Pipelines are widely recognized for their efficiency in material transport, and dredging operations rely heavily on them to convey sediment. In pursuit of more sustainable dredging practices, reducing the flow resistance within the dredging pipelines presents a promising approach, as the lower resistance directly decreases the energy consumption of the pump.

A previous study by Yun Peng Yan investigated the effect of the convex pattern on the wear rate in dry bulk material transport. The finding showed that the convex pattern reduced wear by inducing dilation in the bed layer, temporarily creating free space that allowed the particles to roll more easily. This rolling mechanism, observed under dry conditions, is particularly interesting to examine in wet environments. If the same mechanism also occurs inside the pipe in wet conditions, it could influence the flow resistance and potentially improve the efficiency of the dredging pipe. Since no prior research has tested convex patterns under wet conditions, the present study investigates how a convex pattern plate installed at the bottom of a pipe affects flow resistance and wear distribution during the transport of a gravel–water mixture.

In this study, experiments were carried out using gravel with an average particle diameter of d_50=3 mm. The convex pattern plate was evaluated against a plain plate, which served as a reference. The findings revealed that, consistent with previous dry-condition studies, the convex pattern induced particle rolling along the bed within the sliding-bed regime. However, this rolling mechanism did not lead to a reduction in flow resistance. Instead, the convex pattern configuration consistently exhibited higher resistance in both the stationary-bed and sliding-bed regimes. This increase in resistance also affected the deposition limit velocity, which increased from 0.9 m/s for the plain plate to 1 m/s for the convex pattern plate.

In the sliding-bed regime at lower flow velocities, abrasive wear was observed in both configurations. However, the wear behavior changed at higher velocities within the same regime. For the plain plate, the most severe wear was concentrated in the joints between the pipe segments, where the surface irregularities intensified abrasion. In contrast, the convex plate showed reduced wear at the joints and top surfaces, as the stationary particles in the bed provided a protective layer. However, at higher velocities, the upper layers of the bed began to move more rapidly and collided with the crests of the convex structures. This transition shifted the wear mechanism from abrasion to impact wear, causing the most severe damage at the tops of the convex patterns. In both types of plates, the segment directly exposed to the incoming flow experienced the heaviest wear.

In general, the anticipated reduction in flow resistance from the convex pattern was not achieved. The installation of convex structures at the bottom of the dredging pipes does not improve the transport efficiency and, in fact, may increase resistance, making the approach unsuitable as a general wear-reduction strategy. Interpreted through Newitt’s theory and the Darcy–Weisbach relation, the higher resistance observed in the 40 mm pipe suggests that this effect could be amplified in larger pipe diameters. This is because the water resistance decreases with increasing diameter, and then the solid effect increases to compensate for the higher mechanical friction introduced by the presence of convex structures. However, the additional resistance measured in this study was relatively minor and the deposition limit velocity increased by only 0.1 m/s. Since this increase is small, the improvement may still be advantageous in critical pipeline sections where wear protection is required. Therefore, a localized application of convex patterns could remain a practical solution.

This study is the first to experimentally show two wave mechanisms regarding density wave amplification with long horizontal slurry transport. There is bed-driven and suspended-driven density wave amplification, in which the grain size determines which mechanism is dominant.

Density wave amplification in hydraulic pipeline transport causes significant risk during operation with the consequences of blockage. Current design methodology for pipeline transport considers mixture velocity and density constant over space and time. However, these conditions are only possible in laboratory circuits where conditions can be controlled carefully. In real-world conditions, concentration varies significantly over time due to the natural dredging process in which a dredging vessel takes slurry from the seabed. Density wave amplification can be differentiated into two different flow categories. Both long horizontal transport and a combination of vertical and horizontal transport. With the first category, there are two main theories that explain the amplification of density waves: 'erosion and sedimentation imbalance' and 'the unstable slip point of the bed'. Here, density wave amplification only occurs in the presence of a bed. 
In the second category, there is one theory called the: 'transient accumulation theory' which is applicable to a combination of horizontal and vertical transport. With this density wave, amplification can occur far above the deposit limit velocity. Mixture velocities change when density waves travel from horizontal to vertical orientation and vice versa. When mixture velocity changes density will change. The influence of grain size, concentration and the centrifugal pump on density wave amplification has not been researched yet.

A test loop has been built with an inner diameter of 46mm and a length of 46 meters. The goal of this laboratory circuit is to investigate the mechanisms that result in the amplification of density waves. Two types of density waves were measured: bed-driven density waves occurring with coarse sediments (Dorsilit 7: d₅₀=1040 μm and Dorsilit 8: d₅₀=619 μm) and suspended-driven density waves occurring with fine sediments (Dorsilit 9: d50=316 μm and Zilverzand: d₅₀=240 μm). With bed-driven density waves, there is fast amplification and multiple sharp waves which can result in areas where no concentration is left. With suspended-driven density waves, there is one smooth wave, and amplification takes multiple loop lengths.

Density waves inhibit the stability of (long) pipelines, which cannot be predicted using current day design tools. Density waves are formed due to an inverse relationship between sand eroding and settling in a pipeline. By designing and building a dedicated flow loop, an experimental study on how the erosion and sedimentation of sand in a pipe is influenced by particle size and concentration is conducted. Moreover, it is determined whether the erosion and sedimentation process can be modeled numerically using existing analytical pick-up functions. This could be applied for predicting density waves in horizontal pipelines.
From the experimental study it is found that at higher concentrations, the caterpillar-like movements of the bed, associated with the sedimentation and erosion unbalance at high concentrations, were observed more and more frequently. The influence of the mean grain size diameter can be characterised as the ability of the mixture to trigger amplifying density waves in the horizontal measurement section. Only the coarser sands used in the experiments were found to experience the sliding-stopping behaviour. Implementing different analytical pick-up functions to simulate the experimental study resulted in sufficiently accurate results. Depending on the pick-up function on how much calibration was required the measurements could be simulated adequately.