1. Structural models shall represent all effects contributing to loads, actions, stiffness, etc., of a structure and shall contain sufficient detail to accurately represent the weight, buoyancy, stability, stiffness and environmental force characteristics of the structure.
2. An integrated 3D model of the jacket, piling, major appurtenances, and deck contributing to the stiffness of the structural system shall be used for in-service analyses.
a. For substructure design, the deck shall be modeled in sufficient detail to accurately represent stiffness and interaction with the jacket.
b. The effect of the added thickness of jacket members for corrosion allowance shall not be used in the in-service analyses (i.e., operating storm, design event, seismic, etc.).
3. Pre-service analysis models shall include any pre-installed sections of piling and/or appurtenances.
4. The additional strength of the increased wall thickness for corrosion allowance may be used in the pre-service analyses.
5. If the eccentricity of a brace at a joint is more than D/4 or 12 inches (300 mm), then the eccentricity of the brace shall be modeled. A separate joint shall be coded on the chord member along the line of the eccentricity with the brace/member connected to that joint.
6. Computer models shall include all primary structural members. Joint cans and brace stubs shall be modeled for primary members. In some cases it may be appropriate to model a joint along a member at wall-thickness transitions such as at the ends of thick-walled joint cans.
Supplier shall discuss with Purchaser where this should be done.
7. Dead loads and environmental loads on secondary members and appurtenances shall be included in models.
8. The computer model shall properly take into account the way the piles are connected to the legs.
a. For non-grouted piles, the piles and legs shall be modeled as separate elements using the
“wishbone” modeling technique to properly simulate the pile-leg contact at each jacket bracing level, and a fixed connection at the pile-leg shim connection at the top of the leg.
b. For grouted piles, the Supplier shall discuss with the Purchaser whether modeling a composite section is adequate, or if the pile and leg should be modeled as separate elements, with adjustments made for grout weight and at nodes to facilitate proper punching-shear analysis. Reasons for using separate elements include cases in which the pile and leg are of different material grades.
9. Appurtenances such as conductors, risers, and caissons shall be included as load collectors for wave and current in-service conditions, but shall not contribute to structural or foundation stiffness unless these members are framed such that they contribute to the structural and/or foundation stiffness.
a. The mass (including well casings and entrapped fluid) of these appurtenances shall be appropriately modeled.
b. The ends of such non-structural elements shall be appropriately “released” such that they can only transfer loads and do not contribute to the global stiffness of the platform.
10. Jacket non-structural items such as boat landings and anodes that are included solely to distribute load to the main members shall be modeled as wave load areas and miscellaneous masses.
11. Conductors shall be modeled such that they are not directly attached to the platform.
a. Vertical loads shall be carried by the soil. Lateral loads shall be transferred to the platform structure by bearing against the guides.
b. If the jacket structure has mudline framing with conductor guides, all conductors shall be modeled (preferably as foundation piles) to take proper account of the resistance they add to lateral structure deflection.
c. “Wishbones” with proper releases at the ends shall be used at all horizontal levels with conductor guides to ensure that the conductors transfer lateral loads only, not axial loads.
d. For conductors with a large annulus between the conductor and the guide, a gap element may be used.
12. When conductors are used as foundation elements for global place analyses, a separate in-place stress analysis with no conductors modeled shall be run to assure correct pile design. The reasons for this analysis are as follows:
a. It is necessary to prevent under-sizing of piles.
b. In some cases where soil is strong, the conductors may take out more lateral load at the mudline than they contribute to the structure.
c. Maximum pile lateral shear shall occur when no conductors are present.
13. The effects of marine growth on members in and below the splash zone shall be modeled.
14. For complex nodes or other locations where Finite Element Analysis (FEA) is called for, Purchaser’s guidelines are as follows:
a. Ideally, one FEA model shall be used for detailed component strength analyses and SCF development for input to fatigue analyses.
b. For thin-shell FEA, four-noded shell elements with full integration (four integration gauss points) shall be used.
c. Finite elements, similar to Shell 181 in ANSYS or S4 in ABAQUS, shall be used.
d. For thick-shell analyses, such as castings and annular grout, an eight-noded solid brick element shall be used.
e. In areas of high stress concentration the size/aspect ratio of the FEA mesh should be about t t with a maximum of t 2t, where t is plate thickness.
f. Only rectangular elements should be used in the high-stress concentration areas.
