Foundation Performace of Offshore Jack-up Drilling Rigs
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Transcript of Foundation Performace of Offshore Jack-up Drilling Rigs
FOUNDATION PERFORMANCE OF OFFSHORE JACK-UP DRILLING RIGS 3
By Alan G. Young,1 M. ASCE, Bernard D. Remmes,2
and Barry J. Meyer,3 Associate Members, ASCE
ABSTRACT: An offshore fleet of about 435 mobile jack-up rigs is currently drilling oil and gas wells around the world. About 2.6% of these jack-up rigs annually experience an accident which is far greater than any other type of offshore drilling unit. Over one-third of these accidents are associated with foundation problems. Certain geological conditions pose particular hazards to the foundation performance of mobile jack-up rigs. To reduce the risk of foundation failure, these hazards can and should be identified by investigating subsurface conditions. Considerable interest is being given to assessing the accuracy of the analytical methods now available to predict the foundation performance of jack-up drilling rigs. Some commonly used analytical methods are reviewed and actual case histories are presented.
INTRODUCTION
Mobile jack-up rigs are used extensively offshore around the world to drill oil and gas wells in water depths u p to about 350 ft (100 m). A mobile jack-up rig is a floatable drilling platform (self-propelled or towed by tugs) with legs that can be moved u p and d o w n (Fig. 1). The rig is floated to the well site wi th its legs elevated. When it reaches the site, the legs are lowered and jacked into the sea bottom to produce a foundation; the once floating hull climbs the legs to become an elevated work platform.
Presently, there are 435 jack-up rigs in operation worldwide, with about 450 rigs expected to be in operation by 1984 (in accord with current construction trends). Of all the types of mobile offshore drilling units , jack-up rigs have experienced the worst accident rate dur ing the period from 1955 to the present (17). Considering that today rigs can cost over $50,000,000, the loss of one of these rigs can be disastrous in terms of replacement cost of the rig. The high accident rate may be attributed to the severe design requirements associated with the rigs ability to change from its seabed resting state to a floating condition with legs full retracted, and vice-versa. Roughly one-third of these failures are related to the supporting capability of the seafloor soils under the jack-up rig foundation.
The purpose of this paper will be to critically review the operational problems associated wi th foundation stability and seabed support . The paper will recommend the types of geotechnical and geophysical data
'Presented at the October 25-27, 1982, ASCE Annual Convention, held at New Orleans, La.
'Offshore Project Mgr., McClelland Engrs., Inc., Houston, Tex. 2Offshore Supervising Engr., McClelland Engrs., Inc., Houston, Tex. 'Offshore Supervising Engr., McClelland Engrs., Inc., Houston, Tex. Note.—Discussion open until December 1, 1984. To extend the closing date
one month, a written request must be filed with the ASCE Manager of Technical and Professional Publications. The manuscript for this paper was submitted for review and possible publication on March 23, 1983. This paper is part of the Journal of Geotechnical Engineering, Vol. 110, No. 7, July, 1984. ©ASCE, ISSN 0733-9410/84/0007-0841/$01.00. Paper No. 18996.
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AFLOAT PRELOADING DRILLING
FIG. 1.—Jack-Up Rig Operational Modes
required at each site to help reduce the potential risks of foundation failure. The paper will also present case histories that illustrate and evaluate analytical methods now available to predict foundation performance.
EVOLUTION OF FOUNDATION DESIGNS
One of the main goals facing rig designers over the years was the development of a mobile drilling unit that could be operated under the most severe weather conditions and still remain stable regardless of soil conditions. Unlike the foundations supporting a conventional fixed platform or gravity structure, the foundation supporting a mobile jack-up rig is not custom designed for a specific site. Because offshore soils around the world exhibit a wide range of strength and classification properties, many different foundation designs have evolved since the first jack-up, the Barnsdale Hayward-Breton Rig 20, was used offshore Louisiana in 1949.
Mobile jack-up rigs today can be divided into two broad categories according to their foundation type: (1) Individual footings; and (2) a single large mat, both shown in Fig. 2. With either type, the soil-foundation system must safely carry the dead weight of the unit, the operating loads, and the environmental forces, mainly wind, current, and waves.
FIG. 2.—Typical Mobile Jack-Up Rig Design: (a) Individual Footing Type; (b) Mat Foundation Type
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D 1955 1956
OFFSHORE NO. 52 SCORPION (Circular) (Circular)
1963 1967 DIXILYN 250 PENROD 54
(Truncated-Circular) (Circular)
1975 PENROD 65
(Dodecagonal)
1980 1983 GLOMAR HIGH ISLAND VI MARATHON GORILLA
(Hexagonal} (Dodecagonal)
FIG. 3.—Evolution in Footing (Spud Can) Configuration; 1 m = 3.28 ft
Individual Footing Rigs.—The first jack-up rigs were supported by cylindrical pile legs. The footing or "spud can" was first introduced in 1951 on the Offshore Company's Rig 51 to further reduce leg penetration in soft clays. Early jack-up rigs commonly had 8-12 individual legs. The design featuring three legs first appeared in 1955 on the Zapata Offshore Company's Scorpion, designed and built by Marathon Le-Tourneau. Over 60% of the fleet today are of the three leg design.
