A precise prediction of seabed stability involving the fluid-pipe-soil interaction can lead to significant cost reductions by optimising design. Unlike previous investigations, a three-dimensional numerical model for the wave-induced soil response around an offshore pipeline is proposed in this paper. The numerical model was first validated with 2-D experimental data available in the literature. Then, a parametric study will be carried out to examine the effects of wave, seabed characteristics and confirmation of pipeline. Numerical examples demonstrate significant influence of wave obliquity on the wave-induced pore pressures and the resultant seabed liquefaction around the pipeline, which cannot be observed in 2-D numerical simulation.
Nowadays, many offshore structures have been commonly constructed over the last few decades due to the growing engineering resource in the ocean. Submarine pipelines, as one of the popular offshore infrastructures, have been extensively used for transportation of natural gas and oil from offshore platform, and disposal of industrial as well as municipal waste. To ensure the safety of usage of such submarine pipelines, the coastal engineers have to consider the unexpected loads including the wave, current, and anchor dropping/dredging, which might cause the its stability and decrease its life span. Thus, it is customary to bury the pipeline by trenching and refilling soil whose cost is relatively high and time-consuming (FredsØ e, 2016).
As reported in the literature, two well-known main mechanisms of dynamic wave-induced seabed liquefactions are the momentary liquefaction and residual liquefaction, based on in the field measurements and laboratory experiments (Zen and Yamazaki, 1991). The fist mechanism, momentary liquefaction, can occur beneath wave troughs when the great seepage flow is upward directly. Since this kind of liquefaction may be happen within a short duration as the passage of wave trough, it is also called instantaneous liquefaction. The other mechanism, residual liquefaction, takes place as a result from a compacted and cyclic shearing process that the build-up of excess pore pressure in the seabed (Seed and Rahman, 1978). As mention previously, the waves also can induce shear stress in the soil when the waves propagate, which has been analytically investigated by Yamamoto et al (1978). Whereas the wave-induced shear stress has less impact on seabed instability compared to that caused by the previous two mechanism above. This study only focuses on the wave-induced seabed liquefaction incorporating both instantaneous mechanism.
The investigation of the wave-induced soil response is extremely significant for sake of submarine pipelines. Unlike conventional approaches, in this paper, a meshfree model for the wave-seabed interactions around an offshore pipeline is established. The pipeline is considered to be fully buried or partially buried in a trench layer surrounding impermeable walls. The proposed model is validated with the analytical solution, laboratory experiments and numerical models available in the literature. Then, a parametric study is carried out to examine the effects of configuration of a pipeline on the wave-induced soil response in the vicinity of a pipeline.
Submarine pipelines play an extremely important role for the transportation of offshore energy resources that is one of main concerns for offshore engineering. In general, the vulnerability of underwater-laid pipelines may be exposed due to wave-induced liquefaction of underlying seabed soil layers.
Generally, the fluctuating pressures acting upon the seabed due to progressive motion of ocean waves will further induce excess pore pressure and reduce the effective stress within seabed soil. When the excess pore pressure increases, the shear resistance surrounding pipelines may be loss due to the liquefaction of soil. Therefore, the evaluation of the wave-induced soil response is particularly important for offshore engineers involved in the design of protection of offshore pipelines (Fredsøe. 2016).
