S P E 5758 Dallas, Texas 75206 THIS IS a PREPRINT --- SUBJECT TO· CORRECTION

SOCIETY OF PETROLEUM ENGINEERS OF AIME PAPER 6200 North Central Expressway NUMBER S P E 5758 Dallas, Texas 75206 THIS IS A PREPRINT --- SUBJECT TO· CORRECTION

FOUNDATION ENGINEERING FOR GRAVITY STRUCTURES IN THE NORTH SEA

Knut Schjetne, Norwegian Geotechnical Institute

American Institute of Mining, Metallurgical and Petroleum Engineers, Inc. Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021

This paper was prepared for the SPE/DUT-European Spring Meeting 1976 of the Society of Petroleum Engineers o·f AIME, in cooperation with the Division for Underwater Technology of the Royal Institution of Engineers in The Netherlands (KIVI), held in Amsterdam, The Netherlands, April 8- 9 19,76. Permission to copy is restricted to an abstract ·of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgement of where and by whom the paper is presented. Publication elsewhere after publication in The Journal of Petroleum Technology, the Society of Petroleum Engineers Journals or De Ingenieur, is usually granted upon request to the editor or the appropriate journal provided agreement to give proper credit is made.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Netherland Section of the Society of f'etroleum Engineers, P.O.B~x 228, The Hague, The N~therlands. Such discussion may be considered for publication together with the paper.

ABSTRACT Most of the fixed platforms insta~led The paper outlines the particular have been the pile-supported steel problems related to the foundation jacket type, as developed in Lake design of offshore gravity structures in Maracaibo and in the Gulf of Mexico. comparison with structures on shore. However, new types of structures have The following aspects are dealt with in been developed, those today conunonly more detail: (1) The current NGI method known as gravity structures, i.e. for overall foundation-stability on clay, structures sitting on the sea bottom by (2) the effect of cyclic loading on clay, their own weight with no anchors or pile (3) the dynamic analysis of the soil support. structure system and (4) platform behaviour during the installation phase. The first gravity structure installed in the North Sea was the Ekofisk Tank in the summer 1973. It was a one million INTRODUCTION barrel ~il storage, concrete tank Since 1968, when the Ekofisk oil field designed by C.G. Doris (Figure 2). was discovered, we have witnessed a hectic activity in the northern part of Two years later, in the summer of 1975, the North Sea. Many oil and gas fields three concrete drilling platforms were . have been discovered, most of them along installed, Beryl A and Brent B, both the borderline between the British and the Condeep type designed by A/S H¢yer Norwegian sectors. (Figure 1) Several Ellefsen and Frigg CDPl, an0ther Doris of the fields are now being developed design. At the moment there are 9 more and the Ekofisk field alone is already concrete platforms under construction in producing enough oil to cover Norway's the countries surrounding the North Sea. total petroleum consumption. (Doris 2, Andoc 1, Sea Tank Co. 3 and Condeep 3).

First of all because of the water depths and the hostile weather conditions in References and illustrations at the end the area, the dimensions are huge of paper. compared to on-shore structures. Owing 2 FOUNDATION ENGINEERING FOR GRAVITY STRUCTURES IN THE NORTH SEA SPE 5758

to the wave action, the structures are Several of the analyses have to be subjected to horizontal forces that are carried out for different load very large compared to the weight, and combinations in order to find the in addition these horizontal forces are most critical one. The behaviour of the cyclic. The base area may be of the platform must be acceptable and the 2 order of 10,000 m , the deck may be as satety adequate from the time of much as 200 metres above the sea bottom installation and during all stages of (water depth + 40- 50 metres) and the production for some 20 - 30 years. Local submerged weight of the order of 200,000 contact pressures between base and soil metric tons. may for example have the highest values shortly after installation and before

For on-shore structures the foundation the voids have been grouted. On the Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 load is normally increasing gradually other hand, the shear strength during the whole construction period, characteristics of the soil will change i.e. during months or even years. A with time due to cyclic loading such gravity platform is, on the other hand, that the stability of the foundation may placed on the site during a few days. be smaller after several years of The platform is constructed in sheltered production than it was shortly after waters near the coast, and usually even ins.tallation. deck and production equipment have been installed prior to towing out. In order to check the validity of the engineering assumptions and methods of In the following will be discussed what analysis, the behaviour of the platform particular geotechnical problems a should be monitored. Instruments gravity structure presents, and what measuring deformations of the structure approaches we take at NGI when trying to and pore water pressures in the soil is solve some of them. It will also be the only way to improve the state of the described how instruments are used to art. The real magnitude of the safety achieve a safe platform installation. factors is today unknown.

