Seismic Assessment for Offshore Pipelines

SEISMIC ASSESSMENT FOR OFFSHORE PIPELINES By R. Bruschi/ O. T. Gudmestad,Z F. Blaker/ and F. Nadim4

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ABSTRACT: An international consensus on seismic design criteria for onshore pipelines has been established during the last 30 years. The need to assess seismic design for offshore pipelines has not been similarly recognized. The geotechnical hazard for a pipeline routed across steep slopes and irregular terrains affected by earthquakes are discussed in this paper. The integrity of both natural and artificial load-bearing supports are assessed. The response of the pipeline to direct excitation from soil or through discontinuous, sparsely distributed natural or artificial supports are commented on. Some applications are given in order to point out topical aspects and major design issues for currently operating offshore pipelines crossing seismic active seabeds.

INTRODUCTION In most cases, offshore pipelines are laid either on an even sea floor or on the bottom of an artificial trench. This trench will be backfilled by natural sedimentation processes or by artificial dumping of material other than the local one, due to specific stability or protection requirements (Anselmi and Bruschi 1993). Sometimes offshore pipelines are laid on irregular seabeds where they achieve an equilibrium configuration with free spans of random lengths. Suspended lengths could, however, be critical from the viewpoint of overstresses at the support points or they could be susceptible to vortex-sheddinginduced oscillations. In such cases, additional supports to bear the pipeline at midspans are introduced either before or after pipe installation (Bruschi et al. 1991). In all these circumstances, offshore pipelines are not expected to suffer from any significant damage caused by seismic excitation. Indeed, the almost rectilinear pipeline configuration can sustain considerable soil deformations due to traveling seismic waves without triggering any failure mechanism (Kiyomiya 1983). Nevertheless, the following circumstances are possible:

structural integrity of the envisaged pipeline configuration in operation and whether corrective measures are necessary or not. The analyses are based on design premises pertaining to the seismic environment and to seismic input criteria (St. John and zahrah 1987). A series of seismic hazard assessments regarding pipelines recently installed across the European Continental Shelf have been performed during the last years, notably for the following: 1. The Zeepipe offshore gas pipeline [1,016 rom (40 in.) outer diameter (00)], which links the Sleipner platform located in the Central North Sea with Zeebrugge in Belgium. It is approximately 800 kIn long and crosses the

1. Corridors and their flanks may comprise soil layers that might become unstable during seismic excitation (slope instability due to shear failure or liquefaction), causing slides or turbidity currents potentially interfering with the pipeline. 2. Specific care is necessary when the pipeline has to cross potentially active faults protruding from the bedline, possibly creating a steep discontinuity of the seabed. 3. For a partly free pipeline spanning over the seabed and more or less regularly supported by protruding rocks or artificial supports, it may be necessary to document that the seismic excitation will not threaten the structural integrity of the pipeline or its supports. 4. Subsea connections to other pipelines or structures may be subjected to large earthquake-induced stresses, due to structural discontinuities or abrupt changes of stiffness, which will enhance the concentration of the response to earthquake loading. Under these circumstances, it is necessary to assess the 'R&D Subsea Mgr., Snamprogetti, P.O. 97, 61032 Fano, Italy. 'Spec. Advisor, Prof. of Marine Techno!., Statoil a.s., N-4035 Stavanger, Norway. 'Sr. Vice Pres., Gas Technol. and Services, Statoil a.s., N-4035 Stavanger, Norway. 'Tech. Expert, Norwegian Geotech. Inst., P.O. 3930 Ullevaal Hageby, N-0806 Oslo, Norway. Note. Editor-in-Chief: Jeff R. Wright. Discussion open until February I, 1997. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on November 13, 1995. This paper is part of the Journal of Infrastructure Systems, Vol. 2, No.3, September, 1996. ©ASCE, ISSN 1076-0342/96/00030145-01511$4.00 + $.50 per page. Technology Review No. 12005.

