Study on Fatigue Strength Evaluation of Casting Alloy and Its Welded Joints for Offshore Structures
- Yoon-duck Seo (Daewoo Shipbuilding & Marine Eng) | Chan-hoe Kang (Daewoo Shipbuilding & Marine Eng) | Chang-hyun Lee (Daewoo Shipbuilding & Marine Eng) | Chang-hwan Jang (Daewoo Shipbuilding & Marine Eng) | Sung-gun Park (Daewoo Shipbuilding & Marine Eng)
- Document ID
- International Society of Offshore and Polar Engineers
- The 28th International Ocean and Polar Engineering Conference, 10-15 June, Sapporo, Japan
- Publication Date
- Document Type
- Conference Paper
- 2018. International Society of Offshore and Polar Engineers
- Low cycle fatigue, fatigue strength, High cycle fatigue, Fracture surface, welded part between dissimilar metals, Cast alloy
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- 18 since 2007
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In the offshore structure, the stress concentration area (Node), which is subjected to excessive stress concentration due to the complicated shape, is mainly made of cast alloy. In this study, low cycle fatigue and high cycle fatigue tests were carried out to determine the fatigue characteristics of base metal for domestic and overseas cast alloy. In addition, to evaluate the integrity of the structure since fatigue failure occurs frequently in the welded part of the structure, the strength of the welded part between dissimilar metals is also evaluated by conducting high cycle fatigue test. It is also compared with the S-N curve of the Classification used as the fatigue design criterion of cast alloy. The fatigue life is more associated with defects in the cast steel than fracture position.
In recent, development of offshore energy resources has become active lively due to the increased demand of oil and gas. Unlike ships, offshore structures such as Jacket, Jack-up rig, Semi-submersible Drilling Rig, Tension Leg Platform and FPSO operate during long periods with station keeping on site. Therefore, offshore structure requires high level of structural safety. Especially, semi-submersible Drilling Rig has unique configuration consisting of columns, pontoons and topside structure. And each part of structures are connected each other. These connected areas (i.e. pontoon-column connections, pontoon-brace connections and upper hull-column connections) have higher stress due to the complicated structural characteristics. In order to solve the problem, recently, a high strength, simply designed cast alloy is used.
Korean shipyards mainly use overseas cast alloy to offshore structures. However, due to the long lead time from order to warehousing (i.e. it takes about 11 or 13 months), ship builders have difficulty in design change when unexpected conditions occurring in the detailed design stage.
In this situation, recently, cast alloy used in offshore structure was developed by domestic company. Using this cast alloy, Korean shipyard will be able to solve the delivery deadline and reduce limit of design changes. However, for the application, it is necessary to verify the fatigue strength for cast alloy.
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