Subsea Pile and Foundation Design
The increasingly more robust and specialized anchoring systems with piles for drilling and production systems have found wide applications in the exploration and exploitation of hydrocarbons offshore from shallow water to deepwater. The anchoring systems were designed based on soil conditions and load considerations and can be categorized into three groups:
(1) mudmat,
(2) suction pile, and
(3) pile.
This section provides a general description of the design of suction piles and foundations, including:
- Design methodology: selection of appropriate design code and relative safety factors;
- Geotechnical profiles: interpretation and selection of design soil profiles and geotechnical parameters;
- Pile capacity and sizing: calculation of in-place resistance and pile capacities for pile sizing;
- Detailed structure design: verification of suction pile responses under the design loadings;
- Installation and retrieval: analyses of installation/retrieval.
Contents
Design Methodology
The API RP design codes ensure adequate foundation safety using the working stress design method. The partial coefficient method is used in DNV codes, by which the target safety level for the structure is reached by applying partial coefficients to the characteristic loads and soil parameters and taking into account the reliability of the design data. Suction pile design includes preliminary pile sizing and final structure design. In the preliminary pile sizing, the pile aspect ratio (L/D) versus steel weight should be optimized prior to the final suction pile design. Suction pile sizing assumes that the pump-in suction pressure is acceptable for the available soil strength (i.e., soil plug stability check) and that the pump-out pressure is acceptable for the available soil strength.
The suction pile foundation design should account for the loads in the lift, transportation, installation, penetration, and in-place conditions:
- Seabed slope, installation tolerances, and effects from possible scouring;
- Soil bear capacity and bear stress should satisfy API RP 2A-WSD;
- Stability analysis of the manifold should include overturning, sliding, and shearing resistance;
- Suction loads due to repositioning or levelling;
- Invasion of soil into pile sleeves should be prevented;
- For foundation and skirt systems, arrangements should be made for air escape during splash zone transfer and water escape during seabed penetration. Lift stability and wash-out of soil should be taken into account;
- Structures with skirt foundation should be designed for self-penetration;
- Skirt-system facilities for suction and pumping should, where required, be included to allow for final penetration, leveling, and breaking out prior to removal. The suction and pump systems should be operated in accordance with the selected intervention strategy;
- Settlement of the structures for initial settlement and long-term deformation (during installation and lifetime) should be accounted for. Designersmust ensure that the pump capacity will be sufficient to provide adequate flow at the required pressure in order to allow efficient operations. This must be verified by testing the pump in submerged condition.
Codes and Standards
The following is a list of codes and standards that can be used for the design of suction piles:
- API RP 2SK: Recommended Practice for Design and Analysis of Station keeping Systems for Floating Structures;
- API RP 2A-LRFD: Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms;
- API RP 2A-WSD: Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms;
- AISC Manual of Steel ConstructiondLRFD;
- API Bulletin 2U: Stability Design for Cylindrical Plates;
- API Bulletin 2V: Design of Flat Plate Structures;
- DNV, Rules for Classification of Fixed Offshore Installations, 1995;
- DNV-RP-B401, Recommended PracticedCathodic Protection Design;
- DNV-RP-E303, Geotechnical Design and Installation of Suction Anchors in Clay.
Design Loads
Permanent Loads
The permanent load will not vary in magnitude, position, and direction in the consideration period. It includes the dead load of the structure and any long-term static load applied to the subsea structure.
Live and Dynamic Loads
A live load is applied to the foundation during installation or the considered working period. It may vary in magnitude, position, and direction.
Environmental Loads
The influence of environmental factors such as current and wave action can result in significant hydrodynamic loads on installations, which are typically transferred as dynamic loads to the foundations.
Accidental Loads
An accidental load considers the load due to fishing gear snagging or impact of dropped objects.
Geotechnical Design Parameters
Reliable information concerning the soil layering and properties should be collected. The soil investigation program should include high-quality geophysical and geotechnical surveys. The geophysical survey allows geohazards to be detected and also provides for a regional geological overview. The geotechnical survey should allow for the collection of high-quality soil samples in as nearly an undisturbed state as feasible. In situ geotechnical tests (such as cone penetrometer tests, in situ shear vane tests) should be performed.
The depth of the geotechnical borings should exceed the anchor’s penetration. The number of these borings should be defined as a function of the soil variability. One boring should be performed at each anchor location when lateral variability of the soil properties is expected. The following main soil properties are needed for the design of suction anchors:
- Index properties;
- In situ stresses and stress history;
- Undrained shear strength;
- Drained characteristics;
- Consolidation characteristics;
- Interface strength and thixotropy.
Index Properties
The index properties (including water content, unit weight, plasticity index, grain size distribution, and organic content) are needed to characterize the soil. The unit weight, as deduced from laboratory tests, should be in agreement with the bulk density of the soil evaluated by core logging (gamma-ray absorption).
In Situ Stresses
The in situ effective stresses and stress history are mainly needed to establish the consolidation stresses for laboratory tests. The in situ stresses are evaluated from the following geotechnical parameters: unit weight, pore pressure, and coefficient of earth pressure at rest. The stress history is commonly expressed by an overconsolidation ratio (OCR).
