Innovative Concepts for Marginal Fields (Advanced Monotower Developments)

Abstract

ABSTRACT The braced mono tower provides a safe, functional and cost effective solutionfor topsides up to 500 tonnes, with up to 8 wells and standing in water depthsof up to 70 metres. It is both simple in concept and structurally efficient. The superstructure is supported by a single column which is stayed by threesymmetrically orientated legs. A broad mudline base is also provided to limitpile loads. The final concept offers complete protection to the risers andconductors from ship impact, as all appurtenances are housed within the centralcolumn. The basic design philosophy of the low intervention platform is to minimise theonboard equipment to that vitally needed to produce hydrocarbon. The concepteliminates the life support functions that on a normal North Sea platform cancontribute up to 50% of the topside dry weight. A system of Zero BasedEngineeringis used that ensures each item of equipment contributes more to theNPV of the platform than the fully built-upthrough life cost. This effectivelyeliminates the operator preference factor and the "culture" cost. 1. INTRODUCTION 1.1 General The design of offshore platforms for hydrocarbon production from the North Seahas recently gone through a period of rapid change. This process has occurredas a response to the increasingly difficult economic conditions over the lasttwo years. The increased competitiveness in design proposals has been matchedby a willingness on the part of Operators to accept more economic solutions tooperational requirements. Amongst the areas where developments have shown major improvements is in thedesign of shallow water lightweight platforms. This paper traces the designdevelopment of one group of lightweight platforms for usein the NorthSea. 1.2 North Sea Environment Figure 1 shows the UK sector of the North Sea. Two areas are of specialinterest - the Southern North Sea area which is a gas basin covered by shallowwater less than 50m in depth, and the southern edge of the Central North Seabasin, which contains oil, condensate and gas fields at water depths up to100m. This paper refers to developments primarily in the Southern Basin area. These areas are of special importance, as they are mature basins with a largepopulation of marginally economic small fields awaiting low cost developmentschemes before production can be justified. 2. ECONOMIC BACKGROUND 2.1 Gas Demand The Western European gas market has shown strong growth over the last 15 yearsand is expected to continue to grow at the current rate for some time to come. The development of new high efficiency combined cycle technology andenvironmental concerns have resulted in very strong growth of gas demand forthe power generation industry. This is currently the fastest growing sector ofthe European gas industry. In the UK, the liberalisation of gas supply from the original British Gasmonopoly position has given a very sharp increase in the proportion of gasfired power generation, as shown in Figure 2.