Page 309 - Marine Structural Design

Part I1

Ultimate Strength

Chapter 14 Offshore Structures Under Impact Loads

14.1 General

Large plastic deformation may develop in offshore structures due to severe ship-platform
collisions. Such collisions are considered to be a dynamic phenomenon that has costly
consequences in material, environmental, and human terms. The dynamic collision response of
platforms should be analyzed at the design stage. This precaution ensures that the structure has
sufficient strength to withstand impact and therefore has a low probability of severe collision
damage.
Petersen and Pedersen (1981) and Pedersen and Jensen (1991) pointed out that after a minor
collision, a considerable amount of the available kinematic energy could be stored as elastic
vibration energy in the affected structure. In such cases, the global dynamic load effects can be
significant and the equations of motion of the structural system, for the striking and the struck
structures, should be established and solved. The elastic-plastic deformation modes of the
structural system in a collision may be classified as (1) indentation of the striking ship, (2)
local indentation of the hit member, and (3) overall deformation of the affected structure. In
earlier studies, the response of the affected structure, excluding the hit member, was treated
linearly. This analysis approach overlooked the possibility of analyzing and treating the plastic
deformation behavior of the affected structure.
Based on Bai and Pedersen (1993), this Chapter deals with the dynamic response of the steel
offshore structure. A system of equations is derived which describes the local as well as the
global elastic-plastic behavior of the structural system. These highly nonlinear equations are
then solved in the given time domain. In order to derive these equations, a nonlinear force-
deformation relation that can model the local indentation of a hit tubular member is calculated.
This derivation is based on a linear elastic solution, numerical results from Ueda et a1 (1989)
and experimental results from Smith (1983) and Ellinas and Walker (1983). Thereafter, a
three-dimensional beam-column element is developed which is used to model the global
behavior of the affected structure. A large displacement analysis of the beam-column elements
is established by combining a linear stiffness matrix, a geometrical stifhess matrix, and a
deformation stiffness matrix @ai & Pedersen, 1991). Furthermore, the effects of plasticity and
strain hardening of beam-column elements are taken into account by the plastic node method.
Some basic numerical examples are presented in order to demonstrate the accuracy and
efficiency of the developed beam-column element. Calculated results are compared with
numerical results obtained from general-purpose finite element programs, reported
experimental results and rigid-plastic analysis results. In addition, the dynamic plastic
responses of two offshore platforms in typical ship-platform collision situations are analyzed.