Subsea field is developed using subsea production tree installed on a subsea well tied back to a gathering facility using flowlines and control equipment & distribution system to operate the well. The architecture and layout can be as simple as single satellite tie-back or can be very complex consisting of multiple wells.
The drilling plan should be looked in conjunction with the subsea field development plan to maximize the project value. Well locations and resulting subsea layout is a multi factor optimization problem depending on the – subsea infrastructure cost, drilling cost (vertical vs deviated vs extended reach vs horizontal), well spacing & associated impact on reservoir recovery.
Subsea infrastructure cost depends on the type of development architecture used. Simplest architecture is satellite well, where each well is tied back to the host using individual flowlines. This type of development is more suitable when number of wells is few or for individual well tie-back to existing infrastructure.
Multiple well development can have different architecture based on a number of factors, the main architecture types being:
- Cluster development
- Template development
- Daisy chain
- Daisy chain loop
Typical cluster architecture modelled in SFACE is shown below. In this development, there are 6 production wells and 2 water injection wells. The production wells are drilled using two drill centers and each drill center has a 4-slot cluster manifold to gather production from associated wells. The water injection wells are drilled from a separate drill center and supplied via water injection manifold. A common umbilical distributes power, communication, hydraulic fluids and chemicals to all the wells via UTA and flying leads.
This type of architecture provides cost savings compared to sparsely located single wells which would have individual long flowlines & umbilicals. Since the wells are drilled from a single location (for one drill center) in this architecture, the reservoir coverage could be limited or it might be costly (for drilling & well completion) to reach the optimal reservoir target. Commonly used manifolds are 4-8 slots while larger manifolds could be possible though might be impractical.
Template is a seabed installed structure (structural framework and a foundation) that houses production system equipment and protects them from dropped objects and/or fishing equipment. See detailed definition.
Typical clustering with template manifold as modelled in SFACE is shown below. In this development, there are 6 production wells and 2 water injection wells. The production and water injection wells are drilled using two 4-slot templates. Production from wells is gathered and water is distributed to injection well using a manifold installed within the template. Umbilical is directly connected to the manifold using UTH and controls distribution is done through the manifold.
This type of architecture can enable further cost saving compared to cluster architecture because piping & umbilical interfaces are less expensive and Installation time is reduced due to modularization. For this architecture, template is required to start drilling which could be a schedule constraint. There is even less flexibility in determining well locations. There may also be safety concerns related to simultaneous drilling and production operations.
This architecture is typically used in areas where there is high fishing activity and protecting all the seabed equipment from fishing gear is required. Commonly used template manifolds are 4-slots while 6-8 slots have been used in some projects.
The daisy chained architecture consists of two or more wells joined by a common flowline and umbilical.
Typical daisy chain architecture as modelled in SFACE is shown below. In this development, there are 3 production wells. Production from wells is individually gathered in a common flowline via an ILT (In-Line Tee). Daisy chain architecture can alternatively be achieved by the use of flowbases (installed under the XT) instead of ILTs. Controls distribution to the Trees is done via UTAs or In-Line UTAs (I-UTA) located close to the Trees.
Daisy chained architecture allows for the use of a common flowline by multiple wells (compared to individual flowlines in satellite development), thus reducing subsea infrastructure cost. This architecture allows well location flexibility and optimal coverage of the reservoir, which is especially important in oil fields where low permeability exists.
Daisy chaining is not very suitable in the regions requiring fishing protection for subsea equipment, as extensive and distributed protection is required, driving the cost up. Drilling rig needs to relocate from one well location to another for each well, increasing the rig time and drilling cost.
Daisy chain loop
Typical daisy chain loop as modelled in SFACE is shown below. In this development, there are 6 production wells. Production from wells is individually gathered in a common flowline via an ILT (In-Line Tee). Controls distribution to the Trees is done via UTAs or In-Line UTAs (I-UTA) located close to the Trees.
Daisy chain loop is similar to daisy chain architecture except that it has two flowlines connected in a loop. Pros and cons for daisy chain architecture (discussed above) applies to daisy chain loop as well. However, it has some additional benefits compared to daisy chain architecture,
- Round trip pigging from topside
- Diverting all production into a single flowline if the other is damaged
- Simultaneous testing of two wells
Subsea field architecture applicability
Small number of wells
Brown field development with individual wells
Reservoir targets could be reached from few drill centers
No rigorous requirement of fishing protection
Reservoir targets could be reached from few drill centers
Fishing and overtrawlable protection required
Dispersed wells required
Relative well & host / tie-in locations are such that the cost of combined flowline is lower than the sum of the cost of the individual flowlines, when developed as satellite
|Daisy chain loop||
Same suitability as daisy chain
Frequent operational pigging required