Research and Development of a Three-Piece Tendon for a TLP

Abstract Research and development of TLP tendon is described. Forged tendons are in actual use but their cost increases rapidly as the diameter becomes greater. For the purpose of cost saving, three-piece tendon manufacturing was studied. The three-piece tendon consists of a straight pipe with a couple of threaded forged connectors welded to the both ends. Assuming sea depth in service of 1,500 ft, the basic dimensions postulated for the present study are as 20 inch OD, 1 inch WT at straight pipe and 40 ft in length. Yield strength is more than 80 ksi. The tendon of excellent service performance and economy was successfully developed as intended owing to the followings:Development of high-strength and high-toughness steel and welding material.Development of differential-thickness pipe.Evaluation of the corrosion fatigue and the stress corrosion cracking characteristics to confirm the reliability of the tendon. 1. Introduction In the summer of 1984, the world's first tension leg platform (TLP) was successfully installed in the North Sea Hutton field in the water of 485 ft. This success offered high hopes of deep-sea oil production. However, the manufacture of one-piece forged tendons used in the Hutton TLP is technically and economically difficult for larger tendons for TLPs to be operated in deeper waters. The fabrication of economical pipe tendons with excellent service characteristics are presented here. 1.1 Characteristics Required for Tendons A great many jacket-type platforms are operating in water depths of up to about 1,000 ft. Chords, braces and other tubular parts constitute such platforms. These tubular components may be compared with TLP tendons in terms of basic characteristics required as follows:The TLP is lower than the jacket-type platform in redundancy from the standpoint of structural mechanical. Therefore, higher reliability is required for the tendons.Tendons are generally subjected to high stresses and thus are made of high-strength steel.Tendons are exposed to a high mean stress and pulsating stresses at the same time. Therefore, they must have high corrosion fatigue strength at a high stress ratio.Tendons must be fabricated within strict dimensional tolerances to avoid additional stresses and to ensure desired collapse strength and water tightness. 1.2 Economical Three-Piece Tendon with Necessary Properties Satisfied When installing a TLP, tendons can be effectively joined by means of threaded connectors in a short time. Tendons connected by threads, therefore, are expected to be used generally and are discussed here. One-piece forged tendons were used in the Hutton TLP. The tendons were 10 1/4-inch aD and made of steel with a yield strength of at least 115 ksi. l)