The Study of Larmellar Tearing In Offshore Structure

Abstract This paper describes the mechanism of lamellar tearing occurrence and the method to avoid cracking, for which various studies and experiments were performed on material, welding procedure and designs. The following conclusions, already applied successfully to the offshore structure construction, are effective to reduce the risk of lamellar tearing.Use of special clean steelsPreheating and low hydrogen electrodesPatch method of cut-out method Introduction When rolled steels are joined by means of multiunit fillet weld by which a high reaction stress acts through the plate thickness, there are cases that pull-out type cracks called "lamellar tearing" will occur parallel to the plane of the plate. This cracking differs largely from the other weld cracks as cited hereunder.Inclusions play a significant role in the lamellar tearing occurrence.The occurrence of lamellar tearing is most common in multiunit fillet weld.Even in case that steel plates satisfying usual standard values are used and normal welding procedure is applied to them, cracking is still caused sometimes.It is difficult to detect lamellar tearing by nondestructive inspections and even if these are detected, their repair welding is very difficult. The main structure of the offshore drilling unit, many of which have already been constructed by our company, is composed of thick tubular members. Because welded joints by which high reaction stress acts through the plate thickness, are adopted for the connecting joints between members, prevention of the lamellar tearing is an important matter for the unit. Therefore in this paper, the cause of lamellar tearing occurrence is investigated by conducting various tests in which the lamellar tearing was reproduced experimentally and also a study is made as regards the material selection and welding procedure for preventing this type of cracking. Lamellar Tearing Test Restraint Test Method of Test Restraint cracking specimens were manufactured for giving various reaction stresses through the plate thickness as shown in Fig. 1. By changing the test piece (I) in that figure, it is possible to investigate material problems, which give influence upon the lamellar tearing. In this test, we used two kinds of mild steel for hull structure, which differs in steel making process. The chemical compositions and mechanical properties of these test pieces are shown in Table 1. Also Table 2 shows representative distribution conditions of inclusions and their sculpture prints. As compared with the normal converter steel A, the electric furnace steel B has a lower sculptures content and less inclusions. The influence of reaction stress upon the lamellar tearing can be investigated by changing the restraint length ? in Fig. 1. In this test t, the ? was changed to 4 kinds of ?,. = 60, 100, 150 and 200 room. The reaction stress (0') became 25, 22, 18 and 14.5 kg/mm2 and according as the ? became longer, the reaction stress was decreased linearly