Section 7 Foundation

701. General

1. Application

(1) Scope

Soil investigations, design considerations for the supporting soil, and the influence of the soil on the foundation structure, are covered in this Chapter.

(2) Guideline

The degree of design conservatism should reflect prior experience under similar conditions, the manner and extent of data collection, the scatter of design data, and the consequences of failure. For cases where the limits of applicability of any method of calculation employed are not well defined, or where the soil characteristics are quite variable, more than one method of calculation or a parametric study of the sensitivity of the relevant design data is to be used.

702. Site Investigation

1. General

(1) The actual extent, depth and degree of precision applied to the site investigation program are to reflect the type, size and intended use of the structure, familiarity with the area based on pre- vious site studies or platform installations, and the consequences which may arise from a failure of the foundation.

(2) For major structures, the site investigation program is to consist of the following three phases.

(A) Sea Floor Survey to obtain relevant geophysical data(see Par 2)

(B) Geological Survey to obtain data of a regional nature concerning the site. (see Par 3)

(C) Subsurface Investigation and Testing to obtain the necessary geotechnical data.

(3) The results of these investigations are to be the bases for the additional site related studies which are listed in Par 5.

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(4) A complete site investigation program is to be accomplished for each offshore structure.

However, use of the complete or partial results of a previously completed site investigation as the design basis for another similarly designed and adjacent offshore structure is permitted when the adequacy of the previous site's investigation for the new location is satisfactorily demonstrated.

(5) When deciding the area to be investigated, due allowance is to be given to the accuracy of po-

sitioning devices used on the vessel employed in the site investigation to ensure that the data obtained is pertinent to the actual location of the structure.

2. Sea Floor Survey

Geophysical data for the conditions existing at and near the surface of the sea floor are to be obtained. The following information is to be obtained where applicable to the planned structure.

(A) Soundings or contours of the sea bed

(B) Position of bottom shapes which might affect scour

(C) The presence of boulders, obstructions, and small craters

(D) Gas seeps

(E) Shallow faults

(F) Slump blocks

(G) Ice scour or sea floor sediments

(H) Subsea permafrost or ice bonded soils

3. Geological Survey

(1) Data of the regional geological characteristics which can affect the design and siting of the structure. Such data are to be considered in planning the subsurface investigation, and they are also to be used to assure that the findings of the subsurface investigation are consistent with known geological conditions.

(2) Where necessary, an assessment of the seismic activity at the site is to be made. Particular em- phasis is to be placed on the identification of fault zones, the extent and geometry of faulting and attenuation effects due to conditions in the region of the site.

(3) For structures located in a producing area, the possibility of sea floor subsidence due to a drop in reservoir pressure is to be considered.

4. Subsurface Investigation and Testing

(1) The subsurface investigation and testing program is to obtain reliable geotechnical data concern- ing the stratigraphy and engineering properties of the soil. These data are to be used to assess whether the desired level of structural safety and performance can be obtained and to assess the feasibility of the proposed method of installation.

(2) Consistent with the stated objective, the soil testing program is to consist of an adequate num- ber of in-situ tests, borings and samplings to examine all important soil and rock strata. The testing program is to reveal the necessary strength, classification and deformation properties of the soil. Further tests are to be performed as needed, to describe the dynamic characteristics of the soil and the static and cyclic soil-structure interaction.

(3) For pile-supported structures, the minimum depth of at least one bore hole, for either individual or clustered piles, is to be the anticipated length of piles plus a zone of influence. The zone of influence is to be at least 15.2 m or 1.5 times the diameter of the cluster, whichever is greater, unless it can be shown by analytical methods that a lesser depth is justified. Additional bore holes of lesser depth are required if discontinuities in the soil are likely to exist within the area of the structure.

(4) For a gravity-type foundation, the required depth of at least one boring is to be at least equal to the largest horizontal dimension of the base. In-situ tests are to be carried out, where possi- ble, to a depth that will include the anticipated shearing failure zone.

(5) A reasonably continuous profile is to be obtained during recovery of the boring samples. The

desired extent of sample recovery and field testing is to be as follows.

(A) The recovery of the materials to a depth of 12 m below the mudline is to be as complete as possible. Thereafter, samples at significant changes in strata are to be obtained, at ap- proximately 3 m intervals to 61 m and approximately 8 m intervals below 61 m.

(B) At least one undrained strength test(vane, drop cone, unconfined compression, etc) on se- lected recovered cohesive samples is to be performed in the field.

(C) Where practicable, a standard penetration test or equivalent on each significant sand stratum is to be performed, recovering samples where possible.

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(D) Field samples for laboratory work are to be retained and carefully packaged to minimize changes in moisture content and disturbance.

