Abstract

The Elf TCP2 platform at the Frigg Field was successfully installed on a very dense sand deposit where the sea floor slope gave a difference in elevation of I m over the base area. Experience obtained during penetration of dowels, skirts, and domes into the sea bed are summarized. Recorded penetration resistances are correlated to data penetration resistances are correlated to data obtained from the soil investigations, and empirical correlation coefficients are given.

Introduction

The Condeep TCP2 is the 3rd gravity structure installed for Elf-Norge A/S at the Frigg Gas Field in the North Sea. TCP2 stands for Treatment and Compression Platform No. 2. Previously installed gravity structures at this field are:

  • the Doris platform referred to as CDPL (Concrete Drilling platform referred to as CDPL (Concrete Drilling Platform No. 1) and

  • the Sea Tank structure Platform No. 1) and

  • the Sea Tank structure referred to as TPI (Treatment Platform No. 1).

The distance between the TCP2 and the TPI is approximately 50 m, and the platforms are connected by a bridge, Fig. 1.

Since the Ekofisk Doris Tank was placed in 1973, the number of gravity structures in the North Sea has increased relatively fast. As of July 1978 the total number installed is thirteen.

A detailed description of a typical installation has been given by Eide et al. for the Condeep Brent B for Shell/Esso. Other papers related to the installation of gravity platforms have been by Allcock, Boons, Fjeld et al., Hoeg et al, and Werenskiold.

The main concerns for the installation of the Frigg TCP2 were:

  1. the marine operation of positioning the structure very close to another positioning the structure very close to another platform,

  2. maintaining verticality during platform,

  3. maintaining verticality during penetration of skirts into the sloping sea floor, penetration of skirts into the sloping sea floor, and

  4. keeping control of the contact stresses on the bottom domes when penetrating the very dense subsoil.

Norwegian Contractors were the general contractors for the project. The platform was built in Aandalsnes on the West Coast of Norway south of Trondheim. Norwegian Contractors were responsible for the design, the construction, and all maritime operations including installation and underbase grouting at the field. The design of the steel deck was sub-contracted to Kvaerner Engineering, and the production was undertaken by Aker Group/CMP.

Ballast system and mechanical installations were designed by Kvaerner Engineering. Dr. Olav Olsen and Ing. Chr. Groaner A/S served as structural consultants for the concrete structure. The Norwegian Geotechnical Institute acted as geotechnical consultants and was responsible for the installation phase instrumentation. The Central Institute for Industrial Research was consultant for the data acquisition system. The River and Harbour Laboratory at the Technical University of Norway determined the environmental loading data. Det norske Veritas certified the platform on behalf of the Norwegian Petroleum Directorate. Noble Denton and Associates acted on behalf of the insurance interests.

STRUCTURAL DATA

The Condeep TCP2 is a reinforced and post-tensioned concrete gravity structure. The concept post-tensioned concrete gravity structure. The concept was originally developed by A/S Hoeyer-Ellefsen.

The base structure, or caisson, is designed with 19 closely packed vertical concrete cylinders (cells) with outer diameter 20 m, (Fig. 2). The height of the cylinders is about 40 m. The top and bottom of the cells are dome shaped. Three of the cells are extended upwards as shafts to support the deck structure some 30 m above sea level. The shaft above cell No. 1 is referred to as the utility shaft. All valves, pumps, and generators for the ballast system are placed in this shaft.

The area of the foundation slab was selected so as to give sufficient foundation stability during maximum wave loading conditions.

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