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JOURNAL OF INFRASTRUCTURE SYSTEMS / SEPTEMBER 1996/145

J. Infrastruct. Syst., 1996, 2(3): 145-151

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seabeds of the southern North Sea (Oiamantidis et al. 1992), as shown in Fig. 1. 2. The TransMed gas pipeline system [3 X 508 mm (3 X 20 in.) 00 plus 2 X 660 mm (2 X 26 in.) 001 between Cape Bon, Tunisia and the western part of Sicily. It is routed across the deep waters (more than 600 m) and uneven seabeds of the Channel of Sicily and across the seismically active seabed of the Strait of Messina (Albano et al. 1992), as shown in Fig. 2. 3. The Troll oil pipeline [406 mm (16 in.) 001, which links the floating production platform installed at the Troll West Oil Province with the Mongstad Refinery inside Fensfjorden on the Norwegian west coast. It crosses the Norwegian Trench, and is then routed on sediments overlaying the bottom of the fjord between steep flanks, leaving a corridor of about 500 m of soft sediments (Breivik et al. 1995), as shown in Fig. 3. For each case the seismic characteristics were addressed to the extent required, covering topical issues such as seismic hazard response spectra and relevant time histories of excitation, etc., as well as local effects regarding amplification of ground motions and geotechnical stability. SEISMIC HAZARD The seismic hazard analysis of the area crossed by an offshore pipeline includes: estimation of the recurrence rate of

seismic events within the region, assessment of the upper bound magnitude, selection of an attenuation relationship, and calculation of the probability of exceedance of peak ground acceleration levels for the site. In addition, response spectra relevant to the site and characteristic time histories of seismic excitation are estimated (Nadim et al. 1993). The recurrence rates are estimated by assuming that earthquakes are spatially uniformly distributed within each seismic-tectonic province. An upper bound magnitude for the provinces is normally assumed as one-half magnitude unit higher than any historically observed earthquake event. For the estimate of the probability of exceedance of peak ground acceleration levels, the method proposed by Cornell (1968) is normally used. The method assumes that the earthquake occurrence within the seismic-tectonic provinces follows a homogeneous Poisson process. The contributions from each location in the nearby provinces and the contributions from potentially active faults are added to compute the seismic hazard at the site. To provide information on how seismic parameters attenuate with distance from the origin of an earthquake, different attenuation relations based on worldwide data are normally used as there is a substantial lack of adequate regional studies specific to the site (Trifunac 1980). The evaluation of the characteristics of the earthquake-induced motions are normally based on the shape of response spectra as proposed by relevant regulatory commissions. The design spectra for vertical accelerations may be derived from

146/ JOURNAL OF INFRASTRUCTURE SYSTEMS / SEPTEMBER 1996



FIG. 3.

Troll 011 Pipeline Route Approaching Steep Wall and Bore Hole Exit at Landfall near Mong.tad Refinery, Norway

the horizontal spectrum using, for example, a variable scaling factor equal to two-thirds at low frequencies and equal to one at high frequencies (USAEC 1973). Furthermore, the probability of fault displacements interfering with the pipeline transmission system is calculated, considering the relation of estimated previous fault displacements to fault length and magnitude and the recurrence of historical earthquakes with the different faults (Slemmons 1977). SOIL STABILITY The stability of soils in areas characterized by high seismicity is particularly relevant offshore, where even gentle slopes of marine sediments may fail during a strong earthquake (Rahman 1994). For seismic events, temporary exceedance of the soil shear strength or progressive degradation associated with pore-pressure buildup may lead to permanent deformations of the soil mass (Tokimatsu and Seed 1987). Slope stability is particularly relevant in shore approaches where relatively steep slope angles are not uncommon (Bazzurro et al. 1994). Soil instability may interfere with the pipeline causing the development of failure mechanisms, notably: 1. Impact on the pipeline of turbidity current or debris flow (Norem et al. 1990), which may cause excessive bending or tearing depending on the direction of impact. 2. Differential settlements, which the pipeline has to cope with by developing local curvatures and/or axial deformations (Bruschi et al. 1995). These could possibly trigger instability modes of the pipe section or Euler-bartype buckling, depending on the interference pattern and on the force transfer capacity from the soil to the pipe.

Geophysical and geotechnical survey programs in general help determine pipeline routing which avoids old and known instability-susceptible zones. Irrespective of this, hydrogeomorphological modifications can still affect a pipeline route. Therefore, careful analyses are necessary to support the final selection of the route and of the measures to protect the pipelines, if needed. A probabilistic approach addressing the nature, the occurrence, and the entity of soil settlements is required for a rational decision in a field where the scarceness and sparseness of data is of major concern. Experiences, especially in the Gulf of Mexico, show that a pipeline cannot survive the impact of a large dense mass flow (Bea and Aurora 1983). Nevertheless, remedial measures such as pipe burial or covering could be effective in the presence of turbidity currents, when it is necessary to avoid large displacements of the pipeline and interference with obstacles. Any measure against differential settlements, e.g., the slip surface of a slide (or of an active fault) should be based on an accurate estimate of potential slip modes and magnitudes of the settlement. The earthquake-induced permanent displacement may be calculated by a procedure analogous to that used for analyzing the movement of a sliding block on an inclined plane (Newmark 1965). The displacements of the soil mass are calculated by integrating twice the time history of the accelerations exceeding the yield acceleration, defined as the one for which failure occurs along a potential sliding plane. The inertia of the settling mass, the dynamic resistance from shear forces at the slipping surface, and the duration and number of passages above the yield accelerations are the main parameters. This simple model has been used extensively and satisfactory performance has been experienced in many circumJOURNAL OF INFRASTRUCTURE SYSTEMS / SEPTEMBER 1996/147



stances, suggesting its applicability in the probabilistic assessment of critical situations (Bazurro et al. 1994). These assessments can be critical at shore approaches with steep slope angles. Here the pipe ductility (dependent on the diameter over thickness ratio and on pipe material) on one hand, and the pipeline layout with respect to potential surface slides intersecting the pipeline (it applies also to the crossing of seismic faults) on the other, are design variables which can be adjusted to allow for soil movements interfering with the pipeline integrity (NZS 1985). Specific criteria for such conditions are not covered in international standards for offshore pipelines. However, the criteria can be deduced from general standards, e.g., the ones relevant to load-bearing structures for offshore applications. On the Norwegian Shelf, pipelines are designed according to the Norwegian Petroleum Directorate (NPD) rules (1990), and have to meet criteria relevant to serviceability and progressive collapse limits states (Nadim and Gudmestad 1990). Partial safety factors are introduced for materials and loads. The analyses refer to design and exceptional design events, defined as follows:

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1. Normal operations, which are related to the normal use and operability of the offshore structure, to withstand the loading of the design event without significant damage. 2. Abnormal operations (survival), which are related to the avoidance of failure of the structure when subjected to an abnormal earthquake event. The design level earthquake or strength level earthquake is recommended by API (1993) to be the level of ground motion with a return period several times the projected life of the structure. The NPD regulations (1990) associate this event with a return period of 100 years. The exceptional level earthquake or rare intense earthquake is associated with an event that can have a recurrence interval of several hundreds to a few thousand years. For design checks, NPD regulations (1990) recommend an earthquake with a recurrence interval of 10,000 years to be applied.

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PIPELINE INTEGRITY For offshore pipelines resting on even seaftoors or on the bottom of a naturally or artificially backfilled trench, earthquake analysis of the pipeline considers the distortional effect from each excitation wave traveling across and along the pipeline. The pipeline is one body with the soil and, therefore, is subjected to approximately identical deformations as the soil. The amplitude and length of each wave, generally assumed as incorporating the elastic energy discharged by the earthquake, are the parameters which influence the induced stresses (St. John and Zahrah 1987). For pipelines resting on the seabed, the deformation is felt less than for buried pipelines due to the weak frictional link with the soil. This concept even applies to the case of a fault crossing. However, for discontinuities which are not protrud-

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148/ JOURNAL OF INFRASTRUCTURE SYSTEMS / SEPTEMBER 1996



ing as steep slopes and are not creating excessive free spanning, the approach adopted for on-land pipelines is applicable and the criteria are discussed in different national guidelines (Guidelines 1985; Told et al. 1977). Nevertheless, as mentioned, there are conditions which require a specific attention. Of these, particular emphasis should be given to free-spanning pipelines (Bruschi et al. 1991). Free spans occur as a result of seabed undulations caused by past environmental effects (e.g., by iceberg scours) or by outcropping rocks and deep depressions due to crustal morphogenetic phenomena. These conditions have been experienced across the Channel of Sicily and are present in current projects such as the Strait of Gibraltar crossing (Tam 1995) and the entrance into Norwegian fjords (Fensfjorden and Ramsfjorden), or in upcoming challenges such as the Oman-India pipeline (Mc Keehan 1995). In such circumstances the pipeline in operation presents or is exposed to a sequence of suspended lengths alternating between contacts with the seabed or with artificial features, either gravel berms or mechanical trestles. The design aims at defining a pipeline configuration which satisfies both the strength criteria and the dynamic and fatigue criteria (Bruschi et al. 1991), as follows: 1. The strength criteria mainly address the state of stress at the shoulders of the suspended lengths to be within the allowable limits. The anticipated nominal stresses are based on a theoretical seabed profile extracted from an accurate remotely operated vehicle (ROV) based bathymetry survey. At the locations where the anticipated stresses exceed the allowable ones, additional supports are considered and hence sized in order to provide acceptable as-laid stresses (prelay seabed preparation works) and extrapolated operating stresses (on the basis of an as-built survey). In these conditions, the sequence and combination of functional loads (empty or waterfilled pipe, pressure testing, operation), environmental loads (vortex shedding, waves, and currents), and accidental loads (impact from trawling gear, etc.) must be accounted for to demonstrate that the pipeline meets the design criteria. 2. The dynamic and fatigue criteria are specified to prevent resonant oscillations and associated overstresses (as cross-flow vortex shedding, vibrations of short free spans, or the oscillations induced by waves on very long free spans) and to limit the cumulative fatigue damage

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caused by small amplitude, but frequent oscillations (as the ones due to the in-line lock in of vortex shedding or those due to frequent wave cycles on short span lengths). Earthquake loading effects on the as-laid configuration or the inoperational configuration after correction works are implemented are mainly related to the overall bending stiffness of the residual suspended lengths. Of importance are the pipe mass and diameter, the seabed clearance, spanned length over diameter ratio, bending and axial stiffness, midspan sagging, effective axial load including the pressurized internal medium, as well as the distribution, extent, and nature of the interaction forces with the seabed, etc. The combination of such parameters may give rise to either stiff (first natural frequency greater than 1 Hz) or flexible (first natural frequency less than 0.2 Hz) spanning configurations, even for the same pipeline in different conditions. Figs. 4(a) and 4(b) show the first natural frequencies of a [406 mm (16 in.) 00] 20.5 mm wall thickness (WT) pipeline in the vertical plane, as laid on an uneven and prepared seabed [Fig. 4(a)] and as operating at high pressure/temperature conditions after a number of supports are added to correct the notallowable free spans [Fig. 4(b)]. Special-purpose calculation tools to analyze such an event were used to obtain these figures. In general, the criteria for assessing the structural integrity of the pipeline are dominated by allowable usage factors and no vortex shedding oscillation criteria under near-bottom currents. The dynamic response to a multisupport excitation due to a traveling seismic wave (Datta and Meshal 1988) is generally negligible when the pipeline meets the foregoing criteria. However, attention has to be paid to the stability of the artificial supports supposed to bear the pipeline during the operating life span or, at least, between two annual inspections. Sometimes, these supports must be built or installed on a sloping seabed or on soft soils. Fig. 5 shows a possible configuration of the preparation works which is necessary to support the pipeline during laying. Three-dimensional (3D) processing is necessary to anticipate the extent of gravel dumping needed to reach the required height over the bedline (experience is with heights up to 6 m) and the width (5 -15 m, according to pipe laying criteria) with respect to alignment for installation purposes. Soil and berm stability are investigated where slopes and/or weak surficial layers may slide even during gravel dumping. In this respect,

