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Über dieses Buch

Two main areas of offshore activity are addressed in this book: Site investigation on assessment; and Applications and foundation engineering. The 37 contributions from a wide ranging group of international experts, are resulting from the Offshore Site Investigation and Foundation Behaviour Conference, London, U.K., September 1992.
Adequate determination of site conditions can only be achieved by the integrated approach of using geological, geophysical and geotechnical data.
Developments in data acquisition techniques are illustrated through case histories in the section on Geotechnical Sampling and Testing.
In the section on Advanced Interpretation Techniques and Integrated Interpretations the state of the art of these topics is also illustrated by case histories.
A review of foundation behaviour is presented in the section on Gravity Foundations, Foundation Performance Monitoring, Piling Research and Design Criteria. These topics are illustrated in the light of field experience and recent research, in particular that involving full-scale tests and monitoring.
This book provides many illustrative figures and much pertinent information to exploration and marine geophysicists, petroleum and offshore engineers and for researchers working these fields.





Opening Address

Mr. Chairman, Ladies and Gentlemen, it is an honour for me to open this SUT International Conference on “Offshore Site Investigation and Foundation Behaviour” and a privilege for me to welcome you here on behalf of the Planning Committee and of the Organisers. A meeting such as this with its range of underlying science and engineering is an ideal opportunity for exchange between professionals of the different disciplines involved.

T. D. Patten

Keynote Address: Offshore Foundation Safety

The title of this keynote address is ‘Offshore Foundation Safety’. This is a wide subject and could be controversial, however nothing I am going to say is particularly new rather I am going to present old information with a new emphasis. I will be concentrating on the UK offshore sector but the principles that I will be elaborating on are the same worldwide.

M. Birkinshaw

The Use of Exploration Geo-Science Data in the Planning and Execution of Site Investigations for Offshore Development Facilities

Prior to development of an offshore hydrocarbon field large sums are spent by Operating Companies on the acquisition and processing of exploration geo-science data. These data, augmented by other publicly available data, can be cost-effectively integrated and reviewed to aid field develop­ment conceptual design, selection of suitable sites for development facilities and planning of detailed, site-specific, geotechnical and geophysical site investigations.This paper illustrates the range of geo-science data that normally exists and the uses to which such data can be put. A case history of two field development sites in the southern North Sea is presented to emphasise the benefits of such a data review and the pitfalls that can occur if such a review is not undertaken.The benefits of multi-use of existing data are summarised. Recommendations are made for future geo-science data acquisition to provide further information for subsequent field development site investigations.

M. R. Cook, J. M. Squire, A. W. Hill

Geotechnical Sampling and Testing


Successful Cable Burial — Its Dependence on the Correct Use of Plough Assessment and Geophysical Surveys

Burial of subsea cables began in the early 1980s when excessive cable faults due to increasing and changing fishing activity, was resulting in a significant loss of income to the cable owners. Several methods are now used to bury the cables (usually to a depth of 0.6m), the primary method being the use of a subsea plough pulled and controlled from a host ship. However, water jetting tools mounted on tracked or swimming ROV systems are also used.The BT (Marine) Limited (BT(M)) cable plough is towed behind a vessel, using a share to cut a trapezoidal prism of sediment from the seabed. This is lifted up by an inclined ramp, while the cable is laid underneath it. Before undertaking this type of cable burial, it is very important to assess whether burial depth can be achieved. To this end a geophysical route survey and a plough assessment survey are carried out, and the correct analysis, correlation and interpretation of these determine the success of the burial operation.This paper assesses the present state of these surveys, and considers new methods that could be used to improve the results obtained. The use of high resolution sonar scans, coupled with an improved plough assessment tool, could give an excellent picture of the conditions extant at the sea floor. This would also allow an accurate prediction of the way the plough behaves in layered sediments, helping to control burial depth. The new processes will be used to develop a new strategy to the benefit of cable owners.

