The Asia-Pacific Offshore Conference-APOC2010 Kuala Lumpur, December 13-14, 2010

Comparison of Foundation Strength Modelling of Jacket Platforms in API RP2A codes and ISO 19902

Arazi B. Idrus1, Narayanan Sambu Potty1, Mohd Foad Abdul Hamid2*, Zafarullah Nizamani3 1Associate Professor, 3PhD Student

Department of Civil Engineering, Universiti Teknologi PETRONAS Bandar Seri Iskandar, Tronoh

Email: zafarullah.nizamani@gmail.com 2Principal Engineer - Structural

Scientige Sdn Bhd, Kuala Lumpur Email: mfoad@scientige.com

*Currently on leave of absence from Faculty of Mechanical Engineering Universiti Teknologi Malaysia

ABSTRACT Comparison of three offshore jacket platform codes i.e. API RP2A-WSD (21st edition, 2008) API RP2A-LRFD (1st edition, 2003) and ISO 19902 1st edition, 2007) is made on the foundation strength modelling. A detailed comparison of the provisions of the codes is useful to understand the provisions as well in the process of calibration of the load and resistance factors used in design. Guidance provided in the codes does not specifically apply to problem soils of carbonate material, volcanic sands or highly sensitive clays. All code contains guidance against static and monotonic loading. In this paper, the related equations are identified and compared and the similarities and differences are shown. KEY WORDS: Working stress design, Load and resistance factor design, Jacket Platforms, Pile foundations, ISO, API RP2A. 1.0 INTRODUCTION The growth of design codes(which reflect engineering practice) is an indicator of development of structural design (AME, 1999). Offshore structures were constructed in the Gulf of Mexico (GOM) in 1940’s with no specific guidelines. The first design standard was issued by American Petroleum Institute (API) in 1969. Design equations are developed with adequate assurance that the actual strength will be greater than the nominal value assumed in design. Steel cylindrical pipe pile is considered for design of platform foundations. Driven piles, used in the platform foundations are usually open ended steel tubes. The piles in sand are more at risk of failure than piles in clay (Horsnell 1996). Piles governed by dead loads are more at risk than those governed by extreme environmental loads (Horsnell 1996). Code design is based on static, cyclic and transient loads acting on the platform, and it should have minimum deformations and vibrations. Effects of cyclic and transient loading on the strength of the supporting soil and on the structural response of pile are also considered. During design of jacket platforms movement of sea floor against foundation members should be considered as well as the forces causing such movement. In this paper the basic background information on offshore pile foundations are provided, followed by comparison of clauses, highlighting major differences and similarities in each section of code and summary of conclusions. 2.0 OBJECTIVE AND METHODOLOGY In Malaysia API RP2A WSD is being still used for design of offshore jacket platforms. Probability based limit state design (LRFD method) is being adopted by many countries for all types of design. Thus it is imperative to adopt this methodology for design of offshore structures in Malaysia. For LRFD methodology, the load and resistance factors are optimised by the method of calibration. The first step in this process

is the determination of reliability of structural members of the jacket designed as per existing practice i.e. WSD. The structural design using the LRFD expressions should have the identical reliability. Factor of safety to be used must be selected depending on the reliability of soil data, load estimates, analytical methods and installation techniques (Advanced Mechanics, 1999). Understanding the equations in the different codes and their similarities and differences is useful for the calibration process. Basic background information, regarding offshore pile foundations has been provided in the literature review. 3.0 LITERATURE REVIEW Pile foundations must withstand lateral loads and pile head moments without excessive deformations or pile material overstressing (ICL, 2009). During pile driving and grouting, problems arise like tilting of jacket, piles not driven to predetermined depth due to soil set-up, soil plug removal, chocking of annulus between jacket leg and pile and piles going inside the jacket leg(Agarwal et.al,1978). Foundation collapse can have a significant influence on system reliability due to small redundancy associated with the piles, few adequate alternative load paths and because the uncertainties related with the soils and their strength modelling are huge (Advanced Mechanics, 1999). Pile wall thickness should be sufficient at mud line to carry design lateral moment loading (ICL, 2009). Increased wall thickness requires the weld stress to be relieved through some treatment (ICL, 2009). Pile foundation design considers pile capacity, pile stress, fatigue, deflection criteria, cyclic loading and driveability for operating conditions, design conditions and survival or abnormal conditions (Bomel, 2000). The subsidence related to reservoir depletion should be considered as also its effects on pile stresses (Bomel, 2000). Pile data required for design are length, diameter, L/D ratio, pile tip condition (during driving), and set-up period (time elapsed during driving and testing the pile) (Briaud, 1990). Compression capacity normally governs pile design except where the structure is lightweight (e.g. minimal structure) (OTR 2000/072, 2000). 3.1 Vertical piles compared with battered piles: Vertical piles have benefits like no pile guides and easier stabbing during installation. Significant disadvantages are: increased sensitivity to soil properties, base shear resisted entirely by lateral soil capacity (battered piles only had to resist 42% average laterally), 0.5 second increase in structural natural period, more fatigue, buckling problems with heavy hammers and potential for vortex induced vibration due to currents during installation (Digre,1989).

