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Strengths and Weaknesses Guide Choice of Brazilian Subsea-Development Options

Topics: Subsea systems

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This paper identifies and compares four subsea-development concepts for typical Brazilian presalt deepwater applications on the basis of generic system functional requirements; the strength, weakness, opportunity, and threat analysis method (SWOT); and comparative cost/benefit analysis. The paper provides a fast-track approach to perform screening assessment of multiple subsea concepts.

Introduction

Subsea development concepts for presalt regions such as those offshore Brazil have followed a replication methodology primarily based on a satellite configuration with flexible risers and flowlines. While this concept has several strengths, the associated scope of the operation incurs considerable cost. This concept, in addition to three others, is outlined in this paper. The concepts discussed include the following:

  • Satellite configuration with flexible risers and flowlines
  • Daisy-chain configuration with rigid risers and flowlines
  • Cluster configuration with manifolds, rigid risers, flowlines, and flexible jumpers
  • Hybrid solutions with rigid risers and flexible flowlines (with and without manifolds)

The objective of this paper is not to select the best option; rather, it is to discuss these options on the basis of application, objectives, premises, and field characteristics. Common characteristics of the Brazilian presalt fields are provided in the complete paper, and its Fig. 13 summarizes the SWOT analysis for all four concepts.

Satellite Configuration

In this configuration, each well is connected directly to the host by two dedicated flowlines/risers: The production well has one production and one service flowline/riser, while the injection well has one gas-injection and one water-injection flowline/riser. Each well has an umbilical connected directly to the host. Thus, each tree has three connection modules located on the tubinghead spool (THS), one for each flowline and one for the umbilical. A pigging loop inside the THS allows for fluid displacement, double-sided depressurization, and round-trip pigging of the flowlines. The subsea-controls system is multiplexed, with a subsea-control module on each tree.

Strengths.

  • Flexibility to position the wellhead on the seabed
  • Independent well control from topsides
  • Standardization of subsea systems
  • Installation of flexible flowlines and risers can be faster and cheaper than the installation of rigid pipes

Weaknesses.

  • High number of flowlines and risers (two per well)
  • Use of flexible lines is at the edge of technology for these materials for presalt conditions, so qualification is required
  • Limited inner diameter (ID)

Opportunities.

  • Reduce the number of risers
  • Use free-hang catenary risers instead of lazy-wave risers
  • Incorporate subsurface data to define well location in the reservoir as late as possible

Threats.

  • Because the subsea system is not modular, interventions tend to increase operating costs
  • A lack of multiple vendors capable of delivering the necessary scope of the operation

Daisy-Chain Configuration

In this architecture, the wells are spaced out on the seabed and a trunk line runs across the field, gathering the production (or providing the injection) on each well. Assuming that the production system requires a closed circuit for pigging and fluid displacement, the production loop starts with one riser from the floating production, storage, and offloading (FPSO) vessel and is laid on the seabed, reaching each well along the way, and then returns to the FPSO by another riser. The number of wells on each loop will depend on the flow rate per well and sizing of the loop pipe ID. The wells are connected to the loop by one short jumper, which connects the tree to an in-line tee (ILT) on the loop pipe. The injection system may or may not have a closed circuit. The subsea-controls system is multiplexed. For individual well-production flow-rate measurement, a multiphase flowmeter can be installed in the tree.

Strengths.

  • Simpler riser-balcony design
  • Subsea umbilicals, risers, and flowlines (SURF) capital costs less affected by riser and flowline materials
  • Rigid lines have higher reliability than flexible lines
  • Ability to have vertical wells leads to simplification and lower cost
  • Two separate injection loops, one for water and another for gas

Weaknesses.

  • Less flexibility to change well locations if new subsurface data arrive
  • Wells on the same loop are less independent
  • Installation of rigid risers and flowlines, especially with larger IDs, tends to be more expensive than that for flexibles

Opportunities.

  • Extra ILTs can be installed in strategic points of the loop
  • Deviated wells can provide more flexibility for changing well-location targets in the reservoir
  • Injection loop can be changed to an open circuit with parallel water- and gas-injection flowlines
  • Subsea installation can be independent of FPSO arrival

Threats.

