Investigation of hydrodynamic forces on articulated concrete block mattresses in fluid flow from various horizontal directions

This thesis investigates the optimisation of hydrodynamic stability for Articulated Concrete Mattress (ACM), to improve the cost efficiency of ACMs currently used in industry and enhance ACM stability calculations. ACMs are mainly deployed for subsea structure stabilisation and scour protection. They are most commonly used for subsea pipelines, but are also used in a range of other offshore and coastal applications. ACMs are currently sized in industry using roughly estimated hydrodynamic coefficients and large safety factors to account for the uncertainty in the coefficients. The large safety factors cause drastically increased stability requirements and therefore increase the required cost for each mattress. The aim of this thesis is to decrease the required safety factors by investigating the hydrodynamics of ACMs using novel investigation methods to find more accurate coefficients for industry stability calculations. This thesis also compares various existing block types to determine how variations in ACM block shape affect hydrodynamic stability. Additionally, this thesis will recommend shape optimisations to increase mattress stability and therefore increase cost efficiency. The most effective method for investigating hydrodynamic coefficients is an experimental investigation using a scale model or a full-scale model. The stability of ACMs was investigated through several full-scale experimental investigations at the Australian Maritime College’s (AMC’s) Circulating Water Channel (CWC). The experimental investigations are split into three separate investigations. The first investigation determines the mattress failure location for all incident flow angles, thereby reducing the variables that are needed to be investigated in further testing. The second investigation compares several existing block types to determine their stability and allow for further optimisation of ACM block shapes and sizes. The third investigation analyses the flow around current ACMs and recommends block optimisation along with current shape strengths and weaknesses. The first two sections involve the acquisition of hydrodynamic coefficients with respect to variations in incident current angles. The experimentally acquired hydrodynamic coefficients are then input into a static stability calculation to determine the failure mechanism, location and velocity. To accurately define the failure mechanism of an ACM, the mattress failure location is first investigated. Literature shows that the leading-edge row has far lower stability than any other row of blocks within the ACM. However the failure position within the leading-edge row is not thoroughly investigated. This thesis compares the corner block to the centre block in the leading-edge, concluding that the centre block has lower stability for almost all flow angles and is therefore the earliest mattress failure location in the leading edge row. Through this comparison, further investigations are made more efficient by leaving only the centre block as the necessary point of investigation. Due to this narrowing of the subject matter, several extra ACMs could be investigated. The second stage of this thesis investigates the comparison between three different ACM block types, the 300-series, the 400-series and the 500-series, which are variations of the same base block type. While the 300 and 500-series blocks are symmetrical about the horizontal plane, the 400-series has the bottom half of the 300-series and the larger top half of the 500-series, allowing for simple comparisons between the different block types. From the investigation, it is found that the 400-series has higher stability than the heavier 500-series. Due to its size, the 500-series costs more than the 400-series. Therefore purely based on hydrodynamic stability, the 500-series is obsolete when compared to the 400-series. To improve upon the 400-series’ shape efficiency, it is important to analyse which fluid mechanisms cause the 400-series’ high stability. The third section of this thesis uses experimental and numerical investigations to analyse flow patterns around ACMs. Through these investigations it is found that the 400-series has a stable block shape due to its smaller and more streamlined bottom shell which reduces lift and its sheer top face which increases down force, thus creating an overall reduction in lift. These same factors also increase the drag component of the overturning moment. While drag force is still important, the reduction in lift has a greater effect on stability than the reduction in drag. From this thesis, it is found that an efficient block shape has higher pressures on the top shell than on the bottom shell. It is recommended that the bottom shell of the ACM be hydrodynamically optimised to achieve the most efficient increase in stability