A codecheck methodology is ideal for checking the behavior of offshore assets. The methodology combines complementary aspects of existing industry standard codes, covering strength, serviceability and fatigue limit states. It’s particularly well suited to the post-processing of Finite Element (FE) analyses. The methodology allows additional capacity to be used for safety enhancements and improvements needed to extend the platform’s economic life.
Problems with the Current Code Checking process
Although code checking both framed and simple structures (including large diameter stiffened tubulars) is normally highly automated, the opposite is true of complex structures. These structures are commonplace in the offshore industry and are found in modules, plate girders, semi-submersibles, jack-ups, TLPs and FPSOs.
Extending the life of current offshore assets has become a multi-billion dollar industry, especially since the fall in oil prices since the early 2010s. In theory, it’s possible for infrastructure to continue operating beyond its fatigue design life for a limited time. However, offshore assets endure harsh conditions over the decades they operate, which means there are a lot of uncertainties for both operators and regulators.
The current fatigue checking approach, fatigue life calculation based on Stress Concentration Factor (SCF), dates back to the ‘80s and isn’t appropriate for current requirements due to many of the uncertain factors during this code checking process. Engineers have to use very high safety factors which leads to shortened life-cycle of assets. This leaves the offshore industry in an unenviable position of balancing stringent safety requirements with the need to maximize asset life. At a time when considerable attention is being paid to the intrinsic safety of structures, it appears the codes of practice are lagging behind the current needs of the industry and advancements in technology.
How can operators build the safety case to extend the life of infrastructure long after its intended design life?
Akselos provides integrated analysis and design capabilities for the complete lifecycle of any complex offshore structure. Akselos’ comprehensive built-in Utilization code check, developed in collaboration with LIC Engineering, enables structural compliance of offshore structures to international codes such as ISO, API, DNV and NORSOK or any other standards based on the customer’s needs, and review asset capacities with easy to interpret numerical and graphical displays.
Moving on to Fatigue checks, Akselos includes powerful next-generation technology that allows the use of pure 3D shell elements at the joints, coupled with beam elements elsewhere within a global model. Each joint contains very fine mesh at hot-spot places such as weld zones which are aligned to DNV-GL RP-C203 standards.
Using the previously calculated stress, Akselos performs further processing, combining the stress transfer functions with wave data from inspections. This approach delivers extremely accurate results in determining a component’s fatigue life without resorting to highly conservative and inherently inaccurate stress concentration factors (SCF). Moreover, there is no speed penalty to include 3D joints since 3D joints are as fast-solving as 1D Beam with Akselos Reduced-Basis-FEA technology (RB-FEA).
Due to the continuing research and development of various codes of practice and improvements in FE analysis capability, Akselos believed that there was considerable scope for increasing the asset life of Shell’s Offshore Platform situated in the UKCS Southern North Sea (SNS). The integrity of the structure under this new lifetime was demonstrated using detailed Akselos Reduced-Basis-FE analysis and the above Fatigue codecheck methodology. An additional 20 years of this structure fatigue life has now been added as a result.
By Andrew Young, VP of project engineering and services