Optimum layout of stiffened panels subjected to lateral pressure and inplane loads

Abstract

This thesis has been to develop from the result of project thesis. The main task is now focused on the optimum layout for stiffened panel and girder.

Stiffened panels constitute important structural elements in marine structures, ships and offshore platforms. An example is the pontoons of semi-submersibles. This project analyzes the stiffened plate of the pontoon bottom for an offshore platform (Gjøa platform).

The aim of this thesis is to determine optimal design with respect to weight and costs of stiffened panels for the above platform. A briefly description of the Gjøa platform has been investigated. An introduction of typical loads and characteristic actions used in the design is also discussed. Besides, a review of relevant characteristic resistance formulation given in NORSOK N-004 (DNV-RPC201) for stiffened plates is also performed. Particularly, the theory of PULS code is also described briefly.

The stiffened plate and girder have been analyzed by means of PULS program and DNVRPC201 spreadsheet. With these two programs, geometric parameters which characterizes the stiffened plate are investigated, i.e. stiffener spacings, plate thickness and the stiffener profile. In the present stage of this report, only three different stiffener spaces are considered for comparison of the two programs.

The optimal stiffened panel must be satisfied three parameters of optimum design function, that are maximum usage factor is limited with η=0.9, the weight and the fabrication cost must be always considered during design process.

The optimal design procedure will be divided into two parts with four different stiffener spaces. The weight and the fabrication cost are assessed as function of the plating thickness and stiffener dimension.

The first part will be performed with optimum layout of stiffener and panel dimension; the stiffened panel will be investigated by varying parameters such as plating thickness, stiffener dimension as well as stiffener spacing. In the second part, the girder optimum design shall be performed bases on new optimal stiffened panels have been previously performed.

Buckling assessment of the optimal stiffened panel will be also investigated and the corresponding optimal capacity curves will be investigated in DNV RP-C201 and PULS.

For the DNV RP-C201 the buckling assessment for stiffened plate as well as for the girder is performed by interaction equations. The maximum capacity will be found when the largest utilization ratio found for the four equations is at its minimum.

The current PULS apply six limit state functions for identifying critical conditions in different locations in the panel. A function corresponds to applied loads less than the critical condition in the corresponding point. The ultimate strength is found from the minimum of all defined limit states.

Finally, a new stiffened plate is found by trial and error analysis. The optimal girder and stiffener dimension, stiffener spacing and a new plating thickness are also determined. The objective variables of this optimal design are weight and fabrication cost of the stiffened panel.

With the designed result the conclusions with recommendations for further work are also done at the end of this report. The agreement between DNV RPC202 spreadsheet and PULS is studied. Moreover, a short comparison and discussion about the differences between DNV RP-C201 spreadsheet and program from Aker Solutions Company will be also investigated.