1. Offshore Pile Design
A Brief Project Presentation
April 23, 2015
Pagkratios Chitas

2.  offshore geotechnical engineering has formed an
increasingly developing concept for the recent decades,
as a result of the requirements of the oil and gas industry
to move its activities to deeper waters, and thereby,
introduce more challenging environments to geotechnical
engineers
 most of the available foundation design methodologies,
are based upon empirical approaches in order to capture
the pile behaviour, making the design reliability
questionable
 any over-prediction of pile’s capacity can be proved risky
and lead to significant financial costs for the companies
and sometimes to irreparable damage
 in response to the above assumptions, the presented
report attempts to provide an assessment of the
performance of the most relevant pile design techniques,
on sites with different soil profiles
Overview

3. Aim:
 the assessment of the behaviour and response of offshore driven piles, in normally consolidated
(i.e. Pentre site) and over-consolidated (i.e. Tilbrook site) clayey soils, through the use of both
O-Pile software, as well as the findings from available literature, in order to make a comparison
and evaluate the pile performance in different soil types
Objectives:
 to interpret and extract required design parameters from real measured field data, as they have
been derived from in-situ and lab tests for Pentre and Tilbrook sites
 to insert the parameters from data sampling into the O-Pile software, which will produce the
axial and lateral analysis outputs, through different pile design approaches, so as to assess the
performance on the two sites
 to bring the data together for presentation, so as to provide a reasonable comparison and
commentary on the approach followed
 to select, according to the results, the most suitable method for pile design for each site case
Aims and Objectives

4. Undrained Shear Strength Profile Bulk Unit Weight Profile
Atterberg Limits
(Borehole Data)
Relative Void Index at Yield (Oedometer Data) Unit Skin Friction (CPT Test)
Interface Angle of Friction (Ring Shear Test)
In-situ and Lab Test Data for
Pentre Site

5. Undrained Shear Strength Profile Bulk Unit Weight Profile
Lateral Resistance at 2.3 and 2.8 m Depths (UU Triaxial Test)
Atterberg Limits
Pile Head Load-Displacement
In-situ and Lab Test Data for
Tilbrook Site

6. Additional Design Considerations
Pentre Tilbrook
Pile diameter (m) 0.762 0.762
Total pile length (m) 58.5 33.5
Pile tip depth (m) 55 31
Pile wall thickness 20mm for top 4.5m to withstand driving
stresses, remaining pile length 15mm
pile wall thickness
40mm for top 6.5m to withstand driving
stresses, remaining pile length 30mm
pile wall thickness
Pile unit weight (kN/m3) 78 78
Yield stress (MPa) 470 770
Poisson’s ratio 0.3 0.3
Young’s modulus (GPa) 230 210
Comments: Soil parameters for Pentre, will be
considered from 15m below ground
surface and under, up to 55m, which is
the penetration depth, as the pile used
is sleeved for the first 15m, reducing,
thus, the skin friction in this zone to a
negligible value.
Soil parameters for Tilbrook, will be
considered from 1.8m (i.e. due to
excavation reasons) to 31m below
ground surface, where the penetration
reaches.
Pile Design Considerations for both Sites

7. Pile Design:
 In general, piles represent deep foundations that gain capacity through the distribution of loads
throughout the soil layers, utilising hence, the interface shearing resistance between the soil mass and
the shaft or the tip of the pile.
 The axial resistance to both uplift (tension) and bearing (compression) loads, arises from the skin
friction and the end bearing pressure at the bottom.
 Capacity provided by skin friction is of primary importance when designing in an offshore environment.
 Offshore pile design relies mainly on CPT data.
 The installation occurs with the driving of the pile.
 During offshore design on clayey soil, undrained conditions need to be considered.
Important Design Aspects

8. Factors to consider during capacity design:
 soil features (plasticity, particle-size distribution, stress history, relative density)
 installation method
 post-installation effects
 loading rate
 loading type (tensile, compressive)
 pile length
 pile roughness
 friction fatigue
Adhesion factor (α):
 Alpha forms an empirical parameter, which is often used during pile design in order to account for the
reduction of skin friction after pile driving process in clays. This occurs due to the decrease of some
part of soil’s undrained shear strength, after the lateral vibration of the shaft which is accompanied by
pore pressure generation and strain softening that take place during the pile driving procedure.
Factors Affecting Pile Capacity

9. Main Features of Design Methods
Main Features of Pile Design Methodologies
Method API KOLK NGI ICP F05
(1)
skin friction is a function of
undrained shear strength
based on CPT data CPT-based method CPT-based method CPT-based method
(2) simplistic approach accounts for friction fatigue
accounts for soil
plasticity
requires lab tests
(shear ring and
oedometer)
links skin friction with cone
tip resistance
(3)
no consideration of soil
sensitivity
accounts for stress history
(OCR)
reliable for sensitive
soils
accounts for stress
history
accounts for long-term pile
capacity performance
(4)
no consideration of friction
fatigue
accounts for pile length
effects
alpha-based
method
accounts for friction
fatigue
reliable but
overconservative
(5)
no consideration of
interface friction angle (δ)
no consideration of soil
plasticity
designed to
address defects of
API
reliable mostly for sand
main disadvantages:
measurement
uncertainties and
interpretation difficulties
Parameters
used:
 end bearing
resistance (qbf)
 undrained shear
strength (su)
 Skempton’s bearing
capacity factor (Nc)
 effective overburden
pressure (σv0’)
 effective unit weight
(γ’)
 end bearing
resistance (qbf)
 undrained shear
strength (su)
 effective overburden
pressure (σv0’)
 effective unit weight
(γ’)
 pile geometry (D and
L)
 undrained
shear strength
(su)
 effective
overburden
pressure (σv0’)
 plasticity
index (Ip)
 overconsolidation
ratio (OCR)
 relative void
index (lv0)
 relative void
index at yield
(Δlvy)
 interface angle of
friction at failure
(δ)
 cone tip resistance
(q)
 cone skin friction (f)

