IV Международная студенческая научная конференция Студенческий научный форум - 2012

The research work was done during project Sarctoga (2011), the cooperative project of Norwegian University of Science and Technology and National Research Tomsk Polytechnic University.  The main purpose of the research is to give detailed information to the principles of subsea gas pipelines engineering design. Through the research we pursued the following objectives:

• To investigate the environmental conditions in the Barents sea;
• To develop relevant engineering solutions for these conditions in accordance with Russian and Norwegian standards;
• To compare results by using computing methods of Russian and Norwegian standards.

The first part is dedicated to investigation, description and assessment of environmental conditions and other external factors that influence the subsea gas pipeline design and construction in the Barents Sea, from the Shtokman field to shore. Generally, they can be divided into two groups, strengths and weaknesses. Brief results of the distribution are in Table 1.

Table 1

Strengths

Weaknesses

Relatively shallow water depth (0-340m); Even relief along the most part of the pipeline route; Stable and even coastline.

Unstable and soft bottom sediments (mostly silts and clays); Few potentially hazardous areas along the pipeline route (mostly plate boundaries); High current speed (both surface (0,9 m/s) and bottom(0,3 m/s) current); High probability of storm occurrence (average wave height is 5-10m, maximum wave height reaches 27m); High soil and water corrosiveness (low pH and salinity, high oxygen content); Harsh ice conditions (exaration by hummocks, shallow water icing, soil congelation); Unfavourable conditions of navigation (damage by vessel anchors in the offshore area); "Relatively dangerous" seismic activity (magnitude of possible earthquakes is 4-7)

The second part of the research is based on the first part and also both Norwegian and Russian standard, Norwegian principal standard being presented by Offshore Standard DNV-OS-F101 «Submarine Pipeline System», Russian one - Industrial Standard VN 39-1.9-005-98 «Standards on gas pipeline design and construction» and includes different design calculations.

The key point in the pipeline design is a pipe wall thickness calculation. General principle in accordance with both Norwegian and Russian standards is based on the internal pressure containment (bursting). It´s also essential to check the characteristic resistance for hydrostatic pressure (local buckling) and propagation buckling (in case the external pressure exceeds the previous criteria). The formulations and results are in Table 2.

Table 2

	Norway	Russia
Formulation (the internal pressure containment)	(see DNV-OS-F101 Sec.5 D200, p.46)	(see VN 39-1.9-005-98 Sec.5, p.13)
Minimum pipe wall thickness, mm	20,2 (plus corrosion allowance and fabrication thickness tolerance)	24,2 (plus corrosion allowance and fabrication thickness tolerance)
Formulation (local buckling  - external pressure only)	(see DNV-OS-F101 Sec.5 D400, p.46)	5<D/δ<15   (see VN 39-1.9-005-98 Sec.6, p.15)
Maximum pipeline laying depth, m	1721,2	-
Formulation (propagation buckling)	(see DNV-OS-F101 Sec.5 D500, p.47)	(see VN 39-1.9-005-98 Sec.6, p.16)
Critical depth (below which additional safety against propagation buckling are necessary), m	174,4	122,9

Another important stage of pipeline design is stability calculation. DNV-OS-F101 doesn´t regulate calculation methodology, but recommend the minimum requirements to concrete coating properties (see DNV-OS-F101 Sec.9 C200, p.102). As for Russian Standard, it provides calculation methodology, the minimum requirements being missed. Taking the density of concrete to be 4000 kg/m³, according to VN 39-1.9-005-98 coating thickness equals to 59 mm.

The submarine pipeline system also shall be protected against unacceptable damage caused by e.g. dropped objects, fishing gear, ships, anchoring etc. Dragging anchors are accidental loads which are imposed on a pipeline system under abnormal and unplanned conditions. According to DNV-OS-F101, anchor patterns shall be predetermined for each vessel using anchors to maintain position. Different configurations for anchor patterns may be required for various sections of the pipeline, but maximum distance must be 500m from pipeline. Russian standard VN 39-1.9-005-98 shows that the pipeline must be buried sufficiently deeply in the seabed to prevent the damage. Burial depth varies from 1m along the most part of the pipeline route to 2m within the coastal zone.

In areas where ice may develop or drift, the possibility of ice loads on the pipeline system shall be considered. Such loads may partly be due to ice frozen on the pipeline system itself, and partly due to floating ice. Based on DNV-OS-F101 the ice load can be defined as environmental or accidental depending on its frequency, but exact burial depth isn´t regulated. According to Russian Standard VSN 51-9-86 the pipeline must be buried a meter lower than the depth of ice exaration. DNV-OS-F101 also specifies that the most important protection should be in shallow waters.

As for seismicity, the pipeline route is characterized by "relatively dangerous" seismic activity. According to DNV-OS-F101, loads imposed by earthquakes shall be classified into accidental or environmental loads, depending on the probability of earthquake occurrences in line with accidental loads. Design with respect to accidental load must ensure that the overall nominal failure probability complies with the nominal failure probability target values:  (see DNV-OS-F101 Sec.5 D1200, p.51).  The safety class being high, the nominal failure probability equals to , i.e. . The overall nominal failure probability from accidental loads can be expressed as the sum of the probability of occurrence of the i´th damaging event, i.e. , and times the structural failure probability conditioned on this event, . As a result , and consequently the overall failure probability complies with the target values.

Concerning corrosion processes in the Barents Sea, it tends to be electrochemical with oxygen depolarization. According to both Russian and Norwegian Standards, external corrosion can be decreased by coating and cathodic protection application. Also the possible additional ways to decrease corrosion can be addition of corrosion allowance to the nominal pipe thickness or/and alloy cladding of  the pipeline sections characterized by high corrosion rates. Corrosion allowance commonly ranges from 1,5 to 3 mm. In turn, cathodic protection shall keep electrochemical potential on the "pipe-water" boundary varied between +0.8 and -1.1 V relative to Ag/AgCl/subsea water reference electrode.

In the result, it follows according to the research work that Norwegian and Russian Standards use the similar conceptions and different correcting coefficients in the design calculations.  But taking into account the fact that the values of these coefficients are found based on operating and design experience and Norway has more experience in subsea pipeline construction and design,     Norwegian standards can be considered as  more detailed  and widely applicable than Russian ones.

 

References:

1. Offshore Standard DNV OS-F101: Submarine Pipeline Systems. - Det Norske Veritas, October 2010. - 238 p.; Khasanov, I. (2010) Maintenance Service and Repair Chamber for Pipelines in Bogs and Under Water, from http://www.onepetro.org/mslib/;
2. Offshore Standard DNV OS A-101: Safety Principles and Arrangements. - Det Norske Veritas, April 2011. - 50 p.;
3. Arild Moe, Lars Rowe. Petroleum Activity in the Russian Barents Sea. - Fridtjof Nansen Institute, September 2008. - 26 p.;US20090105000 (2009-04-23) Robert J. Scarborough, Portable surface covering;
4. ВН 39-1.9-005-98 Нормы проектирования и строительства морского газопровода;
5. Официальный сайт компании "Shtokman Development AG"/ http://www.shtokman.ru/;
6. Официальный сайт ФГБУ "Арктический и антарктический научно-исследовательский институт" (ФГБУ "ААНИИ")/ http://www.aari.nw.ru/.

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