1) The questionnaire, mentioned in the S.I.P.M. notes on the project, had not in fact been sent to operating companies, as S.I.P.M. and Fugro agreed that mare useful information would be obtained from visits to the operating companies.
2 B.S.P. would like to receive the manuals in their Final Draft form, to permit scrutiny and comment prior to publication by S.I.P.M. and Fugro. A time limit would be imposed for submission of comments. (say 1 month)
Mr. Toolan would raise this point with S.I.P.M., and B.S.P. should do so independently.
(Action: EDA/22)
3) Updating of Manuals. B.S.P. would like to see the Manuals updated at specific intervals to incorporate new experience by operating companies, new developments in equipment and techniques, and changes in design and installation criteria.
Mr. Toolan agreed that the layout of the Manuals would be designed for ease of revision.
B.S.P. would propose to S.I.P.M. that a provision for regular updating be incorporated into the Fugro/ S.I.P.M. contract.
(Action: EDA/22)
B. PURPOSE
4) Mr. Toolan stated that in his brief these Manuals were intended primarily to cover installation criteria for conductors and piles. They were for presentation of recommended procedures.
5) B.S.P. stressed that in their view an important function of the Conductor Manual would be to itemise and specify the design considerations for the structural, operational, and drilling departments.
6. An additional purpose was to identify areas, in which data gathering would be required.
C. STYLE
7. The style proposed for the Manuals was generally agreed, with the proviso that all formulas used should be derived or referenced, with constants and parameters specified.
In each section, basic principles would be highlighted, with references where subjects were not covered in detail.
D. SCOPE
8. There was a request from B.S.P. for greater emphasis, in the pile Manual, on design methods and parameters.
9. Mr. Toolan stated that to expand the manuals to include all aspects of design would make them very large and somewhat clumsy.
NOTE: In subsequent discussions, certain topics were pinpointed for coverage of design factors in greater depth in the manuals.
10. A section will be included on Freestanding and guyed conductors, mudline suspension wells, and tie-ins to jackets.
11. The main points of interest from Operational Viewpoints were emphasised to be formation strength, conductor setting depths, and casing cementing setting levels.
Mr. Toolan was able immediately to provide information on calculation methods to determine the correct conductor setting depth for avoidance of formation fracture.
More detailed discussions on Conductor criteria are covered in subsequent notes.
E. FORMAT
12. As stated in the notes on the manuals, the main part of the Manuals will be sections covering basic principles in general terms, with detailed specific information contained in appendices.
13. Where sections will cover installation procedures, examples will be quoted wherever possible of operating Companies who use these procedures. Examples of Good Practice and Bad Practice will be included, as well as alternative methods.
SESSION 3
This session concentrated on the Drilling/operational criteria for conductor Specifications.
Drilling Department have found from experiences over the past few years that the old conductor installation criteria dating from about 1972 have not been completely satisfactory and have suggested that those procedures should be revised.
The following points summarise the main areas of concern among Drilling, Operational and Engineering Departments of B.S.P., for inclusion into the Conductor Installation Manual being compiled by Fugro.
1. A general point was made that criteria governing free- standing conductors should be included - with references.
SECTION 3 - Conductor Setting Depths
2. The manual should describe how to calculate conductor setting depths using normal soil data, so as to satisfy the following requirements.
a) Enable Drilling Department to obtain mud returns to surface when drilling surface casing (normally diameter 20")
b) Have sufficient axial conductor capacity.
c) Ensure that pile capacities of structure are not adversely affected.
d) Locate the conductor shoes in clay rather than sand, as far as possible within the above constraints.
SECTION 4 - Spudding-in Procedures
3. The manual should advise that prior to drilling a pilot hole in spudding procedure in the conductor shoe be drilled through to a depth approx. 10' below the conductor shoe, using the largest bit to be used for drilling the next casing.
The reasons for this
are:-a) To check the shoe is not damaged.
b) To centralise the subsequent holes.
c) To confirm that the axial external skin friction capacity of the conductor exceeds it's own weight.
4. There was a strong request that the criteria for cementing the first casing to surface or to mudline be closely examined. This should be covered in depth by the manual.
SECTION 7 - Selection of Installation Method
5. This section or an appendix should include the results of a wave equation analysis for the conductor sizes and hammers commonly used by B.S.P. For this Mr. Toolan has been provided with information on all hammers used in B.S.P. offshore areas, also information on common conductor sizes and material strengths.
