Wave-induced fatigue of multi-span pipelines

doi:10.1016/S0951-8339(99)00009-X

Marine Structures

Volume 12, Issue 2, February 1999, Pages 83-106

https://doi.org/10.1016/S0951-8339(99)00009-X Get rights and content

Abstract

Free spanning of the multi-span pipeline is an important subject for design of pipeline in uneven seabed. The seabed intervention costs are largely influenced by pipeline spanning design, which includes assessment of trawl pullover response, vortex-induced vibrations and wave-induced fatigue. The objective of this paper is to develop a rational design methodology for the determination of the free span lengths based on the multi-span pipeline in-line fatigue assessment. Following the summary of the procedure, a detailed mathematical model for the free span movement and its analytical closed form solution are developed. The fatigue damage models are detailed both in time domain and frequency domain approaches. A numerical example is presented to illustrate the technical models.

Introduction

Excessively long spans of submarine pipelines may cause fatigue of welded joints. Remedial seabed intervention is often used to ensure that pipeline lengths below acceptable limits. Because of the high cost of seabed intervention, the spans have to be carefully designed with respect to vortex-induced vibrations and wave-induced fatigue.

A considerable amount of work has been conducted to develop methods for assessment of Vortex-Induced Vibrations (VIV) of free span pipelines, see [1], [2] Pantazopoulos et al. [3] outlined approximate analytical solutions for VIV of free span pipelines based on the Fourier transformation. Anfinsen [4] reviewed the important parameters in the free span pipeline design. Extensive experimental studies [5], [6] flow-induced vibration and free span support conditions have been performed together with analytical studies in developing the rational design procedure [7]. Recently, Crossley et al. [8] summarized the generalized force model for VIV of pipeline spans and Bryndum et al. [5], [13] used this force model in the pipeline span design.

An official DNV guideline (draft version) for free spanning pipeline was issued in Dec. 1997 and will be issued formally as DNV [9], [14] Guideline No. 14. The guideline provides rational criteria and guidance on fatigue design of free spanning pipelines subjected to combined wave and current loads.

Given these technical developments, methods for assessment of pipeline free span have been tremendously improved in recent years. However, there is a lack of comprehensive mathematical formulation for pipeline span wave-induced fatigue.

The objective of this paper is to develop a rational design methodology for the determination of the free span lengths based on the multi-span pipeline in-line fatigue assessment. Following the summary of the procedure, a detailed mathematical model for the free span movement and its analytical closed form solution is developed. The fatigue damage models are detailed both in time domain and frequency domain approaches. A numerical example is presented to illustrate the technical models.

Section snippets

Fatigue of free-spanning pipelines

Fatigue due to vortex-induced vibrations and wave loads are considered in two directions:

•
in-line direction (horizontal plane, in-line with the wave direction),
•
cross-flow direction (vertical plane, perpendicular to the wave direction).

The following analysis models are used:

•
Amplitude response models
•
Force models

An amplitude response model may be applied when the vibrations of the free span are dominated by vortex-induced resonance phenomena, while a (Morison) force model may be used when the free

Fatigue damage assessment procedure

In the development of the wave-induced multi-span fatigue models, the following assumptions are made:

•
The mass, the axial force, the stiffness and the structural damping are constant in time and along the span.
•
The mean (main) wave direction is assumed to be perpendicular to the span and all the energy is assumed concentrated around the main wave direction.
•
The developed time-domain fatigue model can include a statistical distributed current velocity, or a fixed current velocity, the frequency

Example applications

A computer program was developed to facilitate the mathematical model development earlier in this paper. The program has been validated by comparisons with fatigue calculations by Mørk et al. [7], Mørk and Fyrileiv [17] and Fyrileiv et al. [10] for the following real-case studies.

The span is 60 m long and the boundary conditions are pinned-fixed, current is neglected. The fatigue damage is calculated with a fixed added mass coefficient. The fatigue damage is calculated for one span for two

Summary and conclusions

Following conclusions can be summarized:

The paper developed a fatigue damage methodology to determine the span length of multi-span pipelines in wave dominant situation. This paper presents the time-domain approach, frequency-domain approach, and the hybrid time/frequency-domain approach to evaluate the fatigue damage of free span pipelines.

The numerical example illustrates that there is certain difference between the time-domain fatigue analysis and frequency-domain fatigue analysis. This is

References (18)

Tura F, Dumitrescu A, Bryndum MB, Smed PE. Guidelines for free spanning pipelines: the GUDESP project. Proceedings of...
Tura F, Bryndum MB, Nielsen NJR. Guideline for free spanning piplines: outstanding items and technological innovations....
Pantazopoulos MS, Crossley CW, Orgill G, Lambrakos KF. Fourier methodology for pipeline span vortex-induced vibration...
Anfinsen KA. Review of free spanning pipelines. Proceedings of ISOPE'95,...
Bryndum MB. Application of the generalized force model for VIV in pipeline span design. Proceedings of OMAE'98,...
Bruschi R, Vitali L. Large amplitude oscillations of geometrically non-linear elastic beams subjected to hydrodynamic...
Mørk KJ, Vitali L, Verley R. The MULTISPAN project: design guideline for free spanning pipelines. Proceedings of...
Crossley CW et al. Generalized force model for VIV of pipeline spans. Proceedings of OMAE'98,...
Det Norske Veritas. Design guideline for free spanning pipelines. Report No. 97-3176,...

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