Low Nickel Duplex Steel for Flexible Pipe Armouring
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Solution Overview
Problem
Unbonded flexible pipes for offshore transportation face challenges in balancing corrosion resistance, mechanical strength, and cost, particularly when transporting aggressive fluids in deep water environments, where existing high-corrosion-resistant steel alloys are expensive and may be sensitive to hydrogen-induced cracking.
Innovation Solution
A low Ni duplex steel alloy with specific composition (0.01-0.05% C, 0.15-0.25% N, 20.0-23.0% Cr, up to 3.0% Ni, 3-6% Mn, up to 1.0% Mo, and incidental impurities with balance being Fe) is used for the armouring layers, providing sufficient mechanical strength and corrosion resistance without the high cost of expensive alloy components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high corrosion resistance steel alloys (e.g., Duplex 2205) are used for armouring layers, then corrosion resistance is improved, but cost increases significantly due to high nickel content
Solution Approach 1:
The patent changes the chemical composition parameters of the steel alloy by reducing nickel content from 4.9% to 3.0% and adjusting other alloying elements (chromium 20.0-23.0%, molybdenum 0.05-1.0%, manganese 2.0-6.0%, nitrogen 0.10-0.25%) to achieve the desired corrosion resistance at lower cost
Solution Approach 2:
The patent creates a composite alloy system combining multiple elements (Cr, Ni, Mo, Mn, N, Fe) in specific proportions to achieve synergistic effects where the combination provides superior corrosion resistance compared to individual elements, particularly for resistance to sulfide stress cracking and pitting corrosion
2Strength
If high strength steel compositions are used for armouring layers, then mechanical strength is improved, but sensitivity to hydrogen-induced cracking increases
Solution Approach 1:
The patent optimizes compositional parameters including limiting carbon to 0.01-0.05% to reduce carbide precipitation, controlling nickel to 3.0% maximum, and adding nitrogen (0.10-0.25%) to enhance strength while improving resistance to hydrogen embrittlement through solid solution strengthening
Solution Approach 2:
The patent replaces expensive, high-nickel alloys that are prone to hydrogen cracking with a more economical formulation that achieves comparable or superior resistance to hydrogen-induced cracking through optimized alloying, effectively substituting a problematic material with a superior alternative
3Reliability
If expensive alloy components (nickel, molybdenum) are increased to improve corrosion resistance, then resistance to aggressive fluids is improved, but material cost increases
Solution Approach 1:
The patent optimizes the concentration parameters of alloying elements by reducing nickel to 3.0% maximum and molybdenum to 0.05-1.0%, while compensating with increased chromium (20.0-23.0%) and optimized manganese (2.0-6.0%) to maintain corrosion resistance at lower material cost
Solution Approach 2:
The patent applies different alloying strategies for different functional requirements: chromium and nitrogen provide general corrosion resistance and strength, while controlled amounts of nickel and molybdenum provide targeted resistance to specific corrosive environments, optimizing the overall composition for cost-effective multi-functional performance
Data Source
AI summary
The invention relates to a use of a duplex steel for the production of an armouring layer of a flexible pipe comprising at least one unbonded armouring layer and at least one unbonded sealing layer wherein the duplex steel comprises in weight percent0.01-0.05% by weight of carbon (C)at least 0.15% by weight of nitrogen (N)20.0-23.0% by weight of chromium (Cr)up to 3.0% by weight of nickel (Ni)up to 1.0% by weight of molybdenum (Mo)3-6% by weight of manganese (Mn)up to 1.0% by weight of copper (Cu)incidental impurities andbalance being iron (Fe).