Section 5 Steel Structures

501. General

1. General

(1) The requirements of this Chapter are to be applied in the design and analysis of the principal components of steel structures intended for offshore applications.

(2) Items to be considered in the design of welded connections are specified in Ch 3, Sec 2.

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2. Materials

(1) The requirements of this Chapter are intended for structures constructed of steel manufactured and having properties as specified in Ch 3, Sec 1.

(2) Where it is intended to use steel or other materials having properties differing from those speci-

fied in Ch 3, Sec 1, their applicability will be considered subject to a review of the specifica- tions for the alternative materials and the proposed methods of fabrication.

3. Corrosion Protection

(1) Materials are to be protected from the effects of corrosion by the sue of a corrosion protection system including the use of coatings. The system is to be effective from the time the structures is initially placed on site.

(2) Where the sea environment contains unusual contaminants, any special corrosive effects of such contaminants are also to be considered. (For the design of protection systems, reference is to be made to the NACE SP0176-2007, or other appropriate references)

(3) The design of corrosion protecting device is to be accordance with the discretion of this Society.

4. Access for Inspection

In the design of the structure, consideration should be given to providing access for inspection dur- ing construction and, to the extent practicable, for survey after construction.

5. Steel-Concrete Hybrid Structures

The steel portion of steel-concrete hybrid structures are to be designed in accordance with the re- quirements of this Chapter, and the concrete portions are to be designed as specified in Ch 4, Sec 6.

502. General Design Criteria

1. Application

(1) Steel structures are to be designed and analyzed for the loads to which they are likely to be exposed during construction, installation and in-service operations.

(2) To this end, the effects on the structure of a minimum set of loading conditions, as indicated

in 503. are to be determined, and the resulting structural responses are not to exceed the safety and serviceability criteria given below.

(A) The use of design methods and associated safety and serviceability criteria, other than those

specifically covered in this Chapter, is permitted where it can be demonstrated that the use of such alternative methods will result in a structure possessing a level of safety equivalent to that provided by the direct application of these requirements.

503. Loading Conditions

1. General

(1) Loadings which produce the most unfavorable effects on the structure during and after con- struction and installation are to be considered.

(2) Loadings to be investigated for conditions after installation are to include at least those relating

to both the realistic operating and design environmental conditions combined with other pertinent loads in the following manner.

(A) Operating environmental loading combined with dead and maximum live loads appropriate to the function and operations of the structures.

(B) Design environmental loading combined with dead and live loads appropriate to the function and operations of the structure during the design environmental condition.

(C) For structures located in seismically active areas, earthquake loads(see 303. 6) are to be combined with dead and live loads appropriate to the operation and function of the structure

which may be occurring at the onset of an earthquake.

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504. Structural Analysis

1. Application

(1) The nature of loads and loading combinations as well as the local environmental conditions are to be taken into consideration in the selection of design methods. Methods of analysis and their associated assumptions are to be compatible with the overall design principles.

(2) Linear, elastic methods(working stress methods) can be employed in design and analysis pro-

vided proper measures are taken to prevent general and local buckling failure, and the inter- action between soil and structure is adequately treated.

(3) When assessing structural instability as a possible mode of failure, the effects of initial stresses

and geometric imperfections are to be taken into account.

(4) Construction tolerances are to be consistent with those used in the structural stability assessment.

2. Dynamic analysis

(1) Dynamic effects structural natural in the structure.

(2) In assessing the

are to be accounted for if the wave energy in the frequency range of the frequencies is of sufficient magnitude to produce significant dynamic response

need for dynamic analyses of deep water or unique structures, information re-

garding the natural frequencies of the structure in its intended position is to be obtained.

(3) The determination of dynamic effects is to be accomplished either by computing the dynamic amplification effects in conjunction with a deterministic analysis or by a random dynamic analy-

sis based on a probabilistic formulation. In the latter case, the analysis is to be accompanied by a statistical description and evaluation of the relevant input parameters.

3. Plastic methods of Design

(1) For static loads, plastic methods of design and analysis can be employed only when the proper- ties of the steel and the connections are such that they exclude the possibility of brittle fracture, allow for formation of plastic hinges with sufficient plastic rotational capability, and provide ad- equate fatigue resistance.

(2) In a plastic analysis, it is to be demonstrated that the collapse mode(mechanism)which corre- sponds to the smallest loading intensities has been used for the determination of the ultimate strength of the structure. Buckling and other destabilizing nonlinear effects are to be taken into account in the plastic analysis.

(3) Whenever non-monotonic or repeating loads are present, it is to be demonstrated that the struc- ture will not fail by incremental collapse or fatigue.

(4) Under dynamic loads, when plastic strains may occur, the considerations specified in (1) are to

be satisfied and any buckling and destabilizing nonlinear effects are to be taken into account.

505. Allowable Stresses and Load Factors

1. Working Stress Approach

When design is based on a working stress method(see 504. 1 and 402.), the safety criteria are to be expressed in terms of appropriate basic allowable stresses in accordance with requirements speci- fied below.

(A) Structural members and loadings are to be in accordance with the discretion of this Society.

(B) Where stress in members is due to forces imposed by the design environmental condition acting alone or in combination with dead and live loads the basic allowable stresses may be increased by one-third provided the resulting structural member sizes are not less that those required for the operating environment loading combined with dead and live loads without the one-third increase in allowable stresses.

