**2.7 Design loads**

The loading on offshore ice-resistant oil and gas structures can be split in three groups: permanent, temporary and special loads [1, 2]. Among permanent loads are the loads of the structure weight Рs.w. and self-weight of soil and soil pressure on fixed piles. The temporary loads are subdivided into long and short term, namely:

	- weight of equipment and drilling rig;
	- weight of liquids, bulk materials and stocks of drill pipes and tubing;
	- weight of warehouse equipment and tools; and
	- weight of drilling cuttings (bore mud, etc.).
	- load on drilling rig in and derrick table during drill string trip;
	- snow loads (used for design of bowl type helicopter deck);


**Table 2.** *Accident and threat types.* *Hybrid Modeling of Offshore Platforms' Stress-Deformed and Limit States… DOI: http://dx.doi.org/10.5772/intechopen.88894*


the greatest possible damages (total destruction of OGPD) and a large number

Group U2 group: the threats causing the beyond-design-basis accidents which are followed by permanent damages of the SP critical components with high

Group U3: the threats causing the design accidents followed by standard

Group U4: the threats causing the SP operating mode accidents followed by deviations from normal operation conditions while OGPD is operating in

The loading on offshore ice-resistant oil and gas structures can be split in three groups: permanent, temporary and special loads [1, 2]. Among permanent loads are the loads of the structure weight Рs.w. and self-weight of soil and soil pressure on fixed piles. The temporary loads are subdivided into long and short term, namely:

◦ weight of liquids, bulk materials and stocks of drill pipes and tubing;

◦ load on drilling rig in and derrick table during drill string trip;

**Type of accident Threat causing the accident Type of**

including terrorist attacks.

Design accidents (Type Т3) The affecting factors are known and predictable. U3

controlled.

to predict constructive, technological initiating events and affecting factors of huge intensity,

The affecting factors, the initiating events and damages development are not known in full.

The affecting factors are well understood and

The affecting factors are studied and controlled. U4

**threat**

U1

U2

U5

◦ snow loads (used for design of bowl type helicopter deck);

Hypothetical accidents (Type Т1) Combination of unknown, unlikely and the difficult

outperformance with predictable and acceptable consequences.

Group U5: the threats when an object operates in standard mode.

◦ weight of equipment and drilling rig;

◦ weight of warehouse equipment and tools; and

◦ weight of drilling cuttings (bore mud, etc.).

of the victims.

standard mode.

• Long-term load:

• Short-term load:

Beyond-design-basis accidents

Operating mode accidents (deviations from standard conditions) (Type Т4)

Normal (standard) operating conditions (Type Т5)

*Accident and threat types.*

(Type Т2)

**Table 2.**

**88**

**2.7 Design loads**

level of damages and fatalities.

*Probability, Combinatorics and Control*


The special loads are the seismic ones *Рseism* and those initiated by natural phenomena (structure base subsidence, additional dynamic loads due to impact of ice filed on the structure imbedded in ice); and ice load due to hummocked nature of ice fields (collision of the structure and iceberg). Seismic impacts are taken into account during design of stationary platforms constructed in different regions with seismic magnitude of 7, 8 and 9.

For definition of seismic loads, it is required to have data on seismological parameters of seismic zones: magnitudes, depths of earthquake sources, the epicentral distances, earthquakes frequency, seismicity of the site and spectral characteristics of seismic impacts depending on engineering-geological conditions on construction sites.

Various types of loads on ice-resistant stationary platforms are schematically presented in **Figure 10**.

When calculating the wind and wave loadings, it is expedient to accept load factor for one of loadings equal to 0.9, and for another equal to 1. This assumption is based on more realistic knowledge (from physical point of view) by reference to correlation between these processes. In the case of basic combination, the calculated values of short-term loadings (wind, wave and current) respectively refer to the reliability factor which is equal to 1. For special combinations, these loadings are calculated with factor 0.8, however, at the same time, as well as in the previous case, two possibilities of wind and wave impacts on ice-resistant structures are taken into consideration.

