Structure Strength And Fatigue Analysis For Aged Jack-up

Many offshore platforms have been in service for many years in China. Some of them have been up to design life, even exceeded, and are stepping into later service period. Because these platforms work in the harsh marine environment in a long term, corrosion, deformation, cracks and structural damages inevitably occur, which reduce the service ability of the platforms and even affect the operation security of the platforms. So it is necessary to check the safety of structural strength of the aged platforms.In addition,platforms in marine environment will be affected by the cycled waves, and alternating stresses will appear on the leg structure. Although the stress level is below the yield strength, long-term exposure to the alternating stresses may make the platform suffer the suddenly fatigue damage. So it has important significance to apply fatigue analysis to the legs of platform.The strength and fatigue of a mobile jack-up is analyzed based on the"GUIDELINE FOR SITE SPECIFIC ASSE -SSMENT OF MOBILE JACK-UP UNITS"worked out by Offshore Committee.The integral special shell-beam element model of a jack-up is set up based on the FEM and considering the drilling and surviring.The loads of wind, wave and current is synthely considred to solve the stress distribution,and the static strength of main hull and legs are checked.The modal analysis and fatigue analysis were carried out considering the surviring in the storm,and, the free vibration properties of platform are acquired by the modal analysis. Considering the effect of dynamic magnification factor and stress concentration, accumulated damage of legs of platform subjected to cycled load which is caused by wave is calculated by using specture analysis. Also, the longevity is estimated as well.It is shown that the strength of chord and groove area are the restrict factors of platform strength, and the platform is easily occur sympathetic vibration with low frequency wave. In addition, fatigue analysis shows that fatigue damage of tubular joints in the bonding area of leg and guiding device and in the splash area are largest, that is, their service life are shortest. So, the tubular joints of these areas should be paid more attention.