Defects Assessment in Subsea Pipelines by Risk Criteria

*Anatoly Lepikhin, Victor Leschenko and Nikolay Makhutov*

### **Abstract**

Subsea inter-field pipelines are an important element of offshore oil and gas infrastructure. Leakage or fracture of these pipelines is associated with the risk of large economic and environmental losses. One of the main sources of pipeline fracture is pipe defects. The presented section discusses the methodological aspects of assessing the hazard of defects of subsea inter-field pipelines by risk criteria of accidents. A conceptual approach of defects hazard assessing by risk criteria has been formulated, based on analysis the requirement of modern standards. The risk is defined as the probability of negative consequences, the scale of which is determined by the hazard class of pipeline accidents. The probability and scale of accidents are linked by a risk matrix. A method for a three-level assessment of the suitability of a pipeline for operation after in-line inspection has been developed. The method allows assessing the hazard of the most typical defects in subsea pipelines, such as metal loss, metal delamination, cracks and crack-like defects. The allowable defect sizes are determined for the given risk criteria using partial safety factors. The novelty of the methodology lies in the substantiation of safety factors according to risk criteria corresponding to a given class of damage and loss. A scheme for making decisions on the admissibility of defects by risk criteria has been developed. An example of hazard assessment of defects in subsea pipelines is presented.

**Keywords:** subsea inter-field pipelines, defect, fracture, criterion, risk, calculation

### **1. Introduction**

Subsea inter-field pipelines are an important element of the offshore oil and gas condensate field infrastructure. Leakage or breakdown of these pipelines is associated with the risk of large economic and environmental damage. To ensure the safe operation of pipelines, systematic non-destructive testing is carried out using in-line diagnostics. Production, construction and operational defects of pipes are often by found the diagnostics. The presence of defects in the pipes requires solving the problem of classification and risk assessment of defects. In the classical setting, this problem is solved on the basis of the norms allowable defect sizes [1, 2]. The unacceptable defects are subject to mandatory repair or elimination. This approach is irrationality and has been repeatedly discussed and criticized from various points of view [3–5]. Classifications of defects on the basis of calculation their hazard, taking into account the peculiarities of the operating conditions are more reasonable. At the moment such calculation base on using a number of methods [6–10]. However, these methods also have a number of disadvantages. The most significant disadvantage is that they are based on a deterministic concept of ensuring strength, with deterministic defects sizes, loads values and characteristics of mechanical properties of pipe metal. In real conditions, random variations and statistical scattering of calculated variables always occur, which violate the uniqueness of the estimates of the hazard of defects. Taking this into account, the methods assessment of defects hazard based on the normative approach and deterministic strength calculations can be considered justified during the construction or reconstruction of pipelines. But they are irrational at the stage of pipeline operation, when deviations from design solutions, specified technological modes, environmental conditions and other factors affecting the performance arise. In such conditions, some of the permissible defects can be dangerous, and vice versa, pipelines with defects that are unacceptable according to the norms can be (and often turn out to be) workable.

• construction and pipe metal defects;

*Defects Assessment in Subsea Pipelines by Risk Criteria DOI: http://dx.doi.org/10.5772/intechopen.94851*

–4.21 <sup>10</sup><sup>4</sup>

–7.10 <sup>10</sup><sup>5</sup>

external influences is in the ranges: DOT is (5.52 <sup>10</sup><sup>6</sup>

–9.46 <sup>10</sup><sup>5</sup>

(1.16 <sup>10</sup><sup>6</sup>

(1.01 <sup>10</sup><sup>5</sup>

\$104

(10<sup>6</sup>

–\$10<sup>7</sup>

–10<sup>3</sup>

of risks can be \$10<sup>1</sup>

application.

**151**

also the time of their restoration.

**3. Brief of the problem of defects hazard assessing**

PARLOC is (1.53 <sup>10</sup><sup>5</sup>

• natural impacts (landslides, earthquakes, underwater currents, etc.).

