1. Decks and modules shall be sized, modeled, and analyzed in sufficient detail to ensure structural adequacy for all possible loading conditions experienced during their lives—
pre-service and in-service.
2. The deck shall be analyzed using a three-dimensional space-frame computer model of sufficient detail to accurately represent the stiffness and load distribution within the deck in order to design individual members.
a. Deck and module computer models shall include primary truss and framing members and major deck beams.
b. Deck plate may be modeled to incorporate in-plane shear restraint or to provide lateral bracing to beam flanges.
c. If not explicitly modeled, plate stiffness shall be simulated by X-bracing.
d. Grating shall not be considered to provide in-plane shear restraint or lateral beam-flange restraint.
e. The flare boom shall be included in the in-place model.
f. Conductor, pull tube, bridge supports, and sump supporting members shall be modeled explicitly.
3. For in-service loadings, if at all possible, decks shall be analyzed with a full jacket model.
a. If, however, decks and/or modules are modeled and analyzed separately from the jacket, total structural system performance shall be accounted for, including the effect of jacket stiffness on support conditions.
b. As a minimum, the first bay of the jacket shall be included in the structural model.
6.2 Environmental Loading
6.2.1 Wave and Current Forces
1. The global effect of wave and current forces on the jacket shall be accounted for in the design of the deck.
2. To ensure these effects are included in the deck design, if at all possible, the jacket shall be included with the deck in the computer model and the platform shall be analyzed as an integral structure.
6.2.2 Wind Forces
1. For the global analysis of the deck, the wind forces shall be assumed to act omni-directionally, simultaneously and co-linearly with the wave and current forces on the jacket.
a. The one-minute sustained wind speed associated with the wave condition shall be used for the superstructure analysis.
b. The global wind load on the deck may be determined by taking the projected area of the deck with a shape factor of 1.0.
c. Wind loads on flare boom shall be determined rigorously; i.e., a projected area approximation shall not be used.
2. For the local design of deck appurtenances, such as communication towers, flare booms, flare boom connections, firewalls, splash walls, equipment supports, and supporting deck beams, the wind-only, three-second-gust wind speed shall be used.
6.3 In-Service Analysis
1. The deck beams, major support beams, and deck trusses shall be analyzed and designed for loads including, but not limited to, dead loads, drilling rig loads, crane loads, equipment loads, and production live loads.
2. The platform shall be analyzed for a minimum of eight wave and wind attack directions at 45 degree intervals around the platform.
3. Wave directionality (different wave heights from different directions) shall be considered in the analysis.
4. The deck shall be designed with the drilling rig positioned to produce the maximum possible forces in the skid beams and supporting trusses.
5. Loads shall be varied to include conditions that both maximize and minimize foundation loading.
6. Crane loads shall be included in the operating load cases.
a. For each operating case, the crane shall be positioned at multiple headings with full capacity crane loads.
b. For each operating case, eight crane-load cases based on eight headings (0, 45, 90, 135, 180, 225, 270, 315 degrees) are recommended.
c. Use of a fewer number of load cases in final design shall be acceptable if it can be shown during preliminary design that only a few load cases control the design.
7. Deck loads shall include area-load and equipment-load cases when the data are available.
a. Deck loads shall also include a hydrotest case with equipment filled with water for hydrotest condition.
b. In running the hydrotest case, consideration shall be given to the fact that all equipment will not be filled with water at the same time.
6.4 Transportation Strength Analysis
1. A three-dimensional computer model of the deck and barge shall be used to analyze the stresses in the deck and barge.
a. The deck model shall include sufficient detail to evaluate the stress in all members and joints that will experience significant transportation-motion-induced loads.
b. When a rigid barge model is not sufficient—generally for barges in excess of 300 feet (91 m) in length—the barge model shall contain sufficient detail to capture the overall flexibility characteristics of the vessel and to permit the distribution of hydrodynamic loads to model hogging and sagging effects.
