Comparative Study of Drilling Riser Running/Retrieving Analysis Methodologies

This paper discusses modeling aspects related to dynamic analysis of deep water drilling risers. These risers must have a heave compensator that maintains a near constant tension in the riser independent on platform motions. Traditional riser analysis will apply constant top tension or a simple parametric model that may give approximate tension variation. The present paper describes an alternative analysis procedure that consists of the following step: • Global riser analysis including calculation of dynamic stroke of the heave compensator from platform motions and riser dynamics. A “pipe-in-pipe” approach is used to represent the hydraulic cylinders. • Calculation of dynamic tension variation from an analysis of the hydraulic tensioner system. The dynamic stroke found from the first analysis is applied as known piston motions in this analysis. • Identification of parameters in a simple model for dynamic tension variation from the results from the second analysis. • Use of the simple model in a second global riser analysis. The difference between the two riser analyses can hence be found, which represents the error one must expect from a traditional riser analysis with constant riser tension. A case study with realistic data is reported. The conclusion is that the constant tension model is valid for small heave motions only, while the parametric tensioner model can give almost correct results for tension variation. However, the parametric model must be tuned for each case. Hence, an integrated model that accounts for riser dynamics and pressure variation in the tensioner system should preferably be developed.

Global concurrent cross-scale nonlinear analysis approach of complex CFRD systems considering dynamic impervious panel-rockfill material-foundation interactions

Critical sections of work over strings, with respect to integrity, are components located close to end terminations, near well heads and above drill floors, where recurring bending moments are prevalent. The lifetime of these components are strongly dependent on the stiffness in the components of the string. Connections between these components are often complex, and of a type where the stiffness is unknown, or hard to reveal based on theoretical analysis. This paper considers the feasibility of applying state-of-the-art measurement technology for testing of the physical behavior of specific connections on a landing string to be used for work over operations in harsh environmental conditions, where low fatigue life of components have proven to be a recurring problem. Behavior of joint-connections revealed through measured response from physical testing serve as input for the global finite element (FE) analysis, where accumulated fatigue damage for each sea state is calculated based on site specific met-ocean data. The present work was carried out in advance of an operation on the Norwegian shelf, where a four-point bending test of the actual landing string to be used during the offshore campaign were performed on a section containing two critical couplings, in order to reveal the actual stiffness of the connections. The test string was subject to variable internal pressure, axial tension and bending loads, representative for the applicable work-over riser operational loads. The performance of the system was monitored through strain, displacement and force sensors, in order to relate applied loads to structural response. The results from these tests where later recreated from local FE analysis, where non linear springs was implemented and modified to fit the experimental results at the connections of interest. These springs was later input to the global fatigue analysis, where the complete system, including marine riser and inner work over string, was implemented in one model. Results from the fatigue assessment where used to determine the operational criteria for the work over operations.

This paper presents a multi-dimensional variable-kinematics finite element model for nonlinear static analyses of structures with complex geometries. The approach incorporates higher-order beam models and classical solid finite elements in a unified framework, enabling refined modeling of complex geometries. The finite element procedure proposed follows the Carrera Unified Formulation (CUF) and uses a pure displacement-based methodology. The governing equations are derived within the classical continuum mechanics framework, and weak-form equilibrium equations are established using the Principle of Virtual Displacements (PVD). Within the CUF framework, higher-order beam and hexahedral solid models are defined in a unified manner, and the governing equations are written in terms of invariants of mathematical models used and the theory of structures approximation. A coupling technique is used between the beam and solid elements at the nodal level using superposition. The capabilities of fully nonlinear variable-kinematics models are investigated for the static analysis of various rectangular and curved structures. The numerical results are compared with solutions obtained using commercial software. Finally, the proposed methodology is applied to analyze more complex geometries in engineering applications. The results show the capabilities of variable-kinematics models in terms of both accuracy and computational efficiency for the computation of highly nonlinear deformed states and localized phenomena, such as stress concentrations and buckling.

