Advances in Frequency and Time Domain Coupled Analysis for Floating Production and Offloading Systems

2005 ◽  
Author(s):  
Donogh W. Lang ◽  
Aengus Connolly ◽  
Michael Lane ◽  
Adrian D. Connaire
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Domain Analysis ◽  
Offshore Structures ◽  
Viscous Drag ◽  
Coupled Analysis ◽  
Diffraction Radiation ◽  
Analysis Tool ◽  
Floating Structure ◽  
The Individual

With the move to the development of remote, deepwater fields, increasing use is being made of floating production, storage and offloading (FPSO) facilities from which oil is intermittently offloaded to a shuttle tanker via offloading lines and an anchor leg mooring buoy. The response of the individual components of these systems is significantly influenced by hydrodynamic and mechanical coupling between adjacent components, precluding the use of traditional analysis techniques such as displacement RAOs derived from tank model tests or diffraction/radiation analyses of the independent components. Consequently, the reliable and accurate design of these complex systems requires an analysis tool capable of determining the fully coupled response of each of the individual components of the system. A recently-developed time domain coupled analysis tool has been extended to incorporate a frequency domain coupled analysis capability. This tool combines radiation/diffraction theory with a non-linear finite element (FE) structural analysis technique used for the analysis of slender offshore structures. This paper describes the application of frequency domain analysis to the coupled FE/floating structure problem, with particular consideration given to the linearisation of viscous drag loads on floating structures and the treatment of low-frequency second-order loads in the frequency domain. Results from frequency domain and time domain coupled analyses of a typical West of Africa type offloading system are compared, highlighting areas of application where frequency domain coupled analysis can offer significant benefits when used in conjunction with time domain analysis. Based on this, recommendations are made for the appropriate use of frequency and time domain coupled analysis for this type of system.

Hydrodynamic Interactions and Relative Motions Analysis for Installation of an Extendable Draft Platform

2009 ◽  
Author(s):  
Bonjun Koo ◽  
Jang Whan Kim
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Hydrodynamic Interactions ◽  
Coupled Analysis ◽  
Heave Plate ◽  
The Time Domain ◽  
Stabilized Platform ◽  
Relative Motions

The Extendable Draft Platform (EDP) is a deep draft, column stabilized platform with a deck box support for topsides and a single, deep draft heave plate that provides suitable motion characteristics to enable the use of dry tree top tensioned risers. The EDP can be fabricated with topsides installed on the deck box and commissioned quayside in a typical construction yard. With the columns in the retracted position, the EDP floats on its deck box and can be towed, in this configuration, to the location of interest. Once the EDP is transported to its final site, the columns and heave plate are lowered to their final operating draft. During the lowering sequence, the deck box and the lower hull become two relatively independent bodies, mechanically connected by chains that control the lowering of the columns and heave plate, and the guides between the deck box and the columns. This multi-body system is hydrodynamically coupled because of radiated and diffracted waves. The global performance analyses of the installation process (lowering of the lower hull) are carried out by three different methods. The first method is frequency-domain analysis by WAMIT and a frequency domain motion solver. In the frequency domain analysis, all the mechanical connections are modeled as linear springs. The second method is time-domain, partially coupled analysis using HARP/WINPOST. In this analysis, the off diagonal 6×6 hydrodynamic interactions are ignored. The last method is a time domain, fully coupled analysis using HARP/WINPOST. In this analysis, full 12×12 hydrodynamic interactions are considered. In the time domain analyses, the mechanical couplings between each column and deck box are modeled with linear springs and the chain connections are modeled with slender rods by using the nonlinear finite element method. This paper presents and compares analysis results based on the three methods for relative motions and loads between the deck box and the lower hull during the lowering of the columns and heave plate.

Robustness of an SCR in Gulf of Mexico Hurricane Conditions

2010 ◽  
Author(s):  
Partha Sharma ◽  
Kim Mo̸rk ◽  
Vigleik Hansen ◽  
Celso Raposo ◽  
Srinivas Vishnubhotla
Keyword(s):  
Gulf Of Mexico ◽  
Time Domain ◽  
Offshore Structures ◽  
Coupled Analysis ◽  
Floating Structure ◽  
Steel Catenary Riser ◽  
Strength Capacity ◽  
Critical Components ◽  
Robustness Check ◽  
Robustness Assessment

Recent hurricanes in Gulf of Mexico, most notably Ivan (2004), Katrina & Rita (2005), Ike (2008), were more severe than the local 100 year extremes in the Gulf of Mexico (GoM). As a result API has issued an interim metocean bulletin, API Bulletin 2INT-MET [1]. Concurrently, API also issued API Bulletin 2INT-EX [2] for assessment of existing offshore structures for hurricane conditions. API Bulletin 2INT-EX recommends a robustness check to evaluate floating structure critical components including production and export risers. The robustness check for risers as a minimum should consider the capacity and ductility of the key riser components. This paper investigates the robustness of a steel catenary riser (SCR) suspended from a deepwater tension leg platform (TLP) unit in Central GoM. The robustness assessment is performed for the 1000 year Central GoM hurricane conditions provided in API 2INT-MET. Time domain coupled analysis using the program DeepC is performed to determine the TLP motions and the associated loading on the SCR. SCR strength capacity checks are performed as per the methods outlined in new ISO 13628-12 [3].

