On-bottom roughness analysis
Need for this analysis
Subsea pipeline free spans can become a challenge in pipeline design and operation due to uneven seabed or seabed scouring effects. Determination of allowable free span length plays a crucial role in design of offshore pipelines. The spanning led oscillation can fatigue the pipeline and eventually result in catastrophic failure like a hydrocarbon leak at the weak spot, which is generally the pipe joining girth weld.
The strength failures of the spans are generally caused by the following:
self-weight
hydrodynamic loads
the anchor dragging or fishing gear lateral imposed load
The pipeline on-bottom roughness analysis is conducted to determine the pipe stress and the associated fatigue damage due to internal pressure and temperature fluctuation and vortex-induced vibration (VIV) of free span sections due to seabed unevenness.
Our analysis method
We conducted an on- bottom roughness analysis of the 455mm OD subsea pipeline (WT = 23.0 mm) installed on highly uneven sea bed terrain. Allowable free spans for the empty and flooded conditions were determined as per requirements of the recommended practice DNV-RP-F105 (Free Spanning Pipelines). To assure that the installed pipeline spans are lower than acceptable spans length, on-bottom roughness analysis is carried out using OBS module of the SPDT. All required input data for the analysis were saved in the SPDT centralised database. The pre-processing and post processing steps were carried out in the SPDT.
Result
The integrated onbottom roughness analysis module of SPDT allows engineers to easily configure the pipeline-seabed model by extracting relevant data viz dimensions, geotechnical, bathymetry, hydrodynamic loads etc. from the project centralised database. Once the model is set up, using ANSYS solver, the simulation is completed and results are processed using SPDT post-processing module.
The process sample results for the presented case study are shown in figures below.
Lateral bucking analysis
Need for this analysis
The accurate prediction of lateral buckling is crucial for offshore pipelines that are subjected to high temperature, internal pressure and complex pipe-soil interaction. The excessive lateral deformation from lateral buckling may risk safe operation of the pipeline due to local axial strains that potentially could be severe enough to cause fracture failure of welds or collapse of the pipeline. This case study aims to evaluate the effect of key parameters viz seabed natural unevenness, environmental loads on the pipeline lateral buckling behaviour.
Our analysis method
This work takes into consideration a case study scenario in order to analyse the effect of the uneven seabed on the 455mm OD subsea pipeline’s (WT = 23.0 mm) lateral buckling behaviour. After review of the subsea pipeline using the Hobbs’ analytical method, FEA simulation was carried out to investigate the effect of the actual seabed terrain on the pipeline lateral buckling behaviour. The FEA Using the pre and post processing features of the SPDT, the lateral buckling analysis was carried out in ANSYS.
Results
On completion of preliminary lateral buckling screening assessment using SPDT Hobbs model, further detail analysis was carried out in SPDT integrated FEA global buckling module. The pipe-seabed configuration was modelled automatically by extracting relevant inputs viz pipe dimensions, soil properties, bathymetry, operating conditions, metocean etc. from SPDT centralised database.
The configured model was simulated in ANSYS solver and the results shown in figures below are processed using SPDT post-processing module in an efficient manner.
Comments