Wind Induced Nonlinear Response of Coupled Spar Platform

2014 ◽  
Author(s):  
Mohammed Jameel ◽  
Suhail Ahmad ◽  
A. B. M. Saiful Islam ◽  
Mohd Zamin Jumaat
Keyword(s):  
Deep Water ◽  
Oil And Gas ◽  
Wave Force ◽  
Wind Loading ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Wave Direction ◽  
Spar Platform ◽  
Mooring Lines ◽  
Spar Platforms

The oil and gas exploration has moved from shallow water to much deeper water far off the continental shelf. Spar platforms under deep water conditions are found to be the most economical and efficient type of offshore platform. Several Spar platforms installed in the Gulf of Mexico and North Sea proves its suitability for deep water exploration. Accurate prediction of motions of a Spar hull is very important for the integrity and associated costs of the riser/mooring line. The most common approach for solving the dynamics of Spar platform is to employ a decoupled quasi-static method, which ignores all or part of the interaction effects between the platform, mooring lines and risers. Coupled analysis, which includes the mooring lines, risers and platform in a single model, is the only way to capture the damping from mooring lines and risers in a consistent manner. The present coupling is capable in matching the forces, displacement, velocities and acceleration for mooring line with Spar hull at the fairlead position and riser with Spar hull at the riser keel connection. It can handle possible significant nonlinearities. The output from such analyses will be platform motions as well as a detailed mooring line and riser responses. In actual field problems hydrodynamic and aerodynamic loads act simultaneously on Spar platform, mooring lines and risers. In finite element model, the entire structure acts as a continuum. This model can handle all nonlinearities, loading and boundary conditions. The selected configuration of Spar platform is analysed under wave force together with wind loading and its structural response behaviour in steady state is studied. An automatic Newmark-β time incremental approach in ABAQUS/AQUA environment has been implemented to conduct the analysis in time domain. The wind force acting on the exposed part of the platform encompasses mean and fluctuating wind components. The frontal region includes the topside assembly and the spar hull portion above the sea level. High degree of nonlinearities makes the solutions convergence sensitive and it requires large number of iterations, at each time station. Spar responses in surge, heave and pitch along with top tension in moorings are computed. The coupled Spar experiences significant lateral shift along wave direction due to wind loading. Increase in standard deviation shows the participation of wind loading giving higher fluctuations. The CML tension increases for wind loading but the extent of the tension fluctuations under wind loading is not much due to high pretension of mooring line.

Nonlinear Response of Coupled Integrated Spar Platform Under Severe Sea States

2012 ◽  
Author(s):  
A. B. M. Saiful Islam ◽  
Mohammed Jameel ◽  
Suhail Ahmad ◽  
Mohd Zamin Jumaat
Keyword(s):  
Deep Water ◽  
Beam Element ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Intermediate Water ◽  
Spar Platform ◽  
Mooring Lines ◽  
Spar Platforms ◽  
Water Conditions ◽  
Wave Heights

The oil and gas industry has moved towards the offshore deep water regions due to depletion of these resources in shallow and intermediate water depths. Conventional fixed jacket type platforms and bottom supported compliant platforms have been found to be inefficient and uneconomical for exploring these resources in deep water regions. In view of deep water conditions, Spar platforms have been seen to be the most economical and suitable alternative offshore platforms. Several operational Spar platforms such as SB-1, Shell’s ESSCO, Brent Spar, Oryx Neptune Spar, Chevron Genesis Spar and Exxon’s Diana Spar in the Gulf of Mexico and North Sea have shown the effectiveness and success of such platforms in deep-ocean. In deep water conditions, the severity of sea states has substantial effects on the spar platform. The mooring lines contribute significant inertia and damping because of their longer lengths, larger sizes, and heavier weights. Precise motion investigation of platforms should consider these actions in deep waters. However, proper dynamics cannot be assessed by the commonly used decoupled quasi-static method that ignores all or part of the interaction effects between the mooring lines and platform. Coupled analysis, which includes the platform and mooring lines in a single model, is the only way to capture the damping from mooring lines in a consistent manner. In the present study, coupled analysis of integrated Spar-mooring system has been performed. Cylindrical spar hull is treated as a rigid beam element and catenary mooring line as hybrid beam element. Nonlinear dynamic responses have been evaluated under several severe sea states of dissimilar wave heights and wave periods. Damping due to mooring lines has been assessed. An automatic Newmark-β time incremental approach has been implemented to conduct the analysis in time domain. Wave induced spar hull motion in surge, heave and pitch direction along with maximum tension in mooring line has been assessed for different wave conditions with and without current in 1018 m water depth. The time histories of spar responses follow substantial alteration for larger wave heights and wave periods. Maximum tensions in mooring line are very sensitive with momentous value for extreme sea loading. Mooring tension responses are significantly different reflecting the damping effect of mooring lines.

