With the increase of human activities at sea, it is inevitable that anchors drop into the water due to operating errors, which may lead to failure of pipelines and cause economic damage and environmental pollution. Previous methods of related analysis are mostly based on the DNV-RP-F107 recommended method (hereinafter referred as DNV method). DNV method hardly considers the variation of anchor's size and weight. And it is insensitive to the pipeline geometry and material properties. Based on reliability theory, DNV method is improved to calculate failure probability under the consideration of the above relevant factors. The efficiency of the proposed method is verified by a practical case. Besides, analysis of the influence of various factors on pipeline failure probability is completed in this paper, including anchor weight, size, pipeline geometry and material properties, the distance from the anchor drop point. Meanwhile, considering the variability, the sensitivities of variables to the failure probability are discussed. Study results indicate that the failure probability calculated by DNV method is underestimated in some situations, which can probably cause a loss for pipeline projects. Whereas the proposed method is able to consider much more influences and leads to reasonable results consistent with the actual situation.
Submarine pipeline is seen as the ‘lifeline’ for offshore oil and gas industry. Pipeline safety is one of the most important problems for engineering practice. Recently, anchors dropping into the sea becomes more frequent due to the increasing human activities at sea. The dropped anchors are likely to impact on pipelines and lead to pipeline failures, which can cause economic damage and environmental pollution. In order to reduce the risk and provide safe design, considerable research efforts have been devoted to risk assessment and reliability analysis of pipelines. In general, methods of the relevant research mainly consist of two categories: one is qualitative analysis, which can study the main influence factors on pipeline failures. Among them, fault tree analysis (FTA) is the most popular methodology and has been extensively applied to pipeline failure analysis. (Wang et al., 2007; Dong et al., 2005; Lavasani et al., 2011). The other one is quantitative analysis, which can determine pipeline failure probability and provide reliable reference for safe design. Katteland et al. (1995) developed a model for risk calculation, and applied it to evaluate the risk of all the installations in the North Sea. Det Norske Veritas (2010) proposed a ubiquitously used method for pipeline risk assessment and failure probability calculation (DNV method). Based on statistics of crane accidents, Det Norske Veritas (2013) also gave the falling probability for typical loads and various objects, which provided abundant references for pipeline risk assessment. On the basic of the above research, Liu et al. (2005) proposed a model to calculate the probability of pipeline being impacted under various anchorage conditions. Ding et al. (2010) modified DNV method and made a risk assessment of pipelines due to third-party activities. Yan et al. (2014) proposed a procedure to estimate the pipeline failure probability caused by anchoring activities. Up to now, to the best of the author's knowledge, quantitative analysis methods are mainly based on DNV method. In some situations, this method is hardly to consider the effect of anchor size and weight on pipeline failure probability. What's more, it is insensitive to the effect of pipeline geometry and material properties, which is not consistent with practice and may cause errors. In order to give an insight into those effects, a method based on reliability theory to calculate pipeline failure probability is proposed.
A two-dimensional (2-D) numerical model is applied to study the wave reflection performance of perforated caisson breakwaters. The numerical model adopts a volume of fluid (VOF) method to track free surface and simulates the turbulent flow by using Reynolds Average Navier-Stokes (RANS) and k-ε turbulence model equations. The numerical results for the reflection coefficients of perforated caissons are in good agreement with the experimental data in literature, which means that the present numerical model can well estimate the hydrodynamic performance of complicated perforated thin wall structures. Numerical examples show that when the caisson porosity is fixed, the slit width in the perforated front wall has no significant influence on the reflection coefficient of perforated caisson breakwater. The effects of the slit width and the relative wave chamber width on the flow velocity and the turbulence kinetic energy are also discussed. It is found that the fluid flow through the perforated wall is in a form of jets and the turbulence kinetic energy is mainly concentrated around the region of the wave chamber.