3.2 Failure modes of offshore piles: Pile failure occurs when they become unserviceable as structural element during installation for example buckling, tip damage or fatigue

The Asia-Pacific Offshore Conference-APOC2010 Kuala Lumpur, December 13-14, 2010 during driving (Horsnell, 1996). Six types of failure modes are horizontal deformation at pile head, yielding at top of pile due to combined axial force and bending moment, development of yield hinge at top of pile due to combined axial force and bending moment, shear failure at top of pile, failure of soil with pile in compression and failure of soil with pile in tension (Hansen et al 1995). 3.3 Soil parameters: Briaud and Audibert (1990) lists them as plasticity index, liquid limit, undrained shear strength, vertical effective stress, particle size distribution, moisture content, bulk density and specific gravity. Pile friction is not significantly affected by plasticity of clay. Soil parameters selected for loading in cohesive and cohesionless soil are given in table 1. Table 1: Soil parameters for pile design (Fugro, 1993).

Cohesive soil Cohesion less soil Submerged density Submerged density / Relative

density Undrained shear strength Angle of internal friction Index properties Grading Remoulded shear strength Stiffness (e.g shear modulus) Consolidation characteristics Settlement parameters Over consolidation ratio Coefficient of lateral earth pressure

3.4 Loads effect: Maximum load effect in piles occurs when the largest wave with maximum height passes the structure. During other times the variation of wave loads with time is taken using Turkstra’s rule. Loads which are not dependent on wave loading are taken considering random point in time; loads which are correlated to wave height conditional distributions are used (Hansen et al, 1995). Axial capacity of steel piles driven into sand is influenced by the effects of age and cyclic load history, i.e. high level of cyclic loading causes systematic loss of pile capacity i.e. reducing the strength and stiffness (Atkins, 2000). Therefore the cyclic loading due to storm waves is most significant for offshore design (Matlock, 1970). Pile load tests were conducted first in compression and then in tension up to 1.5 times the design working load (0.75x net ultimate compression capacity and 0.75x net ultimate tension capacity)(Gilchrist,1984). Table 2 shows increments of load to minimum holding time. Table 2. Specified increments of load and minimum holding time, for

compression and tension load tests (Gilchrist, 1984) No  Load as

% age of design

working load

Minimum holding

time

No  Load as % age of design

working load

Minimum holding

time

1  25 1hr 10  125 1hr 2  50 1hr 11  150 6hr 3  75 1hr 12  125 10 min 4  100 1hr 13  100 10 min 5  75 10 min 14  75 10 min 6  50 10 min 15  50 10 min 7  25 10 min 16  25 10 min 8  0 1 hr 17  0 1hr 9  100 6 hr 18  --- ---

Each test pile has four reaction piles, placed at four corners of a square;

test pile position is at centre of square. Reaction piles are joined rigidly at their heads by a steel frame called as platform on which weights for the compression tests are placed (Gilchrist, 1984). 4.0 COMPARISON OF FOUNDATION STRENGTH

PROVISIONS The API and ISO equations for calculating pile capacity is somewhat identical but some differences have been highlighted. ISO uses most severe environmental condition for design consideration for last stage of installation i.e. when structure is straight and placed on mud floor but not permanently connected. ISO calls foundation capacity as axial pile capacity only and terms lateral soil behaviour is related to foundation displacement not capacity. Disturbance to foundation soils occurs during installation of well conductors and shallow well drilling. ISO requires the design to be based on exposure level 1(manned non-evacuated). 4.1 Foundation types: API provides details for three types of foundation i.e. driven, drilled and grouted and bell piles whereas in ISO besides these guidance is provided for vibro driven piles. 4.2 Foundation parameters: Parameters considered in the design of piles by different codes are indicated by tick marks in table 3. Table 3: Parameters in different codes for design of pile foundation

Foundation (Pile) types API WSD & LRFD ISO Design action --- √ Diameter √  √ Penetration √  √ Wall thickness √  √Type of tip √  √ No. Of piles √  √ Spacing √  √ Geometry √  √ Location √  √ Pile head fixity ‐‐‐  √Material strength √  √Installation method √  √Mudline restraint √  ‐‐‐ 

4.3 Pile Design:

Non-linear behaviour of soil is assumed by all codes for foundation response; and force-deflection compatibility between structure and pile-soil system is ensured. All codes require deflection and rotations to be checked for individual piles and total foundation system at critical locations like pile tops, points of contraflexure, mud line and this should never exceed serviceability limits. Foundation capacity is taken as minimum of pile strength and pile axial resistance in ISO and LRFD where as WSD uses pile penetration capacity. Pile strength is checked through combined axial load and bending equation used in tubular strength modelling. Internal pile forces are checked as factored loads in ISO using complete structure/ soil non-linear foundation model. In case of inadequacy of lateral restraint provided by soil, column buckling effects on pile shall also be checked as per ISO or LRFD. In WSD Pile penetration model the design should be sufficient to develop adequate capacity to resist maximum computed axial bearing and pull out loads with factor of safety (these factors have remained unchanged since first edition in 1969) (Advanced Mechanics 1999), where as allowable pile

