Response of a fixed offshore platform to seismic excitations and fatigue damage assessment

Abstract

The expansion of offshore structures (both reinforced concrete and steel) has seen significant growth since the mid-20th century, driven by the increasing global demand for energy and the need to exploit marine energy resources. These structures have been developed at varying sea depths, ranging from shallow coastal waters to deeper offshore locations. Given the harsh and complex marine environment, offshore platforms are subjected to numerous external forces throughout their Service life. These include wave loads, wind pressures, ocean currents, seismic activity, and fatigue damage. As a result, these platforms must be designed as elevated, resilient, and robust systems capable of withstanding these diverse and dynamic loading conditions. The cumulative effect of these external forces presents substantial risks to the integrity of structural elements, potentially leading to partial or total failure of the platform. Such structural failures can result in serious environmental hazards and significant economic losses. This thesis presents a comprehensive static and dynamic analysis of a hypothetical fixed offshore platform situated in the Mediterranean Sea, off the coast of Türkiye. The structural model was developed and analyzed using the SACS V16 finite element software. The study primarily investigates the effects of seismic and wave-induced loads on the platform and evaluates its structural performance under these combined actions. The objective is to assess the platform's safety, durability, and long-term reliability through detailed numerical simulations, contributing to the broader understanding of offshore structural behavior in challenging marine environments. The ability of offshore structures to withstand seismic loading and resist fatigue damage is a critical design consideration universally recognized by engineers and researchers. These factors play a pivotal role in the structural assessment and design of offshore platforms, where the intensity of such demands is largely influenced by the platform's geographical setting—particularly when located in seismically active regions or environments subjected to severe and recurrent wave action. To evaluate the seismic response of the fixed offshore structure, a nonlinear dynamic time history analysis was performed using three ground motion records in accordance with ISO 19901-2:2004 standards. The selected ground motions were carefully chosen based on the criteria outlined in the same standard and were spectrally matched to the target elastic response spectrum with 5% viscous damping using the SeismoMatch software. The time history analysis was conducted employing the implicit time integration method to ensure accurate representation of the structure's dynamic behavior under seismic excitation. A comprehensive report was conducted to evaluate the stress distribution in the tubular joints and the associated fatigue damage in the fixed offshore structure under the influence of environmental and wave loads from all directions. This assessment contributes to estimating the platform's theoretical Service life. The numerical fatigue analysis was based on the S–N curve methodology in accordance with API standards, employing the spectral wave analysis approach to assess cumulative fatigue damage. The findings offer valuable insights into the structural integrity, safety, and long-term reliability of fixed offshore platforms.