The calamities of oil leaking under water in ocean environments have motivated a number of studies on the theoretical mechanisms of oil jetting, oil leaking and diffusion phenomena from subsea oil pipelines. This paper presents results in terms of theoretical and computational fluid dynamics (CFD). Sensitivity studies of leak hole diameters, leak velocities and ocean current velocities are investigated. The analytical results indicate that the underwater oil leaking processes of subsea oil pipelines manifest two different scenarios (a) the jetting and (b) the diffusion. In the jetting, initial oil momentum is dominant. The larger the leak hole diameter and the leak velocities are, the larger the oil jetting column diameters and the lengths are. Also, significant necking phenomena occur in the oil columns, and significant plumes are developed at both sides of an oil column. In the diffusion scenario, larger ocean current velocities, smaller leak hole diameters and smaller leak velocities give rise to oil droplets' growth. Based on results in this study, constructive suggestions are put forwarded to dispose of oil spill pollution. INTRODUCTION Subsea oil pipelines play a fundamental role for offshore oil and gas field explorations and productions. However, the subsea pipelines buried under water all year round may suffer from corrosion, fatigue damage and damage caused by external forces such as human destruction and natural disasters. These result in serious environmental pollution and immense economic losses. For example, the pipelines in the Gulf of Mexico were torn apart, leaking about 5000 barrels of crude oil every day in April 2010. Although the frequency of underwater oil leaking is relatively lower than that of surface or near-surface oil leaking, it is quite difficult to observe the mechanism of transport and diffusion process. The research on the dispersion process and drift of the leaking oil of subsea pipelines is helpful to understand the distributions of the leaking pollutants in water. The research will be beneficial to estimate the risk of the oil leaks, to take emergency response actions and control of the pollutants released.

Scouring, due to the interaction of fluid, pipeline and material of soil, is a complex phenomenon, numerous effective parameters are involved on the scouring phenomenon below the pipelines. In order to predict scour depth accurately, investigators have tried to predict scour depth by artificial intelligence models like support vector machines, radial basis function, genetic programming and group method of data handling. A prediction model of BP neural network is proposed for the pipeline scour depth in this paper, which weights and thresholds of the BP neural network were optimized by genetic algorithm. The scour depth was predicted by this model using nondimensional data sets that were collected from the existing research. The results of the prediction model were compared with empirical equations and BP neural network, which showed that GA-BP neural network predicted the scour depth with R=0.9437(Correlation coefficient), RMSE=0.1072(Root Mean Square Error, RMSE) and MAE=0.0788(Mean Square Error, MAE) compared with empirical equations (R=0.7605, RMSE=0.2662, MAE=0.1730 and R=0.7416, RMSE=0.2790, MAE=0.2015) and BP neural network (R=0.8281, RMSE=0.2323, MAE=0.1736) in live-bed conditions. Thus it can be seen GA-BP neural network has lower error and higher accuracy than the empirical equations and BP neural network. Results of the sensitivity analysis indicated that Fr is the most effective parameter for predicting the scouring depth below the pipeline. Therefore, the method proposed by this paper can be used to accurately predict the scour depth of submarine pipeline, and provide an important role to improve the stability of pipeline. INTRODUCTION Pipelines are used to carry liquids, such as water, oil, and natural gas. These pipelines are basically embedded in the sea bed. When the flood occurs, the water flow oscillation caused by the shedding of wake vortex influence the stability of pipelines, which may cause local scour around the pipe. Therefore, the prediction of the scour depth around the pipeline is an important problem in submarine pipeline engineering. Aiming at the problem of pipeline scour prediction, the experimental research and numerical calculation are carried out(Brørs, 1999; Moncada-M et al, 1999; Chiew, 1990; Myrhaug et al, 2009; Çevik and Yüksel, 1999; Cheng et al, 2014). A large number of empirical equations have been obtained through previous studies. The main disadvantage of these traditional methods is that it is not accurate enough to predict the scour phenomenon. Therefore, artificial intelligence methods are propoesd for predicting this problems. For example, artificial neural networks (ANN), support vector machine (SVM), machine learning methods, adaptive neuro-fuzzy inference systems (ANFIS), genetic programming (GP), group method of data handling (GMDH) have been used to predict scour of underwater structures(Guven et al, 2008; Najafzadeh et al, 2014; Najafzadeh et al, 2015; Azamathulla et al, 2010; Azamathulla et al, 2011; Azamathulla et al, 2015; Etemad-Shahidi et al, 2011; TothE et al, 2011; Parsaie et al, 2019). Recently, the BP neural network optimized by genetic algorithm (GA-BP) are used to solve various problems in engineering field. In this paper, according to the scour influence factors, the BP neural network optimized by genetic algorithm (GA-BP) is used to establish the scour depth prediction model, which is introducing the genetic Algorithm to optimize the weights and thresholds of the network and taking advantage of the characteristics of GA global search, and the generalization performance of the neural network can be improved, so as to improve the prediction accuracy of the network model and provide another scientific method for the prediction of scour depth. The main purpose of this paper is to study the effectiveness of GA-BP model in predicting pipeline scour depth. Also, the results are compared with empirical equations and BP neural network.

