Structural Response of Pipeline Free Spans Based on Beam Theory

Abstract

One of the main risk factors for subsea pipelines exposed on the seabed is fatigue failure of free spans due to ocean current or wave loading. This paper describes how the structural response of a free span, as input to the fatigue analyses, can be assessed in a simple and still accurate way by using improved beam theory formulations. In connection with the release of the DNV Recommended Practice, DNV-RP-F105 “Free Spanning Pipelines”, the simplified structural response quantities have been improved compared to previous codes. The boundary condition coefficients for the beam theory formulations have been updated based an effective span length concept. This concept is partly based on theoretical studies and partly on a large number of FE analyses. The updated expressions are general and fit all types of soil and pipe dimensions for lower lateral and vertical vibration modes. The present paper focus on estimation of simplified response quantities such as lower natural frequencies and associated mode shapes. Hydrodynamical aspects of Vortex Induced Vibrations (VIV) are outside the scope of this paper.

Modal response of short pipeline spans on partial elastic foundations

“…If a pipeline is deformed statically, the curved system responds differently to dynamic loading than a perfectly straight configuration. Most notably, the curvature will couple axial and transverse degrees of freedom, which has a stiffening effect, increasing both the modal response frequency and the associated modal stresses (Fyrileiv and Mørk, 2002;Vedeld et al, 2013). In cases where the deflection is more extreme, the first symmetric mode shape becomes suppressed, and the first anti-symmetric mode attains the lowest frequency (Fyrileiv and Mørk, 2002;Vedeld et al, 2013).…”

“…The current revision of DNV-RP-F105 (2006) includes a recommendation for calculating approximate modal response which is taken directly from the formulation proposed by Fyrileiv and Mørk (2002). Their formulation is highly accurate for a large number of relevant free span configurations and pipeline operational conditions.…”

“…Hobbs (1986) solved for the frequency of a free span resting on soil at each end, where the soil was modeled as a linearly elastic foundation. Fyrileiv and Mørk (2002) expanded on the theory of Hobbs by including an approximation for the effect of axial forces, based on work by Hetenyi (1946), and also considered the stiffening effect of static deflections (Fyrileiv and Mørk, 2002;Vedeld et al, 2013). Fyrileiv and Mørk (2002) adjusted their approximation by curve fitting to the results of a large number of FE analyses.…”

“…A highly accurate semi-analytical method (Vedeld et al, 2013) is applied to create response surfaces in the dimensionless solution space, and various surface fitting techniques are applied to create simple closed-form analytical expressions for these response surfaces. Extensive parametric comparisons are made between the novel analytical solutions and solutions obtained with the semi-empirical formulae of Fyrileiv and Mørk (2002).…”

“…Their formulation is highly accurate for a large number of relevant free span configurations and pipeline operational conditions. However, the semi-empirical formulations of Fyrileiv and Mørk (2002) were mainly developed with typical offshore pipeline free spans in mind. As a result, the lower span length limit in their studies was L s /D s at 30, where L s is the span length and D s is the outer steel pipe diameter.…”