The Asia-Pacific Offshore Conference-APOC2010 Kuala Lumpur, December 13-14, 2010 capacity is found by dividing the ultimate pile capacity by a factor of safety as given in table 4. Table 4: Code Provisions for Pile Capacity API RP2A –WSD API RP2A –LRFD ISO 19902

PD QD γ DEC with drilling loads γ = 1.5 OEC with drilling operations γ = 2.0 DEC with producing loads γ = 1.5 OEC with producing operations γ = 2.0

= 0.8 sistance factor for

(reEEC)

= 0.7 (resistance factor for OEC

,

,

,

,

= 1.25 sistance factor for ,

(reEEC)

, = 1.50 (resistance factor for OEC)

DEC -Design environmental conditions; EEC- extreme environmental conditions; OEC- Operating environmental conditions 4.3.1 Scour Scour (seabed erosion due to wave and current action) is known to occur surrounding offshore piles. Scour is more prominent in waters less than 30m (Fugro 1993). Scour reduces axial pile capacity in sand and both friction and end bearing components can become reduced. Scour does not cause problem in cohesive soils, but it should be considered in cohesionless soils (API-WSD, 2008). All codes include severe scour effect on lateral and axial pile performance and capacity, with design criteria for local and global scour. Scour reduces lateral soil support, which leads to increase in pile maximum bending stress. For an isolated pile, local scour depth of 1.5D and an overburden reduction depth of 6D is taken. Reduction of lateral soil support in cohesionless soils is due to lower ultimate lateral pressure caused by decreased vertical effective stress po, and decreased initial modulus of subgrade reaction modulus(ES) (API-WSD, 2008).

4.4 Axial Pile Capacity:

ISO takes Qp as the sum of the end bearing capacity of internal plug and end bearing on pile wall annulus and for designing in compression, weight of pile is also taken. Where as in WSD and LRFD the total end bearing Q should not go beyond the capacity of internal plug and while calculating pile loading and capacity, the weight of pile-soil plug system and hydrostatic uplift are also considered. All three codes take into account for load capacity of pile, relative deformations between soil and pile and compressibility of soil pile system. All three codes state that skin friction on upper bell surface of belled piles, at certain distance above the bell should not be included in calculating the skin friction resistanceQ . For belled piles, in calculating the total bearing area of the bell, the end bearing area of a pilot hole, if drilled, is not taken into account. Table 5 shows the compression and tensile capacity equations from the codes. Table 5: Code Provisions for Representative Axial Pile Capacity

API WSD & LRFD ISO 19902 Compression Qd= Qf + Qp = f * As + q * Ap Tension Qd= Qf = f As

Compression Qr= Qf + Qp = f * As + q * Ap Tension Qd= Qf = f As

4.4.1 Skin friction and end bearing in cohesive soils: For unconsolidated clays, α is assumed to be 1.0 in all three codes. Average ultimate skin friction and undrained shear strength (α) reduces with increasing level of over consolidation and increasing pile slenderness (Pile length over diameter ratio) (Briaud 1990). Care should be taken in calculating α when, i) C /P > 3, ii) Deep penetrating piles in soils with high undrained shear strength, where as ISO also takes into account here, low plasticity clay soil. Capacity reduction is warranted in long piles, where skin friction degrades on continued displacement. Soil strength (total axial resistance for pile compression) is given by external skin friction, end bearing on pile wall annulus and total internal skin friction or end bearing of plug whichever is less (Hansen, 1995).The bearing pressure acts over the entire cross-section of the plugged piles, where as for unplugged piles only the pile wall annulus is considered. The static evaluation depends on whether a pile is in plugged mode (pile acting as a closed ended) or unplugged mode (shear failure taking place between soil plug and pile shaft), or a pile is driven in unplugged condition but behaves as plugged under static conditions. Failure of pile will be in plugged mode if shear capacity along length of soil plug exceeds the end bearing capacity at base of plug (Randolph et al 1991). ISO/WSD also provides an alternative method for determination of pile capacity in clay. The unit skin friction, f may be less than or equal to undrained shear strength of clay cu. The following limits apply. With highly plastic clays f may be equal to cu for unconsolidated and normally consolidated clays. In case of over consolidated clays f should not exceed 48 KPa for shallow penetrations or equal to cu, for normally consolidated clay for deeper penetrations whichever is greater. For other clay types f varies linearly between the following limits f= cu for cu < 24 KPa (i) f= cu /2 for cu > 72 KPa (ii) Table 6: Pile axial capacity for cohesive soil