As a premise research, performance and optimization strategy for conveying system of hydraulic dredgers is studied. Calculation method of conveying system performance is formed by combing pump performance and pipeline resistance formulas according to available researches and engineering experience, with the restriction of pipe blockage, pump cavitation, and excavation capacity. Three cases of different operation scenarios are applied to verify and analyze the performance of dredger conveying system. Calculated conveying performance is in good agreement with measured in each case. The results show specific properties of dredger conveying system: there is an optimal working point for conveying system for each pump speed(s) under certain operation conditions, characterized by the maximum production and the minimum energy consumption; there is a perfect pump speed when conveying capacity and excavating capacity is matching, which leading to production maximized and energy consumption minimized at the optimal working point; different operation mode (pump combination) leads to different conveying performance. Therefore, the basic idea and strategy of conveying system optimization is to determine the appropriate operation mode, find the perfect pump speed, and keep the optimal working point. Meanwhile, excavation operation shall be kept or adjust to match conveying capacity as much as possible. INTRODUCTION Hydraulic dredger, represented by Trailing Suction Hopper Dredger (TSHD) and Cutter Suction Dredger (CSD), plays an indispensable role in many construction projects of waterway dredging, port construction, land reclamation, river management, contaminated sediment cleaning, submarine pipe laying, trench backfilling, deep sea sand extraction, geological mining, coastal defense construction, etc. (Zhang, 2003). It is a real challenge to keep a dredger working with high efficiency, due to its complex composition and operation processes, coupled with the changeable geological, hydrological and other working conditions. Traditional dredging operation depends on experience and judgment of operators, and operating performance differs greatly between operators (Tang et al. 2008; IHC 2004). As theories and control technology develops, dredging is marching from period of traditional experience and artificial control toward period of automation and intelligence. Neural network, genetic algorithm, expert control, fuzzy control and other intelligent control technologies are applied to solve the problems of working condition variability and complex operation processes, so as to realize intellectualization of dredging operation (Tang et al., 2008; Zhang et al., 2011; Wang et al., 2012; Li et al., 2015). Most of these researches are in exploration stage, and the optimization results depend heavily on pre-training.

Three surveys of an 8km long subsea pipeline laid on a soft clay seabed were performed using side scan sonar profiler. The free span characteristics over the field surveys were examined and compared with the conventional free span design criteria. The comparison results indicate some shortcomings in the conventional design approach, with more refined evaluation of the critical spans allowing for optimization of intervention works. A refined free span acceptance criterion is developed based on the field observations. INTRODUCTION The presence of a pipeline on a seabed promotes local sediment transport that leads to scouring of sediment from around and underneath the pipeline. The scouring causes the pipeline to develop a free span. The scour problem around subsea pipelines has been studied extensively by researchers (e.g. Fredsoe, 1988; Mork, 1999) as well as in the SCARCOST project (Sumer et al. , 2001), MOBILEspan Joint Industry Project (Madeley et al. , 2015) and research work on studying the existing Woodside North West pipeline behaviour by University of Western Australia (Leckie et al. , 2015). The conventional approach to design a pipeline assumes that the seabed itself is stable and so the pipeline span remains stationary. This assumption is difficult to justify because substantial experiments and field evidence have demonstrated that the seabed becomes unstable under severe wave and current condition (Leckie et al. , 2015; Teh et al. , 2003). Because of the uncertainties of the pipeline span characters, Mork (1999) and Madeley et al. (2015) proposed probabilistic assessment to simulate the span development process. However, the probabilistic approach is not commonly used, which may be because of the limited availability of reliable statistical distributions of span characteristic data to consider in the pipeline design. The objective of this paper is to present the field observation of a pipeline from three surveys and to discuss the shortcomings in the pipeline free span design. It is motivated by questions about what the actual behavior of pipeline free spans is over the design life and its impact on the pipeline integrity.

The J-lay and S-lay are two common methods for SCR (steel catenary riser) and pipeline installation. When using the S-lay installation method, onboard welded pipe joints leave the vessel horizontally and are guided to the seabed over a stinger. The pipe is lowered using tensioners. With the advantage of high production rate, S-lay can be a cost-effective solution for deepwater riser and pipeline installation. This paper investigates feasibility of using CNOOC’s HYSY 201 S-lay installation vessel to install Lingshui 17-2's SCRs and pipelines. This paper investigates feasibility of using CNOOC's HYSY 201 S-lay installation vessel to install Lingshui 17-2's SCRs and pipelines. This paper first introduces the S-lay installation vessel HYSY 201 and S-lay configuration. The hydrodynamic motion analysis for a Response Amplitude Operator (RAO) was computed for HYSY 201 in different environmental headings. It then presents the layout and description of SCRs and pipeline for LS17-2 project. With the site-specific metocean design basis, this paper presents an installation procedure, analysis methodology, and acceptance criteria. The study covers different sizes of SCRs and pipelines to investigate the feasibility of S-lay installation. The study starts from the static installation analysis of SCRs and pipelines and includes different installation steps. The acceptance criteria are examined for the pipes at over bend and sag bend regions. The support reactions load on the stinger structure are reported at each step. The dynamic analysis is selectively performed to evaluate Dynamic Amplification Factors (DAFs) of support reaction loads especially for roller box supports on the stinger structure. The sensitivity of DAFs to wave parameters such as wave height and peak period is analyzed as well. The maximum top tension and the stinger's roller reactions are also computed and post processed for installation team to check against HYSY 201's tensioners and stinger's load capacity. This paper finds the feasibility of S-lay installation of deepwater SCRs and pipelines for Lingshui 17-2 project using the installation vessel of HYSY 201.

