13 Applications & Limitations
13.1 General
Risers play an important part in the drilling, production, and transportation of hydrocarbons and other associated fluids with any offshore oil and gas production. For a floating production system, risers provide the link between the floating platform and:
The oil/gas wells close to or underneath the floating platform,
Subsea satellite wells at some distance from the platform,
Other floating or fixed platforms,
Export facilities, either a pipeline to the shore or to a shuttle tanker loading facility.
The risers may handle:
Drilling or production.
Hydrocarbon imports (from remote wells/platforms),
Hydrocarbon exports (via pipelines to shore, another platform or a storage unit).
Water or gas injection (into the reservoir to increase pressure and force the oil up the well).
Gas lift (gas pumped into the bottom of the well or at the riser base to help the oil to flow more rapidly up to the floating production system).
![[Note]](note.png) | Note Other risers such as hydraulic and electrical lines, work-over risers are not covered by this study. |
Risers may be classified either as rigid or flexible or hybrid:
Rigid risers manufactured from steel pipe and generally found in free hanging configuration or in a vertical position (top-tensioned riser).
Flexible risers manufactured from layers of wires and polymers, which are hung in suspended catenary or free hanging configuration.
Hybrid riser systems combine these two types of pipes.
13.2 Flexible risers
Flexible risers have the advantage that they can accommodate larger platform motions than rigid risers. They are suitable, even in high sea states, for use with semi-submersibles and turret moored ships when rigid risers would be unsuitable. Although flexible Flowlines are more expensive than rigid steel pipes (due to sophisticated plastic materials and manufacturing methods) they are utilised for the development of short distance between wells to production facilities, and also the development of deepwater small and marginal fields. In these types of application, the used lines could be recovered from the sea bottom, transported to onshore base and submitted to inspection and refurbishment, to assure a safe and efficient reutilization. In predominant seabed irregularities where flexibility is required flexible pipes are also used.
Despite the inherent advantages in the use of flexible risers, there are technical and economic limitations. It is well recognised by the industry that the use of flexible risers from the floater to the sea floor in deepwater needs exceptional flexible pipe design to withstand the extreme external hydrostatic pressure and large top tension. On top of this, flexible pipes of the sizes anticipated for export lines are at the current limits of manufacturing capabilities.
13.3 Steel risers
The use of risers made of rigid pipe become more economical for deepwater applications as:
The rigid pipe has no specific limitation concerning the water depth (the limits are mainly fixed by the laying capacity of the installation vessel, the deck space and load of floating production system)
The reduction of the difference between the installation cost of rigid riser and flexible lines in deepwater, combined with a lower fabrication cost for the rigid line, turn the rigid pipe riser into a cost effective alternative.
The flexibility of the overall riser in production phase increases with the water depth (i.e. increased riser length induces increased flexibility).
At present, the steel pipe is mainly used in two types of riser configuration:
Top-tensioned risers
Steel catenary risers
13.3.1 Top-tensioned riser tower
The original philosophy for the top-tensioned riser tower, developed by Cameron Iron Works in early 1983 for Placid Oil, was to design a system, for small or marginal fields, that could drain a reservoir and then move on to a new location. By utilising established technologies and fabrication methods, the top-tensioned riser tower has been proven to be a viable and cost effective solution for deepwater projects. This technique is well adapted to marginal fields using subsea drilled and completed wells tied-back to a floating production system located directly above the subsea template/wells.
When compared with an hybrid riser tower, the top-tensioned riser tower has the advantage of optimised jumper length, The coupled motions of the riser and the floater, by means of tensioned system or buoyancy cans, allow the installation of short jumpers, thus reducing the overall project cost.
However, a relatively stable floating production system is required to avoid the disconnection when hurricane conditions are encountered: for a floating production system with a large number of risers it is not practical to disconnect and it is undesirable to lose position. Furthermore, the number of risers, tensioned using tensioner system with multiple wires, hydraulic accumulators and jacks, is limited on the semi-submersible due to space requirements and complexity of the tensioning system.
