In this study, artificial ground motions with temporal-spatial correlation are first synthesized based on a collected offshore recording from Kyoshin Network (K-NET). Second, a finite element model of 400-meter subsea pipeline is created, and the time-domain earthquake analyses are carried out under two patterns of seismic inputs. One is the uniform excitations without considering temporal-spatial variations, whereas the other is the non-uniform multi-point excitations with temporal and spatial coherence functions. Comparisons of the transient dynamic response are therefore conducted. It is shown that the nonuniform excitations tend to trigger a stronger dynamic amplification effect than uniform excitations do in the aspect of maximum acceleration and maximum displacement. Furthermore, the maximum principle stress is approximately 1.33 times under non-uniform excitations as compared to that of the uniform excitations, which is consistent with the trend of maximum relative displacement. The importance to account for the effect of temporal-spatial variations in the seismic ground motions on the dynamic response of subsea seismic pipelines thereby is manifested in this study. INTRODUCTION In earthquake prone countries like China, Japan and the Unites States, the long-span coastal and offshore structures including cross-sea bridges, tunnels, and subsea pipelines are subjected to potential earthquake attack. Especially for subsea pipelines that are commonly employed for oil and gas transportations, most of them are shallowly buried in soft seabed soil or even directly laid on seabed without trenching operations. Oil and gas would leak off once the pipeline is damaged or ruptured by the seismic ground motions. Earthquakes would then cause serious property loss and serious pollution to ocean environment and ecology water. Consequently, it is of great significance to investigate their dynamic responses under strong seismic loading in order to ensure the safety and reliability of subsea pipelines. According to a large quantity of studies based on the strong motion array records (Loh, 1985; Hao, Oliveira, 1989; Roullé and Chávez-García, 2006), it is denoted that seismic ground motions at different sites typically vary in frequency, amplitude and duration, which is known as temporal-spatial correlation. The influence factors are related to the local site effect, incoherency effect and wave passage effect. The local site effect is produced by the anisotropic and damping soil along the wave travelling paths, leading to variation of frequency as well as decay or soil amplification in amplitude. The incoherency effect include reflection, refraction and scattering during the wave propagation, conducing to the frequency variation as well. The wave passage effect accounts for the time delay of the arrival earthquake waves, resulting in phase-lags at different sites. However, in the previous works (Datta and Mashaly, 1990; Wang, et al., 2010; Xue and Wang, 2019; Zhang, et al., 2019), the temporal-spatial correlation of ground motion has been extensively discussed based on the proposed power spectral density (PSD) models (Kanai, 1961; Clough and Penzien, 1975; Du and Chen, 1994), in which the parameters are derived from existing onshore earthquake recordings. In fact, unlike the onshore earthquake ground motions, the offshore earthquake ground motions have unique characteristics in the ratio of vertical-to-horizontal peak ground acceleration and the peak spectrum plateau due to the effects of seafloor soft sediments and water layers (Chen, et al., 2015; Zhang and Zheng, 2019). If the onshore motions are used as seismic inputs, they may lead to erroneous predictions of offshore pipeline seismic responses (Bi, Hao, 2017).

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.

Consolidation settlements of a dual suction anchor (DSA) functioning as foundation structure (FST) of a pipeline end manifold (PLEM) on a soft clayey seabed during different installation and operational stages is evaluated. 3D finite element model of the DSA, including time intervals between installation phases, was created. The results showed the feasibility of DSA as the foundation structure of heavy subsea structures on very soft clays. It was observed that the maximum lateral deformations in the soil develop at shallow depths adjacent to the outside of the skirts, and the maximum vertical displacements occur in the soil plug inside the DSA and adjacent to the skirts. Concentration excess pore pressure was observed around at the tip of the skirts and adjacent to the stiffeners. In addition, it was found that the presence of post-lay rock at the vicinity of DSA considerably affects the settlement of DSA. INTRODUCTION Troll is a gas and oil field in the northern part of the North Sea, about 80 kilometers north-west of Bergen. The field is situated in the blocks 31/2, 31/3, 31/5 and 31/6 and the water depth in the area is 300 to 335 meters. The gas in Troll East is recovered by pressure depletion through 39 wells drilled from Troll A. The gas compression capacity at Troll A was increased in 2004/2005, and again in 2015. In addition to gas from Troll East, gas from the Troll B and C platforms is exported via the Troll A platform. The gas and natural gas liquids are transported from Troll A to Kollsnes Gas Processing Plant in Øygarden via three 36″ pipelines. The oil is transported from Troll B and C in pipelines to Mongstad. The Troll Phase 3 development (stage 1) is a 36’ subsea pipeline tie-back of two templates (W1 and W2) to the Troll A platform (see Fig. 1). Out of three initially proposed foundation structure (FST) alternatives, i.e. a rectangular foundation with peripheral skirts (Størkersen, 2016), a fourbucket suction anchor (Bertrane, 2010) and a dual suction anchor (DSA), the DSA was selected as the best foundation alternative for a pipeline end manifold (PLEM) on a soft clayey seabed. It was found that the DSA has advantages, such as providing a stiffer mainframe structure, keeping the periphery of the suction anchors within the frame extents and providing simple suction system arrangements compared with the four bucket solution, and a better control of installation and leveling process compared to the rectangular skirt solution. TrollWest DSA was installed successfully in July 2019 (see Fig. 2).