Wave-induced scour plays a key role in the stability analysis of coastal structures, submarine pipelines or cables. There is a rich literature in current-induced scour, but more research is needed to understand the characteristics of wave-induced scour and the mechanisms that are important to the scour process. Sediment transport and flow-induced scour are three-phase (air-water-sediment) flow problems in nature and multiphase flow simulation is a useful tools that can provide information diffcult to obtain from physical tests. Most existing numerical models developed for simulating local scours are based on one-way coupling, which neglects effects of sediment phase on hydrodynamics of the flow. The present study uses a three-phase (air, water and sediment) flow model, which allows for a two-way coupling, to simulate wave-induced local scour problems. The three-phase flow model captures the air-water interface using a modified VOF method, and uses an improved rheology for the sediment phase for better results. The model is validated and verified using one set of existing experiment results for local scour around a submerged horizontal pipe. The detailed flow fields of both the sediment phase and the water phase around the scour are analyzed to understand the scour process. All three-phase flow simulations flow simulations on XSEDE's Stampede2 supercomputers. The applicability of the model to other local scour problems is also discussed. INTRODUCTION Sediment transport involves interaction between water/air and sediment, which is a complicated process frequently happening in nature. Understanding sediment transport is critical for some engineering projects involving pipelines or power cables on seafloor and oil platforms. The interaction between sediment and the flow around the structures may cause local morphological changes: an increased flow rate might lead to local erosion while decreased flow rate may result in deposition. The foundation stability will be affected by these processes. Most existing numerical models treat sediment transport as a passive motion (one-way coupling) following the flow (Roulund et al., 2005) and the transport rate is calculated using published empirical formulas (Lee et al., 2019); this is due to the poor understanding of micro-scale hydrodynamics, turbulence and the computing power available at the time when these models were developed. For instance, Roulund et al. (2005) studied, both numerically and experimentally, the flow and local scour around a vertical pile exposed to a steady current and found that the scour depth was highly affected by the local flow pattern around the pile, such as horseshoe vortex and lee-wake vortex. Wu et al. (2000) developed a three-dimensional (3D) sediment transport model, which treats the bed-load and suspended-load with different empirical or semi-empirical equations, and simulated the sediment transport in an open channel flow; however, the agreement between the numerical and experimental results is less satisfactory close to the bank. Wave-induced sediment transport and scour around a pipeline is a multi-phase phase problem in nature. Even though one-way coupling models are suitable for large-scale practical problem, they do not have a direct consideration on the influence of sediment motion on the flow characteristics. The rapid development of computer hardware has made it possible to develop two-way coupled multi-phase flow models to understand physics involved in the interaction between the sediment and fluid phases and to better simulate the sediment transport and scour processes.

Compliant tower platform is a fixed rig structure normally used for the offshore production of oil or gas. The platform consists of narrow, flexible (compliant) towers and a piled foundation supporting a conventional deck for drilling and production operations. Compliant towers are typically used in water depths ranging from 1,000 to 2,000 feet, which exceeds the economic limit of the fixed platform. In this paper, Compliant tower platform working principle was fully discussed, in contrast to a conventional skirted fixed platform, a compliant tower does not have battered legs. The overall configuration is usually slender, tubular steel, and the spatial frame is relatively constant with the cross-section size in the direction of the whole structure height. Compared with the traditional fixed platform, the structure is designed to resist the environmental force and inertia force. In addition, the platform structure is the least sensitive to deck weight load among all platforms in deep waters, and the design stress level, steel tonnage and manufacturing cost are significantly lower than deep-water jacket platforms in water depth of more than 1,000 feet. Since the application of the first deep water compliant tower platform in the Gulf of Mexico in 1983, after more than 20 years of development, there are five compliant tower platforms in the world, three in the Gulf of Mexico and two in Angola, West Africa. Compliant tower platform is not a simple evolution and substitution, but a revolutionary technological development. Through the comparative analysis of the existing deep-water compliant tower platforms, the specific technical characteristics and application difficulties of the compliant tower are obtained. INTRODUCTION Compliant Piled Tower(CPT)platform has a more slender profile that requires less steel to construct. Due to its small base dimension, regular section geometry much more even distributed weight on skid beam, fabrication of CPT is relative easier. Unlike conventional platforms that are designed to resist forces, the compliant tower is designed to flex with the forces of waves, wind and current. The flex- legs are attached to the lower section of the tower with brace members and shear plates and connected to foundation pile with grout connections. The tower is secured to the seafloor with foundation piles. With flex-legs, resonance is reduced and wave forces are de-amplified. Compared with floating systems, such as tension-leg platforms and SPARs, the production risers are conventional and are subjected to less structural demands and flexing. However, because of cost, it becomes uneconomical to build compliant towers in depths greater than 1,000 meters.