2. The deck-barge computer model shall use spacers and tie-downs to simulate actual connection conditions.
a. Vertically oriented spacer members shall be attached at all deck leg locations to support and elevate the deck to its proper height above the barge deck.
b. These members shall support vertical load only.
c. Tie-downs shall be modeled at each deck leg.
d. The barge ends of the tie-downs shall be attached to nearby barge joints using a series of statically determinate rigid links.
3. The analysis of the deck, equipment, access platform, and secondary deck structures shall include the forces imposed from the barge/cargo motion during transportation.
4. The transport structural weight shall include weight contingencies, operator’s reserve, if applicable (as defined in the project weight control report), and the weight of all pre-installed rigging and shipped-loose items.
a. Weight contingencies shall account for different levels of uncertainties at different design stages.
b. Operator’s reserve shall account for potential changes, as requested by the Purchaser.
5. A minimum of eight headings shall be considered, including all combinations of beam seas, head seas, and quartering seas.
a. For the specified significant-wave height, the mean-spectral period shall be varied between the upper and lower bounds in order to maximize inertial forces acting on the deck.
b. The extreme-tow inertial forces shall represent the 1-in-1000 highest values (RMS 3.72) of the motions using a spectral sea model.
6. The lateral component of gravity due to roll caused by the design wave and wind shall be accounted for in the deck stress analysis.
a. Conservatively, the phase differences of the six components of inertial force may be neglected.
b. Linear load combinations shall be formed for each wave direction to combine the extreme global force components using all possible signs.
7. A sequential structural solution shall be performed.
a. The system without the tie-down braces shall be solved for the still-water transportation case (loadout condition).
b. The system with the tie-down braces shall be solved for dynamic load cases, including the static wind-induced loads.
c. The still-water load case shall then combined with other defined cases in the post processor.
d. Therefore, the presence of the tie-downs shall only be active under the dynamic transportation loads.
6.5 Transportation Fatigue Analysis 6.5.1 General
1. A comprehensive spectral-dynamic fatigue analysis shall be performed on the deck in accordance with API RP 2A-WSD and/or API RP 2A-LRFD, as applicable.
2. Transportation fatigue analyses shall be required for decks where tow duration exceeds ten days.
3. A towing velocity of 5 knots (2.6 m/s) shall be assumed to relate encounter frequency to wave frequency.
4. If appropriate and accepted by Purchaser, a higher towing speed may be used for self-propelled tows with a heavy-lift vessel.
5. If expected seastate conditions are used for the tow duration time period, a weather event equal to the magnitude of the design event and equivalent in length to a storm of this type shall be included in the fatigue computations.
6. Stresses shall be calculated at each of eight regularly spaced phase angles of each wave.
7. A minimum of eight headings shall be considered, including all combinations of beam seas, head seas, and quartering seas.
8. A minimum of ten encounter frequencies shall be used to provide smooth-motion RAO functions.
9. For each location at which fatigue life is to be determined, eight points around the circumference shall be checked.
6.5.2 Computer Model
The structural model used for transportation fatigue analysis shall be identical to that used for the transportation strength analysis, capturing both the flexibility and hydrodynamic pressure distribution on the barge.
6.5.3 Stress Concentration Factors
Stress concentration factors shall be determined as described previously in this document.
6.5.4 S-N Curves
Preferred S-N curves shall be as described previously in this document.
6.5.5 Fatigue Design
Preferred methods of improving fatigue life shall be as described previously in this document.
6.5.6 In-Service and Transportation Fatigue Combination
1. Prediction of fatigue life of the deck structure shall include damages resulting from the transportation as well as in-service stress condition.
2. For appropriate safety factor and instructions on combining the damages due to transportation and in-place conditions, see Section 5.9.6.
6.6 Lifting Analysis 6.6.1 General
1. Three-dimensional space-frame lift analyses shall be performed for all structures to be lifted in accordance with API RP 2A-WSD and/or API RP 2A-LRFD, as applicable.
a. The lift weight shall include weight contingencies, operator’s reserve (as defined in the weight control report generated in each project), and the weight of all pre-installed rigging and shipped-loose items.
b. The center of gravity shall be calculated without contingencies.
c. The analyses shall be verified at the completion of fabrication after the structures have been weighed or the final weight report issued.
d. For general guidance and computer modeling for the lift analyses, see Section 5.11.