Most of the design considerations listed above are discussed in earlier GEOTECHNICAL CONSIDERATIONS FOR GRAVITY state-of-the-art papers by NGI (Bjerrum, PLATFORMS 1973 and Eide, 1974). This presentation A great many of the design considerations will consentrate only on a few aspects: of a gravity structure involve soil (1} the overall stability analysis on mechanics. Below is given a list of clay, (21 the effect of cyclic loading analyses which have to be carried out in on clay, (3) the dynamic analysis of the order to demonstrate the soundness and soil structure system and (4) platform ~afety of a platform and its foundation. behaviour during the installation phase.

1. Platform installation, including skirt penetration analysis, the use STABILITY ANALYSIS FOR A CLAY FOUNDATION of under-pressures and grouting The aim of the overall stability procedures. analysis is to demonstrate that the 2. Analysis of the load transfer from platform and supporting soil are able to the structure to the soil. withstand the most unfavourable combination of forces with a de£ined 3. Overall foundation stability with factor of safety. A pseudo-static respect to sliding and overturning of analysis is used with dynamic the platform. - amplification factors .applied to the 4. Initial and long term settlements. computed wave forces. 5. Short term cyclic displacements. In addition to the submerged weight of the platform and loads on deck, the 6. Dynamic analyses of the soil structure system for wave loads and structure must· withstand environmental forces due to waves, wind, current and earthqu~kes. ice. In the North Sea the wave loads 7. Stresses in conductors and risers due are by far the greater of these. The to short and long term displacements design wave is taken as the highest wave and settlements. that with a certain probability will occur once every 100 years, i.e. the so 8. Problems related to the removal of called 100-year wave. At a water depth the platform. of 150 metres in the northern North Sea, SPE 5758 KNUT SCHJETNE 3