The submarine pipeline is an important transport tool in the offshore oil and gas field. Many pipelines are laid on the seabed directly and suffer hydrodynamic forces. The sandy seabed is also scoured under the pipeline in the marine environment. In order to investigate the local scour around the submarine pipeline in extreme sea conditions, the numerical study on the local scour under solitary wave is conducted in this paper. The influence of prominent factors, such as solitary wave height, pipeline diameter and sediment property, on the scour depth has been examined and discussed in detail. Furthermore, the forces on the submarine pipeline are analyzed in the process of local scour. It is found that the scour depth increases with the increase of wave height, and the forces on the pipeline also increase. When the pipeline diameter and sediment grain size change, the forces and scour hole around the pipeline change irregularly. The findings drawn from this study are instructive in the design and application of the submarine pipeline. INTRODUCTION Local scour around submarine pipeline has been the subject of much research. Most research has been in the form of laboratory experiments. In recent years, a large number of numerical investigations have emerged as computational power has increased. The scour around the submarine pipeline involves two stages: tunnel scour in the early stage and lee wake scour in the late stage (Larsen, Fuhrman and Sumer, 2016; Zhao, Shi and Cao, 2016). Zhao, Shi, Qu, Dong and Zhang (2018) investigate the local scour around submarine piggyback pipeline in the steady flow experimentally and numerically. It is found that as the inflow Re number increases, both maximum scour depth and the length of the scour hole increase at the same time. After a systematic study of the scour depth below the submarine pipelines in clear water (Sudhan, Sundar and Rao, 2002; Zhao, Vaidya, Zhang and Cheng, 2015), the results showed that the high and low pressure values were recorded at the top and bottom of pipeline respectively when the waves pass a pipeline buried in a permeable seabed. Besides, when the pipeline is laid on the seabed directly, a critical ratio of the seepage velocity to the main flow velocity corresponds to the minimum scour depth. To date, although numerous studies have been conducted to investigate the local scour and hydrodynamic characteristics of the submarine pipeline, most research (both numerical and experimental) has focused on scouring induced by regular marine condition, such as steady current, regular wave and irregular wave, whereas comparatively few studies have involved extreme environmental conditions. Of these, Lucassen (1984) carried out a series of experiments for a rather limited set of waves and currents, whereas Sumer and Fredsøe (1990) carried out scour depth investigations for a comprehensive range of combined waves and currents. The experimental studies focused primarily on the equilibrium scour depth. More recently, Myrhaug, Ong, Føien, Gjengedal and Leira (2009) focused on scour depth below pipelines under second-order random waves plus currents and investigated the scour propagation speed along the length of the pipeline.

A pipeline inspection gauge (PIG) has been a versatile tool for the maintenance of a long pipeline. However, speed excursions in a low-pressure gas pipeline may damage the pig or the pipe. Due to the gas compressibility, significant speed excursions may occur during the start of the pig inflow pressure gas pipeline. This work presents the numerical model to simulate the behavior of the pig in a low pressure gas pipeline. In this model, the transient gas flow equations are solved by method of characteristics (MOC), and then the Runge-Kuta method is used for solving the dynamic equation of the pig. A parametric study was conducted to investigate the influence of main variables on speed excursions. The obtained results were compared to those for the commercial simulation tool, OLGA. The results show the static friction and pressure has a significant impact on the speed excursion, while the flow rate has a slight impact on it. INTRODUCTION Pipeline pigging is considered as the common way to inspect and maintain an oil and gas pipeline system. In a high pressure pipeline, the pressure creates enough force for the PIG to run inside the pipeline, allowing the pig to perform the inspection mostly successfully. However, there is not enough force to push the pig in the low pressure pipeline, and cause a speed excursion which is repetitive movements that PIG stops and restart at high speed. This phenomenon is called "stick-slip motion", which often occurs in a certain pipeline configuration, such as pipe diameter changes, curved pipes, and PIG start locations (Hendrix et al 2018). The speed excursion of the pig can damage the pipe and pig, and can lead to dangers of pipe breakage. This especially likely when the fluid in a pipeline is gas, the speed excursion would occur more often due to the compressibility of the gas. Hence, the estimation and prevention of_ speed excursions is essential.

It's very common now that more and more offshore oil fields are found with high CO 2 content. In certain offshore new oil field development project in South China Sea, the CO 2 content is about 95%(mole) in several oil wells, so it meets with huge challenges on CO 2 corrosion to equipment and subsea pipeline. In order to reduce the CO 2 partial pressure then to reduce the subsea pipeline cost based on material selection, a new process treatment system is designed. In which, fluids from the wells of high CO 2 content and low CO 2 content are treated respectively in different separators, finally the totally mixed crude oil with 30% water content from the separator with low CO 2 content could be exported into the subsea pipeline using common carbon-steel material instead of more expensive CRA-clad pipeline. So about 1 billion RMB could be saved which is a big cost reduction to the oil field development. Considering the greenhouse effect, it is not recommended to vent the separated high CO 2 gas directly, so a CO 2 reinjection system is designed. In which for the corrosion issue, a molecular sieve dehydration system is designed, and TEG dehydration system is not adopted by detailed comparison. And a 4-stage compression system is designed in which the CO 2 phase envelope diagram is studied to prevent dry ice or liquid CO 2 formation. It is hoped to be a reference for other similar offshore oil fields development. INTRODUCTION With the quick offshore oil and gas field developments, more and more oil fields are found with high CO 2 content in the well fluids. It is noted (Li, Wang, Zhang, and Luo, 2004; Li, Li, Guo, and Zheng, 2003) that it is very common now that the CO 2 content is as high as 20% or 30% (mole). However in certain new oil field development project in South China Sea, the CO 2 content in the experimental flashing gas is about 95% at maximum. So it will meet with huge challenges on CO 2 corrosion and emission issues.

