Deepwater Technology: Riser Systems

Riser concepts for Mexican deepwater production systems

Basic design overviews illustrate several types of potential riser systems for deepwater, subsea field developments in Mexico's Bay of Campeche in the Gulf of Mexico, setting the stage for more detailed assessments as non-generic engineering data becomes available

Otávio Borges Sertã, INTEC Engineering

INTEC Engineering has performed, in the past, studies for potential production system concepts to be employed in Mexican deep water in the Gulf of Mexico. More recently, the author was invited to give a presentation on “Deepwater production systems: Innovation, technology and future developments in Mexico,” in a workshop organized as part of the Conference on Offshore Mechanics and Arctic Engineering (OMAE), June 2003, Cancun, Mexico. The title of that presentation was “Review of riser systems for future Mexican deepwater developments.” This article is based on the research carried out for that presentation. 

A preliminary assessment of potential riser systems for application in deepwater Mexico can be based on a comparison between conditions in the deepwater Bay of Campeche and those in other deepwater areas currently being exploited, namely Brazil (Campos basin), US (Gulf of Mexico) and West Africa (Nigeria and Angola). 

Although a key element of the offshore production system, the riser system should not be selected in an isolated fashion, as it influences other essential elements of the production system. A methodology for riser selection is provided, together with a preliminary assessment for Mexican deepwater conditions. Several different production system concepts/ strategies can be considered for Mexican field exploitation, but in the present context, only those based on subsea systems are addressed. 

RISER SYSTEM SELECTION

Fig. 1 illustrates the generic riser selection process for a subsea production system. The process starts from knowledge of local environmental and reservoir conditions. The latter are the basis for defining the production scheme which, in turn, is the main input for establishing the field layout. Conversely, environmental conditions are the main driver for selecting the surface unit concept. 

Fig. 1. Riser system selection process.

However, field layout and surface unit are elements whose definitions affect each other. Moreover, their design attributes affect the riser system, the physical link between them. In summary, selection of these three elements is a highly coupled process. 

MEXICAN CONDITIONS

The following list summarizes Mexican conditions considered in this article: 

• Geographic region: Bay of Campeche, Gulf of Mexico 
• Water depth: 300 m to 700 m
• Existing infrastructure in water depths between 30 m and 50 m, located 50 km to 100 km from future development areas 
• Reservoir depths: 3,250 m to 6,050 m below mean water level 
• Wellhead shut-in pressure: 5,700 psi to 6,600 psi 
• Maximum expected production rates: Five oil fields ranging from 25,000 Blpd to 150,000 Blpd and one gas field with 60 MMscfd, and
• Typical soil: Soft to medium clay.

ENVIRONMENTAL CONDITIONS

Table 1 presents average environmental conditions offshore the deepwater Bay of Campeche, and equivalent data for the US Gulf of Mexico (GoM) and West Africa. 

A comparison between 100-year design waves in Mexico and the US GoM shows that they are similar, which can be explained by the fact that the design storm condition is the 100-year hurricane for both regions. Wave conditions of Brazil (Campos basin) are known to be milder than in the Gulf of Mexico. 

West African conditions are even less severe. The wave regime is predominantly an almost constant swell from S to S-SW, which allows use of spread-moored FPSOs. The main concern for West Africa is the high-wave period, which may coincide with heave motion peak periods of the FPSO vessels, which produces resonant effects. 

Surface currents of the Bay of Campeche are, conversely, comparable to those of West Africa, and significantly lower than those of the US GoM. The ones of Brazil (Campos basin) are of the same order as those of the US GoM. Deepwater Bay of Campeche current values are similar to those used for designing jackets in the Bay's shallow water. Considering that riser design is strongly influenced by current profile, riser designers must investigate: 1) presence or absence of loop currents in the deep Bay of Campeche, as occurs in the US GoM; and 2) current data for deepwater based on actual measurements, not just extrapolation of shallow water data. 

Fatigue met-ocean conditions are not available for the deep Bay of Campeche region. However, considering that riser designs are often driven by fatigue, and assuming these conditions are similar to those typical of US GoM, based on wave condition similarity between those two regions, it is worth comparing fatigue conditions in Campos basin and US GoM. The omnidirectional wave scatter diagram for these two regions represents centers of gravity at the range 1.4<H_s <1.7, revealing similar fatigue conditions. West Africa's conditions are, again, milder than the other regions. 