Over the years, footing designs have included a large assortment of sizes and shapes, as shown in Fig. 3. The trend towards larger spud cans is demonstrated by the increase in the bearing areas from 220 sq ft (20 m2) on the Offshore Rig 51 to 3,300 sq ft (300 m2) on the Marathon Gorilla, probably the largest rig built to date. The majority of the worldwide fleet today has bearing areas ranging from 1,000-1,800 sq ft (90-165 m2) per footing.
Maximum footing loads typically are 4,000-11,000 kips (18-49 MN), or the maximum bearing pressures are about 4-7 ksf (192 to 335 kPa). The Marathon Gorilla rig, with a 3,300 sq ft (300 m2) spud can, is designed to support a 23,000 kip (102 MN) maximum leg load which develops a bearing pressure of 7 ksf (335 kPa). A few rigs, such as the ODECO's Oceantide, have higher footing bearing pressures in the range of 12-20 ksf (575-960 kPa) to increase footing penetrations, thereby greatly reducing foundation instability from scour in granular soils.
Mat-Supported Rigs.—A number of different mat shapes have appeared with these rigs; the most common design features an A-shaped mat, as shown in Fig. 2(b). All the mat foundations feature a large bearing area to limit penetration below the seafloor to generally less than about 10 ft (3 m). The height of the mat is usually 10 ft (3 m), although the range in height is 6-15 ft (2-4.5 m). Bearing areas for these rigs may
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range from 13,500-30,000 sq ft (1,250-2,750 m2), resulting in average bearing pressures in the range of 0.4-0.7 ksf (19-34 kPa) under sustained dead and operating loads. During storm loading the mat-supported rigs develop an increased and nonuniform bearing pressure upon the supporting foundation soils. For a rig designed to operate in 250-ft (75-m) water depth, the resulting soil pressures at the mat edges during a severe storm vary from 0.25-0.65 ksf (12-31 kPa), although the average seafloor bearing pressure is about 0.47 ksf (22 kPa). Other details on the design and foundation performance of mat-supported jack-up rigs have been previously described by Whitley (20), Hirst, et al. (8), and Young, et al. (23). This paper will not examine causes of foundation failure or analytical techniques applicable to the mat-supported rig since these considerations have been covered in the previously mentioned papers.
FOUNDATION FAILURE MODES
Accident Record.—The accident exposure of mobile jack-up rigs was critically examined in an earlier study by McClelland, et al. (13) for the period 1955 through 1980. This study showed that an average of about 1% of the total rig fleet annually experiences a leg accident. The foundation accidents that occurred in 1981 and 1982, as shown in Fig. 4, confirm this accident rate of 1% or more each year. Considering the current size of the fleet, this percentage suggests that 4 or 5 serious foundation accidents will occur in 1983. Table 1 gives a listing of all footing supported jack-up rig failures resulting in major economic loss that are reported to have been caused by foundation problems.
Review of the mobile jack-up rig accident record indicates that foundation failures or mishaps can be attributed to the following causes:
1. Footing punch through during preload because of a soil profile that includes a strong layer overlying a weaker layer. ("Preload" is the weight taken on by a rig to "proof test" its foundation.)
2. Excessive storm penetration because the maximum footing preload did not exceed the maximum storm loads.
3. Footing instability induced by scour that undermined the footing resulting in reduced bearing area.
FIG. 4.—Annual Leg Accident Rate for Jack-Up Rigs
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TABLE 1 .—Accident Summary of Mobile Jack-Up Rigs"
Year
(D 1955
1957
1959
1965
1965 1966
1966 1966 1968 1969
1970 1970 1971 1972 1973
1974
1974 1976
1976
1979 1979 1980
1980
1980 1980 1981
1981 1982
1982
1982
1982
Rig name (2)
Rig No. 52
Mr. Gus I
Rig No. 10
Penrod 52
Maverick I Sea Gem
Roger Butin Rig No. 52 Dresser II John C. Marthens
Rig No. 59 Rig No. 59 Panintoil II Intrepid Rowan Anchorage
Gemini
Gatto Salvatico George F. Ferris
Kaspii 1
Ranger 1 Pool Rig 142 Western Triton III
Western Triton II
Rig No. 81 Bohai I Rio Colorado I
Don E. McMahon Maersk Valient
Zapoteca
Zapoteca
High Island V
Accident type (3)
Damaged while jacking in Gulf of Mexico
Capsized during relocation in Gulf of Mexico
Capsized while preparing to move in Gulf of Mexico
Capsized by hurricane in Gulf of Mexico
Sank in hurricane in Gulf of Mexico Collapsed while preparing to move
in North Sea Capsized off Cameroon Leg damaged off Nigeria Capsized in Gulf of Mexico Leg damaged during storm in Gulf
of Alaska Leg damaged off Nigeria Capsized on site off Nigeria Damaged by storm in Arabian Gulf Leg failure in Gulf of Mexico Leg collapsed while jacking up off
E. Kalimantan Leg damaged after punch-through
seafloor in Gulf of Suez Leg penetrated off Madagascar Damaged during jacking operations
in Lower Cook Capsized and sank on location in
Caspian Sea Leg failure in Gulf of Mexico Leg damaged in Arabian Gulf Lost leg support during rig posi
tioning in Gulf of Mexico Lost leg support during rig posi
tioning in Gulf of Mexico Lost during hurricane Allen Damaged in Gulf of Bohai Punch-through of one leg offshore
Argentina Jacking down off Zaire Leg problem while jacking down in
Mediterranean Punch-through while jacking up in
Gulf of Mexico Punch-through while jacking up in
Gulf of Mexico Punch-through while jacking up off
Brazil
Damage, in millions of
U.S. dollars (4)
0.3
2.5
3.1
2.5
5.7 5.6
7.0 0.2 2.0 0.1
0.2 4.0 2.8 3.5 3.0
2.0
4.0 10.0
8.0
4.8 1.2 1.2
4.8
30.0
— —
— —
—
—
—
"Summarized from LeBlanc 1981.
