Heavy Wall Hsla Steel Pipes And Heavy Section HSLA Steel Plates For Jack-Up Rigs
Authors: Tagawa Hisatoshi , Taira Tadaaki , Ume Kazuyoshi , Ishihara Toshio 
Published: October 2008

Development has been made of materials meeting various requirements for strength, toughness, weldability and dimensional accuracy with regard to leg and rack materials for jack-up rigs. Fundamental studies were first carried out concerning the optimum manufacturing process and chemical composition with regards to satisfying these requirements. Heavy-wall class 80kgf/mm2 HSLA steel pipes for legs and heavy-section class 80kgf/mm2 HSLA steel plates for racks were then manufactured to industrial scale in compliance with the results of these preliminary studies, and various properties were also investigated.

Leg materials, steel pipes manufactured by controlling the chemical composition and introducing the UOE pipe forming and pipe QT (quenching and tempering) heat treatment process, showed excellent qualities for the entire pipe, particularly for the welded joints which formally had been poor in toughness, as compared with the pipe body. Pipes manufactured by this process are superior to conventional ones manufactured by the bending-roll process in dimensional accuracy, product length, and homogeneous properties.

Concerning rack materials, the effect of the hardenability index, DI, and the Ni content on toughness of high-strength heavy-section steel was investigated through laboratory heat tests. Three typical types of steel Plates meeting strict demand for toughness or strength with a thickness of 5 in. were manufactured in regards to the results of investigation as well as to weldability. Tests were carried out on these plates. According to the test results, there plates are of excellent quality, and weldability. The properties of welded joints are also of excellent quality.

Introduction

A number of jack-up rigs have recently been constructed for off-shore oil and gas drilling. 1 While ship steel is used for the hull, HSLA steel is employed in large quantities for legs and racks which elevate the deck to reduce the weight of rig and increase the stability. There are various requirements including heavy walls for pipe, high strength (e.g., UTS 74kgf/mm2 to 88kgf/mm2), high toughness (e.g., guaranteed Charpy V notch toughness at -30°C to -50°C), good weldability, strict dimensional accuracy, low cost, and in addition, in view of the use in deeper and colder sea waters, and increasing price rivalry, further rationalization in design is required.

The authors developed high-strength heavy-wall pipes by the DOE and pipe QT process2,3 for legs and several types of high-strength heavy-section steel plates for racks both meeting these requirements. This paper outlines the following aspects of leg and rack materials: the design base for the selection of chemical composition, and properties of plates and pipes.

Heavy Wall Class 80kgf/mm2 Hsla Steel Pipes For Legs

1. Basic policy in manufacture

Large-diameter pipes for rigs have conventionally been manufactured principally by the bending-roll process. When considering the dimensional accuracy, stability of property, manufacturing efficiency and cost, application of the UOE process in more advantageous. The UOE process consists, as shown in Fig. 1, of forming a steel plate previously subjected to edge preparation, i.e., crimping, U-ing, Q-ing and then finishing after welding the seam to a prescribed dimensional accuracy through expanding of about 1%.