  • Flexible joints for rigid risers may require specific design and qualification
  • More subsea hardware and connection points
  • Rigid pipes must have internal corrosion-resistant-alloy (CRA) coating
  • Extra gas-lift lines may need to be installed in the future

Cluster Configuration

The architecture with manifolds (or templates) is the one typically used in deepwater basins around the world. The main characteristic of this concept is to cluster wells and equipment in a few locations of the field area on the seabed and to commingle the flow from multiple wells through the manifold, which can provide good operability.

For the specifications defined in this study, each production manifold has two lines connecting to the FPSO. A pigging loop at the end of the manifold provides round-trip capability for these two lines. The injection system also has manifolds with two risers to the FPSO, one for water injection and the other for gas injection. The connection from the tree to the manifold is made by a single short jumper. Because the clusters where the manifolds and wells are located comprise a small footprint of the entire reservoir area, the wells need to be deviated to reach the reservoir target locations. The subsea-controls system is multiplexed. A flowmeter can be installed in the tree.

Strengths.

  • Simpler riser-balcony design
  • SURF capital costs less affected by riser and flowline materials
  • Reduced footprint on the seabed
  • Rigid lines have higher reliability than flexible lines, especially in corrosive environments
  • Manifolds provide good operability

Weaknesses.

  • Wells must be deviated to reach reservoir target locations, increasing well complexity and expense
  • Manifolds usually are bulky, complex, and costly, requiring specialized construction and installation resources
  • Once the scope is frozen at contract award, changing well locations if new subsurface data arrive is more difficult
  • Installation of rigid risers and flowlines, especially with larger IDs, tends to be more expensive than that for flexibles

Opportunities.

  • Some slots may be left available on the manifolds for future infill wells
  • Simplify manifold design to make it lighter, smaller, and more reliable
  • Subsea installation can be independent of FPSO arrival

Threats.

  • Construction lead time and cost may be high
  • Flexible joints for rigid risers may require specific design and qualification
  • Rigid pipes must have internal CRA coating
  • Extra gas-lift lines may need to be installed in the future

Hybrid Solutions

Some elements of the concepts mentioned previously in this study can be combined in many forms, presenting opportunities to optimize the development concept. An alternative arrangement could feature two production loops in daisy-chain configuration and two water-alternating-gas injection manifolds with long flexible jumpers connecting to the injection wells. A single well could also be in satellite configuration; individual wells could be tied in directly to the FPSO as needed.

The production system is the same as the one described in the daisy-chain configuration. The injection system is similar to the cluster arrangement, with manifold configuration, but the jumper connecting the manifold to the tree is more like a flexible flowline that reaches out to the vertical well location. The subsea-controls system is multiplexed. A multiphase flowmeter can be installed in the tree.

Strengths.

  • Simpler riser-balcony design
  • SURF capital costs less affected by riser and flowline materials
  • Rigid lines have higher reliability than flexible lines, especially in corrosive environments
  • Manifolds provide good operability
  • Possibility of vertical wells allows for simplification and lower cost when compared with deviated
  • wells

Weaknesses.

  • System is less standardized, with many different types of equipment
  • Manifolds usually are bulky, complex, and costly, requiring specialized construction and installation resources
  • Once the scope is frozen at contract award, changing well locations if new subsurface data arrive is more difficult
  • Wells on the same production loop are less independent
  • Installation of rigid risers and flowlines, especially with larger IDs, tends to be more expensive than that for flexibles

Opportunities.

  • Extra ILTs can be installed in strategic points of the loop
  • Deviated wells can provide more flexibility for changing well-location targets in the reservoir
  • Some slots may be left available on the manifolds for future infill wells
  • Subsea installation can be independent of FPSO arrival

Threats.

  • Alternating from water to gas (and vice versa) on the single flowlines from manifold to wells is more exposed
  • Flexible joints for rigid risers may require specific design and qualification
  • More subsea hardware and connection points
  • Rigid pipes must have internal CRA coating
  • Extra gas-lift lines may need to be installed in the future.
This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 28051, “Subsea Concept Alternatives for Brazilian Presalt Fields,” by Kevin Buckley and Ricardo Uehara, Shell, prepared for the 2017 Offshore Technology Conference Brasil, Rio de Janeiro, 24–26 October. The paper has not been peer reviewed. Copyright 2017 Offshore Technology Conference. Reproduced by permission.

Strengths and Weaknesses Guide Choice of Brazilian Subsea-Development Options

01 August 2018

Volume: 70 | Issue: 8

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