THE ARUP JOURNAL AUTUMN 1989 Vol.24 No.3 Autumn 1989 Contents Published by Ove Arup Partnership THEARUP 13 Fitzroy Street, London W1P 680 Editor: David Brown Art Editor: Desmond Wyeth FCSD JOURNAL Deputy Editor : Caroline Lucas Ravenspurn North concrete 2 gravity substructure, by John Roberts Rank Xerox, 12 Welwyn Garden City, by Ian Gardner and Roger Johns Les Tours de la Liberte, 17 by Bernard Vaudeville and Brian Forster Matters of concern, 20 by Jack Zunz Front cover: Ravenspurn oil platform (Photo: John Salter) Back cover: View through pod windows at Rank Xerox, Welwyn Garden City (Photo: Jo Reid & John Peck) Ravenspurn North concrete gravity substructure John Roberts Significance Two concrete gravity substructures (CGSs) 1. Impression supporting production decks have been of Ravenspurn installed in the North Sea this summer. North central processing In June the Gullfaks 'C' platform was in­ platform after stalled in the Norwegian sector. At towout the installation of structure weighed 850 OOO tonnes - both decks. reputedly the largest object ever moved by man. At the beginning of August the Ravenspurn North concrete gravity sub­ structure, weighing some 28 OOO tonnes, was installed 80km off Flamborough Head in block 43/26 of the UK sector. It is perhaps surprising that, of the two plat­ forms, the Ravenspurn North CGS is of greater significance to the oil industry. In the UK over the last decade conventional wisdom has held that a steel jacket is the most economic substructure for a fixed plat­ form . In Norway, on the other hand, where there is a more limited indigenous steel­ making industry, the use of concrete gravity substructures has been encouraged. As a result, CGSs have been installed in the Norwegian sector at the rate of about one per year over the last decade. Most of these are the well-known Condeep design developed The main reasons why concrete gravity the topside equipment has been installed by Norwegian Contractors. substructures are now comparatively offshore in comparatively small lifts. A CGS was selected for Ravenspurn North cheaper than steel jackets stern from design. (2) The Ravenspurn North CGS will support because the operator, our client, Hamilton two separate decks, of which only one will In the case of the Ravenspurn North CGS, be installed initially, whereas all previous Brothers Oil and Gas Ltd., was convinced the principal factors were as follows: that the design developed was cheaper than designs supported only a single deck. Thus the steel alternative. That decision has (1) The decks can be installed offshore in a the cost of a second separate support struc­ stimulated considerable interest among single lift using a semi-submersible crane ture has been avoided. other operators, many of whom are now re­ vessel after emplacement of the CGS on the (3) The CGS has been built entirely in a dry viewing the technical and economic merits of seabed. Previous CGSs have been mated dock rather than partly as a floating structure 2 alternative substructures. with their decks inshore prior to towout, or - which has been Norwegian practice. Ravenspurn North CGS Gullfaks 'C' Platform FORMS OF SUBSTRUCTURE Environmental conditions Environmental design conditions Jackets Depth of water at field Depth of water at field : 217m Piled steel jackets are by far the most common form of support structure for (LAT - lowest astronomical tide): 41.6m Height of design wave offshore platforms throughout the world . (100-year return period): 30m Height of design wave They consist of a tubular steel (100-year return period): 18.7m Dimensions spaceframe supported on tubular steel Associated period: 12.3-15.3 sees. Caisson size (base area): 16 000m 2 piles driven after the jacket is located on Surface current Caisson height: 87m a seabed. The word jacket was originally (100-year return period): 2.41 m/sec. Shaft diameter - base 28m adopted as the name for the steel Dimensions -top 14m template placed in the shallow water swamps of Louisiana as a guide for the Caisson size: 62 x 54.5m Shaft height above caisson: 162m Caisson height: 16m piled foundations which supported drilling Skirts - concrete: length 23m equipment. Today virtually all jackets are Shaft height above caisson : 37.5m Materials transported offshore on purpose-built flat Shaft diameter - base 11m Volume of concrete barges and installed either by launching -top 6m (grade C65): 246 000m3 from the end of the barge or by lift Materials Weight of solid ballast at installation using a semi-submersible Volume of concrete towout: 165 OOO tonnes crane vessel. 