Offshore sites in the Arabian Gulf are characterized by the presence of calcareous sediments. Research work on driven piles in calcareous sediments has been developing for over 40 years. Yet, international codes and standards do not provide, exploit or quantify guidance for driven piles in carbonated sediments. Lack of design methods is partly attributed to inability of conventional soil mechanics to predict appropriate engineering parameters in calcareous sediments. Further, the characteristics of the sediments vary between the geographical regions depending on the geological history forming that region. As a result, current industry practice follows a conservative and subjective approach at the mere mention of carbonated sediments. Consequently, reassessment of an existing platform may falsely indicate the need for expensive construction intervention. In this study, we reviewed current practice for assessment of piled foundation in the calcareous sediments of the Arabian Gulf, collated a database of actual pile driving records, developed and implemented a detailed back-analysis procedure and implemented to derive actual pile capacities. The statistics show that the use of a single capacity value, as implied by the deterministic method of codes and standards, is insufficient to describe the various conditions surrounding the as-installed driven piles in calcareous sediments.
Relatively accurate techniques are available to assess structural behavior under given loads, yet the loads themselves remain an estimate based in part on field measurements, in part on professional logic and experience, and in part on trial and error. The design of piled foundations for fixed offshore platforms must consider operating and extreme weather conditions. In the operating condition, the magnitude of live loads on open areas of topside structure is an important consideration. Unfortunately, the design live load intensity that applies to open areas on offshore platforms is not identified in international codes and standards. There does not appear to be any consensus on the value to be adopted in the industry. Some operators suggest the open area live loads need not be considered for pile foundation design, while others stipulate values such as 10 kPa. This is partly due to the variability associated with the different live loads sources. The objective of this study is to obtain a better understanding of open area live loads on offshore platforms and develop a methodology to obtain the long-term and extreme open area live load. A load survey was conducted for the purpose of this study, and a probabilistic analysis was carried out to derive the maximum axial load on piles that is expected during platform lifetime. The results of this study indicate that the use of a single value for the open area live load (OALL) may not be appropriate and suggest appropriate values for Load Resistance Factor Design (LRFD) or Working Stress Design (WSD) methods.
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.
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