Integrated Structural Analysis Methodology for Truss Spars

2007 ◽  
Author(s):  
Michael Y. H. Luo ◽  
Bob L. X. Zhang ◽  
Sudhakar Tallavajhula ◽  
Sanjay Srinivasan
Keyword(s):  
Structural Analysis ◽  
Low Frequency ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Wave Frequency ◽  
Analysis Methodology ◽  
Motion Responses ◽  
Truss Spar ◽  
Low Frequency Motion ◽  
Floating Platforms

Eleven truss spars have been successfully installed in the deep water fields since late 2001. Compared with other floating systems, Truss spars offer significant advantages in motions, stability, and project schedule. One of the unique aspects of a truss spar is that it exhibits both high-frequency and low-frequency motion responses. The high-frequency motions, or wave frequency motions, are peaked around the wave spectral energy, while the low-frequency motions correspond to the natural periods of the spar’s rigid-body motions. Accurate structural design should include loads due to both wave and low frequency motions. The wave-frequency motions can be accurately estimated with potential/diffraction theory, but the low-frequency motions cannot be accounted for using the traditional spectral method. The traditional spectral method may be acceptable to other types of platforms such as Semi-submersibles and TLPs, but can become non-conservative for a spar structure. In the past, this challenge was overcome by performing time domain analysis to design the truss and a combined time and frequency domain analysis to design the other structural components. The procedure proved to be time consuming and inefficient, requiring extensive engineering hours. The hull design process was enhanced by developing an integrated structural analysis methodology. The methodology significantly reduces engineering hours and maintains accuracy in the estimation of loads by a combination of the wave frequency and low frequency motion responses. Efficient use of personnel for the labor-intensive structural modeling tasks was also achieved. Use of this methodology in two spar projects has proved to add significant value. The procedure is also applicable to a range of floating platforms such as Technip’s extended draft platform (EDP) and other deep draft floating platforms. Salient features of the integrated structural analysis methodology for both strength and fatigue analysis of the truss spar are discussed in the paper. Structural loads determined from the integrated methodology are compared with those from a complete time-domain analysis of the truss spar.

Frequency-Domain Calculations of Moored Vessel Motion Including Low Frequency Effect

2008 ◽  
Author(s):  
C. Le Cunff ◽  
Sam Ryu ◽  
Jean-Michel Heurtier ◽  
Arun S. Duggal
Keyword(s):  
Frequency Domain ◽  
Drag Force ◽  
Low Frequency ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Second Order ◽  
Wave Frequency ◽  
Diffraction Radiation ◽  
Floating Structure ◽  
Low Frequency Motion

Frequency-domain analysis can be used to evaluate the motions of the FPSO with its mooring and riser. The main assumption of the frequency-domain analysis is that the coupling is essentially linear. Calculations are performed taking into account first order wave loads on the floating structure. Added mass and radiation damping terms are frequency dependent, and can be easily considered in this formulation. The major non-linearity comes from the drag force both on lines and the floating structure. Linearization of the non-linear drag force acting on the lines is applied. The calculations can be extended to derive the low frequency motion of the floating structure. Second order low frequency quadratic transfer function is computed with a diffraction/radiation method. Given a wave spectrum, the second order force spectrum can then be derived. At the same time frequency-domain analysis is used to derive the low frequency motion and wave frequency motion of the floating system. As an example case, an FPSO is employed. Comparison is performed with time domain simulation to show the robustness of the frequency-domain analysis. Some calculations are also performed with either low frequency terms only or wave frequency terms only in order to check the effect of modeling low and wave frequency terms, separately. In the case study it is found that the low frequency motion is reduced by the wave frequency motion while the wave frequency motion is not affected by the low frequency motion.

Motion Analysis of TAD and TLP Platforms Operating Side by Side

2016 ◽  
Author(s):  
Mohammed Abdul Hannan ◽  
Bai Wei ◽  
Allan Ross Magee ◽  
Ankit Choudhary ◽  
Amit Jain ◽  
...  
Keyword(s):  
Stiffness Matrix ◽  
Low Frequency ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Equivalent Stiffness ◽  
Floating Bodies ◽  
Drilling Rig ◽  
Production Platform ◽  
The Time Domain

A Tender Assisted Drilling (TAD) is typically a support vessel that serves support of a drilling rig. The TAD acts as a platform for supplies and is stationed alongside the drilling rig from which the rig will work off to reduce the loads on the actual rig. While operating, the TAD needs to be structurally coupled with the floating production platform. The coupling makes sure that the Tender does not drift away from the platform and provides stiffness to the relative motions between the TAD and the production platform. Current practice is to couple the TAD and the platform by using nylon hawser ropes. The hawsers provide adequate elasticity to accommodate the low frequency motions and do not allow the TAD to drift away from the production platform. In this study the individual free floating TAD and Tension Leg Platform (TLP) system is model first using the commercial software Hydrostar. After that the technique of developing equivalent stiffness matrix to represent the TLP tendons are applied in order to simulate the influence of the tendons on TLP motions. The response of these individual models are then calculated numerically and validated against existing data. After validation is performed, coupled interactions of this two body is simulated while both the bodies are freely floating side by side. Several possible side by side orientations of the TAD and TLP are tested and the results are compared for this first phase of the study. Later, the freely floating bodies are connected to each other by means of springs. The connection between the two floating bodies is modeled using equivalent stiffness matrix for connections. The results obtained from this first phase frequency domain analysis will help to clearly understand the coupled behavior of such system under various orientations. In later phases, this model will be further developed in order to perform the time domain analysis and several innovative types of connections other than simple hawsers will be tested in order to couple this TAD and TLP system.

