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Yin F.,2H Offshore Inc. | Cerkovnik M.,2H Offshore Inc. | Conle A.,University of Windsor
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2014

The in-service fatigue loading of risers may include contributions from first and second order vessel motions, wave loadings, vortex induced vibration (VIV), vortex induced motion (VIM), pressure variation and slugging. All of these loadings are variable amplitude random loadings where the sequence of cycles can change the fatigue outcome. The loading sequence effects can be addressed in analysis through cycle counting methods and through use of the most appropriate methods of damage accumulation. High loads can be responsible for either fatigue crack growth retardation or acceleration depending up loading sequence. The Rainflow Counting method takes into account the whole loading algorithm and assumes the structure memory sustains all through the loading. The Simple Range Count method counts cycles by every single reversals and does not take into account the load sequence or material memory. Fatigue damage accumulation can be accomplished using a linear model like Palmgren-Miner or a nonlinear model. Fatigue life can be estimated using crack growth methods, S-N or strain life methods. For each method some tools are available that can account for sequence effects. The fatigue analysis of the riser system is illustrated step by step. This study particularly looks at how different cycle counting methods affect the fatigue damage estimate obtained with the S-N approach, strain-life approach and fatigue crack growth approach. In the study Rainflow and Simple Range cycle counting methods are applied to variable amplitude loads typical of deepwater riser systems. Copyright © 2014 by the International Society of Offshore and Polar Engineers (ISOPE).


Mercan B.,2H Offshore Inc. | Mercan B.,University of Minnesota | Schultz A.E.,University of Minnesota | Stolarski H.K.,Precast Engineering Systems | And 2 more authors.
Journal of Structural Engineering (United States) | Year: 2012

Spandrel beams in precast concrete buildings are widely used to support double-tee deck beams, particularly in parking garages. Spandrel beams of deep cross-sections, resisting eccentric loads from double-tee beams, can be susceptible to excessive lateral deformations and serviceability failures before reaching their strength limits. However, closed-form solutions for estimating lateral deflections in such members are not available in the technical literature. In this paper, approximate analytical solutions for the deflection of beams with thin rectangular sections are derived from second-order elastic analysis, and they are proposed for use in estimating maximum lateral deflections in spandrel beams under eccentric and uniformly distributed loads. Continuous lateral support is provided at the elevation of the floor deck to the spandrel beams. Thus, two cases are considered: one for laterally restrained beams under typical service conditions, and a second for laterally unrestrained beams prior to the establishing the floor deck connections, or if those connections fail prematurely during service or under extreme loading. An equivalent loading method is proposed to obtain the approximate analytical solutions, in which the differential equations of equilibrium governing the problem are simplified by replacing the actual loading in the spandrel beams with a substitute loading. Numerical solutions are also obtained from three-dimensional finite element analyses and their results are found to be in close agreement with the analytical solutions for two of the three common types of load-bearing precast, prestressed concrete spandrel beams. © 2012 American Society of Civil Engineers.


Zhang H.,University of Toledo | Zhang H.,2H Offshore Inc. | Fatemi A.,University of Toledo
International Journal of Fracture | Year: 2010

Mixed-mode loading represents the true loading condition in many practical situations. In addition, most of the fatigue life of many components is often spent in the short crack growth stage. The study of short crack growth behavior under mixed-mode loading has, therefore, much practical significance. This work investigated short crack growth behavior under mixed-mode loading using a common medium carbon steel. The effects of load mixity, crack closure, and load ratio on short crack growth behavior were evaluated by conducting experiments using four-point bending specimens with several initial KII/KI mixed-mode ratios and two load ratios. Cracks were observed to grow along the paths with very small K II /K I ratios (i.e. mode I). The maximum tangential stress criterion was used to predict the crack growth paths and the predictions were found to be close to the experimental observations. Several parameters including equivalent stress intensity factor range and effective stress intensity factor range were used to correlate short crack growth rates under mixed-mode loading. Threshold values for short cracks were found to be lower than those for long cracks for all the mixed-mode loading conditions. Crack closure was observed for the entire crack length regime with all load mixity conditions at R ≈ 0.05 and for short crack regime under high load mixity condition at R = 0.5. Several models were used to describe mean stress effects and to correlate crack growth rate data. © 2010 Springer Science+Business Media B.V.


