ABSTRACT This paper reviews some key issues regarding the cyclic loading response of offshore piled foundations. Starting with axial loading it considers: the cyclic loading that can be expected; the fundamental responses of piles driven in clays and sands; frameworks for understanding axial cyclic response and specifying cyclic soil testing; and approaches for practical application in design. The review then moves to consider pile responses to moment and lateral loading, distinguishing between flexible and relatively rigid piles and anchors. A range of possible design approaches is considered and it is argued that current routine practice needs to be reconsidered. Practical methods now exist to address the potentially highly significant effects on axial capacity of piles that experience high ratios of cyclic to average loads. New research and calculation procedures are emerging that offer significant improvements in a broad spread of topics. 1. Introduction Interest in the behaviour of piles under cyclic loading grew in the 1980s to meet challenges posed by inherently fail-unsafe Tension Leg Platforms (with the first TLP being installed at Hutton in 1984) and heavily loaded deeper water fixed platforms, such as the Cognac jacket set in 320m water. Briaud and Felio (1986) assembled for API a database intended to resemble fine marine sediments covering the cyclic behaviour of clays in: laboratory tests, cyclic model experiments and axially cyclic field pile tests. They considered 16 studies on piles with diameters greater than 150mm, most of which were strain-gauged to measure axial load distributions. Local shaft friction, pore pressure and radial stress measurements were attempted in some cases, although these parameters are notoriously hard to sense reliably. The response of piles driven in sands was not addressed. The piles were submitted to significant numbers of load cycles (typically 100 to 1,000) with frequencies generally around 0.1 Hz.

Abstract Significant research effort has been put into the formulation of improved design procedures for the calculation of the axial capacity of open-ended piles in sand. Methods, such as the Imperial College Pile (ICP) and the University of Western Australia (UWA-05), estimate the medium-term capacity of piles at relatively large displacements (typically 10% of the pile diameter). To use these approaches in pile driveability analyses, some additional factors need to be taken into account. In addition to the significant influence of dynamic effects, such factors include the absence of pile ageing and the relatively low displacement mobilised during individual hammer blows. In this paper, pile driving records from the installation of two open-ended steel piles installed in medium dense to dense North Sea sand are considered. The piles are a 0.762m-diameter skirt pile supporting a jacket structure and a 4.2m-diameter monopile, respectively. The piling records are used to test the accuracy of existing pile driveability models by comparing the measured and predicted blow counts. The latter are determined by incorporating the static resistance to driving (SRD) and dynamic damping parameters in a 1D wave equation analysis program. The possibilities of using recent static capacity formulae (e.g. ICP and UWA-05) to predict driveability were also explored. 1. Introduction Pile driveability is an integral component of the process of offshore pile design and is often the determining factor in the selection of an appropriate driving system. Accurately predicting the pile response to driving is becoming increasingly important, with the use of large 3–6m diameter steel monopiles becoming commonplace. Pile driveability assesses the ability for a pile to be economically driven with an acceptably low risk of refusal and, ultimately, to reach a desired penetration or capacity within a reasonable number of blows without overstressing the steel.

Abstract A study of the break-out factor ( Nc ) for plate anchors is presented in this paper. Comparisons among finite element method (FEM) analysis and laboratory results were performed. A soil corresponding to a soft normally consolidated clay was considered. Numerical FEM analyses were performed with the Plaxis® code, using an elasto-plastic model with a Mohr-Coulomb criterion. Undrained soil parameters and an adhesion factor α = 1 were used. Values of Nc factor for axisymmetric and 2D FEM were obtained. Two geometries considering a perpendicular load applied in the anchor area and horizontal anchor position to different depths were studied. In the same way, anchor plates to a reduced scale were tested in a tank containing a soft soil. It was verified that the factors Nc reach constant values beyond a determined depth of the soil. Finally, the experimental and numerical values were compared with suggested methods reported in the literature. 1. Introduction Plate anchors are frequently used as foundation solutions for offshore structures to transmit forces to surrounding soils at various depths. The capacity estimation of the anchor is nevertheless uncertain insofar as there are factors that have a large influence in its behaviour. Examples of such factors include the installation process, soil characteristics, geometry of plates and large numbers of methods used to calculate the holding capacity of the anchors. In this context, this paper presents a study of Nc values, obtained from axisymmetric and plane strain finite element model (FEM) simulations done in Plaxis® code. In addition, it outlines the results of scaled-down plate anchors tested in the laboratory. In both cases, the soil considered was soft clay with deepwater sediment characteristics. The description of specific FEM analyses and laboratory test characteristics taken into account are described. Comparisons with expressions found in literature were made.

