Because of the reduced difference between fracture and pore pressure gradients inherent in deepwater drilling, gas influx during a kick must be limited to much lower volumes than on land or in shallow water. To do this, instruments capable of detecting a kick at a very early stage are needed.Using a recently developed computer program capable of modeling a kicking well, the transient behavior of several well parameters was investigated to determine which parameters are most indicative of a kick and what sensitivity is desirable for instrumentation to measure these parameters. Results show that instrumentation to detect changes in return mud flow rate offers the greatest potential. Pit volume and standpipe pressure instruments can provide valuable backup detection capability, provided they can be made sufficiently sensitive. SLandpipe pressure instruments have potential advantages for floating drilling operations, since they should be unaffected by vessel motion. Desired sensitivity levels, based on the computer analysis, are presented for each type of instrument.
In deepwater drilling, increased emphasis must be placed on early kick detection. Using a recently developed computer program to model kicking wells, various types of kick detection instruments were evaluated to determine which are most useful and what sensitivities are desirable. Kick detection based on return-mud flow rate was found most beneficial. Introduction In offshore drilling operations, increasing water depth reduces the difference between the mud weight required to balance formation pore pressures and that weight causing formation fracture. Fig. 1 illustrates this point at the seat of a 3,500-ft (1070-m) surface casing string. The formation pore pressure gradient is assumed to be that of seawater, equivalent to an 8.5-lbm/gal (1020-kg/m3) mud weight. The casing-seat formation fracture gradient at zero water depth (i.e., on land) is assumed to be equivalent to a mud weight of 13.5 lbm/gal (1620 kg/m3) . Translating the same geological conditions to an offshore environment results in the situation shown in Fig. 1. The pore pressure gradient remains constant since the added overburden is seawater; however, the casing-seat fracture gradient decreases dramatically with water depth. If a 0.5-lbm/gal (60-kg/m3) margin is maintained relative to both the fracture and pore pressure gradients, the range of allowable mud weights is reduced substantially as water depth increases.Another way of illustrating the effect of increasing water depth is to consider the "critical kick size." This is the maximum gas influx volume that can be contained in a shut-in well without fracturing the rock at the casing seat. Influxes larger than this will cause an underground blowout if shut in. Fig. 2 shows the critical kick size as a function of water depth for kicks taken while drilling a 12 1/4-in. (311-mm) hole 5,000 ft (1520 m) below a 3,500-ft (1070-m) surface casing string. The drillstring consists of 5-in. (127-mm) drillpipe with 600 ft (180 m) of 8-in. (203-mm) drill collars. The gas is assumed to be a single bubble at the bottom of the hole. Mud weights of 9.5 and 10 lbm/gal (1140 and 1200 kg/m3) and underbalance conditions of 0.25 and 0.5 Ibm/gal (30 and 60 kg/m3) are considered. While kicks of 150 to 250 bbl (24 to 40 m3) are necessary to produce casing-seat failure on land, in 5,000 ft (1520 m) water, the critical kick size is reduced significantly.While there are numerous reasons for limiting kick size on land or in shallow water, this analysis shows that there is increased incentive in deepwater drilling operations to detect and control kicks at an early stage. The accuracy, sensitivity, and reliability of the drilling instrumentation are key elements for determining at what stage a kick will be detected and controlled by the drilling personnel. We have investigated the time-dependent evolution of kicks under a variety of assumed conditions to define better what instruments are useful for early kick detection and what levels of accuracy and sensitivity are desirable. Here, we used a recently developed computer program that models the transient behavior of a kicking well. Gaskick Program The Gaskick computer program was developed to simulate the evolution of gas kicks in a wellbore. JPT P. 1029
The design, fabrication, and installation of the Lena guyed tower required the development of new offshore technology in several areas. Lessons learned from this experience include an understanding of the consequences of major design decisions and insight into the interactive behavior of the structural components. The guyed tower is now a proven deepwater production platform concept for the Gulf of Mexico and production platform concept for the Gulf of Mexico and is also judged to be applicable for a wide range of water depths and deck loads in the North Sea and other areas of the world. Introduction A major milestone was reached in the summer of 1983 when the Lena guyed tower was installed in 1000 ft (305m) of water in the Gulf of Mexico. The installation was achieved after 12 years of developnent work and represents the efforts of many individuals who contributed innovative solutions to challenging technical problems. The solutions not only have advanced the state of guyed tower technology but also have contributed to offshore platform technology in general. Many lessons were learned as a result of the Lena experience, as would be expected with a first-of-a- kind structural concept. The insight gained will allow future designers to better evaluate alternatives and thus promote further optimization of the concept. The design principles and construction techniques successfully applied to the Lena tower can be adapted to other water depths and environments, thus enhancing the applicability of the guyed tower as a production platform. The brief description of the Lena guyed tower presented below is followed by a discussion of the consequences of several major design decisions. The paper concludes with a discussion of the impact of the lessons learned from Lena and in particular focuses on how this knowledge would be particular focuses on how this knowledge would be applied to North Sea guyed towers. THE LENA TOWER Structural Configuration Above the water level, the Lena guyed tower appears similar to a conventional platform. The most obvious differences below the water line are the shape of the jacket (tower) and the system of guylines. The jacket has a constant cross-sectional dimension of 120 ft (37m) square (Fig. 1). The tower is supported vertically by eight main piles, which are located in a circular array near the center of the jacket. Twisting restraint is enhanced by six short piles driven through guides placed around the perimeter of the base. Twelve buoyancy tanks located centrally in the upper part of the jacket support about 75 percent of the deck load. The tower is supported laterally by 20 guylines symmetrically located around the jacket. Driven piles anchor the guylines to the seafloor. A 200-ton clump weight is attached to each guyline and partially rests on bottom. During severe environmental events the articulated clumps are designed to reduce mud suction forces by allowing the clump segments to "peel" off the bottom. The anchor line (between the pile anchor and the clump) is longer than the depth of the water to allow surface make-up to the clump after installation of the pile anchor. The guylines enter the jacket well below the water surface and are redirected vertically to the deck by bending shoes located near the center of the jacket. Fairleads located around the exterior of the jacket serve to route the guyline through the structure and accommodate any structure-guyline misalignment. The guyline tensions are transmitted to the jacket by wedge-type cable grips located on the +15-ft (4.6m) elevation. The end of the guyline terminates in a field-poured socket located at the +53-ft (16m) elevation. The principal supporting members (the piles, buoyancy tanks, and guylines) are located in the upper region of the jacket. p. 161
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