Extra-Long Tiebacks for Deepwater Development: A Review of Enabling Technologies
It is possible to reduce deepwater development costs by increasing the distance between new assets and existing production hubs or shallow-water areas, or even connecting these assets to shore. However, distances for subsea tiebacks have been limited by conventional development concepts based on chemical injection and double-header manifold structures to loop the subsea architecture with the topside facilities. This paper discusses a research project conducted by Eni to explore the maturity status of new technologies to enable economical development of deepwater prospects with tieback distances longer than 50 km and 150 km, respectively, for oil and gas fields. The paper, a continuation of paper OTC 28839, describes how these new technologies compare to more conventional development schemes and how the company is preparing for potential implementation.
Introduction and Background
Conventional subsea oilfield project development has used dual production flowlines and injection of various chemicals to produce wells and guarantee the management of critical operations such as shutdowns and subsequent restarts. During hot restart scenarios, the double-loop architecture also allows for hot-oil circulation by using topsides hot-oil pumps to limit the chemical injection requirement and duration of restart operations.
The conventional development concept poses several sometimes-insurmountable limitations to tiebacks of 50–100 km and longer, and may compromise the feasibility of the development. These limitations may include the quantities of chemicals to be injected, the amount of inert fluid to be circulated through the loop, and the relevant pump capacity, as well as the economic effort to procure and install a dual-production flowline of extreme length.
Eni has undertaken a research project to explore the maturity status of new technologies that can enable or facilitate the economic development of deepwater prospects with extra-long tieback distances. The technologies that have been identified and studied include the following.
Active flowline thermal management
Subsea multiphase boosting
Subsea electrical distribution
Direct-current fiber optics
Active Flowline Thermal Management
Active flowline thermal management appears to be the most suitable solution for long-tieback development projects, as subsea oil effluents transportation over distances longer than 50 km poses serious challenges to flow assurance from potential hydrates and wax formation. The development field schematic concept that has been studied and developed for a long tieback to an existing FPSO is based on a single heated production flowline that connects the field with the topside treatment and storage facility. The development scenario includes a water injection line that is looped with the production line to allow piggability.
Subsea pipeline heating results as an efficient method to avoid the cost and environmental risk of continuous chemical injection and to substantially simplify the subsea architecture.
Historically, active heating technologies have used hot-water circulation and direct electric-heated pipeline systems. These technologies suffer from relatively poor thermal and heating efficiency. Additionally, their ability to overcome the growing challenges of deeper water, longer tiebacks, and more challenging flow assurance issues poses limitations.
Electrically Trace Heated Pipe-in-Pipe
Considering that in normal production, passive insulation should be enough to keep the fluid warm, the operating philosophy for activating the thermal management system will foresee the following operating cases.
Continuous heating mode: heating during very low turndown production to maintain the fluid temperature above hydrate-formation temperature (HFT) and/or wax-appearance temperature
Keep-warm mode: heating during shutdown of production and subsequent restart operation to maintain the fluid temperature above HFT
Warm-up mode: after a long shutdown for which the line has been depressurized and live oil has been allowed to cool down to ambient temperature or after live oil displacement with injected water (via the water-injection line looped with production line)
Electrically trace heated pipe-in-pipe appears to be an attractive solution for long oil tiebacks to existing infrastructure because of the high passive insulation provided by pipe-in-pipe technology, high electrical efficiency, and low power requirement. Electrically trace heated pipe-in-pipe consists of a standard pipe-in-pipe system with the addition of a traced heating system, made with three-phase thermal cables passing between the internal flowline wall and the thermal insulation.
Fiber-optic cable can also be included for monitoring the temperature throughout the length of the pipeline.
The working principle relies on the Joule resistive effect, which brings active heating through the three-phase electrical tracing cables. The traced heating system is based on a three-phase star configuration: three separate cable cores, each of which conducts one phase, and each phase meets at the star connection remote end, where the sum of current phases is nil. It means that there is no need of a return current cable.
The complete paper devotes significant text to a discussion of the pipe-in-pipe system, including explaining the main differences between the technologies that the main suppliers are developing and implementing on their respective systems.
Subsea Multiphase Boosting System and Electrical Distribution
For a deepwater, long tieback project, the need for a subsea multiphase boosting system (SMBS) is envisaged.The SMBS is a field-proven technology with several applications worldwide, so cannot be considered as a new technology.However, its application on such distances and in synergy with the electrically trace heated pipe-in-pipe is something new and has its specific issues related to the transport of power over long distances and to its distribution to multiple users that have to be considered.
Compared to a conventional topside distribution, a subsea power distribution system will minimize the impact on topside plant in terms of space for new equipment and number of connections, and will also minimize the number of cables that have to be installed subsea. This makes it a viable solution for brownfield tiebacks. On the other hand, it increases the amount of equipment to be installed subsea, whose reliability becomes a sensitive and critical factor.
Direct-Current Fiber Optics
The direct-current (DC) fiber optics is a new submarine communication and power transmission system that is composed of cable and fiber optics, topside power and control equipment, and subsea distribution components for connection to the subsea production system (SPS).
The submarine cable is a standalone cable that brings DC power and fiber optics for communication. The cable has the same cross section regardless of the distance, and the power is provided through fixed DC current at medium voltage.
The power and communication are distributed subsea through splices and subsea nodes. The splice is a fully passive component designed to allow subsea branching from the main trunk submarine cable toward a subsea node.
The subsea node is the battery limit of the system and the subsea interface with the SPS control equipment.
This solution for low-voltage (LV) power and communication may be a valid alternative to the conventional system in specific cases, in particular for long stepouts, considering the following.
It removes the need for LV cables and fiber optics inside the main umbilical, and with long distances, the saving in terms of umbilical size and weight can be significant and facilitate the construction and installation phase.
The standalone cable is not redundant, but it is easily recoverable and repairable in case of damage.
Expandability: even once the system is installed, the end termination of the cable can be recovered, and the main trunkline can be extended up to new nodes. It is an interesting aspect of this technology, in particular, when we may have several development phases or nearby fields that can be developed later and connected.
During the feasibility phases of long tieback projects, the implementation of these technologies can already be assessed and evaluated.
The fine-tuning of these technologies to the specific technical needs of the project may require additional qualification activities that must be taken into account and considered, but that appears manageable in relatively short terms.
A combination of heated, high-thermal performance flowlines, subsea multiphase boosting, subsea power management, innovative preservation procedures, and newest subsea production components, together with a reliable, integrated control system and digital technologies are the key enablers of a very long tieback solution that may work to bring production from a whole area back to a production hub.
Considering such distances, a subsea multiphase boosting system installed close to the production wells is most likely necessary.
The electrical distribution is a key matter since it must provide the power for heating the flowline and supplying the pumps over long distances and must also comply with existing topsides constraints. These may be related to available power or maximum number of cables that can pass through the existing swivel when the FPSO is turret moored.
An alternative solution to conventional communication and power systems is the DC fiber-optics system, which is based on a standalone cable that consists of a combination of DC medium-voltage power and fiber optics over long distances, and replaces the need for LV electrical cables and fiber optics within the umbilicals.
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