| At present,It grows weakly for world land oil and gas,so oil companies around the world are starting to exploit offshore oil.70%of offshore oil resources are in deep water,therefore,the deep water has gradually become the main area of future offshore oil and gas exploration.Derrick is an important bearing equipment for offshore drilling rig of semi-submersible platform,and load on the derrick weight of the accessory equipment,the hook load,the ocean wind load and the platform-motion inertia force.The marine environment is extremely bad,the structural strength and safety of the derrick have become an extremely important part of the research of offshore platform drilling rigs,so it is highly valued by relevant designers and manufacturers.This paper based on the load type and failure mode of the dual derrick of the semi-submersible platform,the mechanical properties of static,dynamic and stability of the dual derrick under different working conditions are studied,the safety of the dual derrick under dangerous conditions is analyzed and evaluated,finally,the size of the dual derrick structure is optimized by the response surface method in Design-expert.(1)Based on the requirements of the drilling rig of offshore platform,this paper is proposed a dual derrick design scheme and established a three-dimensional model of the truss for static analysis.Firstly,the constant load,working load,wind load and platform motion inertia force of the dual derrick are calculated,then,ANSYS is used to study the static characteristics of the dual derrick in the no-wind condition,working condition and storm storage condition.And analyzes the influence rules of mechanical performance of dual derrick of typical factors,such as hook load,wind direction,wind force etc,the results show that the dual derrick meets the mechanical safety requirements,and the 90° wind of storm storage conditions is the limited working condition.(2)In this paper,the dynamic characteristics of semi-submersible platform and dual derrick are considered,the modal and harmonic response of the dual derrick are analyzed,the results are that the dual derrick has the largest vibration displacement in the Y direction,and the natural frequency range is 1.372Hz～9.1556Hz,the displacement peaks is at 1.32Hz,2.31Hz and 3.74Hz.After the transient dynamics analysis is carried out by using the dynamic acceleration to define the dynamic boundary of the platform,the dynamics and static calculation results are compared,the results show that the trends of displacement and stress of the dual derrick are similar,when the wind force is small,the static and dynamic results are basically consistently,when the wind force is large,the dynamic calculation results are larger than static.(3)Based on the static and dynamic characteristics of the dual derrick,this paper studies the overall and local stability of the dual derrick.In this paper,the dual derrick is simplified into a beam that maintains the bending stiffness of the original structure.Firstly,the energy method is used to study theoretically the overall stability of the dual derrick,secondly,the finite element buckling analysis of numerical calculation is used to study the stability of the dual derrick,the minimal error of theory and numerical result is within 6%,finally,the overall and local stability of the dual derrick are calculated by numerical method,the results show that the overall and local stability of the dual derrick meets the safety requirements.(4)Based on the results of statics,dynamics and stability analysis,the size of the dual derrick is optimized.According to the force characteristics of the dual derricks,the beams are classified into vertical beam,horizontal beam and oblique beam,the beam’s sizes are set as the influencing factors,the stress value and the weight of the dual derrick are defined as the response values,the mathematical model of optimization design is established,then,the regression equations of the response variable are established due to RSM,the influence of the influencing factor is analyzed by RSM for the response variable,finally,the method of multi-objective optimization is used to solve the optimal beam’s size of the dual derrick.The final results are that dual derrick weight reduces about 21.2%and safety factor increases about 8.3%.This paper studies the static,dynamic and stability properties of dual derrick in a deep-water semi-submersible platform firstly,then optimizes the structure of the dual derrick,finally,the optimal dual derrick meets the work requirements.It provides systematic and scientific theoretical support for design and manufacturing of the dual derrick. |
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