Optimizing MEG Systems on Long Subsea Tiebacks Patrick Wan DOT PERTH, Wednesday 28 Nov 2012
Presentation Outline Overview Hydrates MEG Management Summary 2
Overview Various flow assurance challenges associated with a long deep water subsea tie-back of a gas-condensate field Hydrate management Most long subsea tiebacks (> 30km) in cold waters tend to be un-insulated Require continuous hydrate inhibition 3
Overview Continuous hydrate inhibition using MEG or LDHI depends on forecasted produced water rates. MEG can be recovered at the host facility (low losses) and recycled, thus reducing OPEX. Depending on the location of the host facility (onshore / offshore), the slug catcher size and MEG storage requirements become important design factors, particularly for offshore facilities. Note: LDHI Low Dosage Hydrate Inhibitor 4
Overview Piggable loop pipeline configuration is employed To cater for turndown flow rates Provision for depressurization either side of a hydrate blockage Inspection pigging etc. 5
Schematic Gas Dehydration Gas Compression Export Gas 22 PL (Production) Slug Catcher MEG Recovery Unit Subsea Manifold 22 PL (Recycle) 6
Gas Recycling Initially, depending on requirement, a single flowline can be used for production and the other as a gas recycle line Gas recycle facility in order to maintain high flow rates / velocities in the single production line and hence limit liquid holdup in the production line. 7
Hydrates Particularly for deep water gas development, hydrates will form in the presence of free water at low temperatures and high pressures Hydrate blockages can result in long production downtimes Long distance tie-backs do not have insulation but rely on continuous hydrate inhibition LDHI injection for low water production systems MEG injection rates proportionally increase with increases in water rates 8
MEG Management MEG injection rates Increase with increasing produced water rates Decrease with decreasing SITHP Decrease with lower degrees of hydrate depression Design MEG injection rates could be based on early or late life production requirements. Once design MEG rate is determined, MEG system can be designed for Subsea MEG injection requirements Gas dehydration requirements Lean MEG storage requirements 9
MEG System Gas Dehydration Gas Compression Export Gas 22 PL (Production) Slug Catcher Subsea Manifold 22 PL (Recycle) Rich MEG Receiving Vessel MEG Recovery Unit Lean MEG Storage 10
Issues - Turndown Slug catcher receives the well fluids Gas to gas dehydration system Condensate to condensate stabilization system Water phase with MEG to the Rich MEG vessel for recovery During turndown condition (low flow), flowline liquid holdups will increase There may be periods of Rich MEG starvation until new steady state conditions at low flows (turndown) are established. The starvation period is important in terms of lean MEG storage, because of continuous MEG injection during this period when no Rich MEG is being returned to the slug catcher for recovery. 11
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By re-circulating gas during turndown conditions via one flowline and producing through the other flowline the liquid holdups do not increase significantly during turndown. This reduces the effects of rich MEG starvation periods, thus reducing lean MEG storage requirements during turndown 14
Issues Ramp Up When flow is ramped up to normal design rates, ramp up slugs are generated. May trip process on high liquid levels Also, water draw-off rate for slug catcher tends to be based on the MEG regeneration units MEG + water processing capacity. In order to avoid a large surge volume in the slug catcher or a high liquid processing capacity (liquid draw-off rate) during flow ramp-up, the flow ramp-up period would need to be over a much longer time. Alternatively a larger rich MEG receiving vessel or some temporary storage can be considered for high liquid draw-off rates for the water / MEG phase from the slug catcher during ramp ups. 15
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Summary There are a number of benefits of employing gas recirculation during low flow rate periods to limit liquid holdups in flowlines for a looped configuration. Reduce Lean / Rich MEG storage requirements Reduce Rich MEG starvation periods Help optimize slug catcher and draw-off rate requirements Allow for faster ramp-up rates from turndown condition Reduce/eliminate the severity of slugging depending on the gas re-circulation rate being employed. 19