High-Strength Low-Alloy Steel Bent Beam Derrick Legs

Abstract The geometric development and application of A.S.T.M. A572, Grade 50 high strength, low alloy steel is presented for use as corner leg material in offshore drilling derricks for low temperature service. The relationship between material ductility, notch toughness and yield point is discussed for A572 bent wide flange beam material. The danger of "open ended" material specifications versus published welding preheat tables is explored. Recommendations are outlined for positioning "low temp" steel in derricks. Illustrations depict major details. Bent beam leg derricks are being used world wide, including the North Sea. Introduction This paper surrounds the case history of the evolution and development of the first major bent beam derrick leg. The experience of developing the proper method of bending steel parallel to the direction of rolling is related. Structural engineers, welding engineers, and quality control people are advised to keep a constant vigil on material Mill Test Reports in order to use proper welding preheat. The paper will aid derrick specification writers in determining what derrick members should be "low temp" material and therefore capable of absorbing greater impact loads thus preventing brittle fracture. Evolution of the Bent Beam Leg The early 1950's was a dynamic era for the radio, microwave and television tower industry. Engineers and Industry joined forces to meet the accelerating demand for tall, strong antenna supporting towers. By 1955, the Television Industry was requiring tower heights in excess of 1000 feet. Triangular cross section, guyed towers were produced with corner legs made from structural hollow sections, solid rounds and open profile shapes, including bent flat plates. One such tower, whose corner legs were made from 60 degree, 10 in wide flange beams, was part of the evolution providing 90 degree bent beam leg derricks for today's drilling operations. The Open Profile The open profile section was selected for several reasons:It lends itself to simple structural details, including welding or bolting.Its column strength to weight ratio is high.Maintenance costs are low; the material may be coated with hot dip galvanizing or inorganic zinc rich paint.The wide flange beam is a popular, readily available shape.Arrangement of the bunt beam with its companion members produces good aesthetics.The section is easily repaired cold or hot from the every day wear and tear of the oil country. The 90 Degree Bent Beam Leg. Fig. 1 shows the unique for step evolution of the 90 degree bent beam leg. First came the classical 90 degree angle which presents symmetry about an axis that bisects the included angle. The axis of symmetry is a simple but powerful tool for the designer in order for him to obtain a minimum number of different connections. This system has been employed in tower design for centuries. The full spectrum of materials from plain carbon steel through high strength, low-alloy steels has been used, including A.S.T.M. designations A7, A36, A440, A44l, A242 and the present popular A572.