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4. Seafloor instability triggered by mass movements of the seafloor sediments that produce destructive lateral forces on the rig.
5. Inability to extract footings when the pullout resistance on the footing exceeds the available uplift force.
McClelland, et al. (13) indicate that every failure mode except item 5 can be predicted when soil data are available from site specific borings. Additional research should help provide methods to predict problems associated with item 5, thereby allowing the rig owner to avoid areas where this failure mode could, potentially, cause a problem.
The records (as shown in Table 1) reveal many accidents in the last 2 yr associated with footing punch-through during preloading. The writers believe that leg accidents in the earlier years were also caused by this failure mode even though the terminology previously employed in the descriptions of accident type may be different. The writers' experience confirms that many other punch-through accidents have occurred that did not reach the public record. The writers believe that the actual occurrence of punch-through failures probably exceeds by a factor of two the rate indicated in the published accident record.
Ballast Preloading.—A standard operating procedure for rigs supported by individual footings is to use a leg loading system called "preloading." The main purpose of preloading is to perform a "proof test" under controlled conditions rather than during storm conditions, such that each footing will be subjected to the maximum loading that it should ever encounter. A jack-up unit with four or more legs can use its jacking system to preload one or more legs at a time, using the dead weight of the unit as a reaction. Preloading of three leg jack-up units is performed by the rig mover in charge and consists of pumping seawater into holding tanks while the rig is elevated with a 5-ft (1.5-m) or less air gap (Fig. 1). As the jack-up hull is loaded, the legs usually penetrate into the seafloor. If the additional penetration exceeds 5 ft (1.5 m), the rig mover may have to dump the preload, raise and relevel the hull, and then resume preloading. Generally, the full preload is held for a minimum
FOOTING BEARING CAPACITY —B~
FIG. 5.—Punch-Through Failure During Preioad
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time of 2-4 hr after all footing penetrations have ceased. The preload is then dumped and the rig is jacked up to the specified air gap, typically 40-50 ft (12-15 m).
The risk associated with the preloading operation lies in the fact that at a few locations, the subbottom soil profile may include a strong layer of soil (with a high bearing capacity) overlying a weaker layer (with a lower bearing capacity), as shown in Fig. 5. If the ballast preload exceeds the bearing capacity of the upper layer, the footing will penetrate until it encounters adequate resistance. In this situation, the foundation can punch through the hard layer so rapidly that the penetration rate exceeds the capacity of the jacking system to adjust and level the rig. At most punch-through sites, excessive movement usually occurs with only one leg, often resulting in structural damage to the leg or, worse, overturning of the rig. Adequate assessment of potential punch-through failure at a site requires specific knowledge of layer thickness and soil strength. This type of information can only by established by drilling and sampling the subbottom materials, as described in the following section.
SUBSURFACE INFORMATION
The writers' experience indicates that the foundation performance of a mobile jack-up rig at a proposed site can be predicted within reasonable limits using geotechnical engineering methods described in later sections, if advance knowledge of subsurface conditions is obtained. In some cases, as described by Young and Noggle (21), geotechnical engineers may make reasonable predictions of foundation performance using nearby soil boring information or published generalized soil stratigraphy maps. The accuracy of foundation performance predictions based on extrapolated information is obviously lower than when based on site specific data; thus, the potential risk of the former is higher. The reliability of predicting foundation performance improves greatly when one or more soil borings are drilled at the proposed site and correlated with geophysical data. The recommended scope of work for each phase of an integrated geophysical survey and geotechnical investigation at a proposed drilling site is described in the next two sections.
Geophysical Survey.—A high resolution geophysical survey will identify potentially hazardous geological features and help create an understanding of areal soil conditions surrounding the drilling site. Ploessel, et al. (18) demonstrated that geophysical data provide not only economic benefits by limiting required borings, but also other benefits that include: (1) Correlation with soil boring data; (2) detection of three-dimensional features over a large area; and (3) understanding of the geological setting that may not be easily detected by data from borings. Geophysical data are generally obtained with a side scan sonar and 1-3 different vertical profiling systems.