SECTION 9 - Conductor Shoes
6. It was recommended that internal conductor shoes should be used generally 1.5 x conductor wall-thickness.
SECTION 10 - Curved Conductors
7. The section of the manual dealing with curved conductors should be expanded to discuss mitred and slanted conductors and any other techniques used in the Group to maximise separation of conductor shoes.
8. A note should be included that it has been B.S.P.'s experience that diameter φ20" casing cannot be run through a diameter φ 26"x 3/4 conductor at a dogleg severity or more than 5°
per 100'.
Furthermore, diameter φ18 5/8" casing has become an important size in B.S.P. and has been run inside a diameter φ26" curved conductor thro' a dogleg severity of 7º per 100'.
Data on the directional survey for Magpie D/P, and installation planning data was provided for Mr. Toolan by OPE/15.
INTERNAL DISCUSSION
Arising out of this discussion, it was agreed that the following installation procedures should be recommended for trial by B.S.P. in the near future.
a) ECO will attempt to drive marine conductors on the drilling platform to or below the setting depths calculated in point (2) above.
Notes have been left with EPE/4 on this procedure, and relevant parameters are being received by Telex from Fugro, U.K.
b) If required by Drilling Department, one conductor will be stabbed by the installation barge, but not driven. This will usually be a conductor furthest away from adjacent piles. Normally, this decision should of course be made before placing of the jacket.
c) Drilling Department can then install this conductor using the most appropriate techniques, and perform formation leak-off test at any depths desired.
The conductor will finally be set to a depth 50' - 150' below the pile tips.
It is intended that the first well from the platform be drilled through this conductor, setting the diameter φ20" casing following current normal practice.
d) Should difficulties be encountered in this first well, either from loss of circulation or from shallow gas pockets, it is agreed that for the remaining conductors on the jacket the
φ20" casing would be set at a shallow depth of typically 75' - 150' below pile tips.
For this situation, it is recommended that, as in the past, sea-water would be used for drilling, with the casing being set and cemented as in normal practice.
APPENDIX II-1
SOIL PARAMETERS REQUIRED FOR CONDUCTOR DESIGN AND INSTALLATION PLANNING
SOIL PARAMETERS REQUIRED FOR CONDUCTOR DESIGN AND INSTALLATION PLANNING
The soil parameters required for the design and installation planning of conductors are given in the Table below. Also included in this Table are the types of laboratory tests which are used to obtain these parameters.
In addition to the soil parameters, it is also necessary to know the specific gravity of any drilling fluids to be used and the discharge height of drilling returns above sea level.
SOIL PARAMETER TEST
_ Visual description
Bulk density density test
Submerged density
Moisture content Moisture content test
Undraine Shear Strength Unconsolidated undrained triaxial test; pocket penetrometer;
torvane; fall cone; consolidated undrained triaxial test (if insitu stresses can be estimated)
Remoulded Shear Strength plasticity index; moisture content; remoulded triaxial test Strain required to mobilise 50% of
maximum soil shear strength
APPENDIX II-2
CALCULATION OF THE COEFFICIENT OF EARTH PRESSURE AT REST (Ko )
CALCULATION OF THE COEFFICIENT OF EARTH PRESSURE AT REST (Ko )
The method of calculation of Ko is dependent upon the soil type under consideration. Basically the methods differentiate between cohesionless and cohesive soils.
COHESIONLESS SOILS
For a cohesionless soil which has not experienced an overburden pressure greater than the existing value, i.e. the precompression ratio (PCR) is unity, the value of Ko is obtained from the
expression:-Ko = 1 - sin φ'
where φ is the angle of internal friction of the soil.
If the PCR of the soil is greater than unity, Schmertmann (Ref. 1) has developed the formula:-Ko = (PCR) 0.42 x (1 - sin φ')
COHESIVE SOILS
For a normally consolidated cohesive soil the theoretical value of Ko is the same as for a cohesionless
material:-Ko = 1 - sin φ'
Due to the difficulty in assessing whether a clay is normally or lightly overconsolidated it is preferable to carry out laboratory tests to obtain a Ko value. These tests are relatively inexpensive and can be performed on standard soils laboratory equipment. In addition to laboratory testing, various relationships have been developed linking Ko to the index properties of the material.
