Design-to-Cost Approach Increases Project Value of Troll Phase 3
Authors: Carpenter Chris 
Published: August 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper OTC 30954, “Troll Phase 3: The Next Step for a Groundbreaking Giant,” by Bjørn Laastad, Knut Ellevog, and Roger Oen Jensen, Equinor, et al. The paper has not been peer reviewed. Copyright 2021 Offshore Technology Conference. Reproduced by permission.

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An important driver for maximizing value creation for the Troll Phase 3 gas project offshore Norway was to identify ways to reduce the pressure drop in the value chain from the reservoir to the onshore terminal. Using a design-to-cost approach in the concept-selection phase has affected design of the wells, subsea production system, pipeline, and a new inlet separator on the platform. The final design has enabled a significant increase of project value by accelerated gas deliveries, reduction of energy consumption, and lowered CO2 emissions.

Troll Phase 3 Business Case and Development Concept
The Troll Phase 3 business case is to develop the vast gas reserves in Troll West, thus recovering approximately 340 billion std m3 gas over a 40‑year period. Production from Troll West will be commingled with Troll East gas production at Troll A. The Troll West volumes will supply approximately 40% of the total Troll A delivery and enable a 5‑year continuation of the current high plateau production level of 36 billion std m3/yr and an extension of field lifetime by 15 years until 2062. A prerequisite for the Phase 3 project is that the annual summer/winter swing production will be from the Troll A wells. Consequently, Troll West production will be stable and not affected by seasonal variations in market demand.
Following a comprehensive concept‑selection analysis made from the perspective of the field’s life cycle, it was decided in May 2017 to develop Troll Phase 3 as a subsea tieback to the Troll A platform. The subsea design facilitates possible future Troll West gas building blocks.
The first section of the complete paper is devoted to a discussion of the flow‑assurance aspects of the development; this component is not included in this synopsis.

Troll Phase 3 Well Design
During the feasibility and concept‑selection phases, an important opportunity to maximize value creation was found to be improving individual well performance by enlarging the tubing size. The standard 7‑in. tubing would have been a considerable production restriction. By enabling a larger tubing size, the production from each well could be increased and the required number of wells could be reduced. The big-bore completion design was new to the operator but had been implemented successfully in other high‑rate gas fields in the industry. Using a design‑to‑value process, the tubing size for Troll Phase 3 was increased to 9⅝ in.
Comparison of a big‑bore well to a standard 7‑in. well in 2030 gives a reduced pressure drop of more than 12 bars/well, with an increased production potential for the big‑bore well of greater than 1000 std m3/d. Consequently, the required number of wells could be reduced by approximately 40% (from 14 to eight). Factoring in the total cost of the final gas producers, six saved wells equals a total cost savings of $150 million ($300 million including the subsea facilities).