In the past few decades, numerous investigations for the wave-seabedstructure interactions by using traditional numerical methods, such as finite di_erence method, boundary element method and finite element method, have been reported in the literature. Among these, Cheng and Liu (1986) proposed a boundary integrated model for wave-induced soil response propagating over a pipeline fully buried in a trench layer. Jeng and Cheng (2000) developed a two-dimensional finite di_erence model in a curvilinear coordinate system to examine the wave-induced pore pressures and stresses around a pipeline. The numerical results depicted the significant influence of pipelines on the soil response. Jeng and Lin (2000) proposed a finite element model to examine the wave-induced seabed response in the vicinity of a pipeline in an inhomogeneous seabed. These models were based on the assumption of no slipping at the interface between pipeline and soil. Luan et al. (2008) considered inertial forces and soil-pipeline contact e_ects in their model. It was found from the results that the interface between soil and pipeline significantly a_ected the internal stresses. Dunn et al. (2006) applied the poro-elastoplastic model (Chan, 1988) to investigate the problem of wave-seabed interaction around a fully buried pipeline in marine environments. A three-dimensional finite element model was proposed by Shabani and Jeng (2008) to examine the behaviour of soil under various wave obliquity, soil characteristics and trench configuration. Zhao et al. (2014) adopted new definition of source term in their residual model and applied to the problem of wave-soil-pipe interactions. Duan et al. (2017) proposed a 2D coupled model for wave and current induced soil response around a partially buried pipeline in a trench. They investigated the water-seabed-pipeline interaction under wave and current loading system, and the process was fully coupled.
Thiberville, Caitlyn J. (Louisiana State University) | Wang, Yanfang (Louisiana State University) | Waltrich, Paulo (Louisiana State University) | Williams, Wesley C. (Louisiana State University) | Kam, Seung I. (Louisiana State University)
Early detection of subsea pipeline leaks is a very serious and ongoing issue for the oil and gas industry with limited successful cases reported. For example, aerial surveillance of pipelines can be applied only for relatively shallow and concentrated areas, and an advanced technology such as fiber-optic cable can be considered at the significant expense of time and cost for installation and equipment. The objective of this study is to evaluate a software-based leak-detection technique through complex multiphase flow mechanics. More specifically, this study investigates (i) how leak-detection problems can be formulated from a fluid-mechanics viewpoint and (ii) how reliable such a technique can be under conditions resembling the deepwater Gulf of Mexico (GOM). In examining a wide range of scenarios, this study proves that software-based techniques have a potential for playing a key role in the future.
First, this study defines a base case selected from the literature review of deepwater GOM flowlines in terms of pressure and temperature conditions, fluid properties, reservoir properties, and flowline characteristics that allows a steady-state flow in pipeline to be determined with no leak present. Next, leaks with certain opening sizes (dleak) at different longitudinal locations (xD = x/L) are positioned, and new steady states in the presence of leaks are calculated. By comparing the two steady-state responses (with and without leak), finally, the changes in two leak-detection indicators [i.e., change in upstream pressure (ΔPin) and change in downstream total flow rate (Δqt out)] can be calculated in a wide range of input parameters. This study presents the results in the form of contour plots for pressure and flow responses.
The major finding of this study is that, theoretically, it is possible to estimate both size and longitudinal location of the leak with the two leak-detection indicators in the software-based leak-detection method. The results from various subsea flowline conditions [such as different gas/oil ratios (GORs) and fluid types, water depths, pressures at the receiving facilities, inclination angles, pipe diameters, water cuts, and so on] show that the reliability of this technique is improved when the sink term (i.e., amount of leaking fluid) is more dominant, which, in turn, means that leaks positioned farther upstream, with larger opening size, and occurring at higher pressure inside pipe are relatively easier to detect. In many of the scenarios considered, Δqt out as a leak-detection indicator shows more than a 10% change in the presence of a leak with dleak > 1 in., allowing relatively easier activation of a leak-warning system, which demonstrates the robustness of this technique. Other scenarios in which the indicators are less than a few percent changes, however, may be challenging—in those cases, additional responses from other methods (hardware-based or transient simulation) will be helpful.
The objective of this paper is to show how a system composed of many individually designed and manufactured components each with independent descriptive and analytical models of varying definition can be developed and managed by an integrated systems model.
Typical engineering design methods require the engineering team to manually coordinate and integrate the engineering domains: electrical, hydraulic, mechanical, software, etc. This effort can take an inordinate amount of time and resources and is rife with errors, which inevitably lead to specification issues. Model-based system engineering (MBSE) is a method used extensively in the military and aerospace industries to reduce component integration and system development time for complex systems. This paper examines methods used to integrate the component models during development of a design for a subsea system. The authors describe the application of a commercially available MBSE toolset extended to integrate with analytical physics based design software.
