Jacket Damage Identification Based On Ansys Optimization Analysis

Jacket is the support structure of offshore platform. It is consisted of hollow legs (pipes). If the platform has worked in harsh environment for a long time, it would tends to damage jackets, for example, corrosion led by sea water and quick sand local bending of main legs and fracture of thin rods resulted from collision. As a result, it is quite essential to study and develop methodologies for damage identification of jacket. Structural damage identification methods are divided into local method and integral method, the damage identification method based on the modal is one of the integral method. The former research needs to collect elements around the damage location in order to detect and identify damage, This subject tries to find a way to identify damage location and severity under deep water by collecting data far from the damage location at the overwater part of the jacket. Main work is as follows:1. Take four legs fixed wellhead jacket service at a depth of about20m in the Bohai sea as an example. APDL was used. PIPE59, which unit can reflect the influence of water was used to establish a1:1finite element model. In the study we set up the environment loads and boundary conditions According to the actual situation, then studied The force of the jacket under normal conditions.2. On the basis of investigating the ocean jacket on the basis of several kinds of common injuries, Element Parametric finite-element models which demonstrated several common damages such as corrosion、Main leg bending damage、and Thin rod bending damage by changing geometric parameters were analysed.3. Methods of modal identification put forward by the predecessors were used to identify damage location and severity.Jacket vibration modal data was extracted and calculated in both intact and damage situation. Results show that the curvature modal method and modal strain energy method is better than the displacement modal method, the two methods both can identify the damage location and damage degree well.4. set several common injury at different positions underwater, extract the jacket vibration modal data above the water or Near the surface of the water, find the change rule of several modal parameters (displacement mode and curvature mode, etc.) in the case of damage location change. It was found that damage location change regularly can lead to the upper part of structure vibration modal data change with a certain rule. Damaged legs can be preliminary identificated by getting modal strain energy at the upper part of the legs. In order to get more accurate position, sensitive nodes overwater and far from the damage location was selected, Vibration data in corresponding nodes of the pre-established damaged model was collected to form the objective function. ANSYS design optimization is used to find a model, the data of which at test nodes is almost agree with the pre-established damaged model at corresponding nodes. Then the damage locations and severity would be identified. The result show that in the case of calculation step in optimization module was set as20,000, the accuracy rates of location and severity identification were all over90%on average when there was only one damage, the accuracy rates of location and severity identification were all over70%on average when there were two damage.