Deoiling is a very common operation and is typically performed during change of service, maintenance and inspection or decommissioning. In all cases the level of cleanliness is critical to prevent contamination of the aquatic environment.
A commonly required level of cleanliness is 30ppm Oil in Water. This is a necessarily strict target and can be difficult, time consuming and expensive to obtain, especially when operating from a vessel with very high daily rental costs.
To make matters more difficult, especially during decommissioning when the pipeline may have undergone many changes during its life time, the pipeline may have different diameters along its length making it unpiggable. In addition access to the pipelines may only be possible through a narrow downline from a vessel.