Accidental Impact Resistance of non-disconnectable bouy type FPSO
The surroundings of offshore structures consist of many components controlled by nature, but also some that are managed by people- ships. Due these factors, ship collisions are a constant threat to offshore installations because human error can occur.
This report consists of two main problems: the effect of ballast water in collisions, and the effects of decoupling the collision problem into internal and external mechanics. Both of the problems are analyzed using LS-DYNA. The finite element models for the analyses are created using MSC Patran.
Ideally, the effects from fluids were to be included in both problems. How to model fluids and fluid structure interaction was therefore learned. A simple analysis where a platform floated with an almost constant draft was successfully performed. The vertical motion was only 8 cm.
To verify the fluid modeling in LS-DYNA, analyses were performed to calculate added mass coefficients. The coefficients were compared to added mass coefficients calculated in Wadam with the same geometry. When comparing the added mass coefficients, it was found that they did not match at all for periods over 10 seconds.
The first collision problem investigated, is the effects of internal fluids in collisions. The case used in the analyses, is a rigid sphere impacting a stiffened tank. Analyses are performed for different filling levels and impact velocities. The energy dissipations and contact forces are compared to that of the empty tank.
The results show that the presence of ballast water has a clear effect on both the contact force and energy dissipation. The contact force, for a given deformation, increases when water is present in the tank. The increase is however small when the impact speed is 2 m/s before the water reaches the ceiling of the tank. For larger impact velocities, the presence of water clearly strengthens the ballast tank for all defomations.
When there is water in the tank, the energy dissipation is larger and the maximum contact force is in the same range as for the empty tank. It is concluded that since the ballast water strengthens the tank, it is conservative to neglect the internal fluid for the tank, and non-conservative to neglect the internal fluid in the tank for the impacting structure.
The second problem considered, was to study the effect of decoupling the collision problem into internal mechanics and external dynamics. Unlike the first problem, analyses including the effects from water without simplification are not successfully performed. A collision between Moss Maritimes Octopus and a shuttle tanker is used as a case. Four different realistic collision scenarios are chosen to be analyzed. The scenarios are analyzed using integrated analyses, and by decoupling the problem into external an internal mechanics.
The results for the decoupled analyses were quite similar to the integrated analyses. The energy dissipation was successfully calculated within 10 \% for all the analyses with an impact angel of 30 degrees and less using simplified methods. It was seen from the analyses that the deformation of the ship estimated decoupling the problem was typically too small, and the deformation of the platform, typically too large. The decoupled analyses were therefore conservative from the platforms point of view, and non-conservative from the ships point of view.
Place, publisher, year, edition, pages
Institutt for marin teknikk , 2014. , 149 p.
IdentifiersURN: urn:nbn:no:ntnu:diva-25091Local ID: ntnudaim:10926OAI: oai:DiVA.org:ntnu-25091DiVA: diva2:730493
Amdahl, Jørgen, ProfessorStorheim, Martin