One-Piece Installation Of A 7,000 Ton Deck Structure On The Ninian Central Platform
Authors: Wortman Edward J. , Dary John B. , Murphy Samuel M. 
Published: December 2007

A 7,000 ton steel truss deck unit was installed on the world's largest concrete oil platform in 1977 using techniques and equipment from both the offshore industry and the bridge building field. Comment is included on the potential for one-piece superstructure unit installation on future U.S. West Coast platforms.

Introduction

The Ninian Central platform is an offshore drilling and production platform being built for installation in the northern North Sea. Figure 1 is an artist's impression of the completed platform, which consists of a concrete gravity base structure topped by a superstructure made up of drilling, production and support system modules. The modules are supported by a deck structure that is the subject of this paper.

As shown in Figure 2, the deck structure basically consists of four parallel main longitudinal trusses tied together with six transverse trusses. Truss members are tubular, with diameters from 30 inches to 54 inches and with wall thicknesses up to two inches. The truss system is supported vertically by four 100 inch and four 72 inch diameter columns which land on the outer wall of the concrete structure. Horizontal loads are taken from the deck into the inner shaft of the concrete structure through shear-carrying weldments. Total weight of the deck structure is 7,000 tons. (Note: all "ton" figures in this paper refer to 2,000 pound tons.)

Chevron Petroleum (UK) Limited, as operator for the Ninian Field group, manages the work of several prime contractors on construction of the Central prime contractors on construction of the Central platform. Howard Doris is prime contractor for design, platform. Howard Doris is prime contractor for design, construction and installation of the concrete base structure and the steel deck structure. Howard Doris built the concrete base structure in and near its own yard at Loch Kishorn on the west coast of Scotland. Installation of the deck structure and facilities modules was done at a semi-sheltered deep water site in the Inner Sound of Raasay, a few miles from Kishorn.

The steel deck structure was fabricated by McDermott Scotland, a Howard Doris subcontractor, at McDermott's Ardersier yard on the east coast of Scotland. Since the deck was installed on the west coast in the Sound of Raasay, an exposed ocean tow around Scotland was an unavoidable step in the deck installation procedure. procedure. Howard Doris set up a Deck Installation Group to plan and execute, under Chevron's close direction, the deck installation operations from load out at Ardersier to attachment to the concrete structure at Raasay. Howard Doris brought in Tokola Offshore to collaborate on the planning and execution of the deck work. Tokola and Howard Doris retained Earl and Wright of San Francisco as consulting structural engineers and naval architects on the project.

SELECTION OF DECK INSTALLATION SCHEME

Several general schemes were proposed for installing the deck on the concrete structure. These can be outlined as follows:Assemble in place with derrick ship in twoor four major piecesSet in place in one piece from bargesTwo barge catamaran tow from fabrication yard