**2.8 Consideration of ultimate limit states at risk assessment of SP condition**

When forming a system of classification of ultimate limit states in routine operating conditions of objects and in case of occurrence of accidents and disasters in comprehensive technical systems, it is required to identify various combinations of states for five groups of situations [1, 2, 5]:


Ultimate limit stress for normal service conditions have to be in full reflected in design codes of potentially hazardous objects, consider a set of design operating modes and proceed from all previous operating experience of similar objects.

In case of violation of normal (i.e., abnormal) service conditions (at any deviation from planned operating procedure causing the necessity to change operating mode or stop an object without necessity to activate or use all safety systems) the given above types of ultimate limit states can be used, or more extensive and wide. Such expansion is caused by the increase of number of work abnormalities and range of operation parameters changes.

When analyzing a design accident requiring the stop of an object and activation of safety systems, in addition to mentioned above types, it is necessary to consider those types of ultimate limit states which occur at increased mechanical, thermal, electromagnetic and other loads at scheduled stages of accident development.

For beyond-design-basis accidents followed by full activation of safety systems, it is not possible to exclude considerable damages of the most critical components and the equipment in general; in this case, the ultimate limit states include not only standard ones, but also new ultimate limit states that are object specific at broad variation of load conditions at all stages of accidents development.

The hypothetical accidents are most severe, hardly probable and poorly studied, and the worst combination of the affecting factors and that is why it is necessary not only to provide the analysis of the ultimate limit states stated above but also to analyze the states at which significant changes of conditions of working substances and structural and mechanical conditions of engineering materials are possible.

When accidents (explosions, destruction, fires, collisions, collapses, chemically dangerous substances release) are occurring in the load bearing structures, the corresponding ultimate limit states are arising. At different stages of accidents development, these limit states can change both in the direction of scaling up of consequences, and in the direction of localization and full stop of the accident development.

infrastructure of *UT<sup>И</sup>* type (transportation, energy, pipeline, telecommunication

Environmental damages *US* defined by summation of damages to soil *USП*,

Damages and losses quantitatively are defined by two types of parameters:

• in physical units—scales (number of damaged objects and injured people,

In statistical estimation of the above damages, the summarized information

In probabilistic estimation of damages, the data from simulation modeling, data on probable areas covered by the affecting factors, and probabilistic and statistical data on vulnerability of objects, the environment and the population at various

In the analysis and risk assessment, various aspects of accidents and disasters occurrence and development including various dangerous processes, the factors initiating events, scenarios of development, objects and personnel pattern damage

The variety of issues to be studied in the analysis process and risk assessment requires application of various methods at various stages of the systems analysis of

Some methods in nature are integral ones; for example, the logical-and-probabilistic method, which includes a graph method, a probabilistic method, a logical reasoning method, event tree analysis and fault tree analysis are probabilistic

about emergencies from the state reports of departments can be used.

examined object safety, as well as their integrated application.

polluted and damaged territories by area); and

• in equivalent economic units (rubles, dollars).

Damages to the personnel and population *UN* are defined by summation of losses from fatalities *UN<sup>б</sup>* and losses from injuries (permanent injuries and health dam-

aquatic *USа*, air *US<sup>в</sup>* environment, flora *US<sup>р</sup>* and fauna *US<sup>ж</sup>* are as follows:

*UT* ¼ *UT<sup>П</sup>* þ *UT<sup>Г</sup>* þ *UTИ:* (16)

*UN* ¼ *UN<sup>б</sup>* þ *UNу:* (18)

*US* ¼ *US*<sup>П</sup> þ *US*<sup>а</sup> þ *US*<sup>в</sup> þ *US*<sup>р</sup> þ *US*ж*:* (17)

systems, etc.):

ages) *UNу*, which are as follows:

*Probability, Combinatorics and Control*

emergencies are used.

**Figure 11.**

**92**

function, etc. can be considered.

methods implementing the graph method.

*Basic scenarious of accident development on sea platform (SP).*

When determining safety of the most important objects, the following types of ultimate limit states have to be considered: plastic deformation and forming; shortduration elastic failure; delayed or fast brittle failure; long-term static fracture; cyclic (low- and multi-cycle) destruction; creep strain accumulation; cyclic strain accumulation; buckling; dangerous vibrations occurrence; coupled units wear; single loading cracks initiation and propagation; cyclic cracks initiation and propagation; corrosion, corrosion and mechanical, cavitation and erosive damages; leakages; and change of structures and a condition of the bearing components.

Between low criticality design elements, the following ones shall be listed:

• design elements of equipment supporter not identified as elements of critical

**2.9 Comprehensive assessment of risk, strength, in-service life, reliability and**

Characterization of initial strength, in-service life, risk and safety of the bearing elements of the sea oil and gas production platform in terms of impact of a complex of loads (including such specific service conditions as collisions with the drifting ice floes, impact of storm and gale-force winds, existence of the corrosive environment, low-temperature embrittlement effects, etc.) is the comprehensive problem considering occurrence of the cyclic dynamic loads corresponding to these conditions and, consequently, nonlinear change in time of the kinetic fields of stresses and deformations in these elements of SP under the impact of irregular loads [1–4]. In this regard in zones of design concentration, the local stresses and deformations have the increased values and the processes of material damage run more intensively leading to appearance of local destructions zones (cracks) eventually developing into macrodestructions (loss of bearing capacity). In such conditions, depending on the nature of loading and the operational environment, various mechanisms of accumulation of damages and destruction are implemented.

For the analysis of operational load of SP (as well as on other objects of energy, transport, oil and gas chemistry) at all stages of the life cycle, curves of the parameters dependent on calculated or real force impact on the bearing elements of the oil and gas production platform (set in the specification or measured during operation) are plotted. Among these parameters are number of loading cycles *N*, time *τ*, temperature *t* as well as service forcing *P*, stress *σ* and deformation *e*. The curves of parameters *P*, *t, σ* and *e* as function of time (**Figure 12**) are plotted for all stages and

• internal structure not involved in provision of general strength; and

*Hybrid Modeling of Offshore Platforms' Stress-Deformed and Limit States…*

*DOI: http://dx.doi.org/10.5772/intechopen.88894*

importance.

**safety**

operational phases.

**Figure 12.**

**95**

*Diagram of operational loads and their basic parameters.*

The ultimate limit states listed above identify methods, structure and criteria of safety analysis by integrated approaches of mechanics, physics and chemistry of disasters.

In the process of design of structure, its components and, at the bottom, the following groups of the ultimate limit states are taken into consideration. The first group with unacceptable plastic strain and damages includes ultimate limit states surpassing of which will cause total unusability of the structure or total (or partial) loss of supporting capacity of the platform substructure. The second group with damages accumulation and development includes the ultimate limit states where surpassing makes impossible the normal operation of the platform substructure.

It should be noted that the above-listed ultimate limit states were taken into account at design of the reinforced concrete support substructure of gravity type for offshore stationary platforms on the sites of the Sakhalin-II project for Piltun-Astokhsky (PA-B) and Lunsky (LUN-A) fields.

The design elements of the platform substructure can be split into criticality categories depending on the external impacts taken into account:

**High criticality design elements**—these are elements whose destruction can cause fatalities, serious damages to constructions and environment contamination.

**Low criticality design elements**—these are elements whose destruction will not cause fatalities, serious damages to constructions and environment contamination.

Between high criticality design elements, the following ones shall be listed:


*Hybrid Modeling of Offshore Platforms' Stress-Deformed and Limit States… DOI: http://dx.doi.org/10.5772/intechopen.88894*

Between low criticality design elements, the following ones shall be listed:

