Abstract

The subsea production system for oil or gas in shallow water area is usually placed in a caisson, and a certain thickness of soil is backfilled above the caisson. During maintenance of the subsea production system, the backfill above manhole on the top of caisson needs to be washed away to expose the manhole. In order to study whether the vertical jet flows from the jet trencher can wash away the backfill above the manhole, a three-dimensional (3D) numerical model was established based on FLOW-3D. According to the design for washing away backfill soil on top of the caisson, the erosion and deposition process of the sediment on top of caisson for an subsea production system were simulated. The results show that the backfilled soil with the median particle size of 0.2mm can be washed away to expose the whole manhole when the buried depth is 0.5 and 1 m, and the manhole area can be fully exposed after the sediment silting. The depth of the scour hole cannot reach the top of caisson when the buried depth is 4m or more.

Introduction

In the navigation area of shallow water depth, the protective facilities and subsea production system can be set below the seabed surface. A circular caisson, open at the bottom and closed at the top, can be used as the structure of protective facilities for subsea production system. Manholes are placed on the top of caisson, which is initially covered by steel plate and can be opened when required. In practice, the caisson is covered by a certain thickness of backfill soil in shallow water area. When the subsea production system needs to be repaired, the soil on the top of steel plate of the manhole needs to be flushed away for maintenance.

According to the design, jet trencher is adopted to wash away the soil above the caisson through the jet flow. Numerical simulation is an important investigation tool to evaluate the processes of sediment erosion and scour by jet flows. Li et al. (2014) carried out a two-dimensional (2D) simulation to study the relationship between scouring depth and target distance, which is measured from the end of jet pipe to the sea bed. Wang et al. (2014) demonstrated the relationship between jet flow velocities and the depth of the scour hole by a 2D model. Huang et al. (2019) studied the key parameters of sediment erosion in FLOW-3D. Gu et al. (2016) carried out numerical simulation of sand bed erosion by double jet pipes and determined the optimal spacing of the nozzle system. Meng (2011) established a 3D jet model with two jet pipes to study the hydrodynamic characteristics of jet flow for seabed trenching. Sun (2019) studied the changes of scour depth and width under different particle sizes by using a 3D double-pipe jet model. In addition, Qian et al. (2011) carried out a 2D numerical simulation of erosion of inviscid sediment bed based on Euler model, and discussed the influence of sand properties on the shape of scour hole. Based on turbulence theory, Huai et al. (2012) calculated the local scour caused by 2D vertical jet flow and studied the development of scour hole. Liu et al. (2015) constructed a numerical model for local sediment scour and propose a new method for the VOF (Volume of Fluid) scheme to reduce the computational time. Ozan et al. (2010) studied the submerged 3D turbulent jet flow behavior around a pile on rigid and sand bed, respectively.

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