3 (grade C50): 9750m Weight of reinforcement: 80 OOO tonnes In operation, hydrodynamic forces Weight of reinforcement: 2750 tonnes Weight of prestressing: 3550 tonnes generated on the structure are Weight of prestressing (27km): 450 tonnes transmitted to the seabed soils through Weights and draughts the piles. The tendency for a jacket to Weight of steel skirts: 350 tonnes Final weight of CGS in air: 850 OOO tonnes Weight of steel deck overturn is resisted by tension in the piles Floatout draught on the upstream side and this load connections: 410 tonnes (skirts and base slab only): 13m condition normally governs the design of Weights and draughts Towout draught: 208m the piles. Final weight of CGS in air: 27 850 tonnes Deck weight: - operating 55 OOO tonnes The popularity of piled steel jackets can Floatout draught - at mating 48 OOO tonnes be traced to the first offshore (on 1.5m deep air cushion): 9m Schedule developments in the Gulf of Mexico. Steel Towout draught: 13.5m (design and build contract) rather than concrete was the material Main deck Contract award: December 1985 most familiar to the American engineers involved in developing offshore technology. Operating weight: 8500 tonnes Start of concept design : December 1985 Lift weight: 6000 tonnes The influence of American oil companies Completion of detailed design: October 1988 throughout the world is probably the main Compression deck Start of site construction: January 1986 reason why steel jackets remain Operating weight: 4000 tonnes Ready for towout: April 1989 predominant. Lift weight: 3250 tonnes Installation at field: May 1989 Gravity-based structures Schedule Project team Gravity-based structures (either steel or Start of concept design: March 1987 Client: Statoil concrete) resist overturning in the same Completion of detailed design: March 1988 Partners: Norsk Hydro manner as a simple pad foundation. To Start of site construction: May 1988 Saga Petroleum prevent uplift the width of the CGS base Ready for floatout: mid.July 1989 Engineering and construction: Norwegian must be such as to ensure the resulting Installation at field: August 1989 Contractors force from horizontal hydrodynamic loading, and vertical loads lie within the (Source: Nor.vegian Contractors) middle third of the base width . Apart from isolated examples such as the Royal Sovereign Lighthouse, concrete gravity substructures had not been used offshore until the early 1970s when a combination of factors created the need for a new design solution. At that time developments in the North Sea were at the forefront of offshore technology, requiring platforms in a deeper and more hostile environment than had been attempted previously anywhere in the world . In addition the platforms required very heavy topsides. At that time the largest offshore crane vessel could only lift about 1OOO tonnes and therefore a large number of relatively small lifts were required to install the topsides on a 217m conventional steel jacket. Concrete gravity substructures were designed to reduce the cost of offshore hookup by mating the CGS with a single-piece deck inshore prior to towout. The CGS and decks were constructed separately and the CGS ballasted down in deep water so that only the tops of the shafts were above the water. The deck was floated over the CGS on barges and the CGS deballasted 2. Ravenspurn North .. .. compared to .. ... Gullfaks 'C'. to achieve the mating. In this way decks of up to about 50 OOO tonnes have been (4) The structure was designed to be simple, the cost of the substructure supporting the installed. highly repetitive and aimed to disassociate production equipment on a platform typically Since the early 1970s concrete gravity the concrete from the more ephemeral pro­ represents less than 5% of the total field substructures have been automatically cess pipework which it supports. development cost, any 'innovative' design associated with this sequence of It comes as a surprise to the majority of civil has to show substantial savings compared construction and installation which has engineers that the oil industry does not with the alternatives, before operators can been adopted for all subsequent CGS readily appreciate the advantages1 of adopt­ accept the risks. designs installed in the Norwegian sector. ing concrete as a structural material in a Potentially, the Ravenspurn design offered The Ravenspurn North CGS is the first marine environment. In fact concrete is seen substantial savings but would not have been concrete structure in the North Sea to as a radical change which therefore involves adopted except for the determination of break with this tradition . greater financial and programme risk. Since Hamilton Brothers to prove the case. 3 4a. Casting the base slab to float out. (2) Installing the decks offshore reduced the N size of the CGS to the point where the entire CGS could be fabricated in any of the existing UK construction docks. Thus the • cost premium associated with completion of t the CGS while floating at an inshore location was avoided. (3) By providing a design which could be constructed entirely within the dry dock, the problem was reduced to one of conventional prestressed concrete construction. The in­ herent economy of the structure therefore depended upon providing a design which Middlesbrough• was simple and highly repetitive and could be constructed from readily available materials.