Undrained Shear Strength
The undrained shear strength is a key parameter for the design. The variation of this parameter with the stress path should be assessed. Anisotropic triaxial compression (CAUc), triaxial extension (CAUe), and direct simple shear tests (DSS) are necessary to derive the undrained shear strength profiles. The in situ cone penetration tests (CPTs) generally provide the trend in the variation of the undrained shear strength with depth. The in situ shear vane allows in situ measurement of the undrained shear strength. The undrained cyclic shear strength has to be evaluated to calculate the capacity under cyclic loads. The undrained cyclic shear strength should be obtained for various stress paths. The remolded shear strength is an important parameter because it is used to calculate the penetration resistance of the skirt walls. The remolded shear strength profile may be deduced from laboratory tests (fall-cone, laboratory vane tests) and in situ tests (mainly from in situ vane tests).
Drained Characteristics
The drained friction angle and its associated cohesion have to be evaluated when the suction anchor is required to support long-term loading.
Consolidation Characteristics
The consolidation characteristics include compressibility modulus, Poisson’s ratio, coefficient of permeability and the OCR. These characteristics are needed for anchor capacity calculations under long-term loads.
Thixotropy
Thixotropy is the increase in the undrained shear strength of remolded clay with time and without any consolidation effect.
Suction Pile Sizing and Geotechnical Design
Pile Axial, Lateral, and Torsional Capacities
Suction anchors can be designed for different loading conditions:
(1) mooring applications for floating production units; in this case the suction anchor should resist to an inclined mooring load where the vertical component may be highwhen fiber ropes are used;
(2) anchoring for TLPs and other structures such as riser towers where the suction anchor should support uplift loading; and
(3) foundations for subsea structures where the anchor should resist to a compression load combined with horizontal load and moment.
For the suction pile foundations used to support the manifolds, the foundations should have sufficient capacities to withstand all loads from the manifold system, including compression, lateral, moment, and torsion under both subsea equipment installation and operating conditions. The safety factors are determined in the design basis. As an example, the following safety factors (FOS) were used in a GoM application:
Operating Conditions
FOS = 1.9 for the axial failure; FOS = 1.9 for the lateral failure; FOS = 1.5 for the torsional failure.
Installation Conditions
FOS = 1.5 for all type of failures.
The safety factor of 1.9 for the axial failure and lateral failure includes a load factor of 1.3 for dead loads and a resistance factor of 0.7 for soil axial capacity. The safety factor for the torsional failure is reduced to 1.5 due to the fact of the consequence of torsional failure is less severe than axial and lateral failures. However, the negative impact of torsional loading on the axial capacity and lateral capacity of the system’s foundation should be considered during the design stage. A safety factor against all type of installation loading is reduced to 1.5 due to the nature of their temporary, short period of presentation, which also agrees with API RP 2A.
Suction piles should be designed to provide adequate lateral and axial capacities. The tasks include:
- Foundation stability;
- Penetration resistance and the accompanying required suction for final penetration;
- Retrieval resistance and the accompanying required overpressure;
- Vertical settlements of the manifold;
- Maximum horizontal displacements;
- Maximum responses due to the impact when landing the suction pile anchor and the interface guide base at the seabed.
Stability Analyses
Stability calculations are performed for the following two separate conditions:
- Vertical load in combination with torsion moment, utilizing the skin friction and plugged end bearing of the suction pile anchor;
- Horizontal load in combination with an overturning moment, utilizing the shear strength of the soil surrounding the anchor. The combined vertical and torsion resistances are solved by computing the allowable skin friction according to API and the maximum value applicable for suction-assisted penetration beyond the self-penetration depth.
Penetration Analysis
A penetration analysis involves the calculation of skirt penetration resistance, underpressure needed to achieve the target penetration depth, allowable underpressure (underpressure giving either large soil heave inside the skirts or cavitations in the water) The penetration analysis is commonly performed using the general principles given in Andersen and Jostad. Such an analysis includes the following parameters:
- Penetration resistance;
- Self-weight penetration of the anchor;
- Required underpressure as a function of depth;
- Allowable underpressure as a function of depth;
- Soil heave as function of depth;
- Maximum penetration depth.
Installation Impact Response
The impact response analysis of lowering the suction pile to the seabed is to check the limiting heave motions of the pile close to the seabed. A range of crane lowering speeds should be used considering the available water evacuation areas, suction pile anchor geometry, and prevailing soil conditions. The check criteria are to avoid soil-bearing capacity failures and to avoid high hydrodynamic pressures within the suction pile anchor that could influence its structural design.
Suction Structural Design
The suction pile should be designed to withstand the following loads:
- Maximum loads applied and equilibrated by the soil reactions;
- Maximum negative pressure (underpressure) required for pile embedment;
- Maximum internal pressure (overpressure) required for pile extraction;
- Maximum loads imposed on the pile during lifting, handling, launching, lowering, recovery, etc.
