This paper develops an experimentally validated computational model based on crystal plasticity for the analysis of two-phase a/b Ti-6242 polycrystalline alloys. A rate-dependent elastic-crystal plasticity model is incorporated in this model to accommodate anisotropy in material behavior and tension-compression asymmetry inherent to this alloy. A combination of microtesting, orientation imaging microscopy, computational simulations, and minimization process, involving genetic algorithms, is implemented in this study for careful characterization and calibration of the material parameters. Size effects are considered in this analysis through a simple scaling process. A homogenized equivalent model of the primary a with transformed b colonies is developed for incorporation in the Ti-6242 FE model. The polycrystalline Ti-6242 computational model incorporates accurate phase volume fractions, as well as statistically equivalent orientation distributions to those observed in the orientation imaging microscopy scans. The effects of orientation, misorientations, and microtexture distributions are investigated through simulations by this computational model. The model is used to simulate constant strain rate and creep tests in compression and tension, and the results are compared with experiments. The effects of microstructure and creep-induced load-shedding on the localization of microstructural stresses and strains are studied for potential crack initiation criteria.

This paper is aimed at identifying key microstructural parameters that play important roles in the failure initiation of polycrystalline Ti-6242 subjected to creep and dwell loading. A finite element model, incorporating rate dependent elastocrystal plasticity, is developed for analyzing evolving variables in material microstructure. The crystal plasticity parameters are characterized by a combination of microtesting, orientation imaging microscopy, computational simulations, and minimization process involving Genetic algorithms (Ga). Accurate phase volume fractions and orientation distributions that are statistically equivalent to those observed in orientation imaging microscope scans are incorporated in the computational model of polycrystalline Ti-6242 for constant strain rate, creep, and dwell tests. The computational model is used for the identification of possible microstructural variables that may result in local crack initiation. Basal normal stress, equivalent plastic strain, and stress in loading direction are considered as candidate parameters, of which the former is chosen as most probable from results of creep and dwell experiments and simulations. Creep induced load shedding phenomena is observed to lead to high value stresses that cause failure. The role of grain orientation with respect to the loading axis and misorientation with its neighbors, in causing load shedding and stress localizations is explored.

Riser VIV response due to ocean current loading is a complex phenomenon governed by both the hydrodynamic and structural properties. In order to obtain better understanding of the global riser VIV response and assist in the improvement of riser VIV design, riser monitoring is being increasingly used. An optimization technique to identify the number of sensors required and the sensor locations for monitoring riser VIV fatigue is presented. The optimization technique has been developed using modal decomposition and linear regression. The paper explains why monitoring at selected locations with limited instrumentation is sufficient to capture global riser response. The principles and methods of using multiple measurement quantities in the optimization technique are also presented along with the adopted methodology, limitations and key conclusions.