API WSD & LRFD ISO 19902

Where,

. for ψ 1.0

. for ψ 1.0 0.5 0.5

α 1.0 ψ c/p q = 9 c

Where,

. for ψ 1.0

. for ψ 1.0 0.5 0.5

α 1.0 ψ c /p q = 9cu

Axial pile capacity in clay is influenced directly by undrained shear strength and effective overburden stress profiles (API-WSD, 2008). ISO/ WSD also provide insight into the length effects on piles. Long piles driven in clay soil also experience capacity degradation due to installation conditions and soil behaviour. These are: i) Continued shearing of a particular soil horizon during pile installation, ii) Lateral movement of soil away from pile due to “Pile whip”, iii) Progressive failure in soil due to strength reduction with continued displacement (softening). Axial pile capacity in clay changes with time. The driven piles achieve ultimate strength in cohesive soil 2-3 years after installation(API-WSD, 2008; ISO 19902, 2007). The rate of strength gain is high soon after driving and it reduces during dissipation process (surrounding soil mass begins to consolidate and thus pile capacity increases with time). This

The Asia-Pacific Offshore Conference-APOC2010 Kuala Lumpur, December 13-14, 2010 process is called ‘set-up’ (API-WSD, 2008; ISO 19902, 2007). The speed of dissipation of excess pore pressure is related to coefficient of radial (horizontal) consolidation, pile radius; plug characteristics (plugged against unplugged pile) as well as soil layering (API-WSD, 2008; ISO 19902, 2007). 4.4.2 Skin friction and end bearing in cohesion less soils: Carbonate soils cover more than 35% of ocean floor. It differs from silica soils. The major constituent of carbonate soils is calcium carbonate (has low hardness) compared to quartz, which is present in dominant amount in silica sediments. Other pitfalls of carbonate soils are: i) it has large interparticle and intraparticle porosity, that results in high void ratio and low density and thus more compressible as compared to silica, ii) Mechanical behaviour of carbonate soils is affected as they are inclined to post-depositional changes by biological and physio-chemical process under normal pressure and temperature which results in formation of irregular and discontinuous layers of cemented material (API-WSD, 2008). Table 7 provides equations for axial capacity of piles. WSD and ISO equations are same but LRFD contains a K and tan factors. The values of K are: Open ended pipe piles under compression loading 0.8 Open ended piles driven under tension loading 0.8 Closed ended piles under compression loading 1.0 Closed ended piles under tension loading 1.0 These were also part of old WSD but after publication of ISO they have now been changed and formatted as ISO. Table 7: Pile axial capacity for cohesionless soil

API –WSD & ISO 19902 API-LRFD f β p q N p

tq p

an N

Table 8 and 10 gives classification of soils and adopts CPT based methods for certain type of soil, whereas LRFD still uses old methods. Table 9 provides relative density of soil. Table 8: API RP2A-WSD & ISO 19902 Provisions for design parameters for cohesionless siliceous soil

Soil type

Relative density Skin friction factor

Limiting skin friction values (f) KPa

End bearing factor (Nq)

Limiting unit end bearing values (q) Mpa

Sand

very loose note 1 note 1 note 1 note 1 Loose Medium dense 0.37 81 20 5 dense 0.46 96 40 10 Very dense 0.56 115 50 12

Sand silt

Loose note 1 note 1 note 1 note 1 Medium dense 0.29 67 12 3 dense 0.37 81 20 5 Very dense 0.46 96 40 10

Silt Medium dense note 1 note 1 note 1 note 1 Dense

Note 1: ISO and API WSD recommends that values given by API LRFD here are unconservative therefore use CPT- based methods for these soils. Note 2: Strength values generally increase with increasing sand fractions and decrease with increasing silt fraction. Table 9: Relative Density (API WSD & ISO 19902)

Soil description Relative density (%) Very loose 0-15 Loose 15-35 Medium dense 35-65 Dense 65-85 Very Dense 85-100

Table 10: API RP2A-LRFD Provisions for design parameters for cohesionless siliceous soil

Soil type

Relative density

Soil pile friction angle

Limiting skin friction values (f) KPa

End bearing factor (Nq)