The side scan sonar records will provide a detailed resolution of sea-floor irregularities such as exposed rock or coral outcrops, pipelines, depressions, or pockmarks left by jack-up rigs previously placed at the proposed drilling site. Geologic features, such as mudflows, associated with seafloor instabilities in delta regions like the Mississippi River, can
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also be detected by the side scan sonar. Depending upon the desired seafloor penetration and resolution, a
number of vertical profiling systems such as the tuned transducer, sparker, or boomer are available to develop vertical profiles of the seafloor. These profiles can be interpreted to help identify deeper geologic features that may present a potential hazard to the rig. Gas charged sediments, areal variations in the thickness of soil strata, and faulting may be detected from the records obtained from the geophysical profiling systems. Results of a geophysical study normally are presented as a series of maps depicting bathymetry, isopach of weak sediments, presence of faults, gas charged zones, and other geologic features of interest.
Geotechnical Investigation.—To develop reasonable predictions of foundation performance of a mobile jack-up rig, site specific data showing the stratigraphy, classification, and engineering properties of the foundation soils are required. These data are obtained by drilling and sampling one or more borings. There are two main options for drilling a soil boring: The boring may be completed by a geotechnical coring vessel, as described by McClelland (12), well in advance of the anticipated arrival of the mobile jack-up rig; or the oil well drilling equipment on the jack-up rig can be used to drill the boring. The disadvantages of this latter option are: (1) The rig may not have adequate leg length to jack up and drill the boring; and (2) a potential punch-through problem disclosed at this time would require the site to be abandoned. Both situations cause a major economic loss because of nonproductive rig time.
The growing practice of drilling the boring from the jack-up rig requires a special set of equipment and usually two or three specially trained geotechnical engineers. The engineers work with the drilling crews in advancing and sampling the boring to a penetration in excess, by one footing width, of the deepest expected footing penetration. They are also responsible for classifying and performing strength tests on the recovered soil samples. Once the boring is complete, engineering analyses are made to predict the foundation performance; the geotechnical engineers then make their recommendations to the rig mover. If favorable conditions exist, the rig mover will carry out the preloading and jack up to the working elevation.
BEARING CAPACITY ANALYSES
After the soil boring and test data become available, analytical procedures used in designing foundations on land may be used to predict the bearing capacity of the soils supporting the rig foundations. The bearing capacity analysis of individual footings should serve at least two purposes. One purpose is to predict footing penetration below the seafloor to determine if the combined air gap, water depth, and footing penetration are less than or equal to the maximum available leg length. This can be done by preparing a plot of bearing capacity (or leg load) as a function of footing penetration, as shown in Fig. 6. The other purpose is to evaluate the risk of a foundation failure caused by a punch-through.
Footings Supported on Uniform Soil.—Several procedures are available for computing the bearing capacity of a foundation in cohesive soils
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FOOTING BEARING CAPACITY —»•
I (3 Z
o o u.
I
FIG. S.—Installation and Preloading of Footings in Normally Consolidated Clay
0 1 2 3
Ralio of Footing Embedment to Footing Diameter, D/B
FIG. 7.—Nc Factors Used in Bearing Capacity Analyses
provided the shear strength is fairly constant. In general, all procedures can be represented by the bearing capacity equation
Q = ANCSU + y'V (1)
in which Q = footing capacity; A = cross-sectional area of embedded footing; Nc = dimensionless bearing capacity factor; S„ = undrained shear strength; 7' = effective unit weight of soil displaced by footing; and V = embedded volume of footing.
In the past, we have used bearing capacity procedures outlined by Skempton (19), Meyerhof (14), and Hansen (7) to estimate Nc for computing rig foundation penetrations. Each of the three investigators recommended slightly different Nc values to be used in Eq. 1. Fig. 7 shows the values recommended by each investigator as a function of footing embedment. These bearing capacity factors do not vary by mroe than a maximum of about 18%, and consequently our experience has been that
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the accuracy of each method is about the same. The writers' experience indicates that accurate predictions of footing penetration is more dependent upon the selection of undrained shear strength than the analytical method; therefore, we recommend the use of the Skempton method in our practice.
The use of Skempton's equation was first suggested in 1970 by Ge-meinhardt and Focht (5). Their study gave results comparing observed jack-up rig footing penetrations with those predicted by the available procedures in clay mentioned earlier. They found that the most satisfactory results with the Skempton equation were obtained when: (1) D is defined as the depth to the footing tip; and (2) S„ is averaged over a depth equal to the footing width below the widest cross section.
The writers have compared numerous observed and predicted footing penetrations since this study. Our experience confirms that greater accuracy is achieved by averaging the strength over a zone equal to B/2 instead of B as originally proposed. We also assume D is defined as the depth to the widest embedded cross section.
Since the most important parameter used in the bearing capacity computations is the undrained shear strength, the writers have closely observed the variations in strength data caused by different sampling and testing procedures. Based on this experience, the writers suggest using laboratory strength data obtained with the miniature vane or unconsoli-dated-undrained triaxial compression tests performed on high quality push samples.
Evaluation of bearing capacity in granular soil depends on the internal friction angle of the material (14). We generally estimate friction angles from the gradational characteristics of the recovered material with some adjustment for mineralogy and sampler driving resistance. Even low estimated friction angles with any of the recommended bearing capacity factors will provide bearing capacity resistance much greater than the applied bearing pressure of the footings. Thus, footing penetration is usually limited and predictions are within a 5-ft (1.5-m) accuracy (4). The major problem with granular stratum is not whether it will support the foundation, but whether the soil below the stratum will support the load.