10. Axial Capacity Results
Total Capacity Comparisons for Pentre
Alpha vs Depth for Pentre

11. Axial Capacity Results
T-Z Curves at 54.75m Depth for Pentre Q-Z or Pile Head Load-Displacement Curves at 0.05m Depth for Pentre
 T-Z and Q-Z curves are used during the assessment of the performance of axially loaded piles, to
provide axial load transfer analysis in relation to the pile vertical displacement (Z).
 T is the mobilised shear stress at the shaft-soil interface.
 Q is the end bearing resistance of soil beneath the pile tip.

12. Axial Capacity Results
Total Capacity Comparisons for Tilbrook
Alpha vs Depth for Tilbrook

13. Axial Capacity Comparisons
Axial Capacity Comparisons for both Sites
Method Skin
Friction
(kN)
End
Bearing
(kN)
Total Peak
Capacity
(kN)
Average Unit
Skin Friction
(kPa)
Average α
(alpha)
Pentre Site (NC Clay)
API 7936 513 8449 68 0.973
APICOOM 4409 513 4922 38 0.557
KOLK 6341 513 6855 54 0.746
NGI 4697 513 5211 40 0.540
ICP 6150 435 6585 52 -
F05 4235 513 4749 36 -
Measured 5170 860 6030 54 0.617
Tilbrook Site (OC Clay)
API 13143 1728 14871 186.6 0.352
APICOOM 16839 1728 18567 239.2 0.500
KOLK 14084 1728 15812 199.8 0.846
NGI 12529 1728 14257 177.8 0.346
Measured 14681 1450 16131 204.4 0.428

14. Lateral Capacity Results
P-Y Curve Comparisons at 2.3m Depth Pile Head Load-Displacement Curve for Peak Displacement 0.1m
 The P-Y curves displayed above, are used for the lateral analysis for the Tilbrook site, and present a
comparison between measured results and the estimated values according to API and Reese methods.
 Where ε50 is the axial strain at 50% of the peak deviatoric stress during triaxial undrained test, as
recommended by Reese.
 The value 0.0025 is taken from the UU tests, whereas 0.005 is generally used in the absence of triaxial
test data for stiff clay (Clarke, 1993).

15. Pentre site (normally consolidated clay):
 Axial Capacity and Alpha: In the case of Pentre, NGI forms a relatively reliable method to predict the
axial pile capacity in soft clayey soil (as it does account for soil’s sensitivity), as well as the adhesion
factor α. In contrary, Fugro method was found to be over-conservative in this case and wouldn’t be
recommended as it would result in overdesigning and unnecessary high costs, whereas the API approach
was found to overestimate the axial capacities.
 T-Z and Q-Z Curves: With regards to the T-Z curves (i.e. the skin friction against displacement), the
methods that capture best the behaviour are KOLK and ICP, whereas for the Q-Z curves, the most
reliable results were derived from NGI method.
Result Interpretations for Pentre

16. Tilbrook site (over-consolidated clay):
 Axial Capacity and Alpha: In the case of Tilbrook, KOLK is found to offer relatively reliable results for
the total capacity modelling (as it accounts for the stress history of the soil and is reliable for clays),
while the API would be recommended for the estimation of the adhesion factor α, as KOLK
overestimates it. On the other hand, NGI was found to be over-conservative, thus it wouldn’t be
recommended, and APICOOM can be regarded as the least reliable method during axial capacity
estimations.
 P-Y Curves: For lateral pile design, interpreting both the results from the P-Y and the pile head load-
displacement graphs, it can be assumed that API method is able to model the lateral response of the
clay in the most approximate way. However, for more conservative purposes, average values between
API and Reese results could be considered.
Result Interpretations for Tilbrook

17. Assumptions:
 Predicting offshore pile capacity forms a complex procedure, and therefore, more than one methods
should be considered during the design process, to enable comparisons between the various obtained
results, while different factors should be taken into account depending on soil type and condition, in
order to provide more accuracy and reliability.
 The results are found to be in a good alignment with the findings of related literature review.
 During pile capacity estimation, skin friction should be regarded as more important than end bearing
resistance, due to the fact that higher loads occur at the top layers, while soil’s undrained shear
strength there is lower than the respective value at the bottom.
 It should be born in mind that for soft clays, sensitivity factors need to be considered during design,
whereas over-consolidated stiff clays offer higher capacity but stress history is important.
Conclusions