In the more severe environments special platform designs will be required. These may be:
Tension Leg Platforms, which are constrained to move approximately on the same arc as the top of the Riser. The TLP riser tensioner is much simpler than the tensioner required on a semi-submersible owing to the very small stroke requirement (i.e. TLP low heave motions), so multiple risers are practical.
SPAR platforms, where the risers are tensioned by internal buoyancy cans in a deep centre well in the platform
13.3.2 Steel catenary risers
Steel catenary risers offer advantages over risers made of flexible pipe since steel catenary risers are much less expensive. Steel catenary risers also offer advantages over top tensioned risers since steel catenary risers need no heave compensation, no subsea connections, and no flexible jumpers to transition to fixed piping at the production deck.
For some applications a disadvantage of steel catenary risers compared to top-tensioned risers is the length of active footprint on the seafloor.
If soft soil conditions are encountered, problems of pipe self-embedment at touch-down point may appear, thus limiting the compliance of this area and increasing stress magnitude.
Another major problem with some steel catenary risers is the high bending stress where the riser touches down. Tension is typically low near touch-down point, so the riser is easily bent. In addition to a high static touch-down curvature, the situation is often exacerbated by severe dynamic response – characterised by waves travelling down the riser.
Theoretically, the problem is a complicated one, involving local tension, drag, mass and weight distributions, as well as global dynamics.
However, it is possible to overcome the problem by:
- Using strakes: it increases in drag coefficient dampens the SCR dynamics
- Repositioning occasionally the vessel: it leads to a corresponding movement of the TDZand the peak fatigue damage location for each mean position of the vessel. This methodology is already being applied in semi-submersible FPS projects in Gulf of Mexico.
- Lightening the TDZ: numerical simulations have shown that short length of buoyancy material applied on TDZ reduces the soil-riser interaction and can improve the SCR fatigue performance by about a factor of 2.
- Using a length of titanium pipe in the touch-down region. Consideration of the higher and lower stiffness of titanium shows that the minimum allowable radius of curvature of a titanium pipe is three to four times less than that of a steel pipe with the same outside diameter and wall thickness.
- Using internally-clad steel pipe
13.4 Hybrid riser system
Flexible riser costs are relatively high due to the complex pipe manufacturing process and materials used. As water depth increases, flexible riser costs become an increasing proportion of the total development cost. Additionally, flexible pipe may be considered limited with respect to high temperature and pressure capability.
This has lead to the development of a number of deepwater riser concepts that utilise all or a majority of rigid pipe. The main objective of these developments has been to reduce the length of flexible pipe required, which is up to 10 times the cost of steel pipe, in order to reduce total riser costs.
The hybrid riser is one of these concepts. Hybrid risers utilise a vertical bundle of relatively low cost steel pipes extending from the seabed up to a point near the sea surface from where short lengths of flexible pipe are used to connect to the vessel. This arrangement does not eliminate flexible pipe but minimises the length required.
The hybrid riser concept is not very sensitive to the water depth as increasing the length of the steel bundle and the wall thickness of the lower part are the main required operations to increase the range of use of an hybrid riser tower in terms of depth.
The hybrid riser tower, as it is not rigidly linked with the FPS, can be used with a floater that has a relatively large dynamic response, such as an FPSO. The compliance of the riser bundle combined with the flexibility of the jumpers allows withstanding its motions.
Moreover, the concept allows decreasing (with reference to a classic catenary configuration) the loads (vertical and horizontal) applied to the FPS as only the jumpers weight is supported by the floater. This becomes important as the water depth and the riser number increase.
Another advantage of this riser concept is to avoid any fatigue problem on the main part of the riser: The steel bundle provides a relative stability along its length. The critical points are mainly the jumpers, their interface with the steel bundle, the subsea buoy, the flex-joint connection, and finally the lower part of the riser anchoring, which require accurate engineering studies.