Zhang H.,2H Offshore Inc. | Fatemi A.,University of Toledo
International Journal of Fracture | Year: 2011

Short crack growth behavior from a notch including crack closure and load ratio effects was investigated. Experiments and analyses were carried out using four-point bending specimens made of SAE 1045 steel, using a blunt notch keyhole specimen geometry. The lower the load ratio, the more notch effect on short crack growth behavior was observed. Short cracks in the notch affected zone had higher growth rates than long cracks. After the crack grew out of the notch effect field, short crack growth rates merged with the long crack growth rates. Several parameters were used to correlate the short crack growth rates including stress intensity factor range, effective stress intensity factor range, and stress intensity factor range based on notch root stress. © 2011 Springer Science+Business Media B.V.


Akhtar W.,2H Offshore Inc. | Cerkovnik M.,2H Offshore Inc.
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2013

A methodology is developed to account for the effect of crack face pressure on the reference stress in high pressure, thick walled, flowlines and risers. A detailed finite element analysis (FEA) is conducted over a range of crack depths (a/t) for long flaws (large crack aspect ratios (2c/a)) for a thick walled pipe (wall thickness/average radius = 0.28). The results from this analysis allows for the implementation of an accurate calculation of the reference stress, for thick walled pipes under high service pressures and temperatures, thereby, allowing an improved fracture prediction. Copyright © 2013 by the International Society of Offshore and Polar Engineers (ISOPE).


Tran L.,2H Offshore Inc. | Cerkovnik M.,2H Offshore Inc.
Proceedings of the International Offshore and Polar Engineering Conference | Year: 2013

Risers, flowlines and other subsea components may be designed using limit state concepts, where displacement controlled loads are allowed to push stresses beyond yield. However, there can be consequences if the lines are damaged or degraded in service. This paper presents a case study comparing the damage tolerance capacity of a deepwater steel catenary riser (SCR) designed with allowable (working) stress criteria and another designed with limit load criteria. The exercise is also conducted for a production flowline. The damage scenario examines the effects of various levels of pitting corrosion and accounts for the potential of accelerated crack growth due to corrosive effects. Copyright © 2013 by the International Society of Offshore and Polar Engineers (ISOPE).


Saglar N.,2H Offshore Inc. | Toleman B.,2H Offshore Inc. | Thethi R.,2H Offshore Inc.
Proceedings of the Annual Offshore Technology Conference | Year: 2015

The offshore industry anticipates the need for production riser systems in ultra-deepwater fields where water depths are between 3,000m to 4,500m. The development of ultra-deepwater fields leads to many challenges on the selection of the riser concept and in some instances such applications may require extending riser technology beyond its current limits. Consequently, there is a need to understand the feasibility of riser systems in such ultra-deepwater applications and the technology gaps that exist. In ultra-deepwater, long suspended riser lengths will significantly increase the riser weight potentially leading to challenges with offshore installation related to laying vessel capability. High external and internal pressure on the riser will lead to the need for heavy wall pipes. Thick-wall riser pipe will bring about riser design challenges in fabrication of pipes, riser pipe welding, riser hang-off system selection, long-term fatigue design, and fabrication of specialty riser joints. By looking into these challenges, it is very important to select the most appropriate riser concept for the ultra-deepwater fields and understand the current pipe manufacturing limits, enabling technology needed for such systems, and technology gaps considering the critical points mentioned above. This paper addresses the key riser design issues considering wall thickness sizing, top tension, axial dynamics, selection of pipe material, design of key components, and installation issues. This paper evaluates feasibility of a number of production and export riser configurations for ultra-deepwater applications based on existing technology, identifies current technology limits, and determines technology gaps that exist. Methods of advancing the current capability to meet the requirements of frontier deepwater applications are also proposed in the paper. Copyright © (2015) by the Offshore Technology Conference All rights reserved.


