Qiao Dai

Interaction and Assessment of Multiple Local Wall Thinning Defects

The safety assessment of pipes with multiple local wall thinning defects (LWTs) is systematically investigated by finite element analyses (FEA) with special attention to the interaction of multiple LWTs. It is interesting that the arrangement of LWTs (axial arrangement and circumferential arrangement), the load condition (pure pressure, pure bending and complicated load) and the relative depth play important roles in the interaction of multiple LWTs. The effective stress area and critical stress can be used to explain their influences. Moreover, existing assessment methods of multiple LWTs in some defect assessment standards such as API 579/ASME FFS, ASME B31G, BS 7910 and GB 19624 are reviewed. It’s noticed that the influences of arrangement, load condition and relative depth are ignored in existing standard methods, but they can influence the assessment results significantly. In order to consider these issues, an improved assessment of multiple LWTs based on API 579/ASME FFS is proposed. This improved assessment method has considered the influences of arrangement, load condition and relative depth, and can give better results.Copyright

Non-Probabilistic Failure Assessment Methodology About Titanium Pipes With Cracks

In this paper, the non-probabilistic failure assessment method for a pipe with crack was proposed, and interval model was applied in this assessment method. Compared with the deterministic approach, the assessment parameters were regarded as interval parameters in the non-probabilistic method, which could reflect the uncertainty of data and avoid the unsafety of results caused by the scatter of these parameters. On the other hand, compared with traditional probabilistic method, the non-probabilistic method only needs the range or bound of the parameters rather than detailed statistics information of the parameters which may be uneasy to be obtained from practical engineering. So the new assessment method reduces the request for detailed statistics properties of parameters. In this paper, load, material mechanical properties and the size of crack were treated as interval parameters, and the non-probabilistic failure assessment method was established based on the level 2 normal assessment of the BS 7910:2005. In this case, the assessment point (Lr , Kr ) is not a deterministic value, Lr and Kr would vary in a certain interval. As a consequence, the assessment point was changed into a rectangle zone, and the non-probabilistic failure measure for pipe with crack could be obtained by the variation of the rectangle. The non-probabilistic failure measure can be defined as the ratio of the area of failure region to the total area of the rectangle. At last, the non-probabilistic failure assessment method was used to evaluate a titanium pipe, and the analysis results were compared with the results of deterministic method, probabilistic method according to Monte-Carlo and partial safety factors method, respectively. The analysis results sufficiently demonstrated that the new non-probabilistic failure assessment method proposed in this paper was feasible and available.Copyright

Finite Element Simulation of Limit Load of Components at High Temperature

The creep of materials makes it difficulty to determine the limit load of component at high temperature. In this paper, limit load was obtained by finite element simulation according to isochronous stress versus cumulative strain data and creep failure criterion at high temperature. Firstly, isochronous stress versus cumulative strain data of P91 steel was generated. In finite element analysis code ABAQUS, isochronous stress versus cumulative strain data was replaced by equivalent elastic-plastic constitutive relation. Then, sustained load versus cumulative strain curves at high temperature during service was obtained after finite element simulation. At last, limit load at high temperature during given working hours was determined based on the restriction of total strain at key point of specific component. The restriction of total strain which could also be regarded as long-term fracture strain was discussed in this paper. Finite element simulation of limit load of component at high temperature is simple and reasonable. Limit load of complex component at high temperature during given working hours can be obtained by this method. Using this method, limit loads of a pipe with local wall thinning defect and a branch junction at high temperature during given working hours were obtained as examples.