A perforated caisson breakwater, containing a chamber between a perforated front wall and an impermeable back wall, has a good capability of reducing wave reflection and wave forces. The conception of the perforated wall breakwater was initially proposed by Jarlan (1961). Since then, research on wave interactions with perforated caisson structures has been ongoing. The relevant studies on the hydraulic performance of various types of perforated breakwaters have been reviewed by Huang et al. (2011).
Tanimoto et al. (1976) carried out earlier model tests to study the reflection of irregular waves by the perforated caisson breakwater. Kondo (1979) presented an analytical approach to estimate the reflection coefficient of a two-chamber perforated wall breakwater. Tanimoto and Yoshimoto (1982 ) theoretically and experimentally studied the reflection coefficient of a partially perforated caisson breakwater. Fugazza and Natale (1992) applied the potential flow theory to examine the reflection coefficients of perforated breakwater with multiple wave chambers. Using the Galerkin-eigenfunction method, Suh and Park (1995) developed an analytical model to predict the reflection coefficient of perforated caisson breakwater with a rubble mound foundation. Suh et al. (2001) theoretically investigated the reflection characteristics of irregular waves on perforated caisson breakwater. Ti et al. (2002) and Ti et al. (2003) used the matched eigenfunction expansion method to examine the oblique wave reflection by single-chamber and doublechamber perforated caisson breakwaters, respectively. Takahashi et al. (2003) employed a numerical technique to investigate the reflection coefficient of perforated caissons. Liu et al. (2007) studied the reflection coefficients of regular and irregular waves by a partially perforated caisson breakwater with a rock filled core. Lee and Shin (2014) earned out a three-dimensional model test to investigate the reflection coefficient of the perforated wall structure. Recently, Neelamani et al. (2017) carried out experimental tests to assess the wave reflection characteristics of a Jarlan-type breakwater with slotted walls.
The impact of dropped objects and trawl board on submarine pipelines are simulated by a non-explicit finite element method. The new method works in three mechanics. The impact process is simulated by adjusting the material properties. The damage of the pipeline is solved using Cowper-Symonds equation. Drucker-Prager model is used to analyze the elastic-plastic properties of soil under impact. Then the present work can take into account the interactions among the dropped objects, pipelines and soil. Furthermore, the effects of the weight, shape, impact velocity and seabed flexibility are discussed in detail.
Submarine pipelines are the “lifeline” of offshore oil and gas production system and are used as one of the primary ways to transport oil and gas for offshore development. The risk of pipeline leakage is increasing with the rapid expansion of submarine pipeline networks. Statistically, more than 50 percent of submarine rupture accidents are caused by third-party damage such as ship anchoring and trawl fishing (Famiyesin et al., 2002; Cao et al., 2010; Ivanovic et al., 2011). In order to reduce the damnification to submarine pipelines caused by third-party damage, the pipelines need to be buried into sea floor reasonably. It is necessary to investigate the deformation of the submarine pipelines for designers. DNV-RP-F107 (Det Norske Veritas, 2002) gives an empirical formula for the dent depth of the pipelines impacted by dropped objects (Alexander, 2007). However, this specification does not consider the absorption of the impact energy by seabed and soil covered on the pipelines, resulting in a conservative assessment. Some scholars have explored the response of submarine pipelines to the impact of dropped objects. The interaction between pipe and soil is a complex process which contains complex mechanism and thus evaluating the damage on submarine pipelines caused by dropped objects is quite complicated. Alsos et al. (2012) discussed the importance of impact velocity and mass during impact, and found that global deformations would be triggered, which implied that the dissipated energy going into local denting is reduced to a fractional value. Yu et al.used a three-dimensional numerical method to study pipeline deformations due to transverse impacts of dropped anchors and the dent depth of the pipe was estimated by the local Galerkin discretization method. The results showed good consistency with experiment. Zeinoddini et al. (2013) carried out a parametric study to examine the effect of bed flexibility and the results showed that the flexibility of seabed plays an important role in impact energy dissipation. Ryu et al. (2015) investigated pipe-soil interaction using finite element technology in which the soil was simulated using the Mohr-Coulomb failure criterion. Robert (2017) used a modified Mohr- Coulomb model to simulate the behavior of pipelines in unsaturated soil. The model was developed considering microscopic and macroscopic suction hardening mechanisms and was implemented into a commercial finite program.
