While the offshore pipelines are designed to withstand an extensive range of combined loads such as the internal/external pressure, axial force and bending moment, the residual strength of these structures accounting for impact induced damage is one of the most detrimental situations in limit state design. The impact might be caused by machinery such as trawl gears or anchors, which could introduce mechanical damage. One specific case is the interaction between the pipelines and fishing trawl boards when they are towed underwater. Depending on the pipe material properties and the impact forms, mechanical damage could be a dent, metal loss or even a crack. Therefore, the aim of this paper is to investigate the residual strength of metallic pipelines with impact induced dent or combined dent & crack subject to dominated bending moment with a combination of axial force and internal pressure under a certain level of absorbed energy. The nonlinear finite element method is deployed and the numerical models accounting for damage are developed. Simulation results are compared with each other and the most unfavorable damage pattern of the pipelines is determined. Research found that the combined dent & crack damage significantly decreased the residual strength of pipe structure under combined loads, and single dent damage had slight effect on pipe residual strength.
It has been estimated that the failure of pipelines resulted from external interference ranged from 70% in Europe, and more than 53% of crude oil pipe structure failures were caused by mechanical damage (AER, (2013); Lancaster and Palmer, (1996)). Among external interference, a typical scenario (see from Fig.1a) is the impact between pipelines and fishing trawl boards when they are towed underwater (DNV, 2010). It will inevitably cause mechanical damage such as a dent, a crack as well as metal loss, which might introduce harmful effect on the residual strength of pipe structures. Meanwhile, within different pipe phases such as installation and operation, pipelines are normally subject to combined loads but with a variation of dominated load. Within operation stage, bending moment could become dominated because of the geological motion of seabed such as earthquakes, mudslides. Therefore, it is essential to clarify the residual strength of damaged pipelines subject to bending moment with a combination of axial force and internal pressure.
The interaction between pipelines and trawl boards can be divided into two stages including initial impact, pull-over (DNV, 2010). The initial impact typically lasts several hundredths of second. Local pipe damage might be introduced from this stage. The second stage is the pull-over, which can last from about 1 second to some 10 seconds. Global response will be mainly introduced from this stage. Besides, another rare but existing scenario is the so called "hooking". The pipelines might suffer force as large as the breaking strength of warp lines when the trawl gear is "stuck". In this paper, we only concern local impact induced damage from the initial impact stage.