RAMFORM - A New Vessel Concept

ABSTRACT The paper presents a unique new design for monohull offshore support vessels. The hull form exhibits extreme beam aft. Maximum beam aft is 40m, while overall length is just 80m. The special hull characteristics give the vessel exceptionallyhigh stability and volumetric load capacity compared to its external dimensions. Thedesign was refined through extensive tank testing and theoretical work. Special motion characteristics result in significant advantages for the operator in terms of all weather capability, offshore safety and cost effectiveness. The vessel design is called Ramform. The paper describes the Ramform design applied as a seismic survey vessel. The large after deck area overcomes traditionallimits to seismic equipment handling and the vessel is designed to tow at least 12fulllength seismic recording streamers, compared to a practical maximum of around half this using traditional seismicvessel hulls. The ultrahigh stability and motion characteristics of the hull allow seismic equipment to be left in the sea without the need for continual retrieval and deployment for periods of inclement weather. MARINE AND SEISMIC SURVEYING Seismic surveying is a means by which oil companies can map subsurface geology. The technique involves a vessel which towsan acoustic energy source and one or more streamer cables which contain highly sensitive pressure sensors known as hydrophores. Acoustic energy sources, typically based on the emission of high pressure air into the water, are fired at regular intervals whilst the vessel traverses over the survey area. Acoustic waves emitted from the source travel through the water column into thesubsurface, with a portion of the acoustic energy reflecting from geological boundary layers that the wavefield encounters alongits propagation path. Upcoming reflected energy is recorded by the sensors contained in the streamer cables. After extensive data processing, principally involving the analysis of travel paths and signal to noise enhancement, the data give a representation of subsurface geological structure. Over the last few years, as improved towing technology has developed, there has been a dramatic increase in thenumber of streamer cables towed by seismic vessels. This increase is related to demands for improved cost efficiency.. MULTIPLE STREAMER CABLES IMPROVES EFFICIENCY Increasing the number of streamer cables towed behind a seismic vessel, increases the width of the data swath recorded during a traverse of the vessel over the survey area. The marginal costs involved with adding streamer cables to existing vessels are relatively low compared to the cost of the vessel itself, so increasing the number of towed streamer cables results in substantially reduced acquisition costs on a per unit of data basis. As towing technology has developed, seismic fleet operators have invested heavily in increased streamer capacity. The first five streamer vessel appeared in 1993, the first six streamer vessels in 1994, and 1995 will see eight and ten streamervessels entering operations for the first time. Each step has seen consequent reductions in acquisition costs for the vessel operator and end user. The cost efilciency of a seismic operation is related to the width of the data.