Fatigue Assessment for Pump Tower Tubular Joints under Partial Penetration Welds

Ozgur Ozguc

Abstract

ABSTRACT In this paper the fatigue capacity of the tubular joints of the tripod pump tower structure installed in the cargo tanks of membrane type LNG vessels is studied using fatigue assessment methodology specified in Class Code such as DNVGL-RP-0005 [1]. The stress concentration factors and the S-N curve used in this methodology are normally valid for the weld toe location where fully penetration weld is applied. The current study is initiated to address the risk of root cracking if the pump tower tubular joints are not welded with full penetration weld. In other words, the influence of the partial penetration weld is evaluated upon the stress level at weld toe on the pump towers in LNG vessels. The risk of root cracking is assessed by means of finite element analyses, where three tubular joint geometries is considered to be representative for the LNG pump tower structure. The effect of the partial penetration weld on the weld toe stress is investigated by modifying the weld geometry of one of the tubular joints to obtain a full penetration weld, and compare the calculated stress at an element at the weld toe between the two models The comparison shows only small differences in weld toe stress, and the standard tubular joint fatigue assessment methodology might also be applicable to the partial penetration welds. It is concluded that weld toe failure is the most probable failure mechanism also for the partial penetration welds. Keywords: LNG pump tower, Fatigue damage, Weld root failure, Tubular joints, Partial penetration weld