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.

Detail pipe soil testing and back analysis investigations of operational pipelines sited on different soil types and seabed terrains has identified time and cyclic displacement varying axial and lateral soil restraint mechanisms. These cyclic Pipe Soil Interaction (PSI) mechanisms have been shown to have significant impact on the through-life displacement response of operational pipelines. The inclusion of cyclically varying PSI input parameters into 3D global buckling and pipe walking models via appropriate Finite Element (FE) sub-routines, have been qualitatively verified on Wood Group projects and enable realistic assessments of pipeline integrity and through-life operational risk. This paper describes the impact of pipe soil modelling issues on operational pipeline responses and the detail axial, lateral and 3D pipe soil sub-routines developed in-house to capture their global behavior within cyclic FE simulations. INTRODUCTION For certain soil types, particularly soft cohesive sediments which allow significant levels of pipe embedment and consolidation with time, the adoption of simple bi-linear first load equivalent axial and lateral friction curves to represent the through-life soil restraint can lead to significant inaccuracies compared to the observed displacement response of operational High Pressure / High Temperature (HT/HP) pipelines. In complex pipe buckling and walking analyses, previously ignored aspects such as seabed undulation, nature of pipelay, operation startup/shutdown frequency and the extent of axial and lateral cyclic displacement levels have all been found to significantly change levels of soil restraint. In addition, recent optimal designs have shown a requirement to include detail analysis of ‘through-life’ soil berm and axial restraint increases for transportable non-cohesive sediments or cemented calcareous sands. To enable optimized cost effective and robust design schemes pipe soil interaction data from specialist geotechnical consultancies has become increasingly complex. This presents a challenge to pipeline design consultancies regarding how to include additional non-linearity and through-life cyclically varying input data within their FE modelling. Compounding the design enginee's problem is the need to develop buckling, walking and spanning mitigation schemes in project time frames. This often necessitates an initial rationalization of the Geotechnical inputs and modelling approach as a requirement of undertaking numerous pipeline response sensitivity studies along long route lengths.

Submarine-pipeline developments are moving into deeper waters. As a result, thicker pipe walls are needed and hence the quotient of the pipe diameter and its wall thickness (i.e., D/t ) reduces. On the other hand, there is a push to optimise properties and requirements for moderate water depths of several hundred metres. Design criteria in the prevailing submarine-pipeline design standard DNVGL-ST-F101 (2017) were developed for straight pipes with 15 ≤ D/t ≤ 60 but applicability is limited to D/t ≤ 45. There can be a desire to go beyond this limit when designing for moderate depths. When pipe walls are becoming thinner, this makes the pipeline more susceptible to collapse. It has been found that, in some cases, the DNVGL-ST-F101 formulations that aim at predicting the collapse pressure of relatively thin-walled pipelines can lead to nonconservative results. This is a concern when the line-pipe manufacturing technique employs cold forming—such as the cross-section expansion step in the UOE and JCOE methods—and the adverse contribution of the Bauschinger effect is not mitigated in a suitable manner. INTRODUCTION The UOE and JCOE methods are used for manufacturing largediameter seam-welded line pipe. Both methods include a coldexpansion step, which can promote the Bauschinger effect. The pipe's compressive stress–strain curve can be affected in two different ways: (1) the plasticity plateau can be lowered (i.e., a reduction of the yield stress), and (2) the curve can become more rounded near its transition from linear-elastic to plastic behaviour. Current design formulations were derived after assuming bilinear (elastic-perfectly plastic) relations between stress and strain, and deal with only the first effect, via the fabrication factor. The second effect can cause significant narrowing of the range in which the material behaves linearly and is particularly pronounced for steels that are not subjected to (light) heat treatment after cold forming. The collapse mechanism is a typical design driver for pipelines in deep and ultra-deep water. However, it can also be important for pipelines in intermediate water depths. Collapse is the failure mode where the pipe cross section becomes unstable due to excessive external pressure acting on the pipe wall. It is typically viewed as buckling or bifurcation behaviour because the cross section will deform significantly only when the acting pressure is very close to the limit capacity. In this range of pressure, a very small pressure increase will result in a relatively large growth of deformations. When the cross section cannot sustain any more pressure increase, it will ovalize excessively and ultimately flatten: this is called collapse.