Footings Supported on Layered Soils.—Geological conditions that can cause a footing to experience a punch-through failure, as shown in Fig. 5, are generally recognized as: (1) A sand or rock layer of limited thickness supported by a weaker underlying clay layer; or (2) a very strong clay layer underlaid by a weaker clay. McClelland, et al. (13) have described a variety of goelogic processes that can produce the hazardous subbottom sequence of a hard layer overlying a weaker layer.
A number of analytical procedures are available to evaluate the margin of safety against a punch-through failure for two-layered soil systems. One simplified procedure involves the projected area method as demonstrated in Fig. 8, in which pt = bearing stress under the footing; Ai = bearing area of the footing; p2
= stress transmitted to top of soft layer; A2 = area over which p2 is distributed; p = total load of footing; p = piA^ ~ PiA2; q2 = ultimate bearing capacity of soft layer; and factor of safety = q2/p2 • This method assumes that the footing loads applied to a strong layer are distributed downward through the layer. An equivalent footing, with effective dimensions that are increased at a rate of 3-vertical
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STIFF LAYER ' I 3 \ / J ^^EQUIVALENT FOOTING \
SOFT LAYER L A . J
FIG. 8.—Projected Area Method for Predicting Punch-Through Failure (from Ref. 22)
OBSERVED NET BEARING CAPACITY, KPa
0 200 400 600 BOO 1000
OBSERVED NET BEARING CAPACITY, KSF
FIG. 9.—Comparison of Layered Soil Bearing Capacity Methods to Model Test Results (Modified from Ref. 10)
to 1-horizontal through the strong layer, is placed at the top of the weaker layer. When the pressure on the equivalent footing equals the bearing capacity of the underlying layer, then the computed factor of safety will be unity. Because punch-through safety for conditions shown in Fig. 5 greatly depends on the strong layer thickness, potential variations in stratum thickness must be considered in the factor of safety.
We have mainly used three different methods to compute the bearing capacity of a footing resting on a sand that overlies a clay stratum. The methods are: (1) Projected area (22); (2) Hanna and Meyerhof (6); and (3) Jacobsen, et al. (9). These three methods are compared in Fig. 9 by a number of model test results (10). The closest fit between observed and predicted capacities is obtained with the Hanna and Meyerhof procedure. Their equation for circular footings is
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Q = A 2D\ tan<))
6SUB + 27iH2( 1 + — )Ks—^ + y'V. (2)
in which SUB = undrained shear strength of the lower soil stratum; H = thickness of upper layer in two layer system; D = depth to widest cross-sectional area; Ks = coefficient of punching shear; <f> = internal friction angle of granular stratum; and 7 J = effective unit weight of granular stratum.
Although the writers currently prefer the Hanna and Meyerhof procedure in their practice, we recommend that additional experimental studies be performed to evaluate the effect of the conical point and large footing size not considered in the model studies. Some field observations suggest that the method may be too conservative for sand layers with low interpreted angles of internal friction.
A bearing capacity procedure, other than the projected area method, for a footing resting on a strong clay overlying a weak clay has been outlined by Brown and Meyerhof (1). Their equation is
Q = A H
3SUT + 6SUB D
+ y'V . (3)
in which SUT = undrained shear strength of upper soil stratum. In checking the suitability of a rig for operation at a location with a two-layer, strong-over-weak soil profile, one of the two following criteria must be satisfied, either: (1) The bearing capacity of the strong layer must be low enough to allow the footing to pass completely through that layer; or (2) the bearing capacity of the strong layer must be high enough to stop the footing and to support it with safety. A jack-up rig will usually meet the first criterion if an upper-bound estimate of bearing capacity of the strong layer is less than the footing pressure resulting from the lightship weight and variable weight. The footing should then pass through the strong layer before the hull clears the water with no application of preload. A jack-up rig will meet the second criterion if the bearing capacity of the strong layer significantly exceeds the maximum foundation load, including full preload. We recommend with the latter criterion, that the soil bearing capacity exceed the foundation bearing pressure under full preload by at least 50%, representing a factor of safety against punch-through of at least 1.5.
CASE HISTORIES
In most instances, soil profiles can be divided into two broad categories—those that have a computed bearing capacity that increases with depth and those that have a hard layer over a softer layer and, thus, are a potential punch-through problem. An example of a soil profile in the first category is a uniform clay deposit with a linearly increasing strength profile or a layer of soft clay overlying a massive sand deposit. In this case, the bearing capacity methods yield a close correlation between computed and observed footing performance, indicating the analytical procedure models actual field performance.
The soil profiles where potential punch-through problems exist are more difficult to model with analytical procedures. Three case histories have
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FIG. 10.—Footing (Spud Can) Configurations and Dimensions for Case Histories (1 ft = 0.3048 m)
Fooling Bearing Capacity, Kips
2000 4000 6000
Interpreted Snear Sltenglh, Kips Per Sq Ft
8000 10,000
Ob I served Penelr i Port ] Siafboard
Bow
Very Sod Clay
Fi'm TO
Very Stiff Clay
/
.
