Design of the Draugen Topsides for the Effects of Gas Explosions
Authors: Cockbain G. , Jermstad A. , Stenhouse D. 
Published: October 2008

The paper describes the approach taken in the design of the Shell Draugen Platform integrated deck topsides to account for the effects of possible gas explosions. The principal elements are:–problem review and definition–explosion simulation analyses by the FLACS code–optimization of topside configuration–derivation of time dependent design loadings–application of non-linear dynamic structural analysis.

INTRODUCTION

Until recently, the effects of a partly confined gas explosion on a platform structure and internals have generally been assessed in a rather simplified manner due principally to inadequate methods for calculating overpressures in large compartments with complex geometry. The recent availability of the advanced numerical tool known as FLACS 1 (FLameAcceleration Simulator) has permitted the designers of the Draugen Platform topsides to follow a more systematic and realistic approach to optimizing layout and establishing time dependent design loadings.

The explosion pressure loadings calculated for many compartments are high compared to both normal operational design loads and to explosion design loads specified for earlier generations of offshore platform designs.

To avoid undue conservatism and subsequent economic consequence, whilst complying with the design acceptance criteria, special design techniques havebeen employed.

Non-linear dynamic structural analysis has been carried out for representative models of the structure in the time domain. Stresses, strains and deformations are calculated for the structural steel elements to demonstrate compliance with the acceptance criteria.

The overall sequence of events related to design for the effects of gas explosion is illustrated in Figure 1.

THE DRAUGEN TOPSIDES

The Draugen Platform will be installed in summer 1993 in 250m water depth on the Halten banken, offshore Mid-Norway. The 22000t operating weight topsides will be supported upon a concrete monotower Substructure (gravity base). Oil production capacity is 110,000 BPD. Associated gas will be re-injected to the reservoir during the first six years, after which it will be exported by pipeline. Up to 10 wells can be drilled within the single water flooded concrete shaft.

The topside which has been designed as a fully integrated deck, is shown in Figure 2.

Asystem of four main girders runs in the east-west longitudinal direction and seven girders in the north-south transverse direction. The deck is supported by the concrete monotower at the four intersections of main girders L200 and L300 with main girders T300 and T500. Plated bulkhead construction is used for all seven transverse main girders and for longitudinal main girders in the non-hazardous quarters and utility areas west of the central wellbay and drilling facility areas. In the process areas (Separation and Compression), located to the east of the platform, the longitudinal main girders are of open truss construction.

Area classification boundaries are compatible with plated bulkhead locations, thus avoiding additional fire and blast walls.

The individual decks, within the compartments formed by the intersecting main girders, are supported by a system of longitudinal deck beams spanning between the transverse bulkheads. In the hazardous classified areas deck plating is replaced by floor gratings to permit explosion venting down towards the sea.