Chapter 7 - Limit-State Design of Offshore Structures

Abstract

This chapter discusses limit-state design of offshore structures. The concept of limit-state design is introduced to allow an assessment considering the limit-states such as ultimate limit states, fatigue limit states, serviceability limit states, and accidental limit states. The structure will need to be checked for all groups of limit states to ensure that there are sufficient safety margins between the maximum likely loads and the minimum resistance of the structure. Extreme care is required in the finite element analysis to ensure that the correct load and resistance factors have been applied, particularly when several models are being used and the results are linearly superimposed. This chapter discusses the ultimate limit-state design under the headings ductility and brittle fracture avoidance, plated structures, shell structures, etc. Fatigue limit-state design is also discussed in this chapter. Several levels of fatigue analysis may be performed, including fatigue screening, detailed analysis, reanalysis of welding improvements, reanalysis of design improvements, and reanalysis of design, and welding improvements. The problems that have been experienced with fatigue cracking in marine structures and the extreme difficulties associated with inspections of all types of marine structures ensure that large factors of safety are needed to truly accomplish safe life designs. Because of the extreme difficulties associated with the inspections of marine structures and the high likelihood of undetected fatigue damages, it is not normally reasonable to expect that inspections will provide the backup or defenses needed to assure fatigue durability.