Triangular-shaped elements should be avoided.
g. A larger mesh size shall be acceptable farther away from high-stress areas.
h. Imposed displacement or load boundary conditions shall be located at sufficient distance from the high-stress region of interest to insure accurate FEA results.
i. Von Mises stresses shall be determined at both the top and bottom side of the shell element and at all element integration points. Von Mises stresses shall be extracted from the connection nodes for direct comparison to allowable stresses.
j. Stresses in hot-spot locations shall be evaluated for their degree of “reality” with respect to the following:
1) Non-optimum mesh size/shape in hot-spot area.
2) Excessive stress gradient over adjacent elements.
3) Usage of thick-shell element sub-modeling instead of thin-shell modeling.
4) Usage of non-linear analysis to determine hot-spot actual size.
5) Physical revision of the component structural geometry to mitigate the hot-spot.
6) Good engineering judgment.
k. Allowable stresses for detailed FEA component strength evaluations shall be as follows:
1) In-place extreme (100-year RP) load condition. The maximum Von Mises stress (top or bottom and at any integration point) in hot-spot locations shall be less than 0.9 Fy.
2) Operating (10-year RP) load condition. The maximum Von Mises stress (top or bottom and at any integration point) away from hot-spot locations shall be less than 0.67 Fy.
5.2 Pile Design
5.2.1 Computer Model
1. A computer solution of coupled, non-linear, pile-structure interaction shall be required for all in-place structural analyses.
2. An integrated, pile-structure computer model shall be used.
a. It shall consist of a non-linear representation of the foundation and a linear-elastic model of the structure.
b. The behavior of the non-linear, soil-pile foundation system shall be modeled under both extreme environmental design and operational loads.
c. All p-y and t-z curves shall be explicitly defined in the soil model and modified for scour.
3. Jacket foundation support for modal and fatigue analyses may be modeled as springs or with direct six-degree-of-freedom stiffness matrices.
4. Foundation spring and linear stiffness values, if used, shall be calculated using platform loads that are representative of design conditions being considered.
5.2.2 Scour
Unless better site-specific data is available, a scour depth of 1.5 times the pile diameter or five feet, whichever is less, shall be assumed.
5.2.3 Underdrive Allowance
1. For piles having thickened sections at the mudline, the pile wall thickness make-up shall be designed to allow for the possibility of pile driving refusal before reaching the design penetration.
2. An underdrive allowance shall be provided by extending the length of heavy-wall material in the vicinity of the mudline by the largest of the following:
a. Twenty feet (6.1 meters).
b. The difference between design penetrations using a safety factor of 1.35 and 1.50 for the 100-year-return-period storm.
c. The difference between design penetrations using a safety factor of 1.80 and 2.00 for the 5-year-return-period storm with associated full operations of the facility.
5.2.4 Overdrive Allowance
1. For piles having thickened sections at the mudline, the pile wall thickness make-up shall be designed to allow for the possibility of pile driving beyond design penetration.
2. In instances where the pile is expected to encounter a specific bearing stratum or when there are uncertainties in the soil data, an overdrive allowance of a minimum of 10 feet (3 m) shall be provided and the length of heavy-wall material in the vicinity of the mudline shall be extended. Allowance shall also be made for grout shear keys, when used.
3. The final value of the overdrive shall be determined by the Supplier with b, taking into account items such as the following:
a. Level of uncertainty in the soil information (distance from nearest soil boring to platform location, number, and quality of soil samples and types of in-situ and laboratory soil tests, etc.).
b. Total length of piles.
c. Percentage of contribution from end bearing in the specific bearing stratum to the total axial compressive pile capacity, etc.
5.2.5 Pile Drivability
1. A pile drivability analysis shall be performed to determine the required pile wall thickness and hammer requirements for both continuous and interrupted driving caused by the installation of add-on sections, the changing of hammers, etc.
2. A pile drivability analysis using a wave-equation program shall be performed in accordance with the recommendations made in the geotechnical reports.
3. Dynamic pile stresses during driving shall be limited to 90 percent of the pile material yield stress.
4. The pile shall be checked with its tip both plugged and unplugged for a range of hammer sizes.
5. For assessing drivability with steam hammers, the assumed global hammer efficiency shall not exceed 70 percent without written verification of higher efficiencies from the hammer manufacturer.
6. For assessing drivability with hydraulic hammers, the assumed global efficiency shall not exceed 90 percent without written verification of higher efficiencies from the hammer manufacturer.
