Block Erection Method of Maui "A" Tower

Abstract

ABSTRACT This paper introduces the block erection method used to assemble the Maul 'A' Tower an offshore platform located 21 miles off the Taranaki coast of New Zealand's North Island. Descriptions are given herein ofthe block erection concept applicable to the various components of an Offshore platform,the materials used in the main sections of the tower, andthe assembly procedures. The principal characteristics of the tower and the shipyard where it was fabricated and assembled are also outlined, as are the tower flotation procedures and the benefits of the block erection method over the conventional panel erection method. INTRODUCTION Shell BP and Todd Oil Services Limited (SBPT) discovered the Maui field some 30 miles off the South Taranaki coast in early 1969 on a prospecting license area of 11, 030 square-miles. The field is being developed by Maui Development Limited, a joint venture company operating on behalf of the New Zealand Government (as Offshore Mining Co. Ltd.), 50%; Shell (Petroleum Mining) Co. Ltd., 18.75%; BP (Oil Exploration) Co. of New Zealand Ltd., 18.75%; and Todd Petroleum Mining Co. Ltd., 12.5%. On behalf of Maui Development Limited, Shell BP and Todd Oil Services Limited is the operator responsible for design, procurement, construction and operation of the platforms, submarine pipelines, and onshore processing facilities. Project plans called for a gas and oil drilling and production platform to be installed at the site in a water depth of 108 meters. In April 1974 our company was awarded the contract by SBPT to build the tower support structure for the platform. Seventeen months later, in September 1975, the tower was delivered to the client. Besides being one of the world's largest structures of its kind, the Maui 'A' tower is also the world's first self-buoyant tower of this type and for this water depth ever erected. Further, delivery involved the world's longest tower tow, some 9,200 km. The principal particulars of the tower are: height, 131.674 m; breadth (leg center to leg center), 21.366 × 48.768 m (top), 47.854 × 48.768 m (bottom); leg diameters, 6.706, 4.877 and 1.829 m; maximum wall thickness, 95.3 mm; total weight, 6,400 metric tons (tower, 5,174 tons; accessory structures and systems, 831 tons; piping systems, electrical equipment and wiring supports, 395 tons). High-strength steel produced to DIN specifications are used in the principal members of the tower, while a steel produced to satisfy DIN specifications for mechanical strength in the through thickness direction is used in the nodes. The tower was assembled at our Tsu Shipyard (refer to Fig. 1) by the block erection method the first time this method has been used to assemble an offshore structure. This was the principal factor that permitted the completion of the tower in a record time of 17 months.