Subsea Operating Envelopes in Ultra-Deepwater;Kaombo Development

摘要

In a deepwater environment,production fluid conditions have to satisfy complex requirements to flowsmoothly to the production facilities on the FPSO.Flow assurance specialists work at turning theseconstraints into operating guidelines.This allows to close the gap between reservoir conditions,optimizeddesign of the subsea network,topsides processing capabilities and operability requirements.In the context of Kaombo,offshore Angola (Block 32),the wide range of reservoir conditions and fluidsplus the extreme specificities of the subsea network called for an innovative approach with the followingobjectives: 1.Empower the operator with a visual decision tool for normal and unplanned operations of thesubsea system.2.Promote collaboration between production,flow assurance & geoscience teams to reach anefficient decision,and minimize production shortfalls.3.Allow a design robust enough to tackle geosciences uncertainties.4.Optimize subsea design margins.This new approach,the"Visual Operating Envelopes",aims at explicitly and visually defining theoperating limitations of the subsea production loops in a multi-parameters environment: A multi-dimensionsmap,function of the six main parameters (basically liquid and gas-lift flowrates,water and gas contents,reservoirs pressure and temperature) influencing multiphase flow into pipeline is hence created to evaluatethe six main operating constraints (thermal and hydraulic turndown rates,wells eruptivity,maximumflowrates) for the full range of Kaombo fields.This"operating envelope"tool can then define the minimum and maximum recommended flowrates fordifferent operating conditions based on the following safe criteria: 5.Arrival temperature above the Wax Appearance Temperature.6.No hydrates risk during preservation.7.No severe slugging effect.8.Production below the flowline design flowrate.9.Velocity below the erosional velocity.In addition,the optimized gas lift flowrate is directly accessible,and the pressure available at everywellhead is compared to the backpressure associated to the operating point to assess the eruptivity of thewells.By having previously defined an overall operating envelope,it is extremely easy to evaluate quickly theimpact of new operating conditions (due to degraded operating conditions,changes in reservoir parameters,modifications in the drilling and wells startup sequence),which makes this new approach very powerful andversatile.It also contributes to the definition of the production forecast during operation phase integratingreservoir depletion and available gas lift rate.Instead of relying on specific simulations for a limited number of cases,this innovative method defines anew approach where operating parameters are evaluated from the start,and boundaries are clearly identified,thus allowing to build a sound production profile for an extensive range of operating conditions.By doingso,system knowledge is improved,bottleneck conditions are anticipated,operators,flow assurance andgeoscience teams are able to tightly collaborate and take smarter decisions together,resulting in moreproduction.Eventually the method applied to a multiphase pipeline is actually transposable to every probleminvolving multi-dimensional inputs with combined constraints.