The Development of a Novel Non-Linear Spectral Model for Analysing Offshore Structures, Part I: Development of Drag Force Terms and System Receptances

2009 ◽  
Author(s):  
M. Hartnett
Keyword(s):  
Drag Force ◽  
Offshore Structures ◽  
Spectral Model ◽  
Non Linear

scholarly journals Resonant Motions of Dynamic Offshore Structures in Large Waves

Fluids ◽  
2021 ◽  
Vol 6 (10) ◽  
pp. 352
Author(s):  
Ove Tobias Gudmestad
Keyword(s):  
Drag Force ◽  
Oil And Gas ◽  
Offshore Structures ◽  
Gas Production ◽  
Mass Force ◽  
Oil And Gas Production ◽  
Damped System ◽  
Loading Function ◽  
Non Linear ◽  
Marine Engineering

In marine engineering, the dynamics of fixed offshore structures (for oil and gas production or for wind turbines) are normally found by modelling of the motion by a classical mass-spring damped system. On slender offshore structures, the loading due to waves is normally calculated by applying a force which consists of two parts: a linear “inertia/mass force” and a non-linear “drag force” that is proportional to the square of the velocity of the particles in the wave, multiplied by the direction of the wave particle motion. This is the so-called Morison load model. The loading function can be expanded in a Fourier series, and the drag force contribution exhibits higher order harmonic loading terms, potentially in resonance with the natural frequencies of the system. Currents are implemented as constant velocity terms in the loading function. The paper highlights the motion of structures due to non-linear resonant motion in an offshore environment with high wave intensity. It is shown that “burst”/“ringing” type motions could be triggered by the drag force during resonance situations.

Conditional Short-Crested Second Order Waves in Shallow Water and With Superimposed Current

2004 ◽  
Author(s):  
Jo̸rgen Juncher Jensen
Keyword(s):  
Shallow Water ◽  
Wave Spectrum ◽  
Ocean Waves ◽  
Offshore Structures ◽  
Second Order ◽  
Stokes Wave ◽  
Wave Crest ◽  
Wave Kinematics ◽  
Wave Approach ◽  
Non Linear

For bottom-supported offshore structures like oil drilling rigs and oil production platforms, a deterministic design wave approach is often applied using a regular non-linear Stokes’ wave. Thereby, the procedure accounts for non-linear effects in the wave loading but the randomness of the ocean waves is poorly represented, as the shape of the wave spectrum does not enter the wave kinematics. To overcome this problem and still keep the simplicity of a deterministic approach, Tromans, Anaturk and Hagemeijer (1991) suggested the use of a deterministic wave, defined as the expected linear Airy wave, given the value of the wave crest at a specific point in time or space. In the present paper a derivation of the expected second order short-crested wave riding on a uniform current is given. The analysis is based on the second order Sharma and Dean shallow water wave theory and the direction of the main wind direction can make any direction with the current. Numerical results showing the importance of the water depth, the directional spreading and the current on the conditional mean wave profile and the associated wave kinematics are presented. A discussion of the use of the conditional wave approach as design waves is given.

Dropped Object Analysis Using Non-Linear Dynamic FE Analysis

2014 ◽  
Author(s):  
Kwanghyun Ahn ◽  
Minsung Chun ◽  
Sangmin Han ◽  
Kibok Jang ◽  
Yongsuk Suh
Keyword(s):  
Material Properties ◽  
Failure Criteria ◽  
Offshore Structures ◽  
Linear Analysis ◽  
Fe Analysis ◽  
Structural Safety ◽  
Design Consideration ◽  
Linear Dynamic ◽  
Non Linear

For the last few decades, necessity of direct non-linear FE analysis has been increasing for the accidental events at the vessel/offshore structures. One of major areas for the accidental design, dropped object analysis using non-linear analysis is indispensable for the verification of structural safety at the design process. This paper is concerned with the methodology, conditions, and design consideration of dropped object analysis using dynamic FE analysis. By comparing the results from direct FE analyses to those from simplified energy method described in DNV-RP-C204, necessities and advantages of direct non-linear analysis can be verified. In this paper, the effect of analysis condition is investigated using parametric study. The results are influenced by the application of failure criteria according to the rule requirements, application of material properties, dropping position, condition of the object, and so on. This study can suggest appropriate determination of the methodology and condition for the dropped object analysis using direct FE analysis.

Nonlinear Analysis of Offshore Platforms Subjected to Earthquake Loading Considering the Effects of Joint Flexibility

2009 ◽  
Author(s):  
S. Samadani ◽  
A. A. Aghakouchak ◽  
J. Mirzadeh Niasar
Keyword(s):  
Structural Analysis ◽  
Nonlinear Analysis ◽  
Offshore Structures ◽  
Offshore Platforms ◽  
Joint Flexibility ◽  
Modeling And Analysis ◽  
Shell Elements ◽  
Tubular Joints ◽  
Two Samples ◽  
Non Linear

In a conventional method of structural analysis, for modeling and analysis of jacket type offshore platforms, member connections are assumed to be rigid. In this method, members are rigidly connected which means there is no axial or rotational deformation at the end of brace member relative to chord axis. However in reality local deformations occur at chord surface due to applied loads from braces, which mean tubular joints are considerably flexible especially in non linear range of deformations. Therefore results of analysis based on rigid connections assumption differ from real behavior of the structure. Various research works have been carried out in the past on tubular joints and different methods have been presented in order to include the effect of joint flexibility in structural analysis. Most of these methods are just valid in elastic range but some non-linear methods have also been developed for simple tubular joints. In order to carry out a nonlinear analysis on a 3-D model of an offshore platform with multi-brace / multi-planar tubular joints, none of these simplified methods is applicable. In this case a complete model of tubular joints by non-linear shell elements is the most accurate one which is not only valid for non-linear analyses but also covers all type of tubular joints. In this paper two samples of offshore platforms are studied. These platforms are modeled using the following approaches: 1. No modeling of joints as structural elements (rigid connections). 2. Modeling of joint can with nonlinear shell elements (flexible connection). Different types of static non-linear analysis (Push over) are carried out and results are compared. In order to evaluate the results and compare this type of modeling with simplified methods included in professional software for the analysis of offshore structures, aforementioned platforms are also analyzed using the Fessler and MSL models to include effects of joint flexibility. The results of these types of modeling are also compared with the previous ones.

Estimation of High Energy Collision Response for Jacket Structures

2015 ◽  
Author(s):  
Gabriele Notaro ◽  
Atle Johansen ◽  
Stine Myhre Selås ◽  
Terje Nybø
Keyword(s):  
High Energy ◽  
Offshore Structures ◽  
Energy Collision ◽  
Offshore Structure ◽  
High Energy Collision ◽  
Collision Response ◽  
Non Linear ◽  
Jacket Structures ◽  
Deformable Bodies ◽  
High Energy Collisions

This paper presents a procedure for evaluating the collision response of fixed offshore structures exposed to high energy collisions with Offshore Service Vessels (OSV) or other floating units. A combination of traditional non-linear frame analyses for screening purposes and more detailed non-linear FE analyses using an explicit code is proposed. In the detailed integrated FE analyses, both the fixed offshore structure and the impacting vessel are explicitly modeled as deformable bodies. The progressive changes in contact and the resistance of both deforming objects are fully accounted for, enabling for more accurate results compared to methods where the resistances of the two objects are assessed separately. Results from case studies are shown. The calculated resistances for the striking objects are compared to the load-indentation curves presented in DNV-RP-C204 [1] and NORSOK N-004 [2].

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