Multiphase transportation technology is widely used in subsea pipelines, while some necessary flow characteristics of the multiphase flow are difficult to obtain by traditional methods. In this study, the emulsified water fraction and equivalent viscosity of oil-water mixture under different flowing conditions were investigated with a novel stirring measurement method. Their relationship with total water fraction, temperature, and shear rate were established and analyzed based on the experimental results. This study is helpful to understand the phase distribution and flow characteristics of the oil-water two-phase flow in subsea pipelines. INTRODUCTION In recent years, the petroleum resource of offshore oil fields has become an important proportion of oil production growth. The distance of subsea gathering and transportation pipelines has been increasing substantially. The phase separation operation is usually done onshore in order to reduce the construction area and costs of offshore platform, so multiphase transportation technology is widely used in subsea pipelines. The agitation and mixing effects from the pipe fittings and shear flow disperse one phase into another, forming complex and varying flow patterns (Sharma et al., 2011; Wang et al., 2011). For oil-water two-phase flow, a small proportion of water may be entirely emulsified to form water-in-oil emulsion with oil phase (Xu, 2007; Sotgia et al., 2008; Klink et al., 2011). Nevertheless, when the water fraction is considerable, besides the emulsified water, the remaining part of water phase is co-existed with the emulsion as free water (Chen and Tao, 2005; Wen et al., 2014). The emulsified water in oil increases the effective volume of oil phase and changes the viscosity of oil-water mixture. Emulsified water fraction and viscosity, as important parameters of oil-water flow characteristics, are essential for the prediction of flow pattern and the hydraulic and thermodynamic calculation. However, they are difficult to be obtained by using traditional measurement methods. The emulsifying behaviors are not only influenced by the properties of crude oil and water, but also closely related with the shear rate and temperature. Since the emulsifying behaviors in pipe flow are different from those in the static condition, emulsified water fraction needs to be determined under flowing condition in order to reflect the flow characteristics in pipelines. However, nearly all of the current research is aimed at emulsifying properties under stationary condition (Nenningsland et al., 2011; Azodi and Nazar, 2013), which is quite different from the actual situation.

To overcome difficulties that Structural Health Monitoring Devices (SHMDs) are sometimes isolated without power supply, this study proposed a real-time SHMD for pipelines using a bellows-type of flexible piezoelectric device (FPED) as a vibration energy harvester to obtain a piezo-voltage frequency response (PFR). The vibration experiments were conducted to find out the characteristics of the frequency distribution of the pipeline. A finite element model was also developed to investigate the disturbance in PFR with cracks and suitable occasions of the energy harvester. The reliability of a bellowstype of FPED was verified by the comparison between the numerical results with the experiment ones. The device shows good performance on monitoring of pipelines and reliability as an energy harvester. INTRODUCTION Structural Health Monitoring (SHM) of pipelines in the Ocean Engineering field is a well-established research area. The specific objective of SHM is to detect degradation caused by strategy landslides, earthquakes, corrosion and fatigue. Huge economic investment and human resources could be consumed if the systems will require access to AC power supply or if batteries have to be periodically replaced. For these reasons, the technologies of SHM meet requirements of harvesting ambient energy sources and surveilling quality of products in real-time. To make clear characteristics of structures such as strain, displacement, curvature, mode shapes, mode shape curvature, modal strain energy and natural frequencies, frequency response function (FRFs) and strain frequency response function (SFRFs) are useful for monitoring identification of structural damage. Especially, strain must be more sensitive than displacement to structural local changes in the previous research (Yam et al., 1996). However, strain monitoring on pipelines in ocean engineering field might be difficult and impossible anytime and anywhere. On the other hand, utilization of materials coupling two forms of energy is one of the most promising ways of developing real-time Structural Health Monitoring Devices (SHMD). The materials in previous research (Sun et al., 1995; Giurgiutiu et al., 1997; Park et al., 1999; Lopes et al., 2000; Park et al., 2000b) have drawn main attention on Piezo-ceramics (PZT) coupling electric and mechanical energy, magnetorheological (MR) fluid or heat coupling magnetic and mechanical energy, and shape memory alloy (SMA) coupling heat and mechanical energy. The device with those could be placed in a closed room and isolated without power supply. Especially, nondestructive evaluation technique utilizing the piezoelectric material, shows great promise for SHMD. At the same time, Real-time SHMD systems should be a strongly positive-correlation between output voltage and strain of piezoelectric material.

Ultra-deepwater hydrocarbon pipelines face a number of design challenges in the form of demanding geotechnical characteristics of the selected route, extreme slope, construction loads arising from suspended catenary and loads introduced during conventional hydrotesting of the installed pipeline. The paper details the design, manufacturing and construction approach adopted by the Turkstream project to eliminate hydrotest requirements. Following completion of pipelay, both pipelines were pre-commissioned with multiple cleaning pigs propelled by compressed Nitrogen and batches of mono-ethylene glycol (MEG) as desiccant for the moisture entrapped in the pipeline during construction. INTRODUCTION South Stream Transport BV (SSTTBV) has constructed and commissioned an ultra-deepwater major gas transmission system in the Black Sea comprising of two (2) offshore pipelines installed in water depth up to 2200 m (Fig. 1). Each pipeline is designed for transportation of 15.75 billion cubic metres of natural gas per year over a seabed distance of some 940 km from Russia to Turkey at maximum operating pressure of 284 barg. Key parameters of the pipeline system are detailed in Table 1. This project is widely considered as one of the most challenging pipeline projects to-date, stretching both pipe manufacturing and offshore construction capabilities of linepipe manufacturers and pipelay contractors. Ultra-deepwater hydrocarbon pipelines generally span across continental shelf and abyssal plain of the intervening ocean. Selection of the shortest technically feasible pipeline route between the pre-determined landfall locations demand thorough assessment of technical limitations set by geotechnical characteristics of the planned route, pipeline material properties and construction methodology. In most cases of ultra- deepwater gas pipeline design, the loads encountered during the construction phase far exceed in-situ pipeline operating loads; tending to dominate pipeline design parameters. Where governing regulatory environment does permit elimination of in-situ hydrotest of the installed pipeline, design optimisation can yield significant economic and environmental benefit to the pipeline operator. Pipeline certification authorities, e.g. DNV-GL, have developed design code (e.g. OS-F101) to enable pipeline designer to secure "hydrotest waiver" via implementing stringent design verification, linepipe material selection, manufacturing quality control, advanced construction quality surveillance and "dry" pre-commissioning of the installed pipeline.

