| Shengli Oilfield of Chengbei offshore oil development zone is located in theabandoned underwater delta junction zone of the Yellow River, water depth and thegeology of the area is particularly suitable for the jack-up platform operation. The keyfactors affecting penetration of pile shoe is the engineering geological conditions ofsoil. The penetration depth of spudcan foundations is typically predicted by followingtraditional bearing capacity theory approaches to assess the depth at which theestimated capacity matches the maximum applied loading. In offshore oil and gasdevelopment history, due to the marine engineering geologic investigation andresearch is not enough, failure of analysis on foundation strength stability, cause theplatform legs suddenly penetrating or slipping to occur from time to time. Therefore,the stability analysis of foundation must be carried on, in order to ensure the operationsafety of the platform.The engineering geology and geomorphology characteristics of research area,especially caused by the Yellow River swinging shoreline change and sedimenttransport law is very complex, the foundation soil is often composed of multilayer soil.The shallow subsoil of research area mainly have three kinds of group types, type1:the thick layer of silt over soft silty clay; type2: thin silt over soft silty clay layer;type3: thick soft silty clay layer over silty soil.The low strength silty clay layer issilty clay of delta facies with shallow marine silty clay, the silt is form from dense siltlayer delta subjected to wave and current action jigging transformation form or thelacustrine silt and silty clay before Holocene transgression.Through the analysis more than30field survey data of the ChengBei area inrecent three years, according to water depth, topography, strata distribution,engineering geological properties, suitable platform types and hazard factors, the research area had to be divided to I, II, III and IV block. The depth of water of I blockis very shallow, which affected by the artificial modification remarkably. And offshoredrilling engineering were rarely implemented in this region. Stratigraphic structure ofII block belongs to the type2with less area landslide, fault depression and otherdisastrous factors, is relatively stable in the research area block. Stratigraphicstructure of III block belongs to the type1where has serious rift, landslides, erosionand other hazard factors, is unstable block. Stratigraphic structure of IV area belongsto the type3where has less landslide, fault depression and other disastrous geologyfactors, is relatively stable block. Offshore drilling engineering are mainly distributedin the area II, III and IV area.According to the research data in the recent three years, the predictions of pileshoe depths were generally conservative by following traditional bearing capacitytheory. And the actual penetration depth is often far less than the theoretical predictiondepth, especially in the blocks which have thick-layer of silt or silty clay layer withthe high shear strength (such as II and III blocks). When the stratum is single such asthick clay (such as IV bolck), the predictions by following traditional bearing capacitytheory match well with the measured results.The failure mechanism of soil at the bottom of pile shoe is very complex whenpenetrating which is influenced by soil conditions, pile shoe size, depth and theloading rate etc. The bearing capacity of the foundation is determined directly by thedifferent soil failure mode and failure depth, which needs to consider all aspects of thecomprehensive factors.The results of finite element analysis (FEA) show that when B/H (the ratio ofpile shoe size and thickness of hard soil) is greater than0.3, the bearing capacity ofhard soil need to consider the effect of subjacent soft layer. The more ratio of B/H，the more influence of soft substratum and the less of bearing capacity. When the ratioof B/H is less than0.3, it can ignore the role of subjacent soft substratum on bearingcapacity.The thickness of hard soil layer is smaller, with the increasing of buried depth ofpile shoe, bearing capacity increases less, and critical penetrating depth is shallower. When the hard soil layer thickness is much larger than the size of pile shoes, the ratioof B/H is very small, the bearing capacity of foundation increases first and thendecreases with the penetration depth increasing, piercing damage occurred in a depthin the hard soil layer. When the hard soil layer thickness is relative small, B/H isgreater than the critical ratio, piercing damage occurred in the surface of hard soillayer, with the penetration depth increasing, the bearing capacity of foundationdecreases all the time.The ultimate bearing capacity of hard soil shell over soft stratum by followingtraditional bearing capacity theory tends to be more reasonable when the actual failuremode matches with the particular failure mode assumed in theory. On the contrary,when the actual failure mode does not comply with the assumption failure mode, theresult will be a larger deviation.On the multilayer foundation, the bearing capacity is closely related with thecharacters of all the effect depth range, embodies the comprehensive bearing capacityof soil. Full consideration should be given to the mechanical properties within alleffect depth range about3~3.5times the pile shoe dimension. And thecomprehensive bearing capacity is reasonable.The failure modes of soil in the research area can be divided into the followingseveral types: sandy soil failure mode mainly includes the local punching shear failuretype, the whole punching shear failure tyoe and the whole punching-through type;clay soil failure mode mainly includes lateral plastic flow type, whole sliding type andlocal shear failure type. Different soil failure modes have a significant effect on thebearing capacity of pile shoes.Using the finite element method simulating the bearing capacity of spudcanfoundation, solves the problems that the assumed failure surface of the theoreticalcalculation formula, and the calculation results can not match well with the actualsituation. It can be applied to more complex models and boundary conditions, such asmultilayer soil, eccentric load, and can reveal the progressive failure mechanism ofsoil. The analysis results are more reasonable and accurate, considering the charactersof all the effect depth range, the more closely to the actual situation, and no longer need to be modified. Through several typical demonstrations of well sites, it showsthat the prediction results by the Finite Element Method match well with the on-sitemeasurement penetration values.The hydrodynamic environment of research area is very complex. The effect ofwind, wave, flow and other environmental factors will cause erosion, scourphenomenon around drilling platform pile foundation. These can cause pile shoepenetration depth reducing and the effective bearing layer thickness decreasing, leadto a reduction in the bearing capacity of the foundation and the extra penetration, eventhrough the instability. Analysis shows that, when the pile shoes are in the type1(thethick layer of silt over soft silty clay) region choosing the thick layer of silt as the pileshoes bearing layer, the scouring process should have significant effect to the bearingcapacity. The appropriate engineering measures should be taken timely to prevent orrespond to pile shoe penetrating. |