The ability to predict offshore pipeline plough performance (e.g. tow forces and progress rates) in a variety of ground conditions is an important part of project planning. This may be estimated based upon experience from nearby locations or similar ground conditions, but existing empirical plough force prediction models are available and recent updates have been made. With the increasing use of digital data, it is hoped that greater reliance can be placed on these models and this process can be automated and integrated into Geographic Information System type applications. This paper uses real offshore plough data from three trenching projects in sand to compare plough performance prediction models. INTRODUCTION To allow appropriate planning for offshore pipeline ploughing installation campaigns it is necessary to reliably predict tow forces and advance rates prior to undertaking works, or at the tendering stage. This is not only important for planning, but also in selecting appropriate tow vessel support and deciding the approach to ploughing i.e. can the ploughing be undertaken in a single pass whilst minimising tow forces and maintaining adequate progress rates, or are further passes required. A multi-pass approach may be adopted where although the design trench depth could be achieved in a single pass, trenching is undertaken in two or more passes with an initial shallow trench depth followed by trenching to the final design depth (Machin, 1995). A multipassing approach may seem counterintuitive because of the associated additional plough handling time and the requirement of pulling the plough across the seabed more than once, but this can be more efficient and save time when compared to a single pass. The potential benefits of a multipass approach are demonstrated by Eq. 1 (Cathie and Wintgens 2001) where the passive pressure component of tow force ( F ) is shown to increase with depth ( D ) cubed and there is a rate dependent term or potential for tow forces to increase with increasing plough velocity ( v ). This latter term is dependent again on the depth of the plough share but also on the soil permeability and its potential to dilate (relative density, or unit weight γ and depth of ploughing) during shearing, resulting in low pore pressures and increased effective stress (Lauder et al., 2012).

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.

Market demand pushes subsea developments into ultra-deep water. Pipelines installed in such environment require very thick walls to cope with the extreme hydrostatic pressure. However, the design criteria for local buckling under combined loading in one of the prevailing submarine-pipeline design standards, DNVGL-ST-F101 (2017), are stated to be valid only for pipelines with a diameter-to-wall-thickness ratio ( D/t ) between 15 and 45. The European Pipeline Research Group (EPRG) evaluated whether the existing design equations can be used for very thick-walled pipelines with D/t below 15 that are subject to external pressure and bending, without affecting the level of conservatism underlying the code framework. The limit-state formulation for load-controlled behaviour is found to be increasingly conservative for thicker pipe walls. INTRODUCTION Background Intecsea reviewed the status of deep-water pipeline technology for the EPRG Design Committee as part of EPRG Project 178/2015. This study covers a gap analysis and was concluded in 2016. Areas that require further research were identified and subsequently included in EPRG's research roadmap. An important gap is the absence of a reliable and valid limit-state formulation for local buckling of thickwalled pipe when loaded by a combination of bending, axial force and pressure. The limit-state formulations for local buckling included in DNVGL-ST-F101, which is widely used for design of subsea pipelines, are stated not to be valid when D/t is less than 15. Deep-water pipelines installed to date require D/t in the order of 20 for external pressure design. This is the case in maximum water depths in the range of 2000–2500 m. When moving to ultra-deep water, the required D/t can be even lower than 15. This asks for technological advancements, which should also be sought in a fundamental review and a potential reformulation of the existing design equations. In addition to the lack of adequate design equations, manufacturability of line pipe with an extremely thick wall, meaning very low D/t , may be beyond the capabilities of the leading pipe mills. This depends on the selected manufacturing method; this paper considers seamless (SMLS) and longitudinally arc-welded (SAWL) pipe. The design of pipelines with D/t below 15 needs documented information on what can be produced, and according to which specification.

