Computer Aided Reliability Based Hydro-Structural Response Analysis of Tension Leg Platforms

Chatterjee, Pratul Chandra (1995) Computer Aided Reliability Based Hydro-Structural Response Analysis of Tension Leg Platforms. PhD thesis, University of Glasgow.

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Abstract

The TLP technology has gained credibility with the offshore industry and the associated engineering community. The TLP concept covers a number of areas where there is a great need for research. The developments associated with the TLP technology are briefly reviewed in Chapter 1. For numerical demonstrations, a TLP model is selected which was originally chosen by the Derived Loads Committee 1.2 of the 1985 LSSC. The principal particulars of the ISSC TLP are discussed in detail. Some important design parameters that influence significantly the configuration of TLPs are outlined in Chapter 1. A complete global analysis of a TLP includes many analytical and empirical methods where some of them are fairly standard but some other may not be well defined. The problem areas involved in the analysis and the future trends in design are also discussed. Chapter 2 examines different components of the environmental loading. Second order effects are included. Drift forces are estimated from simplified analytical solutions. The calculation of external forces described in Chapter 2 are used in the next chapters. Some closed-form expressions are derived that are particularly important for transforming complicated external forces into equivalent nodal loads. A step-by-step procedure combining transformation matrices and results from standard load cases is proposed which deals with complex member loads on a 3-D beam, arbitrarily oriented in space. Chapter 3 describes the development of a rigid body motion analysis program, RBRA. The dynamic equation of motion which takes account of all six rigid body degrees of freedom and the associated coefficient matrices are discussed. A solution technique is proposed which is found quite effective to estimate the linearised rigid body responses. The results from RBRA are compared with the published results from 17 organisations who have used boundary element formulations and diffraction-radiation analysis for their calculations. Chapter 4 discusses the development of a computer program, DCATLP which can bring hydrodynamic and structural aspects together in the dynamic coupled analysis of a TLP. The TLP is modelled as a 3-D frame structure with internal hinges to account for the hull-tether connections. However, main particulars of a TLP are not enough for any structural analysis. For numerical demonstration, the ISSC TLP components are designed realistically with stiffeners to estimate their scantlings. In DCATLP, the global mass and stiffness matrices are stored in skyline arrays. The dynamic equilibrium equations are solved in the time domain. The non-linear time integration algorithm belongs to the Newmark-beta family but it is a modification and combination of a number of existing algorithms. DCATLP can calculate internal member forces in each beam element under the action of environmental loading. The structural displacements, velocities and accelerations at each and every FE node are calculated. The static, quasi-static and dynamic components of the environmental forces are applied simultaneously in DCATLP to include the inertia effects. A different type of model for the ISSC TLP is developed in LUSAS where tethers are replaced by linear springs at each corner. Responses of two different ISSC TLP models (one for DCATLP and the other for LUSAS) under sinusoidal loads are compared. The structural and hydrodynamic responses of the ISSC TLP in a peak storm event are also presented. Chapter 5 presents a step-by-step calculation procedure to find failure probabilities of TLP column structures after assigning appropriate coefficients of variation to the strength and load variables. The calculation of longitudinal and hoop stresses from maximum axial compression, torsion, shear forces, bending moments and hydrodynamic pressure is also shown. The uncertainty modelling is mainly based on the previous work carried out in the author's Department. The failure surfaces for TLP columns are formulated according to API Bulletin 2U and the Model Code of TLP RCC. The chapter also describes some improvements achieved in developing a program BCCNNV, based on the AFOSM method. The program is sufficiently accurate for non-normal correlated variables and it is validated by considering a few classic cases. The main objective in Chapter 6 is to carry out a detailed FE analysis of a part of a TLP structure with the help of the results obtained from a 3-D beam element based global analysis. One ring only stiffened and three orthogonally stiffened cylinders, similar to TLP columns are modelled in LUSAS to compare the numerical predictions with the experimental results available from other researchers' work. To get an initial "feel' for the structure, the buckling loads of the FE models are estimated by eigenvalue buckling analyses. The author also attempts to capture the post-buckling phase of the FE models through rigorous non-linear FE analyses. Chapter 7 contains the final discussion and conclusions and ends with some recommendations for future research work.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Douglas Faulkner
Keywords: Ocean engineering
Date of Award: 1995
Depositing User: Enlighten Team
Unique ID: glathesis:1995-75731
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 18:29
Last Modified: 19 Nov 2019 18:29
URI: http://theses.gla.ac.uk/id/eprint/75731

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