Minimum weight concrete platforms
Authors: Ridley T. , Malone A.W. 
Published: December 2007

SYNOPSIS

A design computer model has been devised which, for given functional requirements and location site criteria, determines the dimensions and structural thickness of the lightest cuboid caisson for a chosen draught and adequate floating stability of the platform during submergence. The deck and towers assumed in the calculations are chosen from a range of standard decks and towers which have previously been designed for adequate performance to suit present design codes.

This model permits sensitivity studies to be carried out to test the effect of the parameter of interest on platform weight, and therefore cost. The purpose of this platform weight, and therefore cost. The purpose of this paper is to illustrate how this model can be used to paper is to illustrate how this model can be used to investigate the effect of a given variable on caisson geometry. In this study the variable is the chosen draught dictated by the water depth in the tow-out channel from the construction site. The water depth at location is fixed at 150 m LAT. Minimum weight platforms have been calculated for a range of chosen draughts between 26 and 40 m. The geotechnical characteristics of these platforms have been calculated.

The results of the model study arc presented graphically and as a rough test of its validity, the characteristics of a number of fully designed platforms described in available literature are superimposed. The agreement is reasonable which allows a number of general conclusions to be drawn, subject to the stated assumptions, on platform weights and geotechnical characteristics.Below about 25/27 m tow-out water depth, caissons become impractically large.Between 25/27 and about 30 m tow-out water depth, platforms will require the additional lift provided by a buoyancy jacket to be cost competitive.Above about 32 to 34 m tow-out water depth, the geotechnical characteristics of the sea bed site will control caisson size, for the minimum weight caisson (based on floating criteria) will be too narrow for Typical North Sea sites.Above about 37 m tow-out water depth further weight saving is marginal.

Introduction

As consulting engineers Ove Arup and Partners Scotland are designers of a family of concrete oil production platforms to operate in the North Sea. production platforms to operate in the North Sea. This experience in the design of concrete gravity platform schemes has led to the development of a platform schemes has led to the development of a computer procedure which carries out rapid preliminary platform design for given functional and site criteria. platform design for given functional and site criteria. The procedure gives, within a couple of days, a solution which is likely to be confirmed as feasible by more complete analysis at the derailed design stage. The object is the elimination of costly and timewasting schemes which would eventually be ruled out by detailed analysis.

This 'Rapid Design Procedure' (RDP) combines the following analyses:For chosen draught, metacentric height and payload the minimum weight caisson is derived, the most payload the minimum weight caisson is derived, the most suitable deck and tower being chosen from a standard range.For a range of wave periods the most onerous combinations of environmental loads on the structure and foundations are calculated.The geotechnical requirements of the derived platform are calculated for the limiting sliding and platform are calculated for the limiting sliding and overturning stability cases. Assuming the worst tolerable soil conditions based on these stability criteria platform motions are determined under extreme storm conditions and for operating sea states by dynamic analysis. For the installation case skirt lengths are sized on penetration and hydraulic stability criteria and caisson-soil pressures are calculated for given bed topography.

Available water depth is a critical factor in the choice of suitable construction sites for concrete platforms, During towing the draught of the structure must be such that, with allowance for tidal variation during the time of passage, the underkeel clearance is always adequate. The passage, the underkeel clearance is always adequate. The available water depth in the tow-out channel will influence the design of the platform, flatter caissons being required for shallower channels. In view of the interest in platform sites throughout the Scottish and continental coast line the RDP was used to determine the influence of chosen draught on platform weight and geotechnical characteristics.

Where the available water depth is limited, considerable advantage can be found in certain cases from the introduction of a 'buoyancy jacket'. This is provided in the form of a number of additional concrete cells around the caisson which reduce floating draught, and which are completely flooded - or removed - after initial tow-out. In such a case the minimum weight caisson is determined for an unassisted draught greater than will apply with the use of the 'buoyancy jacket', with significant weight saving.

Technical brief

The following is the outline technical brief assumed in the RDP for this study:Crude storage facility 1,000,000 barrelsNumber of legs four