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.

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.

Reel-laying rigid pipelines induce cyclic deformation that affect the section. Among other, excessive ovality may jeopardize pipe strength during installation & in-service. Previous works shown that conventional material stress-strain relation is no longer suitable for pipeline reel-lay simulations. This paper introduces an advanced model for material behaviour. The evolution of the material behavior and its variations on the cyclic loads paths is detailed. The model uses stressstrain relations and yield surface evolution from the first load curve, with Lüders plateau leading to stabilized cycle with material property variations in cycles. The model has been implemented in finite element software. This model calibration and validation are discussed in this paper. Simulation accuracy is improved compared to published results. INTRODUCTION The pipeline reeling installation process can be a cost-effective installation method for infield flowlines/risers and smaller diameter export lines under certain economical, logistical and technical circumstances. Saipem has acquired a recent pipeline construction vessel, Constellation, which has been designed as a high-performance construction and lay vessel, capable of performing in the world's hardest environments and designed to meet the requirements of current deep and ultra-deep-water projects. During offshore reeling installation, and possibly later pipeline recovery and re-reeling installation, the pipe is reeled, unreeled and straightened. During the process, part of the pipe cross-section has experienced varying levels of plastic deformation in compression and tension, in a cyclic manner. Throughout these steps, the plasticity deformation has modified the pipe material properties, while the other part remains in the elastic domain. Materials under cyclic loading can harden both kinematically and isotropically (or anisotropically). For applications where cyclic loads stay within a single cycle, hardening process comes mainly from kinematic hardening while yield domain surface evolution is negligible. Within a single cycle, Bauschinger effect drive the model behavior, while variation of this behavior through cycles comes from isotropic hardening. The Bauschinger effect refers to a property of materials where the material's stress/strain characteristics change as a result of the microscopic stress distribution of the material. An increase in tensile yield strength occurs at the expense of compressive yield strength. After increasing the tensile yield strength, the local initial compressive yield strength is actually reduced. The greater the tensile strengthening, the lower the compressive yield strength.

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 As offshore oil and gas industry goes deeper in marine waters for the search and development of oil and gas fields, pipe-in-pipe (PIP) which has excellent thermal insulation performance becomes widely used in the production of these hydrocarbon resources. However, the simplification of PIP as a single pipe for the mechanical design and damage evaluation causes significant errors due to negligence of the influence of bulkhead. In this paper, finite element analysis (FEA) is adopted to study the PIP deployed in China's Bohai Sea, which reveals that the influence of distance between impact point and bulkhead behaves in the form of an exponential function. A single pipe theoretical model is adopted with the impact point on the bulkhead. The PIP is simplified as a Single Pipe Weight (SPW) model when the impact point is far enough from the bulkhead, such that a theoretical model of modified coefficient can be obtained. This study elaborates that bulkhead plays an important role in PIP impact damage. The difference between theoretical model of modified coefficient and FE model is in an acceptable range. The bulkhead can be regarded as a rigid body when the distance between impact point and bulkhead is larger than the bulkhead width of the upper surface ( L 1 ). The shape can be simplified as a ring of length, L 1 , to avoid modeling the complicated structure of the bulkhead since it is outside the scope of this study. INTRODUCTION Nowadays, many accidents have occurred in pipeline resulting in serious economic losses and environmental damage. 27% of failures in liquid transmission pipelines and 31% of failures on natural gas pipelines in the USA (Obeid et al., 2018) are caused by external interference reported by Pipeline and Hazard Materials Safety Administration (PHMSA). Fig 1 shows the statistics based on submarine accidents in China. The marine activities such as anchor impact and trawling are the second important factor leading to the accident. Therefore, the design of impact damage is quite important for safe operations.

