This paper discusses the development of a large diameter threaded connector for tension leg platform (TLP) tendons. The connector is designed to join sections of 48 inch (1.22 meter) outside diameter, 1. 625 inch (0.041 meter) wall thickness pipe that make up the TLP tendons. This work is part of a broader research effort aimed at demonstrating the use of large diameter tendons. Use of large diameter, buoyant tendons results in TLP designs that are relatively insensitive to water depth.
Elastic and plastic finite element analyses were conducted on various axially and radially preloaded connector concepts to study ways to reduce stress concentration factors (SCFs), improve static strength and resistance to jump-out, and evaluate sensitivity to machining errors. The results favored a radially preloaded connector concept. The resulting design has a weight of only 2.4 times the corresponding unit weight of the tendon pipe, a low SCF (less than 1.4), and a static strength greater than the tendon pipe. It also can be machined using conventional, computer controlled machines.
A comprehensive testing program aimed at verifying the structural and functional performance of the connector was carried out. Results confirmed the validity of the analyses. In addition, a full-size connector pair was manufactured to identify possible fabrication problems.
The tension leg platform concept uses tensioned vertical tendons to moor a floating hull or platform over a well template. Excess buoyancy of the platform insures that the tendons remain in tension. This arrangement produces a, compliant structure whose sway, surge and yaw response is at very low frequencies. The tendons constrain vertical motions, like heave, roll and pitch, making them small but leading to large dynamic tension in the tendons. Further, if large diameter tendons are used, the cyclic bending stresses may be of the same order of magnitude as the cyclic axial tension stress variations and therefore must be accounted for explicitly in fatigue life calculations.
The TLP tendons discussed in this paper are large diameter and thin walled, typically 48 inch (1.22 meter) outside diameter (OD) with a diameter to thickness (D/t) ratio greater than 30. The only installed TLP, on Conoco's Hutton field, has 10.25 inch (0.26 meter) OD, thick-wall tendons [1,2). In general, industry is tending toward larger diameters with D/t ratios of 20 to 30. Compared with heavy-wall, small diameter tendons, the advantages of large D/t tendons are that they:
have greater buoyancy, which reduces the weight that must be supported by the TLP hull, thereby reducing the hull size and cost and making the TLP cost relatively insensitive to water depth;
permit a leak-before-break inspection philosophy;
can be fabricated by more efficient pipe rolling methods, thereby reducing overall tendon cost, and
may have greater steel area in each tendon, thereby reducing the number of tendons and tendon support hardware.
Before these larger tendons can be employed, a satisfactory tendon connector must be developed.
