Offshore Australia Research Articles

Crushing of Calcareous Sands During Drained Shear

Abstract Sands are known to crush under stresses at high confinement. Particle crushing is more predominant under shear stresses than under isotropic compression. Stresses in soil under deep foundations often reach the range where crushing becomes significant. Calcareous sands crush more readily than terrigenous quartzitic sands. Calcareous sands occur as skeletal remains of marine organisms and/or as nonskeletal oolites of calcareous material. Skeletal calcareous sand grains are characterized by the presence of intraparticle voids that increase the tendency of these sands to crush under stress. Presence of thin plate-like shell fragments further increases the crushability of calcareous sands. Deposits of calcareous sands are encountered extensively on the continental shelves lying between latitudes 30°N and 30°S. In many such marine areas, jacket-type offshore structures for oil production are being founded on calcareous sand strata – e.g., in the Arabian Sea offshore India, in the Bass Strait offshore Australia, and in the Persian Gulf offshore Saudi Arabia. The foundations of these structures consist of deep-penetration piles. These piles derive their axial load capacity from the skin friction resistance and the end bearing resistance offered by the soil strata they penetrate. Static pile load tests in calcareous sand strata have exhibited unusually low values of unit skin friction and unit end bearing capacity.1,2 It is believed that particle crushing that occurs during pile driving and static loading is one of the causes for the low pile capacity.2,3 For design of deep-penetration piles in calcareous sand strata, low values of limiting unit skin friction and limiting unit end bearing capacity currently are adopted in comparison to those being used for terrigenous quartzitic sands.3 To arrive at a rational design practice for piles in calcareous sands, one must begin by (1) identifying a method for quantitatively expressing the the sands' propensity to crush, (2) determining the extent of crushing of different types of calcareous sands in comparison to terrigenous sands, and (3) isolating the influence of crushing on shear behavior, which partly controls the end bearing and skin friction offered by such sands. This paper describes the results of an experimental investigation designed to achieve these three objectives and, in part, offers an explanation for the low skin friction and end bearing values in field tests.

Evaluation of Fatigue Considerations in the Design of Framed Offshore Structures

This paper evaluates the implications of fatigue criteria on platform design using recent experiences with the Maui Platform A, offshore New Zealand. The importance of realistic fatigue criteria is noted and various methods to analyze fatigue are examined. The impact of fatigue on the weight and cost of a platform is discussed. Introduction Offshore structures installed in the hostile North Sea can be subjected to fatigue loading far exceeding that experienced in the Gulf of Mexico. The China Sea and offshore Australia and New Zealand are other areas that impose a fatigue premium. Several studies have dealt with analysis and design of fatigue, focusing on the estimation of stress-concentration factors at the nodes of frame structures. This study evaluates the significant elements of fatigue design using recent experiences with the Maui Platform A, installed offshore New Zealand. It evaluates the importance of realistic fatigue criteria and examines various methods for analysis. A comparison of stress-concentration factors found using several proposed equations with a finite-element analysis is presented. Weight and cost implications are discussed. Maui Platform A The Maui Platform A (Fig. 1) currently installed in 354 ft of water about 23 miles offshore Taranaki, North Island, New Zealand, is composed of a 432-ft tower, topped by three deck levels for the development drilling, production, and processing of 660 MMscf/D of gas and 20,000 production, and processing of 660 MMscf/D of gas and 20,000 B/D of condensate through 12 wells. The four-legged tower is 70 x 160 ft at the top and 160 x 160 ft at the base, with battered broad sides and vertical narrow sides (Fig. 2). Two legs are 22 ft in diameter, narrowing to 6 ft at the top. These legs served as pontoons to float the tower on a 5,200-mile journey from the manufacturing site in Japan. The other two legs are 6 ft in diameter, increasing to 16 ft in the bottom panel. The tower is anchored to the sea bed with panel. The tower is anchored to the sea bed with twenty-eight 48-in.-diameter steel piles penetrating 240 ft. The tower float-out weight is 14,020 kips. Maximum deck loading is 24,000 kips. A 100,000-kips mass is used to calculate the dynamic properties of the platform. Steel specifications required normal DIN steel with 50-ksi yield strength. To enhance the resistance of the structure to fatigue, improved DIN steel with special impact and through-thickness properties was specified at the joints. Approximately 2,500 kips of this special steel was required. Further discussions of the tower are given in Refs. 1 and 2. Every joint in the structure was analyzed for fatigue. Cyclic stress for the in-place platform was computed using wave forces determined by Morison's equation and Stokes' fifth order or Airy wave theory. A mass coefficient of 1.5 was used for members 10 ft or less in diameter. This coefficient was increased to 2.0 for members greater than 10 ft in diameter. A drag coefficient of 0.5 and 4 in. of marine growth on the diameter of members below the water line were used. The drag coefficient was increased to 3. 0 to consider wave slam. The effect of the towing on fatigue life was considered also. JPT P. 357

Exploration on Continental Shelf Off Northwest Australia: ABSTRACT

Exploration of a 144,000-sq mi rank wildcat area in offshore northwestern Australia began in 1964 with analog seismic surveys. Each year additional surveys have been carried out with a steady improvement in data quality due to advantage being taken of technological advances of the industry. Interpretation of these data and incorporation of available drilling results have permitted structural and depositional models to be rationalized for post-Paleozoic time-rock sequences. Improvements in drilling technology have allowed locations to be sited in greater water depths confirming depositional and structural models. End_Page 637------------------------------ To overcome seismic data problems associated with the high-speed Tertiary section, navigational problems due to the size of the area, drilling problems associated with geo-pressured shales and the highly porous Tertiary beds, new techniques constantly were required and introduced. To integrate the interpretation of the large volume of data and to apply the maximum amount of geologic rationalization require the cooperation and mutual support of competent geologists and geophysicists. Large areas to be mapped and steadily increasing volume of data from this area required the utilization of computer-assisted mapping. By mid-1971 approximately 40,000 mi of seismic data were in the files. At the same time wells 11 (Scott Reef No. 1) and 12 (North Rankin No. 1) were drilling and located what promise to be commercial accumulations of hydrocarbons. However, feasibility studies still are being evaluated and step-out wells have yet to be drilled. Since completion of these wells, other wildcats have been drilled resulting in several discoveries. Future potential in the area looks very bright with source, reservoir, and cap rocks in Triassic, Jurassic, and Cretaceous rocks. The potential of the Paleozoic section is as yet virtually unknown. End_of_Article - Last_Page 638------------