NorMoor JIP: On Safe Mooring Line Design

2012 ◽  
Author(s):  
Siril Okkenhaug ◽  
Jan Mathisen ◽  
Torfinn Hørte
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Line Tension ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Extreme Value ◽  
Mooring Line ◽  
Safety Factors ◽  
Line Design

DNV is currently running a Joint Industry Project, “NorMoor JIP”, on calibration of safety factors for mooring lines together with several oil companies, engineering companies, rig-owners, manufacturers of mooring line components and Norwegian authorities. Our motivation for initiating a study on mooring line safety factors started out with questions raised with regards to the safety level given by the Norwegian regulations. However, this is equally important for other mooring regulations like ISO, API and class-regulations. What we see is that the mooring standards are interpreted and applied in different ways. The reliability level implied by the regulations is not known, and the present safety factors were set when frequency domain analysis was prevalent while time domain analysis is often applied today. DNV carried out the DeepMoor JIP [9] during 1995–2000 using frequency domain analysis and reliability-based calibration. Now, a decade later, the increase in computing capacity makes it feasible to carry out a similar calibration for time-domain analysis of the mooring systems. The objective of the project work is to investigate and compare the characteristic line tension calculated according to design standards with the annual extreme value distribution of the line tension. Further, to calibrate safety factors for mooring line design for the ultimate limit state (ULS) as a function of the target probability of failure. The original proposal for this JIP included calculations for chain and wire rope moorings on a typical drill rig and a turret moored FPSO at three different water depths at Haltenbanken. However, since this JIP has been very well received in the industry, the scope has been extended to include calculations for a production semisubmersible, for fibre rope systems and for Gulf of Mexico environmental conditions. This paper will focus on the reasons for doing this calibration study, and the importance of seeking to agree on unified calculation recipes and requirements. Preliminary results for characteristic tension and annual extreme value distributions of tension for some designs are presented and discussed. The calibration of safety factors will be carried out later in the project when all designs are finalized.

Investigation on the Use of Different Approaches to Mooring Analysis and Appropriate Safety Factors

2012 ◽  
Author(s):  
Sojan Vasudevan ◽  
Paul Westlake
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Line Tension ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Safety Factors ◽  
Dynamic Time ◽  
The Time Domain

This paper presents the results of the analyses of a twelve line catenary mooring system using a quasi-static method in the frequency domain, and uncoupled and coupled dynamic methods in the time domain. The latter is found to produce significantly higher tensions. The reasons for these differences are investigated. The minimum line tension safety factors required by design codes do not distinguish between uncoupled and coupled dynamic analyses and some codes use the same factors even for quasi-static analyses. Consequently, the present mooring system passes the acceptance criteria based on quasistatic frequency domain and uncoupled dynamic time domain analyses but does not meet the same criteria when a coupled dynamic time domain analysis is employed. It is understood that because the coupled time domain analysis determines the vessel motions using all forces the accuracy of mooring line tension estimation will be improved over other methods. Hence the application of less conservative safety factors is proposed.

Summary and Recommendations for Safe Mooring System Design in ULS and ALS

2017 ◽  
Author(s):  
Siril Okkenhaug ◽  
Torfinn Hørte ◽  
Øivind Paulshus
Keyword(s):  
Gulf Of Mexico ◽  
Frequency Domain ◽  
Time Domain ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Mooring Line ◽  
Safety Factors ◽  
Design Code ◽  
Oil Companies ◽  
Reliability Level