The research for the effect of the restitution coefficient (RC) between particles on slurry CFD simulation is limited. In this study, using the Eulerian multiphase model based on the kinetic theory of granular flow, a steady state 3D multiphase hydrodynamic model was established to investigate the effect of the RC on sand slurry transported by horizontal pipelines. Six groups of simulations were conducted with particle sizes ranging from 0.29 to 0.55 mm, pipe diameters ranging from 53.2 to 263 mm and efflux solid volume concentrations ranging from 15% to 34%. In each group, different values of RC were investigated to see their effect on the distributions of solid concentration. The result shows the RC has a significant effect on the coarse sand with high concentration in the small diameter pipe. INTRODUCTION Compared with the other solid transport methods, pipeline is more efficient and can be easily controlled automatically. Because the material transported is isolated from the external environment, pipeline transport is safer, more environment-friendly and is considered the most potential method in deep-sea mining transport. To determine the critical transport velocity and to predict the pressure drop are always two main focuses for the slurry transport community. The satisfying determination or the precise prediction depends on a fundamental and profound understanding of the flow characteristics of these multiphase slurries. Numerous studies about slurry transport especially for the solid concentration distributions have been carried out since the early 20 th century. O'Brien (1933) provided a review of the turbulent theory developed by Osborne Reynolds, Prandtl, von Karman, and Taylor, and of its application to the suspended material distribution. Rouse (1937) studied the concentration distributions of flows in open channels. Recently, as the method of computational fluid dynamics (CFD) develops, many detailed parametric analyses have been conducted with the Eulerian multiphase model based on the kinetic theory of granular flow. Ekambara et al. (2009) predicted the time-averaged solids concentration profiles of horizontal slurry pipeline flows using the CFD software package, ANSYS-CFX. Chen et al. (2010) simulated the flow of coal-water slurries in horizontal pipelines and concluded that the gravity difference and the interaction between particles had significant effects on slurry concentration distributions. Kaushal et al. (2012) compared the slurry simulation results from Mixture and Eulerian twophase models, and concluded that the Eulerian model was better in predicting the mono-dispersed fine particles at high concentration. Kumar Gopaliya and Kaushal (2016) did the similar study as Kaushal with a much larger pipe (263 mm in diameter), which is closer to the practical applications. Li et al. (2018) gave a precise hydrodynamic simulation for the multi-sized high concentration slurry transport. The agreement with the experiment data shows this Eulerian multiphase model based on the kinetic theory of granular flow is good in predicting the high concentration slurry transport. Sultan et al. (2019) summarized and validated the developed CFD model through comparison of its results with experimental data and empirical correlations.

In order to test the localized Skin Effect Current Tracing technology, which will be large-scale applied in offshore oilfields. A large-sized experimental apparatus is set up in the dock of a port in Bohai Bay of China. The results showed that when the SECT is turned on, the temperature along the pipeline increase and reach the target value in 12 hours. Nearly two years passed, the apparatus and the heating and insulation effect in the test pipeline section is keeping in good condition. The results have verified the functional feasibility and physical performance reliability of the localized SECT. INTRODUCTION After the Chinese government propose a requirement for no less than 200 million tons of domestic oil production per year recently years, all the oil & gas companies in China strive for production increasing. According to the state planning, the vast majority of the yield increase will be produced from offshore oilfields. High viscosity crude oil which accounts for 68% of the total reserves of Bohai Bay, and high pour point crude oil which is abundant in the less developed South China Sea and East China Sea, will become the focus of offshore oil development in the future. At present, the measures of blending heat water or heating normally are adopted to improve the heat capacity in system, and reduce the fluid viscosity when pipelining the oil-gas-water mixtures. However, with the further increase of crude viscosity, water depth, transportation distance and operating pressure, the method of blending water or heating cannot meet the requirements of economic and safe transportation. Due to its strong safety performance, high heat tracing efficiency and uniform temperature distribution along the subsea pipeline, the skin effect current tracing technology, SECT for short, will be large-scale applied in the development of offshore high viscosity, low pour-point oil and deep-water oil & gas in China (Wan, Liu and Qian, 2019). In 2009, the first SECT cable and devices are introduced from Japan and laid in Bozhong 13-1 offshore oilfield to increase the heat capacity of produced fluids to overcome the risk of cooling, solidification and waxing during transportation. According to the on-site feedback, the temperature at the outlet of the 13km-pipeline has a rise of 5°C after the skin-effect electric heat tracing apparatus is turned on in winter.

