Geotechnical Design of Suction Caisson

Bearing Capacity of laterally loading Suction Pile

As offshore exploration and development of oil fields reach water depths in the 1,000 to 3,000 m range, suction caissons become attractive alternatives to driven piles in deepwater because of technical challenges and costs associated with the installation equipment.

In addition, suction caissons also provide a greater resistance to lateral loads than driven piles because of the larger diameter typically used .[1]

0. Introduction

Offshore floating structures are tied through mooring lines to anchors in the seafloor . There are several configurations for the mooring system attaching the structure to the anchors.

Current mooring concepts include vertical moorings as in Tension Leg Platforms (TLPs), taut mooring systems as in SPAR platforms, and semi-taut and catenary mooring systems in Floating Production Systems (FPS) and Floating Production, Storage and Offloading systems (FPSO) [2]

Loading Conditions

Suction caissons in these offshore applications are subjected to a wide range of loading conditions. Loading are vertical for Tension Leg Platforms, inclined for Tau Mooring Systems, and mostly horizontal in Catenary Systems [3].

Types of mooring systems (Vryhof Anchor Manual)

Quasi – Horizontal Loading

The most common use of suction caisson is a anchors for catenary moorings lines, where the chain angle at the mudline is close to zero. The loading angle at the caisson padeye will fall within the range 10 to 20 degree from the horizontal [3].

Geotechnical Design

Laterally Bearing Capacity Analysis

Analysis Methods

The commonly used method for Analysis and Design of suction piles, especially under lateral loading, is a straightforward application of the established Finite Element Method.Less commonly used methods include the Limit Analysis Methods ( Limit Equilbrium Method and Plastic Limit Analysis), which involve assuming a number of failure mechanism, or a general mechanism with a number of variables.Some Designers also use semi – empirical methods : beam – Column Analysis procedures otherwise known as the P-Y methods.

P-Y methods

For suction pile with higher L/D ratio(above 1.5), the P-Y method is considered by many designers to be acceptable

The following simplified approach to assessing the pile ultimate lateral capacity has been used to evaluate the bearing capacity factor Nc

•Ignore the fact that the resistance elements depend on the deformation mode and ignore the coupling between the resistance elements. This can lead to large errors, particularly for relatively short piles

•Do not include independent side shear resistance components on active and passive sides to model different relative shear displacements between the soil and the pile on the two sides,

•Do not include the coupling between the horizontal and vertical soil resistance components along the pile sides and thus do not show the effect of inclined anchor forces

•Require input that is not essential to the capacity assessment such as pile bending stiffness and sub-failure soil response and produce output that is of little interest for the analysis such as moment and shear profiles and load deformation response that are probably not very accurate

•Require user intervention to determine the pile capacity. In most beam column programs the ultimate capacity is determined by trial and error, gradually increasing or decreasing applied forces until the minimum force that produces numerical instability (interpreted to be the failure limit) is found

•Require special elements for rotational, vertical and horizontal tip resistance

•Do not explicitly include effects such as soil-pile interface roughness and loss of soil contact on the back side of the pile

Limitation of this conventional beam column model

Limit Equilibrium or plastic limit analysis methods

Limit equilibrium method Plastic limit analysis method

• failure mechanism is assumed, usually described in terms of one or more geometric parameters. The body force distribution, stress boundary conditions, and the stress or force distribution on failure surface re estimated.

• failure mechanism is assumed with added requirement that the mechanism satisfies kinematic constraints.

Advantages Disadvantages

• Automated solutions using these approaches generally require much less input description and are much easier to use than FEM Programs.

• The approximate nature of the analysis and the difficult of generalizing results.

3.2. Analysis and Design tool to determine the capacity of suction caisson can be classified as one of three general methods [7]

1. Finite Element Method

2. Limit Equilibrium or Plastic Limit Analysis Methods ( Models involving soil failure mechanism)

3. Semi –empirical methods (highly simplified models of soil resistance including beam column models)

3.3 Horizontal and Vertical Capacity of Suction Caissons [4]

The horizontal capacity is maximised by positioning the padeye such that the caisson translate at failure without rotating.

Finite Element Analysis is generally used to evaluate the capacity of suction caisson, either full 3-D analyses or 2-D analyses that have been calibrated and are incorporated in design software.

The failure modes identified by 3 D are very simular to that suggested by Murff and Hamilton (1993) for laterally loaded pile.

A suction anchor with no rotation has in principle two translational failure mode components [5]: • vertical pullout due to the vertical load

component at the padeye• Horizontal displacement due to the horizontal load

component

Some important geotechnical considerations in caisson capacity analyses are discussed as follows [8]:

• Loading Magnitudes and Angles at padeye (Load attachment point

• Reduction factors to calculate static axial capacity in drained and undrained loading conditions

• Shear transfer factor for frictional capacity• Trenching factor• Set up factor• Reverse end bearing factor• Sustained load reduction factor• Cyclic loading factor• Axial capacity reduction factor in suction

zone• Post peak reduction on frictional capacity• Torque reduction on axial uplift capacity

III. Challenges [4,10]

• Requires soil data from advanced laboratory testing for design and analysis

• Concern with holding capacity in layered soils• Lack of formal design guide lines• Limited time on set-up time for uplift• Installation in silt layers

Installation in silt :

• It is difficult to do calculations for silts, because it is difficult to determine whether drained and undrained behavior would be appropriate, and partially drained calculations for caisson penetration have not been formulated [10] .

• However, for soil profiles that have sand overlain by layers of silt, there are reasons to believe that it may not be possible to install suction caissons [6].

This may happen because the low permeability of the silt will create a hydraulic blockage, and thus diminish the upward hydraulic gradientrequired to reduce the penetration resistance in the sand to enable caisson penetration. The tendency of the silt to be sucked up, and its effect on the underlying sand, are not clearly understood. Furthermore, the blockage by the silt layer may be less than if a clay layer is present [6].

References:

1. A finite element analysis of the pullout capacity of suction caisson in clay, christina G. Sgardeli, 2006 MIT

2. A computational procedure for simulation of suction caisson behavior under axial load and inclined loads, Dilip R.Maniar, 2004, Texas

3. Calculation procedures for installation of suction caissons, G.T Houlsby and B.W Byrne,2004

4. Challenges of Offshore geotechnical Engineering, Mark Randolph

5. Geotechnical design and Installation of suction Anchors in Clay, DNV-RP-003, 2005

6. Installation of suction Caisson in sand with silt layers, Manh N. Tran; mark F. Randolph, 2007

7. International Standard (amendment) ISO 1990I-7

8. Geotechnical design of suction caisson in clay, Jun Huang, Houston, TX USA,2003

9. Analysis of suction caisson capacity in clay, M.F. randolph. 2002 Hoston, Texas

10.Design procedures for installation of suction caissons in clay and other materials,

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