Non-linear time domain model of electropermeabilization: Effect of extracellular conductivity and applied electric field parameters

2008 ◽  
Vol 66 (5-6) ◽  
pp. 328-334 ◽  
Author(s):  
Sadhana Talele ◽  
Paul Gaynor
Keyword(s):  
Electric Field ◽  
Time Domain ◽  
Applied Electric Field ◽  
Linear Time ◽  
Domain Model ◽  
Extracellular Conductivity ◽  
Non Linear

Frequency and Time Domain Analysis of Vortex Induced Vibrations for Free Span Pipelines

2002 ◽  
Author(s):  
Carl M. Larsen ◽  
Kamran Koushan ◽  
Elizabeth Passano
Keyword(s):  
Time Domain ◽  
Structural Model ◽  
Linear Time ◽  
Time Domain Analysis ◽  
Linear Response Theory ◽  
Domain Model ◽  
Vortex Induced Vibrations ◽  
Non Linear ◽  
New Strategy ◽  
Stress Prediction

The present paper will discuss various models for calculation of vortex induced vibrations (VIV) of free span pipelines, and present a new strategy for such analyses. Applications of traditional models are presented and their limitations discussed. The new approach is based on the combination of an empirical linear frequency domain model, and a non-linear time domain structural model. The first step is to carry out the VIV analysis according to linear response theory, and next introduce the calculated hydrodynamic forces to the non-linear structural model. The benefit from using the non-linear model is to describe stresses at the shoulders more accurately, which is important since fatigue damage in many cases will be largest in this area. The conclusion is that the interaction between pipe and seafloor is crucial for accurate stress prediction, and that a non-linear time domain model will give the most accurate result.

Time and Frequency Domain Analysis of Catenary Risers Subjected to Vortex Induced Vibrations

2006 ◽  
Author(s):  
Carl M. Larsen ◽  
Elizabeth Passano
Keyword(s):  
Frequency Domain ◽  
Linear Model ◽  
Time Domain ◽  
Structural Model ◽  
Linear Time ◽  
Domain Analysis ◽  
Domain Model ◽  
Vortex Induced Vibrations ◽  
Non Linear ◽  
Bending Stresses

Catenary risers in deep waters will experience conditions with insignificant wave forces in combination with strong current. The response will in such cases be dominated by vortex induced vibrations (VIV). Dynamic bending stresses will vary along the riser, but a large peak will almost always be seen near the touch down point. This peak is caused by the restrictions on riser displacements from the presence of the seafloor, and the local bending stresses will be influenced by stiffness and damping propertoes of the bottom. Analysis models based on finite elements will represent the interaction between riser and seafloor by discrete springs, which for the linear case will remain constant independent of the displacements. This type of model may give a significant over-prediction of bending stresses at the touch down point since a linear spring will give tensile forces instead of being released and allowing the pipe to lift off from the bottom. A non-linear time domain model will, however, account for changes by releasing springs if tension occurs and adding in new springs if free nodes obtain temporary contact with the bottom. The results will hence become far more realistic. Traditional empirical models for VIV prediction are based on a frequency domain dynamic analysis with constant stiffness. There is hence an obvious need for improvements when dealing with catenary risers. This paper will describe a new approach that is based on combined use of an empirical linear frequency domain model for VIV, and a non-linear model for time domain analysis. The first step is to carry out the VIV analysis according to linear response theory, and next introduce the calculated hydrodynamic forces to the non-linear structural model. The benefit from using the non-linear model is that stresses in the touch down area are described more accurately. A case study is also reported. Bottom stiffness and friction are varied, and results are compared to a simple model with a hinge at the touch down point. The conclusion is that the interaction between riser and seafloor is crucial for accurate stress prediction, and that a non-linear time domain model will give the most accurate result.

In-Line Vibrations of Flexible Pipes

2017 ◽  
Author(s):  
Jan V. Ulveseter ◽  
Svein Sævik
Keyword(s):  
Frequency Domain ◽  
Vortex Shedding ◽  
Time Domain ◽  
Linear Time ◽  
Domain Model ◽  
Integration Scheme ◽  
Force Model ◽  
Prediction Tool ◽  
Flexible Pipe ◽  
Non Linear

A semi-empirical prediction tool for pure in-line vortex-induced vibrations is under development. The long-term goal is to be able to realistically model the dynamic behavior of free spanning pipelines exposed to arbitrary time dependent external flows at low velocities. Most VIV programs operate in frequency domain, where only steady currents and linear structural models can be simulated. In contrast, the proposed model predicts hydrodynamic forces as function of time, enabling a time integration scheme to solve the equation of motion. Non-linear time domain simulations allow for modelling of excitation from non-steady currents. In addition, non-linear effects such as soil-pipe interaction, varying tension, and response dependent material, stiffness and damping properties may be included in the analysis, when combining the hydrodynamic force model with a structural non-linear finite element model. Hydrodynamically, the proposed prediction tool consists of the general Morison equation plus two vortex shedding forcing terms. The latter two are able to synchronize with the structural motion for a given frequency band, to induce vibrations in lock-in regimes. In this paper, the proposed pure in-line VIV model is compared to the frequency domain model VIVANA and DNV Recommended Practice, simulating experiments with a model-scale flexible pipe exposed to current velocities at which cross-flow vibrations have not yet developed. A few experimental data points are included in verifying the performance of the newly developed time domain model. The effect of changing empirical coefficients in the vortex shedding forcing terms, and allowing only one of the terms to excite structural vibrations during a simulation, is numerically investigated. A goal is to obtain increased understanding of how the proposed time domain model performs when simulating VIV of a flexible pipe, which is more complex than that of an elastically mounted rigid cylinder since several natural frequencies and corresponding modes might be excited.