the 100-year wave may be of the order of constant strength properties with depth. 30 metres high and the corresponding This is seldom the case, at least not in forces and moments acting on a typical the· North Sea. A soil profile may conta platform are shown on figure 3. in several layers of sand, silt and clay, and the shear strength may vary Typically, the static vertical load due considerably with depth. to the submerged weight of platform may be, In order to take these factors into account we believe that the best method Pv max = 200,000 tons of calculation is some form of limiting equilibrium sliding surface analysis Pv min = 150,000 tons (with oil storage) which seeks for the most critical Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 sliding surface (e.g. Morgenstern and The maximum forces. due to waves and Price, 1965, Janbu, 1973). A somewhat current may be, simplified and much quicker method of this type has been in use at NGI for the PH = resultant horizontal force last 3 - 4 years and is continuously = 50,000 tons being improved. The· present version is described in detail by Lauritzsen and M = overturning moment Schjetne (1976), and only the principles 2,000,000 ton metres = of the method are discussed herein !:.Pv = additional vertical load (figure 5) • The base is transformed to = 10,000 tons (in phase.with a square with the same area. The PH and M) vertical load is applied on the effective foundation area only, while The difference in water pressure on the the horizontal force is distributed sea floor adjacent to the platform over the total foundation area. The between windward and leeward side must factor of safety is found by an overall be taken into account in the analysis. equilibrium of all horizontal forces. It may be of the'order of: To account for the three-dimensional situation the shear forces on both sides /:.p = ± 4 t/m2 of the sliding volume are estimated. The undrained shear strength variation It must be verified that the platform with depth is accounted for, and a will not slide horizontally when computer programme varies the angle a in subjected to the maximum horizontal order to find the most critical sliding force, Pa· For a structure without surface. skirts this implies sl~ding along the soil structure interface. For a For a given platform design on a given structure with skirts sliding may take place in the soil below the tip of the soil profile, the effect on the factor of safety by varying for e~ample base skirts. area, platform weight or skirt length is For deep, bearing capacity type readily obtained. An example of such a study is shown on figure 6. failures the simple formulas developed by Brinch Hansen (1970) and Meyerhof As the maximum load has a short duration (1953) are frequently used both for sand it is an "undrained case" in the soil and for clay. The horizontal force is mechanics terminology. In general the taken into account by a correction soil strength is expressed as: factor in the bear~ng capacity formulas. For eccentric loads the concept of an Lf. = c' + cr' • tan ~· "effective foundation area" which is centrically loaded is commonly used and c' cohesion intercept of L-axis cr' - effective stress = cr - u the definition is given in figure 4. cr - total stress The use of these formulas for offshore u - pore water pressure platforms should be used with care and ~· - friction angle is only recommended as a preliminary check on· the stability. One reason for The undrained foundation stability this is that the formulas contain a analysis may be performed by either one number of correction factors and the of the following two methods: available experience is limited to (1) Determine the undrained shear loading conditions for foundations ashore. Furthermore, the formulas strength Lf by subjecting representative soil samples in the assume homogeneous soil conditions and 4 FOUNDATION ENGINEERING FOR GRAVITY STRUCTURES IN TEH NORTH SEA SPE 5758 laboratory to the in-situ·stresses the strength is only partially and the total stress paths they will mobilized (Figure 7). be subjected to under the platform in the field (i.e. sustained + It should be noticed that for the pulsating stresses} • The strength stability analysis of the same structure values determined for the samples soil and loading condition the numerical are used directly along the value determined for SF by method (1) potential sliding surface in the will be different from the numerical limiting equilibrium analysis. The value of the factor discussed in.method strength values used may be very (2). This is not inconsistent, but a , different for the active and passive result of the different ways in which sides of the sliding body. the "safety coefficient" was introduced. Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 This method is often called a "total stress stability analysis." A more CLAY SUBJECTED TO CYCLIC LOADING appropriate name is the stress path When a stability analysis is carried out method (Lambe,l967). It takes full using data from static laboratory tests account of the initial effective or in-situ penetration tests the effect stresses. It simulates the total on the soil of cyclic loading is not stress paths experiences by the accounted for. corresponding elements in the field, and the effective stress paths as In sand the effect of a sustained storm well as strains are recorded in the is a build-up of pore water pressure laboratory. The factor of safety which in some cases may lead to a against instability is introduced by loss of strength. This is treated by divfding the measured undrained for example Bjerrum (1973) who gives a strength Tf with a material factor, numerical example of the effect on the MF (figure 7). (Load factor, LF=l,O) safety factor. In cases where no good soil samples The understanding of the effect of are available, the undrained shear repeated loading on clays has strength values, appropriately substantially increased during the last reduced for the estimated effect of two years thanks to an intensive cyclic loading, must be based on research project initiated by Shell vane shear tests or cone penetration International in Holland. Altogether tests. This has been the case for 13 different oil companies and most foundation analyses performed institutions in th~ United Kingdom, in the North Sea thus far. Holland and Norway sponsored the 400,000 $ project. (2) In the so-called "effective stress stability analysis", the strength is Details about the tests and the introduced by using the effective conclusions may be found in Andersen stress parameters c' and~·. (1975) and Andersen (1976). Furthermore, the pore pressures due to the loading (sustained + All tests were carried out on one pulsating} must be estimated. This particular clay from Drammen in Norway. is the most difficult part of the This is a clay with a plasticity analysis. similar to many of the North Sea clays. It is homogeneous and easily available, This method requires tests on and the material properties in general laboratory samples and cannot are well known from several previous directly use strength values investigations. The results obtained obtained by the vane or the cone were compared to cyclic laboratory tests penetration test which frequently ha already carried out on actual North Sea been the only information available clays. from the North Sea. All samples were preconsolidated to A margin of safety against 40 t/m2 before unloading to different instability is assured by only overconsolidation ratios (OCR). The allowing a certain degree of majority of the cyclic tests were run mobilization of the effective either as, strength parameters. The terms c' - stress-controlled, one-way loading and tan ~· are divided by a factor larger than one to make sure that triaxial tests or as, SPE 5758 KNUT SCHJETNE 5

