Purnendu K. Das

A hydro-structural analysis program for TLPS

Abstract The TLP configuration in this paper comes from the case study of the ISSC Derived Loads Committee I.2 in 1985. Two models, Model 1 and Model 2, are created aiming at developing a computational tool to handle hydro-dynamic and structural aspects together to help the TLP designer. Another objective is to generate relevant information for a non-linear static local stress analysis of TLP components from a dynamic hydro-structural global analysis. Model 2 is developed for LUSAS, a general purpose, well established, FE software, to validate the in-house program DCATLP. Model 1 and Model 2 are purposely made slightly different in the tether modelling so that the non-linear formulations in DCATLP to treat a TLP as a coupled system with the hull and tethers can be verified with a straightforward linear step-by-step dynamic analysis in LUSAS. The code in DCATLP has advanced features to account for some complex aspects of TLPs, such as compliant characteristics. The results from DCATLP can be directly used in the reliability based structural design of TLP components and subsequent optimisation studies.

Energy conservation during a tanker collision

Abstract The energy dissipation on the bow structure is calculated during a “head-on” collision with a rigid body, using finite element analysis. Gerards method ( Gerard, G., 1958. The crippling strength of compression elements. Journal of the Aeronautical Sciences ), an empirical approach, is compared with the results produced by ABAQUS in terms of velocity, energy and penetrating distance. The energy conservation theorem is applied and the contribution of all individual structural members in terms of elastic and plastic energy is calculated. The strain distribution is shown for those members responsible for the water tightness and structural integrity of the structure. The global bending of the upper part of the bow is shown as well as the effect of the total ship mass inertia, phenomena which have not been incorporated into Gerards approach ( Gerard, 1958 ). The final time of rest predicted from both methods is compatible and this validates the reliability of the numerical approach.

FRAME TRIPPING IN RING STIFFENED EXTERNALLY PRESSURISED CYLINDERS

This paper addresses the problem of designing ring frames in externally pressurised cylinders to avoid sidesway tripping. A closed form solution is proposed for the elastic tripping pressure, including the effect of the rotational constraint provided by the shell to the toe of the web as well as the web deformation effect. Such theory compared reasonably well with numerical results given by ABAQUS as well as BOSOR4 and N9E. The differences become unimportant when a column curve is used for inelastic tripping, calculated with a structural tangent modulus approach. A safety factor for preliminary design is proposed based on parametric studies.

Energy conservation during grounding with rigid slopes

In this paper a numerical approach to the grounding problem takes place. It aims to show the contribution of the energy dissipated in the structure due to elasto-plastic deformation. The analytical methods developed until now, neglect this amount of energy, since they are simulating the vessel as a rigid beam. A tanker vessel is modelled with the Finite Element Package ABAQUS and energy conservation during a grounding scenario on rigid slope takes place. The results are presented both analytically and numerically and comparison in the energy quantities is shown and discussed.

An outline of the application of reliability based techniques to structural design and assessment of submarines and other externally pressurised cylindrical structures

The paper outlines the potential application of a general purpose finite element program (ABAQUS) together with reliability based methods in the structural design and analysis of externally pressurised vessels. The approximate closed form solutions validated through linear finite element models are used in reliability based methods, deriving partial safety factors for direct use in design. It is hoped to extend the analysis to the non-linear range in which material non-linearity, shape imperfections and mode interaction effects will be taken into account. Investigations on complex geometries, innovative materials and deeper operation pressures could be guided by such analytical methods, hopefully reducing the number of costly experiments required. These methods could also be used to assess the safety of in-service structures, as well as the evaluation of the most important variables which influence their failure.

A Parametric Study for the Design of Steel Catenary Riser System in Deepwater Harsh Environments

In recent years, offshore reservoirs have been developed in deeper and deeper water environments, where floating production, storage and offloading (FPSO), semi-submersibles, spars and TLPs are considered to be the most economically viable platforms. Steel catenary risers (SCRs) are being considered for these production units in deepwater development such as Northern North Sea. A variety of uncertainties are associated with material behaviour, environmental loading, hydromechanics modelling, structural modelling, and fatigue / corrosion / wear characteristics, especially around hang-off and touch down areas. SCRs used in conjunction with a semi-submersible or a FPSO in deepwater harsh environments present significant design challenges. The large vertical motions at the FPSO or semi induce severe riser response, which results in difficulty meeting strength and fatigue criteria at the hang-off and touch down point locations. To improve the understanding of SCR behaviour and increase the confidence in the design of such systems in deepwater harsh environments, a parametric study was carried out in this paper to deal with the factors that mainly influence the loading condition and fatigue life of the riser. Two cases, one steel catenary riser connected to a semi-submersible and one steel catenary riser connected to a FPSO, were studied and compared. And weight-optimized configurations were applied for both risers. Riflex combined with DeepC was the primary analysis tool used for the long-term response of the nonlinear structure SCR’s simulations, which is high computer time consuming. Hence, the parameters affecting the efficiency and accuracy of the simulations have also been studied during the analysis process.Copyright

Analysis on the Hull Girder Ultimate Strength of a Bulk Carrier Using Simplified Method Based on an Incremental–Iterative Approach

The hull girder ultimate strength of a typical bulk carrier is analyzed using simplified method based on an incremental–iterative approach. First, vertical bending moment is examined by seven different methods. The moment versus curvature curves and the values of the ultimate longitudinal moments at collapse states are determined for both hogging and sagging cases. Secondly, the ultimate strength under coupled vertical and horizontal bending moment is accounted. An interaction curve is obtained corresponding to the results of series of calculation for the ship hull subject to bending conditions with different angles of curvature. It is found that the interaction curve is asymmetrical because the hull cross-section is not symmetrical with respect to horizontal axis and the structural response of the elements under compression is different from that under tension due to nonlinearity caused by buckling. The angles of the resultant bending moment vector and that of the curvature vector are different in investigated cases. The interaction design equations proposed by other researches are also addressed to discuss the results presented by this study.Copyright