Limiting unit end bearing values (q) Mpa

Sand very loose 15 47.8 8 1.9 Loose 20 67 12 2.9 Medium dense

25 81.3 20 4.8

dense 30 95.7 40 9.6 Very dense 35 114.8 50 12

Sand silt

Loose 15 47.8 8 1.9 Medium dense

20 67 12 2.9

dense 25 81.3 20 4.8 Very dense 30 95.7 40 9.6

Silt Medium dense

15 47.8 8 1.9

Dense 20 67 12 2.9 Gravel dense 35 114.8 50 12

For pipe piles in cohesionless soils, unit skin friction f is given by equations in table 8 and 10. LRFD equations have tangent function where as WSD/ISO has different equation without tangent function. β values for driven open ended unplugged piles are given in table 10. For driven, full displacement piles (closed ended or fully plugged open-ended piles) 25% higher values should be taken (API-WSD, 2008; ISO 19902, 2007). End bearing piles, q =N Po, for long piles q does not necessarily increase linearly with over burden pressure. Shaft friction f acts on inside and outside of piles and total resistance in excess of the external shaft friction plus annular end bearing is total internal shaft friction or the end bearing of the plug, whichever is less (API-LRFD,2003). Bearing pressure assumed to act over the whole cross section of pile in case of plugged piles where as it acts on pile annulus only for unplugged piles, additional resistance is offered by frictional resistance between soil plug and inner pile wall. Piles in cohesion less soil show that variability in capacity predictions can exceed piles in clay using this method. It is conservative for short offshore piles (<45m) in dense to very dense sand loaded in compression (API-WSD, 2008; ISO 19902, 2007). Unfamiliar situation warrant us to use conservative design parameters and high safety factors. It is useful in case where force redistributes after the development of maximum resistance occurs which leads to brittle failure for example the case of short piles in tension. ISO / WSD provide guidance for four methods in this regard which are to be used cautiously, where as LRFD do not give any guidance. In annexure of ISO CPT-based methods incorporate length and friction fatigue. Here it is assumed that piles are open ended steel piles with uniform outer diameter. Piles are installed by impact driving up to considerable depths of siliceous sand, these piles are drived unplugged. Due to static load in compression, inner friction causes the pile to act as fully plugged. Drilled and grouted piles in carbonate sands may have higher capacities as compared to driven piles (API-WSD, 2008; ISO 19902, 2007). It is reported that capacity increases in carbonate soils of

The Asia-Pacific Offshore Conference-APOC2010 Kuala Lumpur, December 13-14, 2010 high densities and higher quartz contents and cementation increases end bearing capacity but on the other hand results in loss of lateral pressure as well as decrease in frictional capacity (API-WSD, 2008; ISO 19902,2007). API-WSD, 2008 reports that offshore sands are most of time very dense and often silty. For cohesion less soils CPT based methods for prediction of pile capacity are adopted, which are based on direct correlations of pile unit friction and end bearing data with cone tip resistance values based on CPT. 4.4.3 Skin friction and end bearing of grouted piles in rock: It is reported in ISO that unit skin friction of grouted piles in jetted/ drilled holes in rock should remain below the half (1/2) compressive strength of rock whereas API reports that unit skin friction of piles should not exceed triaxial shear strength of rock or grout. This is made clear when dry compacted shell is exposed to water from jetting, which gets its strength reduced (API-WSD, 2008). ISO tells that end bearing capacity of rock should not exceed uniaxial compressive strength of rock or grout multiplied by bearing capacity. As per ISO end bearing capacity may be ignored or taken less depending on i)pile construction factors (roughness of sides of hole), ii) rock factors (discontinuities). API gives minimum end bearing capacity of rock as 9.6 MPa, ISO calls these values as based on onshore piles and thus does not support it, unless site specific data proves it. 4.5 Pile capacity for axial tension:

All codes specify that ultimate pile pullout capacity should not be greater than total skin friction resistance (Qf). For design of ultimate pullout capacity, the effective weight of pile, hydrostatic uplift, and Soil plug are considered. Skin friction is calculated, for cohesive, cohesionless and rock samples as described above. 4.6 Axial Pile Performance:

ISO defines difference between axial capacity and pile performance as i) axial resistance ii) specified service requirement (from platform owner) like deflections at pile head, no such definition is provided by API. As per ISO axial capacity and performance depend on type of soil, pile characteristic, Installation methods and Characteristic of applied actions. 4.6.1 Static axial Load-deflection behaviour: Pile axial deflections should be within acceptable serviceability limits. These deflections should be compatible with internal forces and structure movements. Analytical method to find out axial pile performance has been discussed in Meyer(1975) and Focht and Kraft(1986). The methods make use of axial pile shear transition vs Local pile deflection (t-z) curves to model the axial support provided by the soil along the pile. Axial behaviour depends on direction, type, rate and sequence of applied actions, installation technique, soil type and axial pile stiffness. The Q-z curve is used to model the tip and bearing vs the deflection response. Soils which exhibit strain softening behaviour and when piles are axially flexible, the actual capacity of pile can be less than representative capacity. The bigger axial capacity under loading rates during storm waves may neutralize the above effects. 4.6.2 Cyclic Axial behaviour: Cyclic (inertial) actions due to environmental conditions (wind, waves, current, earthquake and ice floes) have two counteractive effects on static axial capacity. They may decrease load carrying resistance and accumulate deformation. Rapid action can produce (due to high rates of

change of action): i) increase in resistance/capacity, ii) increase in stiffness of pile While slow action can produce, (due to repeated actions): i) decrease in resistance/capacity, ii) decrease in stiffness of pile. Thus resultant action includes: magnitudes, cycles and rates of change of applied actions, Structural characteristics of pile and types of soil. Factors affecting capacity specified by different codes are listed in table 11. Table: 11 Parameters influencing on capacity of soil. (*Only ISO specify these) Pile properties 

Soil characteristics

Action characteristics 

Cyclic actions

Stiffness Type Number of repeated action

Accumulation of pile displacements

Length stress Magnitude of repeated action

Any of following: i)Stiffening & strengthening of soil around pile ii) Softening & weakening of the soil

diameter History Duration of action *

Response to resistance*

Material Strain rate ‐‐‐  ‐‐‐ Wall thickness *

Cyclic degradation

‐‐‐  ‐‐‐ 

Weight * ‐‐‐  ‐‐‐  ‐‐‐  Earthquake ground motions may cause cyclic straining effects in soils which may affect pile capacity and stiffness. The design pile penetration is confirmed through pile response analysis of pile-soil system under static and cyclic loading. ISO provides performance requirement guidelines for piles for static and cyclic capacity based design as shown in table 12. Table: 12 Static and cyclic capacity based design

static capacity based design cyclic capacity based design Factored permanent and variable actions along with factored extreme environmental actions are compared with factored pile capacity ( integrated shaft and tip resistance)

In addition to static capacity based design, pile should have a capacity which gives reserve above design forces, i.e. pile should not settle or pull out nor build up displacements to the extent which can cause failure of foundation system.

