ترجمه مقاله آزمایش نمونه اولیه برای حذف جزئی و نفوذ مجدد پلتفرم دلفین مهاربند - نشریه الزویر

The partial removal, adjusting, and re-penetrating tests were carried out on a mooring dolphin platform with three-bucket foundations after a seven-year service. When the bucket foundations were penetrated by internal and external pressure differential, the soil shear strength around the bucket skirt was different from that of the intact soil after installation. The side shear strength along the skirt may be increased with time due to the set-up effect. The shear strength factor α is an important time-varying factor to determine the skin friction of the foundation during installation and removal operations. The shear strength factors obtained by prototype records and theoretical methods in cohesive soils are compared, from total stress to effective stress methods. In addition, the prototype tests show that the foundations will be able to resist removal forces that are significantly larger than penetration forces due to the set-up effect after a long working time. The results show an 85% increase in the soil resistance after seven years for the three-bucket foundations with diameters of 6 m and penetration depths of 8.5 m. Meanwhile, the construction procedure and operation of the three-bucket foundations and the analysis of the extraction forces are also given for the partial removal. The test results support the successful removal by a reasonable overpressure without soil plug failure inside the buckets by controlling the pump discharge inside the buckets and reutilizing the structure with multi-bucket foundations after an initial service period of seven years.

1. Introduction

Bucket foundations are a promising cost-effective type of foundation for ocean engineering [1–6]. Among the costs of geotechnical investigation, material, fabrication and installation, the most significant savings are from installation due to self-installed technology. No complex installation equipment and expensive large crane barges are required. The installation time, which is normally within 24 h, is greatly shortened compared with the several days required for a conventional platform foundation. First, the bucket foundation partially penetrates the soils under its dead weight. Then, the differential pressure across the top of the foundation, between the hydrostatic water pressure outside of the bucket and the reduced water pressure inside the bucket, created by pumping out the water trapped inside the bucket chamber from the top of the bucket, in addition to the weight, can trigger further penetration of the foundation to the desired depth. Bucket foundations also have significant bearing capacity, high positioning accuracy, and mobility, which have been successfully applied as effective solutions for anchoring in deep water and have also been applied as the foundations of jacket structures, jack-up rigs, mooring structure and offshore wind turbines [7–12]. Meanwhile, relocatable structures with bucket foundations can be reused at different sites, and the removal of the structure also provides a clean site after exploitation which accommodates environment-friendly concerns.

4. Conclusions

Prototype testing of a mooring dolphin platform with three-bucket foundations for partial removal and re-penetration is described. Removal, adjustment and penetration were achieved by controlling the pump discharge into/out of the bucket foundations. The test demonstrates the removability of MDP2 with bucket foundations after seven service years from 1999 to 2006 and contributes to the development of design procedures for offshore structures supported by bucket foundations. In addition, it is of vital importance that the shear strengths for reconsolidated remolded soil should be considered in the practical calculation with the set-up time. Consequently, the foundations would be able to resist removal forces significantly larger than the penetration forces. The results in this paper show an increase of 85% in resistance after seven years for the three-bucket foundations, with a bucket diameter 6 m and penetration depth of 8.5 m. Therefore, the theoretical calculation of skin friction for bucket foundations is a difficult task, involving a series of variable parameters such as soil characteristics (especially plasticity), set-up effects, and penetration means (by underpressure or by self-weight), etc. Recommendations for the determination of extraction resistance in practical designs would be worked out via further studies on the shear strength of reconsolidated remolded clay around bucket skirts, and the inner and outer interface friction factor increasing with time due to the dissipation of excess pore pressure, increase of horizontal effective stresses and the thixotropy effect.