Jon Noad

A Measurable Classification System for Non-Calcareous Marine Soils

In this paper a practical engineering classification system for non-calcareous marine soils is proposed as an alternative to BS-5930 and ASTM-D2487. The system is based upon measurable data furnished by Atterberg limits and particle size distributions. This system is applied by means of simple flow charts for naming the main soil type (MST), the secondary soil type (SST) and the soil group symbol. These flow charts are easily adaptable in computer software and can be extended to include more categories. A total of 46 soil classes are possible in the new system. This paper also addresses differences between BS, ASTM and the new system and suggests consistency description criteria for fine and coarse grained soils. The system proposed in this paper is currently adopted by Fugro-McClelland Engineers (FME), The Netherlands in offshore soil investigations.

U. F. Karim, M. R. De Ruijter

Geotechnical Properties of Sediments from the Continental Slope Northwest of the British Isles

To the west of the Shetland Islands the continental margin is constructed from a thick sequence of sediments of Pliocene to Holocene age. In this paper the sedimentology and geotechnical properties of five vibrocores are described. The cores selected for study can be divided into two groups on the basis of their sedimentology, which relate to the acoustic signatures of the horizons from which they were collected. The first group consists of clay-silts of probable mass flow origin. These sediments are acoustically transparent and the cores are lithologically very uniform throughout their depth. The second group is drawn from sediments which are acoustically well-layered at the sample sites, and the cores show major facies variations between muds, muddy silts and sandy silts. Consistent differences are seen between the two groups over a wide range of engineering properties, including water content, grading, plasticity limits, undrained shear strength and compressibility. These differences arise from the sedimentological character of the cores, in particular the mud content and the presence of depositional facies variations, which exert a strong control on the engineering behaviour. Those facies which are rich in mud have high plasticity, high water contents, low strength and high compressibility, whereas those facies which are low in mud content are of correspondingly low plasticity, high density and low compressibility.

M. A. Paul, L. A. Talbot, M. S. Stoker

Geosis Project:Integration of Geotechnical and Geophysical Data

The GEOSIS project aims to integrate seismic and geotechnical data for improving the extrapolation of soil data around geotechnical boreholes. The project includes the development and the testing of two techniques: vertical seismic profiling in geotechnical boreholesmulti-channel very high resolution seismic surveying. The paper describes the state of development of these techniques and comments on the first field test results.

J. F. Nauroy, J. Meunier

A Review of Sampling Effects in Clays and Sands

Predictions of the strains caused by tube sampling are combined with a simple framework for soil behaviour to examine both the response of soils to tube sampling and the effect of sampling on the soils’ subsequent behaviour. Factors shown to be important are the soil’s plasticity, stress history and structure, and the geometry of the sampler. Attention is paid to comparing field and laboratory measurements of dynamic shear modulus as a means of evaluating sample disturbance in clays and sands.

D. W. Hight

Recent Developments in In Situ Testing in Offshore Soil Investigations

This paper gives a review of developments in offshore in situ testing since about 1985. Deployment of the devices is now so well developed that it is difficult to imagine that more efficient operations can be achieved. Advances in methods of measurement and data acquisitions have facilitated multisensor devices. This has resulted in very useful tools like the triple element piezocone and the seismic piezocone which can now be carried out on a routine basis. Other useful tools like the dilatometer and BAT probe are also available.After a period of intense equipment development it is thought that the main emphasis should now be on consolidating present measurement techniques and concentrating on interpretation in terms of soil design parameters. A lot of research work has also been done in this field but there is still much room for further development.A recent trend has been to carry out large scale in situ model tests to check out the feasibility of new foundation concepts especially for complex and difficult soil conditions. Some examples are given.

T. Lunne, J. J. M. Powell

The Influence of Stone and Boulder Inclusions on Offshore Site Investigation and Foundation Behaviour

This paper examines the origin of stony and bouldery soils offshore and describes the techniques and equipment available for their investigation. The experiences of the Norwegian Geotechnical Institute and British Geological Survey are reviewed in presenting a detailed discussion of direct drilling techniques, and the usefulness of indirect methods is also discussed. The applicability of statistical methods is briefly covered, and the paper concludes with a consideration of the foundation engineering implications of these geotechnically difficult materials.