4.6.3 Overall axial behaviour of piles: When unusual conditions of actions on piles or limitations on design penetrations are encountered, the static and cyclic forces are imposed on pile top and displacement is determined. After completion of design forces, the maximum pile resistance and displacement are determined. 4.7 Soil reaction for piles under axial compression:

Axial resistance of soil for pile compression, given by all codes, is the sum of axial soil pile adhesion and associated shear transfer along the sides of pile and end bearing resistance at the pile tip. The t-z curves give relationship between mobilized soil pile shear transfer and local pile displacement at any depth, where as Q-z curves give relationship between mobilized end bearing resistance and axial tip displacement.

The Asia-Pacific Offshore Conference-APOC2010 Kuala Lumpur, December 13-14, 2010 4.7.1 Axial shear transfer (t-z curves): The t-z curves for non carbonate soil are given in all codes. The value of residual strain ratio (tres/tmax) for clay range between 0.70 and 0.90. 4.7.2 End bearing resistance-displacement (Q-z curve): Large pile tip displacement is needed to activate full end bearing resistance. In all codes, pile tip displacement up to 10% of pile diameter is required for full mobilization in sand and clay soils. 4.8 Soil reaction for piles under lateral actions:

Pile foundation is designed to resist static and cyclic lateral actions. Lateral resistance of soil near surface level is important for pile design and effects of scour on resistance are taken care of during design. The relationship between lateral soil resistance and lateral displacement p-y curves is given. In all codes, under lateral loading, clay soils is assumed to behave as a plastic material which makes it compulsory to relate pile soil deformation to soil resistance. ISO gives guidelines for scour reduced lateral soil support which leads to increase in pile maximum bending stress. Scour should be considered for cohesionless soil. Types of scour identified by ISO are general scour and local scour. For isolated pile local scour depth equal to 1.5 D and overburden reduction depth equal to 6 D may be adopted (ISO19902,2007). The causes for reduction in lateral soil support are a) lower ultimate lateral pressure due to decreased vertical effective stress, b) decreased initial modulus of subgrade reaction modulus (ISO19902, 2007). 4.8.1 Representative lateral capacity for soft clay: Table 13 provides equations for capacity of soft clay by different codes. ISO considers geometry whereas WSD and LRFD do not. Table: 13 Lateral capacity for soft clay

A IP WSD & LRFD ISO

p 3c γX J DcX

p 9c for X XR

XR 6D

γDc J

Minimum value of XR= 2.5 Pile diameter

Pr = 3*cu* D = p D Jc X but Pr is limited by: P XR

.C .D γ.D J.C

r = 9*cu* D for X> XR

Minimum value of XR> 2.5 Pile diameter

Under static lateral actions, the unit lateral capacity Pr of soft clay in ISO varies between 8cu* D and 12cu* D. Cyclic actions cause deterioration of lateral capacity, which increases as X increases from 0 to XR 3cu* D to 9cu* D. Under static lateral actions, in API WSD and API LRFD, the ultimate unit lateral bearing capacity of soft clay pu varies between 8c and 12c. Cyclic actions cause deterioration of lateral capacity, which increases as X increases from 0 to XR 3cu* D to 9cu* D. 4.8.2 Lateral soil resistance-displacement p-y curves for soft clay Piles in soft clay, have non linear, lateral soil resistance-displacement relationship. p-y curves for short term static actions as well as the case where equilibrium is reached under cyclic actions are given in tables. Here API WSD updated its values following ISO in year 2008 errata, but as API LRFD is yet to be updated so it does not contain some

values in the table 14. Table 14: Lateral soil resistance-displacement p-y curves for soft clay (short term static data)

API RP2A LRFD ( lateral soil resistance – deflection)

API WSD / ISO 19902 (lateral soil resistance – displacement)

Short term Static Load Short term Static Load p/pu y/yc p/pu (WSD)or

p/ pr (ISO) y/yc

0 0 0 0 --- --- 0.23 0.1 --- --- 0.33 0.3 0.50 1.0 0.50 1.0 0.72 3.0 0.72 3.0 1.0 8.0 1.0 8.0 1.0 ∞ 1.0 ∞Pu= Lateral ultimate resistance P= Actual lateral resistance Y= Actu l Lateral deflection a

=2.5* *D

Yc

= strain at one half maximum stress in laboratory undrained compression tests of undisturbed soil samples.

pu= ultimate resistance Pr= Lateral capacity p= Actu lateral resistance al

me*D

y= Lateral deflection/ splace nt di

Yc=2.5* = strain at one half maximum

deviator stress in laboratory undrained compression tests of undisturbed soil samples.

