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An experimental investigation was performed to study a specific axial crush configuration response of steel, square box components under quasi-static testing conditions. For a specific cross-sectional geometry/fabrication process, test specimens were obtained from commercially produced, welded tube lengths of ASTM A36 and ASTM A513 Type 1 plain low-carbon steels and AISI 316 and AISI 304 austenitic stainless steels. Removable grooved caps were used to constrain tube test specimen ends, and collapse initiators in the form of shallow machined grooves were used to control the initial transverse deformations of the test specimen sidewalls. The progressive plastic deformation for all of the test specimens was restricted to the prototype configuration response (fold formation process and the corresponding axial load-axial displacement curve shape) of the symmetric axial crush mode. Crush characteristics were evaluated and, for each material type, observed differences were less than 7% for maximum and minimum load magnitudes and less than 2% for energy absorption, displacement, and mean load quantities in both the initial phase and the secondary folding phase cycles. Overall, results of the study indicate that for a significant range of material strengths, a controlled and repeatable energy absorption process can be obtained for commercially produced steel box components undergoing symmetric axial crush response. Published by Elsevier Ltd.

The purpose of this study is to explore the conditions in which trenches form beneath oscillating cylinderssuch as pipelines, cables or idealised chains-close to the seabed. Experiments are conducted by oscillating a circular cylinder in a direction normal to an initially flat sandy bed. Across a relatively wide parameter space, the transport patterns and trench geometries reveal three transport regimes that are linked to vortex dynamics and depend primarily on the ratio of oscillation amplitude to cylinder diameter (KC number). For KC 4 sediment motion results in bedload transport that is symmetric about the cylinder centreline. This leads to the formation of two parallel trenches with a prominent ridge forming directly beneath the cylinder. For 4 KC 9 sediment motion occurs via localised transport events, which are associated with the motion of vortices shed from the cylinder. These transport events are irregular but occur on both sides of the cylinder and lead to the formation of a symmetric trench geometry. For 9 KC 12 the sediment motion is characterised by localised transport events and asymmetric bedload transport driven by overall vortex dynamics. In terms of trench size, the maximum (equilibrium) depth is found to increase with KC and a mobility number (ψ) defined in terms of the maximum cylinder velocity. The initial rate of trench development also increases with KC number and ψ, with an additional dependency on the cylinder Reynolds number. The cylinder motions required to initiate trenching are predicted well using continuity arguments and an oscillatory boundary layer assumption, provided the KC number and minimum gap between the cylinder and the bed are relatively small. The findings in this study provide insight into the mechanisms and prediction of trench formation. In particular, this study reveals that significant trenches can form in sandy seabeds solely due to fluid flow induced by pipeline/cable/chain motion without direct seabed contact, which has implications for structural fatigue.

Estimating the horizontal rate of scour propagation along a submarine pipeline is a key step in estimating changes in the pipelines burial state and on-bottom stability due to sediment transport. However, whilst recent work has been undertaken to estimate the horizontal rate for non-cohesive uniform sands in steady current, in field conditions currents can vary in time and the sediment can be fine grained and exhibit very different erosion properties to sand. As a first attempt to account for these complications, in this paper we present results from a series of experiments designed to measure the rate of scour along a model pipeline in time varying currents and in a fine grained sediment. The scour experiments are also supplemented by erosion testing, which indicate that the erosion resistance of the fine grained sediment is larger than that predicted by the well-known Shields curve. Based on the scour experiments, it is found that in time-varying currents the scour rate can be predicted using an amalgamation of the results obtained for steady current conditions; this is a convenient result because theoretical predictions already exist for the scour rate in steady current conditions. In the fine grained sediment experiments, it is found that the horizontal rate of scour is much lower than that predicted by existing theoretical models that assume a non-cohesive sandy seabed. To provide an improved estimate of the horizontal rate of scour, a new theoretical model is introduced that relates the horizontal rate of scour to the measured erosion properties of the sediment. This new model is found to agree well with the experimental measurements. Although further experimental testing is recommended, in combination, it appears that these results may be used to better estimate the horizontal rate of scour in both time-varying and fine grained sediment.