Page 11 - 033

10 Ö. ÖZGÜÇ

material library of LS-DYNA is defined for the RPF, PN supports and the CPF Column
structure. The striking vessel is assumed to be rigid and thus RIGID property as defined in
the material library of LS-DYNA is defined for the striking vessel.
By assuming the striking vessel to be rigid, the colliding energy of the striking vessel is
completely transferred to the struck vessel i.e. RPF support, PN support and the CPF

Column structure, thus the results obtained with this assumption is conservative. Initially the
collision analysis is performed with an objective of achieving zero plastic strain on the CPF
column structure that is supporting the RPF and PN supports but later it is realized that it is
practically not possible to design the structures with no plastic strain for the given colliding
energies. Hence a plastic strain of 5% on the outer shell of the CPF column structure is
assumed to be acceptable. Since the final objective is to check any kind of leakage into the
CPF column due to the collision, it is important to control the plastic strain on the outer shell
of the column. Since the plastic strain of the internal members of the column, RPF and PN
structures, doesn’t cause any leakage inside the column, there is no criterion for the plastic
strain of these structural members.

4.2. Loads and Boundary Conditions

The expected kinetic energy for the collision from the north side is 20MJ. The collision
energy from east/west side is 14MJ for side collision and 11MJ for bow/stern collision.

To check whether the collision simulations are carried out properly, both the kinetic energy of
the striking vessel and the absorbed internal energy of FPSO hull structure are plotted based on
time history. It is confirmed that for the all cases the kinetic energy starts from the required
energy level and the internal energy is exactly same as initial kinetic energy level of striking
vessel. From the analysis results summarized below, it is proven that the collision loadings are
properly applied and the simulations are carried out reasonably.

For the collision from north-side:
Mass of supply vessel is 18,000 tons and the expected collision energy from north side is
20MJ. By considering the DNV rules (DNV-OS-A101), the added mass is taken into
account and thus the required velocity of the striking vessel is calculated so as to obtain
the kinetic energy of 20MJ on the striking vessel.

For the collision from east/west side:
The mass of the striking vessel from east/west side is 7,000tons. The kinetic energy for
collision from east/west side is 14MJ (for side collision) and 11MJ (for bow/stern collision) as
per DNV-OS-A101.

The boundary conditions on the FE model are appropriately given to constrain any kind
of rigid body motion.

Load Cases:

The collision capacities of the RPF and PN supports are estimated along the span of these
structures and hence different load cases are considered for each structure, in which each
load case defines the collision scenario at different points of collision. Figure 14~24 show

GiDB|DERGi Sayı 3, 2015