ABSTRACT Rising gas occurs naturally and as a result of drilling, injection and construction activities, such as air jetting, fracturing, jet grouting and tunnelling. The effect of rising gas is difficult to quantify or model, but is known to reduce the bearing capacity of both shallow and deep foundations. Under gravity based structures (GBS) it can cause problems of flotation and subsequent base sliding and internal over pressure. Storm waves can liquefy shallow gaseous deposits. Gas venting facilities are essential but not always provided on offshore structures. The paper discusses rising gas in terms of sources, pathways, flow calculations, gas experiments trials, and calculations, settlement effects and precautions, as well as its effect on GBS, on bearing capacity (especially during storms) and on shallow and deep foundations. 1. Sources of Gases The most obvious sources of gases are hydrocarbons rising naturally or as a result of drilling operations. Additionally gases, such as nitrogen, are injected as liquid into strata to increase well production, while other gases are injected for storage or disposal. Compressed air is used in tunnels, caissons, jet grouting and jetting. There are also volcanic gases, hydrates and organic deposits releasing methane. 2. Flow Pathways Pathways for rising gas are mainly dependent on the lamination and dip of the strata and local anomalies such as faulting, joints or weaknesses. Layers of gas can be trapped below individual laminations or cyclotherms (due to seasonal depositional changes in particle size). Seismic surveys show numerous strong ‘ghost’ reflections, which correspond to the pattern of lamination but cannot usually be distinguished in recovered core. A likely explanation is that trapped gas provides the ghost reflection. The air lifts the cuttings, but can also escape laterally and appear in adjacent boreholes. Shot hole blasts can show a similar behaviour.

ABSTRACT As offshore wind projects move into deeper waters, tripod and jacket structures are becoming more favourable as support structures for offshore wind turbines. These structures are much lighter than traditional jackets for oil and gas platforms, and the cyclic loading is a larger proportion of the loads. The response of piles in dense to very dense North Sea sands subject to long-term cyclic axial loads is complex, and there are no standard methodologies to predict the effect of cyclic loading on the pile capacity. This paper briefly reviews available methods for assessing degradation of axial pile capacity due to cyclic loading. Foundation response under irregular storm loading is considered, and a new analytical approach for calculating pile capacity degradation is proposed. The method has been successfully validated against the Dunkirk pile load tests (Health and Safety Executive (HSE), 2000a) compared with the results of cyclic (T-Z) analysis, and applied to several large wind farm projects in the German sector. 1. Introduction The response of tripod and jacket piles to long-term cyclic axial loads is complex, and there are no generally accepted methodologies to predict the effect of cyclic loading in sand. Moreover, the majority of recent research on the effects of cyclic loading focused on cyclic lateral loads on monopiles. The analysis of cyclic loads for subsequent capacity degradation analysis is also outlined. Section 3 of this paper contains a brief review of the response of the soil adjacent to the pile during cyclic axial loading. During high-intensity cyclic axial loading, the pile capacity will reduce, and non-recoverable displacement at the pile head will accumulate. The effect of cyclic lateral loading on the axial capacity is also considered. Methods for axial degradation analyses are reviewed in section 4, together with the available pile test data.

ABSTRACT This paper describes aspects of the foundation design methodology developed for the Borkum West II offshore wind farm in the German North Sea, comprising 40 turbines supported on piled tripods in water depths of approximately 30m. The foundation design evolved during a technical due diligence process, which offered the opportunity to review the site investigation data and cyclic loads, to reconsider the effects of cyclic loads on pile resistance and to modify pile lengths and wall thicknesses to mitigate pile tip integrity risk during driving in very dense sands. The re-evaluation of the design storm concluded that axial pile capacities could decrease by up to 25% because of cyclic loading at some turbine locations, but could be almost unaffected by cycling at others. The technical review involved a collegiate process that contributed to the development of acceptable foundation designs and mitigated risks relating to pile installation and foundation performance. 1. Introduction The Borkum West II wind farm is currently being developed by Trianel Windkraftwerk Borkum GmbH in the North Sea, approximately 45km offshore northern Germany (Figure 1). The first phase of this project includes the construction of forty 5MW turbines. The hub height is approximately 90m above sea level, and the rotor diameter is 116m. The turbines are supported in water depths of 26m to 33m by tripod structures designed by Offshore Wind Technologie GmbH. Figure 2 illustrates the general arrangement of the steel tripods, which have an outer footprint diameter of 28m. Geotechnical engineering was undertaken for the project by Cathie Associates (CA) SA/NV, Belgium. The ground investigations, interpretation and foundation designs were performed in accordance with the standards for offshore wind farms , published by the Bundesamt für Seeschifffahrt und Hydrographie (BSH, 2007, 2008; also known as the Federal Maritime and Hydrographic Agency).