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gravel dumping is performed through a free vertical pipe hanging from a surface dumping vessel, with the end of the pipe and the vessel being dynamically positioned over the target zone. Earthquake loading on such berms is analyzed to verify that the load bearing of the pipeline is ensured during and after the design earthquake event. One may permit berm settlements requiring minor intervention to reestablish the load-bearing performance of the berm after an exceptional event. Fig. 6 shows the approach of "slicing" the berm in order to determine the critical sections and relevant directions and, eventually, to estimate the need for any end-size stabilizing counterfilling. At the end the pipeline has to meet certain safety criteria related to social, environmental, legal, insurance, and statutory considerations. Indeed, failure consequences may also be serious from the viewpoint of loss of transport and cost of repair. In general acceptance limits for structural failure in terms of annual probabilities, are defined as functions of (l) degree of severity of failure consequences (e.g., loss of life, environmental damage, or economic impact); and (2) type of limit state (serviceability, ultimate or progressive collapse limit state). This concept of implementing safety in design criteria is known as "safety class differentiation" and is recommended for future design practice (Sotberg and Bruschi 1992).

CONCLUSIONS

Current procedures implemented in the seismic design of important pipeline systems on the European Continental Shelf were reviewed. Pipeline projects for which earthquake loadings are of significance were described and commented on. Uncertainties in the definition of seismic loading conditions were analyzed and current safety provisions aiming to cover such uncertainties were reviewed. It is concluded that in most cases offshore pipelines are not expected to suffer significant damage as results of seismic excitation, as the rectilinear route can substain the traveling seismic waves without mobilizing any threatening mechanism. There are, however, some circumstances which need a comprehensive assessment of the structural integrity of the pipeline, both from the viewpoint of the stability of the encountered soils and from the viewpoint of overstresses of the pipeline due to dynamic response or differential settlements. This aspect may be topical in upcoming offshore pipeline projects as, e.g., the Oman-India pipeline, where ultradeep waters, uneven seabeds, potentially unstable soil slopes, and severe seismic environment are crucial issues.

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APPENDIX.

REFERENCES

Albano, G., Lehrizi, M., and De Caro, R. (1992). "The TRANSMED: a technological milestone now under further expansion." Proc.• II th Int. Con! on Offshore Mech. and Arctic Engrg., Am. Soc. of Mech. Engrs. (ASME), New York, N.Y. American Petroleum Institute (API). (1993). "API recommended practice for planning, designing and construction fixed offshore platforms." API RP2A, 20th Ed., Dallas, Tex. Anselmi, A., and Bruschi. R. (1993). "North Sea pipelines: meeting the engineering challanges." Pipeline Industry, (Jan. -Feb.). Bazzurro, P., Pelli, F., Manfredini, G. H., and Cornell, C. A. (1994). "Stability of sloping seabeds seismic damage analysis: methodology and application." Proc.• Strait Crossing Symp., J. Krokeborg, ed., A. A. Balkema, Rotterdam, The Netherlands. Bea, R. G., and Aurora, R. P. (1983). "Design of pipelines in mudslide areas." J. Petr. Technol., (Nov.). Breivik, J., Bruschi, R., and Leopardi, G. (1995). "Troll Oljerfllr-a unique technical challenge." Proc., Offshore Mediteranean Con! Bruschi, R., Curti, G., and Tura, F. (1991). "Free span assessment: a review." Proc.• 1st Int. Offshore Polar Engrg. Con!, Int. Soc. of Offshore and Polar Engrs., Golden, Colo. Bruschi, R., Spinazz~, M., Tomassini, D., Cascuna, S., and Venzi, S. (1995). "Failure modes for pipelines in landslide areas." Proc., 14th Int. Con! on OMAE, Am. Soc. of Mech. Engrs. (ASME), New York, N.Y. Cornell, C. A. (1968). "Engineering seismic risk analysis." Bull. Seismological Soc. of Am., 58,1583-1606. Datta, T. K., and Meshaly, E. A. (1988). "Seismic response of suspended spans of submarine pipeline." Proc.. 7th Int. Con! of OMAE, Am. Soc. of Mech. Engrs. (ASME), New York, N.Y. Diamantidis, D., Gudmestad. O. T., and Manfredini. G. M. (1992). "Probabilistic seismic design aspects for pipeline systems in the North Sea." Seismic Engrg., PVP-Vol. 327-1. Guidelines of the seismic design of oil and gas pipelines systems. (1984). ASCE, New York, N.Y. Kiyomiya, O. (1983). "Stresses of pipelines during earthquakes." Proc.• Offshore Technology Con! (OTC) 4456, Offshore Technol. Conf., Richardson, Tex. Mckeehan, D. (1995). "Oman India pipeline project-technological development." Proc.• Offshore Pipeline Technol. Conf. Nadim, F., and Gudmestad, O. T. (1990). "Reliability of an engineering

