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Extra-Long Subsea Tiebacks Reduce Deepwater Development Costs

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It now appears possible to reduce deepwater development costs by increasing the distance between new assets and existing production hubs and shallow-water areas, or even connecting those assets to shore. The complete paper presents and discusses the authors’ technology-development program regarding very-long oil-tieback architectures (50–100 km) and enabling technologies. The paper describes how the new technological solutions compare with more-conventional development schemes in concept-selection phases and discusses how the operator is preparing for potential implementation. The authors believe that the capacity to invest in and develop technologies required for long and very-long tiebacks will decide the future of the deepwater subsea industry.

Introduction

Over the past few years, Eni and its partners installed more than 100 subsea trees, mainly on deepwater fields in various areas of the globe, in particular Angola, Norway, Indonesia, Ghana, Egypt, and Libya. The growing trend in subsea trees is guiding the operator’s needs and ideas about how to develop fields progressively around its production hubs.

In the evolution of deepwater oil fields, production floaters have been located close above the first fields to be developed. To guarantee the progressive infilling of the production hub, a first series of tiebacks was developed for a range of up to 15 km from the hub. The second series of tiebacks, ranging from 25–30 km from the hubs, is under development. Architectures and technologies to develop these tiebacks have been extensively studied, matured, and optimized, and some of those tiebacks are already in production.

New infilling fields are currently being explored and discovered at distances 30 to 100 km from production hubs, with new extra-long tieback architectures now under study. Those tiebacks will necessitate new technologies to enable managing the growing technical and economic challenges of subsea field developments. Long and very-long tiebacks that allow expanding from a brownfield perspective are likely to enable today’s greenfield programs.  

The paper reviews architectures and operation features of the operator’s currently installed short- and medium-length tiebacks to introduce the drivers and features of very-long tiebacks. The paper addresses sizing, operation targets, flow assurance, technologies, and related maturity.

Short- and Medium-Length Tieback Review

The operator’s initial deepwater fields have been developed with the production hub typically a few kilometers from the first main reservoirs in the direction of shallower water depth, allowing minimum distances dictated by drilling and offloading requirements. The main field is connected to the hub through a conventional wet insulated loop, which collects production from all the field’s drilling centers. Flowline diameters have usually been in the range of 10–12 in.

Second-line fields ranging up to approximately 25–30 km from the main hub have been developed for both deeper and shallower waters through a ­service-production loop interconnected with the main field loop. The paper discusses the production loops, the use of pipe-in-pipe technology, and subsea multiphase boosting stations located close to the production hubs that can be adapted on the loops’ interconnection for distant fields’ enhanced recovery. Gas-lift injection is also discussed.

According to the authors, if a production facility (including topside and subsea systems) is designed appropriately from the beginning as a hub to produce multiple fields with a progressive deep and shallow infilling strategy, available, proven, conventional subsea and flowline technologies have fully demonstrated their suitability to technically and economically develop five fields, or even more, within a 25–30-km radius of the main hub.

Extra-Long Tiebacks

It appears that the enabling technology for extra-long (30–100 km) tiebacks resides in combining heated pipe-in-pipe solutions with subsea multiphase boosting, staggered subsea preservation systems, and advanced subsea production systems. The production flowline will be based on continuous, high-performance, heated pipe in pipe with, typically, a 10–14-in. internal diameter, collecting all fields’ drilling centers in a daisy-chain configuration.

The paper continues the architecture discussion with descriptions of the drilling center and branches, main production flowline, subsea production system, subsea multiphase boosting system, flowline heating system, and powering and communication systems. Water-injection flowlines will be replaced progressively by subsea water treatment and injection systems.

Digitalization will be key for both asset integrity and operations management, with resident autonomous underwater vehicles being deployed for inspection and monitoring, digital data recovery, and nonfrequent interventions, allowing reduction in the extent of offshore-vessel use and the number of remote operable valves installed subsea.

Providing heat locally where needed is the more-efficient and absolute way of resolving nearly any thermal flow-assurance issues, beginning with managing wax and reducing hydrates. As such, a wide range of functionalities can be expected from heated flowlines, even up to enabling production restart after a long period of complete shutdown where no system, including flowline heating, is available. The operator is considering heated, high-­performance pipe-in-pipe solutions with low U‑­values and optimized diameters for efficient long-tieback development. From the extensive design work performed on case studies and tieback configurations, it appears that high—and possibly boosted—flows above 50,000 B/D are workable even without heating over distances in the range of 40–50 km. For higher tieback lengths or lower flow rate, full-pipeline heating systems will remain the only technical solutions for development.

An expandable subsea production system is being considered to cover all generations of tiebacks for both design and supply. The system will be aimed at meeting integrated drilling-center requirements, expanding compact subsea tree technology to other components, and sharing control systems.

Powering and communication will evolve to use high-potential telecommunication technology that allows multiple users (more than 80 subsea control modules per cable) as well as the progressive conversion to all-electric actuation of subsea valves and trees. Subsea control modules should be developed to allow for additional functionalities such as controlling machinery and more-complex production systems, and safety-critical functionalities.

Flowline and Installation

Major installation contractors are working already on heated, large-size, high-performance pipe-in-pipe technologies and verifying that their fleets of vessels are able to install these lines cost-effectively. Different technologies’ maturity and qualification status can be perceived across the market. Considering the significant lengths of this new generation of tiebacks, some vessels may be adapted to lay cables and/or umbilicals simultaneously to the main production line, reducing installation time and associated costs.

Conclusions

  • Recent technology development, qualification, and testing, and the experience gained on the most-recent deepwater projects enable the possibility of considering seriously a full-subsea-development approach to long-distance tiebacks from existing hubs or from shore.
  • For intermediate-case studies, where both conventional and extra-long architectures are possible, the extra-long architectures appear to be cost-effective, proving to some extent their sustainability as a self-standing offshore scope.
  • Connecting to existing amortized production hubs, projected development economics of midsize oil assets in a range up to 80–100 km from existing or planned hubs are encouraging.
  • It is still unclear which contractor category will eventually propose these systems. Few subsea-production-system or installation contractors are proposing their services as a supplier, or at least as an integrator, of these extra-long-subsea-tieback systems, forcing operators to tender components separately.
This article, written by JPT Technology Editor Judy Feder, contains highlights of paper OTC 28839, “Deepwater Innovations: Extra-Long Oil-Tieback Technologies,” by Michael Gassert, Gianluca De Molli, Vito Calabrese, Eniprogetti, and Stefano Magi, Gianfederico Citi, and Fabrizio Rollo, Eni, prepared for the 2018 Offshore Technology Conference, Houston, 30 April–3 May. The paper has not been peer reviewed. Copyright 2018 Offshore Technology Conference. Reproduced by permission.

Extra-Long Subsea Tiebacks Reduce Deepwater Development Costs

01 September 2019

Volume: 71 | Issue: 9

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