Looking Into Shallow Offshore Sites - Article Example

As a result of the dynamic nature of the earth, tripled with earth movements and unstable atmospheric conditions that result in destructive and sometimes life threating experiences, need arises for reliable and relevant data about ground conditions to enable assessment and planning of potential offshore field developments. This cushions structures and developments that configure the offshore areas after the development and their surrounding from destructive but predictable natural disasters. Since offshore ground construction works cannot be carried out in situ where the installations are to be located, comprehensive data acquisition is required in the entire area affected and far down beneath the sea floor. Such processes that encompass offshore site investigations are varied in nature. They range from analyzing marine geological information, scrutiny of available geophysical data which is used to plan the actual investigations. Such processes that lead to the success of the above mentioned range consist of drilling, sampling as well as in situ testing which in essence includes penetrating into the seabed with the help of high technology drilling vessels. These processes encompass what is generally referred to as Geotechnical investigation. Introduction: From The period 1985 to 1982 Lunne and Powell (1992) gave a review of developments in offshore investigations. They explored the various technological inventions that marked this period and discus the contributions of such developments to offshore studies with new in situ tests being tried out in the offshore environments, including several examples of field model testing. Lunne and Powell observed the general trend over the last 6-8 years which was the gradual increase in deep water developments. Due to the difficulty of taking undisturbed samples in deep water there has been a tendency to rely more on situ testing. Special geotechnical problems associated with geo-hazard evaluations have also inspired developments within the field of in situ testing. In addition the general competitiveness of the market has been a driver cost efficient solutions. Ground investigation in shallow offshore sites a.)Soil sampling Borings, the most efficient and probably accurate technique of shallow offshore studies come in two main varieties, large-diameter and small-diameter boring. Large-diameter borings Peres involve offshore drilling with large enormous machines that bore extensive areas. They are rarely used due to safety concerns and expense, but are sometimes used to allow a geologist or engineer to visually and manually examine the soil and rock stratigraphy in-situ. In the first years of offshore investigations, from about 1972 to 1983, seabed rigs used a hydraulic cylinder to push in the cone rods in a discontinuous manner with stroke length of up to 0.9 m. Nowadays the majority of testing is done with continuous penetration. Small-diameter borings are frequently used to allow a geologist or engineer examines soil or rock cuttings from the drilling operation, to retrieve soil samples at depth, and to perform in-place soil tests. Mokkelbost, K.H. and Strandvik, S. (1999). b.)In-situ tests An in-situ dynamic penetration test prepared in such a way to provide leads on the properties of soil, while also providing an idea of a disturbed soil sample for grain-size analysis and soil classification is known as A standard penetration test (SPT). The role of in situ testing is generally even more important in offshore soil investigations than onshore. In most offshore investigations insitu tests play an important role. A trend that is observable in in-situ tests is the rise in the deployment of minirigs. To transport the equipment used in minirigs is easier moreover smaller vessels can also be deployed which renders the whole exercise at pocket friendly prices. The major setback with regard to minirigs is that nonstandard equipment such as, cone penetrometers with cross-sectional areas of 1 to 5 cm compared to the standard 10 cm, is used and the end result on the overall findings is often inaccurate and can not be dependent on to make sound conclusions. When considering whether seabed or down hole deployment should be used the disturbance caused by drilling should be considered, generally higher quality testing can be obtained with seabed testing. This mitigates the effect of trans location wherein in its process changes may occur that may not read from the same palm as the original data. A new very interesting method whereby CPTs can be carried out while drilling is advanced facilitates continuous testing to large depths. Regarding the in situ tools in use today the CPT/CPTU is by far the most important. In the paper examples of its use are included and special discussions are focused on the accuracy that may be achieved in the two operational modes and the use of non-standard sizes. In some geographical areas the vane test is also used regularly. For special purposes the seismic cone and the BAT/DGP probes are used on large important