GEORG Geothermal Workshop 2016 SESSION B3, Midstream. Presentation title: Principles of limit state design for geothermal wells

1. Principles of limit state design for geothermal wells
Magnús Þór Jónsson and Lilja Magnúsdóttir
GEORG Geothermal Workshop 2016 – Reykjavik, Iceland 1

2. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Outline:
- Introduction
- Limit State Design (LSD)
- Failures and Failure Reasoning
- Improvements based on LSD
- Conclusion
2

3. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Introduction
Motivation:
A review of the literature shows that the cost of geothermal
wells is in the range of 20-50% of the total investment cost of a
geothermal power plant.
Therefore, the financial feasibility of the plant is sensitive to the
success rate of the wells.
3

4. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Introduction
Objective:
Geothermal wells are subjected to various
types of loads and deformations arising
from service requirements that may range
from standard to extreme values during
the construction, operation or discharging
process.
The objective is to design a casing and a
wellhead that can withstand such demands
throughout it expected lifetime.
4

5. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Introduction
Traditional Design:
In the traditional allowable stress
design, the design is based on the
criteria that the stresses resulting
from the design loads are lower than
the allowable stresses based on
material strength and safety factors.
As the high temperature wells are
more likely to collapse, a new
methodology is presented to reduce
that risk.
5

6. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Limit State Design
Design Philosophies:
The limit state design is based on
explicit consideration of various
conditions where the structure may
not fulfill its intended function. For
these conditions, the strength is
estimated and used as a limit for
such behavior.
For types of limit states:
Serviceability limit state
Ultimate limit state
Fatigue limit state
Accidental limit state
6

7. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Limit State Design
Limit State of Geothermal Well:
A limit state of a geothermal well is
defined by the characterization of
conditions for which a part or an
entire well fails to perform the
function that is expected of it.
Only two limit states are studied,
ultimate limit state and accidental
limit state.
7

8. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Limit State Design
Failure and Failure Reasoning:
Wells with high temperature and high pressure are more likely to
fail or reach the ultimate limit state.
Failure modes:
Casing or coupling rupture Collapse or buckling
during installation or quenching during discharging
8

9. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Limit State Design
Failure and Failure Reasoning:
Principles of limit state design:
Consider:
Load conditions
Material selection
Dimensions
9

10. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Limit State Design
Failure and Failure Reasoning:
Load conditions:
Axial and bending loads before and
during cementing.
Axial and bending loads when drilling.
Axial thermal loads during discharging.
Radial loads applied by internal and/or
external pressure.
Radial thermal loads during discharging.
Radial loads applied by thermal
expansion of trapped water.
10

11. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Limit State Design
Failure and Failure Reasoning:
Material selection:
Strength Thermal expansion Bauschinger
Temperature dep. effect
strength reduction
11

12. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Limit State Design
Failure and Failure Reasoning:
Dimensions:
Thickness Integrity
12

13. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Limit State Design
Improvements based on LSD:
Decrease the loads. (Construction, Operation and Quenching)
Select material that has optimum performance.
Increase the wall thickness of production casing.
Reduce integrity of the production casing, use welding
connections instead of couplings.
13

14. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Conclustion
Summary:
For the ultimate limit state design, the temperature
dependent yield conditions are considered for ultimate
stresses and strains including the Bauschinger effect
for cyclic loading.
At high temperatures, the stiffness and the strength
of steel are reduced. That is, however, usually not the
only reason of a rupture failure which occurs by a
combination of expansion, material degradation and
geometrical discontinuity.
14

15. GEORG Geothermal Workshop 2016 – Reykjavik, Iceland
Conclustion
Thank you!
Questions?
15