Hydraulic Analysis Limited has extensive experience of the design and operation of dry gas systems. We have undertaken many dynamic analysis studies on gas systems to identify the cause of operational problems and to then eliminate these problems, mainly through the adjustment and calibration of HIPPS settings. We do not study wet gas systems and / or multiphase flow.
Most gas system are not prone to rapidly applied surge pressures due to the relatively high compressibility of gas when compared to liquids. It is still however necessary to analyse gas systems in detail in order to ensure pipeline packing issues do not occur.
One of the most common simulation scenarios which we would investigate is the effects of a blockage in the pipeline. In 99% of cases, any high pressure packing problems which could occur due to a blockage forming or due to a valve closing can be resolved through the prompt tripping of compressors before any damage is incurred.
The setting and optimization of any HIPPS facilities requires careful consideration and must be carried out through dynamic analysis to ensure that systems are adequately protected but to also avoid unwanted (spurious) trips during steady operation of the system.
We have undertaken numerous slugging investigations on gas systems in the past. In our experience, slugs will start to form at the end of the subsea pipeline as it starts to rise into the onshore tie in point. Whilst the flow is stratified in the pipeline, the elevation change will generally be the location where slugs will form until they are swept through into the slug catcher during gas ramp up. We use single phase software which can model the effects of liquid / gas mixtures but not true multiphase or stratified flow. We will effectively consider a discreet pipe full slug of liquid in a gas system.
As the gas pressure builds on the upstream side of the slug, it will eventually dislodge the slug and push it forwards into the slug catcher. This chain of events can then cause a rapid drop in gas pressure as the slug accelerates away from the gas and can often result in vibration issues onshore. This is the normal modelling case which we would consider for sizing onshore pipe supports.