DNV GL is currently running a Joint Industry Project, “NorMoor JIP”, with participants from oil companies, engineering companies, rig-owners, manufacturers and marine authorities. It is a global study covering Gulf of Mexico, Northern Europe and Brazil waters. Our motivation for initiating a study on mooring line reliability was that all the global standards (API, ISO, DNV GL, others) are mostly based on work from late 1990s, when frequency domain analysis was prevalent. The reliability level implied by these regulations is not known, and we also see that the mooring standards are interpreted and applied differently. Thus, there is a need for a mooring design code with a consistent analysis methodology and with safety factors that are in line with this methodology and calibrated at an appropriate target reliability level. This is achieved through reliability-based calibration for a range of different units, mooring systems, water depths and geographical locations. The focus in the present paper is the calibration of safety factors and selection of target reliability level. The underlying probabilistic analysis results used for the calibration are reported in two accompanying papers at OMAE 2017, [1] and [2], dealing with structural reliability analyses for the ULS and ALS respectively. For mobile units frequency domain analyses are common, and although the main attention in the JIP is towards time domain analyses, it is part of the JIP to calibrate safety factors for frequency domain analyses as well. The annual extreme value distribution of line tension for all cases is calculated in time domain and is applied both in the calibration of safety factors for time domain and frequency domain analyses. It is seen that characteristic tensions from time domain analyses are likely to be higher than those from frequency domain analyses. The dilemma of not being penalized when using more refined time domain analyses is discussed, and different safety factors have been suggested for use with time domain and frequency domain analyses. A discussion about target reliability level is included, and the target levels are proposed with basis in the existing mooring design practice for mobile units, where frequency domain analysis is prevalent. Different targets are proposed depending on consequences of failure. Calibration for different design formats are carried out. The current format using a single safety factor is challenged with a format with two safety factors. The objective is to arrive as close as possible to the target reliability for all cases analyzed. A different design philosophy is needed in the Gulf of Mexico in order to achieve acceptable risk, and options are discussed. The present work provides a unique and comprehensive set of results, where advanced reliability methods are used to calibrate a mooring design code where the mooring line tensions are calculated in the time domain. The results provide a basis for regulators, such as ISO, to update their rules. ULS and ALS are covered here, and a potential phase 3 of the JIP will cover the fatigue limit state. When the NorMoor JIP is completed the plan is to implement the results into DNVGL-OS-E301, [5].

scholarly journals Analysis of Three-Phase Wye-Delta Connected LLC

Energies ◽  
2021 ◽  
Vol 14 (12) ◽  
pp. 3606
Author(s):  
Jing-Yuan Lin ◽  
Chuan-Ting Chen ◽  
Kuan-Hung Chen ◽  
Yi-Feng Lin
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Operation Time ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Complete Analysis ◽  
Plane Analysis ◽  
Analysis Methods ◽  
Three Phase

Three-phase wye–delta LLC topology is suitable for voltage step down and high output current, and has been used in the industry for some time, e.g., for server power and EV charger. However, no comprehensive circuit analysis has been performed for three-phase wye–delta LLC. This paper provides complete analysis methods for three-phase wye–delta LLC. The analysis methods include circuit operation, time domain analysis, frequency domain analysis, and state–plane analysis. Circuit operation helps determine the circuit composition and operation sequence. Time domain analysis helps understand the detail operation, equivalent circuit model, and circuit equation. Frequency domain analysis helps obtain the curve of the transfer function and assists in circuit design. State–plane analysis is used for optimal trajectory control (OTC). These analyses not only can calculate the voltage/current stress, but can also help design three-phase wye-delta connected LLC and provide the OTC control reference. In addition, this paper uses PSIM simulation to verify the correctness of analysis. At the end, a 5-kW three-phase wye–delta LLC prototype is realized. The specification of the prototype is a DC input voltage of 380 V and output voltage/current of 48 V/105 A. The peak efficiency is 96.57%.

