Electric Heating of Subsea Pipelines: Design and Testing of an Applicable Ring Core Transformer
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- Institutt for elkraftteknikk 
As it becomes profitable to extract oil and gas from smaller and smaller reservoirs, jumpers (t-offs) connected to the main pipelines become necessary. This master?s thesis has investigated a simple transformer design which makes it possible to heat these t-offs using electrical energy. Two prototypes have been designed and tested. Based on these prototypes, electrical parameters and physical dimensions of a full-scale transformer supplying a 500 meter long eight inches thick t-off have been approximated. In addition, numerical and analytical approaches have been proposed to find the theoretical minimum series reactance of this transformer design.Based on transformer design theory and the special application requirements, a ring core transformer consisting of multiple stabled toroidal cores build up from magnetic tape is found to be suitable. This construction design provides the maximum utilization of magnetic flux with the minimum of magnetic force. Two main reasons distinguish themselves regarding the application requirements. The first is absence of termination points. This enables the use of a piggy-back cable as primary winding, increasing the overall system reliability compared to a system consisting of joints. The other main design benefit is the possibility to connect appropriate winding ratio in an unproblematic manner.Open-circuit and short-circuit tests were performed in order to find parameters for an electrical equivalent circuit. The iron cores were produced by ThyssenKrupp Electrical Steel, but the prototypes were assembled by the author. The finite element analyse software COMSOL Multiphysics 4.3a has been used when creating computer models of the prototypes.The two prototypes have ratings of 918 VA (small) and 7,200 VA (large) at 50 Hz. Their dimensions (inner radius x outer radius x length) are 2.5 cm x 4.5 cm x 48 cm and 5.0 cm x 9.0 cm x 56 cm. The small transformer has a rated voltage and current of 3.4 V and 270 A while the corresponding values of the large transformer are 8.0 V and 900 A. An appurtenant datasheet is attached in Appendix A. Other electrical properties of the prototypes were also extracted from these tests. No unusual electrical characteristics were found, compared with iron core theory and regular power transformers.The t-off could be heated either by use of direct electric heating (DEH) or induction heating (IH). If the transformer is to supply a DEH-system, it must deliver 192 V and have a winding ratio of 1:1. Correspondingly, an IH-system needs 282 V and a secondary winding consisting of two parallel conductor pairs, of two turns each. The transformer core dimensions are 7.2 cm x 34 cm x 2 m for a DEH-system while 10.9 cm x 50 cm x 2 m for an IH-system. Their weights are around 5.2 and 11.2 metric tons. Corresponding efficiencies are found to be 97 % and 96 %. All values are calculated based on the prototype tests.Based on the calculations and considerations performed in this thesis, this transformer type is found to be suitable for this kind of application. Since this proposed design avoids any need for splices in the conductors, the overall system reliability is enhanced in relation to a regular power transformer. In addition, the full-scale transformer size is manageable for installation.