## 1 Introduction
Nevertheless, in most of the established applications of DFOS for pipeline monitoring, standard optical telecommunication cables are employed which are usually installed in the ground with some spatial gap to the pipeline. This very cost-efficient sensor installation is suitable for detection of temperature changes caused by leakages (DTS) and the above-mentioned detection of third-party intrusion by recognizing vibration patterns transmitted via the ground (DAS). However, to enable a true condition monitoring system that is sensitive to small changes in the piping system itself and amongst others needs to pick up circumferential changes of the pipe, installation of the optical fiber sensor directly on the pipe is a prerequisite. The optimization of the application method was one of the main aims of the project AGIFAMOR and in a first investigation comparing different ways of linear fiber application along a pipe the importance of a high quality fiber installation was underlined [28]. Additionally, it was found that close contact of the fiber sensor to the pipe along the whole length of the sensor, e.g., gluing of the whole fiber to the pipe, is crucial. Another way to provide close contact is helical wrapping of the fiber sensor around the pipe. This type of application has already been employed when carrying out DSS measurements on pipes [29; 30] and has proven suitable for transmitting strain changes from the pipe to the sensor fiber. Due to the application of more fiber length per pipe length this geometry provides an increased spatial resolution and a higher sensitivity. For the detection of circumferential changes targeted in this study the helical design provides even further sensitivity enhancement as the optical fiber is sensitive to strain changes only along its axial direction. Optimization of fiber-optic cable design with respect to this directionality is currently addressed in in the context of seismic measurements [31; 32; 33] leading to the deployment of different sensor geometries depending on the propagation mode of the targeted pressure wave. On the down side, helical application of the optical fiber sensor to the pipe reduces the overall accessible length of pipe and is therefore expected to be of interest for short to medium pipe lengths. Additionally, it is noted that the bandwidth of the system is reduced in comparison to a linear fiber application on the same pipe length as the maximum pulse repetition rate depends inversely on the length of the fiber under test. However, in the sense of a feasibility study, we focus on optimal conditions with respect to spatial resolution and sensitivity. Therefore, in the work presented here, a bare fiber is used wrapped helically around the pipe under test without any additional glue to exclude influences arising from the adhesive. Two helical configurations are investigated in the following. The first corresponds to a fully distributed sensor system, where the fiber is continuously wrapped around the pipe with a defined pitch, while the second constitutes a quasi-distributed system, where the fiber is applied in spaced segments of densely wound fiber. The latter resembles the deployment of point sensors--as was for example investigated by [PERSON] et al. [34] for acoustic emission transducers--and may be used in otherwise inaccessible regions in a piping system that may be encountered when retrofitting pipes. Additionally, the quasi-continuous setup bears the potential to reach even higher sensitivity for favorable configurations as summation schemes like e.g., the lately proposed adapted method of random quadrature modulation (RQD) [35] may be used. By picking regions of interest and bridging others by linear fibers this scheme may also provide a way to reach longer distances in a piping system while maintaining the advantages of the helical design in crucial areas. Hence, either configuration or a combination of both may be favorable depending on the actual boundaries and requirements in a specific monitoring task.