Study On Collision Damages, Reliability And Fatigue Life Assessment Of Offshore Platform Structures

The exploration for oil and gas in ever increasing water depths has given an impetus to research efforts on the behaviour of offshore structures under severe ocean environment. These structures are continuously subjected to environmental loading because of waves, wind and current. The safety and reliability of offshore platform structures are one of the most important research areas on ocean engineering with difficulties and challenges. In this paper, Special studies are placed on the large and complicated offshore jacket platform structures, which are introduced in the current trends of development and research. The research works include: the collision mechanism between ship and offshore platform jacket structures, repair and strengthening schemes assessment of the damaged members of jacket structures, the fatigue life reliability analysis of the members and tubular joints of WEN 13-1 platform jacket structures being repaired and strengthened; Strength check and reliability analysis of tubular joints, ultimate bearing capacity structural system reliability analysis, fatigue life reliability assessment and fatigue life evaluation of welded joint crack for BZ28-1 Single Point Mooring (SPM) offshore jacket platforms being served for several years. The major contents are summarized as follows: (1) On the base of the damaged member inspected results of WEN13-1 offshore jacket platform being impacted by the large tonnage Derrick & Lay Barge during installation of steel jacket structures, numerical simulation analysis is applied to the collision course, modeling from simpleness to complexity to simulate the impact load. A mechanics model is presented for the static and dynamic analysis of ship-platform collision system, static equivalent intensity analysis is applied to the damaged X diagonal brace member of platform structure, the value of impact load is 2.45 MN. In the course of dynamic elastic-plastic analysis of offshore platforms, the added mass coefficients for the seawater dynamic effect and the structure-pile-soil mutual interaction are considered in the analysis of computational model. The nonlinear spring is introduced for the simulation of the force-deflection relationship of the local indentation for the damaged X diagonal brace, the curve of indentation deformation ofthe damaged member and different collision contact time is simulated under the velocity of ship being presumed. According to the curve, the formula of indentation deformation of the damaged member and different collision contact time is obtained to determine the maximum impact load being 3.405 MN. The damage extents of total platform jacket structures are also evaluated to choose feasible and reasonable repair and strengthening schemes of grouted clamping for the damaged members.(2) The structural dynamic responses and reliability theory for offshore platform structures under stochastic wave loads are applied to assess fatigue life of the deep-water damaged jacket platform structures having been repaired and strengthened. The accessory structures and added mass having evidently effect on dynamic characteristics and fatigue life of structures are included in the numerical calculation models. The soil reaction is modeled as non-spring and damper, then according to t-z curve% Q-z curve and p-y curve measured in the sea field geology, the effects of structure-pile-soil mutual interaction are systematically studied. The calculating results show that the first-order modal of structures is primary contribution to dynamic response, the fatigue life of the members being connected with the damaged member are obviously decreased in contrast to non-damaged conditions, and they are all satisfied with operating requirements in design life term, the stochastic wave spectrum analysis technique is reasonable and feasible.(3) The study thoughts are presented in this paper about the damage analysis of platform structures for collision between ships and offshore platforms, the assessment of repair and strengthening schemes for the damaged...