scholarly journals STRUCTURAL BEHAVIOUR OF FULLY COUPLED SPAR–MOORING SYSTEM UNDER EXTREME WAVE LOADING

2014 ◽  
Vol 19 (Supplement_1) ◽  
pp. S69-S77 ◽  
Author(s):  
A. B. M. Saiful Islam ◽  
Mohammed Jameel ◽  
Suhail Ahmad ◽  
Mohd Zamin Jumaat ◽  
V. John Kurian
Keyword(s):  
Response Time ◽  
Deep Water ◽  
Mooring Line ◽  
Mooring System ◽  
Wave Loading ◽  
Extreme Wave ◽  
Spar Platform ◽  
Mooring Lines ◽  
Time Histories ◽  
Fully Coupled

Floating spar platform has been proven to be an economical and efficient type of offshore oil and gas exploration structure in deep and ultra-deep seas. Associated nonlinearities, coupled action, damping effect and extreme sea environments may modify its structural responses. In this study, fully coupled spar–mooring system is modelled integrating mooring lines with the cylindrical spar hull. Rigid beam element simulates large cylindrical spar hull and catenary mooring lines are configured by hybrid beam elements. Nonlinear finite element analysis is performed under extreme wave loading at severe deep sea. Morison's equation has been used to calculate the wave forces. Spar responses and mooring line tensions have been evaluated. Though the maximum mooring line tensions are larger at severe sea-state, it becomes regular after one hour of wave loading. The response time histories in surge, heave, pitch and the maximum mooring tension gradually decreases even after attaining steady state. It is because of damping due to heavier and longer mooring lines in coupled spar–mooring system under deep water conditions. The relatively lesser values of response time histories in surge, heave, pitch and the maximum mooring tension under extreme wave loading shows the suitability of a spar platform for deep water harsh and uncertain environmental conditions.

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 New Nonlinear Coupled Analysis Tool for Floating Structures

2012 ◽  
Author(s):  
Zhiling Li ◽  
Carlos Llorente ◽  
Cheng-Yo Chen ◽  
Chang Ho Kang ◽  
Edmund Muehlner ◽  
...  
Keyword(s):  
Performance Analysis ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Analysis Tool ◽  
Wave Load ◽  
Mooring Lines ◽  
Dynamic Interactions ◽  
Floating Structures ◽  
Coupled Method ◽  
Global Performance

For the global performance analysis of a floater, the traditional semi-coupled method models mooring lines/risers as nonlinear massless springs and ignores 1) the inertial effects from mooring lines/risers, 2) the current and wave load effects on mooring lines/risers, and 3) the dynamic interaction between mooring lines/risers and the floater. However, these effects are deemed critical for deepwater and ultra deepwater floating structures as they may have a significant impact on the floaters’ motions and mooring line/riser tensions. This paper presents the development and verification of a time-domain nonlinear coupled analysis tool, MLTSIM-ROD, which is an integration of a recently developed 3D rod dynamic program, ROD3D, with the well-calibrated floater global performance analysis program, MULTISIM (Ref [9]). The ROD3D was developed based on a nonlinear finite element method and merged with MULTISIM by matching the forces and displacements of mooring lines/risers with the floater at their connections. MLTSIM-ROD can thus predict the floater’s large displacement/rotation motions and mooring line/riser tensions including all the coupled effects between the floater and mooring lines/risers. In this paper, global performance predictions for a SPAR in the Gulf of Mexico in deepwater were carried out using MLTSIM-ROD. The results were then verified with those from other coupled analysis programs. The paper also presents the results of motions and mooring line/riser tensions of the SPAR using both the coupled and semi-coupled methods. The results from the coupled and semi-coupled analyses indicate that the floater’s motions and mooring line/riser tensions could be significantly influenced by the dynamic interactions between the floater and mooring lines/risers. Hence, the coupled method needs to be considered for deepwater floating structures.

