ABSTRACT Sampling in deep water (>500 m) is challenging. Gravity core type samplers are frequently used due to operational aspects giving more disturbance than thin wall steel tube piston sampling. Additional disturbance due to large total stress relief can occur when the samples are brought to ship deck. These disturbances may be very large, especially when dissolved gas is present in the pore fluid. During stress relief this gas may come out of solution and expand. This paper presents the first phase of a joint industry project. The objectives of the project have been to examine the problem of sample disturbance in deep water sampling practice. The problems with dissolved gases are not addressed in this first phase. NGI has carried out a comprehensive laboratory test programme on two clays. The low plastic Lierstranda clay (I p = 10–17%) and the medium to high plastic Bothkennar clay (I p = 45%). Benchmark reference soil parameters have been obtained from CAUC triaxial and CRS oedometer tests on high quality block samples. The same laboratory tests have been repeated on samples obtained with piston tube sampling and samples subjected to extra disturbance to simulate thick wall gravity core sampling. A series of special tests have also been carried out where sampling in 1500 m water depth have been simulated in the laboratory. Based on these test series it has been possible to quantify the amount of disturbance caused by thick wall tube sampling, and the also by large stress relief which occurs when sampling in 1500 m of water. The results have been used to develop a new criteria for sample disturbance and will also be used to establish procedures for correcting results of CAUC and CRSC tests to arrive at soil parameters that are as close as possible to in situ conditions. INTRODUCTION Problem definition For deep water soil investigations it is highly probable that many of the obtained samples will be more disturbed than those obtained m shallow water or onshore. The main reasons are: the material may be more sensitive to disturbance due to various geological factors, such as the presence of very brittle ooze material control of the sampling process will be reduced due to large water depth the samples may be obtained using simple sampling equipment from general survey vessels stress relief dung recovery of the sample cause expansion and disturbance; sampling from 1500 m of water may give as much as 0.7% volumetric expansion even If no gas is present. With gas dissolved in the pore water large expansions can occur possible gas hydrates can melt, expand and cause disturbance to the sod structure. These potential disturbances may have a serious influence on the quality of the soil parameters obtained from laboratory tests on these samples. Statoil took the initiative to carry out a study into these effects Subsequently this study led to a larger Joint Industry Project supported by a number of oil companies. The objectives of the study have been to:

ABSTRACT The local shaft capacity of displacement piles is governed by effective stress friction laws. Laboratory tests carried out on clay soils in the ring-shear apparatus described by Bishop et al. (1971) predict reasonably accurately the soil-steel Interface friction angles (d) measured directly m the field with specially instrumented piles. The laboratory test values of d depend on: the soil grading and mineral composition; interface material and roughness; and in some cases previous shearing history. Recognising these factors, Jardine and Chow (1996) recommended in their pile design procedure that in clay sods site-specific soil-steel interface shear tests should be performed that model the above factors. This paper explores (i) the possible sensitivity of dto apparatus type and test procedure, and (ii) differences that can arise between test data and the "default" guidance curves given by Jardine and Chow. Results are presented of a series of comparative soil-steel interface ring shear tests, involving the relatively simple "Bromhead equipment and the more complicated "Bishop" apparatus, where clay soils of varying plasticity were sheared against a standard steel pile interface with the roughness of a typical pile. It is shown that d is not unduly sensitive to the choice of apparatus. Recommendations are made on how the commercially available "Bromhead" apparatus should be used to ad pile design in clays. In addition, a summary of recently performed interface ring-shear test results is used to comment on the "default curves" offered by Jardine and Chow for clays. Site-specific tests are strongly recommended for all new site investigations; they are simple and inexpensive to perform, and provide the most secure means of ensuring a reliable balance between safety and economy in pile design INTRODUCTION Background An extended programme 1 of research carried out at Impend College (IC) has led to a new approach for designing displacement piles m sands and clays The provisional proposals of Bond et al. (1992), Lehane and Jardine (1994), and Lehane et al. (1994) have been developed into the fully fledged design procedures described by Jardine and Chow (1996), which are now being used extensively for the design of offshore driven piles. A key finding of the research was that the simple Coulomb failure criterion applies to the skin friction developed by displacement piles, with the local vertical shear stress, ¿being limited by: (Formula available in full paper) Full scale piles fad progressively from the top downwards. In brittle clays, the local operational values of 6f applying when the pile reaches its peak total capacity will fall between maximum and minimum values of d f , denoted as d peak and d ult depending on the depth of the point of interest and the length and compressibility of the pile. With short rigid piles, such as micro-piles, d peak often dominates, while most points on a long compressible offshore pile will have reached then ultimate conditions before the peak overall capacity is developed. Field research with specially instrumented piles showed that the rate of pile installation could have a strong effect on the magnitude of d peak Bond and Jardine (1991) noted that

