Interactive Hydrodynamic/Structural Analysis for Refloating a Very Large North Sea Structure: Maureen Alpha Platform
Authors: Denise J-P , Tilling G.M. 
Published: December 2007

The Maureen Platform in the UK Sector of the North Sea can be refloated since it is a gravity base structure with large oil storage tanks as legs.

An engineering study based on structural/hydrodynamic computer simulations has been conducted to confirm the feasibility of refloating the platform, without damage and without removing any topside equipment, prior to towing it away and installing it at another site.

Computer analyses have been performed with a model exhibiting both structural and naval architectural features to allow representation of all relevant aspects of hydrostatics, dynamics, structural behaviour, soil structure interaction, also specific features such as variable water ballasting and pressurization inside the tanks.

A refloat simulator has been developed to perform computer based animations of the platform behaviour and display visually all platform parameters (forces, stresses, deformations) from the results of the computer analyses. It provides useful information for overall understanding of the refloat processes, risk assessment and personnel training.

The study has confirmed that the Maureen platform can indeed be refloated safely and is providing valuable guidance for the management of the refloat.

Introduction

The Maureen Platform has been producing in the North Sea since 1983, operated by Phillips Petroleum Company United Kingdom Limited. It is a steel gravity base structure with three large oil storage tanks as legs. With the field now coming to the end of its life, plans are under way to refloat the platform for reuse on another field.

The platform was installed in a sequence of marine operations: float-out of its construction dry dock at Kunterston, mating with its topsides offshore at Kishorn, both in Scotland, tow to the Maureen site and positioning on the sea bed over a pre-installed template. These operations were made possible by the unique design of the platform sub-structure which incorporates a number of ballastable compartments and piping systems which were used to control the attitude of the platform at all phases of its installation.

This design means that the refloating of the platform, with the exception of the close positioning of the structure over the subsea template, can be considered as the reverse of the installation operations. A previous study (ref. 1) has already confirmed that the platform can be refloated without removing any tops ides equipment; it has also identified some critical issues which require additional attention, e.g. pressurisation with water under the bases to extract the tanks from their soil foundations and naval stability when the platform is raised to the surface. In addition the structural integrity of the structure also needs to be verified at each stage of the refloating operation and all potential risks must be identified and assessed.

A numerical study presented in this paper addresses these issues. The other objectives of the study are to help understand the behaviour of the platform during refloating, to identify the most suitable set of operating conditions and to minimise any risks.

This is accomplished by means of numerical simulations performed with:–an overall finite element model of the platform–local finite element models of specific joints of the sub-structure, and–a refloat simulator which serves to visualise the three dimensional behaviour of the platform as calculated by the overall model.

These tools and some of the predicted results are presented below. The predictions demonstrate that the Maureen platform can be refloated and show how best to achieve this operation.

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