- stress-controlled, two-way loading effective stresses increase. Point d simple shear tests. represents ~he undrained shear strength after cyclic loading, compared to point The stress pulses for the two types of f which is the strength of a sample tests are shown on figure 8. Either sheared statically to failure without trapezoidal or sinusoidal pulses with a cyclic loading. period of T = 10 sec. were used. The behaviour of clay in cyclic loading In a typical cyclic simple shear test the can be explained in terms of effective cyclic loading was achieved by applying a stresses. Cyclic shear strains are for cyclic horizontal shear force on the top example uniquely related to the effective of the sample under undrained conditions. stress level, independent of over For each test, the cyclic stress level consolidation ratio or number of cycles.Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 was kept constant, but the stress level The project also showed how the rate of varied from test to test. pore water pressure generation and the number of cycles to failure are related Figure 9 shows a typical test result. to the effective stress level at the The top and bottom graphs show shear start of the test. The effective strain and pore pressure respectively strength parameters c' and~· are versus number of cycles. Both the shear practically not affected by the cyclic strain and the pore pressures are in this loading. particular test increasing nearly linearly with increasing number of cycles Instead of pore water pressure At approximately 350 cycles the strain is development, the shear strain development seen to increase rapidly as the sample is may be used. as a parameter describing failing under cyclic loading. The pore the behaviour during cyclic loading. water pressures have become so high that Figure 12 shows how the relationship of the sample is close to the Mohr-Coulomb total stress level, number of cycles and line. shear strain can be plotted for an over consolidation ratio of 4. The results The increase in shear strain with from cyclic tests with a constant cyclic increasing number of cycles implies a stress level plot along horizontal decrease in stiffness of the clay. This lines in this diagram. Figure 13 shows is illustrated in figure 10, where the the relationship between total stress shear modulus, G, normalized with respect level, number of cycles to reach ±3% to the undrained shear strength, Thfr is shear strain and the overconsolidation plotted versus the effective stress level ratio. At a certain total stress level, at the start of the cyclic test, SLc 0 • the higher the overconsolidation ratio, The shear modulus decreases rapidly with the smaller number of cycles is necessary increasing number of cycles, especially to bring the sampl:e to failure in cyclic at high stress levels. loading.

In gener·al the cyclic part of a simple Because a storm consists of waves with shear test was stopped after either ±3% different wave heights, the clay shear strain.or 2000 cycles, whichever elements beneath the platform will be was reached first. All samples were subjected to varying stress levels subsequently sheared statically, during a storm. A method of undrained to failure. accumulation of shear strains has been developed based on the strain contour In terms of effective stresses figure 11 diagram. This is illustrated in figure illustrates what happens to a typical 14, where a chosen storm spectrum is test in one-way cyclic loading. For a expressed as blocks of different total highly stressed sample the effective stress levels. The figure shows how stress path during the first loading is the strain at the end of the storm is from a to b. Because of the high stress constructed to be Yc = 1.3%. This way level, a considerable residual pore water of strain accumulation was verified in pressure is developed. The top point of the research programme by tests where the stre~s path moves to the left with the cyclic stress levels were varied in increasing number of cycles. After a order to simulate a storm spectrum. certain number of cycles the stress path has reached point c. The final static While the effective strength parameters shearing is represented by the line c to c' and q,' were unaffected by cyclic leading, d in fig. 11. The sample dilates, i.e. the undrained shear strength after cyclic! the pore water pressures decrease and the loading is a function of cyclic shear strain reached and the number of cycles. 1 6 . FOUNDATION ENGINEERING FOR GRAVITY STRUCTURES IN THE NORTH SEA SPE 5758 In general, the reduction is less than diagram shows the transfer function for 25% if the number of cycles is less than the mudline moment for a given structure 1000 and the strain due to cyclic loading This is the mudline moment due to 1 metre is less than ±3%. From figure 15 one may high waves. It shows a resonance period conclude that a strain of 1.3%,as found of 4.5 seconds. This is well below the in ~he numerical example,results in a periods with high energy waves. However, reduction in undrained shear strength of at resonance even low energy waves may only 5%. If one makes the conservative cause large forces and displacements assumption that the 100-year wave hits provided the damping is small. the platform at the end of the 100-year storm, the undrained shear strength of The lower diagram in figure 16 shows the