ABSTRACT Geotechnical design of various foundation solutions and evaluation of geohazards requires a range of soil parameters, including monotonic and cyclic shear strengths as well as deformation and consolidation characteristics. For deepwater soft clays the determination of these parameters poses special problems, but recent research efforts have led to improved wars of talking these problems. Main emphasis is given to how the undrained shear strength can be determined from laboratory tests on soil samples, but with some references to in situ tests. The problems of sample disturbance are dealt with in some detail including how to quantify the degree of disturbance and how to minimize the disturbance effects by taking special care in the sampling and laboratory testing process and in the interpretation of the test results. Remoulded shear strength is needed in addition to intact shear strength, and it is shown how this parameter depends on equipment and procedures used. It is also shown that strain rate and temperature effects need to be carefully considered, as well as thixotrop and creep. INTRODUCTION Need for soil parameters in deep water field development Information about seabed soil conditions are required for a range of structures that are installed in connection with deepwater field development, including, but not limited to : anchoring of floating structures using one of several possible foundation solutions driven piles suction anchor drag in anchor plate anchor Seabed structures founded directly on the seabed, like templates pipelines, flowlines and risers In addition, soil parameters are required to evaluate a number of potential geohazard problems, and especially stability of sub-marine slopes 1 Overview of soil parameters needed The soil parameters that are needed may be grouped into the following categories: intact and remoulded monotonic shear strengths to evaluate penetration resistance and allowable underpressure for installation of structures like skirt penetration of suction anchors 2,3 monotonic and cyclic shear strengths to evaluate holding or bearing capacity of installed 3,4 , stability of sub-marine slopes 1 and behaviour pipelines and risers. Intact and remoulded shear strengths may be needed. For skirted structures, like suction anchors, both the shear strength of remoulded clay after reconsolidation and the thixotropy of remoulded clay are also required to determine the shear strength along the penetrated skirts with time 5,6 . The effective stresses may be low beneath pipelines and risers, and strengths at low consolidation stress may be important. deformation and consolidation characteristics of intact clay calculate displacements and settlements of structures deformation and consolidation characteristics of remoulded clay and unloading characteristics of intact clay to calculate the stress changes in the remoulded zone along penetrated skirts with time 5,6 and beneath pipelines and risers The structures and the seabed may be subject to various kinds of loading, including submerged structure weight, soil weight, sustained loads from current and wind, impact loads from icebergs and trawler equipment, and cyclic loading from waves, earthquakes or temperature variations.

ABSTRACT The sediments encountered by great water depths on the continental shelf of the Gulf of Guinea are composed of very fine particles and are characterised by very high water contents (w>100%), very high plasticity (PI > 80), relatively high sensitivity (3 <St <5) and strong potential in shear strength regain due to thixotropy. Difficulties in determining the organic matter content, the grain size distribution and the mineralogy of these sediments by the standard methods routinely used by soil mechanics laboratories have been encountered and reported 1 . Organic matter content can be determined by a number of methods, including loss of ignition, pyrolysis, dichromate oxidation, infrared spectroscopy and geochemical acidification. Grain size distributions can be obtained from sieving and hydrometer testing or by laser diffraction method, both techniques being associated with various dispersive agents. Mineralogical characterisation is usually carried out by X-ray diffraction (XRD), but quantitative results are often given without precisions on operating procedures and are considered as not significant by specialists. The paper focuses on the determination of organic matter content, grain size distribution and mineralogical determination. Results obtained on typical Gulf of Guinea sediments using the various methods listed above are compared and discussed. Specific testing procedures are recommended for these sediments. INTRODUCTION The sediments encountered at great water depths on the continental shelf of the Gulf of Guinea express the following general feature: very fine constituting particles, very high water contents (w> 100%), very high plasticity (PI > 80), relatively high sensitivity (3 <St <5) and strong potential in shear strength regain due to thixotropy. Such properties are usually characterised by geotechnical tests (index or mechanical tests) performed at a macroscopic scale without any reference to the microscopic aspects 1 . This paper highlights the benefit of combining both approaches for a better understanding of the physical and mechanical properties of the Gulf of Guinea deepwater soils. The mechanical properties of a colloidal dispersion are essentially controlled by the particle size and shape distribution, and by the interparticle interactions (electrostatic interactions, bridging, friction, etc.). In soils and sediments, the individual mineral particles are generally assembled within aggregates of various size and tensile strength, according to the nature and surface properties of the particles, as well as the nature of the interparticle phase acting as a linking agent, such as organic matter, amorphous phases or ions. Better understanding of the mechanisms underlying the geomechanical properties of deep sea soils therefore requires thorough analysis of their micro-scale characteristics. These characteristics include the chemical nature and relative abundance of the constituting mineral and organic phases, which are indicative of generic size, shape, surface reactivity of the solid phases and of the nature of the interparticle interactions. Several methods and standardised procedures aimed at determining these characteristics are available. In the present work, three major aspects were investigated: the particle size distribution, the mineralogical determination and the organic carbon content. The particle size distribution is an essential parameter in the geomechanical behaviour of soils and sediments.

