Concrete Foundations for TLP's
Authors: Karal Karel , Hermstad J.M. , Vigander Kjell , Skjong Jan , Pedersen Øivind 
Published: October 2008

Two major concepts of concrete foundations for Tension Leg Platforms are discussed, i.e. gravity foundation and gravity-embedded foundation. The latter concept is the preferred solution in a majority of applications. Four types of the gravity-embedded foundation are described. One of these has been successfully installed for SNORRE TLP in 1991. The other three types have resulted from further development and optimization, suitable also for great water depth,

INTRODUCTiON

Tension Leg Platforms (TLP) represent a viable solution to the demands of today's offshore oil and gas industry, aimed at producing hydrocarbons from reservoirs in deep water. A most vital element in the TLP design is the method chosen for anchoring the tethers to the seabed. During severe storms the load of such a tether system may exceed 50,000 t and after seconds will drop to a small fraction of this. The sucessful application of a TLP concept is literally dependent upon a safe foundation.

A concrete foundation concept has been applied for a TLP - the largest ever built - at the Snorre field in the North Sea. The Snorre foundations were successfully installed in the summer of 1991 in a depth of 300 m.

This paper focuses on the gravity-embedded concrete foundation. It discusses how the design depends on the static and dynamic tether tension, which variables enter the optimization process, and what characterizes the optimum foundations in various conditions. Some typical foundation designs are described.

TYPES OF CONCRETE FOUNDATIONS

Two types of foundation are considered:Grmdly iQUndatiQil, installed QO the seabed and designed to resist all tether tension by its own weight.Gravity-embedded foundation, provided with cylindrical elements - skirts - that are penetrated into the seabed, and designed to resist the tether tension through a combination of its own weight and the interaction between the structure and seabed.

The gravity foundation is advantageously applied in conditions like very hard or rocky seabed where penetration of skirts is not possible. The latter foundation type, suitable for softer soils, is competitive due to cost and technical advantages such as minimum settlement and precise levelling.

The gravity-embedded foundation - the most common of the two - is suitable for the potential TLP sites studied for the Norwegian Continental Shelf and the Gulf of Mexico. Although the principle of this design remains unchanged different configurations have been developed for a range of tether loads and water depth at the installation site. The design aspects are briefly discussed in the sub-section 'Optimization'.

PRINCIPLES OF GRAVITY-EMBEDDED FOUNDATION

Tether loads on the foundation

Tether tension, T, is composed of two main components, I.e. sustained and oscillating (dynamic) tension. The sustained tension, T., calculated by the design engineer to meet requirements of acceptable motion characteristics of the TLP. Ts occurs when the TLP is free from loads by wind, current and waves. The difference between the actual tether tension and the sustained tension is the oscillating (dynamic) component of the tension, T.