Headspace gases from 26 exploration wells drilled in Japanese oil and gas fields were analyzed. Carbon isotope values and chemical compositions of gases can indicate the different origin of methane. A comparison of δ13C1 and vitrinite reflectance values showed that a significant proportion of gases might have migrated from greater depth. Except for secondary fractionation in the reservoir, DST test gases have almost the same δ13C1 values as those of headspace gases at the same depth. The analytical results of headspace gas analyses widely differ in well to well. These results may reflect the influence of ganeration, migration, and accumulation of hydrocarbons in each area.
Spatial and time discretisations cause truncation errors that are always serious and long-standing hazards for the simulation of multiphase reservoir problems. Especially in field-scale applications, reliability of the simulated results greatly depends on how those errors are controlled within reasonable degee througth so-called “pseudofunctions”. For the purpose of preparing proper pseudofunctions to minimise spatial truncation error, then making the simulated results insensitive to how reservoir is discretised or gridded, the author incorporated a comprehensive process of generating pseudofunctions into the general-purpose black oil reservoir simulator. The method presented here generates dynamic pseudofunctions automatically during the iterative step of the calculation. It is based on the frontal advance theory, and has a dynamic feature in which, in deriving pseudofunctions of certain grid, the zonal segregation of phases, gravity-capillary equilibrium, variation of phase representative depth, and directionality of pseudofunctions are simultaneouly considered. Using 1-dimensional models with different gridding, several validation runs were made. The results showed that the proposed method could give almost identical performances for different vertical griddings.
A number of cases have been reported worldwide, where offshore mobile jack-up rigs, whose platform is brought up and down along the legs resting on the seabed, had their decks sharply inclined due to unexpected penetration, viz, punch-through, of the legs into the sea floor. Particularly, the recent marked trend toward heavier jackups being built to cope with deeper and harsher seas of operation calls for effective countermeasures against such sudden penetration of jackup rig legs. This paper reports the results of a study on the development of two preventive measures against offshore disasters likely to take place from sudden punch-through of legs. These countermeasures conceived are Strata Data Monitor System (SDMS) and Vertical Position Monitor System (VPMS). The SDMS permits to precisely know the properties of seabed soil by means of a device, which, attached to the rig in itself, surveys the seabed nearby or immediately beneath the leg footings (spud cans), to evaluate the safety of the sea floor where to place the legs, and further to suggest safe preloading procedures. The VPMS acquires, monitors and records data on the vertical position of the platform with respect to the sea level and that of the footings with reference to the sea floor, during the operations of jacking up and down and preloading. The two systems newly developed are altogether referred to in this paper as Leg Penetration Monitor System (LPMS), which is intended for safe execution of jacking up and preloading operation. In conjunction with the System development, scale model experiments of punch-through were carried out in a laboratory for the purpose of evaluating the occurrence and the effects of punch-through phenomena. Prototype experiments of the SDMS and the VPMS were also conducted on land and in waters to evaluate their performance, respectively.
If jack-up drilling rig or other leg-type operation platforms are settled on a sandy erodible seabed in a rough environmental condition, they will likely to undergo local scour around their legs. Therefore, it is indispensable to a safety design of such structures to devise the countermeasures against the scour at their design stage. The fixed-bed tests and movable-bed tests in use of cylinders were carried out in order to investigate the characteristics of the local scour and movable-bed tests in use of four model footings of a jack-up rig were carried out in order to investigate the footing configurations effective for mininizing the local scour. In this paper, the results and some considerations are given to discuss the scour protection methods for leg-type offshore structures.
