (MC) method, respectively, to obtain state function and determine safety factors and reliability indexes. A conclusion is drawn that the times of simulation based on LHS method were less than Monte Carlo method. Also the convergence of failure probability is better than the Monte Carlo method. The safety factor is greater than one and the failure probability has reached to 35.45% in condition of earthquake, which indicating that the instability of rockfill embankment slope is still possible.

Optimum length of glass fiber-reinforced polymers (GFRP), as an advantageous type of composites, on the retrofitted columns of RC frames is obtained in this study by conducting seismic reliability analyses. To achieve this goal, maximum reliability index with different FRP lengths of columns of a six-story RC frame was calculated to find the optimal length for FRP-retrofitted columns. The seismic behavior of structure after verification was assessed by nonlinear time history analysis with OpenSees finite element software. Three scaled far-field earthquake records were selected for nonlinear time history analysis. The reliability analysis was performed by first-order reliability method and Monte Carlo simulation in RT software. The optimal length of FRP was obtained on the results of reliability analysis. The findings of the study showed that increased FRP material quantities do not affect the retrofitted reliability index. The findings of the study showed that increased FRP material quantities do not affect the retrofitted reliability index. The convergence of Monte Carlo simulation occurred after a variety of iterations and, consequently, was alternated with the first-order reliability analysis which has very short run time and is lower error methodology. Moreover, in all cases of reinforcement and analyses, the compressive strength of the concrete is extremely important.

Serviceability limit state is an important limit state in the reliability analysis of building structures. One of the serviceability limit states to consider in the design is the lateral displacement or drift check. Due to the uncertainties in the cross-sectional dimensions and the mechanical properties of materials, the lateral drift of structures is a random variable. Second order effects also amplify the lateral displacements and are a function of gravity loads and lateral displacements. In this study, the drift reliability of steel frames and the effects of second order analysis are investigated using geometric nonlinear analysis and Monte Carlo Simulation technique. Results show that for the drift limit state that does not include P–Δ effect, reliability indices do not depend on return period of the load cmbination, the wind to gravity load ratio or the drift limits. On the other hands, for the limit state in which P–Δ effect is incorporated, in low wind to gravity range, the reliability indices are sensitive, and could change dramatically. The limit state that does not have the P–Δ effect is the lower bound of load combination with 50 year return period, but is the upper bound for the other load combination with 20 year return period. Furthermore, the value of drift limit affects the reliability indices for the limit states that include P–Δ effect.

As the largest offshore oilfield in China, Bohai Oilfield has made good exploration discoveries in various target strata in the past decade, especially in the exploration of deep buried hills. Because the shallow favorable targets have been nearly drilled out, the exploration focus of Bohai oilfield is gradually shifting to the middle and deep layers. The buried hill have become the most important research hots pot of Bohai oilfield, and several large gas reservoirs have been discovered in Archean strata. The main reservoir type of buried hill oilfield is fracture, which has complicated characteristics with low porosity, strong heterogeneity and complex fracture distribution. The average depth of buried hill is about 4500m, so it is difficult to accurately predict and describe good fractured reservoir in the exploration and development. Because of the deep buried depth, the quality of seismic data is usually very poor. On the one hand, the effective signal of fractured reservoirs are very weak, and the low S/N seismic data is hard to recognize fractured reservoir. On the other hand, there are a lot of residual internal multiples, and their amplitude is even stronger than the reflected wave of fractures. Taking the normal processing procedure into consideration, most of the multiples like surface-related multiples have been well suppressed during the seismic data processing. In order to clearly identify the real reflection wave of fractured reservoir, it is necessary to suppress the internal multiples and enhance the effective signals. Generally, the methods of suppressing multiple waves are divided into two categories: one is the filtering method based on signal analysis and processing, such as t -p transform and f-k transform; the other is the prediction subtraction method based on wave equation, such as wave extrapolation method and inverse scattering series method. The first kind of methods have certain limitations: the dynamic correction speed of multiple waves and primary waves is similar, which weakens the effectiveness of t -p transformation, multiple and primary waves have similar slopes, which weakens the effectiveness of f-k filtering. The second type of method mainly uses the generation mechanism of multiple waves to predict and suppress multiple waves. It comprehensively considers the kinematic and dynamic characteristics of multiple wave propagation and can adapt to complex underground structure. It uses its own data as a multiple wave prediction operator, requires less or no assumptions about underground structure, and attenuates multiple waves without damaging the primary reflected signal. For internal multiple waves, the current effective method is the inverse scattering series method based on the point scattering model. This method does not need a prior velocity structure model, and has a good adaptability to complex structures, but the calculation is very large.

The recent tailing dam failure in Brazil has again emphasized the need of performing robust stability analysis prior to construction. This paper demonstrates the probabilistic dynamic stability analysis of tailing dams considering an existing rock-fill tailing dam in India. The stability analysis was performed using 2D finite element-based package RS2. In the probabilistic analysis, the strength parameters such as cohesion (c) and the friction angle (φ) were considered as random variables. In total, 3000 numbers of samples were generated assuming a normal distribution. Monte Carlo simulation was used to evaluate the probability of failure (PoF) and reliability index. Strength reduction method was used for the finite element analyses. A pseudo-static seismic loading was incorporated in the strength reduction analysis to check the seismic stability of the dam. A factor of safety (FoS) of 1.15 was observed from the deterministic analysis for downstream slope. For the same case, the probabilistic analysis provided a mean FoS of 1.19 with 5.46% probability of failure. The FoS values and the locations of the critical failure surface obtained by the limit equilibrium method and finite element method were compared. The observed FoS values were found to be higher than the values specified in the IS 7894-1975 (reaffirmed in 1997) and ANCOLD (1999). In the case of pseudo-static approach, the maximum displacement of 0.53 m was observed in the slope. Furthermore, nonlinear dynamic stability analysis was performed to simulate a true earthquake event. The permanent deformation of the slope after the earthquake was found to be 0.40 m. The zone of failure observed in both pseudo-static and nonlinear dynamic stability analyses was found to be the same. Overall, the results revealed that the spatial variability of the soil significantly influences the FoS values.