Offshore Application Of Structural Stainless Steel:Background To A Major New Design Manual
Authors: Dier A.F. 
Published: October 2008

Recent work in the provision of guidance for offshore structural applications of stainless steel is discussed. Existing data from worldwide sources and new data were assessed in the preparation of the design recommendations. The differences between stainless steel and c-steel behaviour in respect of mechanical properties, local and overall buckling and connections are highlighted. Greater use will be made of cold formed and welded members for stainless steel as the availability of hot produced sections is more limited than for c-steel. This has resulted in a synthesis of design practices for cold formed and welded sections, these practices being usually separated for c-steel.

INTRODUCTION

Stainless steel has been used for over 50 years in construction, mainly in fixings, fasteners and cladding applications. Certain industries, such as the offshore hydrocarbon industry, impose greater demands on materials than met in common construction. These demands relate to durability and corrosion resistance and in many instances stainless steel can provide a maintenance-free, cost effective, structural solution. Stainless steel can also provide exceptional ductility, fire resistance and nonmagnetic properties, all of which may lead to the specification of stainless steel as a structural material in particular circumstances.

The main offshore use of stainless steel is in pipe work, either for hydrocarbon transport, for flare systems subject to low temperature design conditions or for certain seawater systems. However, substantial amounts of stainless steel are specified for non process components such as cable trays and ladders, ventilation louvres, floor panels, fire and blast walls, ladders, walkways and module cladding. Despite these and other structural applications there is a surprising lack of design guidance worldwide for structural application of stainless steel, with the one notable exception of the American code for cold formed sections 1. In order to develop suitable design guidance for offshore stainless steel structural applications, a joint industry project was conducted at The Steel Construction Institute under the sponsorship of eight participating organisations. It was agreed that the guidance should be prepared in a limit state format in which separate load and resistance factors are applied to specified loads and characteristic resistances. The following aspects are included in the resulting design manua12:Cold formed and welded membersClassification of cross-sections and local buckling behaviourResistances of beams, columns, beam-columns and tie membersBeam deflectionsBolted and welded connectionsFabrication aspectsFatigue aspectsA full supporting commentary to the design recommendations.

It was decided to initially concentrate on three grades of stainless steel. The three grades chosen were 18Cr 10N1 0.03C (304L), 17Cr 13Ni 2½ Mo 0.03C (316L) and an austenitic-ferritic duplex grade - 22Cr 5Ni 3M0 0.03C (duplex 2205) on the basis of familiarity, corrosion resistance, mechanical properties, weldability, availability in a wide product range, and formability. However, the recommendations for member and connection resistances are applicable to other austenitic grades.