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Hydrodynamic Loads and their Impact
on Foundation Design
Harris King – Senior Structural Engineer
Subsea Expo 2017
• Introduction
• The Problem
• Foundation Design
• Current Methodologies
• Refinement
• Outcome
Agenda
2
• Inspiration came from experiences with a recent
project
• SSIV structure within 500m zone
Introduction – Past Experience
3
• Challenging geotechnical conditions
– Close to 3 different soil formations
Introduction – Past Experience
4
2 kPa
• Initial plan to install 2 structures in 1 campaign – no
longer feasible
Introduction – Past Experience
5
• Obtain site specific soils info
• Generate a more complex design incorporating
retractable mudmats
• Use CFD to better quantify applied loads
Introduction – Past Experience
6
• Gravity based subsea structures subject to
environmental loads
• Can lead to increased weight/footprint to achieve on
bottom stability
• Potential increases in fabrication and installation
complexity
• Cost saving opportunity
The Problem
7
• Installation complexity
The Problem
8
Background – Foundation Design
9
• Methodologies outlined in API, ISO and DNV codes
• Develop a design that avoids:
– Sliding failure (horizontal)
– Bearing failure (vertical)
Background – Foundation Design
10
• Bearing failure (vertical)
• Eccentricity - M
Fv
Fv
M
Fv
Fh Fh
Extract from API RP 2GEO/ISO 19901-4
Background – Foundation Design
11
• Sliding failure (horizontal)
• Increased area (clay) or increased weight (sand)
Fv
Fh
Fv
Fh
Current Methodologies
12
• Morison’s Equation
• L = 4.46m, D = 7.46m, H = 4.05m. A = 30.2m2
• Lateral Load = 306kN @ 3.0ms-1
• Approximate 30% reduction accounting for solidity –
242 kN
Current Methodologies
13
• Assessment within beam FE Model
• Lateral Load = 229 kN @ 3.0 ms-1
Current Methodologies
14
• Reduced drag loads for shielded members using
guidance from DNV RP H103
• Lateral Load = ~200 kN @ 3.0 ms-1
Current Methodologies
15
• 306 kN – Solid Body
• 242 kN – Solid Body (accounting for solidity)
• 229 kN – Assessment within beam FE Model
• 200 kN – Assessment within beam FE Model (with
shielding)
• ?
Refinement
16
• CFD Results
Refinement
17
• CFD Modelling of structure
• Lateral Load = 80 kN @ 3.0 ms-1
Refinement
18
• 67% load reduction compared
to solid body (with solidity
reduction)
• 35% Reduction compared to
solid body (with solidity
reduction)
Refinement
19
• 306 kN – Solid Body
• 242 kN – Solid Body (accounting for solidity)
• 229 kN – Assessment within beam FE Model
• 200 kN – Assessment within beam FE Model (with
shielding)
• 80 kN – CFD approach
• Foundation capacity on Sand
– Sliding resistance generally proportional to on
bottom weight
20
Outcome – Foundation Design
Lateral Load (kN) Minimum Required on
bottom weight (t)
306 100
242 79
229 75
200 66
80 26
Outcome – Foundation Design
21
• Foundation capacity on clay
– Generally proportional to mud mat area
10.0m
6.0m
6.0m
3.6m
• Potential to achieve
– Lighter, smaller structure
– Reduce fabrication quantity
– Reduce deck space, crane size on installation
vessel
– Circa. £10ks vs £100ks
– Lower cost
Conclusions
22
Questions ?
Summary
23