Response to changes in light conditions involves a variety of receptors that can modulate gene expression, enzyme activity and/or motility. For the study of light-regulated effects of Agrobacterium tumefaciens, we used a global analysis approach - proteomics - and compared the protein patterns of dark- and light-grown bacteria. These analyses revealed a significant reduction of FlaA and FlaB - proteins of the flagellum - when the cells were grown in light. The light effect was confirmed by SDS-PAGE with isolated flagella. Quantitative PCR experiments showed a 10-fold increase of the transcription level of flaA, flaB and flaC within 20 min after the transfer from light to darkness. Electron microscopy revealed that these molecular events result in a light-induced reduction of the number of flagella per cell. These changes have major physiological consequences regarding motility, which is considerably reduced with exposure to light. The inhibitory effect of light on the motility is not unique to A. tumefaciens and was also seen in other species of the Rhizobiaceae. Previous studies suggested that the flagella function is significant for bacteria-plant interactions and bacterial virulence. In our studies, light reduced the attachment of A. tumefaciens to tomato roots and the virulence of the bacteria in a cucumber infection assay.

Marine riser is an important component of oil and gas drilling and production system. It is essentially a slender pipe conveying fluid between well-head and floating production unit. They are formed out of three basic types of configuration namely, free hanging, “lazy-wave (SWLR)” riser. Risers are subjected to varied static, quasi-static and dynamic forces. For the safety of design, the behavior of the riser under these forces is thoroughly investigated. 3D Nonlinear dynamic analysis of riser is obtained in the time domain using finite element software package ABAQUS/Aqua. The response histories so obtained are employed for the study of fatigue reliability analyses of riser. It is based on a bi-linear relationship to model fatigue crack growth and incorporates a failure criterion to describe the interaction between fracture and plastic collapse. Uncertainty modeling, especially of fatigue crack growth parameters, is undertaken using bi-linear crack growth relationship. Results pertaining to fatigue reliability and fatigue crack size evolution are presented using Monte Carlo Simulation. The bi-linear crack growth models are found to lead to higher fatigue life estimation. Sensitivity behavior pertinent to limit state adopted has been thoroughly investigated. These findings implicate inspection schemes for components of the marine structures to ensure minimization of the surprises due to wide scatter of the fatigue phenomenon in marine environment.

Parametric study and formulae of SCFs for positive large eccentricity CHS N-joints

Element types that are commonly used in practice for a static finite element stress analysis include the truss element, beam element, plane stress/strain elements, axisymmetric elements, and solid elements.This paper focuses on stress concentrations and static failure associated with these elements and serves as a reference for instructors and practitioners.Stress concentrations and static failure for these elements has not been comprehensively addressed in any finite element textbook.The first author has integrated stress concentrations and static failure into introductory finite element undergraduate courses, graduate courses and industrial short courses.The authors have found through experience that students and practitioners are not familiar with the stress concentrations and static failure associated with these elements. Proceedings of the 2007 Midwest Section Conference of the American Society for Engineering EducationAn understanding of the failure modes associated with each component in a structure is required to ensure that it will not fail under loading.Static failure modes for different components can be readily found in mechanics of materials and machine design textbooks.The most common static failure criteria found in finite element textbooks is the von Mises applied to plane stress/strain elements, axisymmetric elements, and solid elements.A review of finite element textbooks reveals that there is no discussion on why the von Mises criterion was selected and other failure criterion are not considered.The authors are not aware of any finite element textbook that address the static failure modes associated with beam element and there is very limited discussion for the truss element.The goal of this paper is to define the stress concentrations and static failure for finite elements commonly used to carry out a static, linear elastic, stress analysis.The elements considered include the truss element, beam element, plane stress/strain elements, axisymmetric elements, and solid elements as shown in Figure 1.Each element will be considered separately in this paper and the following will be discussed: overview that includes a literature review, stress concentrations, and static failure.This paper will serve as a reference for instructors and practitioners.

