Life Management Technique For Deepwater Drilling Risers

The dissertation focuses on life management technique for deepwater drilling risers, which is a part of the"863"High Technology Research and Development Program of China (No.2006AA09A106-4) of the key technologies of deepwater drilling and completion. Life management technique for deepwater drilling risers is investigated systemically based on theoretical research, numerical calculation and computer simulation, including failure mode identification and damage assessment, comprehensive study on life management method, analysis on monitoring and inspection method and technology, as well as life management system application of deepwater drilling risers. The main works are summarized as follows:1 Failure Mode Identification and Damage Assessment of Deepwater Drilling RisersThe drilling riser system is facing a variety of possible failures with the complicated operating environment and load conditions. First of all, the mechanisms of corrosion, fatigue and wear are analysized respectively, and the damage for each of the three kinds of failure modes is assessed quantitatively. The evaluation of corrosion flaws is based on the Allowable Stress Approach. The effect of the corrosion defects is taken into account multiplying the nominal stresses by stress concentration factors of the riser joints containing corrosion defects. The fatigue analyses are performed considering the wave loads and the vortex-induced vibrations. The wave fatigue is weighted-integrating of fatigue damage of each typical sea state. For ideal joints and joints containing corrosion pits, the fatigue damage is calculated with S-N curve and crack propagation theory. As to the VIV fatigue damage assessment, a modal analysis of the riser is performed and the major response modes are determined according to profile and vibration energy. The stress range of the vibration modes then can be used to compute the fatigue damage. In the wear assessment, the cross section shape of worn riser is analogized as an eccentric cylinder. The rupture capacity of the crescent-shaped wear riser is predicted when the internal circumferential stress reaches to a certain limit.2 Comprehensive Study on Life Management Method of Deepwater Drilling RisersThe life management strategy in a drilling cycle for the drilling riser system is presented in the paper, including the damage and life prediction, applicability evaluation of joints, operation process monitoring, riser inspection, establishment of riser information database, and so on. The life of riser joints containing defects is decided by the critical dimensions of flaws as well as development rate of damage. The critical dimensions are determined in view of different kinds of defects. The critical crack depth is not greater than riser wall thickness, and the critical dimensions of corrosion and wear can be anticipated according to stress intensity criteria. A 5,000ft deepwater drilling riser system is defined as the object of life management during the drilling operation. Damage level of riser is classified based on life prediction results in view of corrosion, fatigue and wear of joints, and the joints with high damage level should be given priority monitoring and inspection. Concerning to real drilling riser system with joints which containing defects, the joints with defects should be arranged where the damage predictions are as small as possible to ensure the reliability of the whole system.The life management strategy for riser joints is proposed using integrity management theory for reference. The probability of failure and consequences of failure are analysized based on failure mode identification and failure mechanism appraisal. Then the risk of failure of joints is decided, which is the foundation of inspection and maintenance. The fatigue, corrosion and wear reliability of riser joints are evaluated respectively. For joints without and with crack defects, the limit state equations are formed according to damage and crack propagation life relationship separately. The maximum depth of corrosion defect is deemed to obey I type maximum value distribution through analysis, and reliability of corrosion is calculated in accordance with corrosion allowance. Reliability of wear is the probability that the wear of riser joint is less than the critical wear. The drilling riser pertains to series system while there is certain correlation among different joints, so the system reliability can be determined by interval estimation method.3 Analysis on Monitoring and Inspection Method and Technology of Deepwater Drilling RisersAfter the analysis of motion and strain monitoring method as well as standalone and real-time monitoring system, the monitoring strategy for deepwater drilling riser is proposed considering monitoring cost, installation, reliability, etc. Using distributed standalone system for motion monitoring, the number and spatial extent of the instrumentation as well as optimum sensor placement are discussed. An optimization approach for VIV monitoring locations is presented based on modal analysis of the riser system. The main modes that have significant contribution to fatigue are firstly identified. For each one of the modes, search for possible locations for installation according to the acceleration in response, considering the riser tilt and gravitational effects. Then the optimum location is that would give a near max-signal for one mode while at the same time being close to a zero-signal location for the other modes.4 Life Management System Applications of Deepwater Drilling RisersThe natural frequency, amplification factor and axial response of the object system are discussed, based on which the hard hangoff of the riser is implemented. During the tripping operation, the stress of whole riser string should meet the criterion of no compressive loading after running or pulling each joint and the axial load is not overload. The hangoff analysis is discussed for riser system with and without BOP. The maximum allowable acceleration is determined by the criterion of no compressive. The axial load of hangoff riser system should not greater than load capacity of drill derrick, otherwise soft hangoff can be taken into account, or a number of joints should be recovered until there is no compressive stress in riser.The damage alleviation for drilling riser system is discussed. As to vortex-induced fatigue, the method of altering top tension and mud density as well as layout of riser joints with staggered buoyancy is discussed. The flexible angle is reduced by adjusting vessel offset and top tension of riser system, and in this way, the riser wear damage can be modified.The information databases for drilling operation and riser joints are preliminary established. The drilling operation information database is the description of drilling riser system and the record of riser management, including environment information, vessel and operation information, drilling riser configuration, monitoring data and operation sequence, etc. The riser joints information database consist of inspection results of joints and evaluation report based on inspection data, which is the reference to applicability evaluation and inspection updating of joints.