SURFACE UNIT TYPES

Table 2 summarizes surface unit types employed in the three deepwater reference regions. Concerning riser interface with the surface unit, two main aspects should be considered: motions and hang-off, as detailed here. 

  Motions. In general, the surface units of Table 2 could be classified in the following ascending order, according to their tendency of inducing vertical motion (the most critical) on the risers: Compliant tower (CT), Tension leg platform (TLP), Spar, Semisubmersible and FPSO. This explains the preference for the CT, TLP and Spar in the US GoM environment. Semisubmersible units have only recently been considered, for ultra-deepwater projects, with appropriate designs to reduce motions imposed on the riser. 

In Brazil, the FPSO solution is quite common, with a large majority being turret-based (weather-vane) systems. The few exceptions are compliant, spread-moored systems. FPSOs currently deployed in West Africa are, in turn, spread-moored systems, due to the favorable weather conditions, as noted above. 

Turrets are traditionally located at the FPSO bow, to provide a natural tendency for these vessels to align with environmental forces. However, this imposes a more critical condition to the riser in terms of induced motions. Alternatively, turrets can be positioned in more central positions. In this case, artificial forces (provided by transverse thrusters, for instance) may have to be used to align the vessel with the environmental forces. 

  Hang-off. Different surface unit types impose different restrictions for riser hang-off. Fig. 2 shows a comparison between the typical available space for hanging-off risers on an FPSO turret and on a semisubmersible, indicating the latter permits a larger number of risers. 

Fig. 2. Typical riser hang-off available space at FPSO turret (a), and semisubmersible (b).

Safety and installation are also important aspects of the hang-off system. In a semisubmersible, risers can be supported at pontoon level. This is beneficial in terms of safety, as they are more protected from collision by approaching vessels. This is also beneficial for platform stability. However, it complicates riser installation. One alternative is to use I-tubes and bring hang-off location to deck level, as suggested in Fig. 3. In this case, protective devices, e.g., reinforced net outside I-tubes and fixed at platform legs, are recommended. 

Fig. 3. Use of riser I-tubes in a semisubmersible.

PRODUCTION SCHEME AND SUBSEA LAYOUT

Several factors are considered when defining the layout of a subsea production system. They can be reservoir requirements or constraints, such as the wellhead mesh and need for water and/or gas injection, or flow assurance aspects, e.g., gas lift, requirement for pigging or thermal insulation as part of the mitigation/ remediation strategy for flowline deposits or blockage. Besides seabed profile/ soil conditions, regulatory requirements can also play a role in layout definition. 

All these factors are boundary conditions for the layout design, which basically comprise: definition of the number of subsea manifolds, if any; position of the surface unit; and flowline route. The riser system and configuration are affected by the subsea layout. Some important characteristics of the layout that affect – and are affected by – riser system definition are: the number of lines, proximity of riser base – or touchdown point (TDP) in case of catenary risers – to subsea equipment (manifolds and trees), and use of insulation. 

RISER SYSTEMS CLASSIFICATION

The riser systems usually considered for deepwater subsea production systems can be classified and described, as follows: 

• Catenary Risers: 1) Flexible; 2) Steel catenary riser (SCR) 
• Tower risers: 1) Bundle; 2) Single
• Tension leg riser (TLR).

Flexible risers. Flexible pipes of multi-layered unbonded construction have been used in subsea and floating production systems. The various layers that comprise the pipe wall can be either metallic wires or thermoplastic extruded material, resulting in pipes with a significantly lower bending stiffness than those of conventional steel. This is an advantage for installation and, in some cases, for system operation, as well. 

The supply of flexible pipes is much more limited than that for rigid steel. In addition, due to the more complex nature of flexible pipe, vendors adopt different design/ manufacturing solutions for their products, e.g., materials, metallic wire profiles and manufacturing techniques. Although API Spec 17J and API RP 17B provide good guidelines, it is strongly recommended that the riser system designer maintain active communication with manufacturers whenever the flexible pipe alternative is considered. 