(6) Samples from the field are to be sent to a recognized laboratory for further testing. They are to

be accurately labeled and the results of visual inspection recorded. The testing in the laboratory is to include at least the following.

(A) Perform unconfined compression tests on clay strata where needed to supplement field data.

(B) Determine water content and Atterberg limits on selected cohesive samples

(C) Determine density of selected samples

(D) As necessary, develop appropriate constitutive parameters or stress-strain relationship from ei-

ther unconfined compression tests, unconsolidated undrained triaxial compression tests, or consolidated undrained triaxial compression tests

(E) Perform grain size analysis, complete with percentage passing 200 sieve, on each significant

sand or silt stratum

(7) For pile-supported structures, consideration is to be given to the need for additional tests to ad- equately describe the dynamic characteristics of the soil and the static and cyclic lateral soil-pile.

(A) Shear strength tests with pore pressure measurements. The shear strength parameters and pore-water pressures are to be measured for the relevant stress conditions

(B) Cyclical loading tests with deformation and pore pressure measurements to determine the soil behavior during alternating stress

(C) Permeability and consolidation tests performed as required

5. Documents

The foundation design documentation mentioned in Sec 1 is to be submitted for review. As appli- cable, the results of studies to assess the following effects are also to be submitted.

(A) Scouring potential of the sea floor

(B) Hydraulic instability and the occurrence of sand waves

(C) Instability of slopes in the area where the structure is to be placed

(D) Liquefaction and other soil instabilities

(E) For Arctic area, possible degradation of subsea permafrost layers as a result of the pro- duction of hot oil

703. Foundation Design Requirements

1. General

The loadings use in the analysis of the safety of the foundation are to include those defined in Par 7 and those experienced by the foundation during installation. Foundation displacements are to be evaluated to the extent necessary to ensure that they are within limits which do not impair the intended function and safety of the structure. The soil and the structure are to be considered as an interactive system, and the results of analysis, as required in subsequent paragraphs, are to be eval- uated from this point of view.

2. Cyclic Loading Effects

(1) This influence of cyclic loading on soil properties is to be considered. For gravity structures in

particular, possible reduction of soil strength is to be investigated and employed in design. In particular the following conditions are to be considered.

(A) Design storm during the initial consolidation phase

(B) Short-term effects of the design storm

(C) Long-term cumulative effects of several storms, including the design storm

(2) Reduced soil strength characteristics resulting from these conditions are to be employed in design. In seismically active zones, similar deteriorating effects due to repeated loadings are to

be considered. Other possible cyclic load effects, such as changes in load-deflection character-

istics, liquefaction potential and slope stability are also should be accounted for when they will affect the design.

3. Scour

to be considered, and these effects

Where scour is expected to occur, either effective protection stallation of the structure, or the depth and lateral extent of vestigation program, is to be accounted for in design.

is to be furnished soon after the scouring, as evaluated in the site

in-

in-

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4. Deflection and Rotations

Tolerable limits of deflections and rotations are to be established based on the type and function of the platform, and the effects of those movements on risers, piles and other structures which interact with the platform. Maximum allowable values of platform movements, as limited by these structural considerations or overall platform stability, are to be considered in the design.

5. Soil Strength

(1) The ultimate strength or stability of soil is to be determined using test results which are com- patible with the method selected.

(2) In a total stress approach the total shear strength of the soil obtained from simple tests is used.

A total stress approach largely ignores changes in the soil's pore water pressure under varying loads and the drainage conditions at the site.

(3) When an effective stress approach is used effective soil strength parameters and pore water

pressures are determined from tests which attempt to predict in-situ total stresses and pore pressures.

6. Dynamic and Impact Considerations

(1) For dynamic and impact loading conditions, a realistic and compatible treatment is to be given to the interactive effects between the soil and structure. When analysis is required it may be ac- complished by lumped parameter, foundation impedance functions, or by continuum approaches including the use of finite element methods. Such models are to include consideration of the in- ternal and radiational damping provided by the soil and the effects of soil layering.

(2) Studies of the dynamic response of the structure are to include, where applicable, consideration of the nonlinear and inelastic characteristics of the soil, the possibilities of deteriorating strength and increased or decreased damping due to cyclic soil loading, and the added mass of soil sub- ject to acceleration. Where applicable, the influence of nearby structures is to be included in the analysis.

7. Loading Conditions

(1) Those loadings which produce the worst effects on the foundation during and after installation are to be taken into account. Post installation loadings to be checked are to include at least those relating to both the operating and design environmental conditions, combined in the fol- lowing manner.

(A) Operating environmental loading combined with dead and maximum live loads appropriate to the function and operations of the structure.

