The Use of Implicit Safety Factors in the Reassessment of Existing Platforms in the Arabian Gulf

Abstract

Current engineering practice provides a safety level in the design of structures through the use of explicit safety factors (ESF), which consist of resistance factors (for working stress design or WSD) and/ or load factors (for load resistance factor design or LRFD). The WSD factors have been derived by experience, judgment and observation of actual behavior of existing structures, while LRFD factors were calibrated from WSD factors to yield, on average, similar safety/ reliability levels. In both methods, the proportioning of each element of a structure, rather than performance of the whole structure, is addressed. A platform is considered “unsafe” if ESF are consumed, which may be due to significant increase in the loading and/ or deterioration in the resistance. However, the presence of other safety factors, termed implicit safety factors (ISF), provides local and global safety levels that could considerably increase ESF, and may be utilized to avoid structural intervention, or at least limit its extent. The ISF are, in essence, available defenses for an existing structure, which contribute in a certain way to enhance the safety level. The recognition and exploitation of ISF may result in avoiding expensive construction intervention and bring about economical benefits without compromising the safety levels implied by design codes. The benefits extend to the decision-making process related to inspection, maintenance and operation of an existing structure. For a new design, the utilization of ISF may reduce structural weight and subsequently procurement, fabrication and installation costs. This paper reveals ISF and presents the basis of a development aiming at a method to account for their effect on the deterministic formulation of a code specific to the Arabian Gulf.