Subsea manifold is a flow-routing subsea hardware (subsea flow router) that connects between subsea trees and flowlines. It is used to optimize the subsea layout arrangement and reduce the quantity of risers connected to the platform. If connected to dual flowlines, the manifold can typically accommodate pigging and have the capability of routing production from a particular tree to a particular flowline.
Subsea manifold types
There are several manifold types and it is the field layout that determines the type of manifold required.
Template manifold is a drill-through structure designed to house multiple subsea Christmas trees on top of it and gather/route flow at the same time. It is required when subsea Christmas trees are grouped side-by-side. Trees are connected directly into the manifold via mandrels when installed.
Template manifold must be fabricated and installed before the start of drilling, which gives less schedule flexibility. It does not allow the re-use of exploration wells because they are typically not grouped together. Statoil Norne in North Sea has template manifold.
Cluster manifold is a stand-alone structure designed to direct fluids for multiple subsea Christmas trees placed around it. It typically can accommodate 4- 8 wells per manifold. Both the TOTAL and Shell's manifolds shown in the picture are cluster manifolds. Whensubsea Christmas trees are grouped closely in a central location (but not side by side), either a cluster manifold or a PLEM is required depending on the number of trees anticipated. A Daisy Chain field layout has the same manifold requirements.
- Provides flexibility to co-mingle or segregate any particular wells
- Can accommodate pigging capability
- Can include chemical injection
- Allow the re-use of exploration wells
- Has reduced risk of dropped objects and SimOps compared to template manifold.
Pipeline End Manifold (PLEM)
It a simpler version of a cluster manifold generally designed to direct fluids for only one or two subsea Christmas trees. A PLEM generally connects directly to a subsea flow line without the use of a pipeline end termination (PLET).
A manifold is typically composed of the following major components:
- Pipework and valves – contains and controls the production and injection fluids.
- Structure framework – protects and supports the pipework and valves.
- Subsea connection equipment – allows subsea tie-in of multiple pieces of equipment. Types include vertical, horizontal and stab-and-hinge-over connections.
- Foundation – interface between the manifold structure and seabed.
- Controls Equipment – allows the remote control of any hydraulically actuated subsea manifold valves and the monitoring of production and injection fluids. Control pods may be either internal or external to the manifold.
Valves on the manifold are essential for directing and controlling the flows. They can be either manual or hydraulically actuated. Sometimes chemical injection valves are placed on the manifold as well.
- Branch valves are generally slab type gate valves (similar to tree valves). Their sizes are based on the production/injection tree size.
- Flowline header valves are also gate type, but ball valves have been used previously. Their sizes are based on the flowline size.
- Materials are chosen for compatibility with production and injection fluids. Most of time, it is CRA-clad.
- Double barrier philosophy generally used against production fluids.
- Two valves in series
- One valve and one pressure cap
- Primary seal is generally a metal-to-metal seal
A wide range of pipework configurations is possible. Each header connects to an individual flowline. the pipework sizing is based on the tree piping size and the flowline diameters. The main circuit is designed to accommodate pigging operations. The material of construction needs to be compatible with production and injection fluids.
- Test headers can be incorporated to test individual or groups of trees
- Test headers can be a second or even third header isolated in the manifold
- Insulation may be required for unscheduled or emergency shutdowns
Control system for the manifolds is the same as the control system for the trees. Multiple options for the control system have been used in the manifold design
- No controls on the manifold. The manifold is controlled by tree subsea control modules (SCMs).
- SCMs on the manifold.
- Manifold with control system distribution units with flying leads going to trees.
The framework is a welded structure to provide support for the pipework and valves and contain the foundation interface structure. The pipework is allowed to float inside the framework within limits and it is not rigidly attached to the frame. The frame can also be used for lifting and landing of the jumper tie-in tools.
- Mud mats – a simple foundation resting directly on the seabed, generally with a short skirt around the perimeter to resist lateral loads.
- Piles – long cylindrical structures embedded into the soil intended to hold a subsea structure above the seabed. Foundations may utilize one or more individual piles.
- Intermediate Structures – an intermediate structure can be used to interface a subsea manifold with a pile foundation to reduce weight of the manifold structure or to ease retrieval of the manifold. Intermediate structures can be either retrievable or permanent structures.
Tie-ins to wells and flowlines
The tie-in hubs placed on the outer edge of the manifold, which are used to tie-in jumpers that bring in fluid from the production wells and export fluid into the flowlines (production manifold). The tie-in sizing is based on the tree piping size and the flowline diameters. and the loads applied from the flowlines
Generally gas manifolds are not insulated and oil manifolds are insulated. For oil production, insulation is necessary to allow adequate cool-down time to treat or remove trapped production water. Gas production is generally treated continuously with chemicals to prevent hydrates.
The following vessels are typically used for manifold deployment:
- Drill Rig: through moon pool or keel-hauled on drill string
- Heavy Lift vessels (Derrick Barges): through moon pool or over side
- Work-class vessels: over side on crane or winch
The following equipments are typically required:
- Manifold hydraulic installation tool
- Sling sets, either wire rope or synthetic fiber
Applicable API Specs
- API Spec 17P - Templates and Manifolds
- API Spec 17D - Specifications for subsea wellhead and Christmas tree equipments
- API Spec 17A - Recommended practice for design and operation of subsea production systems
- API Spec 17H, ISO 13628-8 - ROV Interfaces