An Integrated Methodology for the Design of Mooring Systems and Risers of Floating Production Platforms

2012 ◽  
Author(s):  
Aldo Roberto Cruces Girón ◽  
Fabricio Nogueira Corrêa ◽  
Breno Pinheiro Jacob ◽  
Stael Ferreira Senra
Keyword(s):  
Production Systems ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Design Practice ◽  
Structural Behaviour ◽  
Coupled Models ◽  
Mooring Lines ◽  
Hydrodynamic Behaviour ◽  
Integrated Methodology ◽  
Analysis Tools

Nowadays, coupled analysis tools that allow the simultaneous modelling of the hydrodynamic behaviour of the hull and the structural behaviour of the lines of floating production platforms have been increasingly used. The use of such tools is gradually allowing the introduction of some feedback between the design of risers and mooring systems. In the current practice, that comprises the so-called “hybrid” methodologies, mooring designers have been using these tools to consider the influence of the risers on the platform motions. On the other hand, riser designers can use motions that result from coupled simulations for the analysis of each riser. Such integration is already being implemented in the design practice of Petrobras; however, elsewhere the design of risers and mooring systems may still be performed separately, by different teams, therefore not fully exploiting the benefits that the coupled analysis tools can provide. In this context, this work describes an innovative, fully integrated methodology for the design of mooring systems and risers of floating production systems (FPS). This methodology considers different design stages (from preliminary to advanced), integrating the design activities of mooring lines and risers in a single spiral, allowing gains in efficiency and cost reduction. The initial design stages already include a feedback between riser and mooring analyses. The integrity of the risers can be considered in the mooring design by determining their safe operational zones, and therefore, mooring line pretensions can be modified to improve its structural performance. Then, in advanced stages critical design cases for both mooring and risers systems can be identified and rigorously verified by using fully coupled models. The application of the proposed methodology is illustrated with a case study of a typical FPS, representative of the platforms that have been recently considered for deepwater applications. It should be stressed that the methodology described here does not reflect the current design practice of Petrobras. Presently it is merely a proposal that is being studied and assessed; this work comprises the first draft of the methodology, which will be enhanced and consolidated as the result of current and future studies.

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.

scholarly journals Structural Reliability Based Dynamic Positioning of Turret-Moored FPSOs in Extreme Seas

2014 ◽  
Vol 2014 ◽  
pp. 1-6 ◽  
Author(s):  
Yuanhui Wang ◽  
Chuntai Zou ◽  
Fuguang Ding ◽  
Xianghui Dou ◽  
Yanqin Ma ◽  
...  
Keyword(s):  
Mathematical Model ◽  
Structural Reliability ◽  
Oil And Gas ◽  
Single Point ◽  
Line Tension ◽  
Mooring Line ◽  
Dynamic Positioning ◽  
Mooring Lines ◽  
Oil And Gas Exploration ◽  
Time Control

FPSO is widely used during the deep-sea oil and gas exploration operations, for which it is an effective way to keep their position by means of positioning mooring (PM) technology to ensure the long-term reliability of operations, even in extreme seas. Here, a kind of dynamic positioning (DP) controller in terms of structural reliability is presented for the single-point turret-moored FPSOs. Firstly, the mathematical model of the moored FPSO in terms of kinematics and dynamics is established. Secondly, the catenary method is applied to analyze the mooring line dynamics, and mathematical model of one single mooring line is set up based on the catenary equation. Thereafter, mathematical model for the whole turret mooring system is established. Thirdly, a structural reliability index is defined to evaluate the breaking strength of each mooring line. At the same time, control constraints are also considered to design a state feedback controller using the backstepping technique. Finally, a series of simulation tests are carried out for a certain turret-moored FPSO with eight mooring lines. It is shown in the simulation results that the moored FPSO can keep its position well in extreme seas. Besides, the FPSO mooring line tension is reduced effectively to ensure mooring lines safety to a large extent in harsh sea environment.

