Insights on the design of free-spanning pipelines

Abstract

The design of free-spanning pipelines is performed with the aim of ensuring their integrity against permanent loads generated by seabed roughness, functional loads induced by internal pressure and temperature, and dynamic loads induced by marine currents and direct wave action. In particular, a load and resistance factored design is applied that focuses on extreme environmental loads, and a fatigue limit state approach is applied as a consequence of free-span dynamics due to vortex shedding-induced vibration and direct wave action. The pipeline free-span scenario can be permanent, when generated by seabed roughness, or characterized by short- to long-term evolution, when generated by seabed mobility and scouring in shallow waters. Free-span analysis is generally a task involving a number of disciplines and should be carried out using a multidisciplinary approach. The paper illustrates various themes related to free-span analysis: (i) free-span scenarios, (ii) characterization of the environment from deep to shallow water related to proper seabed properties, (iii) hydrodynamic load regimes, (iv) pipeline free-span design assessment aiming to reduce overstress and fatigue damage, (v) erodible seabed mobility and local scour, and (vi) some experiences of inspection surveys chosen as representative of a free-spanning pipeline in sandy soils.

Free Span Analysis for Submarine Pipelines

“…Return periods of 1 and 5 years are used to analyze at the installation state in which no pipe has been placed under the operation [25]. e friction coefficient can be ranged from 0.1 to 1 depending on the material on the pipe and soil of the seabed [26].…”

“…Here, D fat-life is cumulative linear breakdown, a is the meeting place of the S-N design curve with log N, m is the negative reverse slope of the S-N curve, K is the number of the stress blocks, n i is the number if the stress cycles in the tension block i, and N i is the number of the failure cycles at the range of fixed stress Δσ. Anticipating fatigue life for the marine pipeline can be obtained using cumulative breakdown resulting from D fat-life and η coefficient [26].…”

“…Strainbased approach Failure mechanics approach Figure 4: Basic approaches to fatigue analysis and design [26].…”

Vibration Detection of Spanning Subsea Pipelines by Using a Spherical Detector

“…The research on subsea pipeline spanning mainly focuses on the mechanical response analyses of the spanning sections under waves and currents [6], [10], [11], and the stability and reliability assessment [12], [13] of subsea pipelines. These studies provide forward guidance for how to safely lay subsea pipelines to avoid spanning and how to predict the spanning existence, but do not address how to directly detect existing spanning for pipeline reinforcement [13]. People have also make many efforts on judging the possibility of pipeline spanning based on computational fluid dynamics, structural finite element analyses, and associated fluid-structure interaction [14]- [16].…”

Advances in fluid mechanics for offshore engineering: a modelling perspective

“…These are, typically (but not only), submerged pipelines, extensively used by the oil and gas industry over an increasing range of water depths and seabed conditions. The paper 'Insight on the design of free-spanning pipelines' [13] proposes a review of the theme and sets the scene for the other more specific papers. Among such papers the analytical investigation in the paper 'A theoretical model of asymmetric wave ripples' [14] is aimed at characterizing the small-scale roughness that influences pipeline stability.…”