PTIL – Konstruksjonssikkerhet i Nordområdene (NORD ST19)
In order to gain insight into the consequence of potential impacts between offshore structures and relatively small glacial ice features, the Petroleum Safety Authority Norway (Petroleumstilsynet) initiated several studies of which this study (ST19) is one of them. The ST19 focuses on the interaction between floating spheroidal glacial ice features (<15 m) and a semi-submersible structure in irregular seas and in the presence of sea currents. The ST19 study comprises three stages, namely:
- Stage #1: Analysis of impact events to establish probability distributions for impact velocities and locations on the structure.
- Stage #2: Simulations of hundreds of impact scenarios using the ‘Simulator for Arctic Marine Structures’ (SAMS) to create energy map showing the demand for energy dissipation at each location on the structure.
- Stage #3: Structural damage assessment for the critical impact scenarios.
Innovative methods and a cutting-edge simulator for ice-structure interactions (SAMS) are utilised in the different stages of the project offering a great insight into the problem and leading to many important conclusions. The results for the structure under consideration in ST19 show that the maximum demand for energy dissipation is 7.32 MJ but with a very low probability. Actually, 90% of the simulated impacts result in an impact energy lower than 4.3 MJ. The shared-energy approach for damage assessment shows that the maximum indentation depth into the structure in ST19 can vary from 0.25 m to 0.55 m depending on the impact locations. As the outer shell rupture generally is predicted to occur at 0.6 m-0.9 m, the structure is considered safe from compartment flooding risks with the given impact energy.
Although the results from this study are specific to the given structure and pre-assumed ice features, the approach is quite generic and could be adopted to other structures and ice geometries. It should also be noted that critical structural members such as risers and mooring lines are not considered in this study and they should be included in further studies.