For deepwater applications, external pressure resistance and buckling of tensile armor wires are examples of topics that need special attention. Another special issue arises when sour service is envisaged. Gas permeation modeling through an internal pressure sheath has to be performed to enable correct steel wire material selection in such cases. 

One advantage of the flexible riser alternative is the ease with which it accommodates surface unit and layout constraints. For situations of critical motions induced by the surface unit, it is also relatively simple to adopt alternative configurations by use of concentrated or distributed buoyancy. 

Steel catenary risers. SCR is an elegant solution for riser systems, since it is a simple extension of the static portion of the steel pipeline going from seabed to surface in a catenary configuration. In several cases, this has been found to be the optimal economic solution. The typical design steps and deliverables for an SCR system are illustrated in Fig. 4. 

Fig. 4. Design steps and deliverables for an SCR.

In the feasibility review, the designer should demonstrate concept feasibility by performing a relatively small amount of analyses. If this step is not done in an appropriate manner, it can compromise subsequent development phases, when changes are usually much more difficult to accommodate. 

The detailed design involves:

• Wall thickness sizing, for internal and external pressure, buckle propagation, on-bottom stability and corrosion allowance 
• Extreme loads analysis
• Fatigue analyses, for 1st /2nd order motions, Vortex induced vibration (VIV) 
• Pipe/soil interaction
• Engineering critical assessment (ECA) of welded joints 
• Installation analysis
• Cathodic protection, and
• Materials/components design.

In relation to the flexible riser option, SCRs tend to be more advantageous for larger diameters. However, they are less tolerant to induced motions and may not be suitable for water depths less than about 500 m. The shallowest SCR installation to date is on the Prince field's Mini TLP in the GoM in 455-m water. 

Bundled riser towers. The bundled riser tower is the concept shown in Fig. 5 (a). It comprises: 1) a foundation structure (usually a suction pile); 2) a structural link element between foundation and tower (usually a flexjoint); 3) connections between flowlines and riser lines; 4) the tower body, which contains a structural pipe, fluid conveying lines and thermal insulation/ flotation modules; 5) a top tank to provide additional upthrust; and 6) a flexible dynamic jumper to connect tower and surface unit. 

Fig. 5. Riser towers: in bundle (a) and single line (b).

The feasibility of this concept has been demonstrated by its application in Girassol field (offshore Angola). Some of its advantages are: 

• Less critical dynamic/fatigue loading
• Lower load transferred to floating unit
• Does not require heavy-lift installation vessels (usually towed) 
• Potential for accelerated schedule to first oil, and 
• Potential for thermal insulation improvement. 

Main disadvantages of the concept are: 

• Higher number of components, which increases complexity and risk 
• Onshore works may have to be commenced earlier, and 
• CAPEX tends to be higher.

The bundled tower concept permits uncoupling pipeline/ riser construction and installation activities/ schedule from those for the surface unit. Hence, a riser tower contract would not be affected by some delay in the surface unit schedule, which would not be the case of a riser system that requires the floating unit in position for its installation. Some major aspects of tower design are: 

• Buoyancy is sized to be slightly higher than self-weight. 
• Fatigue damage is high in transportation (or towing phase). 
• Upper tank is sized to guarantee minimum positive tension in all conditions of operational life. 
• Dynamics are not severe in operational phase. 
• Dynamic and interference issues are important for top jumpers. 

And it demands strong effort of detailed engineering for ancillary components such as foundation, bottom joint, bottom connection elements, floaters, top connection elements and ballasting system. 

Single riser towers. These essentially differ from bundled riser towers in their main body, which has a single line instead of a bundle, Fig. 5 (b). It has been argued that this increases system overall reliability, since “all eggs are not put in the same basket.” The installation of such risers from pipelay vessels has been considered. 

Tension leg riser. Similar to the riser tower, the basic TLR concept is to uncouple potential severe dynamics at the riser upper portion, induced by surface unit motions, from those of the riser base or TDP. In the case of the TLR, this is obtained by use of a pre-installed tension-leg, subsurface buoy, as shown in Fig. 6. SCRs are installed hanging from the subsurface buoy and, from there, they are connected to the surface unit by means of flexible jumpers. Although it has not yet been implemented, this concept has potential for economical savings for some scenarios. 

