High-Performance Tendon For TLP Manufactured By Quench And Temper

Abstract For the purpose of cost saving, three-piece tendon, which consists of a straight pipe with a couple of threaded forging connectors welded to both ends, is more advantageous than one-piece forged tendon. Thus R & D of three-piece tendon of PWHT type was presented by the authors in paper OTC-5074 in 1985.The presenting paper is concerned with three-piece tendon quenched and tempered after welding fabrication, which is developed for the purpose of higher performance. The description will mainly concern the fabrication process including heat treatment, R & D in steel plate, welding material and corrosion fatigue behavior. Steel plate with adequate nickel and boron contents and with adequate heat treatment is the most suitable one for the purpose. Welding material which gives oxygen content of less than 200 ppm in the weld metal is suitable to secure toughness of QT type tendon. These techniques were applied to test-fabricate tendon pipe of 20" OD, 1" WT, and 40? length. These trial tendon pipes were quenched by induction heating after welding, and tempered in ordinary furnace. The yield strength was more than 80 ksi and charpy energy at minus 20°C was more than 13 Kg-m. Corrosion fatigue characteristics were investigated and compared to the other data. All these characteristics were found satisfactory for the present purpose. The three-piece tendon quenched and tempered in its final process should be most reliable in strength and toughness. Thus, it should have the highest quality and cost economy at the same time. This newly developed tendon will be suitable for use in severe conditions such as encountered in the North Sea. 1. Introduction The development of offshore oil fields for the next 15 years to 2000 will be carried out in (1) relatively shallow arctic seas and (2) non-arctic seas 300 to 1, 000 m in depth. Which seas will see greater oil field development activities will o-f course depend on the cost of crude oil produced there. Although several production means are being developed for deep-sea oil fields, the Tension Leg Platform (TLP) is one of the most promising means. 1) If techniques are developed to reduce the cost of the TLP fabrication, deep-water oil fields will be more actively developed. The authors have studied TLP tendons of high quality and lower cost than one-piece forged tendons. The TLP tendons on which research and development work has been conducted are:Post weld heat treatment (PWHT) tendon,quenched and tempered (QT-1) tendon andquenched and tempered (QT-2) tendon as shown in Figure 1. Each of the three types of tendons is fabricated by girth welding a forged connector at each end of the differential thickness (DT) pipe which is made by press bending and seam welding a DT plate, that is, one tendon is composed of three pieces.