Offshore pipeline on-bottom roughness analysis is generally performed to understand pipe stress and areas of free spanning. In this paper, the predicted pipeline profiles, by finite element analysis (FEA) modeling, are compared with survey data. Several case studies are presented and parametric effects are assessed. The FEA modeling procedures are presented in this paper along with several project experiences. 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. The analysis presented in this paper can aid the design of pipeline free spans and mitigation, if required. The parametric effects of higher and lower bound soil stiffness, friction resistance, pipeline bottom tension, external / internal pressure, and content density are considered. The predicted pipeline profiles by FEA modeling are compared with surveyed profiles under several loading conditions. A strong correlation between survey data and finite element analysis (FEA) is achieved. This paper shows that the FEA stress prediction is adequate, and the FEA modeling is suitable as an advanced design tool for pipeline on-bottom roughness assessment and free span behavior analysis. A commonly used mitigation method for VIV fatigue is to install VIV suppression strakes. For overstresses due to pipeline bending effect, the common mitigation methods are to install mechanical supports, use heavy wall pipe and correct the seabed unevenness among others. It is important to reliably predict the pipeline profiles in order to precisely assess the pipeline free span response and to provide guidance to determine a proper free span mitigation strategy. The FEA method can be utilized to realistically simulate the pipeline on-bottom roughness behavior affected by the pipe properties, pipe-soil interaction including penetration and soil friction resistance, the internal and external pressure, product content, temperature profile, and the bottom tension from pipe-lay. An accurately built FEA model can provide a reliable prediction for the pipeline profile, free spanning length and gap under all conditions, which are crucial for the stress and fatigue assessment as well as for the design of free span mitigation.
Spans occur when a pipeline is laid on a rough undulating seabed or when upheaval buckling occurs due to constrained thermal expansion. This not only results in static and dynamic loads on the flowline at the span section, but also generates vortex induce vibration (VIV) which can lead a fatigue issue. The phenomenon, if not predicted and control properly, will result in significant damage to the pipeline integrity, leading to expensive remediation and intervention works. There are various span mitigation methods in use for both over stressing and fatigue concerns. The mitigation methods, if not analyzed properly, may result in much unnecessary work or generate more problems or concerns in the future. The mitigation analysis can become very challenging due to many restrictions in the field such as the minimum and maximum heights or lift of mechanical supports or grout bags, and bearing capacity vs. cost of supports. The cost of different mitigation methods and their interactions are the other considerations along with the installation tolerances, challenges associated with the water depth and uncertainties in seabed properties. This paper describes the latest developments in use of finite element analysis to investigate associate mitigation solutions given the governing practical limitations and cost factors. The ULS and fatigue lift criteria are used as the guidelines. The methods presented within this paper are applicable for various span conditions. Conclusions are then drawn to the impact of these various scenarios so that the pipeline integrity can be assured with confidence.
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