Offshore pipelines are required to operate at ever highertemperatures and pressures. The resulting high axialstress in the pipe-wall may lead to unexpected buckling, which may have serious consequences for the integrity of the pipeline if this is not taken into account during the design phase.
Unexpected lateral buckling has been observed in several operating pipeline systems. The offshore industry lacks a complete understanding of lateral buckling, and efficient tools for simulating buckling behaviour early in the design phase would make a valuable contribution to our knowledge.
The computer analysis tool, SIMLA, which has been developed by MARINTEK for Norsk Hydro, can accurately predict and simulate buckling effects. SIMLA includes special-purpose tailor-made nonlinear finite elements, contact algorithms, material models and numerical procedures for advanced structural analyses of offshore pipelines. Furthermore, the full 3D representation of the seabed can be taken into account, a necessity in areas with very irregular seabed topography.
Buckling is very sensitive to the initial configuration. In order to obtain accurate results, the various phases in the life-cycle of the pipeline need to be taken into account in the analysis. SIMLA does this in sequential steps or load cases:
- In the first step, the pipeline is automatically placed along the predefined route. This is handled automatically by the AUTOSTART feature in SIMLA. The correct stresses resulting from the route description, seabed, hydrostatic loads and gravity effects are calculated with minimum input from the user.
- Water filling and pressure test are usually then carried out. After full pressure has been obtained, the pipe is de-pressurised and the water is removed. In SIMLA this is performed by raising and lowering the internal pressure and submerged weight of the pipe.
- Operating or design pressure and the corresponding updated submerged weight are then applied.
- In the final step, the thermal load is applied.
When buckling takes place, the problem is unstable, and a transient dynamic analysis is normally performed to apply the thermal load. SIMLA is able to switch from a static to a dynamic approach within the same analysis. This way, the water filling, pressure test and operating loads may be applied statically for the sake of efficiency, leaving the thermal load to be applied dynamically, thus increasing the numerical robustness.
In a lateral buckling and stability analysis, interaction between the seabed and the pipeline are very important. The 3D representation of the seabed ensures accurate results on the basis of geometric effects. Figure 1 visualises an example of a 3D seabed. The green line defines the route centreline and the yellow pins define the surface normals at discrete points. The route corridor grid resolution in both the axial and longitudinal directions is defined by the user.
In addition to the 3D route corridor, the numerical representation of the interaction between the pipe and the soil is also very important. SIMLA makes several numerical pipe/soil interaction models available. The soil stiffness and friction coefficients may individually be defined as functions of displacements in the lateral, axial and vertical directions. Both hyperelastic and elastoplastic material behaviour may be applied.
In order to verify the buckling analysis capabilities of SIMLA, a 4.5 km section of the Ormen Lange PL-A import line was analysed, using a snake lay route according to the original design produced by Reinertsen Engineering. The route is relatively flat in this area, with a vertical difference between the start and end of the route section of approximately 22 m. The seabed was defined with a resolution of 2 m in both axial and lateral directions; see Figure 1 for illustration. During the pressure test, a pressure of 27.3 MPa was applied. The design pressure was 24.2 MPa, and the design temperature was 31.7ยบ C.
The analysis was defined and run in SIMLA. The results indicate that the pipeline would buckle at two sections, and that the maximum lateral displacement is estimated to be 4.6 m, with a maximum axial strain of 0.24%; see Figures 2 and 3.
The SIMLA results were compared with existing results from ANSYS analysis performed by Reinertsen Engineering as part of the detailed design process. The buckling shapes, moment and distributions of force are virtually identical. The maximum lateral displacements differed somewhat, being about 9% lower in ANSYS than indicated by SIMLA results; see Figure 3.
In order to enable the ANSYS analysis to be completed within a reasonable time, 8 m elements were used in the ANSYS model. The time required for analysis of an 8 m element model in SIMLA is less than 5 minutes on a 1.8 GHz AMD Opteron PC. In order to obtain more accurate results, an element length of 2 m was also utilised in SIMLA, requiring an elapsed time of 18 minutes. The short analysis time indicates that significantly larger sections of the pipe can be analysed in one go with SIMLA without problems.
The existing analysis capabilities of SIMLA enable us to accurately predict and evaluate lateral buckling in subsea pipelines. Automatic algorithms and an engineering-friendly input format significantly simplify the pre-processing and model set-up stages. Efficient numerical routines make it possible to analyse long pipeline section with a high degree of accuracy in a matter of minutes.
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