5.2.6 Pile Lengths 1. Main Piles
a. Pile section lengths shall be designed in accordance with API RP 2A-WSD and/or API RP 2A-LRFD, as applicable.
b. Main piles shall have pile section lengths with stick-up lengths such that slenderness ratio (kl/r) is less than 200.
c. For determining the slenderness ratio, the pile length under consideration shall start at the last set of pile centralizers and an effective length factor (k) of 2.1 shall be assumed.
d. The pile makeup shall be planned to avoid pile add-ons when the pile tip is at a stratum where hard driving is expected such as granular or cemented soils or rock.
2. Vertical Skirt Piles
a. The stick-up height of single-piece, vertical skirt pile shall have a slenderness ratio (kl/r) less than 200, in which the effective length factor (k) is 2.1.
b. The pile length (l) shall be equal to the fabricated pile length minus the self-penetration length and the length from mudline up to the last set of
centralizers in the skirt pile sleeve. An effective length factor (k) of 2.1 shall be assumed.
c. The self-penetration of the pile shall be based on the static ultimate soil resistance with end bearing not to exceed the skin friction between the soil plug and inside wall of the pile, using the remolded shear strength to compute skin friction in clay soils, and with consideration given to the buoyancy of the pile and the hammer weight.
d. Vertical skirt piles shall also be checked for the effects of current-induced VIV using DNV RP-C205 and DNV RP-F105.
e. The lateral design load on the pile shall be the maximum of the following:
1) The maximum wave and current load corresponding to sea conditions representative of the area during pile installation.
2) The lateral component of the gravity load resulting from the largest possible deviation from the true vertical of the pile due to the existing gap between the pile and pile sleeve, fabrication/installation tolerances, and pile deflection due to current and wave loads, or 2 percent of the total hammer weight applied at the hammer center of gravity, whichever is greatest.
f. Pile stress checks shall be performed as follows:
1) Combined axial compression and bending using the static axial (fa) and bending (fb) stresses with no increase in allowable stresses.
2) Combined static stress and driving stresses shall be governed by the more stringent of API 2A-WSD or the following.
3) Combined static stress and driving stresses not to exceed the minimum yield stress of the pile material (fa + fb + fd < Fy).
4) The axial component of pile stress due to pile self weight shall also be checked by using Timoshenko, Theory of Elastic Stability, Section 2.12,
“Buckling of Bar under Distributed Axial Load.”
g. VIV shall be evaluated for the pile stick-up during installation according to the latest edition of DNV classification note 30.5 and DNV RP-F105 with a
maximum of 1 percent damping. Fatigue damage due to pile stick-up VIV shall be taken into account.
5.2.7 Skirt Pile Connection to Jacket
The design for connection of the skirt piles to the structure shall assume a minimum grout strength of 2,500 psi (17 MPa).
5.2.8 Pile Group Effects
1. Pile group effects shall be investigated for pile spacings less than four pile diameters.
2. Design of pile groups or clusters shall include group effects.
3. A reduction in lateral and axial capacity of the soil shall be required depending on pile spacing, type of soil, structure geometry, and pile axial load.
5.2.9 Pile Soil Setup
The ultimate axial pile capacity shall be evaluated to account for time-dependent pile capacity at the time of loading for each of the foundation design conditions.
5.3 Gravity Loads 5.3.1 Dead Load
1. The dead weight of the jacket includes jacket members plus all appurtenances such as J-tubes, risers, caissons, conductors, barge bumpers, boat-landing, mudmats, anodes, installation aids, etc. Pipeline weights including clamps and the gas or liquid that fills the line shall also be considered.
2. For dynamic analyses, the jacket legs, conductors, and pipeline masses shall be calculated based on make-up of the members, fluid inside the pipe (if applicable), and added fluid masses for members submerged in water.
a. For grouted jacket legs, grout, pile, and enclosed water inside the pile shall be considered.
b. For conductors, all pipe strings, grout, and fluids inside the conductors shall be considered.
3. A minimum of 2.5 percent weight contingency shall be added to the theoretical calculated steel weight for mill tolerance.
5.3.2 Drilling Loads
1. The drilling loads shall be established on the basis of the requirement of drilling provided in the design basis.
2. The loads from the drill rig during drilling (operating) and non-drilling (storm case) shall be considered.
3. Detail loads shall include skid beam reactions for each well location that include hook load and associated pipe rack loads and the module weights.
