Paper Information

Journal:   JOURNAL OF FACULTY OF ENGINEERING (UNIVERSITY OF TEHRAN)   May 2002 , Volume 36 , Number 1 (75); Page(s) 25 To 33.



During their operating life on site, jacket structures may be subjected to boat impacts. It is useful to know the boat impact intensity a structure can withstand without considerable damage or disruption of normal operations. Furthermore, it is advantageous to know the maximum impact intensity that could cause platform failure. This paper deals with ship impact analyses of SPD1, one of south pars field wellhead jackets located in Persian Gulf. The collision velocities are considered to be 0.5(m/s), 1(m/s) and 2(m/s). Each case was analyzed for two load cases, i.e. one representing impact on the brace and the other reflecting impact on the leg. The striking structure and struck structure are considered as one structural system connected by contact element. In order to allow for local deformation of the impacted leg or brace, a non-linear spring was used at the point of impact on the appropriate part of impacted member. The supply vessel was modeled at one end of the spring; the ship and impacted member springs were separated by a contact element as to allow rebound to occur. To account for hydrodynamic effect added mass is used. Jacket and piles are modeled together while the pile can slip axially relative to the leg. Dynamical pile-soil interaction is accounted. During simulation of impact scenario the concentrated mass, representing the displacement of the ship was prescribed with the required initial velocity. It is shown that strain hardening and large displacement play an important role in the impact response; therefore a rational collision analysis should take into account the effect of large displacement, plasticity and strain hardening. Impact velocity and impact location have significant effect on the structure response. When brace is impacted the 3-hinge mechanism becomes the predominant absorbent of plastic energy and shipside indentation remains elastic. As impact velocity increases, plastic zone is extended in impacted member section. In impact velocity recommended by API (0.5m/s) impacted leg remains elastic and plastic zone of brace section is bounded to outer fibers of section. It is recommended to use impact force derived in this study to consider the behavior of tubular members under impact loads.

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