The challenges, advantages, disadvantages, and suggested improvements for integrating the models are explored. The authors discuss how their process supports the domains of system engineering including: requirements, behavior, physical architectures and verification strategies, and how this intimately aligns with the typical engineering domains. Finally, the authors discuss the development of an application programming interface (API) to integrate the models and manage the transfer of data between disparate software tool sets and provide suggestions for future projects of similar complexity. The paper discusses how the methods applied reduce duplicate work and specification errors, achieving a reduction in rework, and ultimately, development time and cost.
The MBSE methodology has not typically been applied broadly in energy. This paper highlights a successful deployment of MBSE covering early stages of conceptual design to manufacturing. The API developed to permit seamless integration of multiple systems engineering development tools is unique for this type of application. The interoperability between development tools is highly sought after in the engineering community and enables many advanced capabilities for development teams. Some of these capabilities include: end-to-end traceability, automated model development, automated design verification, and automated document generation and design specification.
Two types of approaches--physical inspection and mathematicalmodel simulation--are used to identify a leak in a gas pipeline. The former method can result in an accurate detection of the location and the size of the leak, but comes with the expense of production shutdown and the high cost/long time to run the physical detection, which is very crucial in a long-distance gas pipeline.
Thiberville, C J (Louisiana State University) | Wang, Y (Louisiana State University) | Waltrich, P (Louisiana State University) | Williams, W C (Louisiana State University) | Kam, S I (Louisiana State University)
Early detection of subsea pipeline leaks is a very serious, and still continuing, issue for the oil and gas industry with limited successful cases reported. For example, aerial surveillance of pipeline can only be applied for relatively shallow and concentrated areas, and an advanced technology such as fiber optic cable can be considered at the significant expense of time and cost for installation and equipment. The objective of this study is to evaluate a software-based leak-detection technique through the complex multiphase flow mechanics. More specifically, this study investigates (i) how leak-detection problems can be formulated from fluid-mechanics viewpoint and (ii) how reliable such a technique can be under the conditions similar to deepwater Gulf of Mexico. Examining a wide range of scenarios, this study proves that software-based techniques have potential in playing a key role in the future. This study, first of all, defines a base case selected from theliterature review of deepwater GoM flowlines in terms of pressure and temperature conditions, fluid properties, reservoir properties, and flowline characteristicswhich allows a steady-state flow in pipeline to be determined with no leak present. Next, leaks with certain opening sizes (dleak) at different longitudinal locations(xD=x/L) are positioned, and new steady states in the presence of leaks are calculated. By comparing the two steady-state responses (with and without leak), finally the changes in two leak-detection indicators (i.e., change in upstream pressure (ΔPin) and change in downstream total flow rate (Δqt out) can be calculated in a wide range of input parameters. This study presents the results in aform of contour plots for pressure and flow responses. The major finding of this study is that, theoretically, it is possible to estimate both size and longitudinal location of the leak, by using the two leak detection indicators in the software-based leak-detection method. The results from various subsea flowline conditions (such as different GORs and fluid types, water depths, pressures at the receiving facilities, inclination angles, pipe diameters, water cuts, and so on) show that the reliability of this technique is improved when the sink term (i.e., amount of leaking fluid) is more dominant, which in turn means, leaks positioned further upstream, with larger opening size, and occurring at higher pressure inside pipe are relatively easier to detect. In many of the scenarios considered, Δqt outas a leak detection indicator shows more than10% change in the presence of a leak with dleak>1-inch, allowing relatively easier activation of leak-warning system, which demonstrates the robustness of this technique. Other scenarios where the indicators are less than a few percent changes, however, may be challenging in those cases, additional responses from other methods (hardware-based or transient simulation) will be helpful.