Fig. 6. Schematics of the Tension Leg Riser.

The TLR concept, like the riser tower, also benefits from the uncoupling nature it has in relation to floating unit construction/ installation. 

CONSTRUCTION AND INSTALLATION ASPECTS

Although usually requiring less expensive installation resources than rigid risers, flexible-riser deepwater installation vessels must be provided with dynamic positioning (DP) systems and high tensioning capacity. The flexible riser installation equipment should preferably be a Vertical lay system (VLS). 

Besides lower daily rates, the flexible pipe installation tends to be faster, not only due to the laying rate, but also to shorter times normally required for subsea connections and topside tie-ins. All these factors usually lead to lower installation cost, compared to the rigid option. 

SCR installations can be performed by S-lay, Reel-lay or J-lay methods. Some recent studies, which included fatigue testing with as-reeled samples, showed that loss of fatigue resistance due to yielding and rectification does not necessarily impair use of Reel-lay for SCRs. However, J-lay is the only method usually considered for installation of risers larger than 16 in. The existing J-lay barges (and their towers) for deepwater installation of large-diameter pipeline systems are large size vessels, as shown in Fig. 7. 

Fig. 7. J-lay system for large pipe in deep water.

Bundle towers are built onshore and towed to the offshore location. In the case of Girassol, a surface tow method was adopted. Since the towing phase can impose critical fatigue damage to the tower structure, a sub-surface tow has been proposed as the preferable method for locations with more severe fatigue conditions. Nevertheless, this is a more challenging and risky operation, since it requires use of ballast, and/or chains, and/or retrievable (and reusable) flotation devices, to control the bundle at mid-depth levels during transportation. 

Single-line riser towers are more suitable for J-lay installations. Thus, construction/ installation of those risers involve reduced onshore work and increased offshore activities in relation to the bundle option. For this reason, development schedule and contractual strategies will vary substantially from one case to another. 

The installation of riser lines up to the sub-surface buoy of a TLR system is similar to a conventional SCR installation. However, installation of the sub-surface buoy itself involves ballast/ deballast operations, which tend to increase operation performance risk. 

OPTIMUM PRODUCTION RISER SYSTEM FOR DEEPWATER MEXICO?

It has been shown that overall conditions for the Mexican deepwater fields considered (including environmental conditions) are within the range covered by fields currently being exploited in other deepwater areas. This means that, in general, there is field-proven technology available. 

The intention of this article is to cover a high-level assessment of potential riser systems for deepwater Mexico. A detailed assessment could not be considered, because only generic data is available. It is important to obtain detailed, site-specific data for riser design. However, in view of the general data presented, and of the considerations given above regarding the riser selection process and overview of referenced scenarios, one can conclude that there are a significant number of riser system candidates available for deepwater Mexico. The final decision should be based on an economic analysis. But two aspects deserve further comments at this stage, as noted here. 

   Use of FPSOs. Conditions in Mexico are not favorable to use of spread-moored FPSOs. However, use of turret-based FPSOs can be envisaged, considering successful application of the Floating, storage and offloading (FSO) in Cantarell field in shallow, Mexico GoM water. This option would probably have to be compared with the option of using the existing shallow-water infrastructure and its potential spare export capacity. Surface-unit concepts to compete with the FPSO could be any of those currently used in deepwater systems in the US, Table 2. 

Riser system. The use of turret-based FPSOs would favor the flexible pipe option in relation to the SCR. Alternatives like towers or TLRs could also be considered, but they would depend on detailed risk assessment and economical attractiveness. Such concepts have low track records and no history at all in the US GoM. Conversely, if other surface unit types are used, the overall scenario approaches the ones of the US GoM, and the SCR tends to be a strong candidate.   

THE AUTHORS
	Otávio Sertã, sr. project engineer, INTEC Engineering, Houston, holds a BS degree in civil engineering (1985) from Catholic University of Rio de Janeiro, and an MS degree in subsea engineering (1999) from Federal University, Rio de Janeiro. He started with Petrobras in 1987 working on offshore structures, pipelines and risers for deep water. He joined INTEC in 2001, where he is involved in US and west Africa deep water. Mr. Serta is a member of the Society for Underwater Technology (SUT).