Stiffener placement optimization for stressed skin topsides

Abstract

In this thesis, an automated buckling check method has been developed and validated for thin plated stiffened structures used in stressed skin topsides.

Thin plated stiffened structures are widely used in, for example, aircraft, bridges and ships. Recently, it has become competitive to use this type of construction to offshore topsides as well. Originally, topsides were built as a lattice structure, mainly for the oil and gas industry. Over the years, it was found that these lattice structures are sensitive to fluid and air leakage. Due to the later development of offshore wind farms with high voltage equipment, topsides were now also required as transformer houses for these farms. This necessitated topsides less sensitive to leakage, using a stressed skin structure.

Stiffened plated structures in compression are prone to fail by buckling. Therefore, each panel must be checked for every possible load case in the lifetime of the structure. There are three methods in which stiffened panels can be checked for buckling: full-scale experiments, FEA and by design code. Full-scale experiments are not cost-efficient because each topside is exposed to many different load conditions. Therefore, the only economically viable methods are FEA and design codes. Compared to non-linear plastic FEA, design codes are most economical and therefore the most cost-efficient choice for larger structures like topsides. However, the design code method yields more conservative results compared to non-linear plastic FEA.
Due to the use of many panels in a stressed skin topside, a demand has arisen for an automated application of the DNV-RP-C201 buckling standard for stiffened plated structures. This automated method opens up the possibility of stiffener placement optimization. This means that the stiffeners no longer need to be modelled in the design. In other words, during the design phase, non-stiffened panels can be modelled. At a later point in time, the optimization method can determine the number and type of stiffeners and stringers needed in the structure to ensure stability according to the DNV-RP-C201 design code.

Buckling analysis is dependent on a large set of variables and design considerations. The developed method sets clear boundaries for the applicability of the method and justifies the choices made.

Design codes have their limitation concerning assumptions which are not close to real conditions. Panels are subjected to uneven, distributed loads. Design codes provide rules for linear distributed loads over non-stiffened panels, but not for stiffened panels. For stiffened panels, because of the effective width method, an average stress distribution over a plate-stiffener is assumed. It was unclear whether large stress distributions over plate-stiffeners would cause a degrading effect on their ultimate resistance. Therefore, a validation study has been performed for uni-axially loaded stiffened panels. Non-linear plastic FEA has been used to determine the ultimate resistance.

From these “numerical experiments”, it can be concluded that the effective width method can be applied in most cases of stiffened panels. However, in the case of a plate slenderness between 3 and 4.28, large stress distributions can have a degrading effect on the ultimate resistance of the plate stiffener. Therefore, for plate of such slenderness, the maximum of a stress distribution should not exceed twice the minimum.

The stiffener optimization method developed in this thesis allows engineers to design stability-governed structures without modelling each individual stiffener. This enhances design flexibility and majorly simplifies the FE model. Later, the method can quickly generate a stiffener placement optimum, which would not be feasible by hand.