Physical Simulation of the Res onse of the Enserch Garden Banks 388 FPF/Free Standing Rigid Riser to Wind, 8 aves, and Current
Authors: Edwards Roderick  Y. , Johnson Thomas  L. , Johnson Robert  P. 
Published: October 2008

This paper describes large scale hydrodynamic model tests of the Enserch Exploration Garden Banks 388 Floating Production Facility (FPF). This unique facility consists of a semi-submersible connected to a template on the sea floor 2096 feet below the surface through a buoyant, free standing "rigid" riser. The top of the riser and its manifold are located well below the surface to minimize the forces exerted on the riser by waves. Numerous flexible conductors connect the semi-submersible to the manifold on the top of the riser.

The emphasis of the model test program was to confirm the estimates of:The structural response of the buoyant, rigid production riser and an adjacent drilling riser,The relative displacements between the semisubmersible and the production riser,The relative displacements between the production riser and drilling riser,The forces which must be endured by the tensioners and flexible hoses which join the semi-submersible to the production riser.

The above "system responses" were determined in realistic wind, wave and current conditions for the proposed area of deployment. The model test program was unique in that it involved the simultaneous simulation of the wind, waves and current profile from the sea surface to the sea bottom in 2096 feet of water. Furthermore, the model test program involved the physical simulation of the prototype structural properties of the drilling and production risers.

The FPF was tested in seven different configurations which represented realistic operational scenarios ranging from the simplest - Le. the buoyant freestanding riser alone, such as might occur in the extremely unlikely event of disconnection of the semi-submersible to the most complex wherein the retrieval of the Blow Out Preventer (BOP) stack was simulated in progress.

The paper describes the planning and preparation of the model components, the unique aspects of the simulation of the environments and the structural properties of the models, the execution of the experiments and the handling of the large number of measurements necessary to characterize the response of the multi-component system. Finally, the data analysis tools which were employed to provide an understanding of the responses of the system are discussed

INTRODUCTION

Enserch Exploration plans to develop the Garden Banks 388 field. The field is located in the Gulf of Mexico off Louisiana in 2096 feet of water. A Floating Production Facility (FPF) and drilling system will be used to develop this field.

The main elements of the FPF and drilling system are:A converted 12 column semi-submersible drilling vessel,A buoyant "rigid" production riser,A drilling riser,Flexible hoses which connect the semi-submersible to the buoyant rigid riser,The tensioning system which the buoyant production riser is tethered to the semi-submersible,The mooring system.

Figure 1 is a sketch of all the system elements excluding mooring system. Figure 2 is an underwater view looking up at the keel of the semi-submersible. The production riser, hoses, semi-submersible (and its forward sponsons) are recognizable in the photo.

The design life of the FPF is to be twenty (20) years.