This paper details the methodology and knowledge obtained from a recent riser concept study for a deepwater development in the Gulf of Mexico. The proposed development is a wet tree development tied back to a floating facility with sour production service. Semi submersibles, Spars and Tensioned Leg Platforms are considered in combination with steel catenary riser, lazy wave riser and single vertical import riser (SVIR). The riser design is challenging due to the requirement of artificial lift, resistance to high production pressures, accommodation of sour service upon onset of water injection and the need for large diameter export lines. It is prudent to be conservative in the initial phase of design in order to account for the possibility of detrimental design changes in the later phases of the project. A review of the conservatism involved in the preliminary riser concepts study is conducted in this paper. The demands made by such conservatism for exotic and novel strength and fatigue mitigation concepts such as lumped masses on the SCRs, titanium touch down zones and light weight coating is discussed. It is observed that fatigue knockdown due to sour service on steel, titanium or clad pipes is the most contributing factor in driving production riser design towards the requirement for novel technology. The need for knowledge of accurate sour service knockdown is highlighted. In this paper, the relative performances of the aforementioned vessel riser combinations are presented. The effectiveness and previous track record of the fatigue mitigation technologies for sour service are reviewed. Finally, the benefits and limitations of each vessel and riser system are compiled and the factors considered by the operator in selecting one particular system are discussed. Introduction Subsea riser design is one of the most challenging engineering aspects of a deepwater field development. Risers constitute the conduit that connects the floaters at the surface to the subsea wellhead. The primary challenge in riser design emanates from the fact that these are dynamic structures highly susceptible to environmental and operational loads. As the global demand for hydrocarbons has increased, offshore projects have moved deeper and deeper and riser design has become more challenging than ever before, involving novel technologies and materials. The latest field developments in the GoM are often in the order of 5,000 ft water depth or more; requiring extensive engineering in order to come up with an optimum riser design. This paper describes a case study of a GoM deepwater riser design in the pre-FEED stage. The water depth at this location is greater than 5,000 ft and a variety of riser-vessel combinations are assessed. The technical performances of the range of riser designs are presented with a summary of benefits and limitations of each type. Design data and constraints such as sour production are highlighted and the demand for unconventional technologies driven by the harsh environment and pre-FEED robustness requirements is demonstrated. The commercial feasibility and track record of the different riser solutions are also discussed. Finally, the recommendations and learnings from this study are summarized. The material presented in this paper provides ample insight on how deepwater risers behave in the harsh GoM environment. It is understood that not many project analysis findings are presented in the public domain. In that light, the sections herein are indeed valuable guidance to a designer in the preliminary stages of project development. Certain riser technologies presented herein are considered novel and to date have not been implemented in the field. It is expected that offshore projects will utilize more of these technologies in the coming years. The work conducted in this paper is a stepping stone for such technologies to be realized.

Vortex induced vibration (VIV) due to steady current flow can be a significant driver in the design of offshore riser systems, affecting riser global configuration, component details and overall subsea architecture. Helical strakes are known to reduce VIV but the degree of effectiveness can vary considerably depending on strake pitch, fin height and more importantly, current flow regime. In addition, the amplitude of VIV and the effectiveness of VIV suppression strakes depends on the inclination of flow to the riser (incidence angle) and presence of wake effects from adjacent risers. Test and field data regarding suppression of riser VIV by strakes is not extensively available in the public domain. This is primarily due to the proprietary nature of the tests conducted in industry. In this paper, a program of testing is devised to better understand strake effectiveness as a function of current incidence angle and the presence of adjacent risers. Experiments have been conducted on single and tandem pipe arrangements in air in order to evaluate strake suppression efficiency. Aluminium cylinders are tested in a wind tunnel in the structures laboratory of The University of Western Australia (UWA). Two sets of experiments are conducted: the first to evaluate cylinder VIV response at angles of incidence ranging from 30 to 90 degrees and the second to evaluate VIV response of the downstream pipe in a dual pipe arrangement with varying spacing between the pipes. In both cases the bare cylinders are first tested at varying flow speeds. Helical strakes are then added to the single cylinder, and downstream cylinder in the tandem pipe test, and the vibration response is recorded at varying flow speeds. From the experimentation, it can be seen that downstream cylinder motions are amplified by wake induced instability. This phenomenon is of particular concern for tightly spaced top-tensioned risers (TTR) in wellbays of tension leg platforms (TLP) and deep draft floaters. The VIV motion of the downstream, bare, wake-affected pipe, is magnified to approximately 1.3–2 times the motion of a single bare pipe. When strakes are added to the downstream cylinder, the magnification factor of the downstream cylinder response is largely increased due to the wake of the upstream bare cylinder. However, the actual VIV motions of the downstream cylinder are largely reduced when strakes are incorporated. The present work demonstrates that helical strakes provide an effective means of suppressing vortex induced vibrations of risers in riser arrays, though the degree of effectiveness is reduced in a downstream tubular compared to suppression levels for single pipes.