ABSTRACT In this thesis, a physical model test is designed to calculate the fluid damping coefficient of Hall anchor and analyze the variation trend of anchorage velocity in water. Combined with the numerical simulation results from finite element software ANSYS/LS-DYNA, the influence of anchorage load on the damage degree of submarine pipeline under different factors is determined. The research results of this thesis can provide theoretical reference for the buried depth and protection of submarine pipelines. INTRODUCTION In case of emergency when the ship encounters some force that is difficult to be resisted during navigation or mooring, the ship may temporarily anchor in the designated pipeline protection zone, affecting the safe operation of the pipeline. In order to protect the pipeline from the impact of ship accident and avoid the influence of drag force to cause deformation and breakage of the pipe, before construction, the way to make sure that pipeline is not anchored by the impact on the basis of the third party damage is very important because it can save the cost for the layout of pipeline. Therefore, the optimal embedding depth of the pipe should be evaluated in a scientific and reasonable method and into a meaningful research content. As early as 1993, Al-Warthan and Chung(1993) applied discrete element method to the analysis of the factors of pipeline damage under three different loads. The establishment of physical model simulation in this thesis also contains the use of the previous analysis of pipeline impact substrate by Gan and Yan(2012) for reference. In this thesis, a physical model test is designed to analyze the variation trend of the anchorage velocity in water. With data obtained from finite element software, the damage degree of the submarine pipeline affected by anchorage load is determined under the influence of different factors. The method utilized in this process is numerical simulation. Previously, Bai(2013) and Yu(2016) respectively conducted researched the mechanical damage of pipes under load impact and Yu Jianxing(2016) also obtained the normal distribution of energy transfer induced by transverse impact of pipes. Zeinoddini(2013) carried out extensive experiments on the dynamics and quasi-static behaviors of steel pipe candidate materials under transverse stress. In this thesis, three-dimensional simulation is also carried out to analyze the damage caused by the anchor to the pipeline. Based on previous researches, the buried depth of the pipeline corresponding to different degrees of damage is obtained. It is of great significance for guidance for engineering regarding the buried depth of the pipeline. The research results can provide theoretical references for the protection of the submarine pipeline, too.

ABSTRACT Fiber reinforced thermoplastic pipes (RTPs) consisting of inner liner, middle reinforced layer and outer cover are gradually applied in marine industry to transport high temperature and high pressure oil and gas resources. Due to uneven seabed and human factors, initial imperfections of RTPs are produced during their laying, assembly and maintenance. The combination of initial imperfections in RTPs and high temperature of oil is prone to cause lateral bucking, jeopardizing the structural integrity of the RTPs. In this paper, a numerical study is carried out over a range of initial imperfections and length of RTPs to investigate the influence of the aforementioned factors on lateral buckling. Moreover, the effect of wound angle of fiber tapes in middle reinforced layer on lateral buckling is discussed by numerical method. The lateral buckling of RTPs is characterized by critical temperature and internal energy in this study. The results show that initial imperfection amplitude has more significant influence on the critical temperature and internal energy than that of initial imperfection length. At last, strength analysis after lateral buckling exhibits that the stresses perpendicular to fiber direction and shear stresses in the out-of-plane within the middle reinforced layers are the crucial parameters to evaluate the integrity RTPs in the design process. INTRODUCTION With the exploration and exploitation of deep-sea oil and gas resources, flexible pipe has many obvious advantages over traditional steel pipe in terms of install, transportation and maintenance. As an important class of flexible pipe family, fiber reinforced thermoplastic pipes (RTPs) with multilayer construction is already in application for water injection pipe, oil gathering pipe and gas export pipeline (Bai, 2014). To ease the flow and prevent the solidification of the wax fraction, oil and gas resources usually maintain high temperature and high pressure in RTPs. Owing to human and natural factors, initial imperfections are produced in the process of pipeline laying and service. The combination of high temperature and high pressure as well as initial imperfections is prone to drive lateral buckling, jeopardizing the structural integrity of the RTPs.