Validation of Applicability of Low Frequency Motion Analysis Theory Using Observation Data of Floating Offshore Substation

2018 ◽  
Author(s):  
Haruki Yoshimoto ◽  
Hisafumi Yoshida ◽  
Ken Kamizawa
Keyword(s):  
Potential Theory ◽  
External Force ◽  
Low Frequency ◽  
Demonstration Project ◽  
Wave Frequency ◽  
Offshore Wind ◽  
Observation Data ◽  
Frequency Wave ◽  
Short Period ◽  
Low Frequency Motion

In recent years, the social demands for the introduction of renewable energy are increasing, demonstration projects of floating offshore wind power generation are being implemented and planned around the world. In Japan, a demonstration project named Fukushima FORWARD (Fukushima Floating Offshore Wind Farm Demonstration Project) has been conducted since 2011. Fukushima FORWARD is carried out by the Ministry of Economy, Trade and Industry. The project is the world’s first floating offshore windfarm with a total capacity of 14 MW, including three floating offshore wind facilities and one floating offshore substation. In Fukushima FORWARD, Japan Marine United Corporation is in charge of floater part EPCI (Engineering, Procurement, Construction and Installation) of one floating offshore wind facility and one floating offshore substation. This floating offshore substation is installed in order to observe meteorological and oceanographic data and motion data as well as boosting the generated electric power. Since the installation in 2013 it continues to record various kinds of continuous data. The substation is an advanced spar type floater moored by four spread catenary mooring lines. In the design of the mooring system for offshore structure, the motion of the structure under environmental external force is very important. The motion of the moored floating structure is divided into wave frequency motion, which is a motion of a relatively short period, and low frequency motion caused by mooring restoring force and variable external force, both of which are important elements in the design. Among them, wave frequency motion is known to be accurately estimated by potential theory as a result of research on various types of structures. On the other hand, in addition to the existence of various calculation methods including time domain analysis, its statistical characteristic and applicability are entirely depending on the target structure. Also, observation data of low frequency motion have been very few. In this paper, long-term data observed at the floating offshore substation in Fukushima FORWARD was analyzed with focusing on low frequency motion and its statistical properties were clarified. Furthermore, we analyzed the low frequency wave force spectrum and motion by conventional low frequency motion theory using the wave drifting force calculated by the potential theory. And, we compared the calculated value with the analysis result of the observation data and validated the applicability of the simplified low frequency motion theory.

scholarly journals Comparative study of heart rate variability in normotensive young adults with family history of hypertension

2021 ◽  
Vol 9 (2) ◽  
pp. 371
Author(s):  
Ovais K. Wadoo ◽  
Sheikh I. Sayeed ◽  
Mariya R. Tramboo
Keyword(s):  
Heart Rate ◽  
Heart Rate Variability ◽  
High Frequency ◽  
Low Frequency ◽  
Vascular Diseases ◽  
Test Group ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Control Group ◽  
Sympathovagal Imbalance

Background: Hypertension is a risk factor for the development of cardiovascular and cerebro-vascular diseases. Autonomic nervous system plays a crucial role in the development of hypertension. The integrity of autonomic modulation of heart rate is evaluated by analysing heart rate variability (HRV), which refers to oscillations in the intervals between consecutive heartbeats or R-R intervals. The present study was designed to analyse the indices of heart rate variability in the offsprings of hypertensive parents and off springs of normotensive parents to understand if there is any autonomic imbalance between the two groups.  Methods: The study was conducted in the Department of Physiology, Government Medical College, Srinagar. The test group consisted of 30 healthy normotensive subjects studying in 1st year of MBBS with hypertensive parents and the control group consisted of healthy normotensive of 1st year of MBBS with both parents normotensive. In time domain analysis the standard deviation of all normal-to-normal intervals {SDNN(ms)} was taken as index of overall HRV. Frequency domain analysis was done with respect to low frequency (LF) analysis and high frequency (HF) analysis. Low and high frequency power were expressed in normalized units.Results: The SDNN was reduced in cases but was not statistically significant. RMSSD was also reduced in cases though not statistically significant. LFnu was found to be significantly higher in cases. The HFnu was significantly reduced in cases. LF/HF ratio was found to be higher in cases and the difference was statistically significant.  Conclusions: Our study reveals that incidence of prehypertension and the risk of cardiovascular dysfunction in relation to sympathovagal imbalance is more in the off springs of hypertensive parents than in the off springs of normotensive parents. Sympathovagal imbalance in the form of increased sympathetic drive and decreased parasympathetic drive can lead to prehypertension in these genetically predisposed individuals.  

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