(B) Design environmental loading combined with dead and live loads appropriate to the function and operations of the structure during the design environmental conditions

(C) Design environmental loading combined with dead load and minimum live loads appropriate to the function and operations of the structure during the design environmental conditions

(2) For areas with potential seismic activity, the foundation is to be designed for sufficient strength

to sustain seismic loads.

8. Anchoring System

(1) Where the anchoring utilizes piles, the requirements in these Rules applicable to piles are to be used. The loads at the mooring line attachments are to be calculated and the pile's local strength is to be checked.

(2) Where the anchoring utilizes gravity anchors, the requirements in these Rules applicable to grav- ity based structures are to be used.

(3) Where platform are permanently and partially supported by a mooring system, the analysis of the

platform's foundation is to include the interactive effects of the mooring system.

(4) Other types of anchoring will be specially considered.

9. Loads and Soil conditions Due to Temporally Situated Structures

(1) Changes in soil conditions due to temporarily situated platform such as self-elevating drilling units, workover rigs or tender rigs placed near the structures are to be assessed and investigated.

(2) These changes and their influence on the structure are to be incorporated in the foundation de-

sign to ensure that structure's function and safety are not impaired.

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704. Pile Foundation

1. General

(1) The effects of axial, bending and lateral loads are to be accounted for in the design of in- dividual and group piles.

(2) The design of a pile is to reflect the interactive behavior between the soil and the pile and be-

tween the pile and the structure. Methods of pile installation are to be consistent with the type of soil at the site, and with the installation equipment available. Pile driving is to be carried

out and supervised by qualified and experienced personnel, and driving records are to be ob- tained and submitted for review.

(3) Should unexpectedly high or low driving resistance or other conditions be encountered which lead to a failure of the pile to reach its desired penetration, a reevaluation of the pile's capacity is to be carried out considering the parameters resulting from the actual installation.

(4) Where necessary, the effects of bottom instability in the vicinity of the structure are to be assessed.

2. Axial Piles

For piles in compression, the axial capacity is to be considered as consisting of the skin friction, ĄX developed along the length of the pile, and the end bearing, ĄỲ at the tip of the pile. The ax- ial capacity of a pile subjected to tension is to be equal to or less than the skin friction alone. Predictions of the various parameters needed to evaluate ĄX and ĄỲ are to be accomplished using a recognized analytical method or another method shown to be more appropriate to the conditions at the site. When required, the acceptability of any method used to predict the components of pile re- sistance is to be demonstrated by showing satisfactory performance of the method under conditions similar to those existing at the actual site. The results of dynamic pile driving analysis alone are not to be used to predict the axial load capacity of a pile.

3. Factors of Safety for Axial Piles

(1) When the pile is subjected to the three loading cases described in 703. 7 and the ultimate ca- pacities are evaluated using the above cited API method, the allowable values of axial pile bearing and pullout loads are to able values of axial pile bearing and pullout loads are to be determined by dividing the ultimate capacities obtained above. When ultimate capacities are cal- culated using the method described in Par 2, the minimum safety factor obtained about 703. 7 (1) (A) is 2.0, and 1.5 about (1) (B) or (1) (C).

(2) For the design earthquake, the factor of safety is to be specially considered.

4. Laterally Loaded Piles

(1) In the evaluation of the pile's behavior under lateral loadings, the combined load-deflection char- acteristics of the soil and pile, and the pile and the structure are to be taken into account. The representation of the soil's lateral deflection when it is subjected to lateral loads is to adequately reflect the deterioration of the lateral bearing capacity when the soil is subjected to cyclic loading. The description of the lateral load versus deflection characteristics for the various soil strata is to be based on constitutive data obtained from suitable soil tests.

(2) Reference is to be made for a procedure to evaluate the load-deflection characteristics of later- ally loaded piles. However, the use of alternative methods is permitted when they are more ap- propriate for conditions at the site.

(3) Where applicable, the rapidly deteriorating cyclic bearing capacity of stiff clays, especially those exhibiting the presence of a secondary structure, is to be accounted for in the design.

5. Anchor Piles

(1) When lateral loads are directly applied to a pile such as in the case when it is used to anchor a mooring line suitable load factors greater than 1.0 are to be used to increase the magnitudes of the lateral load effects resulting from the load conditions of 703. 7.

(2) Calculation of the soil capacity and the pile stresses is to be based on consideration of the modified loads.

6. Pile groups

(1) Where applicable, the effects of close spacing on the load and deflection characteristics of pile groups are to be determined.

(2) The allowable load for a group, both axial and lateral, is not to exceed the sum of the appa-

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rent individual pile allowable loads reduced by a suitable factor.