ABSTRACT During the pipe-laying process, the contact force between ship stinger and pipeline is largely affected by the correlated movement of hull, stinger and pipeline. Because many nonlinear and stochastic factors exist in the model coupling problem, it is difficult to guarantee the experimental accuracy in the conventional experimental pool. Based on the previous research results, and taking "Offshore Oil 201" as the analysis object, the hydrodynamic analysis of the hull and stinger structure is carried out. Next, the calculation of roller non-uniform contact force is performed. Then the contrastive analysis of different sea conditions is made. The law of distribution of roller contact force as well as the influencing factors are summarized. At last, the problem of dynamic response fluctuation is analyzed. INTRODUCTION Pipe-laying ships are responsible for the underwater pipeline laying and the installation of the riser system [Song 2003]. Research on the contact force of the roller of the deep-water S-pipe-laying vessel was carried out by many researchers. Song and Sun [2013] applied the contact mechanism between the pipeline and the idler and combined numerical calculation to simulate the coupling effect of the susceptor, hull and pipeline. Zhu and Yue [2014] applied the semi-physical semi-simulation hybrid experimental method to analyze the load characteristics of the roller and the force characteristics of the A-frame and suggested that the structural fatigue is caused by the dynamic response of the roller. Sun and Song [2015] derived the calculation formula of pipeline force in deep water S-pipe-laying based on the lumped mass method. They also established a three-dimensional numerical model, and solved the equation with generalized integral. The obtained results were well fitted with relevant software calculations. Xie and Zhou [2010] simulated the S-pipeline laying process based on the dSPACE simulation platform and carried out the research on the contact force between the pipeline and the idler in the medium-large scale ratio. Wang and Zhang [2017] carried out research on numerical modeling and analysis methods of beam-suspended marine pipelines. On the whole, the research on the non-uniform contact force of the roller mainly involves numerical calculation and experimental simulation. However, the numerical calculation cannot fully consider the influence of the coupling motion of the ship, the stinger and the pipeline on the roller load. The experimental simulation on the susceptor has advantages in the coupling between the stinger and the pipelines, but it is difficult to carry out better simulation due to the load transfer between the susceptor and the hull. Therefore, a new type of roller non-uniform contact force research is carried out based on analytical methods.

ABSTRACT The present paper addresses the stresses developing, inside the End-Fitting (EF), on the tensile armour wires of a metal based unbonded flexible pipe under axial tensile operational loads. To this end, a bidimensional finite element model that estimates the stress state of tensile armour wires inside the EF is developed. The stresses induced during the EF assembling procedure as well as the Factory Acceptance Test (FAT) are considered in the developed numerical model. An analytical approach to this problem, suggested by Campello (2014) and Campello et al. (2016), is utilized as a benchmarking tool in order to evaluate the developed numerical model's performance. The results obtained show good agreement between the developed model and the analytical approach. A high concentration of stresses on the tensile armour wires inside the EF, at a key point near the EF's entrance, is observed under all operational load cases. Moreover, the results suggest that the EF assembly procedure and the FAT test need to be indeed considered, as they yield significant residual stresses on the tensile armour wires inside the EF. INTRODUCTION A Flexible pipe is a composite construction of multiple layers of different material, that form a pressure containing pipe, which allows large amplitude deflections without adverse effects on the pipe itself (API, 2008, 2011). The main concept behind flexible pipes is combining an internal polymeric fluid barrier layer, that transports the pipe's contents without leaks, with helically wound steel armour layers, that provide the pipe with the required structural strength, and an outer polymeric fluid barrier layer that shields the armour layers from wear due to the pipe's operational environment e.g. seawater interacting with the steel armour (Bai et al., 2017). This design results in pipes with low bending stiffness in comparison to axial tensile stiffness that also exhibit large volume stiffness and strength (Vargas-Londoño et al., 2014).

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).

ABSTRACT Magnetic detection technology is a special detecting technique developing for submarine detection and location without being affected by light and underwater sound attenuation effect. The aim of this study was to design the reasonable magnetic array with some inductive deep-sea magnetometers to improve the detection efficiency and realize effective location of the submarine pipeline. The Targeting-Location-Theory (TLT) including the magnetic array was constructed by the calculation model. The error analysis was carried out, and the data system and the terminal interface were displayed. It indicated that there is a possibility of developing the magnetic array into real application. INTRODUCTION With the investigation of marine resources and the development process accelerating today, submarine pipeline detection and location has become a necessary part of modern marine undersea engineering. Nowadays, the position information of a lot of submarine pipeline is lost because of various reasons. For instance, the original submarine pipeline doesn't have the map, the map of the pipeline setting is missing, and the previous position of the pipeline is not accurate and so on. Since the diameter of the submarine pipeline is relatively small, the efficient detection and the effective location of the submarine pipeline are going to be two technical focuses. Submarine detection technology development began in the 1960s. Initially relying on divers into the water to detect, but with the increasing depth and the increasing detection range of the submarine object to be detected, relying on artificial direct detection couldn't meet the need. From the beginning of the seventies, Britain and the United States began to develop a variety of submarine pipelines underwater detection equipment (Palshin, 1996). After years of development, it formed three systems including optical detection, acoustic detection and electromagnetic detection in the underwater detection technology. Magnetic detection technology was a special detecting technique developing for submarine detection and location without being affected by light and underwater sound attenuation effect (Caruso, 2000).