the particular clay layer investigated predicted resulting mudline moment with Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 should /be reduced by 5% in a "total the design values for periods of 16 - 17 stress stability analysis." seconds. It is important to note the peak for the more frequent waves with In order to investigate the long term periods of 4 - 5 seconds. Similar behaviour of a foundation, a number of diagrams could be given for horizontal tests were carried out to simulate shear at mudline and for moments and drainage between storms. For normally shear forces at the top of the towers. consolidated clays drainage after cyclic loading implies consolidation and In Norway a computer program for dynamic increased undrained shear strength. The analyses of a gravity structure has factor of safety of the platform will recently been developed (Boland 1976). then increase with time. For highly The computational model is shown on overconsolidated clays one may fear that figure 17." The stiff base is drainage implies swelling and thereby a represented by a pyramid and each of the reduction of shear strength. The three towers as high stiffness beams overconsolidated samples tested indicated rigidly fixed to the base. The deck is th~t preshearing and drainage would represented by two beams, of course much lower the resistance of the material to too simplified for a detailed study of subsequent cyclic loading. the complex deck structure. The masses are concentrated in ·nodal points where also the wave forces are applied. In this model the soil foundation is DYNAMIC ANALYSES OF SOIL-STRUCTURE SYSTEM represented by three springs and The dynamic analyses are used to: corresponding dashpots. The springs .1) calculate the amplification of forces slimllate the soil stiffness. Even for and displacements f.or the design static loads, the soil is highly non linear. In addition the stiffness, or wave, and the shear modulus, G, decreases with 2) calculate the much higher increasing number of cycles especially amplification of forces and for high stress levels, as demonstrated displacement for the frequent short in the research project on Drammen clay period waves which may govern (Andersen 1975). This implies that liquefaction of the soil or the under a platform, the shear modulus will fatigue of concrete and steel. not only vary from one soil element to another depending on the. static stress The complexity of dynamic analyses in level, but will also vary with time general is one reason why most of the during a storm. A redistribution of foundation analyses for a gravity stresses will take place as the most structure are carried out with static highly stressed zones become weaker. environmental forces. With increasing Complex analyses of this type may only water depths, however, consideration of be performed by using a finite element the dynamic behaviour has proved to model for the soil. become more and more important. Until these factors can be taken properly Figure 16 illustrates some important into account we recommend to calculate principles. The upper diagram shows how the various spring constants for a range the wave energy is distributed as a of shear moduli. For the design, the function of wave period. The highest most unfavourable value within the range energy waves including the 100-year should be chosen. design wave have periods of 15 - 20 seconds. The dotted line in the same SPE 5758 KNUT SCHJETNE 7 The response of the structure in the contact earth pressure on tip of the vicinity of resonance is sensitive to the domes, structural strain in the domes amount of damping which is chosen for the and in the lower part of the cell walls, soil. There are principally two types of and finally short term settlement. damping in soil. The most important one is the geometrical or radiation damping The data acquisition system was which is the amount of energy lost due to manufactured and installed by the wave propagation through the soil away Central Institute for Industrial from the platform. A theoretical value Research (CIIR}, Oslo. In addition to can be calculated for a homogeneous, strip chart recording, automatic elastic halfspace. In reality, the scanning of the transducers and punching radiation damping is most often less than on paper tape, any transducer or group this theoretical value due to reflection of transducers could be read on a TV Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 from boundaries between layers. d~splay just by pres•ing a button.

The second type of damping is the An example is illustrated on figure 25, internal of hysteretic damping due to where the earth pressures on the tip of internal friction in the soil. The all 19 domes are given. Also one energy loss may be determined by the could get these values in a table or in hysteretic loop of the stress-strain the form of a histogram. In addition, curve for a cyclic laboratory test. the computer stored the data such that for any particular transducer one could get the time history for the last 5 INSTALLATION OF PLATFORMS minutes, 2 hours or 48 hours. Figure 26 The two first Condeeps, one for Mobil at shows the time history for one partie~ the Beryl .field and one for Shell/Esse at earth pressure cell for the last 5 the Brent field, were successfully minutes. installed on schedule last summer. NGI has been acting as consultants to the Figure 27 shows the dome contact designer, A/S H¢yer-Ellefsen, and to the stresses from Brent B. As the upper contractor, Norwegian Contractors. We part of the soil profile cbnsists of have also been responsible for the stiff clay, one would expect maximum design and installation of the various earth pressure values as indicated on ·instruments for monitoring the the figure. Except for two domes, all installation of the platforms. measured values were below these predicted maximum values. Cell no. 15, Figure 18 through 23 show some photos however, behaved differently, as the from the construction period, during deck contact pressure increased rapidly installation in Stavanger and during tow shortly after touch-down. The reason out ·to the site. Figure 24 shows two for this was most probably a local sand sections of the Brent B platform designed layer just below cell no.· 15. Because for 140 metres water depth. The platform the value approached the maximum allowable consists of 19 cells with an overall ~ia value for structural reasons, the earth meter of approx. 100 metres. The deck is pressure on this particular cell limited supported by 3 towers. The base is the penetration of the structure. equipped with both concrete and steel skirts, extending 0.5 m and 4 m below the Figure 28 shows how the instruments were domes respectively. Also, there are 3 used during the installation in order to steel dowels extending 4 m below the tip keep the tilt of the platform close to of the steel skirts in order to break zero., The skirt penetration resistance platform movements before steel skirt varied from one side of the platform to touch-down. the other due to local unevenesses of the sea floor and due to local variations in Details about the Condeep Brent B the soil properties. The figure shows installation is given by Eide, ~arsen how the tilting moment was regulated as and Mo, 1976. a function of the measured tilt and how this successfully controlled the tilt. In order to ensure a safe installation, The moment was achieved by means of a number.of control sensors were built eccentric ballasting and by means of into the construction (figure 24}. The excess pressure or suction in the· various parameters measured were, draught of the skirt compartments. The platform ended platform, water level in the cells, water up with no tilt in the x-direction and pressure in skirt compartments, platform approximately 0.1 degrees in the inclination, stresses in the steel dowels, y-direction. 8 FOUNDATION ENGINEERING FOR GRAVITY STRUCTURES IN THE NORTH SEA SPE 5758

SUMMARY AND CONCLUSIONS REFERENCES The paper has in some detail described Andersen., K. H. ( 19 7 5 J. Research project, four of the many aspects of the repeated loading ·on clay. Summary and foundation engineering of gravity interpretation of test results. NGI platforms. These represent areas where we 74037-9, 15. Oct. 1975. believe NGI has contributed to some progress during the last couple of years. Andersen., K.H. (1976). Behaviour of Clay Subjected to Undrained Cyclic The stability analysis of a platform Loading. To be presented at Boss '76, should preferably be carried out with Int. Conf. on Behaviour of Off-Shore some form of sliding surface analysis Structures, Trondheim, Aug. 1976. which seeks for the most critical Bjerrum., L. (1973). Geotechnical Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 sliding surface. Because the design load Problems Involved in Foundation of has a short duration, an undrained Structures in the North Sea. Geotechnique analysis should be carried out. Two Vol. 23, No. 3,1973, p. 319-358. Also different methods for carrying out an publ. in: NGI, Publ. 100, 1974. undrained analysis are described: The so-called "total stress stability method" Hansen., J. Brinch (1970). A revised and and the "effective stress stability extended formula for bearing capacity. method. " Mainly because of the Geoteknisk institutt, Kobenhavn. difficulties in predicting pore pressures Bulletin, 28, p. 5- 11. for the latter method, the "total stress" Eide., 0. (1974). Marine Soil Mechanics. method is the most commonly used. It Application to North Sea Offshore should, however, be based on undrained Structures. Lecture presented at shear ·strength values as determined in Offshore North Sea Technology Conference the laboratory on samples subjected to and Exhibition. Stavanger 1974. Also the same total stress path as under the publ. in: NGI, Publ. 103, 1974. platform in the field. Eide., O.T . ., L.G. Larsen and 0. Mo (1976) The two Condeep platforms installed last Installation of the Shell/Esse Brent B summer proved the importance of Production Platform. To be presented at instruments and a data logging system in the 1976 Offshore Technology Conference, order to achieve a controlled installation Houston, Texas, May 1976. Holand., I. (1976). Svingninger og dyna In spite of the skirt resistance being miske pakjenninger av bygningskonstruksj. much greater on one side of the platform Forelesning pa Kursdagene ved NTH, jan. than on the other, it was possible to 1976. (Lecture given at "Kursdagene ved keep the platform level. Furthermore, NTH", NTH, Trondheim, Jan. 1976). the maximum penetration of the platform was governed by the earth contact Janbu., N. (1973). Slope stability pressures against the bottom domes. computations. Embankment Dam Engineering. These were monitored, and the penetration Casagrande Vol. Ed. by R.C. Hirschfeld was. terminated before critical values and S. J. Poulos. N.Y., Wiley. Pp. 4 7-86. were reached. Lambe., T.W. (1976). "Stress Path Method". American Society of Civil EngineeTs. The most important step forward has Proceedings, b. 93, No. SM6, 1976. undoubtedly been the research project of repeated loading on clay. We have by no~ Lauritzsen., R.A. and K. Schjetne ·(1976) a good basis for un~erstanding the Stability calculations for offshore behaviour of clays when subjected to gravity structures. To be presented cyclic loading. The next important step Offshore Technology Conference, should be to build this knowledge into 8. Houston, Texas, May, 1976. our analytical models and compare the Meyerhof., G.G. (1953). The bearing results with the actual behaviour as capacity of foundations under eccentric measured on the platforms. and inclined loads. Intern. Conf. on Soil Mech. and Found. Engineering, ACKNOWLEDGE.MENT 3. ZUrich 1953. Proceedings, Vol. 1, pp. 440- 45. This presentation is to a large extent based on results obtained by the author's Morgenstern., N.R. and V.E. Price (1965) colleagues at the Norwegian Geotechnical The analysis of the stability of general Institue. This is truely acknowledged. slip surfaces. Geotechnique, Vol. 15, Special thanks are due to the Director, No. 1, pp. 7 9 - 9 3. Dr. Kaare Hoeg, for valuable comments and corrections to the manuscriot. THISTLE:'~~AGNUS • OIL FlELO iwt'fJr."'-'·. • STATFJORO "; • GAS FIELD TER~JO~lr,.o~~- HEATHe~ /•_ sHCri.AitD NINIA~/ ISLMOS f ALWYN BERYL-·rfl-=~~~AL .