Gao, Huijun (Ocean University of China) | Wang, Lvqing (Ocean University of China, Navy Rearch Institute of P.L.A) | Liang, Bingchen (Ocean University of China) | Pan, Xinying (Ocean University of China)
The peak over threshold (POT) method and the generalized Pareto distribution (GPD) model are studied based on 22-year hindcasted wave data. The initial data are divided into many groups with the fixed time interval to ensure the independence of the maximal value within every group. To realize this function, 3-day and 5-day are selected as the fixed time intervals, which are used to obtain the sample to extrapolate extreme significant wave heights with return periods of 100-year, 200-year and 500-year at 4 locations in the Yellow Sea (YS).
Adequate extrapolation of extreme significant wave heights is highly important for the design, installation and operation of the coastal and ocean structures (Soukissian and Kalantzi, 2006). A reasonable design wave height can guarantee the security of structures and reduce the engineering expense, which are two key points for the engineering practices.
To reasonably determine the design wave height, a reliable sampling method and an appropriate distribution method are needed. The sampling methods can be grouped into the annual maximum (AM) method (Carter and Challenor, 1981; Soares and Scotto, 2001), the peak over threshold (POT) method (Petruaskas and Aagaard,1971), the total sample method (Ochi,1992), the triple annual maxima method (Sobey and Orloff, 1995), and the annual N-Largest method (Weissman,1978). Because of the utilization efficiency of initial data, the POT method has widely been employed to select the samples for extrapolation (Caires and Sterl, 2005; Soukissian et al., 2006; Mazas et al., 2014; Shao et al., 2017). According to the samples of the POT method, the generalized Pareto distribution (GPD) model is commonly accepted to extrapolate the extreme values for different return periods with reasonable uncertainties (Tancredi et al., 2006; Izadparast and Niedzwecki, 2010; Os et al., 2011).
The POT method extracts a series of independent peak significant wave heights above a selected value (the threshold) from the initial data. To ensure the independence of the sample, many methods have been proposed, such as the meteorological phenomena method (Shao et al., 2017), the time window method (Thompson et al., 2009) and the double-threshold method (Mazas and Hamm, 2011; Bernardara et al., 2014). In this study, the initial data come from the hindcasted wave data in the Yellow Sea (YS), a winter storm-dominated area. The time window method is more applicable for selection of independent peak significant wave heights, due to the characteristic of winter storms. After preparing the initial sample from the hindcasted wave data, the threshold needs to be determined. The graphical diagnostic referred to as the GPD parameter plot (Coles, 2001) is usually used as the threshold selection method, due to the graphical inspection and comprehension of data and the assessment of the candidate thresholds.
With the development of marine oil&gas exploration technology, flexible risers are frequently used in marine engineering.The paper introduces key procedures and measuring methods of the off shore experiment of this new-type non-metallic composite flexible risers. It inspects the actual experimental operating performance, anti-tension and anti-compression strength, reliability, etc. Results indicate that non-metallic composite flexible risers perform well in anti-bending and anti-compression. It present an atypical catenary shape in water. Bending curvature near the touch down point increases obviously; the top tension is effected by bulge process and wave-current. The bigger the internal pressure is, the smaller the tensile force is. Because of buoyant force, the tensile force along the axis decreases gradually; the internal pressure shared by skeleton layer among all the layers is the largest, arriving at 54.5%.