ABSTRACT This paper reviews the research and debates that have led to substantial changes being made in 2007 to the API-RP2A recommendations for assessing offshore driven pile axial capacity. The reasons for the conventional Main Text method's large scatter, strong skewing and significant biases are explored, with particular emphasis on piles driven in sand. Recent alternative design frameworks are reviewed critically and conclusions are drawn regarding their practical application. Comments are also made on predicting load-displacement behaviour, assessing the impact of load cycling, group interaction effects and aspects of foundation disturbance by drilling. INTRODUCTION The technologies associated with the manufacture and installation of offshore piles are relatively mature; very large piles may now be driven routinely in a wide range of water depths and geotechnical settings. However, the understanding of the ground's reaction to driven pile installation and loading has lagged behind the impressive developments made by the offshore construction industry, as design approaches are still in an imperfect state of evolution. Severe problems have arisen during pile installation in some major projects1. Considerable mismatches have been found in other cases where it has proved possible to check Industry-standard design expectations by static tests on large offshore scale piles 2, 3, 4 . Research in several centres has emphasised the scientific weaknesses of the industry-standard American Petroleum Institute (API) RP2A 5 methodologies, which have remained practically unchanged between 1993 and 2007. While most practitioners have continued to use the conventional methods, alternative geotechnical design frameworks have been proposed that have been applied comprehensively in some sectors 6 . Vigorous debate has taken place over several years, prompted by industry-sponsored reports, academic papers, conference proceedings and meetings of the relevant API/International Organization for Standardization (ISO) review panels. Important changes are included in the 2007 API-RP2A recommendations for piles driven in sand that will also affect the ISO documents and industrial practice. However, progress is being made cautiously and further evolution of design practice can be expected. This paper offers one perspective on some of the issues raised in the recent debates, referring to background research and highlighting physical aspects of pile behaviour that are important to practice. Particular emphasis is placedon the question of axial capacity, as this is arguably the most important issue and has proved to be the focus for most discussion. Consideration is also given to the assessment of load displacement behaviour. While movement prediction and control is emphasised more strongly in onshore foundation projects, as reflected in the recent review by Mandolini et al. 7 , offshore engineers may become more concerned with making better fatigue life predictions for critical structures. Assessing limits for acceptable displacements also could become more important as a means of monitoring platform safety in critical cases. It is useful to consider at the outset the ranges of pile sizes specified by offshore engineers. Piles with diameters of ~5m have been driven for offshore wind turbines. Smaller diameters of 3 to 4m have been specified for such structures in the North Sea, where piles with diameters of up to 2.5m

ABSTRACT Correlations between cone penetration test (CPT), tip resistance, q c' , and pile shaft friction, t f , have been shown to be reliable for evaluating axial pile capacity. The correlation between t f and q c is not direct, and pile shaft friction is influenced by many more factors than those which affect cone tip resistance, namely differences between open- and closed-ended piles/penetrometers; the reduction in local friction with continued pile penetration (friction fatigue); changes in radial stress during loading; and interface friction angle. This paper presents results from analytical studies, model pile test results and field pile load tests in siliceous, micaceous and calcareous sands to assess the influence of sand grain mineralogy on input parameters for CPT q c based shaft friction calculations. While the data for calcareous and micaceous sands are limited, observations based on laboratory and field studies are consistent. The paper concludes that while input parameters may differ, the same framework is valid for evaluating shaft friction in siliceous, calcareous and micaceous sands from CPT data. Calcareous and micaceous sands appear to have higher rates of degradation of local friction than siliceous sands, but this degradation tends to be bounded by a minimum shaft friction value. Input parameters to a general expression for shaft resistance based on the UWA-05 design method are proposed for each sand type. INTRODUCTION A sound design method leads to a safe and cost effective engineering solution with consistent levels of reliability for the anticipated range of situations encountered over the design lifetime. For piled foundations, previous successful experience and static or dynamic load testing plays a significant role in developing such design methods. However, experience alone cannot guarantee reliability, and there is a clear need to develop design frameworks that reflect the current best understanding of the underlying factors controlling pile capacity. Such understanding is critical for offshore piles currently being considered in new regions and in soil conditions that have not been previously encountered 1 . Of the 600 failures of civil engineering systems reviewed by Bea 2 , a majority of failures during operation and maintenance were attributed to flawed engineering design. While these structures and foundations may have been designed to accepted standards, failures occurred due to limitations and imperfections embedded in the standards 2 . This paper is concerned with the design of axially loaded driven piles in sand. The comments of Bea 2 are particularly relevant to this topic, as the underlying behaviour governing the installation and subsequent axial capacity of piles driven in sand is poorly understood. This uncertainty is compounded by a lack of relevant data to support design formulations. Virtually no measurements of the axial capacity of a driven pile with the dimensions relevant to new offshore developments exist. The databases used to calibrate current design methods predominantly comprise short, small diameter piles. Every design of a full-scale offshore pile is therefore reliant on the design method providing a correct extrapolation from the database pile geometry to the field conditions. It is therefore essential that the design method formulation captures the underlying mechanisms as closely as possible.