(C) When considering loading combinations which include earthquake loads(see 503.) on in- dividual members or on the overall structure, the allowable stress may be taken 1.7 times the basic allowable stress of the member.

(D) The allowable stresses specified in (B) are to be regarded as the limits for stresses in all structural parts for the marine operations covered in Sec 9, except for lifting, where the one third increase in the basic allowable stress is not permitted. The lifting analysis should adequately account for equipment and fabrication weight increase.

(E) For any two or three dimensional stress field within the scope of the working stress for- mulation, the equivalent stress(e.g., von Mises stress intensity) is to be limited by an appro- priate allowable stress less than yield stress, with the exception of those stresses of a highly

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localized nature, In the latter case, local yielding of the structure may be accepted provided it can be demonstrated that such yielding does not lead to progressive collapse of the over- all structure and that the general structural stability is maintained.

(F) Whenever elastic instability, overall or local, may occur before the stresses reach their basic allowable levels, appropriate allowable buckling stresses govern.

2. Plastic Design Approach

(1) Whenever the ultimate strength of the structure is used as the basis for the design of its mem- bers, the safety factors or the factored loads are to be formulated in accordance with the re- quirements of this Society or an equivalent code.

(2) The capability of the principle structural members to develop their pacity is to be demonstrated.

predicted ultimate load ca-

(3) For safety against brittle fracture, special attention is to be given to details of high stress con- centration and to improved material quality.

506. Others

1. Structural Response to Earthquake Loads

(1) Structures located in seismically active areas are to be designed to possess adequate strength and stiffness to withstand the effects of strength level earthquake, as well as sufficient ductility to remain stable during rare motions of greater severity associated with ductility level earthquake. The sufficiency of the structural strength and ductility is to be demonstrated by strength and, as required, ductility analysis.

(2) For strength level earthquake, the strength analysis is to demonstrate that the structure is ad- equately sized for strength and stiffness to maintain all nominal stresses within their yield or buckling limits.

(3) In the ductility analysis, it is to be demonstrated that the structure has the capability of absorb-

ing the energy associated with the ductility level earthquake without reaching a state of in- cremental collapse.

(4) The design criteria for earthquake is to be in accordance with the discretion of this Society.

2. Fatigue Assessment

(1) For structural members and joints where fatigue is a probable mode of failure, or for which past experience is insufficient to assure safety from possible cumulative fatigue damage, an as- sessment of fatigue life is to be carried out. Emphasis is to be given to joints and members in the splash zone, those that are difficult to inspect and repair once the structure is in service, and those susceptible to corrosion-accelerated fatigue.

(2) For structural members and joints which require a detailed assessment of cumulative fatigue damage, the results of the assessment are to indicate a minimum expected fatigue life of twice

the design life of

strophic failure of dundancy does not

the structure where sufficient structural redundancy exists to prevent cata-

the structure of the member or joint under consideration. Where such re- exist or where the desirable degree of redundancy is significantly reduced as

a result of fatigue damage, the result of a fatigue assessment is to indicate a minimum expected

fatigue life of three or more times the design life of the structure.

(3) A spectral fatigue analysis technique is recommended to calculate the fatigue life of the struc- ture, Other rational analysis methods are also acceptable if the forces and member stresses can

be properly represented. The dynamic effects nificant to the structural response.

3. Stresses in Connections

should be taken into consideration if they are sig-

(1) Connections of structural members are to be developed to insure effective load transmission be-

tween joined members, to minimize stress

shear.

concentration and to prevent excessive punching

(2) Connection details are also to be designed to minimize undue constraints against overall ductile behavior and to minimize the effects of postweld shrinkage. Undue concentration of welding is to be avoided.

(3) The design of tubular joints may be in accordance the discretion of this Society.

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4. Structure-Pile Connections

(1) The attachment of the structure to its foundation is to be accomplished by positive, controlled means such as welding or grouting, with or without the use of mechanical shear keys or other mechanical connectors.

(2) General references may be made to the discretion of this Society where the ratio of the diame- ter to thickness of either the pile or the sleeve is less than or equal to 80. Where a ratio ex-

ceeds 80, special consideration is to be give to the effects of reduced confinement on allowable bond stress. Particulars of grouting mixtures are to be submitted for review.

(3) The allowable stresses or load factors to be employed in the design of foundation structure for steel gravity bases or piles are to be in accordance with 505. 1 or 2, and with regard to later- ally loaded piles in accordance with 704. 5.

5. Structural Response to Hydrostatic Loads

Analyses of the structural stability are to be performed to demonstrate the ability of structural parts to withstand hydrostatic collapse at the water depths at which they will be located.

6. Deflections

The platform deflections which may affect the design of piles, conductors, risers and other struc- tures in way of the platform are to be considered. Where appropriate, the associated geometric non- linearity is to be accounted for in analysis.

7. Local Structure

(1) Structures which do not directly contribute to the overall strength of the fixed offshore struc- ture, i.e., their loss or damage would not impair the structural integrity of the offshore structure, are considered to be local structure.

(2) Local structures are to be adequate for the nature and magnitude of applied loads. Allowable stresses specified in 505. are to be used as stress limits except for those structural parts whose

primary function is to absorb energy, in which case sufficient ductility is to be demonstrated.