A Tale of Two Trees: Flow Assurance Challenges for Wet Tree and Dry Tree Systems in Ultradeepwater

摘要

As oil and gas exploration moves into deeper watersrnoffshore, flow assurance challenges become morernprevalent and system design must address these issuesrnfrom a fresh perspective. In addition to structural andrneconomic issues, the decision to employ either a wetrntree (subsea) or dry tree solution needs to especiallyrnconsider the flow assurance aspects that ultradeepwaterrnproduction presents. The riser, rather than the flowline,rnwill now dominate the overall hydraulic andrnthermodynamic performance of the system. Anrnincreased liquid column height in the riser, a greaterrnpotential energy change, and an enhanced Joule-Thomsonrneffect will all contribute to significantrntemperature and pressure drops for an ultradeepwaterrnriser, relative to its shallow water counterpart.rnDry trees are traditionally perceived as having lessrnsevere flow assurance issues than subsea tiebacks,rnprimarily because of easier access to the tubing string ifrnwax or hydrates are formed. However, aside fromrnsignificant temperature losses in the riser, dry trees inrnultradeepwater are particularly susceptible to hydraternformation upon shutdown. Because of warmer seabedrntemperatures and lower reservoir pressures, shallowrnwater dry tree risers can provide adequate cooldownrntimes with only minimal insulation. For ultradeepwaterrndevelopments, the same shallow water dry tree designrncan fall into the hydrate formation region within two tornthree hours.rnFor both the dry tree and wet tree riser systems, it mayrnnot be possible to depressurize the system belowrnhydrate formation conditions without gas lift (which mayrnnot be available for an emergency shutdown). The focusrnof this discussion is to evaluate the merits andrndrawbacks for both wet and dry tree systems from a flowrnassurance standpoint in ultradeepwater.