Abstract Exploration Jack-up drilling units in S E Asia routinely encounter punch-through hazards due to peculiar geological conditions prevalent on the Sunda Shelf. The majority of these phenomena are identified prior to rig emplacement and with controlled preload measures this hazard is managed. However, in recent years, rigs have been encountering unexpected punch-throughs that have resulted in both ng damage and lost drilling time at a rate of 1 incident per annum, with consequential costs estimated at between US$1 and US$10 million. The S E Asia Drilling Contractors and Operators have well established site assessment procedures m place, but jack-up foundation failures continue to occur at an alarming rate. This begs the question "why is it that such incidents continue to occur?" This paper identifies some of the problems associated with site assessment practice and seeks to promote adoption of more rigorous standards of investigation and analyses in known punch-through territory. Introduction Worldwide lowering of sea levels during the Pleistocene period exposed a large landmass of approximately 3 million km2 m South East Asia. This ancient landmass is known as the Sunda Shelf and during the general rise m sea levels in the past 20,000 years, marine clays have been deposited on this shelf. As a result, zones of stiff desiccated "crustal clays" formed by sub-aerial exposure are often sandwiched between overlying very soft to soft and underlying soft to fm clays These stiff over soft crusts are typically encountered between depths of 5 to 20 m below mudline which is the zone within which most rig spudcans are founded. These crustal features and their potential hazard to jack-up's have been well documented m the past (Castleberry and Prebaharan, 1985, McClelland et al., 1982) but despite this jack-up foundation failures continue at an alarming rate of approximately 1 per annum in the Far East. The cause of these incidents may therefore also be due to mechanisms other than those previously described. Clearly it would seem that there may be room for improving current site assessment procedures in this region. This paper seeks to identify some of the possible areas for improvement. In addressing this topic to avoid ambiguity, we propose the following definition for "Punch-Through" be adopted: "An unexpected jack-up footing rapid settlement resulting in consequential lost drilling time". Unexpected, or planned "Rapid Leg Penetrations" (or "leg runs") occur more frequently than "Punch-Throughs" during jack-up installation and we propose that these are defined as: "Rapid jack-up footing settlements not resulting in consequential lost drilling time". SE Asia punch-through situation The number of jack-up's operating in the Far East has increased four-fold over the past few years and the rig-time spent at each location has decreased, due to increased drilling and operational efficiency. This has resulted in a significant increase in the number of rig moves per year in this operating area which is known to have a wide occurrence of "punch-through" hazards.

Summary The first string of well casing is usually called a conductor The conductor is the well foundation and its main function is to resist the axial and lateral loads imposed at the wellhead. In stiff clays and sands conductors are generally grouted into pre-drilled oversized holes. However, a technique analogous to wash boring, called Jetting, is a feasible alternative for installing conductors in soft clays that are often encountered in deep water. Jetting can reduce rig operating-time and save about $0.5million per well in deepwater exploration The technique is also inherently safer than open-hole drilling and cementing in very soft sediments but it can be counter-productive because of poor installation practices. The most common operational problem with jetted conductors is excessive settlement under the weight of the next casing string Failures of this type, which arise from insufficient short term axial capacity of the jetted conductor, are episodic rather than frequent but each incident can cost $2million or more if the well has to be respudded. Jetting is used routinely in the Gulf of Mexico (GoM) but drilling and cementing has remained popular in other regions. In the past 3 years BP has promoted the use of jetted conductors m deep water in and outside GoM and has carried out studies to reduce the incidences of axial being capacity failures arising from this initiative. This paper reviews the jetting technique, summarizes BP Exploration's method for estimating the axial capacities of jetted conductors m soft clays and describes the successful adaptation of the method for deepwater exploration wells m Angola. Introduction Deep-water Wells Deep-water wells are drilled from anchored or dynamically positioned semi-submersible drill rigs or drill ships. They can be broadly categorised as Exploration Wells or Development Wells. Exploration wells are drilled during the prospecting stage of a potential field. These include wells to prove the presence of hydrocarbons, often referred to as ‘Wildcats’, and Appraisal Wells used to define the size, quality and extent of a discovery Development wells include production wells to recover hydrocarbons, and gas and water injection wells used to enhance recovery and/or dispose of fluid bi-products (produced fluids). Both types of wells are lined by a series of concentric steel tubes called casing strings that are cemented in place to stabilise the bores and reduce the risk of fracture of the formation while drilling with weighted mud. Unless adapted for production, exploration wells have relatively short design lives. They are also generally planned with &ma1 knowledge of geology, formation pore pressures, and service life loads. Development-wells are drilled later so are usually planned with better information on the subsurface conditions. The design loads for development wells are also better defined than for exploration wells. In each case the main objective of well design is to achieve the total depth safely with the most cost-effective numbers and sizes of casings. An example deep-water exploration well casing design used by BP in Angola is shown on Figure 1.