ABSTRACT Calcareous sand present a particular challenge for offshore foundation engineers. The paper presents a summary of the properties of calcareous sands pertinent to pile design and reviews modifications that have been proposed to make the standard design methodologies more suitable for application in calcareous sands. Analyses of 33 pile load tests performed in calcareous sands are presented, the measured capacities from each of which is compared with the predicted values computed using the API RP 2A (1993) recommendations with the modifications proposed by Datta et al. (1980) and the Jardine and Chow (1996) method. The results of the analyses clearly demonstrate that the application of the Imperial College pile design method to calcareous sands is dangerous, and that predictions based on the moditications proposed by Datta et al (1980) are also, on average, non-conservative, However, simple modifications are proposed which yield predications for shaft resistance of driven displacement tubulars In calcareous sands which are consistently safe , and statistically more reliable than compared methods. INTRODUCTION Recent research into the behabiour of piles driven in silica sands has identified important shortcomings in the physical models that underpin the widely adopted API RP 2A offshore pile design procedures (Lehane, 1992). Jardine and Chow(1996) have proposed new design methods which capture the key features of behavior more realistically, and so allow axial capacity to be predicted with greater accuracy. However, Jardine and Chow specifically state that their procedures are unsuitable for use in calcareous sand. This limitation is inconvenient because calcareous sand present perhaps the most taxing challenge to offshore pile designers. It it well known that methods for estimating the axial capacities of steel tubulars in silica sand can be severely non-conservative in calcareous formations The current API RP 2A document (API 1993) notes that : "..Available data suggests that driven piles in these soils may have substantially lower design strength parameters than those recommended" cementation may increase end bearing capacity, but results in a loss of lateral pressure and a corresponding decrease in frictional capacity?. While Khorshid et al (1998) reported that at the North Rankin site (offshore Western Australia): " Following the initial pile penetration, approximately 47 metres, this pile free fell to 114 metres below mudline as a result of the first hammer blow and no further piles of this type were installed." Several modifications have been proposed to make the standard design methodologies more suitable for design in calcareous sands, but none has yet received sufficiently broad approval to be incorporated sufficiently broad approval to be incorporated into the forthcoming API/ISO recommendations, and so be adopted for industry-wide use. This paper considers how the Jardine and Chow (1996) procedures might be modified for application in calcareous sands. Some introductory comments are made first on the properties of calcareous sand, and a brief review given of existing proposals for design of piles within them. A database of field tests and site investigations is then described which is used to test both an existing ?calcareous sand? design approach

SUMMARY This paper reviews the potential for conductor and shallow well drilling operations to affect the safety of offshore structure foundations. It is written from a geotechnical perspective and draws both on the Authors? experience and on a confidential study carried out for the UK Health and Safety Executive (HSE) which included a literature survey and discussions with operators? drilling and geotechnical engineers, drilling contractors, structural designers and geotechnical consultants. Major changes have taken place which could potentially increase any detrimental effects of conductor and shallow well drilling on offshore structure foundations, in particular, on driven piles. Firstly, conductors and wells are often installed from mobile units through drilling templates, prior to jacket structures to become narrower and to have vertical piles, with a consequent reduction in pile to conductor spacing. Conductors may be installed by drilling, by driving, or a combination of both. Where drilling is undertaken, problems can occur due to hydraulic fracture or to wash-out. Hydraulic fracture occurs where drilling fluid pressure is too high and fluid is lost into the formation, possibly softening the surrounding soil. Wash-out generally occurs in granular soils and may, in part, be induced by high circulation rates of the drilling fluid, Wash-out leads to increased stress relief in the soils. Where such problem occur, the drilling solution is often to use ?lost circulation material? to stem fluid losses, or fill voids with a cement grout. Damage to surrounding soils by softening or stress relief is not drilling consideration. Since lost circulation is not unusual, drilling engineers tend to alert geotechnical engineers only where a problem is severe. Therefore, foundation soils may be weakened without those responsible for the design or verification of the foundations being made aware of this Most operations do not have dedicated geotechnical engineers and so there may be no in-house expertise to evaluate potential conductor installation problems. It is clear from the Authors? research that many drilling engineers are not aware that their activities may affect foundation stability, although some operators do have close, albeit informal liaison between drilling and geotechnical engineers. While structure designers or geotechnical engineers are likely to consider pile penetration requirements when recommending conductor setting depths, the final decision regarding depth and method of installation is likely to be established by drilling engineers in isolation. It is rare for geotechnical engineers to monitor conductor installation or shallow well drilling. Furthermore, there are many differences in the way operators establish and undertake these operations. Case histories of problems encountered during, or as a result of, conductor installation or shallow drilling activities include: Deviating and clashing conductors; Weakening of foundation soils due to significant loss of circulation of drilling fluid; Drilling fluid coming up around a jacket mudmat; Interconnection between adjacent drilled holes; Drilling-induced seabed craters around conductors; Blowouts; and Loss of contact beneath a gravity base structure The major finding is that problems of occur, during these operations, which could have a significant impact on the safety of foundations.