Structural integrity assessment of self-elevating units with fatigue and corrosion damage based on actual load history

Abstract

Seafox owns and operates a fleet of 12 Jack-ups. The design life of these Jack-ups is typically around 30 years. However, some have remained operational after exceeding their design life. It is therefore critical to gain an understanding how long and under what conditions these jacks-ups are still able to operate safely. There are two main deterioration processes in Jack-ups as they age: metal fatigue and corrosion. Fatigue is a process that weakens material due to repetitive loading and unloading. Corrosion is the deterioration of a metal caused by an electrochemical reaction between it and its environment. In this thesis these two deterioration processes are assessed and quantified in two Seafox Jack-ups. Fatigue is assessed on the Seafox 2 and corrosion on the Seafox Burj. The structural characteristics of a Jack-up result in a significant dynamic response. Therefore, a dynamic analysis is conducted to determine the stresses in the jack-up legs. Furthermore, various non-linearities justify a time domain finite element analyses conducted in USFOS. The thesis presents a structural model of the Seafox 2 using a simplified hull and spring elements to represent the hull-leg and leg-ground connections. Environmental loading is determined by using the actual wave records kept for each operating location of the Seafox 2 in a simulation. The simulation identifies a critical joint with the highest stress range. The hot spot stress range in this joint is determined using stress concentration factors (SCF), using two methods: parametric equations of Efthymiou and finite element analyses. From the hot spot stress ranges and the number of recorded stress cycles at each location the fatigue life of this critical joint is calculated with a S-N curve. The critical joint is used as proxy to establish the design fatigue life of the Seafox 2 based on actual wave loading. The analysis shows a design fatigue life of the Seafox 2 of 262,2 years. This supports the conclusion that the rig can remain operational. Although the analysis conducted is a conservative one, the non-linearity of the S-N curve (small fluctuations in in stress cause large changes in fatigue life) makes it advisable to continue to check critical joints for cracks when the Seafox 2 is docked. The Seafox Burj went into docking in 2015. Ultrasonic thickness measurements made it clear that the Burj had significant steel diminution due to corrosion in the legs. For commercial reasons Seafox wanted to know whether the legs needed to be replaced unconditionally, or that the Burj could still operate in lower water depths. The ultimate limit state for the Burj, with steel diminution due to corrosion, is calculated for three locations. Two are possible locations where the Burj might be deployed and one chosen as a model benchmark. The Burj is modelled in the same manner as the Seafox 2. From the results it can be concluded that steel diminution has a considerable impact on the ultimate limit state. The analysis indicates that the Burj, cannot operate in deep waters anymore without costly leg repairs. The analysis indicates that it can still operate safely in shallow waters. Therefore, narrowing the work scope of the Burj to shallower waters is a viable way to avoid costly leg repairs.