For a heavily overconsolidated clay, the following procedure is used to assess the Ko value of the
soil:-1) Calculate the ratio of (c/p') on the basis of the results of the laboratory testing where c = the undrained shear strength
p' = the effective overburden pressure
2) Calculate the ratio of (c/p') nc applicable for a normally consolidated clay, based on the relationship proposed by Skempton (Ref. 2)
(c/p') nc = 0.11 + 0.0037 (PI)
where PI is the plasticity index of the soil
3) Determine the ratio of (c/p')/ (c/p') ncand use this value to determine the overconsolidation ratio (OCR) of the clay, based on the relationship proposed by Ladd and Foott (Ref. 3).
4) Estimate the value of Ko from published relationships between Ko, the OCR and the PI of the soil, Brooker and Ireland (Ref. 4), Vijayvergiya (Ref. 5).
References
1) SCHMERTMANN, J., "The measurement of in-situ shear strength". Proceedings ASCE specialty conference on in- situ measurements of soil properties Vol. 2 1975.
2) SKEMPTON, A.W., Discussion on "The planning and design of New Hong Kong Airport".
Proc. Institution of Civil Engineers 7. 1957.
3) LADD C. and FOOTT, R. (1 974), "New design procedure for stability of soft clays", A.S.C.E., J-GED, July, p 769.
4) BROOKER, E.W., and IRELAND, H.O., (1965) "Earth Pressures at rest related to stress history" Canadian Geotechnical Journal Vol. II No. 1 (Feb.).
5) VIJAYVERGIYA, V.N. "Procedure for computing axial pile capacity" Fugro Internal Paper.
July 1977.
APPENDIX III-1
THE USE OF THE WAVE EQUATION IN DRIVABILITY ANALYSIS
THE USE OF THE WAVE EQUATION IN DRIVABILITY ANALYSIS
General
For most conductor installations, use is made of driving methods for at least part of the installation.
With computer programs based on the wave equation, the ability of a particular hammer to drive a conductor to its required penetration may be assessed. Such programs can also determine whether or not the conductor is overstressed during driving.
To use the wave equation correctly it is necessary to have a basic understanding of the phenomena involved in pile driving. The impact of the hammer generates a compressive stress wave in the conductor. This stress wave travels down the conductor at the velocity of sound in steel, 5000 m/s. If the wave meets a resistance or discontinuity a proportion is reflected back up the conductor, reducing the magnitude of the downward travelling wave by an equal amount. Once the wave has reached the portion of the conductor shaft within the soil, the downward movement of the conductor behind the wave front generates a frictional resistance at the soil/conductor interface. This causes a reflection which reduces the intensity of the wave. Eventually the wave reaches the conductor tip. If there is sufficient power left in the wave to cause permanent deformation in the soil below the conductor tip, or if only elastic movements occur in the soil below the tip, the conductor will be in the same position after the blow as before it. In this situation the conductor is said to have refused.
Most conductor driving analyses use computer programs based on one dimensional wave transmission. Those available within the Shell Group employ finite difference techniques for the numerical analyses. The hammer is modelled as a falling weight striking a cushion and/or anvil, see Fig. 1. The conductor is modelled as a series of lumped masses of the conductor. The soil springs are defined by; an ultimate static resistance (Ru); a displacement over which the soil behaves elastically, the quake (Q); and a damping factor which increases the static soil resistance as a function of the conductor velocity (J). The soil springs are shown on Fig.2.
The input data for hammer and conductor are simple to prepare and field measurements indicate that the mathematical model is sufficiently realistic. Standard values have been established for soil quake and damping. These values have been backfigured from instrumented driving tests in conjunction with soil resistances obtained from static load tests. The only input data which has to be specially prepared by the user is the Soil Resistance at time of Driving, SRD.
Experience gained from back analysing driving records indicates that the SRD is not equal to the ultimate static capacity. For conductors in soft clays the capacity tends to increase with time and so the SRD is generally much less than the static capacity. In hard clays the increase in capacity with time is less, marked and the SRD may be of the same order of magnitude as, or greater than, the calculated static capacity. Generally in all types of clay the majority of the SRD is developed in the skin friction whereas in sand a large proportion may be developed at the tip. In sands the calculated SRD is greater than the calculated static capacity, see Refs. 1 and 2 for Shell Group experience.
There are a number of published methods for calculating the relationship between SRD and depth (Refs. 3 to 7). Since in each case SRD's, quakes and damping values have been developed in tandem, it is not necessarily valid to use the SRD's calculated from one reference with quakes and damping factors quoted in another.