projects. Mokkelbost, K.H. and Strandvik, S. (1999). Figure 1. Fugro’s downhole XP system (after Hawkins & Marcus, 1998). Advantages of the down hole type testing: Down hole type testing can penetrate up to 150 m and more With Down hole type testing, hard layers can be penetrated by drilling it is possible to do a combination of different types of in situ tests and /or sampling in the same bore hole C.)Laboratory tests Loading Rig Oxford has developed a three degree-of-freedom loading rig. Martin, (1994) purports that, initially it was designed to explore the behavior of spud can footings on clay. Since then it has seen several changes and is adaptable to any soil medium. One notable aspect is that an arbitrary displacement path can be applied to the model footing, using computer controlled stepper motors and moreover Independent control of the three components of displacement is accomplished by using separate bearing arrangements, and by superposition of the different motion systems. Its displacement ranges for the vertical, horizontal and moment are given as 300mm, 50mm and 30° respectively. The main work of the footing is determined by getting the scores for the resultant loads using a ‘Cambridge’ load cell, whilst foundation displacements are accurately found using a system of LVDTs. The primary merit of using this displacement-controlled apparatus is the ability to explore strain softening behavior. Dry Sand With sand, two different types are used. It is tested while dry so that only its drainabilty is analyzed. For instance, According to Palmeira, (1987); Schnaid, (1990) ,White and Yellow 14/25 Leighton Buzzard Sand are very uniform silica sands (coefficient of uniformity of 1.3) with an angular grain shape, and have been used in a number of experimental studies. The sand has a D 50 of 0.24mm and a coefficient of uniformity of 2.75. The loose samples are prepared by carefully placing the soil within the sample container from a scoop. This method enables very loose sand samples to be prepared with relative densities of about 20%. To prepare denser samples of sand a vibration is applied to the tank until the appropriate density is reached (Byrne, 2000; Lau, 1988). The main parameter used to characterize the dry sand samples is the relative density. Vertical Loading Tests Vertical loading tests are essential for developing expressions to describe the hardening law within the plasticity models. Martin (1994) and Cassidy (1999), purport that the vertical load-displacement relationship can be employed as the less complex description of the hardening law. Villalobos et al. (2003) give out a series of tests that analyze the load penetration curves for various skirted footings on dry sand. The variables for the footings are given as- diameter 51mm and wall thickness of 1.6mm but with skirt lengths varying from 0mm to 102mm. A series of tests carried Windfarm Investigation, Lincolnshire Coast out, including tests on loose and dense silica sand and tests on loose carbonate sand give the relative density as 88%. In the beginning, the tests followed a common load-penetration curve as the skirts were forced into the sand. Once the base makes contact with the surface of the sand the load increases quickly until a peak is reached. Conclusion The rising desire to find out more about deeper waters has led to the rise of light minirigs which can be used to carry out shallow tests from less expensive ships compared to the traditional more heavy equipment. Besides cutting down the cost, this type of equipment can be assembled with utmost speed and efficiency thus rising up to the urgency of the moment as compared to the bulky nature of traditional cumbersome equipment. One problem that ought to be addressed is the use of non-standard sizes of in situ tools; e.g. cone penetrometers with a cross sectional area of 1 to 5 cm2 are used instead of the standard 10 cm2 . For ease of operation data from the tests are frequently stored in a memory unit instead of being transferred through a umbilical. Systems utilizing transfer of data by acoustic telemetry are also entering the market. Lunne, T., Robertson, P.K. and Powell, J.J.M. (1997). References Lunne, T., Robertson, P.K. and Powell, J.J.M. (1997). Cone Penetration Testing in Geotechnical Practice. Spon Press, London. Marchetti, S. (1997). The flat dilatometer; design applications. Keynote lecture. Third Geotechnical Engineering Conference, Cairo University, January 1997. Mokkelbost, K.H. (1996). Shallow gas detection using NGI’s BAT probe. Proc. 4th International Conference on Gas in Marine Sediments. Varna, Bulgaria 28-30 September 1996. Mokkelbost, K.H. and Strandvik, S. (1999). Development of NGI’s Deepwater Gas Probe, DGP. Proc Conference on Offshore and Nearshore Geotechnical Engineering, Geoshore, Panvel, India, December, 1999. pp. 107-112. Peuchen, J. (2000). Deepwater cone penetration tests. Proc Offshore Technology Conference, OTC, Houston, May 2000, Paper No.12094. pp. 469-477. Powell, J.J.M. (2001). A comparison of different sized piezocones in UK clays. Submitted for Publication. Read More