Section 4 General Design Requirements

401. General

1. General

This section of the Rules outlines general concepts and considerations which may be incorporated in design. In addition, considerations for particular types of offshore structures are enumerated.

402. Analytical Approaches

1. Format of Design Specifications

The design requirements of these Rules are generally specified in terms of a working stress format for steel structures and an ultimate strength format for concrete structures. In addition, the Rules re- quire that consideration be given to the serviceability of structure relative to excessive deflection, vibration, and, in the case of concrete, cracking. The Society will give special consideration to the use of alternative specification formats, such as those based on probabilistic or semiprobabilistic limit state design concepts.

2. Loading Formats

(1) With reference Sec 2 and Sec 3, either a deterministic or spectral format may be employed to describe various load components.

(2) When a static approach is used, it is to be demonstrated, where relevant, that consideration has

been given to the general effects of dynamic amplification.

(3) The influence of waves other than the highest waves is to be investigated for their potential to produce maximum peak stresses due to resonance with the structure.

(4) When considering an earthquake in seismically active areas(see Sec 3), a dynamic analysis is to be performed. A dynamic analysis is also to be considered to assess the effects of environ-

mental or other types of loads where dynamic amplification is expected.

(5) When a fatigue analysis is performed, a long-term distribution of the stress range, with proper consideration of dynamic effects, is to be obtained for relevant loadings anticipated during the design life of the structure. (see 506. 2, 603. 4)

(6) If the modal method is employed in dynamic analysis, it should be recognized that the number of modes to be considered is dependent on the characteristics of the structure and the conditions being considered.

(7) For earthquake analysis, a minimum number of modes is to be considered to provide approx- imately 90% of the total energy of all modes.

(8) The correlation between the individual modal responses in determining the total responses is to

be investigated. The complete quadratic combination(CQC) method may be used when the com- bining modal responses is small, the total response may be calculated as the square root of the

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sum of the squares of the individual modal responses.

(9) For extreme wave

and fatigue analyses, dynamic response is to be considered for structural

modes having periods greater than 3.0 seconds. For significant modes with periods of 3.0 sec- onds or less, the dynamic effect need not be considered provided the full static effect(including flexure of individual members due to localized wave forces) is considered.

3. Combination of Loading Components

(1) Loads imposed during and after installation are to

(2) In consideration of the various loads described in to be combined consistent with their probability

be taken into account.

Sec 3, loads to be considered for design are of simultaneous occurrence. However, earth-

quake loadings may be applied without consideration of other environmental effects unless con-

ditions at the site necessitate their inclusion.

(3) If site-specific directional data is not obtained, the direction of applied environmental loads is to be such as to produce the highest possible influence on the structure. Loading combinations cor-

responding to conditions after installation are loadings. (See 203.)

(4) Reference is to be made to Sec 5, Sec 6,

to reflect both operating and design environmental

Sec 7, regarding the minimum load combinations

to be considered. The operator is to specify the operating environmental conditions and the maximum tolerable environmental loads during installation.

403. Overall Design Considerations

1. Design Life

The design life of the structure is to be specified by the Operator. Continuance of classification be- yond the Design Life will be subject to a special survey and engineering analysis.

2. Air Gap

(1) An air gap of at least 1.5 m is to be provided between the maximum wave crest elevation and

the lowest protuberance of the the design.

(2) After accounting for the initial

solidation and subsidence in a

superstructure for which wave forces have not been included in

and expected long-term settlements of the structure, due to con- hydrocarbon or other reservoir area, the design wave crest ele-

vation is to be superimposed on the still water level and consideration is to be given to eave

run-up, tilting of the structure and, where appropriate, tsunamis.

3. Long-term and Secondary Effects

Consideration is to be given to the following effects, as appropriate to the planned structure.

(A) Local vibration due to machinery, equipment and vortex shedding

(B) Stress concentrations at critical joints

(C) Secondary stresses induced by large deflection(P-△ effects)

(D) Cumulative fatigue

(E) Corrosion

(F) Abrasion due to ice

(G) Freeze-thaw action on concrete and coatings

4. Reference Marking

(1) For large or complex structures, consideration should be given to installing permanent reference markings during construction to facilitate future surveys. Where employed, such markings may consist of weld beads, metal or plastic tags, or other permanent markings.

(2) In the case of a concrete structure, markings may be provided using suitable coatings or perma- nent lines molded into the concrete.

5. Zones of Exposure

(1) Measures taken to mitigate the effects of corrosion as required by 501. 3 and 601. 3 are to be specified and described in terms of the following definitions for corrosion protection zones.

(A) Submerged Zone : The part of the installation below the splash zone.

(B) Splash Zone : The part of the installation containing the areas above and below the still water level which are regularly subjected to wetting due to wave action. Characteristically, the splash zone is not easily accessible for field painting, nor protected by cathodic protection.

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(C) Atmospheric Zone : That part of the installation above the splash zone.

(2) Additionally, for structures located in areas subject to floating or submerged ice, that portion of the structure which may reasonably be expected to come into contact with floating or sub- merged ice is to be designed with consideration for such contact.