M. R. Cooper, T. Lunne, T. By

Advanced Interpretation Techniques


Improvement of Geophysical Interpretation by Use of DELPH1 Processed Data

The paper presents several pinger, boomer and sleeve gun data acquired for drilling site investigation. The data were digitally recorded and processed by DELPHI. Different processing modules (noise control, swell filtering, true amplitude recovery, deconvolution, time varying filtering, scaling, screen interpretation, ...) show the enhancement of the seismic data leading to an improved interpretation.

T. Des Vallieres, T. L. Armstrong, R. Girault

The Extraction of Geotechnical Information from High-Resolution Seismic Reflection Data

There are strong empirical and theoretical correlations between the geotechnical and seismic properties of a marine sediment. With the development of modern digital acquisition techniques, and the advent of PC/workstation-based data processing methods it is becoming possible to extract geotechnical information from seismic data in a cost-effective manner.In a collaborative project between U.C.N.W., Bangor and Applied Geology (NW) Ltd., attempts are being made to develop improved capabilities in high-resolution seismic data acquisition and processing for geotechnical site investigation purposes. The ultimate objective is the creation of a rapid and cost-effective method by which the physical properties of seafloor materials can be determined. The potential users of such a method might include offshore contractors working within the hydrocarbon industry (for rig-site surveys, investigation of pipeline routes, etc.), coastal engineers, dredging companies, and river and harbour authorities.It is envisaged that via the development of robust computer programmes designed to extract a number of different seismic parameters (velocity, acoustic impedance, attenuation) it will be possible to produce depth profiles (to 20–30 metres below seabed surface) of a marine soil’s bulk properties (e.g. density, void ratio, moisture content and grain size). Given this information, it is then theoretically possible to calculate order-of-magnitude estimates of parameters such as shear strength and permeability. Ultimately, with a limited amount of borehole control, it should be possible to improve upon these initial estimates and provide information on the spatial variability of the in situ geotechnical properties.

R. Haynes, A. M. Davis, J. M. Reynolds, D. I. Taylor

Workstation Data Integration Techniques for Offshore Site Investigation

Over the last five years a significant amount of effort has been directed towards the use of workstations in conjunction with engineering seismic data for the detection of shallow gas accumulations. In the main, this effort has tended to focus on shallow gas as a drilling hazard and development work has concentrated on improving techniques for detecting shallow gas accumulations in the depth range to approximately 1000m below seabed. Overpressurised gas in this interval can be a major hazard to safe drilling operations and there are a number of cases, both in the North Sea and worldwide, where such accumulations have been encountered unexpectedly with severe consequences.

J. P. Williams

Integrated Interpretations


Problems Associated with Seismic Facies Analysis of Quaternary Sediments on the Northern UK Continental Margin

Seismic facies analysis is increasingly being used in the interpretation of high-resolution seismic reflection data. Existing depositional systems models, based primarily on seismic data, have been constructed on the basis that acoustic character can be directly correlated with relatively consistent lithologies and geotechnical properties. However, borehole and shallow core data from the northern UK continental margin indicate significant inconsistencies in the use of this technique as a predictive tool. Changes in acoustic texture can occur laterally and vertically, in both lithologically homogeneous and heterogeneous sequences. The successful application of this technique can only be achieved by the integration of seismic data with other subsurface information, with interpretation based on sound geological concepts and models.

M. S. Stoker, F. S. Stewart, M. A. Paul, D. Long

The Engineering Geological Approach to the Siting of Offshore Structures in the Ravenspurn North Field

This paper uses the development of the Raven spurn North Field by Hamilton Oil Company Limited as a case study to demonstrate the advantages of an engineering geological approach to the siting of offshore structures. In 1986, 1987 and 1988 engineering geological investigations were performed in the Ravenspurn North Field in Blocks 42/30 and 43/26 of the southern North Sea. These investigations comprised engineering geophysical surveys and geotechnical borings followed by geological and geotechnical laboratory testing programmes. These data along with relevant publicly available information were integrated to produce an engineering geological appraisal of ground conditions for the entire field, enabling the selection of optimum platform locations and the planning of detailed engineering, geophysical surveys and site investigations for both concrete gravity base and pile-supported steel jacket structures.

