Reliability of Floating Structures: Extreme Response and Load Factor Design
Authors: Winterstein Steven R. , Kumar Satyendra 
Published: October 2008

The reliability of a floating offshore platform against extreme offsets is studied. Methods are illustrated to perform more refined analysis of extreme loads and load effects. These include the effect of nonlineardiffraction under random wave excitation. They permit inclusion of randomness in significant t wave height H8, in peak spectral period Tp, given H8, and also in the extreme load effect given both H8, and Tp.

Numerical effects are demonstrated by applying these methods to a specific floating structure: a deep-draft spar buoy. Design of the spar has been considered in two deep-water sites, one in the GuJf of hfexico and another in the Northern North Sea. Appropriate joint contours of significant t wave height and peak eriod are developed, and used to develop load andresistance factors for each of the two sites.

INTRODUCTION

Floating structures are an attractive option to support oil and gas production in deep water. They promise relatively economic designs, with little sensitivity to increases in water depth, Corresponding challenges arise from complex fluid-structure inter- actions, which lead to basic hydrodynamic questionsof estimating applied forces and restoring properties such as damping. The novelty of these structures suggests the need to study their reliability, and considerpractical LRFD (load- and resistance-factor design) procedures for such structures. These would parallel recent LRFD guidelines for fixed structures (API, 1993).

We consider here one specific floating structure: a deep-draft cylinder known as a spar buoy (Figure 1). This concept has gained considerable industry interest. We focus on a particular spar buoy (Figure 2), which serves as the "theme structure" of the NSF Center for Offshore Technology Research (OTRC) in Texas. This spar has draft h=198m, diameter d=42.7m, and a center well to protect drilling and production risers. We specifically study its reliability against extreme surge offsets, based on state-of-threat nonlinear diffraction force models (Kim and Yue, 1989). Reliability consequences, and necessary loadand resistance factors, are considered for typical sites in the Gulf of Mexico and in the North Sea.

Load Factor Design

In load- and resistance-factor design, nominal loads and resistances are scaled by separate factors, ?L and ØR, to achieve desired reliability levels. The use of several factors, rather than a single multiplier on the net safety margin, aims to achieve more uniform reliability over cases where loads show not ably more uncertainty than resistances, as well as other cases where they may show comparable variability or even less. Similarly, different factors may be applied to separate load contributions which show different variability, Examples include separate factors for dead and live loads, or the separate factors recently suggested for static, wave-frequency, and slow-drift loads on floating structures (Banon et al, 1994).

Here we consider the extreme offset of the spar under wave loads only. In this case slow-drift forces dominate, so we retain only a single load factor ?L L along with a resistance factor ØR.

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