2. The lifting-eye design shall include sufficient reserve strength to allow for future weight growth, load distribution changes, and final selection of rigging.
a. Lifting eyes shall be configured to permit the final angle or orientation to be established as late in fabrication as possible.
b. Lifting eyes for major deck and module lifts shall be located so that they will be permanent, not requiring removal.
6.6.2 Single-Crane Lifts
1. The installation contractor’s lift analysis procedures shall be used in the lift analyses.
2. In the absence of the installation contractor’s procedures, the following procedure shall be followed for a single-crane, four-point lift.
3. The structure shall be analyzed for two conditions:
a. A 75 percent to 25 percent load distribution shall be assumed between diagonally opposite pairs of slings.
1) Sling reactions shall be increased by 35 percent for design of the lifting eye and connecting members.
2) This condition does not apply where use of a spreader bar prevents a load distribution of 75 percent to 25 percent.
b. The following may be used in lieu of 3a above, with Purchaser acceptance:
1) The structure and rigging shall be analyzed with the center of lift points within a 3 ft radius envelop of the calculated CG of the lifted structure.
2) The sling lengths must be certified to industry standards such as IMCA M 179, ISO 7531, and ISO 2408.
c. The sling reactions shall be calculated using a rigid-body lift analysis, which properly accounts for the distribution of weight in the structure and for any hanging/skew angles.
1) Sling reactions shall be multiplied by a factor of 2.0 for design of lifting eyes and connecting members.
2) All other members shall be designed using a load factor of 1.35.
3) Side loads on the padeyes and padears shall be calculated, taking into account the deck out of level, but in no case shall the side load be less than 5 percent of the static sling load.
4. To prevent damage, deflections during lift shall be limited for structures with deflection-sensitive appurtenances (such as quarters building) or coatings (such as fireproofing).
6.6.3 Dual-Crane Lifts
1. If the installation contractor has been selected at the time of design, the installation contractor’s lift analysis procedures shall be used in the lift analyses.
2. In the absence of the installation contractor’s procedures, the Purchaser and the Supplier shall agree upon a dual-crane lift procedure for use in the design.
6.7 Loadout Analysis
1. The deck shall be designed for stresses occurring during loadout.
2. The analysis shall fully consider the method of loadout, behavior of the foundation, and characteristics of the barge.
3. A minimum of the four most critical deck support conditions shall be investigated during loadout. The maximum allowable deflection of the barge supporting the deck shall be determined for each stage of loadout.
4. Unless a specific loadout plan dictates otherwise, loadout analysis shall include the following:
a. All columns supported.
b. Three-point support; loss of support one column at a time.
c. Maximum upward/downward displacement of unsupported columns of 2 inches (50mm).
d. A horizontal jacking force on the two lead columns resisted by friction at all four columns.
6.8 Barge Stability
Barge stability criteria for the deck transportation shall be the same as stated for the jacket transportation above.
6.9 Local Vibrations
1. Local vibration response to harmonic forces caused by reciprocating or rotating machinery shall be determined.
2. Elements in the superstructure that support mechanical equipment shall be designed so that their natural frequency of vibration is either less than 70 percent or greater than 140 percent of any equipment operating or transient frequencies.
3. Detailed methods and procedures for performing this analysis shall be submitted to Purchaser for acceptance.
4. In developing the equipment and structural layout, an attempt shall be made to locate
reciprocating compressors and pumps where deck beams and girders can be rigidly supported.
a. Cantilever or mid-span of unsupported main or secondary girders shall be avoided.
b. Preferred location for placing the equipment shall be on the top of the deck columns, top of the deck trusses, and near deck trusses.
c. Low-rpm rotating equipment shall always be supported by stiff beams.
