Study On Ultimate Strength Of Offshore Platform Under Corrosion And Crack Damage

Corrosion and crack damages are two forms of damages that are unavoidable for offshore platforms.They not only weaken the strength of components of platform,but also threaten the overall safety of platform.Therefore,it is important for structural maintenance and service life extension to correctly analyze and evaluate the ultimate strength of structural members and the entire structures under corrosion and crack damages.In this paper,the circular pipe members,joints and overall structure of the jacket platform were taken as research objects,and much attention focused on the adverse effect of crack configuration(crack location,crack length),corrosion form(pitting corrosion,uniform corrosion)and corrosion intensity on the ultimate strength.The specific work and conclusions are as follows:(1)The ANSYS finite element software was used to establish the finite element model of circular pipe members,joints and multi-scale platform with corrosion and crack damages.The correctness of the model was verified by comparison of results with the existing literature.(2)The influences of corrosion and crack damages as well as their combined damages on the ultimate load of the circular tube under axial load(axial pressure and shaft tension)were analyzed.The results show that the ultimate load of the circular tube decreases with the increase of the crack length,increases with the increase of the inclination angle of the crack,and decreases with the increase of the corrosion intensity.With respect to the ultimate strength of the cracked pipe,the inclined crack can be simplified into a crack with no inclination angle by multiplying the crack length(?)by the cosine of the inclination angle(?).Under the same corrosion volume loss,the pitting damage causes larger reduction of ultimate strength than the uniform corrosion.In addition,under the combined damages of corrosion and crack,the ultimate load of the cracked tube decreases with the increase of the crack length,increases with the increase of the inclination angle of crack,and decreases with the increase of the corrosion intensity.(3)The influences of corrosion and crack damage on ultimate strength of TT-type joint under axial load on its one branch were analyzed.The results show that the crack at the weld of the loaded branch results in much greater ultimate strength of the joint than those at other locations.At the critical position,the longer the crack,the smaller ultimate strength of the joint.The ultimate strength of the joint is mainly affected by the corrosion damage of the main pipe compared to the branch pipe,decreasing with the increase of the corrosion intensity.Under the same corrosion volume loss,there is little difference in the reduction of the ultimate strength of the joint for pitting corrosion and uniform corrosion damage.The ultimate strength of the cracked joint decreases with the increase of the crack length,and also increases with the increase of the corrosion amount,under the combined damages of corrosion and crack.(4)The effects of corrosion and crack damages on ultimate strength of overall structure of the offshore platform subjected to horizontal ice load were studied based on the multi-scale finite element analysis method.The results show that only if the crack situates at the particular location of the joint,it has a significant impact to the overall structural strength,and it will gradually decrease with the increase of the crack length.Under the same corrosion volume loss,uniform corrosion on the main joint of the platform causes greater reduction of strength of the platform compared to pitting corrosion.Under the combined damages of corrosion and crack,the ultimate strength of platform will increase with the increase of the crack length at the critical position,and decreases as the corrosion intensity increases.(5)Comparisons were conducted on reductions of ultimate strengths of tubular member,TT-type joint and overall platform structure due to corrosion damage,crack damage and their combination.It was found that the reduction of ultimate strength caused by the combined damages of corrosion and crack can be conservatively approximated by the summation of the reductions obtained from a single factor related to corrosion and crack.