Comparative Appraisal Of Cast And Welded Tubular Joints In High Strength Steels - Feasibility Study For Pre-Fabricated Leg Block System
Authors: Noda Tadayoshi , Kai Toshio , Okabayashi Nobutaka , Shimada Mitsuo , Ikeda Masahiko , Ogura Yutaka 
Published: October 2008

This is a continuation of the previous experimental work on various prefabricated tubular joints (OTC paper 3524 by Ohtake et al. l) Past studies have demonstrated that nodes in cast steels are apparently superior to the conventional welded ones in resistance to fatigue, ductile and brittle failures because of their smooth surface and variable wall thickness. However, several practical aspects still remained to be assessed.

To facilitate fair comparison of these nodes from viewpoints of fabrication technique and cost, two full-scale leg segments were constructed using either cast or welded nodes for each. Both the leg segments were fabricated under the same design specifications as those for an actual in-service jack-up drilling unit concerning dimensions, materials welding procedures, tolerances and inspections (namely, leg type: trussed triangular, length of leg segment: 9m, chord center to center: 6m, weight: approx. 35 metric ton, chord: 762mm O.D. × 32mm W.T., specified tensile strength of materials: 60 and 80 kgf/mm2, jacking system: rack and pinion). Through the fabrication, numerous data on welding, assembling and inspection were collected. Measurements were taken on mechanical properties of base and welded materials and also on residual stresses with a greater density in the portions around the nodes.

The conclusions drawn from the investigation were in favour of the cast node, especially when applied to pre-fabricated block system. The advantages of the cast node were:It is easy to meet tight tolerance requirements because of less deposited metal and, therefore, of less shrinkage.It is possible to simplify fabrication processes such as cutting of braces, assembling and welding. A comprehensive nondestructive inspection is easy to carry out.Fabrication time can be decreased almost by half.The thickness of the chord can be decreased when the nodes are prefabricated pieces. In consequence the total weight of the leg becomes less with the cast nodes as compared with the conventional welded type designed under the same reliability (strength) levels.

INTRODUCTION

Recently offshore development on drilling, production, transportation and storage of crude oil and gas has been much activated world widely. And it is estimated that needs for offshore structures and equipments related to the above developments will be constantly increased. Among them, jack-up type drilling rig is evaluated to be the most valuable unit, shared more than 40% in total drilling units, building more than 20 units per year lately.

It is said that one of the most significant portion in design and fabrication of offshore structures is tubular joint, so-called node. High tensile strength steel of which yield strength is more than 70 kgf/mm2, has much used for the leg of jack-up type rig and, therefore, it is required extremely high techniques to assure the joint strength in the conventional welded node type, but still very difficult to accomplish it satisfactorily in economic viewpoint.