Stress and Free Span Analysis of an Offshore Pipeline in the Gulf of Guinea

Abstract

Offshore pipelines are critical infrastructure for hydrocarbon transportation from subsea production facilities to processing systems. However, seabed irregularities and hydrodynamic forces often lead to unsupported free spans that threaten pipeline structural integrity through excessive bending stresses and vortex-induced vibration (VIV). This study assesses the stress and free span behaviour of an 18-inch API 5L X70 offshore pipeline in the Gulf of Guinea to determine the maximum allowable free span length under combined static and dynamic loading conditions. Analytical modelling was performed in MATLAB using beam theory, Morison’s equation, vortex-shedding relationships, natural frequency, and free span formulations. A three-dimensional pipeline geometry was developed in SolidWorks, while ANSYS Fluent was used to estimate hydrodynamic loading and ANSYS Mechanical was employed for finite element stress, modal, and S-Lay installation analyses. Results demonstrate that natural frequency decreases from approximately 1.45 Hz at 10 m span to 0.120 Hz at 50 m span, representing a 96% reduction, indicating increased vulnerability to dynamic excitation with increasing span length. Bending stress increases from approximately 5 MPa at 10 m to 75 MPa at 50 m but remains below the allowable limit of 350 MPa. Hydrodynamic analysis reveals that maximum allowable free span length (MAFSL) reduces from approximately 35 m at 0.5 m/s current velocity to 17 m at 2.0 m/s. Stress distribution during S-lay installation shows peak stresses of approximately 290 MPa in the overbend region and 280 MPa in the sagbend region, remaining below the allowable installation limit of 0.65 of yield strength (291 MPa). Validation against finite element results shows less than 10% deviation, confirming the reliability of the analytical approach. The study concludes that dynamic behaviour, rather than static stress capacity, governs allowable span limits, and safe design spans should be maintained within 25–30 m to minimize VIV-induced fatigue risks under moderate current conditions, reducing to 17 m under high current velocities.