This paper gives a summary of the design basis and requirements of the Norwegian Field Development, Snorre B hull. It describes the fabrication of the hull at the Dragados Offshore yard in Spain, and highlights the critical and important features in the design. It describes the method used, the main elements in the hull and the shape of the final product. The paper includes a description of the fabrication sequence, load out, launching and towing to Norway. Experiences gained during fabrication will be discussed and summarised. A full description of the fabricated sections will be performed and this will be visualised with photos and drawings from the yard.
The Snorre B platform is a floating drilling and production unit (SSPV) consisting of a steel hull and deck structure. The hull consists of four "square rounded" columns supporting the deck. The columns are connected in the corners at the ring pontoon, which is rectangular in shape.
The deck structure has two main truss-lines in both the transverse and longitudinal directions. In addition to these trusses, there are also two major transverse bulkheads. Each corner of the deck consists of a "rounded square" box. There is a main deck and a cellar deck including a double bottom structure. Fig. 1 shows the floater, hull and deck.
Fig. 1. Snorre B deck and hull
Most of the design work for Snorre B was initiated in 1998. Key factors accounted for in the structural design basis are:
Functional and Operational Requirements
Snorre B is designed for a 100 years storm condition for the location in the North Sea at a water depth of 350 metres. The extreme significant wave height for the area is 15.5 metres ULS, respectively 18 metres PLS. This is combined with a storm wind of 41 m/s. The design has an allowance as shown below:
Margins: Mai 1999 August 2001
Company: 3,000 tonnes 1,570 tonnes
Contractor: 1,850 tonnes 1,170 tonnes
The design life is 20 years. Factors are used for the fatigue life calculations.
Outside hull below Splash zone: 2
Splash zone: 3
Above Splash zone: 1
Inside hull: 1 and 2
Deck general: 1
Critical areas not possible to inspect: 10
The deck shall accommodate both drilling (RAM RIG), production equipment and aluminium living quarters.
Snorre B has 16 anchor lines which combine wire and 145 / 137 mm chain. These are connected to suction anchors.
Based upon experience gained from more recent platform development in the North Sea, design criteria and rules were established for the structural design.
Det Norske Veritas Rules for Mobile Offshore Units (DNV MOU) including supplementary requirements of Norwegian Petroleum Directorate / Norwegian Maritime Directorate and NORSOK, among other, were used as basis for design.