No abstract

In offshore drilling operations, accidental loads such as dropped objects pose a high potential threat to human safety, asset integrity, environment as well as reputation and business of the operator. A safe design of offshore exploration and production facilities for accidental collisions requires the risk assessment of such accidental events. This paper proposes the methodology to assess such risk. For the frequency assessment part of the risk, the paper proposed an extended version of the approaches outlined in industry guidelines such as DNV-RP-F107. For the consequence analysis, it has been demonstrated that advanced analysis method is capable and suitable for understanding the response of structures to accidental loadings, not only to remove conservatism inherent in simplified approach but also to assure a safer and economical design. Furthermore, this paper has demonstrated that by accounting for fluid-structure and structure-soil interactions, more detailed insight can be obtained of the response of the subsea pipeline protection system that otherwise could be easily missed without consideration of the fluid and foundation and can lead to ineffective design and connection details.

Structural integrity assessment and fire response analysis of offshore topsides structures focus on protecting topsides primary structural steel, hydrocarbon equipment supports, secondary steel along the primary escape routes, pressurized hydrocarbon pipes and vessels from damage and fracture due to pool and jet fires, and providing safe escape routes for emergency evacuation of the personnel during a specific period of time. In order to achieve these purposes, it is very critical to apply sufficient amount of Passive Fire Protection (PFP) on the topsides structural steel members, pressurized vessels and piping to supplement active fire protection systems like deluge, foam systems, etc. On the other hand, excessive use of PFP on the structure results in considerable additional cost and extra added dead load to the structure. Simplified and conservative approaches are available to estimate the extent and amount of the PFP on the offshore structures. However, the main concern with simplified approaches is that they can lead to over-application of the PFP resulting in substantial increase in the topsides weight. These methods may also result in under-estimation of the required amount of PFP, which can compromise the topsides structural integrity. With the use of advanced engineering analysis and knowledge of critical load path, an optimized PFP scheme can be developed that would maintain the structural integrity of the structure and also provide sufficient escape time for personnel during fire. The risk-based approach for PFP scheme development and optimization is introduced in this paper. In this method, ductility level fire response analysis is discussed, which is coupled with the probability of failure and probability of escalation to estimate risk to individual, asset, environment, etc. The ductility level analysis accounts for material and geometric nonlinear behavior of steel and load redistribution when heated members are over-utilized. The risk-based method is compared against the conventional method of PFP optimization using ductility level analysis recommended by API RP 2FB. It was concluded that using the risk-based approach can result in a significant reduction in the amount of required PFP.

The response of a fixed offshore platform subjected to extreme wind and wave loadings (lateral loads) under corrosion conditions is studied. The advanced simulation techniques to model corrosion damage and to perform nonlinear pushover analysis is outlined. The locations of corrosion perforations and thickness reductions can be identified based on an inspection report. A finite element model of the jacket is developed with a focus on these critical locations. The corrosion perforations can be represented as certain analytical shapes, for example, elliptical openings, at different locations, and sizes. The thickness reductions due to corrosion can be approximately modeled as shell elements with varying thickness. The platform capacity under extreme environmental loading is characterized in terms of the platform's Reserve Strength Ratio (RSR). Thus, a non-linear pushover analysis is performed to assess the strength of the corroded structure by quantifying the Reserve Strength Ratio (RSR) value. The nonlinear pushover analysis is carried out using the general finite element package Abaqus.

Significant improvements in technology include the development of subsea systems capable of retrieving and pre-processing oil and gas at sites located miles away from the host platform. The use of subsea systems at deep water sites in seismic regions has become increasingly common in recent years due to the growing exploration and continuing advances in the construction, placement and maintenance of subsea systems. Seismic events may impact the operability, stability and safety of subsea structural systems. Thus, the focus of this study is to evaluate the response and vulnerability of subsea structures during a seismic event and provide a quantitative sensitivity assessment for various parameters affecting the analysis.An analytical parametric study for a subsea structure and connecting pipe system was performed. The structural configurations of the system, surrounding soil conditions and the expected seismic demands along with the intensity and frequency characteristics of the ground motions are crucial for accurately modeling and predicting the expected structural response during earthquakes.Computer models using non-linear finite element method analysis procedures are developed to assess subsea systems under seismic loads. Sensitivity studies are conducted on parameters such as soil damping, soil-pile interface gapping, and multilevel excitation to simulate the response of subsea structural system. Computer model was generated, and nonlinear seismic response was simulated in CAPFOS software.In the case study, it was observed that the soil radiation damping effects, pile-soil interface gapping and uniform versus depth-varying seismic load application can have considerable effect on the response of the subsea structural systems. The results of this study show that the response of the subsea structural system is sensitive to parameters considered. Therefore, specific design information for a subsea system is critical in making a final assessment of the expected performance under site specific ground motions. This study may help offshore engineers to gain a quantitative understanding of how different parameters influence the results of seismic response of subsea structural systems.