Actionable Intelligence at your Fingertips
Floating Production, Storage, and Offloading vessels (FPSOs) are critical offshore installations that are subject to constant and dynamic stresses as a result of waves, wind, and operational conditions such as the tank loading levels. This cyclic loading leads to fatigue damage that directly affects the life of the vessel. It can also compromise the structural integrity and safety of the FPSO. So to ensure continued safe operation, it is necessary to carry out a fatigue assessment. Given the enormous size of these structures, this is not an easy task. In this blog post, we will describe the different steps in calculating fatigue using Akselos’ software, highlighting the different features that make our approach unique and that give you actionable results no other software on the market can.
Step 1: Build the Model and Update it with Data from Operations
Prior to carrying out a fatigue assessment for an FPSO, or any other integrity analysis, we ensure there is a ready-to-use model. As an Akselos customer, we provide you with this model based on the drawings and/or models that you may have in other software.
What is unique about the Akselos approach:
- We use a component-based approach, in practice, this means dividing the structure into many ‘lego-bricks’ and pre-calculating how these bricks interact under a wide range of conditions. This approach enables us to give you insights at the mm level across the entire structure in one single model, which is critical when assessing fatigue locations.
- Our models can be fed with all (relevant) load history data, such as wave height, wave period, current velocity, wind speed, and as well as inspection, repair and loading level information. This is key as it enables us to calculate fatigue based on how the vessel has actually been used (versus making assumptions or averaging)
Step 2: Carry out Preliminary Fatigue Screening to Determine Hotspots
In this step, we run an assessment which evaluates fatigue strength of thousands of structural details in the different regions of the entire cargo region to identify critical locations for more refined analysis. The analysis considers all load cases that the FPSO may experience during its service life, such as extreme waves and hurricanes. The fatigue screening is done to industry standard (as introduced in DNVGL-CG-0129 guidelines). For one customer, we screened over 15,000 fatigue locations and were able to narrow this down to 230 critical hotspots.
What is unique about the Akselos approach:
- No more sub-models: as a result of our component-based approach, you no longer require a sub-model when you need a higher-resolution analysis of a certain part of your structure. This is a really important feature of our approach since sub-models introduce conservatism because they can’t ‘interact’ with the rest of the structure and, as such, require (conservative) assumptions at the component boundaries. For you as a user, our RB-FEA technology gives you more accurate results in a much shorter simulation time.
Step 3: Run Spectral Fatigue Analysis and Low-Cycle Fatigue Assessment
After locating the potential hotspots from the previous step, these locations are then analyzed with a refined mesh size (1t x 1t, with t being the minimum plate thickness around the hotspot) and evaluated for fatigue strength by running a Detailed Fatigue assessment.)
Concurrently, ‘Low-Cycle Fatigue’ is carried out by running a spectral fatigue assessment to determine the total fatigue damage arising from changes in stress level caused by loading and unloading of the FPSO.
What is Unique about the Akselos Approach:
More compute power: conventionally developed procedures for legacy FEA analysis utilize PC Windows-based Software, limiting its processing capability. Apart from the underlying technology differentiation in calculation methods, Akselos also utilizes a cloud-based or server-based application, giving its users more power to run these complex and intensive simulations at ‘the speed of operations’.
Step 4: Present Output in Actionable Format
Finally, the user will get access to the fatigue assessment results through an intuitive user interface, with the option to generate an export PDF fatigue report (these reports can be customized to your or regulatory standards as needed). The report can, for example, describe the detailed locations of the identified critical hotspots and summarize their fatigue lives. This makes it very convenient for customers to understand the key weak points of their FPSOs, and propose practical plans for maintenance, inspecting, and/or repairing their vessels.
What is Unique about the Akselos Approach:
- We provide you with an update on the fatigue life across your entire unit as new data comes in (such as weather info, new inspection data, tank loading levels, etc.). This means that you can always make decisions based on the asset ‘as-it-is today.’
- As a user, you have full access to the fatigue results and can seamlessly move through the vessel, zooming in and out, to understand where your critical hotspots are. This is essential to set informed maintenance, inspection, and repair strategies.
Conclusions
With Akselos’ RB-FEA technology, large-scale FPSOs can be fatigue assessed in their entirety with incredible speed and accuracy due to fast response time, e.g. 1000x faster than conventional finite element analysis (FEA). The resulting fatigue lives of potential critical hotspots provide shipowners/ operators with actionable information to implement the appropriate maintenance, inspection and repair strategies for their vessels. In addition, it improves safety through an improved understanding of risk, so why settle for anything less?!