404. Considerations for Particular Types of Structures

1. General

(1) In this subsection are listed specific design considerations which are to be taken into account for particular types of structures.

(2) Where required, the interactive effects between the platform and conductor or riser pipes due to

platform motions are to be investigated. For compliant structures which exhibit significant wave induced motions, determination of such interactive effects may be of critical importance.

2. Pile-Supported Steel Platforms

(1) Factors to be considered

Factors to be considered in the structural analysis are to include the soil-pile interaction and the loads imposed on the tower or jacket during towing and launching.

(2) Installation Procedures

Carefully controlled installation procedures are to be developed so that the bearing loads of the tower or jacket on the soil are kept within acceptable limits until the piles are driven.

(3) Special Procedures

Special procedures may have to be used to handle long, heavy piles until they are self-support- ing in the soil. Pile driving delays are to be minimized to avoid set-up of the pile sections.

(4) Dynamic Analysis

For structures likely to be sensitive to dynamic response, the natural period of the structure should be checked to insure that it is not in resonance with eaves having significant energy

content.

(5) Instability

Instability of structural members due to submersion is to be considered, with due account for second-order effects produced by factors such as geometrical imperfections.

3. Concrete or Steel Gravity Platforms

(1) Positioning

The procedure for transporting and positioning the structure and the accuracy of measuring de- vices used during these procedures are to be documented.

(2) Repeated Loadings

Effects of repeated loadings on soil properties, such as pore pressure, water content, shear strength and stress strain behavior, are to be investigated.

(3) Soil Reactions

Soil reactions against the base of the structure during installation are to be investigated. Consideration should be given to the occurrence of point loading caused by sea bottom irregularities. Suitable grouting between base slab and sea floor can be employed to reduce con- centration of loads.

(4) Maintenance

The strength and durability of construction materials are to be maintained. Where sulphate attack is anticipated, as from stored oil, appropriate cements are to be chosen, pozzolans incorporated

in the mix, or the surfaces given suitable coatings.

(5) Reinforcement Corrosion

Means are to be provided to minimize reinforcing steel corrosion.

(6) Instability

Instability of structural members due to submersion is to be considered, with due account for second-order effects produced by factors such as geometrical imperfections.

(7) Horizontal sliding

Where necessary, protection against horizontal sliding along the sea floor is to be provided by means of skirts, shear keys or an equivalent means.

(8) Dynamic Analysis

A dynamic analysis, including simulation of wave-structure response and soil-structure inter- action, should be considered for structures with natural periods greater than approximately 3

seconds.

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(9) Long Term Resistance

The long term resistance to abrasion, cavitation, freeze-thaw durability and strength retention of the concrete are to be considered.

(10) Negative Buoyancy

Provision is to be made to maintain adequate negative buoyancy at all times to resist the uplift forces from waves, currents, and overturning moments. Where this is achieved by ballasting oil storage tanks with sea water, continuously operating control devices should be used to maintain the necessary level of the oil-water interface in the tanks.

4. Concrete-Steel Hybrid Structures

(1) Horizontal Loading

Where necessary, the underside of the concrete base is to be provided with skirts or shear keys to resist horizontal loading; steel or concrete keys or an equivalent means may be used.

(2) Steel and Concrete Interfaces

Special attention is to components.

(3) Other Factors

be paid to the design of the connections between steel and concrete

Pertinent design factors for the concrete base listed in Par 3 are also to be taken into account.

5. Minimum structure

(1) These Rules are to be

applied in design of minimum structures wherever applicable. Minimum

structures generally have less structural redundancy and more prominent dynamic responses due to the flexible nature of the structural configuration.

(2) Dynamic effects on the structure are to be considered in the structural analysis when the struc- ture has a natural period greater than 3 seconds. The pertinent design factors listed Par 2 are

to be taken into account.

(3) Connections other than welded joints are commonly used in minimum structures. For these me- chanical connections such as clamps, connectors and bolts, joining diagonal braces to the col- umn or piles to the minimum structure, the strength and fatigue resistance are to be assessed by analytical methods or testing.

6. Site Specific Self-Elevating Mobile Offshore Units

(1) Self-Elevating Mobile Offshore Units converted to site dependent platform structures are to be designed in accordance with these Rules along with the Rules for Classification Mobile Offshore Units, wherever applicable.

(2) When selecting a unit for a particular site, due consideration should be given to soil conditions at the installation site.

(3) The bearing capacity and sliding resistance of the foundation are to be investigated. The founda-

tion design is to be in accordance with 705.

(4) Structural Analysis

(A) In the structural analysis, the leg to hull connections and soil/structure interaction are to be properly considered.

(B) The upper and lower guide flexibility, stiffness of the elevating/holding system, and any spe-

cial details regarding its interaction with the leg should be taken into consideration. For units with spud cans, the leg may be assumed pinned at the reaction point.

(C) For mat supported units, the soil structure interaction may be modelled using springs.

(5) While used as a site dependent platform structure, the calculated loads are to demonstrate that the maximum holding capacity of the jacking system will not be exceeded.

(6) Units with spud cans are to be preloaded on installation in order to minimize the possibility of significant settlement under severe storm conditions.