system under a strong earthquake with application to offshore platforms." Proc.• 9th Eur. ConI. on Earthquake Engrg., 2, 97-105. Nadim, F., Dahle, A., and GUdmestad, O. T. (1993). "Consistent treatment of uncertainties in earthquake hazard evaluation." Proc.• 2nd Eur. Con! on Struct. Dynamics (EURODYN 1993), I, 11-18. Newmark, N. M. (1965). "Effects of earthquakes on dams and embankments." Geotechnique, London, England, 15(2), 139-160. Norem, H., Locat, J., and Schielolrop, B. (1990). "An approach to the physics and modelling of submarine flow slides." Marine Geotechnol., Vol. 9. Norwegian Petroleum Directorate (NPD). (1990). Guidelines to regulations relating to pipeline systems in the petroleum activities, Stavanger, Norway. Norwegian Petroleum Directorate (NPD). (1985). Regulations relating to loadbearing structures in the petroleum activities, Stavanger, Norway. Rahman, M. S. (1994). "Instability and movements of ocean floor sediments: a review." Proc., 4th Int. Offshore and Polar Engrg. Con!, Int. Soc. of Offshore and Polar Engrs., Golden, Colo. Slemmons, D. B. (1977). "State-of-the-art for assessing earthquake hazards in the United States, report, faults and earthquake magnitude." Rep., Mackay School of Mines, Univ. of Nevada, Las Vegas, Nev. Sotberg, T., and Bruschi, R. (1992). "Future pipeline design philosophy -framework." Proc., llth Int. Con! on Offshore Mech. and Artic Engrg., Am. Soc. of Mech. Engrs. (ASME), New York, N.Y. Standards New Zealand (NZS). (1985). "Code of practice for high pressure gas and petroleum liquid pipelines." 5223, Part I. St. John, C. M., and Zahrah, T. F. (1987). "A seismic design of underground tunnel." Tunnelling and Underground Space Technol., 2(2). Tam, C. K. W. (1995). "The Gibraltar submarine gas pipelines: seabed intervention rationale." Proc.. Offshore Pipeline Technol. Con! Toki, K., Fukumori, Y., Sako, M., and Tsubakimots, T. (1977). "Recommended practice for earthquakes resistent design of high pressure gas pipelines." Earthquake Resistant Lifelines, PVD, Vol. 77-1. Tokimatsu, K., and Seed, H. B. (1987). "Evaluation of settlements in sand due to earthquake shaking." J. Geotech. Engrg. Div., ASCE, 113(8). Trifunac, M. D. (1980). "Effects of site geology on amplitudes of strong motion." Proc., 7th World Conf. on Earthquake. United States Atomic Energy Commission (USAEC). (1973). "Design response spectra for seismic design of nuclear power plants." Regulatory guide 1.60, Washington, D.C.

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