ABSTRACT

Static deflections of submarine pipeline with span segment under self-weight and thermal expansion are analyzed in condition of elastic soil. The closed-form deflection and internal forces functions of both segments are yielded. Taken the static deflection of the span segment for mode shape function, mode shape factor and the first order frequency are calculated by energy balance method. Then, the VIV response at any current velocity can be determined according to DnV codes.

INTRODUCTION

The flow around structures, ranging from clarinet reeds to skyscrapers, always causes destructive vibrations. Such nature phenomena is called flow-induced vibration. Currents also bring offshore platforms crashing into the ocean (Blevins,1977). The interaction of flow and structures forms a coupled nonlinear vibration system. Among flow-induced vibration, VIV is of importance in ocean engineering.

Free spanning of pipelines results from unevenness of sea bed or current scouring. Det Norske Veritas (DnV) issued a special code for free spanning pipeline (DnV, 2002). A lot of researches about free spanning have been carried out (Choi et al., 2001, Reid et al., 2000, Mork et al. 1999,1997, Fyrileiv et al.,1998, Park et al., 1997, and Bryndum et al., 1989), and some regulations are developed (DnV 2000a, 2000b). Park et al.(1997) analyzed static and dynamic free spans of pipelines, and proposed an allowable length of free span. The variation of allowable lengths is examined for specialized boundary conditions, where free spanning is modeled as a beam with transversal and rotational springs at each end.

Considering tension and compressive force, Choi (2001) derived a closed form solutions of the beam-column equation for the various possible boundary conditions. The natural frequency is calculated by energy balance method. Some calculations are to present the sensitivity of the axial forces on the allowable free spanning lengths.

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