#### **Figure 10.**

*Symbolic diagram of application of external loads on ice-resistant stationary platforms: 1—derrick; 2—deck; 3—jack structure; and 4—bottom module. For loads, the following symbols are used: Рsw—gravity force; and Рх, Ру—horizontal (shear) and vertical (transverse) reactions.*


*<sup>R</sup>* <sup>¼</sup> <sup>X</sup>*<sup>n</sup> i*¼1

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

*<sup>P</sup>* <sup>¼</sup> *Fp*f g *PN*ð Þ*<sup>t</sup>* , *PT*ð Þ*<sup>t</sup>* , *PS*ð Þ*<sup>t</sup>* <sup>¼</sup> <sup>X</sup>

*<sup>U</sup>* <sup>¼</sup> *FU*f g *UN*ð Þ*<sup>t</sup>* , *UT*ð Þ*<sup>t</sup>* , *US*ð Þ*<sup>t</sup>* <sup>¼</sup> <sup>X</sup>

environment *S* correspondingly*.*

probability of fatalities.

approach is applied.

same time.

**91**

probability will be defined by the formula:

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

nature.

nario No. *I* and *n* is the number of possible outcomes of an emergency.

where *Р(Аi)* is the probability of causing damage *Ui* to a technological object and other objects, the population and the environment in case of the emergency sce-

Generally, the probability *P* of occurrence of analyzed unfavorable event (or its components *Pi*) is defined as the function of function (functionality) depending on sources, corresponding affecting factors and objects subject to damage: person *N*, off-shore technosphere object *T* and environment *S*; taking into account (3), the

*i*

*PN*ð Þ*t* , *PT*ð Þ*t* , *PS*ð Þ*t* are the probabilities of occurrence at time *t* of unfavorable (dangerous) event initiated correspondingly by human factor, technosphere or

*i*

*UN*ð Þ*t* , *UT*ð Þ*t* , *US*ð Þ*t* are damages caused by unfavorable (dangerous) events at time *t* from which suffered population *N*, objects of a technosphere *T* and the

At the present stage of technical regulation, it is recommended to estimate the quantities of damages *U* and total risk *R* from unfavorable events by two indicators: economic—in dollars, rubles (conventional units) and in human losses (fatalities or non-fatal outcomes). Human losses should be estimated by the number of injured or

Taking into account expressions of (13) and (14), components of damages and probabilities of accidents can be calculated separately by use of various methods of risk assessment. Also from the expression of the risk (12) presenting the summation of risks of different emergencies, it becomes clear that to define the total risk, the various methods for definition of its components can be used, i.e., the combined

Combined risk analysis is based on the systematic approach that provides review of the system of interest in a formalized manner, i.e., by studying of subsystems' components by considering structural and functional features of this system at the

The damages and losses *U* caused by technogenic accidents and disasters are

where *UT* are damages to off-shore technosphere objects; *US* are environmental

defined by three basic components by taking into account expression (4):

damages; and *UN* are damages to the population (to the person and society). Damages *UT* are defined by summation of damages from destruction of industrial buildings and constructions of *UT<sup>П</sup>* type; damages from destruction of civilian (residential) objects of *UT<sup>Г</sup>* type; and damages from destruction of

population *N*, objects of a technosphere *T* and the environment *S* as follows*:*

The general damage *U* or its components *Ui* defined by damages affected by the

*FUi UNi*

*FPi PNi*

ð Þ*t* , *PTi*

ð Þ*t* , *UTi*

*U* ¼ *UT* þ *US* þ *UN*, (15)

ð Þ*t* , *PSi* ð Þ ð Þ*t :* (13)

ð Þ*t* , *USi* ð Þ ð Þ*t :* (14)

*P*ð Þ А*<sup>i</sup> Ui*,ð Þ *i* ¼ *1*, … , *n* , (12)

#### **Table 3.**

*Factors of loads combinations.*

As an example of the case when simultaneous impact of the wide spectrum random loadings on ice-resistant structures for sea of Okhotsk conditions can use the approach based on factors of loads combination shown in **Table 3**.