This paper describes a work flow for quickly and semiautomatically mapping geohazards in an arctic setting along with geohazard assessment and pipeline route determination. The authors emphasize the importance of route selection in the early stages of project development to guide subsequent data collection and assessments for detailed site analyses. Often, pipeline routes are predetermined on the basis of regional, low-resolution geophysical data or the location of existing infrastructure or developments. By the time detailed assessments are completed, significant geohazards that were not initially recognized may be identified along the proposed route, and costly changes may be required to revise the route and avoid these geohazards.
Hydrodynamic characteristics of VIV are expected to significantly change when the circular cylinder is placed close and parallel to a free surface comparing to that of deep water condition. In addition, when a hybrid method is applied, it is of significate importance to understand the feature of vorticity for both free surface and vortex shedding in the wake. A 2D circular cylinder horizontally placed beneath a free surface subject to both laminar and turbulent flow was numerically studied using OpenFOAM in this paper. The main purpose of this study is through the investigation of the influence of the submergence depth (h/D), Froude number (Fr) and Reynolds number (Re) to figure out the critical boundary for the models coupling in the hybrid method, which can provide more insights into the hybrid method with the presence of the free surface.
Vortex-Induced Vibration (VIV) is one of the significant physics that are often encountered in the engineering practice, e.g., cylindrical structures like platforms, risers systems and mooring line systems, offshore risers, sub-sea pipelines, chimneys, bridges all subject to the effects caused by VIV. Especially, when the circular structure subject to the impact of the free surface, such like the riser system that service for the FPSO close to the free surface, facing more challenges in the prediction of the structure response, dynamic characteristic and how to depressive the significant vibration and undesirable forces caused by VIV.
In terms of the flow past the circular cylinder, the most studies cases are the stationary circular cylinder subject to the in line steady flow. However, fewer about consider the influence of the free surface. It is generally agreed that the pressure distribution and the near wake structure of the circular cylinder near the free surface is very different to that deeply submerged cylinder situations. Miyata et al. (1990) conducted the experiments and numerical simulations at Re=4.96*104 and Fr=0.24 and observed asymmetric pressure distributions around the cylinder surface when the cylinder is located close to a free surface. Two-dimensional flow past a cylinder close to a free surface at a Reynolds number of 180 is numerically investigated by Sheridan(1997). The wake behavior for Froude numbers between 0 and 0.7 and for gap ratios between 0.1 and 5.0 is examined. The simulations reveal that this problem shares many features in common with flow past a cylinder close to a no-slip wall. Sheridan et al. (1997) conducted experiments using the PIV technique and found that close to a free surface the near-wake structure falls under a number of modes which are very different from those of the deeply submerged cylinder wake. Carberry (2002) observed three different wake states as h/D decreases. Similar physics also found by other experimental (Sheridan et al., 1997) and numerical (Reichl et al., 2005) studies. This suggests that the flow is largely governed by geometrical constraints in the low-Froude-number. Huang (2010) experimentally study the Keulegan-Carpenter number from 5 to 27, reduced velocity from 3 to 19, the ratio between the oscillating velocity and total velocity is varying from 0.1 to 0.8. The conclusion is the combined wave and current flow is significantly different from that in current or wave only. The most influential parameter is the ratio between the current and wave velocity. Despite their success, the clarification of the interaction between free surface and shedding mode is rarely found considering the submergence depth, Froude number, and Reynolds number.
