Wet tree vs. dry tree criteria

July 1, 2015

For offshore field development planners, the selection between wet or dry tree completion systems represents a key planning decision. Granherne compares the two and Jeannie Stell captures the details.

Robert Eichenlaub

A key decision made by offshore field development planners is whether to select wet tree or dry tree completion systems during the pre-FEED (front-end engineering and design) phase. Although both systems have been successfully used in offshore developments, they are not equally appropriate for all field developments.

As a result of its recent US Gulf of Mexico (GOM) field development case study in 4000ft water depth, KBR’s consulting subsidiary, Granherne, developed a detailed set of technical and economical metrics for use in offshore development concept selection. The metrics help planners choose whether to proceed with dry tree, wet tree, or both types as they develop feasibility studies and progress into later stages of development planning. The study considered the influence of field development decisions, drilling and intervention methods, facilities size and other operational issues to provide guidance on completion system selection.

Figure 1: Selected Gulf of Mexico facility types vs. water depth.

Dry trees applications

When considering dry tree systems, planners should be aware of several considerations. Dry tree systems are typically used in water depths ranging from 500ft to 5600ft (see Figure 1), and are suitable for all reservoir geometries where a single drilling location can reach all well locations.

With dry tree systems, the surface tree is installed on the production deck. “Using dry tree systems creates a higher payload (weight) on the hull due to the large number of risers, tensioner systems and the drilling facility itself,” says Robert Eichenlaub, technical advisor for Granherne. “This requires a larger deck area on the facility.”

Dry tree systems limit hull selections to SPARs and tension leg platforms (TLPs). Semisubmersible platforms might be candidates for dry tree systems in the future, but they have not been used to date.

“The dry tree facilities are designed around a particular well bay size, so operators must pre-determine the required number of well slots when designing the facility,” Eichenlaub explains. “The onboard drilling rig capabilities dictate step-out distances achievable using dry tree systems and placement within the development area.”

The advantages of employing dry trees is the system’s direct vertical flow path, which minimizes flow assurance issues and provides direct vertical access for well intervention. In areas where pipeline infrastructure is not present, the host facility can be paired with a floating storage offloading unit (FSO) to allow in-field storage. However, this is an additional investment in the facilities.

With a dry tree platform, the wells are drilled and completed from the facility once it is installed and commissioned. Operators have the option to predrill wells by using a mobile offshore drilling unit (MODU), and then complete the wells after the floating facility is installed. Thus, the dry tree application allows several drilling options for the operator, but the operator must wait for the onboard rig to complete the wells before production can be ramped up.

With a dry tree system, where the risers are in close proximity to each other, a higher level of simultaneous operations (SIMOPS) occurs and increased safety planning is required. Drilling and simultaneous production require a higher level of management during operations.

Wet tree applications

Wet tree, or subsea trees, can be used with any hull form, but are typically used in conjunction with FPSOs, which represent about 70-80% of global floating facilities, Eichenlaub says. These systems are suitable to all reservoir geometries and areal sizes, and allow increased flexibility in field layouts, such as multiple individual wells and multiple drill centers. They also remove the need to pre-design the number of well slots on the host facility.

Wet tree systems can be used in deeper water, reaching the industry’s current capability of subsea completion technology for operating in water depths up to 9600ft. MODUs and drillships are used for drilling and completion activities. Wells are tied back through a common production system to the floating facility.

“The systems are only limited by the seafloor topography,” Eichenlaub says. “Operators might have to go over canyons or other seabed features, but wet tree systems do allow varied well placement and provide for long-distance tiebacks with these systems.” However, wells sited at significant distances from a host facility can result in reduced well performance due to pressure drops in flowlines and flow assurance issues.

Because wet tree systems are segregated from production operations, the operator can pre-drill and complete wells faster. Operators can quickly ramp up production once the floating facility is commissioned.

As an added benefit, fewer risers are required because subsea flowlines are grouped together and routed into a limited number of risers. Fewer risers mean reduced hull payload, which allows for greater flexibility of hull selection, topsides weight, layout and room for future expansions.

Case study

 

The impact of dry tree versus wet tree selections was examined in Granherne’s recent US GOM case study.

The case study included a one-year pre-drilling scenario, as well as other key assumptions (see Figure 2). The methodology included creating the reservoir description, developing a production profile and well counts, creating development plans for SPARs with wet trees (production SPAR) and dry trees (drilling and production SPAR), and creating schedules, CAPEX and OPEX estimates, and economic modeling.

“Granherne deals mostly in pre-FEED work, which is concept selection, concept definition, and feasibility-study work,” says John Vitucci, field development manager for Granherne. “We found that with a CAPEX sensitivity of +40%, -20% the net present value (NPV) overlap is 60% between wet tree and dry tree development plans resulting in quite a bit of overlap between the two solutions at this level of concept definition.”

Given the immaturity of the data companies have in the early planning process, and assuming both wet trees and dry trees satisfy the same technical criteria, the most advantageous development planning strategy is to carry both types of developments forward into the feasibility study, as opposed to excluding one type in preference to the other. The economics that result from more detailed planning for some of the major drivers, such as drilling and completion programs, can be more useful when making a final development determination.

Fig. 3: Economic comparison with CAPEX sensitivity. The Granherne case study included various key assumptions.

Conclusions

The typical overlap of wet tree and dry tree completion systems in the offshore industry is in water depths between 600ft and 5500ft. This range covers the water depth for about 50% of offshore projects, Eichenlaub says, and half of those can be candidates for dry trees or wet trees. The case study resulted in some general conclusions or advice, including:

  • Technical considerations, investment levels and economics are all considerations in the dry tree versus wet tree concept selection decision.
  • The level of uncertainty in the data, particularly drilling and completion data, will play an important role in selection of wet tree versus dry tree concepts.
  • A comprehensive evaluation methodology should be used during the concept selection and an understanding of the uncertainty in the data should be maintained throughout the decision process.
  • During the concept selection and pre-FEED phases of engineering definition, selection of either a wet tree or dry tree solution might be premature.
  • Concept selection is a time-based decision as more definition is achieved.

Fig. 4: Field development capital expenditures. The selection of a wet tree instead of a dry tree increased capital investment by more than $1.04 billion to the cost of the development.

At the conclusion of the case study, the preference of the wet tree technology over the dry tree added more than US$1 billion of additional capital investment to the cost of the offshore development (see Figure 4). The facilities capital cost differential is a result of the dry tree facility having an onboard drilling and completion rig. The installation and commissioning capital cost differential is a result of additional capital costs for installation of the subsea equipment, flowlines and risers.

However, the more expensive wet tree solution allowed the initial subsea wells to start production about seven months earlier than the development solution that employed the dry trees.



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