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D ·I

PH: 50,000 t

»> ))) )/ h ,,, »>

Fig. 3. Examples of forces acting on E· a gravity platform ~

8 2e

; / / / /

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Fig. 5. Pr i nc i p l e s of s l1i p surface Fig. 4. Principles used in defining method effective foundation area 3.0 u.. ~ a:: SHEAR 1

0 2.5 1 1- STRESS (..) - '( <( u..

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Fig. 6. Example of a parameter study Fig. 7. Definition of material factors

4 A. ONE-WAY LOADING: ;-! I T = 10 seconds u ./ )0.. 2 ~ . ~ + z -, ·~c "'a:: <( :n 0:: a:: .... 0 .. IJ) "'::1: Ill -~c 0:: 0 <( ~ UJ -2 0 s 10 12 14 :c TIME I second I IJ)

-4 - 0 100 200 300 400"" 500 NUMBER OF CYCLES, N 8. TWO-WAY LOADING: 2 0 100 200 300 400 500 T: 10 seconds I • E ~ 0:: "' ....w ;::) <( w 0:: ~ :::> IJ) 0 lU IJ) a::: lU 0 0:: 0.. 0.. -1

Fig. $. Type of load pulses. Fig. 9. Example of a cyclic simple shear a) one-way loading test b) two-way loading StMP\.E SHEAR CYC\.IC 000 TWO-WAY ~CR•t,4 110 soo \

zoo \ \\ 100 \ ~ '(j' so \\" \\ H~t~ ...... zo \.1 \ '-I --- a Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 H • 400 •1000 10

s Fig. 11. Effective stress paths for samples in cyclic loading z

I %0.2 :t0.4 :o.a :o .• SL c,o

10 100 1000 10000 1.0 Fig. 10. Shear modulus as a function of stress level and number of cycles :o.g ~ 0.1 tO.I 0,8 ., ...... , :o.1 ' 0.7 'tt~c ~~ .... ~ ~ ""'t.to.a """ ...... o.a I~ • "~ r-.:: ~"'-.... ,..,. .... o.s ~ I - ~r- "' ~ :0.4 ...... 0.4 SYII.OL OCII ~~ (l/mJ I " 1 ~ ::::: ~~ l'~~ !0.3 0 NC 8.7 I"'"' O.l • 4 7.1 ti,O !0.2 o. 2 .1.0 X 10 5.4

! 0.9 ! 0.1 25 4. 4 0.1 a' 50 3.5 t0.8 1 .... ! 0,8 o.o 0.0 1 10 JO so 100 - soo 1000 1000 5000 10000 ~ r-... 3.. .,. ~ 0.7 NUM8ER OF CYCLES TO "tc :a L 3 "/• 1.0 ~ ~--~ ~ --< ~· ~0.6 -11.!11 ·~o !( Oi\ 10 0.51 Fig. 13. Number of cycles to reach yc=±3% G.IO 0,71 ~~ ... ::--...... :o.s .0.5 for different overconsolidation O.ll 1--~-- Clc - ~ ~ ::::~ ~ t0.4 .0.4 ratios ~71 I. 1'- O.l - r-!11 G. lis o.s t 0.3 .0.3

