1. A method of monitoring a subsea pipeline system connecting one or more wells to a floating production system, wherein the pipeline system is at least partially flexible, the method comprising: installing a continuous optical fibre distributed sensor as part of the pipeline system, the sensor capable of providing a distributed measurement of temperature, vibration, pressure or strain, or any combination thereof; using the sensor to obtain a distributed measurement of temperature, vibration, pressure and/or strain along at least part of the pipeline system indexed to a length thereof; and using the distributed measurement to predict the actual condition of the fluid, the pipeline system and/or the adjacent sea water using a model.