ABSTRACT Submarine pipelines are key infrastructures in offshore oil and gas engineering. They are widely used for transportation of the oil and gas from production wells to onshore facilities. With the development of the port engineering and the exploitation of ocean oil and gas resources, overlap may occur between the harbor channel area and the landfall part of submarine pipelines. There is an occurrence that a buried submarine pipeline in a channel is damaged by the collision of falling objects from third party activities, e.g., a falling anchor for a ship. In order to seek reasonable protection measures to ensure the safe operation of the pipeline, it is necessary to analyze the dynamic response of the pipeline caused by the dropped objects. In this study, different protective measures on the pipeline, including pure rock layer, concrete mattress + rock layer, concrete mattress + rock layer + rubber pad and compound flexible pad + rock layer, are physically modelled in the tests to study the impact effect of a dropped anchor on the pipelines. The dynamic responses of the pipeline are measured with a series of Fiber Bragg Grating (FBG) sensor on the surface of the pipeline to analyze its maximum value and the distribution of the strain. The most effective protective measures and its thickness can be determined based on the experiments. The present work can provides the significant support for the design and maintenance of submarine pipelines. INTRODUCTION One of the most significant challenges in operating offshore pipelines is assessing the effects of external damage. When a direct impact on a pipeline occurs, assessment is required to determine whether the pipeline should continue to operate, operate with reduced pressure, or completely shut down the pipeline. Because of the tightness of port shoreline resources, there are often cases that the landing part of a submarine pipeline crosses the channel. If the original grade of the channel is not enough, it needs to be upgraded and reconstructed with excavation to increase the effective depth. This will inevitably affect the safety of the submarine pipeline under the original channel seabed. For example, there being an offshore pipeline crossing the channel of Dongying Port, China, the pipeline is covered by a protective layer of 4.0m thickness of sand filling in the original design of the pipeline. However, with the necessity of upgrading the channel, the water depth of the channel will be increased by 2m after excavation, so that only 2.0m protective thickness of sand filling is left, which is definitely unsafe according to the original design of the pipeline. In order to meet the requirements of pipeline protection after channel dredging, different protective method have to be tested to find a best solution.

ABSTRACT With the improvement of welding technology, applications of electric resistance welded (ERW) pipes have been extended to offshore pipelines. A main design factor for offshore pipelines is resistance to external pressure. The structural performance is affected by influence variables changed by the manufacturing process of pipes. So, it is necessary to evaluate the structural performance considering the manufacturing process. In this study, the ERW pipe manufacturing process is simulated by the finite element model and the effect of the ERW pipe manufacturing process on the pipe structural performance against the external pressure is evaluated. INTRODUCTION Along with the improvement of the energy industry, demand for oil and gas pipelines has increased, and engineers has focused on more economical pipe design. Among the various kinds of pipes, ERW pipe has been expanding its range of applications, which is advantageous in terms of productivity and price. In order to be applied to an offshore pipeline, it is necessary to secure resistance to external pressure. The resistance capacity is expressed as collapse strength. Previous studies shows that ERW pipes are applicable to deep water (Nagata and Tsuru, 2016; Tsuru et al., 2015), and the studies about the influence variables on collapse have continued. Kyriakides and Corona (2007) confirmed that collapse pressure is improved by decreasing diameter and thickness ratio (D/t ratio), and also by decreasing the variation of the diameter and thickness in the circumferential direction. Bastola et al. (2014) confirmed that collapse strength was reduced by residual stresses, and the residual stresses in the circumferential direction had more significant effect than the residual stresses in the longitudinal direction. Influence variables on collapse strength are affected by the ERW pipe manufacturing process. ERW pipes are made by continuously passing through rollers, where unintentional plastic deformation leading to the Bauschinger effect or strain hardening occurs. Since plastic deformation caused by repetitive loading and unloading changes the initial material properties of the steel, it is necessary to accurately predict the final properties of the pipe as well as an accurate understanding of the manufacturing process. Lee et al. (2017) analytically simulated the ERW pipe manufacturing process, and confirmed that the tensile yield strength in the longitudinal direction can be changed by adjusting the manufacturing process. Nishimura et al. (1997) confirmed the distribution of residual stresses according to the ERW pipe manufacturing process.

ABSTRACT Twin pipelines are often constructed in one trench for effectiveness of water management. However, in the Japanese current design for pipelines, interaction of twin buried pipes is not considered at all and they are treated same as single pipes. In this study, in order to clarify the influence of the sheet pile extraction on twin buried pipes, model experiments were conducted. As a result, it has been revealed that the deflection of the twin buried pipes is less than that of a single pipe and the reduction of the deflection depends on the distances between the twin pipes. INTRODUCTION Twin pipelines are often constructed in one trench for the efficient distribution of agricultural water as shown in Fig. 1. In the current Japanese design guidelines for irrigation pipelines (MAFF, 2009), the distance between twin pipes has been described. However, this distance was decided only due to the workability and economy for the construction of pipes regardless of the soil-pipe interaction. In other words, the influences of the distance on the mechanical behavior of pipes have not been considered yet. Additionally, sheet piles have often been used in Japan to install pipelines under poor ground conditions such as the soft ground and high ground-water levels or because of limited work sites. The installed sheet piles are generally extracted after the completion of backfilling. During sheet pile extraction, large deformation and change of stress condition are caused in the surrounding soil. Hence, it has been reported that excessive deformation and cracks occur in flexible and rigid pipes respectively. In many past researches, the influence of the sheet pile extraction on single pipes has been studied. For instance, Mohri et al. (1987) conducted field tests for single buried FRPM (Fiberglass Reinforced Plastic Mortar) pipelines with a large-diameter to examine the changes of the deformations of pipes and the earth pressure distribution due to the sheet pile extraction. Yoshimura et al. (1997) conducted centrifuge tests for single buried flexible pipes to investigate the mechanical behavior of the pipes with the extraction. As the study on twin buried pipes, Kawabata et al. (1991; 1992) conducted a series of loading tests and FEM analyses for twin buried flexible pipes, and investigated the mechanical behavior of the pipes. Mohri et al. (1999) conducted field tests for twin buried pipes with large-diameter, and evaluate the safety of the twin pipes. Moreover, Young et al. (1984) proposed the calculation methods for the vertical pressure applied to the twin or more rigid pipes in single ditch, but these methods didn't include the case of the flexible pipe and the coefficient of subgrade reaction.

