Grid Tensioning System: A New Approach To Deepwater Riser Tensioning

ABSTRACT The move towards deeper waters for oil and gas production over the past decade, has resulted in higher development costs. The use of floating structures with surface completion and rigid risers is an attractive approach for development in this scenario. This paper presents a new riser tensioning concept designed for a so-called "minimum heave" semi-submersible (ref.1). Sixteen risers are connected to a common grid-like structure supported by twelve pairs of tensioners placed around its outer perimeter. Computer based analysis highlights the significant design considerations for this concept and, finally, a comparison is made with individual riser tensioning. INTRODUCTION Their small sensitivity to increases in water depth, make floating systems a natural option for deep water hydrocarbon developments On the other hand, the costs of subsea equipment, their installation and maintenance as well as the higher risks involved in subsea completion (sand production, wax formation, leakage control, etc...) pave the way for floating systems with surface completion. This option involves, however, motion compensation and minimization. The traditional method of providing motion compensation for a large number of steel risers on a floating platform is to provide a separate set (usually four) of tensioner assemblies for each individual riser. This approach works quite well when the vertical stroke of riser top is limited to around 6 to 9 ft (1.8 to 2.7 m) as in the case of TLP's. When designing a semi-submersible with dry completion, however, where the stroke can surpass 20 ft (6 m), the wellhead arrangement becomes very complex if individual riser tensioning is to be used. Other undesirable consequences in this case, are long stroke tensioners of relatively low tension capacity, leading to either unstable, long rod cylinders of normal working pressure, or heavy and costly, low working pressure cylinders of a more stable structural design. Direct tensioning becomes also difficult to accommodate due to the large vertical clearances needed in this case and their quite negative consequences to the platform's stability. A different approach to supporting a number of risers from the seabed to a floating vessel has been developed. This approach is loosely referred to as the "grid tensioning concept"and is suitable for both semisubmersibles, TLP's or any other type of offshore system where motion compensation/riser tensioning is of concern. The concept consists of a group of risers connected to a common grid-like structure. The grid itself is allowed to articulate with respect to the vessel, and is connected to it via a series of compensated tensioners arranged around the perimeter of the grid. The grid is allowed to move vertically within the vessel's moon-pool and to tilt about any axis in the horizontal plane. The concept was applied to a so-called "minimum heaven semi-submersible (ref.1), where 16 rigid risers are supported by a grid using 12 duplex tensioners.