Table 15 shows lateral soil resistance-displacement values which are updated in API WSD but yet to be incorporated in LRFD. Table 15 Lateral soil resistance-displacement p-y curves for soft clay (equilibrium conditions of cyclic actions)

API RP2A LRFD ( lateral soil resistance – deflection)

API WSD / ISO 19902 (lateral soil resistance – displacement)

Cyclic Load Cyclic Load x>xR x<xR x>xR x<xR p/pu y/yc p/pu y/yc p/pr y/yc p/pr y/yc 0 0 0 0 0 0 0 0 --- --- --- --- 0.23 0.1 0.23 0.1 --- --- --- --- 0.33 0.3 0.33 0.3 0.5 1.0 0.50 1.0 0.50 1.0 0.50 1.0 0.72 3.0 0.72 3.0 0.72 3.0 0.72 3.0 0.72 ∞ .72 x/xR 15.0 0.72 ∞ .72 x/xR 15.0 --- --- .72 x/xR ∞ --- --- .72 x/xR ∞

4.8.3 Representative lateral capacity for stiff clay: All three codes specify that unit lateral capacity Pr of stiff clay, (c > 96 kPa) is same as for soft clay as shown above. Due to deterioration under cyclic actions, lateral capacity is reduced for cyclic design. 4.8.4 Lateral soil resistance- displacement p-y curves for stiff clay: It is reported in all codes that stiff clay has non-linear stress strain relationship and they are brittle than soft clays. Rapid deterioration of lateral capacity occurs at large displacement for stiffer clay. 4.8.5 Representative lateral capacity for sand: The lateral capacity at shallow and deep depths due to static lateral load in sand is given by two separate equations provided in table 16.

The Asia-Pacific Offshore Conference-APOC2010 Kuala Lumpur, December 13-14, 2010 Table 16: Lateral capacity of sand

lateral p and ca acity for sWSD LRFD ISO Pus= Pud=  

Pus= D γX C X CPud= C D γ X 

Prs= Prd=

4.8.6 Lateral soil resistance- displacement p-y curves for sand: All three codes use same equations for lateral soil resistance-displacement behaviour (p-y curves) for piles in sand (Table 17). The equations are nonlinear and their internal angle of friction lies between 34 and 42 . Table 17: Lateral soil resistance displacement p-y curves for sand API WSD & LRFD ( Replace H by X notation)

ISO

. t. an ..

.

A= 3 . HD

0.9 (static loading) A= 0.9 (cyclic loading) 

. . tan ..

.

A= 3 . XD

0.9 (static actions) A= 0.9 (cyclic actions)

ISO gives values of rate of increase with depth of initial modulus of sub grade reaction k as table 18. Whereas WSD and LRFD give a graph of relative density to find k values separated by sand above water table and sand below water table. WSD and LRFD use ultimate capacity in place of representative capacity as used by ISO. Table 18: Values of sub grade modulus K

K (MN/m3) K (MN/m3) 25 5.4 35 22 30 11 40 45

4.9 Pile group behaviour:

Effects of closely spaced piles on resistance-displacement characteristics of pile group are considered for design. When pile spacing is less than eight diameters, group effects are to be ascertained. Elastic single pile resistance to general actions (axial, lateral and torsional) are determined. Group resistance is calculated by taking into account elastic pile-soil-pile interaction. 4.9.1 Axial behaviour: Piles in clay have their group capacity less than single pile capacity multiplied by number of piles in group, whereas piles in sand have their group capacity higher than sum of capacities of isolated piles. Group settlement in clay or sand is larger than that of single pile subjected to average action per pile of the pile group. The group effects depend on pile group geometry and penetrations and thickness of any bearing strata underneath the pile tips. 4.9.2 Lateral behaviour: Piles with same pile head fixity, in cohesive or cohesion less soils, group experiences more lateral displacement as compared to single pile subjected to average action per pile of corresponding group. The factors affecting group displacements and actions pointed out by all codes are pile spacing, ratio of pile penetration to pile diameter, pile flexibility relative to soil, dimensions of group, variations in shear strength and

stiffness modulus of soil with depth. 4.10 Pile wall thickness:

Wall thickness of pile varies along its length and is controlled by load conditions or requirements. Restraints put over pile by the structure and by soil are used to check allowable pile stresses for section by the pile not laterally restrained by soil. Pile buckling below mud level is considered only if pile is thought to be laterally unsupported because of extremely low soil shear strength, large computed lateral deflections. Pile wall thickness near mud level is controlled by combined axial force and bending moment due to design loading conditions of platform. It is assumed in design that axial load is removed from pile by soil at a rate equal to ultimate soil-pile adhesion divided by the appropriate pile safety factor (API-WSD, 2008; API-LRFD, 2003). 4.10.1 Check for load case due to weight of hammer during hammer placement: The load case is limiting factor for maximum length of add-on sections in case of piles driven or drilled in batter. Checks are to be made for the design conditions of static bending, axial forces and lateral forces generated during initial hammering procedure. WSD takes pile projection as freestanding column with minimum effective length factor of k is 2.1 and minimum reduction factor Cm is 1.0. WSD also gives guidelines for bending moment and axial loads, i.e. full weight of pile hammer, cap and leads acting at corresponding centre of gravity of their combined masses along with weight of pile add-on- section with consideration to pile batter eccentricities. LRFD takes full factored weight of hammer, pile cap and leads which acts at C.G. of combined masses ( γD 1.3 or γL 1.5 weight of add on sections taking into account batter and centre of mass eccentricities(API-LRFD,2003).However bending moment determined should not be less than corresponding to load equal to 2% of combined weight of hammer, cap and leads applied to pile head and perpendicular to its centreline (API-WSD, 2008; API-LRFD,2003). In this case one third increases in stresses are not allowed (API-WSD, 2008). 4.10.2 Stresses during driving: Stresses in free standing pile section during driving should satisfy the criterion: Sum of stresses due to impact of hammer (dynamic) and stresses due to axial load and bending (static) should not exceed minimum yield stress of steel (API-WSD/ISO 80%-90% of yield). Dynamic stresses are determined through wave propagation theory (API-WSD, 2008; API LRFD, 2003; ISO 19902, 2007). It maybe assumed column buckling will not occur due to dynamic component of driving stresses. Static stresses include weight of pile above point of evaluation plus pile hammer weight supported by pile during hammer blows, including related bending stresses (API-WSD, 2008;). WSD does not allow one third increase in stresses (API-WSD, 2008), whereas LRFD takes a dead load factor of 1.6 for all static loads. 4.10.3 Minimum wall thickness: The D/t ratio of a pile shall be small so that local buckling is avoided at stresses up to yield strength. API WSD/ LRFD give minimum pile wall thickness, where continued hard driving of 820 blows per meter with biggest size hammer is used which is: 6.35 . Table 19 show the standard API values for pile diameter and thickness. ISO provides some checks in case of high D/t ratio; minor local damage near pile tip can spread during installation which can cause deformation and collapse of pile. Besides that pile bottom section should be checked for all load cases occurring during handling to avoid

The Asia-Pacific Offshore Conference-APOC2010 Kuala Lumpur, December 13-14, 2010 local damage. Table 19: Minimum Pile wall thickness in API-WSD and API-LRFD

Pile dia (D) mm

Nominal wall thickness (t) mm

Pile dia (D) mm

Nominal wall thickness (t) mm

610 13 1829 25 762 14 2134 28 914 16 2438 31 1067 17 2743 34 1219 19 3048 37 1524 22 ----- -----

4.10.4 Allowance for under drive and overdrive: Piles having thick sections at sea floor, extra length of heavy wall needs to be provided in vicinity of sea floor so that pile wall not to be overstressed at this point if design penetration is not reached i.e. under drive. Similarly overdrive allowance is allowed in the case of expected bearing stratum is not reached at anticipated depth. 4.10.4 Driving shoe: It assists piles to penetrate through hard layers or reduce driving resistance. If an internal driving shoe is provided then it should be ensured that high driving stresses do not occur at and above transition point between normal and thickened section of pile tip. ISO reports that pile buckling and pile refusal occurring in very dense sands are associated with external chamfers at the pile tip. 4.11 Length of pile sections

Based on all codes, the important points in the selection of pile section lengths are: capacity of lift equipment to raise, lower and stab the sections, potential of lift equipment to place the pile driving hammer on the sections to be driven, Possibility of downward pile movement due to penetration of jacket leg enclosure, development of stresses in pile section while lifting, Wall thickness and material properties at field welds, avoid interference with planned concurrent driving of adjacent piles, Type of soil where pile tip is positioned during driving interruptions for field welding for additional sections, Each pile should contain cut-off allowance due to damaged material from pile driving hammer, which is 0.5 to 1.5 meters per section. This section is placed at accessible elevation. 5.0 CONCLUSIONS The API and ISO code equations for calculating pile capacity are somewhat identical but some differences have been highlighted. API WSD has been updated now to be at par with ISO 19902 especially for axial capacity of cohesionless soil, soil reaction for laterally loaded piles. The conclusions from the study are summarised below. ISO includes vibro driven piles in addition to other types of Pile foundations which is not included by API. There is difference in resistance factor for operating conditions for API LRFD and ISO. WSD safety factors have not been changed since its first edition. There are differences between recommended values for the parameters used in finding out capacity in cohesionless soil. ISO and API WSD omits gravels in soil classification whereas API LRFD includes it. For very and loose sand, loose sand-silt, medium dense silt and dense silt, ISO and API WSD (21st edition-errata, 2008), calls API LRFD design parameters as unconservative and recommends CPT based methods. ISO has a different equation for calculating shaft friction for cohesionless soil replacing K*tan with a factor . The values for open

ended pipe piles are same in both ISO and API. Whereas API LRFD takes plugged or closed end piles also, which are no more part of ISO and API WSD (21st edition-errata, 2008). ISO and WSD uses CPT based methods to find capacity for certain soils where as LRFD has not been updated and uses old method to calculate capacity. API give various D/t ratios where as ISO provides some checks in this regard.

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