ABSTRACT This paper presents a calculation method to predict the installation process of a suction caisson in sand overlaid by clay. Calculation methods are first described for homogenous clay and sand profiles, with the approach for the latter case being based on the measured cone resistance profile and a simple linear reduction in resistance with increasing suction. Example calculations for a sand profile are compared with measurements to help validate the approach. Modifications to account for an overlaying clay layer are then discussed, and two alternative approaches are described, depending on the extent to which the clay layer remains intact within the caisson. A flow model is introduced that balances the water pumped from the caisson with that displaced by the caisson and seepage flow through the soil. By combining the installation resistance models with the flow model, predictions are made regarding the complete installation process over time for given suction caisson geometry, soil conditions and pumping speed. Theoretical predictions are compared with model test data and are provided in the paper. INTRODUCTION Installation of a suction caisson consists of a self-weight penetration phase and a suction-assisted phase. During the latter phase, water is pumped out from inside the caisson creating a differential pressure between the inside and outside of the caisson. This differential pressure provides an extra driving force to help overcome the soil resistance during installation of the caisson. In the case of relatively permeable soils the reduced internal pressure will also induce seepage flow through the soil, leading to reduction in the effective stresses and hence penetration resistance. The installation resistance of a suction caisson comprises internal and external friction along the caisson wall and bearing resistance at the tip. The unit resistance, either friction or bearing, must be estimated from some measure of the strength of the soil. In clay soils the obvious choice is the undrained shear strength, s u , which is almost universally adopted in practice 1 . In sands, shaft friction and bearing resistance values for suction caissons and piles have traditionally been evaluated from fundamental properties such as the friction angle, F', and in situ effective stresses 2, 3 . However, the most direct measure of strength is the cone resistance, q c , and design methods based directly on qc have become increasingly popular over the last decade 4–6 . This approach will therefore be used here to predict the installation resistance in sand, following the basic methodology described in Senders and Randolph 7 . For caisson installation in sands, it has been shown that dramatic reduction in installation resistance occurs due to suction-induced seepage 8–10 . This has been implemented in the proposed installation resistance calculation for caissons in sand, using a simple linear reduction in internal friction and tip bearing resistance with increasing suction 7 . In a soil profile comprising clay overlying sand, the question arises as to the extent that the clay will impact the seepage, and thus limit the decrease in tip resistance during penetration of the underlying sand. Of particular concern is the potential lifting of the clay plug

ABSTRACT The Ormen Lange gas field is located in about 900 to 1100 m water depth in the slide scar of the enormous Storegga Slide that occurred about 8000 years ago. The slide left steep and high headwalls above and below the planned field development area. Today's stability of the headwalls is a major concern for the field development work The area under consideration is large and has been mapped extensively with 2D and 3D seismic profiling The number of geotechnical borings is limited and integration of geological, geophysical and geotechnical information was required to develop a geotechnical model of the area. Stability analyses have been carried out for critical sections of the headwalls. These involved long-term drained analyses under gravity loading and undrained analyses considering the effects of earthquake-loading and possible Influence from field installations like rockfill supports for pipelines and anchors. Focus has been set on explanation of slide mechanisms involved in the Storegga slide and comparison of the stress-strain-strength conditions in the headwall at the tune of the slide and today. Work is still ongoing and under review and the conclusions presented here are thus to be considered as preliminary. INTRODUCTION The Ormen Lange gas field was discovered m 1997 within the slide scar of the Storegga Slide The recoverable gas reserves are 400 billion sm3 and production is planned to start in 2007. Norsk Hydro is the operator for development and construction, and Norske Shell wll be the operator for the production. The other partners in the Ormen Lange license are Statoil, BP, ExxonMobil and Petoro. Three papers are presented on the Ormen Lange and Storegga Slide topic at the 2002 International Conference on Offshore Site Investigation and Geotechnics. These are Establishing a geological model for the regional understanding of the Storegga Slide, by P Bryn et al. Ormen Lange geobonngs by T.I Tjelta et al. Ormen Lange slope stability (this paper) by T J. Kvalstad et al. The Ormen Lange gas field is located in the Norwegian Sea about 130 km WNW of Kristiansund. Figure 1 shows the location and the approximate extension of the Storegga Slide The Storegga Slide is one of the largest submarine slides m the world (Bugge, 1983 and Bryn et. al., 1998) and occurred about 8200 calendar years ago. The upper headwall scar has a total length of about 290km. The estimated soil masses involved were (according to Bugge, 1983) in the order of 5600 km3 More recent updates indicate somewhat lower volumes Evidence of a tsunami generated by this slide event has been found along the coast of Norway, Scotland and the Faeroe Islands. Figure 2 shows an exaggerated bathymetric picture of the central, upper part of the slide showing the upper and lower headwalls of the slide.