The methods given in this Appendix will provide SRD values suitable for use with the wave equation programs available within Shell . An example calculation is presented in Appendix III-2 and the results in the form of a plot of SRD versus depth are given in that Appendix. These SRD values should be used in conjunction with quake and damping factors quoted on Fig. 2. Comparisons between predictions and field experience indicate that the method will predict harder than average driving, i.e. it provides reasonable correlations for the conductors which were hardest to drive (Ref. 5).
This is deliberate. if it predicted average driving behaviour there would be a danger of a considerable proportion of conductors in the field refusing at shallower penetrations than anticipated. This could have costly consequences. The other methods listed in the references have been found to predict average driving. In addition to calculating the SRD during continuous driving, it may be necessary in clay soils to compute the SRD after set-up. This is simply done by substituting the static skin friction
by API method 1 or 2 (whichever is higher, see Appendix IV-1) for the calculated shaft friction during driving. The rest of the computation is as before. In some hard clay soils the SRD computed after set-up may be less than that calculated for continuous driving. This indicates that set-set-up effects may be ignored.
Calculation of SRD versus depth relationship
The SRD at any depth is determined from the following considerations. If the soil inside the conductor (soil plug) remains stationary during driving the SRD must be made up of inside and outside friction and wall end bearing. In this situation the magnitude of the inside friction which can be mobilised may be limited by the end bearing capacity of the soil plug. Alternatively the soil plug may move down during driving in which case the inside friction must be equal to or greater than the plug end bearing.
Generally the conductor will behave in the manner which produces least resistance to penetration.
Thus at any depth the SRD will be the least
of:-where : fs' = unit shaft friction during driving (outside) fi' = unit shaft friction during driving (inside) qp' = unit point resistance during driving These concepts for predicting SRD are shown on Figs.3 and 4.
Calculation of point resistance during driving
The Cone Penetration Test (CPT) is a model test for the penetration of a conductor. The point resistance during driving (qp') may be calculated from the cone resistance (qc). When the conductor is plugged (i.e. the inner soil plug moves down with the conductor) the unit base resistance may be calculated by the method shown on Fig. 5. No limits are applied. Thus,
qp' = qu ……….. ... (3)
When the conductor is unplugged (i.e. the inner soil plug remains stationary as the conductor tip penetrates the ground) the unit resistance acting on the conductor wall annulus is taken to be equal to the cone resistance at that depth,
qp' = qc ………… ... (4)
In clays in which CPT's have not been made a cone resistance for use in drivability studies can be estimated as 18 times the undrained shear strength. In sand strata it is difficult to assess drivability without the results of CPT's.
Calculation of shaft friction in cohesive soils
The contribution of skin friction to total SRD is calculated on the basis of laboratory test results. Along the length of the conductor, the soil at the interface with the conductor wall is strained to failure by every hammer blow. In a cohesive material the soil is. compressed to accommodate the volume of the conductor as it penetrates. The displacement, shearing and compression remould the soil and cause excess pore water pressures to be developed. Thus during continuous driving in a clay:
In many cases it may prove impossible to perform sufficient meaningful remoulded triaxial tests in the time available. This is particularly the case with heavily overconsolidated clays which may have to be ground down, reconstituted and reconsolidated prior to shearing.
In order to overcome this problem, considerable reliance is placed on empirical relationships between remoulded shear strength and other properties of a soil such as those developed by Skemption and Northey (Ref. 8) and Houston and Mitchell (Ref. 9). The remoulded shear strength according to Skempton and Northey depends on a relationship between ιr and plasticity and liquidity indices derived from laboratory measurements. This relationship is shown on Fig.6.
Calculation of shaft friction during driving in granular soils
In granular oil it is assumed that the unit shaft friction during driving is equal to the static unit shaft friction
:-and may be calculated using the cone resistance as described previously. A limit of 0.12 MN/m² should be applied but no allowance should be made for lateral displacement effects. Equation (6) should be applied at all levels.
Results of Wave Equation Analysis
The input to the wave equation program may be divided into three parts, hammer data
soil data pile data
The output includes the permanent set per blow (the reciprocal of the blowcount) and the stresses in the conductor during driving. Normal practice is to vary the SRD, while keeping all other input data constant. In this manner for a given hammer, conductor penetration and soil profile a relationship
The output includes the permanent set per blow (the reciprocal of the blowcount) and the stresses in the conductor during driving. Normal practice is to vary the SRD, while keeping all other input data constant. In this manner for a given hammer, conductor penetration and soil profile a relationship