ABSTRACT A series of wave flume experiments are conducted to investigate the wave-induced pore pressure around a trenched pipeline in a silty seabed. The pipeline is half or full buried with three kinds of backfill sand ( d 50 =0.15mm, 0.3mm, 0.5mm), each is coarser than seabed soil ( d 50 =0.04mm). Results show that the buried soil reduces the excess pore pressure around pipeline and provides a degree of resistance to seabed liquefaction, and the excess pore pressure around the circumference of pipeline or beneath the pipeline is larger in the coarser backfill sand, and decreases with the increase of backfilled depth. INTRODUCTION In submarine structure, pipeline is an important structure to transport oil and gas, and the wave-induced seabed dynamic response has received great concern among coastal engineers and researchers for protection or exploitation, as such, extensive investigations have been carried out in both theoretical and experimental approaches. The mechanism of wave-induced seabed response mainly embodies in the manner of pore pressure generation, it can be divided into two mechanisms as reported by Zen and Yamazaki (1990), one is the oscillatory mechanism, the pore pressure oscillates periodically with the wave loading, and accompanied with the amplitude damping and phase lag (Madsen, 1978; Jeng, 2013), the other one is the residual mechanism, the excess pore pressure accumulates to a certain degree caused by contraction of soil under the cyclic wave loading (Seed, 1978), when the accumulation of excess pore pressure is excessive, the seabed will act as liquid without any resistance to shear loads, leading to failure of seabed (Zen et al., 1998), and be harmful to the stability of pipelines. Therefore, in a liquescent seabed, evaluations of wave-induced seabed response are essential for the safety of pipelines. In engineering practice, pipeline trenching is needed to decrease the effect of the uneven seabed surface, or to be a shelter against the trawl gear, anchors. Besides this, backfill is also a good way to enhance the safety of pipeline. Zhai (2018) conducted a series of experiments to study the backfill influence of a trenched pipeline in a sandy seabed, the result showed that there is no accumulation of excess pore pressure in the seabed and no occurrence of liquefaction phenomenon, as for the amplitude of oscillating pore pressure, increases with the increase of the backfill material grain size. For the numerical analyses, most studies are based on Biot's consolidation model (Biot, 1941). Jeng (2001) used a finite element model to examine the internal stresses within a pipeline, and was extended to investigate the interactions between nonlinear waves, a buried pipeline and a porous seabed by Gao (2003). In a trenched condition, Gao (2006) developed a two-dimensional finite element model to study the non-linear wave-induced response of soil around a trenched pipeline. It was shown that shear modulus and permeability of backfill soils significantly affect the excess pore pressures around trenched pipeline, and that the effect of wave nonlinearity becomes more pronounced and comparable with that of trench depth, especially at high wave steepness in shallow water. Zhao (2014) considered the preconsolidation due to the specific gravity of the pipeline, used a numerical simulation to study the development of liquefaction zone beneath the pipeline, found that section of seabed is more difficult to reach the residual liquefaction state with larger backfilled depth and specific pipeline gravity. Lin (2016) studied the stability of a trenched pipeline under wave condition, and results indicated that a definite buried depth can decrease the possibility of arising scour and the maximum transient liquefaction depth around the pipeline, in some cases, partial or full burial is unable to restrain the occurrence of liquefaction, but the thickness of cover layer above the seabed surface to a certain degree can support the pipeline to be away from the liquefaction zone. Duan (2017) proposed a 2D coupled model with current condition involved, and investigated the influence of the water depth, backfilled depth, seabed saturation, permeability and the parameters of wave and current.

ABSTRACT Offshore pipelines have been commonly used in the transportation of ma-rine energy resources that is one of the main concerns of offshore oil and gas industry. In general, seabed stability around a pipeline and its resultant damage of the pipeline is one of the critical factors to be considered in the design of offshore pipeline project. In this study, an integrated numerical model for wave-seabed interactions around a subsea pipeline is established using OpenFOAM. The numerical examples demonstrate that a significant influence of different configurations of the protective layer on the distribution of excess pore-water pressure near the pipeline, which further affects the occurrence of seabed liquefaction. Based on the proposed model, four different engineering options for offshore pipeline protection are examined through numerical simulation. INTRODUCTION Many offshore structures have been commonly constructed over the last a few decades due to the growing energy resources development in the ocean. Offshore pipelines, as one of common marine installations, have been extensively used for the transportation of natural gas and oil from the offshore platforms, and disposal of industrial and municipal waste. To ensure the safety of such subsea pipelines, coastal engineers have to consider the environmental loads including the wave, current, anchor dropping and dredging, which might cause instability of the pipeline and decrease its lifespan. Thus, it is customary to bury the pipeline by trenching and refilling the soil. However, the cost is relatively high and it is time-consuming (Fredsøe, 2016). To date, a few FEM (Finite Element Method) numerical models have been proposed for investigating a more complicated Wave-Seabed-Structure Interactions (WSSI) involving a trenched pipeline or a multi-layered and anisotropic seabed (Gao et al. , 2003; Gao and Wu, 2006). Note that all aforementioned investigations have only examined a subsea pipeline which fully buried within the seabed; and the effects of the linear or non-linear wave was evaluated. Consequently, these models may not be able to fully predict the seabed response around a pipeline, which is partly buried or mounted on a seabed. To remedy these limitations, Zhao et al. (2014) firstly studied the build-up of pore pressures around a trenched pipeline. Later, Zhao and Jeng (2016) further investigated the protection of backfill depth on residual soil liquefaction in a relationship with both soil and wave parameters. Lin et al. (2016) developed an integrated FEM model to investigate the potential of momentary liquefaction around a trenched pipeline with various configurations. This framework was further extended by considering the effects of ocean currents (Duan et al ., 2017).