Research On Mooring Positioning Technology And Anchor Foundation Mechanics Of Truss Spar Platform In Deep Sea

Truss Spar platform has become one of the main platform types for deep sea oil and gas development in the word for its excellent performance and relatively low cost.Mooring system and anchorage foundation are important components of Truss Spar platform in deep sea.Instead of anchor mooring line,multi-components mooring line is widely utilized in deep sea.How to design a mooring system which meets the design specification with the lowest cost is the researchers’ concern.What’s more,the anchor foundation suffers from tension from the mooring system.The interaction between anchor foundation and seabed soil would affect the pulling resistance of the anchor.In this paper,the mooring positioning technology and pullout resistance of anchor foundation of Truss Spar in deep sea are studied.Based on the catenary equations,the static characteristics of both anchor mooring line and three-components mooring line are analyzed.The latter is more suitable for mooring in deep sea for its high horizontal recovery stiffness.Simulation analyses are conducted to acquire their dynamic characteristics by ABAQUS/AQUA software.The effect of external excitation and environmental load on dynamic tension is analyzed by orthogonal test.Results show that external excitation and environmental load are important factors affecting the dynamic tension of the mooring line.The dynamic effect of the mooring line tension cannot be ignored.Taking typical Truss Spar platform as an example,the heave response of the platform in frequency domain is analyzed by theoretical calculation and AQWA software simulation respectively.And the influence of Heave Plate on the heave response of the platform is studied.Based on the theoretical calculation results,the fitting formula for numerical value of manual damping in the AQWA hydrodynamic analysis is presented.According to the design specification of the mobile platform,the coupled dynamic characteristics of Truss Spar platform/mooring system are obtained by AQWA software in four different working conditions.The damage of one mooring line in the mooring system has a great impact on the sway and surge,but little impact on heave,yaw,roll and pitch.Next,the motion response and tension response of Truss Spar with two different mooring schemes are compared.It concludes that mooring arrangement scheme(b)is better than mooring arrangement scheme(a)and(c).Based on the time domain response of Truss Spar/mooring system,the fatigue life of mooring line is calculated by OrcaFlex software.The most dangerous point of the mooring line occurs at a distance of 3445 m from the top of the mooring line,which is in the area where the mooring line is in contact with the seabed.In order to reduce the cost of mooring system,BP neural network and particle swarm optimization(BP-PSO)optimization model is proposed to optimize the composition of threecomponent mooring line.The mooring system should meet the specification requirements,which are constraint conditions of optimization.BP neural network model is used to simulate the coupled dynamic analysis of Truss Spar/mooring system.The diameters of threecomponents mooring line are defined as the input variable of BP neural network,which are also the design variable of the optimization problem.The bottom length,the tension extremum of the mooring line and the extreme value of the motion response of the platform are output variables of BP neural network.Results show that BP neural network algorithm can accurately predict the output variables.The particle swarm optimization algorithm and the harmonic search algorithm are used to solve this problem.Comparing the performance of them,we find that particle swarm optimization performs better in terms of calculation time and calculation accuracy.Aiming at three different failure modes of suction anchor: local shear failure mode,bottom tension failure mode and global failure mode,the finite element analyses are carried out to analyze the pullout resistance of suction anchor by ABAQUS software.Regardless of whether the suction anchor is subjected to horizontal load,vertical load,or inclined load,the pull-out resistance of suction anchors can be sorted from big to small: global failure mode> bottom tension failure mode> local shear failure mode.The horizontal pull-out resistance are analyzed when the mooring point is located at different depths.The optimal depth is 0.7L blow the mud line.When the load of anchor located at the optimal point,the mooring line in soil is modelled in “inverted catenary” shape.Then,ultimate pulling force is calculated.A new type of suction anchor is put forward,named as bionic suction anchor.The pullout resistance of the bionic suction anchor is calculated by ABAQUS and compared with that of suction anchor.Results show that the pull-out resistance of biomimetic suction anchor is greater than that of conventional suction anchor by increasing the contact area with the soil.When the distance between the main and sub-chambers is small,the mutual influence of the adjacent cylinders is large,resulting in a large additional displacement,which reduces the actual pull-out resistance of the anchor.