Design Approach And Its Application For Deepwater Drilling Risers

The dissertation focuses on design approach and its application for deepwater drilling risers, which is a part of the"863"High Technology Research and Development Program of China (No.2006AA09A106-4) and the project of"Research on Deepwater Riser Engineering Methods and Trace Study of Dual-Gradient Drilling Technology"sponsored by CNOOC. Design approach and its application for deepwater drilling risers is investigated systemically based on theoretical research, numerical calculation and computer simulation, including analysis of influencing factors for design of ultra-deepwater drilling risers system, comprehensive study on (quasi)static performance of drilling risers, parametric sensitivity analysis and research on response prediction model of drilling risers, nonlinear dynamic analysis of risers subjected to random wave loads and vessel motion, axial dynamic behavior of hung-off drilling risers and long-term wave fatigue analysis of deepwater drilling risers, and so on. The main works are summarized as follows:1 Analysis of Influencing Factors for Design of (Ultra)Deepwater Drilling Risers SystemEnvironmental and operational factors are the main factors that affect the design of (ultra)deepwater drilling risers system. The former include water depth, wave, current, and the latter include drilling fluid density, hung-off mode after disconnection of LMRP(Lower Marine Riser Package) with BOP(Blowout Preventer), and vortex-induced-vibration suppression devices, buoyancy modules distribution, service pressure of choke and kill lines, et al. This paper presents a systemic description of the factors affecting the design of (ultra) deepwater drilling risers system, identifies the mechanism of the factors affecting the design of drilling risers system, and analyzes the relationships between the influencing factors and the design of drilling risers system. The research shows that water depth and current are the most significant factors influencing the design of the drilling riser system, and the design of it should be conducted between hung-off and connected mode to obtain the optimal configuration of the drilling riser system.2 Comprehensive Study on (quasi)Static Performance of Deepwater Drilling RisersTheory, principle and method for quasi-static analysis of deepwater drilling risers are presented in the paper. Quasi-static analysis system for drilling riser by use of ABAQUS in the background is developed in C++ Builder environment. Then, the automatic search for the maximal wave phase as well as the nonlinear quasi-static analysis of the top tensioned drilling riser is implemented. Based on the above research, the results are compared between static and quasi-static analysis, and the effect of wave phase angle on riser quasi-static performance is investigated. In the end, a comprehensive study of riser static performance is made from the viewpoint of engineering application.3. Parametric Sensitivity Analysis and Response Prediction Model of Deepwater Drilling RisersGeometry parameters, seastate parameters, buoyancy modules parameters and operation parameters are the main factors affecting the response of deepwater drilling risers. Geometry parameters include outside diameter and wall thickness of the drilling risers, seastate parameters include wave height, wave period and current profile, buoyancy modules parameters include outside diameter, length and configuration position, and operational parameters include TTR(Top Tension Ratio), drilling fluid density and mean offset of the drilling vessel. The purpose of parametric sensitivity analysis is to disclose the effect of those parameters on drilling riser response, and to provide reference for the design and configuration of drilling riser system. Sensitivity analysis of parameters affecting the response of the riser is conducted based on orthogonal test approach in this paper. L50(511) orthogonal table is built after 11 factors noted previously being considered in orthogonal test with 5 levels adopted in each factor. Finite element calculation is carried out by use of quasi-static analysis system, and range and variance analysis of test results is made. As a consequence, the effect of parameters on riser response is analyzed qualitatively and quantitatively.Research on response prediction model for deepwater drilling risers is to provide a rough estimation on riser response which meets the need of engineering application without complicated finite element calculation, and to provide a simple and applicable solution at the stage of preliminary design of the drilling risers. Response prediction model based on support vector machine(SVM) is investigated and proposed in this paper. SVM is trained by orthogonal test results, and its predication accuracy is checked by randomly generated new test data. The difference in accuracy between SVM and regression predication model is compared, and the result indicates that predication model based on SVM meets the need of engineering application.4 Nonlinear Dynamic Analyses of Deepwater Drilling Risers Subjected to Random Wave Loads and Vessel MotionBased on the theoretical research on mathematical model and hydrodynamic loads model for riser dynamic analysis, analysis techniques of dynamic response for deepwater drilling risers are discussed thoroughly. An approach is proposed for calculating the nonlinear dynamic response of deepwater drilling risers subjected to random wave and vessel motion with finite element solver ABAQUS/Aqua in time domain. Firstly, wave trains and the associated sea surface elevation time history based on the simulation of the long-crested random wave should be determined, then dynamic vessel motion based on the wave train and the specified Response Amplitude Operator(RAO) of the drilling vessel should be determined, and finally finite element dynamic analysis can be performed after definition of wave series, boundary condition, structural and environmental loads is completed.Response analysis technique of the hung-off riser after disconnection is studied, and axial dynamic behavior of it is analyzed. The first step in dynamic analysis of the hung-off riser is to determine the vessel heave motion. The calculation of the maximum permissible sea state for a drilling riser string in a hard hang-off configuration is generally based on a criterion of no compressive loading. By use of numerical calculation method, heave motion of the drilling vessel is determined according to wave spectrum and heave RAO, which is the dynamic boundary condition of the hung-off riser. Then the axial stress time history in the riser at different position is obtained. The related conclusions can provide reference for selection of hung-off mode, drilling riser system configuration and its optimization.5 Long-term Wave Fatigue Analysis of Deepwater Drilling RiserAs a dynamic, fatigue sensitive structure, long-term fatigue performance of the deepwater drilling risers is an important design consideration. An approach of long term wave fatigue analysis for deepwater drilling riser is presented, which can be used to calculate the long term fatigue of drilling risers subjected to 1st order wave force, wave frequency vessel motion and 2nd order low frequency vessel motion. By use of the proposed method, according to the short term fatigue case defined by the wave scatter diagram, random wave and random vessel motion require to be determined based on the dual parameters of wave spectrum and the RAO of the drilling vessel. Then, nonlinear dynamic analysis of the drilling riser under random wave loads and vessel motion boundary condition can be performed. Based on the response time history obtained by random nonlinear dynamic analysis, considering the probabilities of short term cases in the wave scatter diagram, a self-developed special program is employed to calculate the long term wave fatigue. As an example, long term wave fatigue of the 1,500m deepwater drilling riser in South China sea is calculated. The effect of drilling vessel motion, top tension and wall thickness of the riser joint on fatigue damage of drilling riser is mainly investigated.