ENGINEERING ASSIGNMENT

Engineering Assignment

Engineering Assignment

Oil and gas are by far Norway's largest export industries, being more than 50% of the export value in 2006. The first oil in Norwegian territory was found in 1969 at the Ekofisk field, located at 70-75m water depth. The first production started there in 1972. Since then, knowledge and experience have developed, and the petroleum industry in Norway presently has expertise on offshore installations in harsh environment and deep water, which is based on disciplines like hydrodynamics, structural mechanics, oceanography, automatic control, material science, etc. However, as new oilfields are explored and developed, new challenges arise and new knowledge and research are needed. New contributions are searched for within each discipline. It is believed that by integrating the different disciplines, further progress can be made. At the Center for Ships and Ocean Structures (CeSOS) at the Norwegian University of Science and Technology (NTNU), the disciplines of hydrodynamics, structural mechanics and automatic control are integrated as a strategy to contribute to the innovation of ships and ocean structures.

Offshore petroleum production began in the most shallow waters. As these reservoirs are exploited together with an increasing demand for oil and gas, the petroleum industry has been moving to ever deeper waters. Riser technology is an important issue both when considering field development costs and technological feasibility. In deeper waters interference between adjacent top tensioned risers in an array is an issue of considerable concern. Collision may lead to dents in the riser pipe and also damage in the coating, with fatigue and corrosion as possible consequences. Even a single collision event may be damaging if the collision takes place with sufficiently high impact.

If the riser spacing and properties are kept constant, the risk of collision will increase with increasing water depth, since the static deflection due to the uniform current drag is proportional to the square of the length. In deep waters this means that even a relatively small difference in static forces may lead to mechanical contact. In addition flow separation and shielding effects between risers in an array can change the local flow velocity. This difference in current forces may cause large relative motions and lead to contact between neighboring risers.

For deep water production systems, riser solutions are traditionally divided into two main groups; (1) subsea wellheads (wet trees) with flexible risers up to a floater like a semi-submersible or a production ship (floating production storage offloading - FPSO), and (2) tensioned risers with wellhead on a compliant platform, like tension leg platform (TLP), spar1 or deep draft floater (DDF). These are the dry tree solutions for floating production systems. Dry tree systems are often the preferred solution for production as they provide easy access to the well for maintenance, intervention and workover. Alternative platform solutions, while still using dry tree systems, are proposed by Often (2000) for a semisubmersible with heave compensation system, and by Mortazavi et al. (2001) for deep draft caisson vessel with buoyancy ...