The jack-up rig is set on the seabed and supported on the seabed ground. There are many problems arising from the seabed ground (such as penetration and punch-through of footing, liquefaction of supporting sand foundation due to earthquake and wave, scouring due to wave and current, suction force on bottom being freed, etc.), which are extremely important in connection with the basic planning and the operation of a jack-up rig and might have been the major cause of its accidents. Of these problems, in this paper, an analysis method of punch-through of footing and a method for reducing suction force are discussed. The analysis method of punch-through of footing is first described. It was used in analyzing an actual example in certain waters based on the local soil and load conditions and the results were compared with the actual values. The main conclusions reached are: (1) an analysis method of punch-through has been proposed, and (2) an actual example of punch-through was analyzed by this method based on the actual soil and load conditions, resulting in good agreement with the actual values. Next, a natural inflow suction breaker was used as a means for reducing suction force. The footing is installed with connection pipes that are allowed to naturally run sea water existing over the footing top therethrough under the suction produced on the bottom being freed, thereby reducing such suction force. A reconsolidated sea-silt ground was prepared in a soil tank. An about 1/20-scaled footing model was placed thereon to penetrate into it for a bottom freeing test. In this test, the suction force, the amount of pullout of footing, the amount of water flow through the suction breaker and the suction on the footing bottom were measured as changing the location of the suction breaker nozzles, water depth and footing pullout speed. The main conclusions obtained in this test are: (1) it is desirable to arrange the suction breaker nozzles evenly around the area close to the footing edge, (2) the relation between the efficiency of the suction breaker and the inflow rate of water into the void under the footing is almost proportional, and (3) the efficiency of the suction breaker has an optimum pullout speed.
In the installation operation of a jack-up rig, the legs will be subjected to impact loads when colliding with the sea bottom due to the rig's motion in waves. To avoid the damages of legs and jacking mechanisms, several operational manuals restrict an operation up to 1.5m in wave height. This standard, however, is not reasonable because it is determined with no relation to an wave period and a bottom rigidity. Excessively conservative standards may lead to much down-time in operation. Then, both from structural and operational viewpoints, it is important to clarify the characteristics of impact loads on legs. In this study, experiments on a model rig subjected to both vertical impact load and horizontal frictional loads from bottom are carried out in regular waves. Theoretical analyses in time domain are also performed using newly developed analytical model where legs are treated as elastic bodies. Based on these results, the occurrence mechanisms and response characteristics of vertical impact loads and impact bending moment at platform bottom are investigated. The accuracy and the reasonableness of conventional simplified formula to estimate impact loads based on the energy concept are also discussed, and finally, an allowable wave height on a proto-type rig is investigated.
The self elevating offshore drilling platform “HAKURYU 9” observed further penetration of the rig's legs after preloading operations were completed at the Bay of Bengal. On the 19th June 1987, “HAKURYU 9” commenced drilling operations under the leg penetrations 1) 3.90 2) 4.10 3) 4.60 metres. By the 2nd July leg penetrations had increased to 1) 5.47 2) 5.13 3) 5.55 metres when drilling was suspended and the rig preloaded again. However, the leg penetrations continued after resumption of driling, and developed the potential risk of punch-through foundation failure. On the 12th August, the penetrations reached to 1) 7.53 2) 7.60 3) 7.94 metres and decision was made to suspend drilling activities, secure the rig and start leg jetting untill leg penetrations into the clay zone below 15.8 metres were achieved. It was understood that at this location there were approximately 4m of silty sand followed by silt to 15.8m and soft to firm clay to 35.6m. This report will introdue the states of the penetrations and the jetting operations mentioned above and inquire into the cause of penetrations.
Semi-submersible rigs are more versatile than ship shaped drilling rigs for operations in deep water areas due to its better ship motion characteristics. Presently, a number of semi-submersible rigs used as the production platforms for small-sized oil fields are increasing. However floating rigs including semi-submersible sometime have difficulties in coping with environmental factors such as deeper working water depth and harsh meteorological condition. Workability of semi-submersible rigs utilized in the areas can be calculated based on operating limited value of the rig behavior constructed elements which consist of riser, mooring and hull. An optimum system as well as operational safety can be determined by maximizing the workability in the field. This paper shows the outlines of; (1)analysis of consisting elements, (2)spectral analysis method, (3)calculating method of workability with irregular response. The computer analysis is used to accurately calculate workability of the semi-submersible rig operation in relation to environmental condition in the field. As the results of above methods and analysis, the optimum system of the semi-submersible rig for the preselected field can be determined.