5. Specific to turbo-machinery skids, the maximum combined bending and twisting deflection shall not exceed those specified in Table 15.
6.10 Member Design
Member design criteria for the deck shall be the same as stated for the jacket above.
Table 15: Deflection Criteria for Beams Supporting Vibrating Machinery
Sagging –0.13 inches (–3.3 mm) over 28-foot (8.5 m) span (approximately L/2500) Hogging +0.13 inches (+3.3 mm) over 28-foot span (8.5 m) (approximately L/2500) Maximum twist Not to exceed ±0.006 degrees per foot
6.11 Tubular Joint Design
Tubular joint design criteria for the deck shall be the same as stated for the jacket above.
6.12 Vortex Shedding
1. Member susceptibility to wind-induced vortex shedding shall be assessed in accordance with Appendix A and the one-minute sustained wind speed of the 5-year storm for the site under consideration.
2. The maximum limiting damping value shall not exceed 0.2 percent for members in air.
3. Members shall be designed to withstand vortex-induced loading.
4. As required, vortex suppression devices may be used.
5. Temporary suppression devices shall be designed for easy removal prior to platform installation.
6. Permanent suppression devices shall be designed, fabricated, and inspected to meet long-term service requirements.
6.13 Primary Beam and Plate Girder Design 6.13.1 Deflections
1. Vertical deflections of primary beams and plate girders shall be limited to criteria based on equipment operating requirements specified by equipment suppliers or the specifications in Table 16, whichever is more stringent.
2. Lateral deflections of primary beams, plate girders, and secondary beams shall be limited to criteria based on equipment and piping operating requirements, supplied by Purchaser.
3. If lateral deflection criteria are not provided by Purchaser, the recommendations from the applicable equipment and piping vendors shall be used.
6.13.2 Plate Girder Guidelines
Plate girders shall be designed in accordance with AISC’s Manual of Steel Construction and shall incorporate the following guidelines:
1. The shear coefficient (Cv) shall be as specified by AISC. As design conditions allow, Cv shall be less than or equal to 0.80.
2. The top and bottom flanges at a given section shall be of equal area and of the same grade of steel.
3. Longitudinal web stiffeners shall not be used.
Table 16: Deflection Criteria for Beams and Plate Girders
Support Condition Allowable Deflection
Live Load Dead Load + Live Load
Both Ends Supported L/360 L/240
Cantilever L/240 L/180
Beams Supporting Rotating Equipment L/480 L/480
Note: L is the member length from face to face of the beam.
4. The yield strength of the flange steel shall be equal to or greater than the yield strength of the web steel.
5. The bearing and intermediate stiffeners shall use the same yield strength steel as the web.
6. All plate girders shall be compact sections as defined by AISC.
7. An increased plate girder web thickness may be required within the intersection zones of connecting truss members.
a. Unless agreed otherwise by Purchaser, computer modeling of plate girder truss connections shall use multiple-joint node connection locations appropriately located along the plate girder length to accurately capture the plate girder shear transfer with intersecting truss members.
b. Using a single-joint node model at the plate girder truss connection location may result in unconservative plate girder shear load distribution and inadequate plate girder web thickness design.
6.14 Dropped Objects
1. Critical areas of the deck shall be designed to withstand impacts by objects that are accidentally dropped in the course of expected platform operations. The design shall be in accordance with Appendix C.
2. The Supplier shall utilize the generic design guidance presented in Appendix C with other project-specific, basis-of-design information to develop relevant dropped object load cases and analysis procedures. These shall be submitted in writing to Purchase for acceptance.
6.15 Fire and Blast
1. As appropriate—depending on the types of fluids handled on the platform and arrangement of equipment—critical areas of the deck shall be designed to withstand fires and explosions in accordance with Appendix D.
2. The Supplier shall utilize the generic design guidance presented within Appendix D together with other project-specific, basis-of-design information to develop relevant fire and blast load cases and analysis procedures. These shall be submitted in writing to Purchaser.
7.0 APPURTENANCE DESIGN CONSIDERATIONS