A Review of Mooring Analysis Methodologies for Permanent Offshore Installations

2020 ◽  
Author(s):  
Spiro J. Pahos ◽  
Georgina Maldonado ◽  
Paul C. Westlake
Keyword(s):  
Time Domain ◽  
Line Strength ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Mooring Line ◽  
System Response ◽  
Mooring System ◽  
Deterministic Approach ◽  
Safety Factors ◽  
Analytical Approaches

Abstract Traditionally mooring line strength assessment is based on a deterministic approach, where the mooring system is evaluated for a design environment defined by a return period. The mooring system response is then checked against the mooring strength to ensure a required factor of safety. Some codes adopt a deterministic approach [1], [2], [3]. Other codes like [4] adopt a partial safety factor format where uncertainties are addressed through load factors for load components and material factors for line strength. Industry practices give guidance on mooring analysis methodology together with analysis options like coupled, de-coupled, time domain, frequency domain and the associated line tension safety factors. Prior work has demonstrated that discrepancies in mooring line tensions are observed when different analytical approaches are used [5]. Namely, the mooring line tensions of a semi-submersible unit in a coupled time domain analysis, were found to be non-compliant, whereas those calculated using a decoupled time domain analysis returned compliant tensions. This work focuses on a coupled dynamic analysis where all inertial, hydrodynamic and mechanical forces are assessed to determine the subsequent motions. Despite being considered the most accurate to capture the true dynamic response, a coupled analysis is also the least efficient in terms of the required computer resources and engineering effort [1]. This paper presents further discussion on the above observation in mooring tensions and also considers differences in the installation’s excursion. All responses are evaluated in the time domain where the nonlinear dynamic behavior of the mooring lines, slowly varying wave drift forces and coupling effects are captured. Agreement is found in the present computations, carried out with two renowned hydrodynamic codes, which validate former results and reiterate the need to distinguish between time domain methods and recommended appropriate safety factors accordingly.

Global Performance Analysis of Deep Draft Semi-Submersible Designed for Standard GOM Application

2019 ◽  
Author(s):  
R. H. Yuck ◽  
D. H. Kang ◽  
E. S. Kim ◽  
M. S. Kim ◽  
T. M. Kim ◽  
...  
Keyword(s):  
Time Domain ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Hydrodynamic Characteristics ◽  
Mooring Line ◽  
Global Motion ◽  
Hull Form ◽  
Global Performance ◽  
Motion Performance ◽  
Line Design

Abstract A deep draft semi-submersible hull has been developed by SHI (Samsung Heavy Industries) and Williams as a standardization concept which can support the topside structures up to the facility weight for the specific level of daily oil production in GOM (Gulf of Mexico). The designed hull has the optimized dimensions of pontoon and column which secure the sufficient level of GM for the stability and ballast capacity for coping with the weight change of topside structures. The hull form also has the good global motion in waves to be able to use the SCR (Steel Catenary Riser) as a deep draft semi-submersible concept. Two hull forms are designed to have 4 columns with two different cross sections and the conventional ring pontoon. The compartmentation and basic hydrodynamic analysis are performed at the hull sizing stage to achieve the sufficient stability and the motion performance as well. The mooring systems are also designed for two different water depths of 1000m and 3000m as a standard design concept of hull including mooring lines. To verify the feasibility of the proposed hull concept with regard to the hydrostatic/hydrodynamic characteristics and mooring line design, the numerical global performance analyses are carried out. Hydrostatic stability is investigated for intact and damage condition of operational loading condition and the proper tank compartments are verified under the given topside weight and the environment condition in GOM. The global motion is validated for all the possible combinations of wave, wind, current for a site in central GOM. Through the frequency-domain analysis and quasi-static time-domain analysis as well, the essential items such as the maximum offset, mooring line tension/fatigue, air-gap and the extreme acceleration at topside are examined and confirms that the certain design criteria of Semi FPU (Floating Production Unit) operation are satisfied. The possibility of SCR usage is also investigated with the fully coupled time domain analysis using the preliminary design of risers and confirms that the designed hull form has the suitable hydrodynamic characteristics to permit the minimum motion performance for SCR in the sense of extreme load and fatigue life.