In recent years, there are more and more oil-gas explorations and production activities. Applying new technology and developing present technology are becoming the theme of modern ocean engineering field. The increase of world petroleum industry and frontier of technology innovation are gradually depending on oil and gas exploitation in deep water (Krishnan, Asher, Kan, and Popelar, 2016). Deep-sea risers are the key parts in connecting subsea production systems and surface unit. In structural shape, deep-sea risers can be divided into Steel Catenary Riser(SCR), Top Tension Riser(TTR), Flexible Riser and Hybrid Riser (Dumitrescu, Pulici and Trifon, 2003). The development of flexible risers may be dated back to late 1970s. Although it started late, it developed promptly. High-rigidity spirals inside the compound structures can strengthen metal layer to guarantee density and the low-rigidity polymer sealant can ensure the integrity of the fluid. In the outside and inside loading conditions, these layers can glide mutually to ensure that flexible risers are equipped with the feature of low warp rigidity (Bectarte and Coutarel, 2004). Besides, spirals can be replaced by various materials to strengthen metal layers in order to improve and enhance mechanical properties. Because of these features, flexible risers are becoming the focus of risers worldwide in recent years. In the area of riser laying technology, the experiment of marine risers were conducted through special experimental equipment including pipelaying vessel, etc. Due to sea current and waves, the progressing of pipelaying vessels will lead to the change of pipeline stress, therefore various riser laying methods are formed. The forms mostly used are S-Shaped laying(Baker and McClure, 2002), roll-shaped laying (Choi, 1999), J-shaped laying (Féret, and Bournazel, 1987), trailer-laying (Li, Zhong, Jiang, He, and Sun, 2016), etc. The paper depends on the actual and correct evaluation of system performance in the process of experiment through roll-shaped laying. This evaluation not only explores the details of technology and the function limits of every designing, but also analyzes the reliability of the designing and the interface requirements, and the cost. The paper introduces the process of new non-metallic composite flexible risers installation experiment, analyzes the measured data and confirms the posture, stress and law of motion.
Jia, Jing (Ocean University of China) | Li, Bei (Ocean University of China) | Gao, Huiying (Ocean University of China) | Sun, Hai (Ocean University of China) | Liang, Bingchen (Ocean University of China) | Wang, Kun (Ocean University of China)
Most islands rely on long-distance power supply from the mainland, such as diesel power supply, overhead cable power supply and submarine cable power supply in China, while these island are rich in clean and renewable natural resources like wave power, tidal power, wind power and solar power. This paper studies the optimization model of Island Multi-energy Complementary Power Supply System (IMCPSS) for the decision-making of the optimal Energy Portfolio of island power supply system. The objective function of this model contains Construction Cost (CC), Operation Cost (OC), Maintenance Cost (MC), Failure Cost (FC) and Decommission Cost (DC), which covers the Life Cycle Cost (LCC). The constraint conditions are designed to meet the regular requirements of electricity on island and the reliability of the island power supply, with the goal of minimum LCC. Genetic algorithm is adopted to solve the model to realize the optimal allocation of the IMCPSS. Daguan island of Qingdao is taken as an example to apply this model, and the result is analyzed.
Liu, Rui (Ocean University of China) | Liang, Bingchen (Ocean University of China) | Pan, Xinying (Ocean University of China) | Wang, Lvqing (Ocean University of China, Navy Rearch Institute of P.L.A)
A porous sea-access road is required to connect the land and an artificial island for the offshore oil exploitation in the Yellow River Delta, China. The effects of a porous sea-access road on hydrodynamic characteristics and suspended sediment transport dynamics are investigated using a three-dimensional ocean model. The model results suggest that symmetrical eddies and the dipole structure of suspended sediment concentration variation occur around the construction. Compared with the traditional imporous sea-access road, the porous construction can diminish the effects on local circulations, sediment transport process and morphology evolution.
