ABSTRACT

As Tendon strings of Tension Leg Platform are used in seawater under high alternating load and the fatigue strength of high tensile strength steel is remarkably lower in seawater than that in air, the most important performances are the seal integrity and the fatigue strength of their thread connection.

It was found that "the distributed seal interference" concept, a design with interference in pin - box shoulder parts, was effective to get stable seal integrity of thread connection, and also found that the fatigue strength of thread connection under high stress ratio could be accurately estimated by combination of Goodman's diagram with stress analysis by FEM.

On the base of these results, the newly designed connection with double shoulders has been developed and its performance properties, i.e., fatigue strength, seal integrity and joint strength were proved to be good by means of the 1/4 scale model test.

1 INTRODUCTION

Recently, as offshore oil development are moving forward to deep sea, Tension Leg Platform (TLP) has been developed as one of the deep sea offshore structures available over the depth of Jacket-type Platform.

In the summer of 1984, the world's first TLP was successfully instable in the Hutton field in the water of 485 ft.

As the structure of TLP is a floating hull moored to a foundation template on the sea bed by vertical mooring tendon, the tendons are responsible for the safety of a whole structure of TLP.

When installing a TLP, tendon should be effectively joined in a short time. The thread connections satisfy this purpose and were used in Hutton field.

Tendon strings are used in seawater under high alternative loading as a consequence of vertical and lateral wave movement. Therefore, the most important performance of thread connection is high fatigue strength. Furthermore, the seal integrity of thread connection is required, as the fatigue strength of high tensile strength steels in seawater is? remarkably lower than that in air.

The authors designed a new type thread connection which has high fatigue strength and stable seal integrity, based on the tendon with HY80 modified steel and O"30.D.xl" W.T. pipe size as a representative one.2),3) As shown in Fig. 1, the newly designed connection consists of a pin and box member with new designed large buttress thread form and external double shoulders. One shoulder has a metal-metal seal surface and is a primary torque shoulder, the other is preliminary torque control shoulder in order to avoid the galling of primary shoulder seal surface.

This paper presents the design concept for the newly developed thread connection based on FEA. Experimental results with 1/4 scale model tests were shown to be compared well with the predicted excellent performance properties.

2 OPTIMAL DESIGN OF THREAD CONNECTION DESIGN CONCEPT

The principal performance properties required for a thread connection of tendon are high fatigue strength, stable seal tightness and high joint strength. When the premises described below for performance properties were given, the optimal design conditions to realize the performance properties in the balanced manner were discussed.

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