LNG Book

Wet tree vs. Dry tree

  • For the dry tree system, trees are located on or close to the platform, whereas wet trees can be anywhere in a field in terms of cluster, template, or tie-back methods.
  • Globally, more than 70% of the wells in deepwater developments that are either in service or committed are wet tree systems.

Wet tree systems

  • Subsea cluster wells

: gathers the production in the most efficient and cost-effective way from nearby subsea wells, or from a remote /distant subsea tie-back to an already existing infrastructure based on either a FPSO or a FPU

Wet tree system benefits

  • Tree and well access at the seabed isolated from people
  • Full range of hull types can be used
  • Low cost hull forms are feasible
  • Simplified riser/vessel interfaces

Wet tree risers challenges

  • Steel risers

: Fatigue critical requiring good quality offshore welds and fatigue testing requirement

  • Flexible risers

: Water depth (collapse) limitations

: Pipe diameter limitations for deep water and higher internal pressure

: Prone to external sheath damage during installation

: Potential of internal sheath (PA11) aging due to high water cut

: Potential end fitting integrity issue

Dry tree systems

  • the main alternative to the subsea well cluster architecture
  • surface well architectures provide direct access to the wells
  • system architectures consist of an FPDU hub based either on a TLP, on a Spar, or even (in some cases) on a compliant piled tower (CPT)
  • Risers for dry completion units (DCUs) could be either single casing, dual casing, combo risers (used also as drilling risers), or tubing risers and could include a split tree in some cases.
  • The riser tensioning system also offers several options such as active hydropneumatic tensioners, air cans (integral or nonintegral), locked-off risers, or king-post tensioning mechanism

Dry tree system benefits

  • Tree and well control at surface in close proximity of people
  • Drilling conducted from the facility –reduced CAPEX
  • Direct vertical access to wells for future intervention activities
  • Minimal offshore construction
  • Enable future drilling and expansion

Dry tree system challengs

  • Safety concern due to well access at surface
  • Large vessel payloads due to the need for supporting risers
  • Require high cost vessels such as Spar, TLP due to design sensitivity to vessel motions
  • Complex riser design issues
  • Limited by existing riser tensioner capacity
  • Riser interface with vessel require specialty joints, e.g. keel joint, tapered stress joint
  • Heavy lift requirement for riser installation

System selection

  • Economic factors: Estimated NPV, internal rate of return (IRR), project cash flow, project schedule, and possibly enhanced proliferation control initiative (EPCI) proposals (if any available at the time of the selection) will most certainly be the key drivers of this choice.
  • Technical factors: These factors are driven primarily by reservoir depletion plans and means, field worldwide location, operating philosophy, concept maturity and reliability, feasibility, and industry readiness.
  • External factors: These factors are in the form of project risks, project management, innovative thinking, operator preferences, and people (the evaluation method may vary between each individual).

Wet tree vs dry tree summary

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