4. Appropriate crane loads during drilling and operating condition shall be considered.
5. Setback loads and pipe-rack loads shall be combined in a logical fashion to obtain the necessary worst cases sought for the design.
5.3.3 Production Loads
1. The production loads are defined as the combined dead weight/mass of equipment and skids, cranes, pipes, electrical, and instrumentation and their supports, plus liquid loads within the vessels and pipes, plus stored commodities and consumables.
2. These shall represent all loads on the platform other than the structural weight of the deck and the drill-rig weight.
5.3.4 Uniform Area Loads
1. Uniform area loads may be used for preliminary deck design during the initial stage of the project, when actual equipment loads are not defined.
2. Unless specified otherwise in a design basis for a specific project, the deck surfaces (plating or grating), deck beams, trusses and walkways shall be designed for the area load as shown in Table 5.
3. A reduced percentage of area loading may be used for the design of truss girders, deck columns, and jacket structures and pilings.
4. The reduction factors shown in Table 6 shall be used unless specified otherwise in the design basis.
Table 5: Uniform Area Loads for Deck Design
Drilling deck 500 psf (2440 kg/m2)
Specified storage and laydown areas 500 psf (2440 kg/m2)
Drill deck (well bay area) 350 psf (1710 kg/m2)
Mezzanine deck 150 psf (730 kg/m2)
Mezzanine deck (separator and manifold area) 350 psf (1710 kg/m2)
Production deck 250 psf (1220 kg/m2)
Production deck (generator area, MCC building) 350 psf (1710 kg/m2)
Cellar deck 200 psf (980 kg/m2)
Walkways 100 psf (490 kg/m2)
Table 6: Load Reduction Factors
Descriptions
Secondary Girders &
Floor Beams
Truss Girders
Deck Columns
Jacket &
Pilings
Main deck area loading 100% 75% 67% 67%
Cellar deck area loadings 100% 75% 67% 67%
Walkways 100% 20% 0% 0%
5. When accurate actual equipment operating loads are defined, these actual equipment loads, used in combination of DL and associated open-area LL (in lieu of uniform area loads mentioned in Item 1 and 2 above), shall be used for subsequent detailed design of topsides main steel and deck beams.
a. All deck beams shall be checked for the actual (wet) weight of the equipment.
b. For equipment placed directly on the deck surface without skids, the deck plate shall be strengthened using a beam underneath the deck plate unless it can be proved that the deck plate is adequate to support the load.
5.4 Environmental Loading
5.4.1 Wave and Current Forces
1. Wave and current forces for in-service analyses shall be computed using the Morison equation and an appropriate wave theory, and shall assume that the current acts simultaneously and co-linearly with the wave propagation unless stated otherwise on the project metocean report.
a. Stream Function wave theory shall be used unless a different wave theory is provided in the site-specific metocean study.
b. Current and wave particle velocities shall be added prior to force determination in Morison’s equation.
c. A wave-kinematics factor shall be taken from the site-specific metocean study or based on the guidance in API RP 2A-WSD or LRFD if the site-specific report does not contain a value.
2. For in-service spectral fatigue analyses, a wave kinematics factor of 1.0 shall be used.
3. For dynamically-sensitive structures such as deepwater fixed platforms or compliant towers, in-service wave and current forces shall be computed using random-wave analysis techniques using superimposed Airy wavelets. Supplier shall submit to Purchaser the details of intended hydrodynamic analyses, both static and dynamic.
5.4.2 Wave Force Coefficients
1. Hydrodynamic wave coefficients for use in Morison’s equation for extreme and operating conditions shall be determined in accordance with API RP 2A-WSD and/or API RP 2A-LRFD, as applicable.
2. Due consideration shall be given to wake-encounter effects for nearly vertical members (15 degrees from vertical or less) where the Kuelegan-Carpenter number falls below 30. Supplier shall submit to Purchaser the details for appropriately including wake-encounter effects.
3. Basic drag and inertia coefficients for use in the extreme and operating storms shall be as provided in API RP 2A-WSD or LRFD, as applicable.
4. Hydrodynamic wave coefficients for use in Morison’s equation for the spectral fatigue analysis shall be constant with no variation for wake effect.
5. Drag and inertia coefficients for use in the spectral fatigue analysis shall be as provided in API RP 2A-WSD or LRFD, as applicable.
5.4.3 Wave Shielding
1. For extreme and operating conditions, wave shielding factors that account for wave
1. For extreme and operating conditions, wave shielding factors that account for wave