Drilling risers are regularly deployed in depths beyond 1500m with large sections covered in buoyancy. The smooth cylindrical shape of buoyancy modules can result in significant Vortex Induced Vibration (VIV) response, causing overall amplification of drag experienced by the riser. In turn, operations can be suspended due to drag effects on top and bottom angles, and high current speeds can lead to a halt in operations - even complete disconnection and retrieval of the riser string.

1. Adrian Eassom
Senior Engineer, AMOG Consulting
AOG 2017
LGS Technology for Drilling Risers -
Reducing Costs by Increasing
Operability whilst Improving Safety

2. > What is “LGS”?
• LGS = Longitudinally Grooved Suppression
• LGS is a VIV Suppression and Drag Reduction Technology
• Specifically applicable to Drilling Riser Buoyancy Modules (DRBMs)
> What is VIV?
• VIV = Vortex Induced Vibration
• VIV is a phenomena that causes:
 Increased drag
 Increased fatigue damage rate
 Reduced drilling operability
Introduction

3. Introduction to VIV
Image: Monash University Water Channel Testing Lab. Used for small scale VIV testing.

4. Introduction to VIV

5. VIV Suppression Solutions
> Although there are existing solutions for VIV suppression in
drilling risers, there is still a demand for a VIV suppression
technology that combines the following:
• A solution that effectively suppresses VIV without an increase in static drag.
• A solution that is reliable, increases operability and improves safety during
drilling operations.

6. LGS Technology

7. LGS Development
> Concept
• Inspired by the Saguaro cactus.
> Small Scale Testing
• Geometry Optimisation.
> Large Scale Testing
• High Reynolds Number testing of the optimal LGS design.
> Full Scale Deployment
• Deployed in July 2016 in the Gulf of Mexico.

8. Small Scale Testing
> Testing performed at Monash University
Water Channel
> 35 Geometries
> +100 Tests

9. > VIV and drag behavior for a cylindrical object is dependent on
the Reynolds Number.
What about High Reynolds Number?
DragCoefficient
Reynolds Number
High Re Testing
Low Re Testing

10. What about High Reynolds Number?

11. > NRCC Towing Tank Test Facility (St John’s Canada)
> The optimal LGS design was tested
> Grooves run longitudinally alternating down the buoyancy
module
> Scale= 1:3.5
Large Scale Testing

12. Large Scale Testing

13. Key Results: Amplitude of Vibration
Bare Cylinder
LGS Low Re results
LGS Testing Results
LGS High Re results
Diameter, D, based on the Outer Diameter

14. Key Results: Fixed Drag Coefficient
LGS High Re results
Fairings Results

15. > LGS® Technology has clear hydrodynamic advantages and
desirable characteristics for use in drilling risers.
> These advantages and characteristics translate into the following
benefits:
• Increased drilling operations uptime
• Increased fatigue life of drilling risers
> The following concept evaluations are based on results from
High Reynolds testing, and analysis performed with industry
leading VIV prediction software SHEAR7.
Integrating LGS® technology into Drilling Riser Buoyancy Modules
Drilling Riser Application

16. Concept Evaluation – Riser Stacks

17. Concept Evaluation Results –
Extreme Current Offset Performance
> Extreme current
• Surface speed =
2.3 m/s
> LGS out-performs
both Conventional and
Conv. Buoyancy with
Fairings:
• LGS has lower top angle
• LGS has lower offset
0
200
400
600
800
1000
1200
1400
0 1 2 3
Depth
[m]
Current Speed
[m/s]
0
200
400
600
800
1000
1200
1400
0 5 10 15 20 25 30
ArcLengthDownTheRiser
[m]
Offset
[m]
LGS
Buoyancy
Top 675ft
w/Fairings
Conventional
Buoyancy

18. > For a particular example site,
all loop current profiles that
persist over a year were
simulated
> Took account of:
• Magnitude variations
• Annualised Probability and
persistence
Concept Evaluation Results –
Annualised Operability

19. > Relative increase in operability = 33%
> Increase in operational hours = 366 hours (~15 days)
> Overall, approximately a 25% increase in absolute operability during annual
Eddy currents is expected at the example site. This is based current conditions
on an annualised basis.
LGS® was shown to increase operability uptime in GoM during
Eddy Current Episodes
Concept Evaluation Results –
Annualised Operability
Operability Uptime
Measure
Conventional Circular
DRBMs
Conventional DRBMs with
Fairings
LGS
DRBMs
% of Eddy Current Period
Operable
67% 86% 89%
Annual Hours Increase
(Relative to Conventional DRBMs)
0
322
(13.4 days)
366
(15.3 days)

20. > Damage Rate relative difference:
• Conv. : R8 Ratio
• 50 : 1
> Over a 1 Yr period, conventional
buoyancy riser has been damaged at a
rate 50 times faster than the LGS®riser.
LGS® Technology demonstrates improved fatigue performance over
conventional buoyancy modules
Concept Evaluation Results –
Annualised Fatigue Performance
Fatigue Performance
Measure
Conventional
Round DRBMs
Conventional
DRBMs with
Fairings
LGS
DRBMs
Annual Damage Rate [1/Yr] 8.8E-02 1.3E-03 1.8E-03
Design Life (unfactored years
of continuous service) [Yr] 11.4 769 556
Design Life (with Safety Factor
of 10) [Yr] 1.14 76.9 55.6

21. First deployment:
July 2016 Gulf of Mexico
Deployment

22. Expertise. Innovation. Value.