A Frequency Domain Analysis of Learning Control

1994 ◽  
Vol 116 (4) ◽  
pp. 781-786 ◽  
Author(s):  
C. J. Goh
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Linear Time ◽  
Planning Horizon ◽  
Time Domain Analysis ◽  
Learning Control ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Convergence Criterion ◽  
The Time Domain

The convergence of learning control is traditionally analyzed in the time domain. This is because a finite planning horizon is often assumed and the analysis in time domain can be extended to time-varying and nonlinear systems. For linear time-invariant (LTI) systems with infinite planning horizon, however, we show that simple frequency domain techniques can be used to quickly derive several interesting results not amenable to time-domain analysis, such as predicting the rate of convergence or the design of optimum learning control law. We explain a paradox arising from applying the finite time convergence criterion to the infinite time learning control problem, and propose the use of current error feedback for controlling possibly unstable systems.

Analysis of Electrocardio Signal Based on Approximate Entropy

2012 ◽  
Vol 429 ◽  
pp. 195-199
Author(s):  
Xiao Lei Zhao ◽  
Ming Rong Ren ◽  
Ya Ting Zhang ◽  
Pu Wang
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Approximate Entropy ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Spectrum Estimation ◽  
Transform Method ◽  
Time Frequency ◽  
Relationship Of

The research and detection of heart disease depends on the analysis of the characteristic of electrocardio signal. Current analysis methods mainly include: (1) time domain analysis is a common used approach. With experience learned by observation and calculation, researchers examine errors and interferences to calculate means and variances directly within time domain. Analysis quality of this method demands higher request for researchers’ experience and skill although it’s a direct and significant result. (2) Frequency domain analysis, such as spectrum estimation, is largely applied to electrocardio signal researches and clinical applications. The analysis reflects abundant electrocardio activities, but failed to show details of the characteristics due to lack of time information. (3) time-frequency domain analysis describes energy density under different time and frequency of electrocardio signal at one time. It clarifies the relationship of signal frequency’s changing along with time such as wavelet transform method. (4) Nonlinear analysis is generally applied to biomedicine signal research in recent years. Correlation dimension, kolmogorov entropy, lyapunov component are major research methods to estimate some nonlinear dynamic parameters to represent the characteristic of electrocardio signal.

Comparison of Time and Frequency Domain Analysis With Full Scale Data for the Horn Mountain Spar During Hurricane Isidore

2006 ◽  
Author(s):  
Arcandra Tahar ◽  
John Halkyard ◽  
Mehernosh Irani
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Time Domain Analysis ◽  
Domain Analysis ◽  
Frequency Domain Analysis ◽  
Full Scale ◽  
Frequency Domain Method ◽  
Drag Forces ◽  
The Time Domain ◽  
Scale Data

The Horn Mountain Spar is located in 1,654 m of water about 135 km from Venice, Louisiana in the Gulf of Mexico. The facility was instrumented extensively to measure key spar and riser response parameters (Edwards et. al. 2003). Halkyard et. al. (2004) and Tahar et. al. (2005) have compared measured spar responses such as motion and mooring line tensions with numerical predictions. This paper extends the work done on comparison of the full scale data during hurricane Isidore. All previous numerical simulations were based on a time domain analysis procedure. One concern related to this method is that it is computationally intensive and time consuming. In the initial stages of a project, a frequency domain solution may be an effective tool compared with a fully coupled time domain analysis. The present paper compares results of time domain and frequency domain simulations with field measurements. Particular attention has been placed on the importance of the phase relationship between motion and excitation force. In the time domain analysis, nonlinear drag forces are applied at the instantaneous position. Whereas in the frequency domain analysis, nonlinear drag forces are stochastically linearized and solutions are obtained by an iterative procedure. The time domain analysis has better agreement with the field data compared to the frequency domain. Overall, however, the frequency domain method is still promising for a quick and approximate estimation of relevant statistics. With advantages in terms of CPU time, the frequency domain method can be recommended as a tool in pre-front end engineering design or in a phase where an iterative nature of design of an offshore structure takes place.

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