A new numerical method for the problem of nonlinear long-short wave interactions

Thesis (S.M. in Ocean Engineering)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Cataloged from PDF version of thesis.Includes bibliographical references (p. 98-100).The scope of this thesis is the development of a new numerical method to address the problem of nonlinear interactions of free-surface gravity waves. More specifically, this study addresses the case of wave interactions of short waves ridding on longer waves up to second order of nonlinearity M. The study of the nonlinear interactions between long and short waves is an active research area that has numerous applications. Most recently the interest on the subject has been regenerated due to the need for reliable prediction of the evolution of large-scale ocean wave-fields using remote sensing techniques for internet-based mapping applications. Accurate deduction of the ocean wave-field elevation requires detailed understanding of the modulation of the wave characteristics due to nonlinear interactions, from both a qualitative and a quantitative perspective. Initially, we will present the necessary background for the mathematical formulation of the general Nonlinear Wave Interactions (NWI) problem. We will then focus on the High-Order Spectral (HOS) method, a specific mode-coupled method used extensively for the solution of NWI. This method is studied with the objective of developing a new Mapped-Domain Spectral Method (MDSM) that plans to incorporate the strong points of the HOS method and also to extend its capabilities for a broader range of wavelength ratios of NWI. The new numerical scheme is based on the mapping of the original Boundary Value Problem (BVP) and on the devolopment of a mixed Fourier-Chebyshev spectral method for the solution of the transformed BVP, while ensuring exponential convergence with respect to the Chebyshev order of expansion K.(cont.) The investigation of the new method has been conducted for the case of two interacting waves taking into account terms up to M = 2. The results agree very well with HOS for cases of wavelength ratios up to AS/AL = 0.1, as well as for short time evolutions. For cases where either the wavelength ratio is smaller than 0.1, or the run time is significantly larger (t > 10Ts), the new method succeeds in providing results, while prior methods are limited by the appearence of divergent terms proportional to ALkS.by Kirki N. Kofiani.S.M.in Ocean Engineerin

Ductile fracture and structural integrity of pipelines & risers

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Page 246 blank. Cataloged from PDF version of thesis.Includes bibliographical references (p. 240-245).The Oil and Gas (O&G) industry has recently turned its interest towards deep and ultra-deep offshore installations in order to address the global increase of energy demand. Pipelines and risers are key components for the production and transportation of oil and gas both in the offshore and onshore environment. The structural integrity and fracture control of pipes, which are major components for the exploration, production and transportation of fossil fuels have been the subject of extensive analysis in the past decade using classical fracture mechanics approaches, especially for the offshore case. The main driving force for this line of research was the fact that both the cost and the technical challenges increase disproportionally with water depth. In the deep and ultra-deep water environment the technical challenges include higher permanent and operational loads, extreme environmental conditions and the presence of corrosive agents. All the above mentioned parameters demand the use of modern fracture mechanics approaches. At the same time, the inaccessibility to structures located at depths of two to three kilometers, results in extreme repair costs. Due to the magnitude of environmental and financial consequences in the event of failure, the industry has established extremely conservative safety requirements resulting from outdated approaches for those types of structures. Furthermore, the O&G industry is reluctant to adopt novel fracture models, unlike other industries, such as the automotive and aerospace. Pipelines and risers need to be evaluated both from a structural and a financial perspective. The current thesis is proposing a new physics-inspired technology and computational capability for the prediction of fracture and structural failure of pipelines and risers operating in extreme conditions, such as deep and ultra-deep water environments subjected to extreme conditions and accidental loads. The computational tool employed in the current study is derived from a variational principle, combined with a cumulative measure of damage that is developed to control the fracture initiation. The calibration process of this methodology is achieved through a hybrid numerical experimental procedure. The material selection for this study was chosen naturally from the O&G and pipeline community. Traditionally, the O&G and pipeline industries have been using not only conventional fracture methods, but also conventional low-grades of steels for pipelines and risers, such as X60 and X70. However, deep and ultra-deep applications and the demand for increase of daily flow production pose new challenges in terms of harsh environmental conditions, increase of external diameter and higher operational loads. The industry is well aware of the fact that Advance High Strength Steels (AHSS), such as X100 and X120, can address those issues, but is not yet ready to introduce them, due to incomplete understanding of their material properties and structural behavior in the plastic and near failure range. Therefore, the current thesis offers a comprehensive study of two representative grades from both categories (X70 and X100), comparing their mechanical properties and completing a preliminary analysis quantifying the financial difference between the two for pipeline construction. Pipeline and riser installations are extremely capital intensive. They need to be evaluated both from a structural and a financial perspective, so that operating companies can quantify the integrity of their investments. The proposed thesis will develop a method using representations of oil prices and material costs along with a fracture mechanics model to improve the decision process of the material, the design, and the operating conditions of pipeline installations. This technique will not only attempt to account for the mechanical properties and structural integrity of the tubular component of interest but also to quantify the financial benefit of AHSS in the Oil and Gas community.by Kirki N. Kofiani.Ph.D

Experiments and fracture modeling of high-strength pipelines for high and low stress triaxialities

This paper provides results from a comprehensive study on mechanical characterization of high-strength pipeline steel, grade X100 using experimental and numerical methods. The material was characterized for anisotropic plasticity, fracture initiation for various states of stress, (pre-cracked) fracture toughness and uncracked ductility. The experimental program included tests on flat butterfly-shaped, central hole, notched and circular disk specimens for low stress triaxiality levels; as well as tests on round notched bar specimens and SENT fracture mechanics tests, for high values of stress triaxiality. This program covered a wide range of stress conditions and demonstrated its effect on the material resistance. Parallel to the experimental study, detailed numerical investigations were carried out to simulate all different experimental tests. Using an inverse method, a 3-parameter calibration was performed on the Modified Mohr-Coulomb (MMC) fracture model. Subsequently, the predictive capabilities of the MMC were evaluated by the comparison to the fracture toughness tests results, used extensively in the pipeline industry. The capabilities of the MIT fracture model have been demonstrated on an example of high strength offshore steel, X100. The outcome of this study was not only to provide, the overall characterization of the fracture behavior of this material as an example, but also to present the methodology on how to use the MMC model as a practical tool in pipeline design