This paper describes the linearized and non-linear motion response of floating structures to regular waves. Special emphasism is laid on taut line moorings and linearized and non-linear response curves are shown for a version of the Aker Tethered Production Platform. The motion esponse of conventional semisubmersibles is also discussed and response curves for the Aker H3 are shown. The linearized frequency domain method of analysis and the non-linear time history method are briefly discussed. An approximate method for solving the nonlinear equations of motion based upon a Newton-Raphson type solution is proposed. This method, which is only valid for near linear systems, is shown to have practical applications for floating structures. 1. INTRODUCTION This paper presents some of the results of a three year research project at Aker Engineering, in collaboration with the Department of Mechanical Engineering, University College London. The tethered floating production platform the TPP - developed in recent years by the Aker Group is well known. Due to the high anchor stiffness, this structure will have different motion characteristics to that of the conventional semisubmersible. It was not apparent that the usual assumptions of linear motion could be applied to a tethered structure. The objective of the research was therefore to develop a computer based analysis of linearized and nonlinear motion response. The program to achieve this has two principal uses:Spectral analysis is the most common approach to dynamic studies of motion response yet it is dependent on the response being linear. A check on the linearity of the system is therefore desirable in order to justify this approach.The program can give the non-linear response for a given position in a given design wave and hence the instantaneous non-linear forces acting on each structural element can be calculated. These forces provide the input to a three dimensional structural space frame program, and the resultant element stresses are thus found. This paper will only deal with the six rigid body degrees of .freedom, but the principles herein could also be extended to structure distortion coordinates. Whilst the work on the tethered production platform provided the initial motivation for this project, the computer program will also handle semisubmersibles in a similar manner. The computer program named SEARESPONS is a module of the system SEACON which is currently being developed by Aker Engineering. SEARESPONS was used for all dynamic analysis of the Aker TPP. This includes; motion response, mooring line design, loads for the structural analysis and offsets for marine riser design. 2. LINEARIZED ANALYSIS 2.1 Linear Forces The first order (Airy) wave theory is implicit in linear analysis to calculate fluid particle accelerations and velocities and dynamic pressures. The corresponding wave forces are calculated by the well known Morison equation (see refs. 1,2,3) which is assumed to be valid as long as the cross-sectional dimensions are less than one fifth of the wave length.
The paper describes a highly effective method for computing the dynamics of the catenary-shaped suspension of flexible hose systems. The method accounts for a number of nonlinearities, it is 3-dimensional and it is performed in the time domain. The paper addresses the analysis method and demonstrates its effectiveness on a sample flexible riser analysis.
Drilling and production in deeper water set new requirements to the design of marine risers. In this paper the most common methods of dynamic analysis of such risers are reviewed and discussed with reference to a set of full scale data. The measurements were taken by CONOCO in 1975 on a drilling riser operated from a semisubmersible. A spectral approach is applied in evaluating these data and the results include standard deviations, response, spectra and transfer functions. The quality of the data is found to be generally good. Some doubt is expressed as to the exact conditions at the top support, but it is still considered that important trends are revealed. In the theoretical analysis four different computer programs are investigated, covering six different methods. The programs employ either linearized frequency domain solution or time integration techneques with varying degrees of non-linearities included and for either regular waves or irregular sea. Most of comparisons with measurements are carried out in terms of linearized transfer functions. The validity of this approach is discussed as part of the evaluation of the results.
A computer based metod for comprehensive analysis of Offshore Loading concepts is discussed. The method differs from much of the previous published work in that all ot he following effects are accounted for:
The paper gives an overview of the analysis procedures involved in analyzing the global motion performance of tension leg platforms such as the Snorre and Heidrun platforms. The Aker Engineering TLP analysis system involves a suite of specially adapted purchased computer programs as well as in house developed programs. These programs are assembled into procedures for efficient and highly accurate analysis of all motion dependent parameters important for TLP design such as:Offset, set-down, air-gap etc. due to wave, wave drift and wind gustRiser and Tether extreme loads and fatigue lifeTemporary phases such as towing, station keeping and tether installationStructural analysis The paper addresses the hybrid panel/space frame model where all radiation and diffraction effects are accounted for and superimposed on stochastically linearized viscous drag which is particularly important to achieve correct damping values for the important slowly varying effects of wave drift and wind gust. Sum-frequency excitations and its effect on tether fatigue life and extreme forces are also discussed. The implementation of coupled TLP/tether dynamic analysis is outlined. INTRODUCTION To the hydrodynamicist, the award of the Snorre TLP Main Engineering Contract represented one of the most challenging involvements that one could hope for in a life time. The compliancy of such a structure, the considerable dynamic amplification of all 6 degrees of freedom resonances as well as its critical dependency on the tethers meant that state-of-the-art analysis techniques had to be stretched to the limit. For some aspects of the global motion performance, techniques only available in research institutions had to be transformed into practical design analysis tools for the first time. Presently, the Aker Engineering hydrodynamics are engaged by Norwegian Contractors, which is another Aker Company, to assist in all major analysis wrt. the Heidrun Tension Leg Platform. Due to the deep draft of the Heidrun TLP. which will be the first concrete floating production platform ever to be constructed, the demand for even further refinements of the hydrodynamics analysis procedures has been pressing. This paper reviews some of the more important adaptations made to the procedures and it outlines how the suite of tightly connected computer programs performs all hydrodynamic and global motion performance analysis tasks required for the TLP hull, tethers and risers. TLP ANALYSIS TOOLS The following is a summary of the major components of the TLP motion performance analysis tools:(available in full paper) GEOMETRY MODELLING All radiation and diffraction data is derived from a wetted surface discretization of the TLP hull, whilst the viscous drag is represented by projected areas in combination with drag coefficients according to the well known Morison equation. This means that the TLP hull has to be modelled by both a panel and a space frame model. All global motion analysis will, however, be performed on a space frame representation of the TLP hull. To achieve a transformation of panel based radiation and diffraction pressures to a space frame Morison type coefficient representation a table of panel numbers versus space frame members must be established.
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