The Development of Casting Leg Nodes for a Jackup Rig
Authors: Ohba H. , Susei S. , Sakai Y. , Atsuta T. , Ohkuma Y. , Ohminami R. , Tamura A. 
Published: December 2007

Ohba, H. Kawasaki Heavy Industries Ltd. Susei, S. Kawasaki Heavy Industries Ltd. Sakai, Y. Kawasaki Heavy Industries Ltd. Atsuta, T. Kawasaki Heavy Industries Ltd. Ohkuma, Y. Kawasaki Heavy Industries Ltd. Ohminami, R. Kawasaki Heavy Industries Ltd. Tamura, A. Kawasaki Heavy Industries Ltd.

This paper discusses the development of casting leg nodes for a jackup rig. Two types of nodes were designed, based on theoretical analysis. A low-alloy cast steel with high tensile strength, ample fracture toughness, and weldability was developed. A number of nodes were molded to study the workability in casting, and their strength was confirmed by full-scale tests with three nodes.

Introduction

With the continuing progress in ocean development and in the oil industry, more efficient, economical and reliable' offshore structures are in demand. Jackup rigs undergo severe meteorological and environmental stress, which requires high-quality material and thorough analysis of the member connections (nodes) of the leg structures.From the points of view of strength and fabrication, the use of casting nodes is of great advantage to directly welded joints. However, only a few rigs have been constructed with casting leg nodes because of (1) the unavailability of suitable cast material to meet the severe offshore requirements, (2) difficulties in molding light and thin-walled tubular joints to meet the weight requirement, and (3) increased costs.To overcome these problems, a low-alloy cast steel was developed with which casting leg nodes were designed for a standard jackup rig for 300-ft water depth, as shown in Fig. 1. Fig. 2 shows a part of the leg structure with two types of casting nodes: Node L (leg chord casting) and Node K (K-brace casting).

Leg Nodes

Generally, the leg structure of the jackup rig consists of high-strength tubular members whose joints tend to be complicated and heavily welded constructions, causing several problems:highly concentrated stress at toes of weldments,lamellar tearing under bead due to surface traction,property change in heat affected zone,residual stress and strain restraint, andwelding defects and hydrogen-induced cracking.

Therefore, when using welded tubular joints, precise analysis, thorough quality control, and a high degree of skill are required in every stage of production, from design and selection of material through fabrication and inspection.By use of casting leg nodes, most of these problems can be solved because of (1) reduction of stress concentration due to smoothly changing wall thicknesses, (2) homogeneous strength inherent to the castings, (3) separation of weldment from the structural discontinuity, and (4) constant quality assured by lot production with standardized inspection at the shop. Thus, with casting leg nodes it is possible to produce highly reliable leg structures as described in the following.

Design of Leg Nodes

Fig. 3 shows theoretically obtained stress distributions in a welded tubular joint. The hot spots, where stress concentrates and yielding initiates, are located at junctions of members designated by the dots in Fig. 3. It is known that the hot-spot stresses consist mainly of bending stresses in the cylinder walls generated by out-of-plane bending moments along the intersection of tubular members.

JPT

P. 1271＾