At the same time, the average failure rate due to corrosion is in the range

) 1/km year (PARLOC data) and in the range

) 1/km year. According to the data [13] for the period 1970–2009 years 6183 accidents of subsea pipelines occurred in the world. The main number of accidents was recorded in the North Sea (3505) and the Gulf of Mexico (1658). In the Mediterranean Sea, the number of accidents was 45, in the Black and Caspian Seas – 29 accidents. At the same time, up to 41% of accidents occur due to external reasons, and up to 47% of accidents due to pipe defects. According to [14], 95 accidents occurred on the continental shelf of Great Britain in 2012–2013 years, of which 49 accidents occurred due to mechanical reasons (defects, fatigue, corrosion, erosion). Of particular interest are assessments of damage from pipeline accidents. Unfortunately, such data are rarely published. In the above-mentioned work [12], it is noted that the total damage from 125 accidents of subsea pipelines in 2012 year amounted to \$138,757 million, which gives an average damage per accident of about \$1,11 million. According to [15], the total direct economic losses from accidents on US gas pipelines for the period 1986–2012 years amounted to \$558,778 million. According to [16], the average damage from accidents at gas and oil pipelines is

, excluding the cost of gas losses. The actual gas losses reach 10<sup>4</sup> m<sup>3</sup>

) 1/km year. Therefore, these values can be considered as the initial

–\$10<sup>4</sup> per accident. It should be noted that these values only

As follows from the data presented, the frequency ranges of accidents for various water areas, pipelines and their operating conditions are within the range of

ones for substantiating the criteria for assessing the hazard of defects. Taking into account these frequencies and the amounts of damages presented above, the range

include direct damages. Taking into account consequential damages, the risks can be significantly higher. It should also be emphasized that recently, risk assessments have taken into account not only the cost of restoring objects after accidents, but

The problem of assessing the safety of pipelines arose at the turn of the 50s - 60s

due to the aging of pipeline systems in the United States. Later it became relevant for pipeline systems in other countries. The initial approaches to its solution were based on the methods of fracture mechanics, since the most largescale accidents were caused by the development of cracks. For a number of reasons (the need for special tests, imperfection of models, the use of steels with increased crack resistance in pipes, etc.) they have not found wide practical

The pipeline transport development in the 1970s adduce three significant changes: pipeline systems swept the all world; the problem of ensuring the safety of pipeline systems, taking into account their aging, has become global; methods of in-line inspections (ILI) are become widely used. The ILI showed the presence of various types of defects in the pipes that reduce the efficiency of pipelines. Takin this in to account the defects hazard assessment began to occupy a special place in

) 1/km year (DOT data). The average failure rate due to

–1.3 <sup>10</sup>–<sup>4</sup>

) 1/km year;

.

Probabilistic risk analysis methods develop to assess the operability of structures with defects [11]. In these methods, the defect hazard is determined by the level of risk of pipeline destruction. This ensures, on the one hand, taking into account the probabilities of violation of the strength conditions in the presence of defect, on the other hand, taking into account the severity of the consequences of accidents. This article discusses the methodological aspects of assessing the defects hazard in subsea inter-field pipelines according by the criteria of the risks of destruction. Risk is understood as the probability of losses from leakage or pipeline failure, caused by the considered defects. This formulation of the problem differs significantly from the above-mentioned traditional approaches to assessing the defects hazard, based on strength calculations.

## **2. Accident analysis of subsea pipelines**

The safety of operation subsea pipelines is ensured by using modern methods of design, manufacture, operation and maintenance, regulated by the rules and regulations. Nevertheless, the practice of operating pipelines is accompanied by cases of fracture with negative consequences. Currently, a large amount of statistical data has been accumulated on accidents of onshore and subsea pipelines. Statistical data on emergency conditions for subsea pipelines qualitatively and quantitatively differ from statistics on emergency and underground pipelines due to differences in operating conditions and modes. Therefore, the statistical data for surface and underground pipelines can only be taken into account for qualitative comparisons.

Accident rate statistics for subsea pipelines are mainly presented for the water areas and continental shelf of the North Sea (PARLOC database) and the Gulf of Mexico (DOT database) [12]. These data cover the period from 1984 to the present, with operating experience over 480 thousand km year (PARLOC base) and over 650 thousand km year (DOT base). According to PARLOC data the average failure rate is 8.79 <sup>10</sup><sup>5</sup> 1/km year, and according to DOT data is 3.51 <sup>10</sup><sup>4</sup> 1/km year. For comparison, according to the UKOPA database (Great Britain), which includes statistics on underground pipelines, with experience over 700 thousand km year, the average failure rate is 4.86 <sup>10</sup><sup>5</sup> 1/km year. According to the EGIG database (European Union), which also includes data on the accident rate of underground pipelines, with experience over 3150 thousand km year, the average failure rate is 3.70 <sup>10</sup><sup>4</sup> 1/km year. According to statistics, the main reasons for failure of subsea pipelines are:


• construction and pipe metal defects;

disadvantage is that they are based on a deterministic concept of ensuring strength, with deterministic defects sizes, loads values and characteristics of mechanical properties of pipe metal. In real conditions, random variations and statistical scattering of calculated variables always occur, which violate the uniqueness of the estimates of the hazard of defects. Taking this into account, the methods assessment of defects hazard based on the normative approach and deterministic strength calculations can be considered justified during the construction or reconstruction of pipelines. But they are irrational at the stage of pipeline operation, when deviations from design solutions, specified technological modes, environmental conditions and other factors affecting the performance arise. In such conditions, some of the permissible defects can be dangerous, and vice versa, pipelines with defects that are unacceptable according to the norms can be (and often turn out to be) workable. Probabilistic risk analysis methods develop to assess the operability of structures with defects [11]. In these methods, the defect hazard is determined by the level of risk of pipeline destruction. This ensures, on the one hand, taking into account the probabilities of violation of the strength conditions in the presence of defect, on the other hand, taking into account the severity of the consequences of accidents. This article discusses the methodological aspects of assessing the defects hazard in subsea inter-field pipelines according by the criteria of the risks of destruction. Risk is understood as the probability of losses from leakage or pipeline failure, caused by the considered defects. This formulation of the problem differs significantly from the above-mentioned traditional approaches to assessing the defects hazard, based

*Issues on Risk Analysis for Critical Infrastructure Protection*

The safety of operation subsea pipelines is ensured by using modern methods of design, manufacture, operation and maintenance, regulated by the rules and regulations. Nevertheless, the practice of operating pipelines is accompanied by cases of fracture with negative consequences. Currently, a large amount of statistical data has been accumulated on accidents of onshore and subsea pipelines. Statistical data on emergency conditions for subsea pipelines qualitatively and quantitatively differ from statistics on emergency and underground pipelines due to differences in operating conditions and modes. Therefore, the statistical data for surface and underground pipelines can only be taken into account for qualitative comparisons. Accident rate statistics for subsea pipelines are mainly presented for the water areas and continental shelf of the North Sea (PARLOC database) and the Gulf of Mexico (DOT database) [12]. These data cover the period from 1984 to the present, with operating experience over 480 thousand km year (PARLOC base) and over 650 thousand km year (DOT base). According to PARLOC data the average failure rate is 8.79 <sup>10</sup><sup>5</sup> 1/km year, and according to DOT data is 3.51 <sup>10</sup><sup>4</sup> 1/km year. For comparison, according to the UKOPA database (Great Britain), which includes statistics on underground pipelines, with experience over 700 thousand km year, the average failure rate is 4.86 <sup>10</sup><sup>5</sup> 1/km year. According to the EGIG database (European Union), which also includes data on the accident rate of underground pipelines, with experience over 3150 thousand km year, the average failure rate is 3.70 <sup>10</sup><sup>4</sup> 1/km year. According to statistics, the main

• mechanical damage (hooking with anchors and trawls, falling heavy objects);

on strength calculations.

**2. Accident analysis of subsea pipelines**

reasons for failure of subsea pipelines are:

• corrosion and aging processes;

**150**

• natural impacts (landslides, earthquakes, underwater currents, etc.).

At the same time, the average failure rate due to corrosion is in the range (1.16 <sup>10</sup><sup>6</sup> –4.21 <sup>10</sup><sup>4</sup> ) 1/km year (PARLOC data) and in the range (1.01 <sup>10</sup><sup>5</sup> –7.10 <sup>10</sup><sup>5</sup> ) 1/km year (DOT data). The average failure rate due to external influences is in the ranges: DOT is (5.52 <sup>10</sup><sup>6</sup> –1.3 <sup>10</sup>–<sup>4</sup> ) 1/km year; PARLOC is (1.53 <sup>10</sup><sup>5</sup> –9.46 <sup>10</sup><sup>5</sup> ) 1/km year.

According to the data [13] for the period 1970–2009 years 6183 accidents of subsea pipelines occurred in the world. The main number of accidents was recorded in the North Sea (3505) and the Gulf of Mexico (1658). In the Mediterranean Sea, the number of accidents was 45, in the Black and Caspian Seas – 29 accidents. At the same time, up to 41% of accidents occur due to external reasons, and up to 47% of accidents due to pipe defects. According to [14], 95 accidents occurred on the continental shelf of Great Britain in 2012–2013 years, of which 49 accidents occurred due to mechanical reasons (defects, fatigue, corrosion, erosion).

Of particular interest are assessments of damage from pipeline accidents. Unfortunately, such data are rarely published. In the above-mentioned work [12], it is noted that the total damage from 125 accidents of subsea pipelines in 2012 year amounted to \$138,757 million, which gives an average damage per accident of about \$1,11 million. According to [15], the total direct economic losses from accidents on US gas pipelines for the period 1986–2012 years amounted to \$558,778 million. According to [16], the average damage from accidents at gas and oil pipelines is \$104 –\$10<sup>7</sup> , excluding the cost of gas losses. The actual gas losses reach 10<sup>4</sup> m<sup>3</sup> .