The Yellow River delta is one of the most important regions of petroleum production in China, where the second largest oil field in the country is located (Wang et al., 2005). A dike construction (hereafter referred as the sea-access road) is required to connect the land and an artificial island for the offshore oil exploitation. Meanwhile, this delta is a typical ecosystem of littoral wetland, providing key habitats for a variety of wildlife (Xu et al., 2004). The traditional imporous sea-access road could fundamentally block the local circulations and alter the topography and might profoundly exert even greater pressures on coastal ecosystems by threatening the fragile wetlands (Künzer et al., 2014). The development of sea-access road has been a source of socioeconomic and environmental conflicts (Bi et al., 2011).
The use of porous sea-access road provides a way to resolve the contradictions. Porous constructions are widely used in coastal engineering (Ma et al., 2014), which are not only environmentally friendly but also functional as well as the imporous constructions. A great number of numerical models have been developed to simulate the interaction between coastal flows and porous structures in the last few decades, for instance, Delft3D (Chatzirodou and Karunarathna, 2014), OpenFOAM (Higuera et al., 2015) and Truchas (Hu et al., 2012; Wu et al., 2014).
Li, Zhichuan (CNOOC Research Institute) | Yu, Ting (CNOOC Research Institute) | Wu, Yonghu (CNOOC Research Institute) | Yue, Juan (CNOOC Research Institute) | Zhang, Li (CNOOC Research Institute) | Xiao, Gang (CNOOC Research Institute) | Zhang, Liang (Harbin Engineering University) | Wang, Shujie (Ocean University of China)
500kW Ocean Energy Isolated Power System demonstration project is one of the first batch of projects funded by the National Marine Renewable Energy Special Fund. The 2 × 50kW gravity base support structure horizontal-axis tidal current energy generation device and the 2 × 100kW floating support structure horizontal-axis tidal current energy generation device have been researched, designed and constructed. The both devices have completed demonstration operation and project acceptance. The research and construction of the both devices have accumulated valuable experience for the design of tidal current energy generation device in China. The purpose of this paper is to post-evaluate and analyze the design of the both devices, and summarize the accumulated design experience. The space of improvement has been given finally through the analysis, which can provide design experience for the future development of China's tidal current energy.
Since 2010, China has enhanced the support for marine energy research, and has established National Marine Renewable Energy Special Fund which is to support marine energy technology research and development, including industrialization and demonstration projects. 500kW ocean energy isolated power system demonstration project is one of the first batch of projects funded by the National Marine Renewable Energy Special Fund. The total installed gross capacity of this project is 500kW which includes 300kW tidal current energy. This project has completed the development and construction of the 2×50kW gravity base support structure horizontal-axis tidal current energy generation device and the 2×100kW floating support structure horizontal-axis tidal current energy generation device, completed the test run and has been accepted in 2015.
Under the background of China's energy structure adjustment and the development of low carbon green, the ocean energy has been paid more and more attention (Fraenkel, P, 2006). From 2016, a series of regulations and policies have been introduced to vigorously support and encourage the development and utilization of marine technology. The National Development and Reform Commission (NDRC) and the State Energy Administration jointly issued the “Revolutionary innovation action plan for energy technology (2016-2030)” in April 2016 (PRC Central Government Website, 2016a), in this plan, it is clear that “Strengthen the development and utilization of Marine Energy, studying the high efficiency power generation equipment and building a megawatt demonstration power plant.”; In June 2016, The National Development and Reform Commission(NDRC), the Ministry of industry and information technology and The National Energy Board issued the “China made 2025-Energy Equipment Implementation Plan”, in the implementation of this scheme(PRC Central Government Website, 2016b), it has also clearly pointed out that “the development of megawatt power generation power generation equipment as the direction of technology attack”;In January 2017, the State Oceanic Administration released the “Marine Renewable Energy Development Plan”, in this plan, tidal energy was listed as the one of the most important marine renewable energy development key point during the 13th Five-Year (National Bureau of Oceanography, 2017).
