Scour monitoring system of subsea pipeline using distributed Brillouin optical sensors based on active thermometry

Abstract

A scour monitoring system of subsea pipeline is proposed using distributed Brillouin optical sensors based on active thermometry. The system consists in a thermal cable running parallel to the pipeline, which acquires frequency shift of optical sensors during heating and cooling, directly indicating temperature change. The free spans can be detected through the different behaviors of heat transfer between in-water and in-sediment scenarios. Three features were extracted from temperature time histories including magnitude, spatial continuity and temporal stability. Several experimental tests were conducted using the proposed system. The results substantiate the monitoring technique.

Introduction

The increasing world demand for and decreasing inland storage of oil and gas bring up a worldwide expansion of offshore hydrocarbon exploration. As the lifeline of subsea oil and gas transmission, subsea pipeline systems are widely used and greatly valued. Scour under subsea pipelines is a serious problem, perplexing engineers and researchers who try to maintain the pipeline systems throughout the service life. Typically, subsea pipelines are embedded into sea bed with depth of 1–1.5 m during installation process. However, the current around the pipes will gradually remove the cover and foundation of the pipes. That is the reason for hanging problem. Once the hanging length exceeds the design limit, local stress beyond ultimate stress in the pipes will lead to the structure failure. Furthermore, vortex-induced vibration can also lead to fatigue damage of the pipeline in a less hanging length. Therefore, the hanging length due to scour is a key parameter that determines the safety of pipes. According to Arnold's statistical analysis, scour and seabed movement were the major cause for subsea pipeline failure in Mississippi River delta from 1958 to 1965, which accounted for 36.2%, much higher than corrosion (29.2%) and machinery damaging (26.6%) [1]. According to Demars’ analysis, corrosion, scour, third party activity and seabed movement were the four main causes of pipeline failure in the Gulf of Mexico during 1967–1975 [1]. A survey conducted on the 61 pipelines in Cheng Dao oil field of China in 2004 showed that only five (8%) of them did not suffer hanging problem due to scour [2]. Therefore, scour under subsea pipelines needs to be timely monitored, reasonably evaluated and effectively controlled.

In recent years, optical fibers have been increasingly used in many fields for their peculiar advantages such as small size, high sensitivity, immunity to electromagnetic interference, low signal decay, accessibility to harsh environment, long-term measurement stability, etc. These advantages promise wide application of optical fibers in a subsea pipeline system, which is miles long and surrounded by complex and turbulent hydrodynamic and geographic environment. Due to the long distances to be monitored and the linear nature of pipelines, among the many optic sensing techniques, distributed fiber optic sensing techniques show significant advantages by providing distribution of the strain and temperature along the sensing optical fiber in both space and time domain. Fully distributed optical fiber sensors are based on Optical Time Domain Reflectometry technology. It uses a fiber, every bit of which acts as sensing element as well as data transmitting medium, to play the role otherwise played by numerous isolated sensors, reducing cost largely. To batter scour problem, many researchers attempted to use distributed optical fiber sensors to detect free spans of pipelines. Jin et al. successively proposed to detect free spans through natural vibration frequency of a pipeline (2003) [3] and an adapted Auto-Regression Model (2006) [4]. Elshafey et al. (2011) proposed strains variation on a pipe surface as an indicator of inception of free spans [5]. All of the methods mentioned above are based on distributed optical fiber sensors to monitor strain. While these attempts contribute to significant progress, they all focus on indirectly measuring free span vibration or strain. There are two limits of research work mentioned above: First, for non-distributed strain sensors and accelerometers, only local information of pipelines can be monitored. Second, for distributed optical strain sensors, it is very difficult to install them along the pipelines.

In this paper, a scour monitoring system is presented based on active thermometry and distributed Brillouin optical fiber sensors. The distributed Brillouin optical sensors are used for sampling temperature, in order to monitor distributed scour status of subsea pipelines. Active thermometry is an effective way to measure thermal parameters [6], [7]. Krishnaiah et al. measured thermal resistivity, thermal diffusivity and specific heat of rocks using thermal probe (2004) [8]. Keith L. demonstrated the capability of thermal probe by measuring thermal properties and volumetric water content of unsaturated sandy soil (1998) [9]. Freifeld et al. had developed a borehole methodology to estimate the ground surface temperature history by monitoring thermal conductivity profile along the length of a wellbore using fiber-optic distributed temperature sensor (2008) [10]. Sayde et al. demonstrated the feasibility of the active thermometry implemented with Raman fiber optical temperature sensors to obtain accurate distributed measurement of soil water content (2010) [11]. Cote developed a water leakage monitoring system of dam using distributed optical fiber temperature sensors (2007) [12]. The proposed monitoring system consists in an armored thermal cable running parallel to the subsea pipelines, which consists of three main components: a heating belt, optical fibers, and packing elements. The heating belt radically releases heat and the fibers concurrently measure temperature. The surrounding environment, sediment or water, can be identified by analyzing the acquired data of frequency shift, thus estimating the interface position and the free span length. The armored cable can be installed along the pipes in the vicinity, and does not need to be mounted on the surface of the pipes. This means that when sour happens, the pipes and the cable will be exposed to water at the same time and same location. Because during the pipelines construction, the pipes are welded together on boats, and then putted into the sea bed, it is almost impossible to install the distributed fiber optical strain sensors along the pipes. Scour monitoring system proposed in this paper can make the installation of distributed fiber optical temperature sensors along pipes possible. The mechanism of scour monitoring breaks the above two limitations. This flexibility saves many construction problems and makes it highly applicable to practice.