Josefsson P.M.,2H Offshore Inc. | Dincal S.,2H Offshore Inc.
19th Offshore Symposium 2014: Emerging Technologies in Offshore Drilling and Production | Year: 2014

One of the factors affecting offshore drilling uptime is the rig's ability to disconnect the riser in a timely manner in case of a drift-off scenario. A drift-off may occur as a result of complete or partial failure of the Dynamic Positioning (DP) system and the severity of the environmental conditions affect the allowable time to emergency disconnect the drilling riser. The conventional assessment to determine point of disconnect (POD) is typically based on a simplified approach that does not accurately capture partial DP loss and/or dynamic effects and may result in unnecessary downtime. The most common methodology is to assume a full DP loss, even though the likelihood of this is much smaller than the likelihood of partial DP loss, i.e. single or multiple thruster failure or generator failure that results in reduced capacity on several thrusters. In this paper, partial DP loss cases are investigated, by assuming some thruster capacity is available to counter-act the environmental drift loading. In addition, this paper illustrates the benefits of including the transient effects during drift including vessel and riser coupling. The paper contains comparisons between results for a DP semisubmersible subjected to typical Gulf of Mexico environmental conditions. A base case assuming full DP system failure and conventional riser offset limits is presented for the purpose of comparison. Multiple cases with partial DP system failure, coupled vessel/riser response and/or with dynamic drift-off riser limits are presented to illustrate how the vessel may be allowed to stay connected in harsher environment than when using the conventional approach. Copyright © (2014) by the Society of Naval Architects and Marine Engineers Texas Section All rights reserved.


Patent
2HOffshore Inc., BP Exploration and Production Inc. | Date: 2014-12-22

A system and method is provided for determining curvature for subsea riser system, including but not limited to drilling risers, steel catenary risers, lazy-wave catenary risers and riser jumpers, comprising the steps of: periodically measuring acceleration in a first lateral direction at said vertical position to obtain a first acceleration timetrace processing said first acceleration timetrace to obtain a first acceleration spectra; applying a transfer function to said first acceleration spectra to obtain a first curvature spectra; and processing said first curvature spectra to obtain a first curvature timetrace. Preferably the transfer function is determined by a method comprising the step of modelling the riser as a Tensioned Timoshenko Beam. The curvature may be used to determine stress and fatigue damage in a structure from motions measured at a single location or a combination of motions measured at a single location with or without tension measurement. The method can be used to determine curvature and hence stress and fatigue damage from any source of excitation, for example the excitation at the tension ring by the top tensioner system, and the vortex induced vibration locked in at any water depth.


Patent
2HOffshore Inc. | Date: 2015-06-24

A system and method is provided for determining curvature for subsea riser system, including but not limited to drilling risers, steel catenary risers, lazy-wave catenary risers and riser jumpers, comprising the steps of: periodically measuring acceleration in a first lateral direction at said vertical position to obtain a first acceleration timetrace processing said first acceleration timetrace to obtain a first acceleration spectra; applying a transfer function to said first acceleration spectra to obtain a first curvature spectra; and processing said first curvature spectra to obtain a first curvature timetrace. Preferably the transfer function is determined by a method comprising the step of modelling the riser as a Tensioned Timoshenko Beam. The curvature may be used to determine stress and fatigue damage in a structure from motions measured at a single location or a combination of motions measured at a single location with or without tension measurement. The method can be used to determine curvature and hence stress and fatigue damage from any source of excitation, for example the excitation at the tension ring by the top tensioner system, and the vortex induced vibration locked in at any water depth.

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