Tor Inge Tjelta

Skirted spud-cans — Extending operational depth and improving performance

Abstract A conventional spud-can may provide some moment fxity, but the level of this fixity is uncertain, in particular for a spud-can with shallow embedment, which is exposed to scour and erosion. Furthermore, the moment fixity is, in general, low, in particular on sands. If an increased, stable moment fixity can be documented, it will be possible to extend the qualified water depth range for a given jack-up unit at given environmental conditions. This paper describes how an existing spud-can may be equipped with skirts, and shows that this modification will improve the moment fixity substantially, compared to a conventional spud-can. In a sand, the increased moment capacity will be due both to an increased effective foundation area and due to a suction capacity in the sand at loads of short duration. This suction capacity has been documented both through model tests and finite element analyses. A case study is included, quantifying the improved overturning moment capacity of a skirted spud-can.

RELIABILITY ANALYSIS OF A PILE DESIGN

Abstract This paper presents a stochastic pile design for a wellhead platform designed for Danish waters. The relevant failure modes for both ultimate and serviceability limit states are defined. Failure modes for the pile as well as failure in the soil are thereby considered. Reliability criteria are suggested based on requirements formulated by the Nordic Committee for Building Structures. An uncertainty modeling for the environmental conditions, the response and the soil stiffness and strength is suggested. The choice of distribution types by and large follows that suggested by the Nordic Committee for Building Structures. The effects of spatial variation of soil properties are assessed both in terms of varying positions of layer boundaries and in terms of spatially varying soil properties. The model uncertainty in the soil strength for axial compressive and tensile loading is quantified. The most important uncertainties are due to uncertainties in the extreme environmental parameters and load model parameters, and for the soil failure modes also due to the model uncertainty. The uncertainties in soil stiffness, strength and spatial variation play a very small role in all failure modes.

Development and Structural Design of the Bucket Foundations for the Europipe Jacket

The installation of the Statoil Europipe 16/11 E jacket with bucket foundations in the summer of 1994 represent the dawn of a new era for jacket foundation engineering. With traditional piled foundations giving way to the novel alternative termed ``bucket`` foundations, the installation of the Europipe jacket provides the first real opportunity to examine properly the benefits of this solution. The bucket foundations are circular plates with skirts installed at the base of each jacket leg, and as such they represent a radical departure from traditional piled foundation solutions. The bucket acts as gravity foundations with the skirts increasing the vertical compression and shear load capacity as well as providing considerable tension load capacity. This paper outlines the development of the bucket foundations, describes the structural design aspects and compares the results from the structural monitoring system installed on the jacket with the installation structural analysis results for the foundations.

How understanding past landscapes might inform present-day site investigations: a case study from Dogger Bank, southern central North Sea

The integration of geophysical and geotechnical datasets acquired during a site survey for the Dogger Bank wind farm has enabled a new litho- and seismo-stratigraphy to be established. Although previously believed to be a relatively simple “layer-cake”, the data reveal that the sedimentary sequence within the foundation zone includes a complex series of buried landscapes with implications for both foundation siting and design. The most significant is a Weichselian glacially derived landscape dominated by a large thrust-block moraine complex buried beneath a thin Holocene sequence. This glacial landscape profoundly affects the structure and physical properties of sediments within the foundation zone due to locally intense glaciotectonic deformation and the occurrence of sub-aerially desiccated horizons recording fluctuating palaeo-climatic conditions. Understanding these landscapes, coupled with the geophysical and geotechnical data, enables the development of a predictive “geo-model” that may be used to target areas of uncertainty, reducing the requirement for boreholes (over Cone Penetration Tests) at every potential foundation location.

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.