Cr
•
si
Ultimate Leg Load, MN 50 100
Interpreted Shear Strength, kpa
FIG. 11.—Case I—Influence of Clay Crust on Bearing Capacity for Footing with Large Apex Angle
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been selected from the writers' files to illustrate this point. The footing configurations and dimensions used in the three cases are presented in Fig. 10.
Case History I.—A mobile jack-up rig with three independent footings was set up at a site in the Java Sea. The rig footings are symmetrical about the vertical axis, as shown in Fig. 10. The largest cross-sectional area of the footing is 1,170 sq ft (109 m2), and the total displaced volume of the footings is about 9,800 cu ft (278 m3).
The soil conditions consist of a near surface, desiccated crust of very stiff clay, as shown in Fig. 11. The 12.5-ft-thick (3.8-m) clay crust has a shear strength of about 2.3 ksf (110 kPa) as revealed by a boring drilled at the location. The clay crust is underlaid by a weaker clay of the same geologic age that reveals a strength profile decreasing with depth.
Using the Brown and Meyerhof method, a curve of footing bearing capacity versus penetration was computed, as shown in Fig. 11. Actual leg load versus penetration information for the three footings is plotted along with the predicted curve. The comparison between the observed and predicted footing penetrations indicates the Brown and Meyerhof method provides a good prediction for this case history. It is important to note the variation between each leg load curve for the three legs. One curve does not reveal the presence of a crust, whereas the other two curves show an increase and then a decrease in soil resistance, indicating the crust is present in the area of these two footings. This case history illustrates the potential variability in soil conditions as reflected in the varying maximum ultimate leg loads required to penetrate the crust.
The footings in this case did not experience a rapid, uncontrolled penetration through the crust probably because of two reasons. First, the strength variation between the two clay layers was not extremely pronounced and the strength of the lower clay is almost equal to 1 ksf (48 kPa), which is about the strength required to support the footings at full preload. Second, the rig mover kept the rig hull at an air gap varying from 0-3 ft (0-1 m); therefore, the hull buoyancy helped reduce the loading with only very minimal footing penetration. This case history illustrates the importance of closely monitoring the preloading operation to avoid potential damage to the rig. The case history also shows that the Brown and Meyerhof procedure give good predictions of footing performance, considering the variability of the clay crust.
Case History II.—This case history involves a mobile jack-up rig supported by tank type spud cans attached to the three independent legs. Each footing is roughly cylindrical in shape with a long conical point extending 18.5 ft (5.6 m) below the widest cross section, as shown in Fig. 10. The largest cross-sectional area of the footing is 960 sq ft (89 m2), and the total displaced volume is about 37,000 cu ft (1,048 m3).
The rig was jacked up at a location in the Gulf of Mexico, offshore Texas. In this particular area, very little soils information existed, so a soil boring was drilled from the rig. This operation was conducted before the full ballast preload was taken onboard.
Ihe soil boring revealed a thin surficial layer of Recent clay underlaid by a desiccated, overconsolidated clay. Below this clay crust, the shear strength dropped off to a fairly constant value of 1 ksf (48 kPa). This soil profile is fairly typical of many areas offshore Texas and western
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Fooling Bearing Capacity, Kips
2000 4000 6000 8000 10,000
Interpreted Shear Strength, Kips Per Sq Ft
- ^ Upper Bound
Lower Bound w
0 I
served Penetr i Port ] Starboard 3 Bow
lions
^
Very Soil Clay
tower
1
A Bound
•
Ultimate Leg Load, MN
FIG. 12.—Case II—Effect of Long Footing Point on Bearing Capacity
Louisiana. The field engineer computed the bearing capacity onboard the rig by the Brown and Meyerhof method using the upper bound interpreted shear strength profile shown in Fig. 12. The predicted footing tip penetration was about 22-24 ft (6.7-7.3 m) under a maximum leg load of 6,900 kips (31 MN).
The rig was preloaded and the observed tip penetrations at full preload were 29-35 ft (8.8-10.6 m). The geotechnical engineer onboard repeated the calculations, but in this case it was assumed that the full shear strength of the crust would not be mobilized. These results are shown as the lower bound curve in Fig. 12 and results are in excellent agreement with the observed penetrations at full preload. This case history illustrates how the long conical point can severely disturb the higher strength crust, thereby reducing its bearing resistance upon the entire footing. The writers believe that this phenomenon is not properly modeled by the Brown and Meyerhof- method, since flat bottom foundations were used in their experimental work.
Case History III.—Within the last couple of years, punch-through type failures have occurred at four different sites around the world, even though a "natural" crust was not revealed by post-failure borings. Similar soil conditions consisting of a constant or slightly increasing shear strength with depth were present at each site. This type of soil profile is not generally associated with a punch-through failure.