S. Thomas

Gannet Site and Piperoute Surveys — An Integrated Interpretation

The Gannet Development in the Central North Sea, consists of a platform, seven subsea developments and associated infield and export pipelines.This paper provides a case history of the site survey aspects of the field development. It describes the advantages of a carefully planned, comprehensive site survey programme and also the advantages of a detailed integration of all available data to provide a reliable interpretation of geophysical data. This is of particular importance in an area such as Gannet where there is shallow gas and highly variable shallow soils.Several seabed surveys and site investigations have been undertaken in the Gannet area over the last ten years. These commenced with reconnaissance surveys in the early eighties. As the field development plans were finalised, a programme of detailed seabed surveys and site investigations was established, supplemented by a shallow gas pilot hole.Interpretation of the seismic data required careful integration of all the data sets. The result of this interpretation was a good understanding of the shallow soils and shallow gas within the area.Shallow gas prognoses were made for all the proposed locations and to date these have been confirmed by wells at five of the locations.Piling and anchoring conditions have been confirmed at the platform and drilling centre locations and trenching conditions for the pipelines were as predicted.

J. H. Sommerville, P. M. Walker

Integrated Geohazard Study Along the Krishna-Godavari Delta Slope, East Coast India

The continental slope of the Krishna-Godavari delta on the East coast of India exhibits a number of important geological phenomena which should be taken in consideration for offshore foundation design. A detailed geophysical and geotechnical investigation provided the data for an integrated study for the site selection for the foundation of an SBM oil terminal. The geophysical survey consisted of sparker, side scan sonar and bathymetry. Soil borings, including cone penetration testing, were performed at five alternative locations to a depth of 150 m.The soil profiles generally consist of normally consolidated very soft clays at mudline, becoming stiffer with depth. Geophysical data revealed two phases of faulting and mudsliding; an older phase (over 3000 years old) showing relict structures and a recent phase of mudsliding affecting the seabed topography.Stable and unstable foundation zones were identified.Stability calculations based on geotechnical properties of the soil profiles confirmed the delineation of these zones. Wave induced seabed loading analyses showed a mudslide risk for the original proposed SBM location and stable conditions for alternative sites.

J. J. A. Hartevelt, G. L. Van Der Zwaag

Rigs and Reef Geology: A Site Survey in the Mafia Channel Offshore Tanzania

A suite of high quality, high resolution seabed survey data was used to investigate the flank of the Dira Reef, offshore Tanzania. The reef occurs on the upthrown side of an offshore normal fault related to the East African rift system. The fault is likely to have moved at a rate of approximately 2.5 mm.yr−1 for 16,000 years. This extension has opened up migration pathways which allowed biogenic gas to vent into the near-seabed soils sequences. The activity of the fault network also determined the nature and distribution of the shallow overburden sequences. They comprise a single fining-upwards depositional cycle compatible with formation during a relative sealevel rise. The Rufiji Delta drains the rift-flank area of the continental hinterland to the SW, and will eventually enroach the area and bury this sequence with coarser sediment. The relationships are therefore a small-scale geological analogy for features commonly occurring around ‘break-up unconformities’ buried beneath continental shelves in mixed carbonate/clastic settings. The complexities of the near seabed sediments, the presence of gas accumulations related to fault structures, and the restrictions imposed by the design of the available rig made for a challenging site survey operation.

