Hydrodynamic Performance Effect of Steel Catenary Risers on Wave Frequency Motions of a Semi Submersible

2016 ◽  
Author(s):  
Chen Gang ◽  
Zhao Nan ◽  
Zhang Wei ◽  
Yuan Hongtao ◽  
Li Chunhui ◽  
...  
Keyword(s):  
Time Domain ◽  
Wave Frequency ◽  
Hydrodynamic Performance ◽  
Coupled Analysis ◽  
Phase Lag ◽  
Floating Platform ◽  
Mooring Lines ◽  
Performance Effect ◽  
Steel Catenary Risers ◽  
Depth Increase

The analysis of the influence of risers on the motions of a floating platform is often conducted and simplified by uncoupled method. As the number of risers and water depth increase, this method would not predict system motion accurately. Coupled analysis method in time domain becomes a very convenient approach in response calculation since it automatically includes the interaction among platform, mooring lines and risers. This paper introduces a full coupled approach by AQWA-NAUT to include viscous damping of the semi submersible and effects of steel catenary risers on the wave frequency response of platform in time domain motion analysis. The main conclusion of this paper is that full coupled method can accruately predict semi submersible Response Amplitude Operator (RAOs) comparing to the case without risers. Other conclusions are that risers have an important effect on the wave frequency motion of semi submersible and also lead to a phase lag with respect to platform motions.

Understanding the Dynamic Coupling Effects in Deep Water Floating Structures Using a Simplified Model

2008 ◽  
Vol 130 (3) ◽  
Author(s):  
Ying Min Low ◽  
Robin S. Langley
Keyword(s):  
Frequency Domain ◽  
Deep Water ◽  
Time Domain ◽  
Low Frequency ◽  
Wave Frequency ◽  
Computational Time ◽  
Coupled Analysis ◽  
Floating Platform ◽  
Coupling Effects ◽  
Mooring Lines

The global dynamic response of a deep water floating production system needs to be predicted with coupled analysis methods to ensure accuracy and reliability. Two types of coupling can be identified: one is between the floating platform and the mooring lines/risers, while the other is between the mean offset, the wave frequency, and the low frequency motions of the system. At present, it is unfeasible to employ fully coupled time domain analysis on a routine basis due to the prohibitive computational time. This has spurred the development of more efficient methods, including frequency domain approaches. A good understanding of the intricate coupling mechanisms is paramount for making appropriate approximations in an efficient method. To this end, a simplified two degree-of-freedom system representing the surge motion of a vessel and the fundamental vibration mode of the lines is studied for physical insight. Within this framework, the frequency domain equations are rigorously formulated, and the nonlinearities in the restoring forces and drag are statistically linearized. The model allows key coupling effects to be understood; among other things, the equations demonstrate how the wave frequency dynamics of the mooring lines are coupled to the low frequency motions of the vessel. Subsequently, the effects of making certain simplifications are investigated through a series of frequency domain analyses, and comparisons are made to simulations in the time domain. The work highlights the effect of some common approximations, and recommendations are made regarding the development of efficient modeling techniques.

A Simplified Model for the Study of Dynamic Coupling Effects in Deepwater Floating Structures

2005 ◽  
Author(s):  
Ying Min Low ◽  
Robin S. Langley
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Low Frequency ◽  
Wave Frequency ◽  
Computational Time ◽  
Coupled Analysis ◽  
Floating Platform ◽  
Coupling Effects ◽  
Fundamental Vibration ◽  
Mooring Lines

As the exploitation of hydrocarbon moves towards deeper waters, the global dynamic response of a floating production system needs to be predicted with coupled analysis methods to ensure accuracy and reliability. Two types of coupling can be identified: one is between the floating platform and the mooring lines/risers, while the other is between the mean offset, the wave frequency and the low frequency motions of the system. At present, it is unfeasible to employ fully coupled time domain analysis on a routine basis due to prohibitive computational time. This has spurred the development of more efficient methods that account for the various couplings, including frequency domain approaches. It is paramount for the complex coupling mechanisms to be well understood before appropriate simplifications and assumptions can be made. In this paper, a simplified two degree-of-freedom system representing the surge motion of a vessel and the fundamental vibration mode of the lines is examined which captures the important underlying physics. Within this framework, the frequency domain equations are rigorously formulated, and the nonlinearities in the restoring forces and drag are stochastically linearized. The model allows key coupling effects to be identified: among other things, the equations demonstrate how the wave frequency dynamics of the mooring lines are coupled to the low frequency motions of the vessel. Subsequently, the effects of making certain simplifications are investigated through a series of frequency domain spectral analyses, and comparisons are made to simulations in the time domain. The work highlights the effect of certain common approximations, and recommendations are made regarding the development of efficient modeling techniques.