The events leading to each rig failure are very similar. The rigs were moved onto location and preloading operations were undertaken. As the rigs took on their preload, the footings continuously penetrated the clay at a fairly constant rate, which would be expected with this type of soil profile. When the preload ballast was within 75-90% of the maximum, the operation was stopped, and in some instances the preload was dumped. The delay time in the preloading lasted from 4-24 hr. After the delay, preloading continued until a punch-through type failure occurred. Interestingly, the ballast taken on after the delay did not cause
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Footing Bearing Capacity, Kips
5000 6000 7000
Interpreted Shear Strength, kst
Ob
• t Leg
Plunge 14-ft
\ erved Penelra
O Starboard Leg
, 1 , 1
Bea
ring C
apaci
ty
Incr
ease
,
Aft
er D
elay
,
Load
Inc
reaa
F
rom
Rig
Ti
l
,
J,
So to Clay
"
\
,
I • 4
I
-
, -
fi
1
1
1
Ultimate Leg Load. MN Inteipreted Shear Strength, kPa
FIG. 13.—Case III—Punch-Through Failure Caused by Preload Delay
the footings to penetrate at a constant rate as would be anticipated. The only real movement of the footings occurred at failure.
One of these three rig failures has been selected for Case History III. The mobile jack-up rig used at the location has three legs and spud cans, as shown in Fig. 10. The rig footing has a maximum cross-sectional area of 1,345 sq ft (125 m2), and a maximum volume of 43,550 cu ft (1,233 m3). Fig. 13 presents the shear strength profile at this location along with the computed leg load versus footing penetration curve. The bearing capacity curve developed for the rig shows a general increase with depth as expected for this type of strength profile. A foundation failure could occur only if the bearing capacity increased after the preload delay or if the gross load on the footing increased suddenly. The writers believe the former is the better explanation. The increase in bearing capacity was probably caused by an increase in soil shear strength with time.
An increase in shear strength may have caused an artificial crust to develop such that the observed leg load versus penetration curve developed a bulge, as shown in Fig. 13. When the leg load exceeds the increased bearing capacity, the footing will plunge at least to a depth where it intersects the original bearing capacity versus penetration curve. The footing penetration in many cases will go beyond this point because the leg load will substantially increase as the rig tilts.
After the delay in preload, the writers believe that the shear strength (bearing capacity) increased because of thixotropy. The cohesive soils that supported the rigs before failure have very low permeabilities, indicating minimal consolidation would occur during the 4-24 hr delays. On the contrary, the soil strength could increase dramatically because of thixotropy, thereby causing an artificial crust.
Fig. 14 shows the thixotropic strength gain for a Gulf of Mexico clay, similar to the soils where this type of failure occurred. The 10-25% increase in shear strength required to cause failure at the sites was achieved in about 1-4 hr with this soil. Therefore, the writers believe the major
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FIG. 14.—Thixotroplc Effect in a Typical Gulf of Mexico Clay
cause of the foundation failure was the interruption of the preloading operation. This example illustrates how the rig mover must carefully monitor the preloading sequence even when a natural crust does not exist at a site. The writers believe additional experimental work is needed to help define how the rate of loading may help reduce the risk of this type of foundation failure.
CONCLUSION
Clear differences exist between foundation design for mobile jack-up rigs and that for permanent offshore structures that require some special considerations. For example, foundations for mobile jack-up rigs are not custom designed for a specific site as are foundations for permanent offshore structures. Also, the foundation of a jack-up rig is "proof tested" at every location to the expected storm conditions, while a permanent structure might not be exposed to assumed design loading during its entire life. For areas like the Gulf of Mexico, permanent structures are exposed to maximum loads during a hurricane, and the standard practice is to remove personnel from permanent structures before the storm strikes. Thus, the risk is only financial. On the other hand, jack-up rigs usually have full crews aboard during preloading operations; thus, there is also a risk of loss of life.
The accident record confirms that foundation failure of mobile jack-up rigs can be attributed to: (1) Punch-through during preload; (2) excessive storm penetration; (3) seafloor instability; (4) footing instability induced by scour; and (5) inability to extract legs. Classical procedures used in designing onshore foundations and described in this paper can be used to predict the hazard of foundation failures described earlier. However, site specific soil data is usually needed from a geotechnical investigation and preferably supplemented with data from a geophysical survey. The case histories illustrate how further refinement of the analytical procedures for layered soil profiles may be needed to predict punch-through failures. Thus, the writers believe that the geotechnical engineer has the opportunity to play a more active role in reducing foundation accidents.
In the past, the standard operating practice has been to move these giant rigs to a new site without specific knowledge of subbottom soil conditions, relying instead on the extendable foundations and preload-
Liquid Limit: 81 Plastic Limit: 31 Water Content: 55 %
"s<
.01 0,05 0.1 0,5 1.0 Time After Remolding, t-j , Days
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ing to adapt to existing conditions. Since neither regulatory agencies nor insurance companies have required more than occasional or token departure from this risk-taking policy, there is a wide range of approaches taken by various owners with regard to geotechnical site investigations. A few rig owners show extreme precaution and require at least one boring at all proposed drilling sites; other owners may never require it. However, the writers have observed a current trend towards more reliance on geotechnical site investigations, and we believe this t rend is warranted and will continue. Closer cooperation between the rig owners and geotechnical engineers should help in finding practical, economical ways to reduce foundation failures of mobile jack-up rigs.