Numerical Study on Mathieu Instability of DDMS Platform

2010 ◽  
Author(s):  
Binbin Li ◽  
Jinping Ou
Keyword(s):  
Numerical Study ◽  
Mooring Line ◽  
Early Investigation ◽  
Spar Platform ◽  
Mooring Lines ◽  
Natural Period ◽  
Ballast Tank ◽  
University Of Technology ◽  
Mathieu Instability ◽  
Decay Tests

Deep draft multi-spar (DDMS) is an innovative platform which is specially designed for deepwater drilling and production in 2009 by Center for Deepwater Engineering, Dalian University of Technology. The hard tank of DDMS is composed of four columns at corners and a novel moonpool protecting the top tension risers at center. In addition, the top tension and self-weight of rigid risers are provided by air-cans in the moonpool. At the foot of hard tank, the pontoons and horizontal bracing are used to connect the separated columns and moonpool. It is noted that two heave plates are directly integrated with the hard tank in order to reduce the heave response. The middle section consists of 4 columns of smaller diameter which connect the hard tank and ballast tank. The early investigation indicates that the global hydrodynamic and motion behavior of DDMS are similar with Spar platform, and furthermore the heave natural period is close to the half pitch natural period. Therefore the DDMS platform may have possibility to trigger the Mathieu instability which has been validated on Spar platform through the numerical and experimental method. In this paper, a coupled heave and pitch motion equations of DDMS platform are established with accounting the time-varying restoring heave and pitch restoring stiffness. A damping case matrix is generated considering the heave plate damping, mooring line damping and hull drag damping. The damping ratios are identified by free-decay tests. The nonlinear motions under the action of regular waves of different periods and heights are numerically solved by the 4th order Runge-kutta method. The calculational results reveal that the heave damping significantly influences the occurrence of pitch instability, meanwhile the damping contribution of heave plates and mooring lines also play an important role in suppressing the instability. The phenomenon of Mathieu instability is owing to the energy exchange in this paper, and the mechanism of this phenomenon is amply studied as well as 3 different ways of instability are summarized.

Study on Weight Control Methods of Single Point Mooring System of FPSO in Deepwater

2015 ◽  
Author(s):  
Yuan Hongtao ◽  
Zeng Ji ◽  
Chen Gang ◽  
Mo Jian ◽  
Zhao Nan
Keyword(s):  
Weight Control ◽  
Linear Time ◽  
Single Point ◽  
Line Tension ◽  
Coupled Analysis ◽  
Mooring Line ◽  
Mooring System ◽  
Control Methods ◽  
Analysis Method ◽  
Mooring Lines

This paper applies 3D potential theory and non-linear time domain coupled analysis method to analyze motion response of FPSO and dynamic response of mooring line of single mooring system. In addition, respectively to calculate mooring line tension of tension type and composite mooring line type and added buoy in mooring line. There the paper analyze different mooring lines to affect on the weight of single point mooring system of deepwater FPSO. Which expects to provide a theoretical basis for single point mooring system design and weight control.

Optimal Design Methodology for Multi-Component Mooring Systems in Deep Water

2010 ◽  
Author(s):  
Long Yu ◽  
Jiahua Tan
Keyword(s):  
Optimal Design ◽  
Deep Water ◽  
Design Methodology ◽  
Element Model ◽  
Gas Production ◽  
Mooring Line ◽  
Mooring System ◽  
Offshore Platforms ◽  
Mooring Lines ◽  
Restoring Force

Multi-component mooring systems, one of the crucial equipments of offshore platforms, play an important role in deep water oil&gas production because of relative low cost and light weight. A single mooring line can be constructed by combination of wire ropes, chains, fiber ropes, buoys and connectors etc. and provide adequate restoring force at fairlead point of platforms. Although the static and dynamic analyzing approaches for a determined multi-component system have been studied already, it is still hard to design and predetermine an appropriate mooring system that can satisfy the codes with multi-component lines. Referred to the conventional mooring system design method, this paper brings out an optimal design methodology for multi-component mooring systems. According to quasi-static method, at extreme offset position of the platform, an optimization model for designing the multi-component mooring line with biggest tension in deep water has been provided. Then, with the aid of design wave method and morison equation, a finite element model has been used to calculate mooring line dynamics at each fairlead point in time domain. The nonlinear interaction of mooring lines and seabed has also been investigated. Heave and surge of the platform have also been considered. Both 2D and 3D mooring system models have been built to search the interference of the lines and directional influence of environment loads like current and wave. The paper applied this set of analyzing methods and processes into a deep water semisubmersible serving at South China Sea. Compared with the results calculated by other software, the methodology mentioned in the paper got similar result with less weight and bigger restoring force.

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