Modelling Hydrate Plug Capacity of Flexible Pipes

Abstract

Abstract Hydrate plug detection and removal in flexible pipe has gained increasing attention from operators and at the same time not much has been published on the subject. When operating flexible pipe for hydrocarbon applications there is a risk of hydrate formation. Hydrates can block the bore of the flexible pipe and differential pressure can form across the hydrate plug. This differential pressure can damage the inner layers of the flexible pipe. The objective of the present study is to estimate the differential pressure capacity of the flexible pipe. Hydrates most often form during production stop periods. When the well is started up the lack of flow can be used to detect a hydrate plug. Either during this detection or during the removal of a hydrate plug, the pipe can be subject to a differential pressure across the plug domain. The differential pressure can lead to excessive loading of the pipe layers, especially the carcass and pressure sheath layers. A numerical model has been developed that can be performed with reasonable computational effort using an in-house pre/post-processor and FE solver. The model adopts the axisymmetrical framework, and the enmeshed domain is limited to the innermost layers, i.e. carcass, pressure sheath, and pressure armour. The model simulates the hydrate plug by applying Boundary Conditions corresponding to the rigid body motion of the surrounding carcass windings and converting the reaction forces monitored in the model into a differential pressure. The pressure sheath principal strain and carcass plastic strain are chosen as limiting criteria. The driving factor is the load transfer across each layer while the carcass windings are displaced axially (together with the plug) caused by the load from the differential pressure. For a given plug length, the differential pressure capacity is evaluated at the “time-step” where the governing strain criteria is exceeded — the reaction forces acting on each carcass winding are integrated into a total reaction force. Finally, the total force is converted into a differential pressure, which is denoted the hydrate plug capacity of the pipe. An important finding is that the differential pressure capacity increases for longer plug length in a non-linear manner. The non-linear plug length dependence is in competition with the hydrate plug shear capacity assumed to scale linearly with the plug length. The applied differential pressure can then be compared to the hydrate shear capacity thereby establishing if the plug is removed by shearing. The applied differential pressure is subsequently compared to the pipe’s hydrate capacity, showing that the differential pressure is below the pipe capacity for the possible range of plug lengths.