Influence of Nonlinear Mooring Stiffness on Hydrodynamic Performance of Floating Bodies

2009 ◽  
Author(s):  
Huilong Ren ◽  
Jian Zhang ◽  
Guoqing Feng ◽  
Hui Li ◽  
Chenfeng Li
Keyword(s):  
Equilibrium Position ◽  
Equations Of Motion ◽  
Offshore Structures ◽  
Hydrodynamic Performance ◽  
Coupled Analysis ◽  
Mooring System ◽  
Main Function ◽  
Ocean Engineering ◽  
Mooring Lines ◽  
Floating Structures

Coupled dynamic analysis between floating marine structures and flexible members such as mooring lines and risers, is a challenging work in the ocean engineering field. Coupled analysis on mooring-buoy interactions has been paid more and more concern for recent years. For floating offshore structures at sea, the motions driven by environmental loads are inevitable. The movement of mooring lines occurs due to the excitation on the top by floating structures. Meanwhile the lines restrict the buoy’s motion by forces acting on the fareleads. Positioning is the main function of mooring system, its orientation effects can’t be ignored for floating structures such as semi-submersible, FPS, and TLP, especially when the buoy’s equilibrium position shifting to another place. Similar as hydrostatic restoring forces, mooring force related with the buoy’s displacement can be transformed into mooring stiffness and can be added in the differential equations of motion, which is calculated at its equilibrium point. For linear hydrodynamic analysis in frequency domain, any physical quantity should be linear or be linearized, however mooring stiffness is nonlinear in essence, so the tangent or differential stiffness is used. Steel chains are widely used in catenary mooring system. An explicit formulation of catenary mooring stiffness is derived in this article, which consists of coupled relations between horizontal and vertical mooring forces. The effects of changing stiffness due to the shift of equilibrium position on the buoy’s hydrodynamic performance are investigated.

scholarly journals Design and Analysis of a Mooring Buoy for a Floating Arrayed WEC Platform

Processes ◽  
2021 ◽  
Vol 9 (8) ◽  
pp. 1390
Author(s):  
Sung Youn Boo ◽  
Steffen Allan Shelley
Keyword(s):  
Power Generation ◽  
Time Domain ◽  
Design Parameters ◽  
Power Cable ◽  
Floating Platform ◽  
Mooring Lines ◽  
Wave Energy Converters ◽  
Industry Standards ◽  
Global And Local ◽  
Fully Coupled

This paper presents the design and analysis of a mooring buoy and its mooring systems to moor a floating platform mounting an arrayed Wave Energy Converters (WECs). The mooring buoy allows the WEC platform to weathervane around the mooring buoy freely by the prevailing environment directions, which enables consistent power generation. The WEC platform is connected to the buoy with synthetic hawsers, while station-keeping of the buoy is maintained with catenary mooring lines of chains tied to the buoy keel. The buoy also accommodates a power cable to transfer the electricity from the WEC platform to the shore. The WEC platform is designed to produce a total of 1.0 MW with multiple WECs installed in an array. Fully coupled time-domain analyses are conducted under the site sea states, including extreme 50 y and survival 100 y conditions. The buoy motions, mooring tensions and other design parameters are evaluated. Strength and fatigue designs of the mooring systems are validated with requirements according to industry standards. Global and local structural designs of the mooring buoy are carried out and confirm the design compliances.

