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.
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.
In this paper numerical simulations are utilized to study the transformation of internal solitary waves (ISWs) of depression type propagation on an underwater slope in a two-layer fluid system. Gravity collapse method is used to generate the depression type of internal solitary waves. The flow evolution of a depression ISW in different physical conditions (i.e. lock length x0, step depth η0 and upper/lower layer depth ratio h1h2) is considered in an incompressible free-surface flow problem. The continuity equation and Navier-Stokes equations are utilized to simulate the flow problem. In order to simulate the deforming and breaking effect of wave and flow during the interaction between internal solitary waves and the slope, the Renormalization-Group (RNG) k-ε model is chosen as the turbulence model. The generation, propagation, breaking phases and reflecting energy of large amplitude internal solitary waves interaction with a uniform slope have been investigated in a numerical flume. ISWs main features depend on the geometrical parameters that define the initial experimental setting. The relations between ISWs geometric and kinematic features and the initial setting parameters are analyzed and compared with the existing empirical relations. The energy dissipation during wave propagation is investigated. We find that the attenuation rate of amplitude and energy of ISWs decrease as η0 increases, however, this trend tends to flatten as the internal wave amplitude increasing. The investigated slope values range from 7.2° to 43.5°. Based on both wave properties and slope values, different breaking types have been found. We also find that different slope values and breaking types cause different reflected wave amplitudes, which means different energy dissipation during the interaction between ISWs and the uniform slope. The energy dissipation during the interaction between the internal wave and the slope decreases with the slope value increasing, and the energy dissipation rate is between 64.6% and 96.2%. Such an energy dissipation during the interaction between internal solitary waves and the slope may cause mixing of the two-layer fluid.
Over the past four decades, the combination of in situ and remote sensing observations has demonstrated that long nonlinear internal solitary-like waves are ubiquitous features of coastal oceans. In a continuously stratified fluid, the interface between warm and cold fluid or between fresh and salt water can oscillate forming an internal solitary wave. Ocean internal waves typically have wavelengths ranging from hundreds of meters to tens of kilometers and periods from several minutes to several hours (Massel, 2016). Internal waves in the ocean are primarily important as they affect mixing through the transport of energy. ISWs may also affect nutrients transport towards the surface, in particular when mixing occurs (Lai, 2010), and also may cause lateral transport of nutrients (Lamb, 1997).
Duan, Chenglin (Ocean University of China) | Dong, Sheng (Ocean University of China, Shandong Provincial Key Laboratory of Ocean Engineering) | Wang, Zhifeng (Ocean University of China, Shandong Provincial Key Laboratory of Ocean Engineering) | Tao, Shanshan (Ocean University of China, Shandong Provincial Key Laboratory of Ocean Engineering)
Variability of winter sea ice in the Barents Sea and its correlations with external atmospheric forcing have been investigated in a statistical approach based on NSIDC sea ice concentration (SIC) and ECWWF surface air temperature (SAT), wind velocity datasets for the period 1979-2016. The relative SIC, defined as the ratio between sea ice area and extent, indicates that the ice regime is towards lower-concentration conditions. The SIC regression results reveal that the most remarkable ice loss occurs in the northeastern Barents Sea, particularly between 74 -78 N and 42 -67 E. The empirical orthogonal decomposition of SIC has distinguished two principal modes of SIC anomaly. The first principal mode describes 57.7% of the total variance and represents SIC multi-year decreasing trends. The SAT and wind field play a fundamental role in sea ice loss. Additionally, the second principal mode (11.3%) behaves as southeast/central anomaly seesaw, revealing sea ice anomalies are in anti-phase in both corresponding zones. Besides, the wavelet variances of the time coefficient of the two principal components for SIC, SAT and wind velocity anomaly have been analyzed. The most significant peaks for three parameters are with similar variation periods. Accurately, the sea ice is experiencing a decreasing process with uncertain oscillations in some periods due to unsteady synoptic process.
Dynamic thermodynamic calculation data of sea ice thickness since 1947 to 1996 in Chengdao oilfield is applied here to calculate corresponding design ice thickness. The fitting curves for these observations are selected from Gumbel, Weibull, lognormal, Pearson type-3 and maximum entropy distribution. Corresponding return values given by these curves are regarded as the best design sea ice thickness parameters. Based on distribution fitting tests and comprehensive consideration, lognormal distribution is chosen as the best fitting curve of annual extreme data for sea ice thickness conclusively. Then different return values can be deduced under different return periods, and maximum likelihood method is applied to determine interval estimations of these return values. The calculated results can provide a reference for disaster prevention and offshore structures design.
A very flexible bivariate joint probality distribution based on Copula function of wave height and wind speed is applied in this paper for use in joint statistical analysis of winds and waves. Bivariate Normal Copula and four kinds of common-used Archimedean Copulas (Clayton Copula, Frank Copula, Gumbel-Hougaard Copula and Ali-Mikhail-Haq Copula) are presented here to construct joint probability distribution of annual maximum wave height and corresponding wind speed on a jacket platform in Bohai Sea, which are applied to verify the efficiency of these joint distribution models. Besides, considering constraint conditions of offshore platform responses (base shear, capsizing moment and deck displacement), the joint design criteria of wave height and wind speed is built. The joint probability design based on Copula decreases the design criterion of ocean environmental factors.
A refined grid mathematic model is established to simulate astronomic and typhoon storm of Sea area around Shandong Province of China in this paper. Based on the Advanced Circulation Model (ADCIRC), an astronomical model is established by adding four main constituents to open boundary. The typhoon storm model is established based on typhoon wind field model. Three typical typhoon processes, namely No.8509, No.9216 and No.9711, are hindcasted numerically. The spatial and temporal distributions of storm surges are analyzed. After simulating the series of typhoons from 1960 to 2011 by using the verified ADCIRC model, the 100-year return values around Shandong Peninsula are estimated by a Poisson-Gumbel distribution. The return values will be adopted for storm surge disaster mitigation plan in the nearshore area of Shandong Province.
Ji, Qiaoling (College of Engineering, Ocean University of China) | Dong, Sheng (College of Engineering, Ocean University of China) | Cao, Shujun (College of Engineering, Ocean University of China) | Tao, Shanshan (College of Engineering, Ocean University of China)