The maximum horizontal and vertical loads should be used for the global structural design of piles. A structural finite element model may be used for the global structural pile analysis to ensure that the pile wall structure and appurtenances have adequate strength in highly loaded areas. The structural components of the suction pile should be designed in accordance with the applicable provisions of API RP 2A, AISC, and API Bulletins 2U and 2V. In general, cylindrical shell elements should be designed in accordance with API RP 2A or API Bulletin 2U, flat plate elements in accordance with API Bulletin 2V, and all other structural elements in accordance with API RP 2A or AISC, as applicable.
In API RP 2A and AISC, allowable stress values are expressed, in most cases, as a fraction of the yield stress or buckling stress. In API Bulletin 2U, allowable stress values are expressed in terms of critical buckling stresses. In API Bulletin 2V, allowable stresses are classified in terms of limit states. Two basic limit states are considered in API Bulletin 2V: ultimate limit states and serviceability limit states. Ultimate limit states are associated with the failure of the structure, whereas serviceability limit states are associated with adequacy of the design to meet its functional requirements. For the purposes of suction pile design, only the ultimate limit state is considered in design.
Allowable Stresses and Usage Factors
For structural elements designed in accordance with API RP 2A or AISC, the safety factors recommended in API RP 2A and AISC should be used for normal design conditions. For extreme design conditions, the allowable stresses may be increased by one-third. For structural elements analyzed using finite element techniques, the von Mises (equivalent) stress should not exceed the maximum permissible stress as shown here:
s = h0sy
where h0 is the basic usage factor, and sy is the material yield strength. The basic usage factor h0 is 0.8 for the maximum in-place loading condition and 0.6 for normal operating, transportation, lifting, lowering, and recovery conditions. The permissible stresses are based on the fiber stresses for simple beam analyses, and the membrane or midthickness stresses for FEAs using plate elements. For laterally loaded plates also exposed to in-plane (e.g., membrane) stresses, the surface von Mises stress computed at the middle of the plate field (e.g., midway between stiffeners and/or girders) should not exceed the following:
sp = ðh0 þ 0:1Þ sy
The nominal elastic stress calculated in the middle of the plate field due to lateral pressure acting alone should not exceed h0sy.
Buckling Checks
Hydrostatic buckling calculations should be performed in order to check the capacity of the shell wall of a pile during pile embedment for local shell buckling. Although the buckling of the shell wall is not a concern for operating conditions due to the soil supporting the structure, it is a potential concern during installation. When the pile is penetrating the soil and is subjected to differential embedment pressures, there is a potential for local buckling due to axial and hoop stress interactions. The design minimum buckling collapse pressure of the pile should have an adequate safety margin to ensure the shell of pile is strong enough to resist the maximum possible suction pressure. In addition, an adequate safety margin between the design minimum buckling pressure and the required suction pressure for embedment of the pile should be applied. A closed, unstiffened cylinder under hydrostatic pressure will buckle between end supports by forming a pattern of circumferential lobes.
The lobes formed around the circumference, or the buckling mode, are a function of the unsupported shell length-to-diameter ratio L/D and cylinder D/t ratio. Buckling may be purely elastic or a combination of elastic deformation and elastoplastic deformation. In the case of suction piles, the D/t and L/D ratios are such that elastic buckling is the predominant mechanism. Two analytical approaches are available for determining the buckling capacity:
- Design code-based buckling capacity analysis methods using an estimated unsupported length of shell;
- Finite element analysis of buckling behavior incorporating full soil/structure interactions to explicitly model the degree of soil support to the shell of a suction pile.
A number of design codes provide analysis methods to determine the design buckling strength of a suction pile. The methodology detailed in API RP 2A for combined axial compression, hydrostatic pressure, and bending may be used to check the pile wall for local buckling during the embedment process. The semiempirical technique used in API RP 2A is appropriate for pile diameter–to–wall thickness ratios of less than 300 (D/t < 300), the methodology must be adapted to account for multiple wall thickness cans between ring frames. This is accomplished by using a weighted average wall thickness in the calculations that is a function of effective buckling length. The effective buckling length will decrease as the pile penetrates the soil and the part of the pile sticking up above the mudline decreases. As the pile penetrates deeper into the soil, the soil will progressively support the pile wall and increase the buckling capacity by forcing the wall structure into a higher mode of buckling failure.
Transportation Analysis
A transportation analysis should be carried out for the following procedures:
- The pile structure is handled and transferred to the transportation barge.
- During transportation to the installation site, the piles in the transportation barge are normally supported at two locations with cradles. The cradles will take the roll forces, while pitch plates welded to the pile and the barge deck will take pitch forces.
- Before installation into the seabed, the pile structure is upended from a horizontal to a vertical position.
The loads considered in the transportation analysis should include the self-weight of the suction pile and inertial loads developed from a motion analysis. A dynamic load factor of 2.0 should be used in the design calculations according to API RP 2A-WSD. The calculations for padeye design can be carried out by hand, and the padeye support structures should be checked by an FEA method for the simulation of all pile structures. Lift analysis should be performed to check the structural integrity of the suction pile and lift attachments under installation and removal onto the transport barge and pile lowering and recovery.
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