Mooring- and Riser-Induced Damping in Fatigue Seastates

2002 ◽  
Author(s):  
D. L. Garrett ◽  
R. B. Gordon ◽  
J. F. Chappell
Keyword(s):  
Coupled Model ◽  
Rational Approach ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Floating Platform ◽  
Mooring Lines ◽  
Equivalent Linear ◽  
Floating System ◽  
Linear Damping ◽  
Stiffness And Damping

Viscous damping due to drag on mooring lines and risers is seastate dependent and significantly affects the motion of a floating platform in deep water, particularly in everyday seastates. This in turn impacts design of the risers, which are typically controlled by fatigue. The dynamic interaction between the platform, mooring and risers cannot be evaluated using conventional uncoupled analysis tools, where each is analyzed separately. Rather, coupled analysis is required to provide a consistent way to model the drag-induced damping from mooring lines and risers. We describe a coupled, frequency domain approach (RAMS – Rational Approach to Marine Systems) for calculating the dynamic response of vessel, mooring and risers. In coupled analysis, the risers and mooring lines are included in the model along with the floater. In this way, damping of the floater motion due to drag on the mooring lines and risers is incorporated directly. It is also valuable to estimate the linear damping factors from the full, coupled analysis results. These damping factors may then, for example, be used in an equivalent linear model of the floating system in which the stiffness and damping effects of the mooring and risers are represented as additions to the floater stiffness and damping matrices. Such a model could be used to efficiently design a subsystem (e.g.; an export riser). We describe a technique to determine the equivalent linear damping factors from the coupled analysis results. This paper also illustrates the use of these methods for a West Africa FPSO. The need for coupled analysis is shown by comparing results from the fully coupled model with those obtained using an uncoupled method in which the mooring line damping is approximated.

A Comparison of Time Domain and Frequency Domain Approaches for the Fully Coupled Analysis of Deepwater Floating Systems

2006 ◽  
Author(s):  
Ying Min Low ◽  
Robin S. Langley
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Coupled Analysis ◽  
Integration Scheme ◽  
Global Coordinate System ◽  
Mooring Lines ◽  
Frequency Domain Methods ◽  
The Time Domain ◽  
Fully Coupled ◽  
Fully Coupled Analysis

The recognition of the need for a fully coupled analysis of deepwater floating production systems has led to the research and development of several coupled analysis tools in recent years. Barring a handful of exceptions, these tools and available commercial packages are invariably in the time domain. This has resulted in a much better understanding and confidence in time domain coupled analysis, but less so for the frequency domain approach. In this paper, the viability of frequency domain coupled analysis is explored by performing a systematic comparison of time and frequency domain methods using computer programs developed in-house. In both methods, a global coordinate system is employed where the vessel is modeled with six degrees-of-freedom, while the mooring lines and risers are discretized as lumped masses connected by extensional and rotational springs. Coupling between the vessel and the mooring lines is achieved by stiff springs, and the influence of inertia and damping from the lines are directly accounted for without the need for prior assumptions. First and second order wave forces generated from a random environment are applied on the vessel, as well as drag and inertia loading on the lines. For the time domain simulation, the Wilson-theta implicit integration scheme is employed to permit the use of relatively large time steps. The frequency domain analysis is highly efficient despite being formulated in global coordinates, owing to the banded characteristics of the mass, damping and stiffness matrices. The nonlinear drag forces are stochastically linearized iteratively. As both the time and frequency domain models of the coupled system are identical, a consistent assessment of the error induced by stochastic linearization can be made.

Efficient Time-Domain Simulation of Side-By-Side Moored Vessels Advancing in Waves

2007 ◽  
Author(s):  
A. S. Murthy Chitrapu ◽  
Theodore G. Mordfin ◽  
Henry M. Chance
Keyword(s):  
Time Domain ◽  
Wind Waves ◽  
Hydrodynamic Performance ◽  
Forward Speed ◽  
Time Domain Simulation ◽  
Mooring Lines ◽  
Close Proximity ◽  
The Us ◽  
Naval Engineering ◽  
Skin To Skin

Evaluation of hydrodynamic performance of two vessels in close proximity that are either stationary or advancing in waves is of paramount importance for many offshore and naval engineering applications. Hydrodynamic interactions between the vessels combined with nonlinear mechanical interactions due to mooring and fendering systems make the problem more complicated. An efficient time-domain method is presented for evaluating the seakeeping and maneuvering performance of proximate vessels advancing with forward speed. The method computes the 6 degree-of-freedom motions of a pair of hydrodynamically interacting vessels subject to wind, waves, currents and maneuvering effects at zero and nonzero speeds in regular or random seaways. Model tests conducted to validate the method are described and results presented. The validation efforts conducted so far have yielded satisfactory comparisons, thereby reinforcing the confidence in the method and its applicability to such problems. The method has been used to predict safe operational limits of two vessels in skin-to-skin operations conducted by the US Navy. A similar analysis is presented herein for a different pair of vessels. Since it is based on time domain simulation, this method also allows the inclusion of non-linear effects due to mooring lines, fenders and effects of viscous roll damping, which is not possible with two-body hydrodynamic interaction solutions in frequency-domain. It is concluded that this method provides an efficient tool to predict the performance of hydrodynamically interacting vessels that are stationary or moving with forward speed. To date, it has proven very useful in the early stages of the design/concept development process in which many configurations are evaluated.