AbstractThe paper presents an integrated probabilistic geohazard assessment (iPGA) method developed in recent years by several leading offshore oil and gas companies for the evaluation of risk to subsea pipelines and facilities. The approach builds on the practice of establishing regional ground models for land-based engineering projects, in which the complete geological history of the site is used to predict the performance of the ground in response to engineering work and to identify the likely presence of geohazards. A considerable challenge in deepwater subsea environments is reconciling the scale and frequency of geohazard features and processes about which little is known and which at first sight can appear to be showstoppers to development. The iPGA method has many benefits over the more typical deterministic approach as it provides a systematic, iterative and rational approach to addressing uncertainties and quantifying the likelihood and impact of geohazards on subsea pipelines and facilities over the life of development. Presentation of outputs in the form of numerical values of geohazard event frequency, probability of a hit and damage outcomes, together with probability maps, is more readily understood and transferable to support engineering design and risk reduction. Experience from major projects around the world demonstrates the considerable value of the iPGA method in finding safe ground for the routing of pipelines and siting of facilities. The paper illustrates the various stages of the iPGA method which provides the industry with a new standard for the evaluation and mitigation of geohazard risk to deepwater development.
Deng, Haifeng (CNPC Research Institute of Engineering Technology) | Luo, Xiaoqiao (CNPC Research Institute of Engineering Technology) | Cao, Wenran (CNPC Research Institute of Engineering Technology) | Qi, Xuhao Lei (CNPC Research Institute of Engineering Technology)
Based on generalized Biot’s dynamic consolidation theory and random wave theory, a FEM model for simulating seabed-pipeline interaction is established. The difference of excess pore pressure and vertical effective stress along seabed depth, and also internal stress of pipeline and excess pore pressure along the surface of pipeline under random and representative waves are analyzed, with emphasizing on the influence of the contact between the pile and soil and inertial effects on the dynamic response of pipeline under random wave, and carrying out the numerical simulation about the influence of burial depth, pipe diameter and the permeability coefficient of seabed on the pore pressure around pipeline and vertical displacement and internal stresses of pipeline. Numerical results conclude that soil-pipeline contact effect significantly affects the dynamic response of pipeline, but the inertial effect is not obvious. Neglecting the stochastic feature of wave will remarkably underestimate the dynamic response of pipeline.
Submarine pipeline is a kind of engineering structures, which is laid on or embedded in the seabed a certain depth to mainly transport petroleum and natural gas. As an effective means of transport, the unstability of pipeline will induce oil or gas leakage, which further causes economic loss and even massive environmental pollution (Herbich, 1981). So with the growing of offshore oil fields, it is of essential importance to properly evaluate the security and stability of pipeline. When the pipeline is buried in the seabed a certain depth, the wave induced-pore pressure around the pipeline will constantly fluctuate and accumulate, which further causes soil liquefaction and destruction of pipeline, Clukey (1989) elaborates the importance of wave-soil-pipeline interaction. But this problem has not been fully understood because of the complicated soil behavior, geometry of the pipeline and wave form.
Most previous dynamics researches of seabed are only concentrated on linear or nonlinear waves. The uplift force of buried pipeline under wave has been studied through a finite element method (Magda, 1997), furthermore, the influences of degree of saturation and wave parameters on the dynamic response of seabed are discussed. The pore pressure around pipeline and the internal stresses of pipeline are derived with considering the shear modulus and permeability coefficient of seabed a function of depth. Jeng (2001) considered a similar case with a wider range of the shape of pipeline and soil parameter to study the dynamic response. Similar related research also includes Bai (2011). Luan (2007 and 2008)studied the pipeline- seabed interaction under nonlinear wave with FEM. With considering the nonlinear wave and wave current simultaneously, the dynamic response of seabed was studied through a finite element method (Wen, 2012). But the real wave is of high irregularity, thus it is unreasonable to describe the dynamic response of pipeline with the computing method based on linear and nonlinear wave theory. Recently, there is few research to solve the dynamic response of pipeline with considering the stochastic behavior of wave, Kumar (2005) and Neelamani (2011) employed laboratory model tests to study the problem respectively. Based on dynamic elastic consolidation theory and Biot dynamic consolidation theory respectively, Bie (1998) and Wang (2008) studied the dynamic response of free seabed under random wave. With the random wave spectrum analysis method, Yuan (2009) conducted a finite element model aimed at analyzing the interaction between pipeline and soil of Yellow River Estuary Cheng-dao oil field, but the study provides no information for the internal stresses.