|Publisher||Offshore Technology Conference||Language||English|
|Content Type||Conference Paper|
|Title||IN SITU BEARING CAPACITY EVALUATIONS|
|Authors||Thomas H. Stremlau and Stevan G. Spencer, McClelland Engineers Inc.|
Offshore Technology Conference, 5-8 May 1980, Houston, Texas
|Copyright||1980. Offshore Technology Conference|
The exact depth of initial support for fixed offshore jacket structures may be very critical for relatively deep water sites having very soft bottom conditions. The prediction of temporary support for mud mats is often very difficult because of physical problems in obtaining reliable soils data in the very soft near bottom soils when operating from a floating platform and using standard offshore sampling techniques. In addition, the accuracy of water depth determinations can be uncertain because of the gradational transition from semi-liquid soil "fluff" to very soft clay.
This paper describes improved methods for obtaining water depths measurements and in situ shear strength determinations for near bottom very soft soils and presents data obtained at two sites in the Gulf of Mexico. A special electrical bottom sensor seated in a drill bit was used to measure water depths. The McClelland Remote Vane mounted on a bottom seated "Halibut" seafloor support device was used to measure and evaluate the variation of in situ undrained shear strengths at depths of 4 to 18 feet below the mud line. In addition, plate bearing tests were performed at each site to provide a more direct measurement of bearing capacity.
Construction of offshore platforms on very soft clay and mud slide prone areas of the Mississippi River Delta has increased in the past few years. The soils at the mudline are typically very soft clays. In some instances "fluff" of semi-fluid soil particles may grade into a very soft clay. In such conditions the mudline must be arbitrarily defined as the surface where a minimum given soil shear strength exists. There are some indications that fathometer readings are affected by the presence of the fluff and the depth being measured often is in question. Pressuremeter readings can also be difficult to interpret since these readings are dependent on the depth of penetration of the pressure sensor into the bottom and on an accurate determination of the specific gravity of the water column. Off the mouth of the River, the specific gravity may be quite variable when river currents are particularly strong. Measuring water depth with a weighted wireline and calibrated counter may also be subject to error because of loss of calibration, slippage of the cable on the calibrated wheel, and unknown seafloor penetration of the weight.
The jacket structure is temporarily supported by mud mats until piles can be driven and fixed to the platform. It is, therefore, important to know the bearing capacity of the near bottom soils for support of the mud mats, the depth to the bearing surface, and the variability of mud mat bearing capacity across a structure site.
This paper discusses investigations at two sites near the mouth of the Mississippi River in about 400 ft of water. Measurements were made to determine water depths at the points of interest using an electrical sensor seated in the bit of the drill pipe. Field bearing capacity tests were performed using a simple test frame fabricated for this project..
|File Size||505 KB||8|