Liu, Fushun (Ocean University of China) | Cui, Gaojie (Ocean University of China) | Gao, Shujian (Ocean University of China) | Wang, Bin (PowerChina Huadong Engineering Corporation Limited) | Shen, Jinning (PowerChina Huadong Engineering Corporation Limited) | Zhou, Hu (PowerChina Huadong Engineering Corporation Limited)
Many time-frequency analysis methods, such as Short Time Fourier Transform (STFT), Wavelet Transform (WT), Hilbert-Huang Transform (HHT), and the newly developed method -Short Time Prony Transform (STPT), have been developed to investigate the time-varying performance of structures. In this paper, these four methods will be employed to investigate their characteristics when applied to offshore wind turbines. Specifically, a field test on an offshore wind turbine located in the Jiangsu Province of China is introduced, and measured data are used. The results show that the offshore wind turbine clearly has frequency-varying characteristics, and the STPT method can be used to better reveal the time-varying performance of structures. Meanwhile, this method can provide more suitable signal time durations for modal analysis.
Offshore structures, such as offshore wind turbines, have dynamic characteristics that change over time due to time-varying environmental loadings such as waves, winds, currents, and operational loadings; this constitutes a time-varying system in modal analysis. In other words, measured frequencies of time-varying offshore structures might be discontinuous and changeable over time, which requires suitable time- frequency analysis techniques before implementing modal analysis of offshore wind turbines in the field. In recent years, the analysis of time- varying structures based on measured dynamic responses has been researched thoroughly, and time-frequency analysis has become an effective approach, because signals are presented in a time-frequency-amplitude/energy density 3D space. Hence, both the constituent frequency components and their time variation features can be revealed.
Traditional signal analysis methods are based on the Fourier transform (Champeney, 1982). In the Fourier transform, a signal, as a whole, is decomposed into different frequency components. The whole and localized characteristics cannot be considered in the time and frequency domain simultaneously. By only conducting a pure frequency analysis, information about the occurrence or duration of the dominating frequencies cannot be obtained. To retain time information, many scholars generalized and even revolutionized the Fourier transform. The joint function of time and frequency was used, and a series of theories to process the signals were developed. Therefore, the energy and magnitude of a signal can be characterized in the time and frequency domain, and characteristic parameters, such as instantaneous frequency and instantaneous bandwidth, can be used to analyse the signal.
Zhang, Min (Ocean University of China) | Wu, Qihao (Ocean University of China) | Wu, Yanjian (Ocean University of China) | Du, Junfeng (Ocean University of China) | Xu, Yu (Ocean University of China) | Xu, Xiaolong (Siemens Corporate Technology)
In order to ensure the safety of the offshore wind turbines (OWTs), the fatigue damage assessment is a key factor at the design and maintenance stages. To evaluate the fatigue damage accurately, the coupled effects of wind-wave loads should be taken into account seriously. The purpose of this paper is to investigate the influences of the coupled wind-wave loads on the fatigue assessment. Both of a fixed tripod-type OWT and a floating OWT are chosen as the target structures. To study the coupled effects, 3 load cases are performed, which are wind load only; wave load only and combined wind and wave loads. The dynamic response under 3 loads cases are calculated respectively. The time histories of responses at failure-critical locations are also derived. The fatigue damages are assessed with the rain-flow counting method and Palmgren-Miner rule in time domain. Finally, by comparing the results of uncoupled and coupled load cases, the coupled effects of wind and wave loads on the fatigue damage are concluded.
Wind energy plays a very important role in renewable energy utilization. Due to limit of installation space for onshore wind turbines, offshore wind energy has a broad prospect of application. And some more advantages like the more stable and higher wind speed, less visual disturbance and noise promote the rapid development of offshore wind technology.
The support structure has been identified as a vital contribution to cost-effective installations (Seidel, 2007). It has been subjected to the environmental loads such as wind, wave, current, ice and so on during the long service years. As a result, the accumulated fatigue damages are unavoidable and have an important effect on the operation and maintenance of the offshore wind turbine (OWT) support structures. The assessment of fatigue damage is required to ensure that the structure will fulfill its intended function by industry standards in the design phase (DNV, 2013).