Effect of Internal Pressure on Free Spanning Pipelines

Abstract

Free span assessment has more and more become an important part of modern pipeline design. The reason for this is partly that the remaining hydrocarbon reservoirs are located in more challenging places, e.g. with very uneven seabed. Another explanation is that the pipeline design codes a few decades ago did not allow for vibrating free spans, while the modern, state-of-the-art pipeline codes, such as DNV-OS-F101 “Submarine Pipeline Systems” (2007) [1] and its Recommended Practices, opens for long spans that are allowed to vibrate as long as the structural integrity is ensured. By opening for longer free spans significant seabed intervention costs associated with trenching, rock dumping and supporting spans by other means are saved. One of the governing parameters to ensure the structural integrity of free spans is the natural frequency of the span. This is a parameter that the designer can to some degree control by means of moderate seabed intervention, e.g. span support. Since the natural frequency of the span together with the water flow velocity normal to the span determine the vibrations and the cyclic loading it is of vital importance to be able to estimate a realistic value of this frequency. The natural frequency is influenced by several effects. One of them is the effect of the internal pressure. This may represent a challenge since the effect of the pressure is the opposite of what one instantaneously thinks is correct. Quite recently some discussion about the effect of internal pressure on free spans were raised and some experimental data presented that claimed to prove that the way the internal pressure was handled in the DNV-RP-F105 “Free Spanning Pipelines” (2006) [2] is wrong. The intention of this paper is to show how the internal pressure influences on the structural response of free spans, and that the DNV codes and standard non-linear FE software, e.g. Abaqus, handle this effect in an adequate manner.

Influence of longitudinal force and internal pressure on the frequency of free vibrations of an underground oil pipeline

“…For the thinness parameter h/R=1/50, other things being equal, the frequency 21 increases from 14.66 to 31.94 Hz, which corresponds to an increase of 45.9 %. The data on the increase in frequency characteristics are confirmed by the experiment described by the group of Brazilian scientists André Luiz Lupinacci Massa et al [6], and the statement of Olav Fyrileiv [5] about the decrease in natural frequencies with increasing internal pressure is refuted.…”

“…in this case, the effect of a stationary fluid flow on the pipeline wall is taken into account in the normal component of inertia forces X3: (4) Solving equation 3using the assumptions of the semi-momentless theory of cylindrical shells [11,20,21], after transformation, we obtain the differential equation of the pipeline motion in displacements: (5) where u, v, w -components of displacements of the middle surface of the shell, related to the radius R, ϑ2-angle of rotation, p0 -internal pressure in the pipe, ρ -soil lateral pressure coefficient, H -squeezed layer thickness, γ -volumetric weight of soil, Eelastic modulus of pipe material, R -median surface radius, √ -relative shell thickness parameter, μbj -added soil mass per unit of pipeline length, κcoefficient of elastic soil resistance for a pipeline exposed to the action of internal working pressure [14], presented in the form: (6) The resulting system of equations (4) contains four unknown functions of coordinates and time t: u, v, w, and ϑ2. Based on the Fourier method (method of separation of variables), we represent the function w(ξ, θ, t), satisfying the condition of hinged support of the ends of the oil pipeline and periodicity along the circumferential coordinate θ, in the form:…”

“…Taking into account that free vibrations of the shell move according to the harmonic law, we have: (9) where mn -first frequency of free bending vibrations in shape, m, n = 1, 2, 3… Substituting (7) -(9) into equation (5) and equating the coefficients at the same trigonometric functions cos(mθ) for m, n = 1, 2, 3..., we obtain an infinite system of homogeneous linear algebraic equations with respect to the unknown amplitude values bmn of the radial component of the displacement w. The coefficients of the unknowns in these equations will be denoted by aij:…”

Study on pure IL VIV of a free spanning pipeline under general boundary conditions

“…Choi (2001) established a rigorous procedure on the free span analysis of offshore pipelines, and derived the closed form solutions of the beam-column equation for various possible boundary conditions. Fyrileiv (2010) showed the influence of internal pressure on free spanning pipelines. Lou et al (2005) investigated the effects of internal fluid on the CF VIV of free spanning pipelines, and found the resonance frequency shifting to a lower frequency value.…”