Research On The Assessment On The Bearing Capacity Of The Offshore Workover Rig Substructure In Service

Offshore workover rig consists of derrick and substructure,plays an important role in offshore platform workover operation.Substructure bears the whole offshore workover rig weight and working load,as a result its capacity directly influence the saftyness and production of the workover rig.Many bearing beams of the offshore workover rig substructure in service of which working time approach the design life have the problem of serious corrosion for the marine serious environment and limitation of maintence,resulting in huge reduction of the substructure capacity and danger to the production of the substructure and workover rig.At present,the acutual substructure stress distribution can not be achieved for the lack of equipment to conduct offshore workover rig substructure in service field stress test.Therefor people mostly use the finite software to analyze substructure stress and displacement distribution where the substructure establishment drawings and beam thickness data measured are used to establish the substructure finite calculation model.However,point corrosion damage,local section corrosion damage and perforation corrosion damage contained in the substructure beam are not included in the finite model,causing large calculation error.In the dissertation,offshore workover rig substructure in service main beam strength effect law caused by corrosion damage was studied and a marine platform offshore workover rig substructure in service bearing load was assessed with substructure corrosion damage taken into account.In order to conduct offshore workover rig substructure in service stress test,a loading device for offshore workover rig substructure in service was designed.The following are the research conclusions.1.When the substructure is at work,the strength of the main bearing beam which suffers larger stress than other beams can be important to the substructure bearing capacity.Firstly,the types and features of the common corrosion damages contained in the offshore workover rig substructure main beam in service was inducted through the field test and previous research results:point corrosion damage,local section corrosion damage and perforation corrosion damage.And damage coeffient was put forward to represent the influence of the corrosion damage on the substructure main beam strength.Then the effect rule of the offshore platform workover rig substructure main beam strength caused by the common corrosion damage above was analyzed using ANSYS software,in which the stress distribution of the corrosion area,corrosion damage coeffient and their change laws were attained.Based on the study,the specified method of the substructure main bearing beam finite element model with corrosion was introduced.2.An effective process of the assessment on the bearing capacity of the offshore workover rig substructure in service with corrosion was produced in this dissertation,and the process turned out to be scientific and correct through experiments.The substructure was tested and measured,which indicated that the substructure was serious eroded.The FEA modeling method that uses solid element and beam element respectively to simulate the beams with corrosion damage and beams without corrosion damage costs a lot of time and needs too much field work.For this reason,the dissertation used beam 189 element to simulate the whole substructure model based on the field test data,and the substructure stress and displacement distribution under loading conditions were attained through FEA numerical analysis.The substructure intensity calibration was done according to SY6326.Then main bearing beam with corrosion damage solid FEA model was established and main bearing beam stress distribution was analyzed through ANSYS software.The substructure main bearing beam with corrosion damage intensity calibration was done through the fourth strength theory.The assessment conclusion was that the maximum rated static hook load,the maximum static rotary load,and the maximum rated setback load that the substructure can bear was respectively 1200000N,900000N,and 590000N.3.A loading device was designed to apply static hook load and rotary load to offshore workover rig substructure in service in field,which provides effective approach for more accurate assessment on offshore workover rig substructure in service bearing capacity.