Finite element analysis of fracture of a ram BOP for deep gas wells

As the search for oil and gas moves to deeper waters and more challenging environments such as the Atlantic margin and South America, the operational risks and consequences of equipment failure increase significantly. Industry guidelines, such as API RP 16Q [1] and ISO 13624-1 [2], define standard scenarios to include in a riser analysis that cover many of the various phases of installation and operation of drilling risers. These scenarios include assessments of the riser in both connected and disconnected modes such as storm hang-off. It is industry practice, as per API RP 16Q/ISO 13624-1, to assess these scenarios independently. Unplanned disconnect of the drilling riser in a storm involves transitioning through intermediate stages to get to the final hang-off configuration. While recoil analysis (assessment of riser disconnect phase) of the riser is commonly addressed, standard industry approaches fail to address other key intermediate and transitional phases of operation required to position the riser into the storm hang-off configuration. This paper presents a study that highlights the importance of analyzing all pertinent stages of drilling riser disconnect and hang-off operations. The study demonstrates that these intermediate phases of operation are often limiting and, if not assessed adequately, have the potential to be catastrophic in harsh environments. A detailed set of revised riser analysis requirements are generated along with the development of a comprehensive harsh environment WSOG (Well Specific Operating Guideline) that optimizes operational performance and improves safety for harsh environment drilling. These revised analysis guidelines and associated modelling that are necessary for harsh environment locations represent a level of pre-drilling assessment that is ahead of that typically undertaken by the industry today.

PurposeThe purpose of this paper is to investigate the effect of pressure angle, and module of spur gear teeth on stress concentration factor, using photoelasticity method, and numerical MSC/NASTRAN finite element package.Design/methodology/approachThe stress concentration factor is determined as a ratio between maximum stress (determined in the fillet radius by photoelastic and finite element methods), and nominal stress (calculated by a common standard formulas). In order to specify the geometric parameters (height and thickness) of gears, both standard Deutsches Institut für Normung (DIN)/Japanese Gear Manufactures Association (JGMA), and five other non‐standard approaches are used.FindingsThe results show that the stress concentration factor increases by decreasing the pressure angle. In addition, the values which are obtained by finite element analyses exhibit more uniformity than photoelastic method.Practical implicationsAn accurate determination of stress concentration factors will limit both over and under design of the gears.Originality/valueThe results show that one of the suggested non‐standard approaches gives the highest stress concentration factor than the standard approaches.

Development of deep water oil reservoirs is presently being considered in the Gulf of Mexico (GoM) where the flowline product temperatures approach 177°C (350°F), and water depths to 3,000 meters (10,000ft). The design of these flowlines requires the use of advanced ‘local’ and ‘global’ Finite Element (FE) analysis models, integrated with limit state design methods. This paper presents the ‘local’ and ‘global’ FE models developed to investigate the limit state design of Extra High Pressure High Temperature (XHPHT) flowlines. The ‘global’ PIP FE model is used to conduct design sensitivity analyses and provide the input loads for limit state design checks. Using the global FE model, investigations are undertaken to examine design issues such as effectiveness of thermal expansion buckle management, application of inner pipe pre-tension and load sharing device, and the effect of operating temperature. The ‘local’ full size PIP FE model is used to verify the global PIP FE model at a local level. Global and local FE models present comparable results in term of pipeline stress response. A 3D helical arrangement of the inner pipe subjected to high temperature is observed. This helical shape is sensitive to temperature and material thermal expansion coefficient. The investigation's goal is bridging technology gaps by pushing the boundaries of PIP designs for XHPHT applications in the GoM. These innovative, cutting edge finite element models could be used by the pipeline for future deep water PIP XHPHT projects. Introduction The pressure and temperature of the oil and gas contents coming out of the ground and well head may be high, and this can be a key design challenge for pipelines. High Pressure / High Temperature (HP / HT), with pressures in the order of 700bar (10,000PSI), and temperatures being considered up to 160°C (320°F) are not uncommon. High Pressure and High Temperature (HPHT) reservoirs are typically, but not exclusively, gas condensate fields. They are technically more complex to develop because of the inherently higher energy in the well fluid. Although initially developed exclusively by dry tree systems, subsea HP / HT developments are now common place. To keep the heat in a pipeline, and especially over long tie-back lengths, is a real engineering challenge. One way that this is overcome is to use a PIP design. This is where the pipe carrying the contents is inside an outer pipe separated by an annulus filled with thermal insulation. PIP technology is a design challenge, and is normally used when a low Overall Heat Transfer Coefficient (OHTC) value is required (<1 W/m2K). High Pressure (HP) has a major impact on the design of wellhead and other equipment, such as manifold valves, in terms of strength, materials and reliability. For piping, flowlines and risers, HP can also lead to very high wall thicknesses. Equipment manufacture and linepipe fabrication and installation become more complex. High Temperature (HT) has a wider impact, as the whole system has to operate over a greater temperature range between non-producing situations, such as: installation and shut down, and the operational case. Also, due to the uncertainty of material response at elevated temperatures, the industry Codes of Practice is approaching their limits of applicability. Additionally, corrosion and corrosion protection throughout the system becomes a challenge for hotter systems.

