Study On Drivability Of Large Diameter And Deep Penetration Pile Of Offshore Platform Foundation And In Situ Test

With the rapid exploitation of the offshore oil and gas resources, the constructionof the fixed jacket platforms that have been widely used in the shallow sea is in theascendant. Pile foundations of the jacket platform are taking on some new characters,such as larger diameter, longer length and deeper penetration. These characters canreduce the steel consumption in foundation structure and decrease the cost, but at thesame time, they make the pile driving more difficult. Recent years, several pile refusalcases have been reported during the pile driving of the platform construction in BohaiGulf, and caused enormous economic losses. This highlights three essential problemswith the pile drivability analysis, which need urgent concern in the pile construction ofthe offshore oil platform. Firstly, the conventional pile drivability analysis methodsfocus on the situation when the pile is driven continuously, without considering therestarting drivability after assembling pile segments, while pile refusal is more likely toocure at this period of time. Secondly, there is lack of effective measures to ease therisk of pile refusal during construction. Such as weather, waves and mechanical failurescan intermit the successive pile penetration for some time on the sea, and that will raisethe feasibility of refusal greatly. It is hardly possible to consider those unexpectablefactors in the pile drivability analysis in the design stage. Therefore, the effectivemeasures to relieve the likelihood of pile refusal are quite important for the pile drivingon the sea. Thirdly, there is no field testing method to evaluate the bearing capacity ofthe offshore platform pile in domestic up to now. Either driving to the designpenetration or refusing during the penetration, the key to pile foundation is whether thepile bearing capacity can satisfy the design requirement. In this thesis engineeringpractices and the driving records were discussed; laboratory simulation tests and fieldtests were carried out in order to establish an improved pile driving analysis program.Based on the improved program, the causes of pile refusal was analyzed andengineering measures to low the risk of pile refusal were suggested. Also theengineering practice in Bohai Gulf has proved the adopted measures very effective.The content of the thesis covers the following aspects:(1) The simulation of soil fatigue in analyzing the pile driving continuously canaffect the results greatly. Based on laboratory dynamic triaxial tests, the mechanism of strength fatigue decrease of clay soil around the pile caused by pile driving is studied.The comparative theoretic analysis for sandy soil is also studied. Based on this, animproved method for simulating soil fatigue during the pile continuously driving is putforward.(2) The high strain tests are carried out in an offshore platform project to monitorthe process of the pile drivability and check the bearing capacity by re-strike. The piledrivability analysis results can be verified by the high strain testing results during piledriving. The pile bearing capacity is checked through re-strike a week after penetrating.Based on this, strength changes of soil surrounding the pile after penetrating arestudied.(3) Based on the method of finite difference on one-dimentional wave equations, asimulation model for the restarting drivability analysis after pile segments assemblingis proposed. A complete pile drivability analysis program is presented, it can analyzethe drivability of driving continuously with the improved soil fatigue simulationmethod, and consider soil strength recovery after pile segments assembling caused bythe dissipation of excess pore water pressure.(4) Pile refusal case study is carried out. The prediction analysis of pile drivabilityfor one refused pile is performed with the pile drivability analysis program proposed bythis thesis. And the causation of pile refusal is analyzed by the pile drivability analysisprogram. The measures to low risk of refusing by changing the structure of the pile areintroduced. And these measures have been applied to other platforms in the same areaand approved very effective.