OCR t.,.lttm{ t 0.2 .0, 2 4 I 7.1 t 1.0 t 0.1 --- 0.1 End of storm !1 l.mmcdiatc cyclic strain (determined 0.0 Ill o.o I 10 JO so 100 lOG 500 1000 1000 5000 10000 0.8 from Fig. 9.49 I 'The lncrcau in c:yctic strain during c:yctic loading 'T nf 0.1

Fig. 12. Strain-contours for two-way 0.4 simple shear tests. OCR = 4

0.2 Results: end of storm: lc•~t.l"/• o.o Hcq¥ :: 7 10 100 1000 10000 NUMBER OF CYCLES

Fig. 14. Accumulated cyclic shear strains X 2000 e392 >- vf>f. (.!) 1.0 ., z a:: 179~~000 •55 0 lJ.J ~470133 •79• z WAVE .100 •47 ...... lJ.J SPECTRUM J o884 zU 7 LJ.JZ ~ ENERGY 0,8 @)1000( ~ ®350 ~::> ::> ou.. a::..... A(MUDL I NE ~a:: u I MOMENT wLJ.J lJ.J zU.. a_ I\ TRANSFER 0.6 ..... ---·-···-···- ··-·-- _V) V) I FUNCTION -.JZ I Q<( lJ.J I I ::>a:: > I \ ~~-- <( 0.4 3 I OCR:4 0 5 10 15 20 WAVE PER I 00, T. SEC basic tests _ 1--- Numbers indicate 0.2 ~----· 0 T:20sec. no. of cycles during ~ ..... Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 @ storm-loading cycloc jloadlog I @ strain-controlled ..... MUOLINE )( one-way loading z 0.0 lJ.J MOMENT 0 ~1 ~ 0 ~ lJ.J z a....J Fig. 15. Remaining undrained shear ::> strength after cyclic loading ~ 0 10 15 20 WAVE PERIOD. T. SEC

Fig. 16. Principles for dynamic analysis

ELEMENT

Fig. 17. Dynamic analysis. Computational model Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021

18 19

21 20

23 22

Fig. 18-23. Condeep under construction, during deck installation and tow-out j j

aO L.A.T. Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021

I~STRUMENTATION FOR INSTALI.ATION

Wot.r IAv.l in the Cells. Draught. Water P,..ssure in the Skirt Compartments. Inclination of the Platform St~- in the Steel Pipe Dowels. EP T0W1'12 Contact Earth Pr.ssure at the Tip of the Domes. Structural Strain in the lower Domes. TIME= 07.14 09=2~ Structural Strain in th• lower Part of the Walls. DRAUGHT: 126. 63 METER Short T•rm s.ttl•ment.

Fig. 25. Example of "hard copy" of earth pressures

Fig. 24. .Instrumentation of Condeep Brent B

EP-4312 TflVH2 220.0

219.0

218.9

217.9

216.9

21~.9

214.9

213.9

212.9

211 a

-19-18-17-16-15-14-13-12-tl-19 -9 -s -7 -6 ~ -4 -3 -2 -1 e Tit£ e7 16 21:38 RESQ..UTICH: 15 SEC. OF'Uf"..HT 127 1€1 I'ETER PERIOO: 3ee SEC

Fig. 26. Example of time history of earth pressure N 200~------~------E 0 esign Maximum Allowable Value -UJ... a:: ~ 150 1------ V) L&J a:: a. t- Downloaded from http://onepetro.org/SPEEURO/proceedings-pdf/76EUR/All-76EUR/SPE-5758-MS/2054230/spe-5758-ms.pdf by guest on 02 October 2021 u 100 1------~z 0 c.,) Expected Maximum Values UJ ~ 50 t---+----- 0 c

1 2 3 4 5 10 15 20 OOME NUMBER

Fig. 27. Dome contact stresses, Brent B

DOME PENETRA T I Otj

~-~001~-~------4------~~~--~------~

a a, 0 f---+------11-/ 1- ~ 0 1--+-----\ ·A--f----\------,#1>1.~~~---~-~.....::::~~~

...J ~ -0.1° f--4------/-<1------~+------+-+------

Fig. 28. Tilting moments and platform inclination