The elastic stress and strain fields in a plate of finite thickness containing an elliptical hole are systematically investigated using the 3D finite element method. It is found that the stress and strain concentrations are different in the plate of finite thickness even if the plate is in an elastic state. The relation between the stress and the strain concentration factors depends on Poisson’s ratio, the hole’s geometric configuration and the plate thickness. The stress concentration factor is equal to the strain concentration factor only at the notch root of the plate surface. The stress (or strain) concentration factor at the notch root of the plate surface decreases rapidly with increasing thickness and becomes lower than the stress and strain concentration factors corresponding to the plane stress state or at the notch root of the mid plane. It is too low to reflect the overall stress concentration as the thickness increases or as the b/a ratio decreases. The maximum stress concentration factor occurs on the mid plane only when the plate is thinner than the transition thickness of the stress concentration factor. When the plate is thicker than the transition thickness of the stress concentration factor, the distance between the location of the maximum stress concentration factor and plate surface tends to be constant with increasing thickness for the plate with a given b/a ratio. The differences between the maximum value and the surface value of the stress and strain concentration factors increase rapidly and tend to their respective constant values with increasing plate thickness. The smaller the b/a ratio, the larger these differences. The difference of the stress concentration factor is larger than that of the strain concentration factor in the same plate.

A full-scale uniplanar completely overlapped tubular K(N) joint specimen is tested to determine the hot spot strains at the joint intersections for the verification of the finite-element (FE) model. The test results reveal that the maximum strain concentration occurs on the through brace saddle near the lap brace. On the chord, the strain distributions perpendicular to the weld toes are linear but on the through brace and the lap brace, they are nonlinear. Based on the FE analysis results, the effects of through brace and lap brace geometric parameters on the joint stress concentration factors (SCF) are found most crucial. The use of the existing Y-joint formulas has considerably underestimated and overestimated the SCF on the through brace saddle near the chord and the lap brace, respectively. At a relatively large gap size, the SCF values at the joint intersection of the through brace and the lap brace are close to that of the Y joint.

Interaction mechanism between the intralaminar damage and interlaminar delamination of composite laminates is always a challenging issue. It is important to consider the progressive failure and strain softening behaviors simultaneously during the damage modeling and numerical simulation of composites using FEA. This paper performs three-dimensional finite element analysis of the progressive failure and strain localization of composites using FEA. An intralaminar progressive failure model based on the strain components is proposed and the nonlinear cohesive model is used to predict the delamination growth. In particular, the nonlocal integral theory which introduces a length scale into the governing equations is used to regularize the strain localization problems of composite structures. Special finite element codes are developed using ABAQUS to predict the intralaminar and interlaminar damage evolution of composites simultaneously. The carbon fiber/epoxy composite laminates with a central hole demonstrates the developed theoretical models and numerical algorithm by discussing the effects of the mesh sizes and layups patterns. It is shown the strain localization problem can be well solved in the progressive failure analysis of composites when the energy dissipation due to the damage of the fiber, matrix and interface occurs at a relatively wide area.