Condition monitoring of offshore pipelines using vibration based methods

[Truncated abstract] Subsea pipelines are essential structural systems to transport natural oil or gas from offshore oil wells to an onshore location. Damage along a subsea pipeline might occur because pipelines are directly and continually exposed to the hydrodynamic environment. Free spanning is a very common problem both in the design phase and during operation of pipelines. Such spans may be subjected to fatigue damage caused by direct wave and vortex induced vibrations (VIV). Therefore appropriate amount of in-service inspection, monitoring and assessment to ascertain their conditions are essential. In industry, in addition to the visual inspection by divers, there are three techniques that are normally used to identify free spans along pipelines, named side-scan sonar, multibeam echosounder and video/camera. However, as all these tools need a remotely controlled/towed vehicle to carry the inspection tools and an appropriate vessel to support this performance, this kind of inspection can only be performed at scheduled intervals, which may cause the pipelines operates under dangerous conditions. Therefore an effective and reliable method that can monitor the subsea pipeline conditions continuously will be very useful. This is the motivation of exploring and applying vibration-based methods to subsea pipeline free-span monitoring. This thesis contains four parts. In the first part, numerical simulation is performed to explore the feasibility of using vibration properties of subsea pipeline structures, and model updating technique to identify pipeline conditions. A finite element model of subsea pipeline is developed, which takes into consideration the fluid-pipe-soil interaction. Ambient excitation for marine environment is simulated based on JONSWAP spectrum.