10.2.1 Foundation Requirements
The primary function of the foundation system is to anchor the tendons.
Load transfer to the soil can be accomplished in a number of ways. For example, through tendons directly attached to piles or suction anchors, through templates, which distribute tendon forces to the soil via piles, or through a gravity base.
The use of a template structure requires consideration of several factors including: template configuration, structural strength, installation feasibility, required positional and alignment tolerances, connections with the tendons, risers, and if applicable, connections between the template and piles.
The design of the foundation structure should ensure that permissible limits of stress, displacement, and fatigue are not exceeded during and after installation. Particular attention should be given to loading eccentricity arising from tendon/riser force variations within a group, tendon/riser installation sequences, and possible tendon/riser retrieval and redeployment during the platform’s operational life. The permissible soil stress and displacement should be established considering variations in soil properties resulting from cyclic tensile and lateral loadings, and in the case of pile supported foundations, potential creep due to sustained axial tension loadings. Consideration should also be given to the loss of foundation capacity due to scour or other soil instabilities (e.g. mudslides and liquefaction).
The foundation system above the mud line should include provisions for inspection and maintenance. The extent of inspection, timing of the inspection, and maintenance should be commensurate with the redundancy relative to overall safety and performance.
10.2.2 Site Characterization 10.2.2.1 General
Primarily, the type and function of the platform to be installed, the availability and quality of data from prior site surveys, and the consequences that would result from a partial or complete foundation failure should guide requirements for site investigations. Special problems include deepwater sites and unusual loading conditions. It is recommended that a high-quality, high-resolution geophysical survey, combined with a
realistic geological interpretation of that survey be combined with the geotechnical data in order to assess restraints imposed on the design by geological features, to serve as a guide to develop a prognosis for the vertical and horizontal extent of geotechnical investigation and to aid in the interpretation of the geotechnical data obtained during a site investigation. Some examples of these integrated geoscience studies are given by Doyle (1998)[126] and Jeanjean, et al. (1998)[161].
The measured properties of soil samples retrieved from deep waters may be different from in-situ values.
Without special precautions, the relief of hydrostatic pore pressure and its resulting effect on any dissolved gases can yield soil properties significantly different from in-situ conditions. Because of these effects, in-situ or special laboratory testing to determine soil properties is warranted. Since installation sites may be remote from areas for which extensive site data are available, regional and local site studies to adequately establish soil characteristics may be required. Previous site investigations and experience may permit a less extensive site investigation. Some of the geotechnical tools available when rotary drilling techniques are employed for deepwater investigations are discussed by Dutt, et al. (1997)[132]. Coring with “jumbo” or “long” coring devices has also been shown in recent studies (Young, et al. 2000 [251]; Borel, et al. (2002) [101]) to provide shear strengths equivalent to those obtained by rotary drilling methods and holds promise as an alternative coring method.
The upward static and dynamic loadings are different from those typically experienced by a jacket-type structure. Piled TLP foundations are subjected to constant and cyclic tensile load components that can result in tensile creep of the foundation and excessive deformations associated with cyclic loading. Tests to ascertain the soil-pile response when subjected to these loadings should be performed.
A site investigation program should be accomplished for each platform location. The program should, as a minimum and preferably in the order listed.
10.2.2.2 Background Geophysical Survey
Regional geological data should first be obtained to provide information of a regional character which may affect the analysis, design and siting of the foundation. Such data should be used in planning the subsurface investigation, and to ensure that the findings of the subsurface investigation are consistent with known geological conditions. Site-specific background data should include a re-examination of the 3D, multichannel data obtained for exploratory purposes and a review of the “geohazard” study used to site the exploratory wells. The 3D data set should be re-processed to enhance its high-frequency content. Suggested reading for further information is given in Doyle and Kaluza (2001) [128].
10.2.2.3 Seafloor and Sub-bottom Survey
A site-specific, high-resolution geophysical information should be obtained relating to the conditions existing at and near the surface of the seafloor. The survey should include the mapping and description of all seafloor and sub-bottom features, some of which may be as follows:
a) contours of the seafloor and shallow stratigraphy;
b) position of bottom shapes, which might affect scour;
c) the presence of sea floor bottom objects such as, slumps, boulders, obstructions, and small craters;
d) gas seeps;
e) shallow faults;
f) slump blocks;
g) the effect of drill cuttings on the foundation installation;
h) previous usage of seafloor;
i) gas hydrates.
The geophysical study should be evaluated within the context of a geological model to determine the existence of any geological feature(s) that might have an effect on the design (i.e. a geological risk assessment shall be performed to assess the affect of geological features on the TLP performance).
The survey should use geophysical equipment and practices appropriate to the water depth of interest and provide high-resolution imaging of the seafloor as well as detailed stratigraphic information to a reasonable penetration below the zone of influence of the structure. In addition, the survey scope should encompass both the lateral and vertical extent of possible geological features that may be a constraint to the design of the foundation. The bedding resolution of the survey should be such that it can be used to interpret the geotechnical data.
10.2.2.4 Geotechnical Investigation
The subsurface investigation should obtain geotechnical data concerning the stratigraphy and the lateral variability of the soil. The sampling and in-situ testing intervals should ensure that a reasonably continuous profile is obtained within each significant stratigraphic layer. The design soil parameters in various soil strata should be determined from a field program that tests the soil in as nearly an undisturbed state as feasible.
Because the quality of soil samples can be expected to decrease with increasing water depth, the use of in- situ testing techniques are recommended for deepwater sites. In addition, soil samples will be required to characterize the soil types and provide other basic engineering property data.
The number and scope of the borings will depend on the quality and interpretation of the high-resolution geophysical study. Based on the geophysical survey, significant lateral stratigraphic variability may require that more than one boring be obtained. For pile foundations, the minimum penetration of at least one boring shall exceed the anticipated design penetration. For piled or non-piled gravity foundations, the minimum penetration of each boring should be related to the expected zone of influence of the loads imposed by the base. Appropriate in-situ tests should be carried out, where possible, to a penetration that will include the soil layers influenced by the foundation components. Additional shallow sampling and testing may be necessary to allow accurate predictions of near-surface soil-foundation interaction, and to assess the variation of soil stratigraphy across the site. Recovered samples, which are to be sent to an onshore laboratory, should be carefully packaged to minimize disturbance, changes in moisture content, and temperature variations.
Samples should be labeled and the results of the initial inspection of the samples recorded, including soil fabric, color and sample disturbance.
10.2.2.5 Soil Testing Program
The soil-testing program should consist of in-situ and laboratory tests to establish classification properties for all significant strata and initial estimates of the soil’s strength and deformation properties. When applicable, testing should be performed in accordance with ASTM or other applicable standards. Additional testing should be performed to define the creep and cyclic behavior of the soil to allow prediction of soil structure interaction due to sustained and cyclic loading. Some examples of the scope of deepwater investigations and the interpretation of data are given in Doyle (1998)[126]; Jeanjean, et al. (1998)[161]; Andersen and Lauritzen (1988)[90]; Andersen, et al. (1988)[89]; Andersen (1991)[91]; Dutt, et al. (1992)[130]; and Pelletier, et al. (1997)
[207]. Consideration should be given to the performance of permeability and consolidation tests in order to understand setup effects for piled structures and capacity consideration for suction caissons.
Additional Studies—as applicable, additional analytical studies or scaled tests should be performed to assess the following effects:
a) scouring potential;
b) hydraulic instability and occurrence of sand waves;
c) earthquake ground response studies or analysis;
d) seafloor instabilities in the area where the foundation system is to be placed;
e) setup effects.