Wet Tree and Dry Tree Systems
Two kinds of subsea production systems are used in deepwater fields: 1. dry tree systems and 2. wet tree systems
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. Dry tree platforms have a central well bay for the surface trees, providing direct access to the wells for workover and recovery. Tension leg platforms (TLPs) and spars are normally utilized in a dry tree system. The size of the central well bay on dry tree platforms is dictated by well count and spacing. Topside equipment has to be arranged around the well bay. The surface trees are designed for full reservoir shut-down pressures. A large production manifold is required on deck, and a skiddable rig is required for individual well interventions.
For wet tree systems, the Christmas tree and its associated components are exposed to the ambient seabed conditions. In deepwater fields, the wet tree system normally utilizes a remotely controlled subsea tree installation tool for well completions, but for shallow water the diver can assist with installation and operation.
Wet tree platforms have a central moon-pool for running marine risers and trees, which might also be preferred for installing other equipment, such as the manifold and blow-out preventer (BOP), if the sizes are suitable. Wet tree systems are suitable for widespread reservoir structures. They provide a degree of vessel and field expansion flexibility with simplified riser interfaces, but at the expense of high drilling and workover costs.
In recent years, operators have been compelled to reappraise their strategy of rapid development in ultra-deepwater areas due to the commercial competitiveness and technical issues related to dry tree versus wet tree systems. Globally, more than 70% of the wells in deepwater developments that are either in service or committed are wet tree systems. These data demonstrate the industry’s confidence in wet tree systems.
Compared to wet tree systems, dry tree systems require motionoptimized hulls to accommodate the riser systems; they are considered to be limited with respect to water depth and development flexibility. Globally, most subsea field developments that are either in service or committed to are wet tree systems. Although widely used for developments in shallow to medium water depths, the dry tree units are still not considered optimum for deepwater and ultra-deepwater situations.
Wet Tree Systems
For wet tree system, the subsea field layout usually comprises two types: subsea wells clusters and direct access wells. Direct access is only applied in marginal field development. All such developments are usually based on semi-submersible floating production and drilling units (FPDUs) with oil export either via pipeline or to a nearby floating storage and offloading (FSO) unit, providing direct and costeffective access from the surface to the wells to allow for workover or drilling activities directly from the production support, especially for deepwater interventions.
The FPSO usually utilizes a ship- shaped or barge-type vessel as the host structure that is moored either via a turret and weathervaning system to allow for tandem offloading or spread moored with offloading via a distant buoy or still in
tandem mode.
For both subsea well clusters or subsea direct-access wells, the three main riser options would be vertical top tensioned risers, steel catenary risers, and flexible risers. Pending use of the above field architectures and risers in conjunction with particular flow assurance design criteria, these wet tree system would be the most effective.
Dry Tree Systems
Dry tree system production systems are the main alternative to the subsea well cluster architecture. Their surface well architectures provide direct access to the wells. Current dry tree 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) concept. Alternative concepts in the form of barge or deepdraft semi-submersible (DDF) floaters could also be considered as possible options in some cases but have not yet materialized.
Deepwater surface well architectures in the form of a wellhead platform (WHPor FDU) associated with either an FPSO or an FPU are starting to emerge in other parts of theworld (West Africa, Southeast Asia). This type of association between a WHP and an FPSO could also eventually become reality in the Gulf of Mexico as the FPSO concept becomes progressively authorized. Existing (or actually planned) WHP hubs are currently all based on a TLP concept with either full drilling capacities or with tender assistance (mini-TLP). However, other concepts such as Spar, barge, or deep-draft semi-submersible units could also be considered as alternatives.
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.
Systems Selection
A thorough and objective assessment of wet and dry tree options should be conducted during the selection process and include costs, risks, and flexibility considerations to ensure that the development concept that best matches the reservoir characteristics is selected.
Operators are often faced with the problem that commercial metrics, such as capital cost or net present value (NPV), are inconclusive. In the base case of developments examined, the commercial metrics are shown to generally favor the wet tree concept over the life cycle of the system, but the benefit over the dry tree concept is not overwhelming. When this is the case, the operator must thoroughly assess other important differentiators before making the final selection.
The best tree system matchup with the reservoir characteristic can be selected by experience and technical analysis. The following are the basic selection points:
- 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).
References
[1] C. Claire, L. Frank, Design Challenges of Deepwater Dry Tree Riser Systems for Different Vessel Types, ISOPE Conference, Cupertino, 2003.
[2] M. Faulk, FMC ManTIS (Manifolds & Tie-in Systems), SUT Subsea Awareness Course, Houston, 2008.
[3] R. Eriksen, et al., Performance Evaluation of Ormen Lange Subsea Compression Concepts, Offshore, May 2006.
[4] CITEPH, Long Tie-Back Development, Saipem, 2008.
[5] R. Sturgis, Floating Production System Review, SUT Subsea Awareness Course, Houston, 2008.
[6] Y. Tang, R. Blais, Z. Schmidt, Transient Dynamic Characteristics of Gas-lift unloading Process, SPE 38814, 1997.
[7] DEEPSTAR, The State of Art of Subsea Processing, Part A, Stress Engineering Services (2003).
[8] P. Lawson, I. Martinez, K. Shirley, Improving Deepwater Production through Subsea ESP Booster Systems, inDepth, The Baker Hughes Technology Magazine, vol. 13 (No 1) (2004).
[9] G. Mogseth, M. Stinessen, Subsea Processing as Field Development Enabler, FMC, Kongsberg Subsea, Deep Offshore Technology Conference and Exhibition, New Orleans, 2004.
[10] S.L. Scott, D. Devegowda, A.M. Martin, Assessment of Subsea Production & Well Systems, Department of Petroleum Engineering, Texas A&M University, Project 424 of MMS, 2004.
[11] International Standards Organization, Petroleum and Natural Gas Industries-Design and Operation of the Subsea Production Systems, Part 1: General Requirements and Recommendations, ISO 13628-1, 2005.
[12] O. Jahnsen, G. Homstvedt, G.I. Olsen, Deepwater Multiphase Pumping System, DOT International Conference & Exhibition, Parc Chanot, France, 2003.