In the given case, it is proposed to analyze the following loads combinations:


In special combinations, the seismic load of calculated earthquake with magnitude 8 is accepted allowing the possible side dynamic effects: liquefaction of soil in the construction bottom and relevant subsidence, additional hydrodynamic loadings from ground shaking in case of open water and impact of ice fields on construction jacks during the winter period. However, depending on the earthquake source location, the specified side effects can happen with considerable time lag with respect to ground shake time, and summation of the caused by them dynamic impacts on a construction with seismic loads does not happen. Impact of the hummocky ice-fields can have very serious consequences for a construction; therefore, such case has to be separated as special loading and be analyzed in other special combination of loads.

In terms of (1)–(5), the total risk *R* of SP operation as mathematical expectation of incurred damages *U* should be presented as follows [3]:

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

$$R = \sum\_{i=1}^{n} P(\mathbf{A}\_i) U\_i, (i = 1, \dots, n), \tag{12}$$

where *Р(Аi)* is the probability of causing damage *Ui* to a technological object and other objects, the population and the environment in case of the emergency scenario No. *I* and *n* is the number of possible outcomes of an emergency.

Generally, the probability *P* of occurrence of analyzed unfavorable event (or its components *Pi*) is defined as the function of function (functionality) depending on sources, corresponding affecting factors and objects subject to damage: person *N*, off-shore technosphere object *T* and environment *S*; taking into account (3), the probability will be defined by the formula:

$$P = F\_p\{P\_N(t), P\_T(t), P\_S(t)\} = \sum\_i F\_{P\_i}(P\_{N\_i}(t), P\_{T\_i}(t), P\_{S\_i}(t)).\tag{13}$$

*PN*ð Þ*t* , *PT*ð Þ*t* , *PS*ð Þ*t* are the probabilities of occurrence at time *t* of unfavorable (dangerous) event initiated correspondingly by human factor, technosphere or nature.

The general damage *U* or its components *Ui* defined by damages affected by the population *N*, objects of a technosphere *T* and the environment *S* as follows*:*

$$U = F\_U\{U\_N(t), U\_T(t), U\_S(t)\} = \sum\_i F\_{U\_i}(U\_{N\_i}(t), U\_{T\_i}(t), U\_{S\_i}(t)).\tag{14}$$

*UN*ð Þ*t* , *UT*ð Þ*t* , *US*ð Þ*t* are damages caused by unfavorable (dangerous) events at time *t* from which suffered population *N*, objects of a technosphere *T* and the environment *S* correspondingly*.*

At the present stage of technical regulation, it is recommended to estimate the quantities of damages *U* and total risk *R* from unfavorable events by two indicators: economic—in dollars, rubles (conventional units) and in human losses (fatalities or non-fatal outcomes). Human losses should be estimated by the number of injured or probability of fatalities.

Taking into account expressions of (13) and (14), components of damages and probabilities of accidents can be calculated separately by use of various methods of risk assessment. Also from the expression of the risk (12) presenting the summation of risks of different emergencies, it becomes clear that to define the total risk, the various methods for definition of its components can be used, i.e., the combined approach is applied.

Combined risk analysis is based on the systematic approach that provides review of the system of interest in a formalized manner, i.e., by studying of subsystems' components by considering structural and functional features of this system at the same time.

The damages and losses *U* caused by technogenic accidents and disasters are defined by three basic components by taking into account expression (4):

$$U = U\_T + U\_S + U\_N,\tag{15}$$

where *UT* are damages to off-shore technosphere objects; *US* are environmental damages; and *UN* are damages to the population (to the person and society).

