Structural Design Offshore

SKG11: Civil, Structural, Pipelines

Structural Design Offshore Lecture 6

Foundation Design


Lecture Objective

U d t di ff h t t Understanding offshore structures foundation system and foundation designdesign

2


Lecture Outline

1. Offshore Structures d h ll f d• Deep and shallow foundations

2 Foundation Design2. Foundation Design• Foundation modelling• Pile designPile design• Class exercises

3


Offshore Structures- Fixed Platforms

Load TransferLoad Transfer

1.Topsides

MSL2. Jacket

Seabed / Mudline

3.3.FoundationFoundation-- PilesPiles

4-- SoilSoil


Deep Foundations : Pile

Main Pile Skirt Pile

5


Pile type

Main Pile Skirt Pile

• Good for shallow water• Welded connection

• Good for deeper water• Grouted connection

l6

• Grout sys. vs material cost


Shallow Foundations : GBS

7


Shallow foundations : Jack-up

8


Pile Design

The pile lateral and axial capacity ofp p yfoundations shall be designed in accordancewith the latest edition of API RP 2A andAppendix III of PTSAppendix III of PTS

9


Soil Conditions & Scour

Soil Conditions:The design shall be based on the actual soil The design shall be based on the actual soil conditions as per soil boring reports

Scour:Unless otherwise specified, the foundation/jacket shall be designed for a local scour of 900 mm.shall be designed for a local scour of 900 mm.

To use coupled nonlinear pile/structure i t ti (PSI) i l di t i ff t f interaction (PSI), including geometric effects for the foundation/jacket design.

10


Foundation Modeling

Adhesion or T-Z

11


Nonlinear Soil Springs P-Y CurvesP Y Curves

12


Axial Pile Load Transfer-Displacement Curve (T-Z)Displacement Curve (T Z)

13


Effect of Spudcan on Piles

To consider effects of soil being displaced by the spud-being displaced by the spudcan on the adjacent platform piles

To use modified P-Y and, where necessary, T-Z data for inclusion in the lateral pile inclusion in the lateral pile analysis.

Pile centre lines shall be more

1.5 x D

Pile centre lines shall be more than 1.5 times x spud can diameters of the spud cans d

Spud can Diameter, D

14

edge.


Pile Group Effects

For pile or conductor spacing less than eight (8) diameters, group

Diameter=D

effects may have to be evaluated

For piles embedded in clays, the group capacity may be less than a

Spacing < 8xD

single isolated pile capacity multiplied by the number of piles in the group

For piles embedded in sands the pgroup capacity may be higher than the sum of the capacities in the isolated piles

Group Effects

Pile group would normally experience greater lateral deflection than that of a single pile under the average pile

15

Group Effectsload of the corresponding group


Bearing (Q-Z)

In the absence of more definitive criteria the following curve is recommended for both sands and clays. (API section 6.7.3)recommended for both sands and clays. (API section 6.7.3)

16


Bearing Area

Bearing AreaArea

Plugged

Unplugged

17


Bearing Area : Exercise

Calculate the end bearing area for the following case : Bearing Area the following case :

1. Pile Properties : OD = 76 2 cm ( 30”)

g

OD = 76.2 cm ( 30 )WT = 3.175 cm (1.25”)

Area = 707 in22 Pil S ti P ti AISC 6 21

Plugged

707 in2 = 456,036 mm22. Pile Section Properties , AISC pg 6-21

UnpluggedArea = 113 in2 = 7.29 E+4 mm2

Unplugged

18

7.29 E 4 mm2


Pile Unity CheckFFvv

22

19

SKG11: Civil, Structural, Pipelines Pile Unity Check

If Storm Case Pile Unity Check > 1.0, then :

20


Pile Axial Capacity

Exercise

• Pile radius r = 2 ft• Pile radius , r = 2 ft• Pile length , l = 100 ft• Skin Friction , f = 2100 lb/ft2

• Determine The pile axial capacity load? Neglect end bearing

00 ft

2 * P

I * r

* h2

* PI *

2 ft

* 1 0

Pile

are

a =

(A

s)

=

21Qd =f As = 2100 x (22/7) x 400

= 2,640,000 lbs = 2,640 K


Pile Axial Capacity curve

PTS Factors of Safety on pile axial loads :

Operating loads alone .................2.0 Operating loads & soft mooring ....1.5 Extreme storm loads ...................1.5 Boat impact 1 0

22

Boat impact ...............................1.0 Earthquake - SLE........................ 1.5


Pile Axial Capacity : Exercise

Pile Axial load = 2.0 MN for l ti normal operating case

Determine the minimum pile ppenetration depth required?

Safety Factor = 2Safety Factor = 2

.:. Require = 4.0 MN

From Pile Axial Capacity curve, required penetration

23=> 26 m approx


Pile Reaction

The most important aspect of structural d i design :

•What is the deflection mode?

•Tension and compression member

STM2A2

A1

Y

STM2STM1 STM3

A BB2

B1

X

24

21

OPR1 OPR2 OPR3


Check Pile Reaction : Example a p e

A 4 legged platform with pilehead numbering of :• 9010• 9020• 9030

90309040

• 9040

Under Load ConditionUnder Load Condition (X –direction)

Q : What is the deflection mode?

90209010

25

Q

SKG11: Civil, Structural, Pipelines Pile Reaction - Results

9030

- 3253.18 KN

90309040

- 54.17 KN

- 3387.39 KN

90209010

28 21 KN

26

- 28.21 KN


End of

Structural Design Offshore Lecture 6

Foundation Design

27

Documents

Structural Design Offshore