scholarly journals Prediction of deepwater riser VIV with an improved time domain model including non-linear structural behavior

2021 ◽  
Vol 236 ◽  
pp. 109508
Author(s):  
Sang Woo Kim ◽  
Svein Sævik ◽  
Jie Wu ◽  
Bernt Johan Leira
Keyword(s):  
Time Domain ◽  
Domain Model ◽  
Structural Behavior ◽  
Non Linear ◽  
Deepwater Riser

Concrete Modeling for Extreme Wave Slam Events

2017 ◽  
Author(s):  
Kasper Wåsjø ◽  
Terje P. Stavang ◽  
Tore H. Søreide
Keyword(s):  
Time Domain ◽  
Linear Time ◽  
Limit State ◽  
Material Model ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Capacity Control ◽  
Extreme Wave ◽  
Conventional Design ◽  
Non Linear

Experience from model tests has initiated a growing attention towards extreme wave slam as a critical load situation for offshore large volume structures. Most of the problem is related to the local slam pressure, which may go up to several MPa’s for 100-year and 10 000-year waves. The paper deals with modeling techniques for marine concrete structures under extreme slam loading from waves where dynamic effects together with material softening play a major role for the response. Different analysis approaches for ultimate limit state (ULS) and accidental limit state (ALS) controls are discussed in view of reliability philosophy as basis for conventional design approach. The present paper is devoted to the local impact scenario and the alternative approaches for response and capacity control involving non-linear time domain analyses. Conventional design schemes as based on linear elastic models for response calculation together with code specified capacity control often come out more conservative than non-linear approach. The paper demonstrates by case studies how softening of the structure in general reduces the response in terms of section forces. A key issue when going from conventional linear approaches into non-linear techniques is to still keep an acceptable reliability level on the capacity control. Load and material factors are normally based on structures with limited non-linearity where linear response modeling is representative. Implementing non-linear material model in time domain analysis has a major challenge in limiting the sensitivity in response and capacity calculation. The paper demonstrates the way material model of concrete affects the section forces to go into local capacity control, and concludes on needed sensitivity analyses. Practical approaches on the concrete slam problem together with resulting utilizations from the control are demonstrated. The full non-linear technique by response and capacity control in one analysis is also handled, using average material parameters and justifying safety factors for the effect of implementing characteristic lower strength of concrete in the capacity. The paper ends up in a recommendation on non-linear time domain analysis procedure for typically slam problems. A discussion is also given on applicable design codes with attention to non-linear analysis.

Evaluation of Impact Loads on Offshore Jacket Platform During Float-Over Mating Operation

2019 ◽  
Author(s):  
Gurumurthy Kagita ◽  
Mahesh B. Addala ◽  
Gudimella G. S. Achary ◽  
Subramanyam V. R. Sripada
Keyword(s):  
Time Domain ◽  
Linear Time ◽  
Random Wave ◽  
Impact Loads ◽  
Transient Effects ◽  
Seed Selection ◽  
Mooring Lines ◽  
Contact Points ◽  
Non Linear ◽  
The Impact

Abstract In the mating phase of float-over operation, the topsides deck load from the vessel is transferred onto the jacket either by ballasting the vessel or by the combination of ballasting and hydraulic jacking system. During this phase of operation, the topsides and jacket experience impact loads through the contact points in a short duration of time. To evaluate the impact loads and to capture the transient effects precisely, a non-linear time domain hydrodynamic analysis is required. To obtain the design loads, generally the numerical jacking simulation is initiated at the time instant of maximum wave height when the jacking system is used. However, the conservative response may also depend on the relative velocity between the jacket and topsides legs. In this paper, a series of non-linear time domain as well as linear frequency domain hydrodynamic analyses are performed to evaluate the impact loads between 9000 tonne integrated topsides deck and a 4-legged jacket in a water depth of 50 m during float-over mating operation. The simulations are performed using MOSES software. The float-over hardware such as LMUs (leg mating unit), DSUs (deck support unit), Jacks, Fenders and Mooring lines are modelled as appropriate linear / nonlinear springs. The principle of the mating operation is considered through a combination of vessel ballasting and jacking operation. This paper discusses about random wave seed selection, effect of vessel response and wave headings on the impact loads of LMUs and Jacks/DSUs.

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