Study On The Fatigue Life Prediction Methods For FPSO Structures

Floating Production Storage and Offloading vessels (FPSO) are the ship-shaped offshore installations, which will subject to much harsher environmental loads compared to trading oil tankers. Therefore, particular fatigue endurance and fracture strength should be satisfied for the hull structures of FPSO to ensure the continuous operation for more than 40 years during the entire production period in a supposed field.According to the investigation on damaged FPSOs, fatigue damages are prone to arise in the structural sites or units as cutouts and connections of longitudinal/frame transition, which bears out the necessity to improve the practical structural form based on fine finite element numerical calculations. Since the loading and unloading are quite frequent for FPSO compared to oil tankers, the low cycle fatigue damage must be certainly considered. Fatigue cracks may be initiated by many factors as structural misalignments, the design and operational flaws, and harsh environmental conditions. During the crack initiation stage at the weld toe, cracks can be repaired by grinding treatment to elongate the fatigue life of the structure. To date, the research work related to the fatigue strength of FPSO is mainly on how to prevent crack initiation but neglects the treatment of damaged structures. As the post-research of"beyond rule research on large ship structures", the following efforts have been made in this thesis:(1) A literature review of the related research works, including the current fatigue strength study methods of ship structures, the research progress of low-cycle fatigue strength and the existed conclusions for the fatigue strength of FPSO, is summarized, in which a description of the existing approaches, development procedure and applied conditions is introduced.(2) Practically investigated the damage conditions and distributions of the hull of a FPSO repaired in dry dock and a certain bulk carrier, the analyses on the obtained data are carried out.(3) On the basis of the investigation results for the fatigue damages of FPSO cutouts, three dimensional finite element numerical calculations are carried out to calculate the cutout stress concentration factors. And a new structural form which is more convenient for construction compared to current rules is put forward accordingly with the reference values of stress concentration factors.(4) On the basis of the investigation results for the fatigue damages at connections of longitudinal/frame transition of aged ships and according to the advised connection form in JTP rule, three dimensional finite element calculations are carried out to obtain the stress concentration factors at the connection of longitudinal/frame transition in the water line of FPSO, using the solid element model with the consideration of weld. And the possible improved structural forms were analyzed when the demanded fatigue life is not satisfied.(5) The effects of the low cycle fatigue damage on the hull structures of FPSO are analyzed and the fatigue life of a connection of longitudinal/frame transition for a certain FPSO hull structure is calculated using the fatigue assessment method considering low cycle fatigue correction.(6) The weld repair method is proposed for the cracked T-joints of FPSO hull structures, and the simplified two dimensional finite element calculations are carried out on the repaired T-joints under the tension and bending load conditions. Accordingly, the stress concentration factors are solved and the optimum repair profiles with the relative lower stress concentration factor are adopted in three dimensional finite element analyses with solid elements, which are used for analyzing the effects of plate thickness and repair profile length on the stress concentration factors of the joints.(7) Considering the condition that surface cracks may arise at the bottom of the repair profile of T-joints, a series of finite element models are established combined with the theory of linear elastic fracture mechanics for the calculations of the stress intensity factors at the bottom and the end point of surface cracks with different dimensions under tension and bending load conditions. Some empirical equations for the stress intensity factors of surface cracks initiated at the bottom of the repair profile of T-joints are obtained by analyzing the calculation results.(8) An experimental study on the weld repair by crack grinding treatment at the weld toe of the T-joint is made to validate the feasibility of the repair method for elongating the fatigue life of the structure. And the engineering applicability is proved to be satisfactory by comparing the results from the pre-cracked test and the finite element analyses. Through the studies,the following main conclusions can be drawn:(1) The stress concentration factors at the sites of cutouts and the connections of longitudinal/frame transition are analyzed by finite element method using solid elements with the consideration of weld for better modeling the practical structures. Under the current construction conditions of the ship yards, substituting the reinforcement of cutout with Type G will be more convenient for construction and effectively decrease the cutout stress concentrations. Soft toe setting at the weld toe of stiffener, tripping bracket and backing bracket is proved to be favorable for lowering the stress concentration factor while the structure with a backing bracket most obviously decreases the stress concentration factor as it reduces the space of longitudinals.(2) The low cycle fatigue damage caused by frequent loading and unloading affects a lot on the FPSO fatigue strength calculations. The reasons of the facts with unsatisfied demanded fatigue life could be illustrated by considering the low cycle fatigue together with high cycle fatigue.(3) Surface cracks may arise at the bottom of the weld repairs when repair by crack grinding treatment is implemented for the cracked T-joints of FPSO hull structures. A series of simplified two dimensional finite element models and more rational three dimensional models are established for the calculations of the stress concentration factors of T-joints with repair profiles (surface flaw) under tension and bending load conditions. It can be shown that the stress concentration factor of elliptical weld repair profile would be lower than that of U-shaped weld repair profile when the surface half width of the repair profile is larger than its deepness. The stress concentration of the surface flaw at the weld toe would be increased when neglecting the effect of weld toe, decreasing the thickness of the base plate and increasing the length of the repair profile. And the simplified method of substituting the surface flaw at weld toe by that in a plate would bring more conservative results.(4) Based on the theory of linear elastic fracture mechanics, a series of finite element analyses are carried out to calculate the stress intensity factors at the bottom and the end point of surface cracks with different dimensions under tension and bending load conditions. And empirical equations for the stress intensity factors of surface cracks initiated at the bottom of the repair profile of T-joints were obtained by analyzing the calculation results.(5) An experimental study on the short repair at the weld toe of the typical T-joints of FPSO is made to show that the crack grinding repair method is feasible for elongating the fatigue life of the structure even when the weld toe cracks had propagated to a certain extent. And the engineering applicability is proved to be satisfactory by comparing the results from the pre-cracked test and the finite element analyses.