Wave Exciting Forces of a Free Floating Semi Submersible in Regular Waves

2015 ◽  
Vol 74 (5) ◽  
Author(s):  
Hassan Abyn ◽  
Mohammad Rafiqul Islam ◽  
Jaswar Jaswar ◽  
Amin Mahmoudi ◽  
C. L. Siow ◽  
...  
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Domain Analysis ◽  
Offshore Structures ◽  
Source Distribution ◽  
Linear Wave ◽  
Distribution Method ◽  
Exciting Forces ◽  
Domain Models ◽  
Wave Exciting Forces

Drilling and production of oil by semi submersible take place in many locations throughout the world. Generally, floating structures play an important role in exploring the oil and gas from the sea. The force and motion prediction of offshore structures may be carried out using time domain or frequency domain models or model tests. In this paper the frequency domain analysis used because it is the simplified and linearized form of the equations of motion. The time domain analysis, unlike frequency domain models, is adequate to deal with non-linearities such as viscous damping and mooring forces, but it requires sophisticated solution techniques and it is expensive to employ. In this paper, the wave exciting forces of a free floating semi-submersible were carried out using 3D source distribution method within the scope of the linear wave theory. The results obtained from computations were also compared with the results obtained using commercial software MOSES and WAMIT.  

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.

Extreme Response Analysis of Floating Structures Using Coupled Frequency Domain Analysis

2011 ◽  
Vol 133 (3) ◽  
Author(s):  
Ying Min Low ◽  
Andrew J. Grime
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Low Frequency ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Coupled Analysis ◽  
Statistical Techniques ◽  
Frequency Domain Methods ◽  
Extreme Response ◽  
The Mean

In the dynamic analysis of a floating structure, coupled analysis refers to a procedure in which the vessel, moorings, and risers are modeled as a whole system, thus allowing for interactions between various system components. Because coupled analysis in the time domain is impractical owing to prohibitive computational costs, a highly efficient frequency domain approach was developed in a previous work, wherein the drag forces are linearized. The study showed that provided the geometric nonlinearity of the moorings/risers is insignificant, which often holds for ultradeepwater systems, the mean-squared responses yielded by the time and frequency domain methods are in close agreement. Practical design is concerned with the extreme response, for which the mean upcrossing rate is a key parameter. Crossing rate analysis based on statistical techniques is complicated as the total response occurs at two timescales, with the low frequency contribution being notably non-Gaussian. Many studies have been devoted to this problem, mainly relying on a technique originating from Kac and Siegert; however, these studies have mostly been confined to a single-degree-of-freedom system. The aim of this work is to apply statistical techniques in conjunction with frequency domain analysis to predict the extreme responses of the coupled system, in particular the modes with a prominent low frequency component. It is found that the crossing rates for surge, sway and yaw thus obtained agree well with those extracted from time domain simulation, whereas the result for roll is less favorable, and the reasons are discussed.

Extreme Response Analysis of Floating Structures Using Coupled Frequency Domain Analysis

2010 ◽  
Author(s):  
Ying Min Low ◽  
Andrew J. Grime
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Low Frequency ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Coupled Analysis ◽  
Statistical Techniques ◽  
Frequency Domain Methods ◽  
Extreme Response ◽  
The Mean

In the dynamic analysis of a floating structure, coupled analysis refers to a procedure in which the vessel, moorings and risers are modeled as a whole system, thus allowing for the interactions between the various system components. Because coupled analysis in the time domain is impractical owing to prohibitive computational costs, a highly efficient frequency domain approach was developed in a previous work, wherein the drag forces are linearized. The study showed that provided the geometric nonlinearity of the moorings/risers is insignificant, which often holds for ultra-deepwater systems, the mean-squared responses yielded by the time and frequency domain methods are in close agreement. Practical design is concerned with the extreme response, for which the mean upcrossing rate is a key parameter. Crossing rate analysis based on statistical techniques is complicated as the total response occurs at two timescales, with the low frequency contribution being notably non-Gaussian. Many studies have been devoted to this problem, mainly relying on a technique originating from Kac and Siegert; however, these studies have mostly been confined to a single-degree-of-freedom system. The aim of this work is to apply statistical techniques in conjunction with frequency domain analysis to predict the extreme responses of the coupled system, in particular the modes with a prominent low frequency component. It is found that the crossing rates for surge, sway and yaw thus obtained agree well with those extracted from time domain simulation, whereas the result for roll is less favorable, and the reasons are discussed.

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