As follows from the data presented, the frequency ranges of accidents for various water areas, pipelines and their operating conditions are within the range of (10<sup>6</sup> –10<sup>3</sup> ) 1/km year. Therefore, these values can be considered as the initial ones for substantiating the criteria for assessing the hazard of defects. Taking into account these frequencies and the amounts of damages presented above, the range of risks can be \$10<sup>1</sup> –\$10<sup>4</sup> per accident. It should be noted that these values only include direct damages. Taking into account consequential damages, the risks can be significantly higher. It should also be emphasized that recently, risk assessments have taken into account not only the cost of restoring objects after accidents, but also the time of their restoration.

### **3. Brief of the problem of defects hazard assessing**

The problem of assessing the safety of pipelines arose at the turn of the 50s - 60s due to the aging of pipeline systems in the United States. Later it became relevant for pipeline systems in other countries. The initial approaches to its solution were based on the methods of fracture mechanics, since the most largescale accidents were caused by the development of cracks. For a number of reasons (the need for special tests, imperfection of models, the use of steels with increased crack resistance in pipes, etc.) they have not found wide practical application.

The pipeline transport development in the 1970s adduce three significant changes: pipeline systems swept the all world; the problem of ensuring the safety of pipeline systems, taking into account their aging, has become global; methods of in-line inspections (ILI) are become widely used. The ILI showed the presence of various types of defects in the pipes that reduce the efficiency of pipelines. Takin this in to account the defects hazard assessment began to occupy a special place in

the security problem. To solve this problem, the methods ASME B31, APT1160, RSTRENG, DNV and others focused on the analysis of the most common defects in the form of corrosion damage [17] were developed. Parallel to this, the methods of breaking mechanics have developed and improved, which are reflected in the standards BS7910, API RP579, SINTAP.

**4. The concept assessing for hazard of defects by risk criteria**

*Defects Assessment in Subsea Pipelines by Risk Criteria DOI: http://dx.doi.org/10.5772/intechopen.94851*

can be write as:

defect size in pipeline.

deterministic values.

*f*(*li*) and *f*(*lr*).

**Figure 1.**

**153**

*Probabilistic scheme of the defects hazard analysis.*

The above analysis shows that pipeline will invariably contain defects at some stage during its life. These defects will require a "fitness-for-purpose" assessment to determine whether or not to repair the pipeline. The full-scale tests of pipelines with defects and limit state functions method are used for such assessment. The limit state function method allows determining the limit size of defect upon

reaching which the pipeline will fail. The limit state function L for pipe with defect

where *P* is operation pressure; *Q* is external loads; *σ<sup>f</sup>* is fracture stress; *D* is outside diameter of pipe; *t* is wall thickness of pipe; *lr* is allowable defect size; *li* is

The defects sizes *li* are established during ILI. The allowable defects sizes *lr* are determined by calculation methods by the specified criteria for the strength and durability of structures, taking in to account the operating conditions and the character of the mechanisms of deformation and destruction [6–10]. It should be emphasized that in these methods the sizes of defects *li* and *lr* are assumed to be

In reality, the defects have inevitable random dispersion of sizes. For detected defects, these are caused by the random nature of the defects, as well as by statistical errors and the probabilistic nature of the operational characteristics (sensitivity and detectability) of non-destructive testing methods [16]. The dispersion of the calculate sizes of defects determined by statistical scattering loads, operating conditions and scattering of mechanical properties. A certain contribution to the possible dispersion of defect sizes is made by idealization of the shapes and schemes of defects. Taking this into account, instead of single-valued sizes in the calculations, it is necessary to use the probability densities distribution functions of defect sizes

Using the functions *f*(*li*) and *f*(*lr*) gives reason to believe that there are always nonzero probabilities *P* presence of defects with sizes *li* larger than *lr* (**Figure 1**):

<sup>L</sup> *<sup>P</sup>*, *<sup>Q</sup>*, *<sup>σ</sup> <sup>f</sup>* , *<sup>D</sup>*, *<sup>t</sup>*, *<sup>l</sup>* <sup>¼</sup> *lr <sup>P</sup>*, *<sup>Q</sup>*, *<sup>σ</sup> <sup>f</sup>* , *<sup>D</sup>*, *<sup>t</sup>*, *<sup>l</sup>* � *li* <sup>¼</sup> <sup>0</sup> (1)

