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.

Lift installation of a major marine or OFFSHORE structure necessitates detailed evaluation of inter-dependent engineering and construction constraints that influence the feasibility, safety and cost-effectiveness of the lifting operations. Due to the advancements in heavy lift technology, large modularized ship blocks may be fully outfitted, and then lifted and joined to form the entire ship. Similarly, an OFFSHORE structure may be fabricated in a yard, transported to the selected OFFSHORE location, and then installed by lifting. The objectives of this paper are to find the locations of optimum positions of OFFSHORE platform heavy lift points using the method of evolution strategies either minimizing the moment or maximizing the natural frequency. The results obtained are compared with the published results. The optimal positions of simple supports are located to maximize the fundamental frequency of beam or plate structure taking into account both elastic and rigid supports. The minimum stiffness of a simple support (or point) that raises a natural frequency of a beam to its upper limit is investigated for different boundary conditions. The solution also provides insight into dynamics of a beam with an intermediate support for more general boundary conditions.Finite element approach is used for solving the problems. The computer packages such as PREWIN/FEAST, SAP90 are used for performing finite element analysis.

Most of oil and gas OFFSHORE PLATFORMS are located in the seismic regains. Therefore, Seismic vulnerability evaluation of the OFFSHORE PLATFORMS is one of the essential vital issues in the structural systems. In this article, jacket type OFFSHORE PLATFORMS are examined by incorporating the pushover analyses and nonlinear time history analyses, in such a way that, first some push over analyses are performed to detect the more critical members of the jacket platform in consonance with the range of their plastic deformations. Subsequently, nonlinear time history analyses are performed, concentrating on the critical members, to examine the vulnerability of the jacket platform under intensive earthquake loads. Pursuant to the numerical results, the combination of the push over analyses and nonlinear time history analyses proposes a reliable and swift seismic assessment procedure to evaluate the seismic vulnerability of the OFFSHORE PLATFORMS. Moreover, seismic vulnerability of the OFFSHORE structures is dependent on the critical member locations and their load bearing situations in the OFFSHORE structures.

Due to the severe environmental condition, steel jacket type OFFSHORE PLATFORMS are highly vulnerable to damage; so, ensuring its proper performance and detection of probable damage is very important and undeniable. The assessment of existing OFFSHORE PLATFORMS is relatively a new process and has not yet been standardized as the design has. To do so, seismic assessment of the existing 4-legged steel jacket type OFFSHORE platform placed in the Persian Gulf (SPD12) is presented in this paper. Based on an actual platform structure and its mechanical model, the parameters which may affect the rate of shock absorption are analyzed, such as the condition of the site where the platform is located. Assessment is done by finding the seismic damage spread in the structure. Therefore, by using some documents such as FEMA-356 and ATC-40 developed for seismic assessment of buildings, seismic damage of the jacket-type platform is predicted with pushover analysis, and damage spread in this structure is predicted. This method could be very simple and detect damage spread precisely. As a result, knowing the weaker points of the structure which will be the first and third levels of the PLATFORMS similar to SPD12 in structural system can help to detect the damaged places, and by improving the capacity of the structure locally, the structural damage spread will be delayed.