Design and testing of deep water sealine connector with non‐linear finite element analysis

Analyses were performed during the conceptual design stage of a 20in. threaded connector for deep water J‐pipe lay, as part of a research project developed by Tecnomare and partly funded by the EEC. The joint consists of two parts, namely a pin and a box, provided with cylindrical threads. It was essential for the joint design to be fully leak‐proof for both internal and external pressure and this requirement had to be satisfied also under the maximum bending moment allowable for the sealine. Sealing was accomplished on a cone surface by screwing the pin into the box until yield was reached. The FEM analysis was carried out primarily to check that the pin and box remain pressed to one another over the sealing surface in every design condition with adequate pressure to prevent leakage. For this purpose, the analysis was a powerful design technique, as it gave an easy understanding of the structural behaviour and provided proper stiffness by making the joint either larger or thinner wherever required. The main characteristic of this work is that FEM analysis has been utilized as a design method rather than as a check. The analysis was performed by means of ADINA (Automatic Dynamic Incremental Non‐linear Analysis) program. Contact pressure between sealing surfaces, as achieved during the joint screwing phase, was modelled through thermal elongation. Pressure loads and external forces were superimposed through a step‐by‐step procedure, by accounting for the elastoplastic behaviour all around the sealing surface. In order to verify the behaviour of the mechanical joint, six prototypes have been fabricated and tested under the design loads of the lay phase and the operative life. The results of the tests confirmed the correct design and the results of non‐linear finite element analysis. The most important performances of the joint can be summarized as follows: (1) the make‐up phase is rapid and easy: no problems of frictional pick‐up took place; (2) no leakage happened during the internal pressure tests: the pressure of 300atm (1.5 times the design internal pressure) was maintained for 12h; (3) the load conditions of the second series of tests were: 200atm of internal pressure and the maximum allowable bending moment relevant to the pipe: after 2h no leakage happened. This paper describes the model used for the analysis, discusses its implications and the most important results achieved in comparison with the tests of the experimental phase.