R. McElroy, B. P. Meier, P. M. V. M. Gabriels, C. D. Green

Deepwater Engineering Geology and Production Structure Siting, Northern Gulf of Mexico

The engineering geology of the continental slope in the northern Gulf of Mexico is probably among the most complex of any offshore area in the world. This complexity is due largely to past and ongoing salt movement and to landsliding and other depositional processes during Quaternary sea-level lowstands. Conditions which can adversely affect siting and design of production structures include: steep slopes and rugged, sometimes rocky, topography; active faults; shallow overpressured zones; both modern and ancient landslides; gas hydrates; seafloor erosion; and variable materials ranging from weak soils to rock. Engineering assessment of deepwater sites may require collection and comprehensive integration of a wide variety of geophysical, geological, and geotechnical data.

K. J. Campbell, J. R. Hooper

Gravity Foundations


Review of the Design Development of a High Performance Anchor System

Particularly onerous positioning restraints, performance criteria and ground conditions called for a design review of available anchor systems and a consequential detailed ground investigation and design. High quality geophysical and geotechnical surveys were performed to establish the nature and characteristics of the ground conditions. The data acquired was used for the selection and design of an anchor system. The soil-structure interaction of the anchor was modelled and analysed for installation, operation and survival conditions.

J. J. Osborne, R. D. Colwill, D. Rowan, D. Phillips

Optimization of Underbase Drainage Systems for Gravity Structures on Sand

For many off-shore sites, gravity structures can provide cost-effective solutions for the provision of support to topside facilities. Uncertainties in drivability and longterm performance of piled foundations, particularly in new and unusual ground conditions, can lead to the adoption of relatively shallow foundation systems to carry the forces collected by the superstructure. Installation of a gravity structure can be undertaken without the requirement for an expensive heavy lift vessel in attendance while a steel jacket structure is set onto the site seabed and piled. The need for such a vessel at the platform site can be completely avoided if the topsides are mated to the structure in the construction yard or inshore. Furthermore, the large volumes associated with gravity structures can be utilised for oil storage off-shore.

N. J. O’Riordan, J. W. Seaman

The Non-Piled Foundation Systems of the Snorre Field

A new milestone has been passed in offshore development with the completion of the Snorre (Phase I) field installations this summer. The installation are located in more than 300m water depth supported on very soft soil. The foundation systems represent a major breakthrough for their simplicity, cost-effectiveness, and technical reliability.The Tension Leg Platform (TLP) and the worlds largest Subsea Production System (SPS) are secured to the seabed without the use of piles. Skirted gravity systems are used which have minimized construction costs and installation time. Stricter requirements to removal can more easily be met with these systems. Future subsea wells in the area are planned to be fully protected within subseabed silo structures, minimizing impact on and from the fishing activities.

Hans Peter Christophersen

Foundation Performance Monitoring


Foundation Behaviour of Gullfaks C

The Gullfaks C platform is a heavy concrete gravity structure resting on soft soils. To resist the design storm loads both a solution with circular concrete walls (skirts) to great depth and active improvement of soils were necessary. This paper describes the solution and gives some results from a continuous monitoring programme of the structure and foundation.

Tor Inge Tjelta

Foundation Monitoring on the Hutton Tension Leg Platform

The Hutton tension leg platform (TLP) was installed in the North Sea in the summer of 1984. This structure was a world first in adopting the tension leg principle for a full scale floating oil production platform. The platform is held in place by 16 vertical tension legs, four at each corner, which are attached to foundation templates piled into the sea bed. The tension legs are held taut by the inherent buoyancy of the platform.This type of loading places unique conditions on the foundations as they are required to work almost exclusively in cyclic tension. As design methods were unproven for these conditions in the early 1980s, higher factors of safety were adopted in their design than would have been used for the equivalent foundation under compression. A monitoring system was installed to identify any deformations of the foundation system.This paper describes briefly the salient features of the monitoring system and summarises the data obtained between 1984 and 1991. Reference is also made to recent reanalyses of the piled foundation, which have been performed using a more advanced Effective Stress method.

P. J. Stock, R. Jardine, W. McIntosh

Magnus Foundation Monitoring Project — Summary of Static and Dynamic Behaviour

The Magnus Foundation Monitoring Project (FMP) was a major instrumentation project involving the measurement of the actual performance of the foundation system of one leg of the Magnus platform under service conditions. This paper describes some of the methods used in analysing the data collected during the project and summarises key aspects of the behaviour of the platform foundations.

