Scour Research Of The Foundation Of Subsea Oil And Gas Production System

With the growing demand of energy, the emphasis on the development of marine oil and gas field is gradually improved, and the attainable depth is become much deeper as the technology matures. A new mining system named subsea oil and gas production system has been widely focused as the greatly improved cost induced by water depth. The application range of subsea production system is extended from a few more yards to thousands of meters, but the construction cost is almost the same.The subsea production system is mainly composed of subsea wellhead, Christmas tree, manifold, jumper tube, umbilical cable, subsea pipeline, riser, control system, subsea processing system, and so forth. It delivers the oil-gas-water produced from wells under the sea to supported equipment or onshore terminal through remote controlling. Most of the foundation of subsea production system is mud mat, and it’s fixed into mud through skirt plate. For the foundation located in the fishing area, the upper manifold needs protective cover in order to avoid the tow of the fishing net caused by the manifold placed barely. As a result, its existence will change the flow field around it and cause scour, and it may lead to uneven settlement of mud mat even slippage. Then accordingly the additional load is induced in the other parts of the production system, and the potential risk will occur in the normal operation.Previously there’s no research on the protection cover of the foundation of subsea production system with the characteristics that its incident and lee sides are both not perpendicular to the seabed, and it allows the water flow through the preformed openings at the front and back. In addition, there’s also few study of scour for the actual size of engineering structures. So here physical modeling experiment and numerical simulation are applied to study on the scour around the foundation of subsea production system with protection cover.Firstly the modeling experiment is conducted for the local scour around the foundation in the normal condition; then the scour numerical model coupling with flow movement, suspended load diffusion, bed load transport and elevation variation is applied to verify the results of physical modeling experiment, in which flow around the foundation is simulated by solving the Reynolds-Averaged Navier-Stokes equation with a RNG k-ε turbulence closure, and its free surface is traced by VOF method. The results demonstrate the feasibility of numerical simulation method. In view of the results of modeling experiment with laboratory scale are difficult to invert to the prototype of the foundation of subsea production system, then the scour of the prototype under normal and extreme conditions is simulated numerically. If three dimensional numerical model simulates the real depth, its calculation amount is quite large; and the bed shear stress caused by the water particle velocity is the dominate element which influences the sediment movement, so in order to simulate the flow field with wide range, it needs to decrease the depth under the premise of the water particle velocity near bottom.Submarine water particle velocity induced by wave is smaller relatively under the normal condition with one-year return current and one-year return wave, so it could be ignored and normal condition can be regarded as the case with pure current. The physical modeling and numerical simulation for subsize foundations both indicate that the scour of indicate and lee sides are starting at the parts with amplification current velocity. In the beginning, the scour occurs around the preformed opening of lee side, but the height of the model above the sand surface is quite small, and the intensity of trailing vortex is lower, so the development of scour in this side is slow. Meanwhile, the scour of incident side is beginning at the sharp corners, in the process of developing, the scour pit expends to the middle rapidly with the aid of auxiliary down flow. So the scour depth near the corners is deeper than the middle opening. On the other hand, the scouring simulation of the prototype under this condition shows that, the development of trailing vortex makes scour range become larger in the lee side because of the sharp increase of the elevation of the protective cover. The scour pit occurs firstly near the preformed opening in the lee side extends around quickly in the acceleration of trailing vortex, and the scour depth of some individual positions is bigger than the thickness of the first layer bed sand. Though the scour in the incident side is beginning at the sharp corners, the scouring pit in the middle is much deeper in the extending process because of the downflow induced by the height of mud mat at the position of preformed opening.In the extreme condition, the water velocity produced by one hundred-year return current and one hundred-year return wave is large, and the boundary layer is thinner caused by the wave, so the whole sand surface appears erosion. The scour formed initially around the corners in the incident side is weakened in the process of wave oscillation. Finally, the obvious scour pits form near the two preformed openings, especially the scour depth near the opening behind has reached the thickness of two layers of sand.In addition, the physical model tests with different scale showed that the scour around the foundation became serious with the increasing of scale under the same water depth, this indicated that the relative ratio between the depth of the water and the height of the foundation has important effect on erosion. In order to study the effect, the paper simulated the scour around the same foundation model under different water depth, the results showed that the scour will no longer be affected by water depth and relatively stable when the ratio is greater than 7.5.The study on the scour of the foundation of subsea production system needs to consider the interaction of water, sand and foundation. Comparing with the surface-piercing foundations, addition item is presented in the trailing vortex of the flow field of submerged foundations. This item is developed from the current flows through the upside of the foundation, and its strength is related to the height of foundation above the sand surface. The inside of the foundation studied in this paper is permeable, so it needs to consider the influence of the water outlet, all above make the scour in the lee side much more complex.Based on the physical modeling experiment and numerical simulation, the scour range and depth of the foundation of subsea production system in current and in combined wave and current are obtained to provide guidance for the latter design and installation, and it also presents reference for the scour simulation of the large models with other size.