Experimental Evaluation of Grade V150 Buttress Casing and Its Use in Arctic Offshore Drilling Operations
Authors: Marshall David W. , Weiner Peter D. , Yazaki Youichi , Maruyama Kazushi 
Published: December 2007

Dome Petroleum Ltd. currently is involved in drilling deep, high-pressure wells in the Canadian Beaufort Sea and as a consequence is a major user of Grade V150 buttress casing. Details of a laboratory research program to evaluate the integrity of V150 buttress casing under simulated well conditions are included. In addition, our procedures for the successful use of such casing in offshore arctic wells are described. The results of the research justify the continued use of such casing in an arctic environment provided that it is manufactured and inspected under stringent quality assurance procedures.

Introduction

High-strength casing with yield strengths greater than 1034 MPa [150,000 psi] represents a small proportion of the total casing production of the world's major steel mills. This is a natural consequence of the limited demand for such casing. Of the 5 × 10 6 Mg [5,050,000 tonnes] of seamless casing manufactured during 1982, 24 × 10 3 Mg [24,000 tonnes] or 0.47% were V150 or equivalent. With the increasing trend toward the drilling of deep, high-pressure wells, the use of V150 or similar high-strength casing greatly extends the casing design limitations imposed by lower grades. Despite this, there is strong concern within the oil industry about the integrity of such casing. An informal survey of many major oil companies operating in the U.S. indicates an almost complete condemnation of grade V150 casing because of its apparent unreliability and, particularly, because of its supposed lack of fracture toughness. While this survey reflected opinions and hearsay as much as actual experience with V150 casing, it provides a fair representation of prevailing opinion within the industry. The alternative to using V150 casing, at least in the U.S., is to use a thicker walled, lower grade of casing. Even with this option the casing seldom can be set as deep as the equivalent V150 with the same nominal wall thickness. In deep, sour gas wells, however, there is no alternative since high-strength casing is incompatible with a severely corrosive environment. The purpose of this paper is to show, by means of the results of detailed experimental studies, that superior quality, high-strength casing can be used safely and successfully in deep, high-pressure wells, even in hostile environments such as the Canadian arctic.

Beaufort Sea Casing Design

Dome Petroleum is the major operator in the Canadian Beaufort Sea. Four drillships are used to drill in the short, ice-free operating season from early July to late October. Since the exploratory drilling program began in 1976, 20 wells have been completed, 14 of which have been tested successfully. With total well depths exceeding 5000 m [16,400 ft], bottomhole pressures of about 90 MPa [13,000 psi], and typical well costs of more than $70 million, casing design is a critical part of the overall well design. As a consequence of the high pressure prevalent in Beaufort Sea wells and the desire to set the casing as deeply as possible, Dome is one of the world's major users of Grade V150 buttress casing. Since the initial use of such casing in 1979 the company has successfully run eight strings of 340-mm [13.375-in.], seven strings of 244-mxn [9.625-in.], and eight strings of 178-mm [7.0-in.] casing for a total amount of 51 500 m [169,000 ft] or 4,000 Mg [4,000 tonnes].A casing design for a typical Beaufort Sea well is shown in Fig. 1. While the actual setting depths vary between wells, those shown in Fig. 1 are typical. The use of 406- and 127-mm [16.0- and 5.0-in.] contingency strings depends on prevailing pressures and hole conditions and is required in less than 20% of the wells. Because of the restricted ID of the 406-mm [16.0-in.] casing, an externally flush connection must be used on the 340-mm [13.375-in.] casing instead of a buttress connection. Similarly, the restricted ID of the 178-mm [7.0-in.] casing necessitates the use of an externally flush connection on the 127-mm [5.0-in.] casing instead of a buttress connection. At the outset, the use of 1034-MPa [150,000-psi] minimum yield buttress casing was considered to involve no greater risk than use of any other grade. Nonetheless, we retained a metallurgical consulting firm to design detailed specifications for the manufacture, inspection, and handling of all casing products to be used in the Beaufort Sea.

JPT

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