D. E. Sharp, R. M. Kenley

Piling Research


Large Diameter Pile Test Programme — Summary

This paper summarises the large diameter pile test programme undertaken between 1985 and 1988 in the UK on behalf of a consortium of oil companies. The purposes of the work were to improve the axial capacity design method for offshore sized pile foundations and to gain more insight regarding the fundamental behaviour of long piles in cohesive soils. The test programme was recently reported in detail at a specialist conference (Clarke, 1992).

J. Clarke, M. D. Lambson

Response of Piles in Soft Clay and Silt Deposits to Static and Cyclic Axial Loading Based on Recent Instrumented Pile Load Tests

The Norwegian Geotechnical Institute (NGI) has carried out an extensive series of static and cyclic axial load tests on well instrumented piles, respectively in a soft plastic clay deposit at Onsøy (Norway), a silty clay deposit at Lierstranda (Norway) and a mainly clayey silt deposit at Pentre (UK). Three types of test piles were used. Test piles type A were a closed-ended 219 mm o.d. steel pipe pile with length of 10 m. They were driven through cased boreholes to tip penetrations of 15 to 37.5 m. Test Pile B was a tubular open-ended steel pile with o.d. of 812 mm driven to tip penetration of 15 m. Test Pile C, only used at Onsøy, was of the same type as Pile A, but was 30 m long and driven to 35 m tip penetration.Each pile was subjected to a series of static (monotonic) and cyclic axial load tests after the excess pore pressures generated during pile installation had essentially fully dissipated. The paper summarizes the main aspects of the test results and how these tie in with various theoretical models and design methods and some previous pile tests in similar clay deposits.

K. Karlsrud, B. Kalsnes, F. Nowacki

Factors Affecting the Shaft Capacity of Displacement Piles in Clays

There has been concerted effort in recent years to investigate the fundamental processes that govern the behaviour of displacement piles in clay soils. Attempts have been made by various research organizations (e.g. MIT, NGI, Oxford University, and Imperial College) to measure the effective stresses acting on pile shafts using high quality instrumented model piles. In addition, a joint industry group led by British Petroleum has tested two large diameter (Ø762mm) instrumented driven piles at sites within the UK (Mullis, 1992).

A. J. Bond, R. J. Jardine, B. M. Lehane

Assessment of an Effective Stress Analysis for Predicting the Performance of Driven Piles in Clays

Researchers have advocated systematic analyses, which model changes in effective stresses and soil properties through successive phases in the life of a pile, as a rational method for understanding the factors which control pile performance. Work at MIT has included the development of analytical models which simulate soil disturbance effects associated with pile installation (Strain Path Method), and constitutive models (e.g., MIT-E3) which describe the effective stress-strain behaviour of normally and lightly overconsolidated clays (OCR ≤ 4) through successive phases in the life of the pile. This paper summarizes the role of these analyses in predictions of pile shaft behaviour. The results illustrate the effects of soil properties, mode of pile installation and other factors affecting the limiting skin friction which can be mobilized at the pile shaft. Predictive capabilities and limitations of the proposed ‘objective analysis’ are reviewed based on comparisons with high quality field data measured by the piezo-lateral stress (PLS) cell and by instrumented model pile tests.

A. J. Whittle

Shaft Friction of Piles in Carbonate Soils

The paper describes the results of an investigation into the behaviour of friction piles in carbonate soils prompted by problems at the North Rankin site, and following on from work by Coop (1990) on the fundamental mechanics of these soils. Firstly a series of shear box tests was conducted which was aimed at examining the behaviour of the soil at the pile interface. These were followed by a short series of triaxial tests which confirmed that the mechanics of the soils were qualitatively similar to those tested by Coop. The in situ states of the Rankin soils were then examined within a critical state framework, correlating the soil state at any particular depth with published pile test data for the site. It is suggested that the flexibility of the pile may be an important influence on the capacity of steel piles. This may be the result of progressive failure, as the steel interface shear box tests revealed brittle strain softening behaviour.