Installation of a Submerged Buoy for Supporting Risers (BSR) System in Campos Basin Site

2011 ◽  
Author(s):  
Jairo Bastos de Arau´jo ◽  
Jose´ Carlos Lima de Almeida ◽  
Antonio Carlos Fernandes
Keyword(s):  
Numerical Analysis ◽  
Model Test ◽  
Water Depth ◽  
Water Model ◽  
Floating Platform ◽  
Mooring Lines ◽  
Sea Bottom ◽  
Campos Basin ◽  
Steel Catenary Risers ◽  
Field Location

The BSR (Buoy for Supporting Risers) concept is composed by a submerged buoy anchored to the sea bottom by tethers and intended to support risers coming from the bottom (probably SCRs — Steel Catenary Risers) and going to the floating platform (probably with flexible jumpers). For the case under analysis here, the main dimensions of the BSR prototype are 27.2 m length × 27.2 m width × 5.0 m depth. The paper describes all final full scale installation step so that the BSR may be considered a suitable technology. The installation indeed was the great challenge of this design due the size of the hull. The present work also evaluates numerically and experimentally a specific new manner to install the BSR with the support of auxiliary mooring lines among with the four tethers connected to it. One of the installation premises was to make use of Anchor Handling Supply Vessels instead of Crane Vessels. After this numerical analysis, the work went on by performing model tests that simulates the operation in a deep water model basin using 1:40 scale. The model test anticipated several problems such as the chain stopper weakness in the operation and others as discussed in this paper. As a conclusion the work was devised the most important parameters during the system installation and suggested ways to improve the methodology. In November 2009 the BSR was installed in 500 m of water depth at Congro field location, Campos Basin, offshore Brazil. The tethers were adjusted in January 2010 and in March 2010 two risers were installed. Thenceforward the last edge of this knowledge was considered over passed.

Experimental and Numerical Investigation of the Stabilizing Effects of a Water-Entrapment Plate on a Deepwater Minimal Floating Platform

2005 ◽  
Author(s):  
Christian A. Cermelli ◽  
Dominique G. Roddier
Keyword(s):  
Time Domain ◽  
Laboratory Experiments ◽  
Interaction Model ◽  
Coupled Analysis ◽  
Diffraction Radiation ◽  
Floating Platform ◽  
Ship Model ◽  
Testing Facility ◽  
Radiation Theory ◽  
The Time Domain

The stabilizing effects of a water-entrapment plate at the keel of a small three-legged semi-submersible platform are determined using laboratory experiments and time-domain simulations. Motion predictions were carried out in the time-domain using coupled-analysis between the vessel and its mooring, linear diffraction-radiation theory, and an empirical wave-viscous interaction model. Model tests were conducted at the U.C. Berkeley Ship Model Testing Facility to determine the validity of the numerical model.

Ultradeepwater Monobuoys

2003 ◽  
Author(s):  
Mauro C. Oliveira
Keyword(s):  
Numerical Simulation ◽  
Time Domain ◽  
Calibration Procedure ◽  
Coupled Analysis ◽  
Global Behavior ◽  
Test Results ◽  
Mooring Lines ◽  
Analysis And Design ◽  
Deep Waters ◽  
Hull Forms

This work deals with the analysis and design of monobuoys for deep waters. The monobuoy performance evaluation is carried out using a time domain computer program due to the non linearities present in this problem. This program is used to simulate the behavior of the monobuoy under the action of waves, wind and current. A coupled analysis between the floater and the mooring lines, considering its inertia, is also employed. Initially a validation study is conducted comparing the numerical simulations with model test results for a 400 meters water depth CALM buoy. The test comprises an operational condition with a tanker connected to the buoy under the action of wave, current and wind loads. From these results a calibration procedure of the numerical simulation is proposed and different hull forms are assessed in order to verify its global behavior. The main objective is to check if there are improvements in comparison with more conventional shapes relatively to the riser forces in the connection point.

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