Subsea pipes under high-mass low-velocity impacts
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Subsea steel pipes are often used to form networks for transporting oil and gas over large distances. Such pipes can potentially be subjected to actions characterised by high loading rates and intensities stemming from accidental loads caused by high-mass low-velocity impacts. In order to ensure that such networks can continue to operate after being subjected to such extreme loading conditions, it is essential that the behaviour of the pipes is characterised by a certain level of resilience. The short duration and high intensity that often characterises impact loads can potentially result in large strain-rates being exhibited within the pipes. To study the effects of the loading-rate on the material behaviour of steel and identify the causes that trigger the experimentally observed shift in specimen behaviour with increasing loading rates compared to that established under equivalent static testing, a review of the relevant experimental evidence is carried out. A review reveals that the specimen behaviour is significantly affected by the developing inertia forces and the interaction with the experimental setup. This suggests that the available test data describes structural rather than material behaviour, thus raising concerns regarding the validity of current practices to employ such data for the development of constitutive models capable of predicting material behaviour under high loading rates. A numerical study is carried out investigating the behaviour exhibited by steel pipes under impact loading, accompanied by a limited number of drop-weight tests. The numerical predictions, which are validated against relevant test data reveal that number of parameters associated with the characteristics of the impacting object, the geometry and the support conditions of the pipes, the level of axial loading as well as the level of internal and external pressure imposed onto the walls of the pipes can significantly affect, often detrimentally, the exhibited behaviour under impact loading. Existing assessment methods employed in practice for predicting the level of damage sustained by pipes during impact do not accurately consider the effect of the above parameters. As a result questions rise concerning their ability to realistically predict the level of damage sustained by such pipes under impact. The numerical predictions are presented in the form of simple diagrams quantifying the individual and combined effect of the above parameters on the level of damage sustained by the pipes when subjected to impact. The latter predictions can potentially form the basis for the development of more advanced analysis methods suitable for practice and leading to the development of more effective design solutions capable of safeguarding the intended level of resilience required to characterise the behaviour of subsea pipes. Finally, it is shown that the use of coatings, constructed from reinforced concrete or engineered cementitious composites, can potentially further reduce the level of damage sustained by pipes due to impact loading, however, further – more detailed – studies are required in order to accurately quantify these benefits.