7. Connections Between Piles and Structure

(1) The loads acting on the platform may be transferred to the piles by connecting the jackets legs or pile sleeves to the piles by welding, grouting the annulus between the jacket leg or pile sleeve and the pile, or by use of mechanical devices such as pile grippers. The design of the grouted pile to structure connection should consider the use of mechanical shear connectors as their presence increase the strength of the connection, and eliminates any effect of long term grouting shrinkage.

(2) Adequate clearance between the pile and the jacket leg should be provided for proper placement of the grout.

(3) Reliable means for the introduction of the grout to the annulus should be provided to ensure

complete filling of the annulus and to minimize the possibility of dilution of the grout and the formation of voids in the grout.

(4) If mechanical devices are used their strength and fatigue characteristics are to be adequately

demonstrated by analysis, testing or experience.

705. Gravity Structures

1. General

(1) The stability of the foundation with regard to bearing and sliding failure modes is to be inves- tigated employing the soil shear strengths determined in accordance with 702. 4 and 703. 2.

(2) The effects of adjacent structures and the variation of soil properties in the horizontal direction

are to be considered where relevant.

(3) Where leveling of the site is not carried out, the predicted tilt of the overall structure is to be based on the average bottom slope of the sea floor and the tolerance of the elevation measuring device used in the site investigation program.

(4) Differential settlement is also to be calculated and the tilting of the structure caused by this set- tlement is to be combined with the predicted structural tilt. Any increased loading effects caused by the tilting of the structure are to be considered in the foundation stability requirements of Par 2.

When underpressure or overpressure is experienced by the sea floor under the structure, provi- sion is to be made to prevent piping which could impair the integrity of the foundation. The influence of hydraulic and slope instability, if any, is to be determined for the structural loading cases of 703. 7 (1).

(5) Initial consolidation and secondary settlements, as well as permanent horizontal displacements, are to be calculated.

2. Stability

(1) The bearing capacity and lateral resistance are to be calculated under the most unfavorable com- bination of loads.

(2) Possible long term redistribution of bearing pressures under the base slab are to be considered in order to ensure that the maximum edge pressures are used in the design of the perimeter of

the base.

(3) The lateral resistance of the platform is to be investigated with respect to various potential shearing planes. Special consideration is to be given to any layers of soft soil.

(4) Calculations for overturning moment and vertical forces induced by the passage of a wave are to include the vertical pressure distribution across the top of the foundation and along the sea

floor.

(5) The capacity of the foundation to resist a deep-seated bearing failure is to be analyzed. In lieu of a more rigorous analysis, where uniform soil conditions are present or where conservatively

chosen soil properties are used to approximate a non-uniform soil condition, pezoidal distribution of soil pressure is a reasonable expectation, the capacity to resist a deep-seated bearing failure can be calculated by standard bearing

and where a tra- of the foundation capacity formulas

applicable to eccentrically loaded shallow foundations. Alternatively, slip-surface methods, cover-

ing a range of kinematically possible deep rupture surfaces can be employed in the bearing ca- pacity calculations.

(6) The maximum allowable shear strength of the soil is to be determined by dividing the ultimate

shear strength of the soil by the minimum safety factors given below.

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When the ultimate soil strength is determined by an effective stress method, the safety factor is to be applied to both the cohesive and frictional terms. If a total stress method is used, the safety factor is to be applied to the undrained shear strength.

(7) The minimum safety mulation and various for (1) (A), and 1.5

factors to be obtained, when employing a standard bearing capacity for- trial sliding failure planes with the loading conditions of 703. 7, are 2.0 for (B) and (C). The safety factors to be obtained when considering the

Design Earthquake will be specially considered.

(8) Where present, the additional effects of penetrating walls or skirts which transfer vertical and lateral loads to the soil are to be investigated as to their contribution to bearing capacity and lateral resistance.

3. Soil Reaction on the Structure

(1) For conditions during and after installation, the reaction of the soil against all structural mem- bers seated on or penetrating into the sea floor is to be determined and accounted for in the design of these members.

(2) The distribution of soil reactions is to be based on the results obtained in 702. 4.

(3) Calculations of soil reactions are to account for any deviation from a plane surface, the load-de- flection characteristics of the soil and the geometry of the base of the structure.

Where applicable, effects of local soil stiffening, non-homogeneous soil properties, as well as the presence of boulders and other obstructions, are to be accounted for in design.

(4) During installation, consideration is to be given to the possibility of local contact pressures due to irregular contact between the base and the sea floor. These pressures are additive to the hy-

drostatic pressure.

(5) An analysis of the penetration resistance of structural elements projecting into the sea floor be- low the foundation structure is to be performed.

(6) The design of the ballasting system is to reflect uncertainties associated with achieving the re-

quired penetration of the structure.

(7) Since the achievement of the required penetration of the platform and its skirts is of critical im- portance, the highest expected values of soil strength are to be used in the calculation of penetration.