Nonlinear Finite Element Analysis of Spar Platform

Ocean Engineering, 2017

Several advanced floating structures have been proposed and developed with varying cost effectiveness and productiveness in deep water exploration. Among them, Spar platforms have been accepted as an efficient platform for the exploration. Many research works have been conducted on floating structure but a few on Spar platform. Nonlinear dynamic analysis of a 3D model of floating Spar platform structure is a resourceful tool to predict the responses, where the main body of the Spar hull and mooring lines are considered as an integrated coupled system. To define accurately the interaction between the Spar and mooring lines, coupled dynamic analysis was found to be appropriate for studying responses in the deep sea. Numerical simulation and motion analyses were carried out with the ABAQUS/AQUA. The responses of Spar platform were extracted and evaluated in time histories along with Response Amplitude Operator (RAO). The behaviours of coupled Spar platform have been investigated under real sea environments for increasing water depth to ultra-deep together with the load variability employing sea current for surge, heave, pitch and mooring tension responses. Motions show the consistency in the behaviour of Spar platform responses. Surge response indicates the static offset of the platform due to the static current force under wave plus current. The current force compresses oscillations and reduce heave and pitch magnitude. For larger water depth the platform responses reduce significantly due to the increased damping of mooring line.

Ships and Offshore Structures, 2016

A coupled numerical analysis has been carried out for the structural responses and motion behaviour of a classical spar structure subjected to irregular waves represented by JONSWAP (Joint North Sea Wave Project) spectrum. The motion of the floating body is restrained by the four catenary mooring lines with the generation of tension due to change in their nonlinear profile. The surface of the spar is represented by hydrodynamic pressure panels while cables are discretised using a series of Morison elements. A comprehensive sensitivity analysis under two sea depths is carried out by changing (1) the length of mooring lines, (2) the vertical position of fairlead point, (3) incident angle of long crested unidirectional single spectrum, and (4) the number of short crested sub segmented spreading spectra. The effect of second-order hydrodynamic loading on the structure is also taken into account in the analysis procedure by using quadratic transfer function.

2013

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.

2011 National Postgraduate Conference, 2011

Various types of offshore structures have been designed since the beginning of oil and gas discovery. They are conventional fixed platform, compliant tower, tension leg platform, spar, and semi-submersible. The oil and gas exploration is now focusing in the deepwater regions as the natural sources from the shallow water regions are facing depletion. In Malaysia, further studies have to be done in order to develop the technology for oil and gas exploration in deepwater. It is well known that the environmental forces are the main sources of lateral loading acting on offshore structures. When the waves propagate towards the large structure, the existence of the currents in the ocean changes the characteristics of the forces acting on it. So, it is important to know the characteristics of the wave-current interaction on the structure. Some possible interaction mechanisms between waves and currents include surface wind stress, bottom friction, wave climate, wave field, depth and current refraction and modulation of the absolute and relative wave period. The two basics methods of dynamic analysis for offshore floating platforms are the frequency domain and time domain analysis.

academicjournals.org

Exploration of oil and gas resources has been accelerated towards deeper waters due to depletion of their reserve in shallow water depth. Malaysian sedimentary basins signpost the existence of these energy sources below its sea-bed. In deep sea depth, traditional fixed types of offshore structures to explore these resources have become incongruous and they have resorted to new configurations. Spar platform is treated as a cost-effective and efficient compliant floating platform in this locale for drilling, production, processing and storage of ocean deposits. This study deals with the oil and gas eminence in Malaysian sea along with structural response behaviors of spar platform subjected to hydrodynamic

Journal of Marine Science and Technology, 2018

The investigation of the interaction of floating structures with very high waves, also known as freak or rogue waves, is of crucial importance for the analysis of their ultimate design conditions. The representation of such waves is usually achieved through computationally intensive numerical simulations. In this paper, a deterministic approach is proposed, to represent extreme wave groups in the space-time domain. The free surface profile for a Gaussian sea is obtained by means of the Quasi-Determinism theory, and the corresponding dynamic response of a spar-type support for floating offshore wind turbines in parked rotor conditions is analyzed. The Quasi-Determinism theory and the nonlinear equation of motion of the structure are coupled through an in-house time-domain numerical code. Wave forces and structure motions in surge, heave and pitch are obtained. A parametric analysis is carried out to investigate the effects of the criteria used for the definition of the extreme wave, its position of occurrence and the initial conditions in terms of body motions. The results obtained give a clear insight into the physics of the wave-structure interaction phenomenon for extremely high waves in Gaussian seas and allow to identify a few load combinations, corresponding to the severest wave conditions for the floating structure. Keywords Extreme waves in Gaussian seas • Quasi-determinism theory • Floating offshore wind turbines • Spar

Journal of Civil Engineering and Management, 2013

Spar platforms are treated as cost-effective and resourceful type of offshore structure in deep water. With increasing depth there are significant changes in its structural behaviour due to coupling of spar hull-mooring line along with radical influence of mooring line damping. So these phenomena should be precisely counted for accurate motion analysis of spar mooring system. In present study, spar platform are configured as a single fully coupled integrated model in ABAQUS/AQUA. Non-linear dynamic analysis in time domain is performed adopting Newmark-β automatic time incrementation technique. Non-linearities due to geometric, loading and boundary conditions are duly considered. Displacement and rotational responses of spar and mooring tensions are obtained during long-duration storm. spar responses get significantly modified and mean position of oscillations gets shifted after longer wave loading. The surge, heave and pitch responses are predominantly excited respectively. The ener...

Oil and gas exploration have 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. The number of 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 prove the effectiveness and success of such platforms in deepwater conditions. For platforms in deeper waters, mooring lines generally contribute significant inertia and damping due to their longer lengths, larger sizes, and heavier weights. Accurate motion analysis of platforms in deep waters requires that these damping values be included. 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 and mooring lines. Coupled analysis, which includes the mooring lines and platform in a single model, is the only way to capture the damping from mooring lines in a consistent manner. The present coupling is capable in matching the forces, displacement, velocities and acceleration at the fairlead position along with all possible significant non-linearities. The output from such analyses will be platform motions as well as a detailed mooring line response. The computational efforts required for coupled system analysis considering a complete model including all mooring lines are substantial and should therefore mainly be considered as a tool for final verification purposes. In actual field problems hydrodynamic loads due to wave and currents act simultaneously on Spar platform and mooring lines. In finite element model, the entire structure acts as a continuum. This model can handle all non-linearities, loading and boundary conditions. The commercial finite element code ABAQUS/ AQUA is found to be suitable for the present study. The selected configuration of the offshore Spar platform is analysed under the regular wave loading and its structural response behavior in steady state is studied. The response of Spar-mooring system is obtained after 1 and 3 hours of storm. The result shows the effect of mooring line damping due to fully coupled analysis of Spar-mooring system.

KSCE Journal of Civil Engineering, 2017

Spar platform is a type of floating structure utilized for oil and gas exploration and production in deep and ultra-deep waters. Coupled analysis in present study considers Spar mooring lines as an integrated system. It incorporates the contribution of drag and inertia forces of mooring lines. A rigid classical Spar cylinder connected by four tensioned catenary mooring lines has been modeled and analysed using finite element approach. Mooring system has been modeled as hybrid beam elements. The Studies cover surge, heave, pitch and mooring line tension responses, highlighting the coupling effect. The non-linearities present in the coupled system leads to irregular behavior under regular sea states. The coupled model noticeably confirms its importance in terms of hydrodynamic damping on mooring system. There is a key variance in behavior found with and without drag and inertia forces on mooring system.