M. R. Coop, J. D. McAuley

Friction Coefficients for Piles in Sands and Silts

The results from a series of direct shear interface tests on a range of cohesionless soils are presented. The tests used steel interfaces with properties comparable to those of industrial piles and investigated the influence on the shearing resistance of relative density, mean particle size and stress level. These results are compared (and seen to be in good agreement) with measurements obtained using a heavily instrumented displacement pile installed in a medium dense sand. Some of the more important factors affecting the friction coefficients developed by piles in cohesionless soils are identified and their implications for design are discussed.

R. J. Jardine, B. M. Lehane, S. J. Everton

Design Criteria


Offshore Experience with Laterally Loaded Piles

The soil reaction for laterally loaded piles is generally determined by the methods described in API Recommended Practice 2A. The number of offshore platforms currently installed worldwide is testimony to the success of these methods. These design methods are based, for reasons of cost, on small diameter piles. Large diameter piles are rarely instrumented for lateral capacity, and such data as are available from jacket piles are subject to axial as well as lateral loading. Pile dead- man anchors used for pipeline initiation are subject to lateral loads — depending on pipe size — in the range 10–100 tonnes, and constitute a data source for confirmation of design data at high loading. This is particularly true for short piles where the pile toe is allowed to move so that any permanent set will be a function of soil plasticity rather than pile elasticity. This Paper presents a comparison between theoretical and observed pile performance under these conditions.

R. Martin, E. Burley

Analysis of Long Term Jack-Up Rig Foundation Performance

Jack-up rigs are now being considered for use in deeper waters and more hostile environments, and for long term functions in hydrocarbon development projects. Much work has been undertaken over the past few years to better understand the, performance of jack-up rig foundations. This paper discusses some of the important aspects considered when evaluating long-term jack-up rig stability.

Derek W. F. Senner

Novel Foundation Concept for a Jacket Finding Its Place

A major departure from traditional foundation practice for steel jackets has been developed based on the experience with skirts for huge concrete gravity platforms. The method, using plate-skirt foundations or perhaps better described as “skirted mudmats” or skirtpiles, will provide bearing capacity for compressive loads in the same way as gravity structures. Overturning uplift forces will be resisted by suction. This is a new concept in sand, and has been verified by offshore field tests and onshore laboratory tests. This is a description of the philosophy and efforts behind the development. The principles will be applied for the Europipe riser platform at Norwegian block 16/11.

T. I. Tjelta, G. Haaland

A Review of the Design and Certification of Offshore Piles, with Reference to Recent Axial Pile Load Tests

Design of offshore piling should commence with a carefully planned, integrated site investigation including geological appraisal of the immediate location, a shallow geophysical survey and an adequate number of detailed geotechnical boreholes. Brief guidelines for such investigations are presented. Certification includes independent interpretation by the certifying authority of the data resulting from such site investigation, to derive appropriate design parameters for use in its foundation and structural analyses.

R. Hobbs

The Evolution of Offshore Pile Design Codes and Future Developments

This paper reviews the evolution of offshore codes, and in particular API RP2A, in relation to axial pile capacity design procedures and comments on the need for, and scope of, future changes. It also discusses progress towards a new international code. The conclusion is that the API system has shown itself responsive of industry needs and that any new system should try to emulate this. With regard to design practice, the paper concludes that this has improved significantly over the years for piles in clay, but much less so for piles in sand.

F. E. Toolan, M. R. Horsnell

Closing Address

The papers to this Conference cover a very wide range of topics and experience. It is clear that a lot of careful thought has been given to the choice of topics. To pick up a major theme of the Conference — the programme and content has been carefully integrated to give a coherent and exceptionally complete coverage of the whole field of offshore site investigation and foundation design, installation and performance. In view of this, to attempt to summarise the Conference would be an impossible task and would only diminish its value. All I can offer are a few very subjective and personal reflections.

J. B. Burland
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