Damages *UT* are defined by summation of damages from destruction of industrial buildings and constructions of *UT<sup>П</sup>* type; damages from destruction of civilian (residential) objects of *UT<sup>Г</sup>* type; and damages from destruction of

As an example of the case when simultaneous impact of the wide spectrum random loadings on ice-resistant structures for sea of Okhotsk conditions can use

In the given case, it is proposed to analyze the following loads combinations:

II. combination of loads during construction and assembling works in ice-free

IV. combination for calculation of maximum efforts in structures of the topside

V. special combination allowing ice loads occurring during freeze-up period;

In special combinations, the seismic load of calculated earthquake with magnitude 8 is accepted allowing the possible side dynamic effects: liquefaction of soil in the construction bottom and relevant subsidence, additional hydrodynamic loadings from ground shaking in case of open water and impact of ice fields on construction jacks during the winter period. However, depending on the earthquake source location, the specified side effects can happen with considerable time lag with respect to ground shake time, and summation of the caused by them dynamic impacts on a construction with seismic loads does not happen. Impact of the hummocky ice-fields can have very serious consequences for a construction; therefore, such case has to be separated as special loading and be analyzed in other special

In terms of (1)–(5), the total risk *R* of SP operation as mathematical expectation

VI. basic loads combination during freeze-up period depending on cycles'

the approach based on factors of loads combination shown in **Table 3**.

**Types of calculated loads Combinations**

Dead loads 1.0 1.0 0.9 1.0 1.0 1.0 Long-term live loads 0.95 — 0.8 1.0 0.95 0.95

• *ice load* (*h* = 0.8 m); — — 0.8 — — 1.0 • *wave load* (repeated once in 100 years); 1.0 1.0 —— — — • *wind load;* 0.9 0.9 0.8 1.0 0.8 0.9 • *current load* 0.9 0.9 0.8 — 0.8 0.9

• *ice load* (*h* = 2.5 m); — ——— 1.0 — • *seismic load* — — 1.0 —— —

**I II III IV V VI**

I. basic combination of loads during ice-free season;

III. special combination allowing for seismic loads;

of incurred damages *U* should be presented as follows [3]:

season;

Short-term live loads:

*Probability, Combinatorics and Control*

Special loads:

*Factors of loads combinations.*

**Table 3.**

facilities;

number.

combination of loads.

**90**

and

infrastructure of *UT<sup>И</sup>* type (transportation, energy, pipeline, telecommunication systems, etc.):

$$U\_T = U\_{T\bar{\Pi}} + U\_{T\bar{\Gamma}} + U\_{T\bar{\Pi}}.\tag{16}$$

The main possible events chains for scenarios of accidents on OGPF are presented in **Figure 11**. The main events (faults) causing accidents are the leak and rupture of technical pipelines. These faults cause development of accidents in various scenarios and corresponding damages. All these possible scenarios and

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

**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

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

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

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

corresponding damages have to be taken into account.

• ultimate limit states for regular service conditions;

• ultimate limit states for abnormal service conditions;

• ultimate limit states for beyond-design-basis accident; and

variation of load conditions at all stages of accidents development.

of states for five groups of situations [1, 2, 5]:

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

• ultimate limit states for designed accident;

• ultimate limit states for hypothetical accident.

range of operation parameters changes.

development.

**93**

Environmental damages *US* defined by summation of damages to soil *USП*, aquatic *USа*, air *US<sup>в</sup>* environment, flora *US<sup>р</sup>* and fauna *US<sup>ж</sup>* are as follows:

$$U\_{\rm S} = U\_{\rm SII} + U\_{\rm Sa} + U\_{\rm Sn} + U\_{\rm Sp} + U\_{\rm Sc} \,. \tag{17}$$

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 damages) *UNу*, which are as follows:

$$U\_N = U\_{N\delta} + U\_{N\text{y}}.\tag{18}$$

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


In statistical estimation of the above damages, the summarized information about emergencies from the state reports of departments can be used.

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 emergencies are used.

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 function, etc. can be considered.

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 examined object safety, as well as their integrated application.

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 methods implementing the graph method.

**Figure 11.** *Basic scenarious of accident development on sea platform (SP).*

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

The main possible events chains for scenarios of accidents on OGPF are presented in **Figure 11**. The main events (faults) causing accidents are the leak and rupture of technical pipelines. These faults cause development of accidents in various scenarios and corresponding damages. All these possible scenarios and corresponding damages have to be taken into account.