APPENDIX.—REFERENCES
1. Brown, J. D., and Meyerhof, G. G., "Experimental Study of Bearing Capacity in Layered Clays," 7th International Conference on Soil Mechanics and Foundation Engineering, Vol. 2, 1969, pp. 45-51.
2. Ehlers, C. J., Young, A. G., Focht, J. A., Jr., "Advantages of Using In Situ Vane Tests for Marine Soil Investigations," presented at the 1980 International Symposium on Marine Soil Mechanics, Mexico City.
3. Emrich, W. J., "Performance Study of Soil Samples for Deep-Penetration Marine Borings," Sampling of Soil and Rock, STP, 483, American Society for Testing and Materials, 1971, pp. 30-50.
4. Endley, S. N., et a l , "Prediction of Jack-Up Rig Footing Penetration," Proceedings, 13th Offshore Technology Conference, Vol. 4, 1981, pp. 285-296.
5. Gemeinhardt, J. P., and Focht, J. A., Jr., "Theoretical and Observed Performance of Mobile Rig Footings on Clay," Proceeding, 2nd Offshore Technology Conference, Vol. 1, 1970, pp. 549-558.
6. Hanna, A. M., and Meyerhof, G. G., "Design Charts for Ultimate Bearing Capacity of Foundations on Sand Overlying Soft Clay," Canadian Geotechnical Journal, Vol. 17, No. 2, May, 1980, pp. 300-303.
7. Hansen, J. B., "A Revised and Extended Formula for Bearing Capacity," Bulletin No. 28, Danish Geotechnical Institute, Copenhagen, 1970, pp. 5-11.
8. Hirst, T. J., et al., "Performance of Mat-supported Jack-up Drilling Rigs," Proceeding, 8th Offshore Technology Conference, Vol. 1, 1976, pp. 821-830.
9. Jacobsen, M., Christensen, K. V., and Sorensen, C. S., "Gennemlokning af tynde sandlag," Vag-och Vattenbyggaren, Sevenska Vag-och Vattenbyggares Riksforbund, Stockholm, 1977, pp. 23-25.
10. Kraft, L. M., Jr., and Helfrich, S. C , "Bearing Capacity of Shallow Footing, Sand over Clay," Canadian Geotechnical Journal, Vol. 20, No. 1, Feb., 1983, pp. 182-185.
11. LeBlanc, L., "Tracing the Cause of Rig Mishaps," Offshore, Vol. 41, No. 3, Mar., 1981, pp. 51-62.
12. McClelland, B., "Techniques Used in Soil Sampling at Sea," Offshore, Vol. 32, No. 3, Mar., 1972, p. 51.
13. McClelland, B., Young, A. G., and Remmes, B. D., "Avoiding Jack-up Rig Foundation Failures," presented at the 1981 Symposium on Geotechnical Aspects of Offshore and Nearshore Structures, held at Bangkok, Thailand.
14. Meyerhof, G. G., "The Ultimate Bearing Capacity of Foundations," Geotech-nique, Vol. 2, 1951, pp. 301-332.
15. Meyerhof, G. G., "Ultimate Bearing Capacity of Footings on Sand Overlying Clay," Canadian Geotechnical Journal, Vol. 11, No. 2, 1971, pp. 223-229.
16. Myslivec, A., "Die Grenztragfahigkeit eines Zweischichtigen Baugrundes," Proceedings, 4th Budapest Conference on Soil Mechanics, 1971, pp. 677-685.
17. National Research Council, Marine Board Committee on Assessment of Safety of OCS Activities 1981, Safety and Offshore Oil, National Academy Press, Washington, D.C., 1981.
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18. Ploessel, M. E., Campbell, K. J., and Randall, R. G., "High-Resolution Geophysical Surveys for Siting Offshore Structures," Soundings, McClelland Engineers, Inc., Houston, Tex., Vol. 2, No. 3, Fall, 1980, pp. 9-13.
19. Skempton, A. W., "Bearing Capacity of Clays," Division 1, Building Research Congress, London, 1951, pp. 180-189.
20. Whitley, J. O., Jr., "Some Aspects of the Structural Design of a Three Column, Mat Support, Self-Elevating Mobile Drill Platform," presented at the 1970 Annual ASCE Texas Section Meeting, held at Galveston, Tex.
21. Young, A. G., Noggle, M. O., "Sediment Strength Distribution for Planning Mobile Rig Operations-Louisiana Continental Shelf," Proceedings, 10th Offshore Technology Conference, Vol. 4, 1978, pp. 2101-2110.
22. Young, A. G., and Focht, J. A., Jr., "Subsurface Hazards Affect Mobile Jack-Up Rig Operations," Soundings, McClelland Engineers, Inc., Houston, Tex., Vol. 3, No. 2, Summer, 1981, pp. 4-9.
23. Young, A. G., et al., "Foundation Performance of Mat-Supported Jack-up Rigs in Soft Clays," Proceedings, 13th Offshore Technology Conference, Vol. 4, 1981, pp. 273-284.
24. Young, A. G., Quiros, G. W., and Ehlers, C. J., "Effects of Offshore Drilling, Sampling, and Testing on Undrained Soil Shear Strength," presented at the 1981 Annual ASCE Texas Section Meeting, held at Houston, Tex.
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