Further research and development, sponsored by major international oil and gas companies (BP, DNV, Shell, Statoil, Total, and others), lead to the development of the Pipeline Defect Assessment Manual (PDAM). PDAM is based on a comprehensive critical review of available methods and full-scale pipe test results [18]. The scope of PDAM includes steel pipelines manufactured to API 5 L or equivalent national and international standards. The methods given in PDAM are applicable to defects in surface, underground and subsea pipelines. In these methods the following types of defects are considered: corrosion damage, scoring and marks, dents and corrugations, welding defects, delamination and cracking of the metal. These methods take into account the interactions of defects. The methods take into account the main and additional loads. At the same time, it should be noted that many PDAM methods are empirical, with a limited scope.

A significant drawback of PDAM methods is the use of a deterministic approach to defect hazard assessing. The dimensions of defects, loads and characteristics of the mechanical properties of steels are considered as deterministic, unambiguously given values. The partial safety factors used in the calculation methods are based on empirical data and are not directly related to the inevitable random variations of these parameters. Due to these circumstances PDAM methods are not combined with the developed concepts of Risk based performance management and Risk based Inspection (RBI).

Comparative analysis of methods for hazard assessment of pipeline defects allows us to draw the following conclusions:


In conclusion, it should be noted that pipeline defects are random, unique and complex in shape and their sizes are depend on the operating conditions and the properties of the external environment. The characteristics of defects cannot always be described by the current norms and calculation methods.

the security problem. To solve this problem, the methods ASME B31, APT1160, RSTRENG, DNV and others focused on the analysis of the most common defects in the form of corrosion damage [17] were developed. Parallel to this, the methods of breaking mechanics have developed and improved, which are reflected in the

Further research and development, sponsored by major international oil and

development of the Pipeline Defect Assessment Manual (PDAM). PDAM is based on a comprehensive critical review of available methods and full-scale pipe test results [18]. The scope of PDAM includes steel pipelines manufactured to API 5 L or equivalent national and international standards. The methods given in PDAM are applicable to defects in surface, underground and subsea pipelines. In these methods the following types of defects are considered: corrosion damage, scoring and marks, dents and corrugations, welding defects, delamination and cracking of the metal. These methods take into account the interactions of defects. The methods take into account the main and additional loads. At the same time, it should be noted that many PDAM methods are empirical, with a

A significant drawback of PDAM methods is the use of a deterministic approach to defect hazard assessing. The dimensions of defects, loads and characteristics of the mechanical properties of steels are considered as deterministic, unambiguously given values. The partial safety factors used in the calculation methods are based on empirical data and are not directly related to the inevitable random variations of these parameters. Due to these circumstances PDAM methods are not combined with the developed concepts of Risk based performance management and Risk

Comparative analysis of methods for hazard assessment of pipeline defects

parameters are the relative depth *h*/*t* and the relative length *l*

1.For the bulk defects in the form of metal loss (corrosion) and dents the main

2.The calculated ratios used for the limiting sizes of defects differ in terms of

3.Defect hazard assessments are carried out for given limit states function of pipes, defined as L ¼ <sup>Φ</sup> *<sup>P</sup>*, *<sup>Q</sup>*,*Cf* , *<sup>D</sup>*, *<sup>t</sup>*, *<sup>h</sup>*, *<sup>l</sup>* , where <sup>Ф</sup> is a function of a given form, *P* is operation pressure; *Q* is external loads; *Cf* is the strength criterion of a pipe with a defect; *D*, *t* are pipe diameter and wall thickness; *h*, *l* are depth

In conclusion, it should be noted that pipeline defects are random, unique and complex in shape and their sizes are depend on the operating conditions and the properties of the external environment. The characteristics of defects cannot always

be described by the current norms and calculation methods.

rectangular (*A* = *hl*), parabolic (*A* = 2*h*/3*l*), combined (*A* = 0.85*hl*). It is not possible to single out a more accurate approximation on the available results of field tests of pipes with defects. Taking into account random variations in the shape and size of real defects, any approximation with an undefined error

the shape of approximation of the area *A* of defect cross-sections:

size of around the circumference of the pipe is usually not taken into account. For the flat defects (crack, delamination) the main parameters are length and

2

/(*Dt*). The defect

gas companies (BP, DNV, Shell, Statoil, Total, and others), lead to the

standards BS7910, API RP579, SINTAP.

*Issues on Risk Analysis for Critical Infrastructure Protection*

limited scope.

based Inspection (RBI).

depth of the defect.

can be used.

**152**

and length of defect [19].

allows us to draw the following conclusions:
