Oil Skimming Vessels: Structure-Seaway Interaction Problems and Provisions for Wave Attenuation Systems
Authors: Clauss G.F. , Kuhnlein W.L. 
Published: October 2008

The paper analyzes hydrodynamic problems of oil skimming vessels with special emphasis on the flow field in the intake area in waves. It covers the two-phase oilwater flow phenomena as well as wave reflection and attenuation at vertical hull walls which are equipped with slender wave absorbers. The paper illustrates the related hydrodynamic and functional problems of three different oil recovery systems and proposes solutions for a smooth hydrodynamic design including wave damping devices. For the rational optimization of wave absorbers a design procedure is proposed.

Introduction

The rising offshore crude oil production and transportation has increased the danger of oil spills. Accidents generally occur in near shore area, which are recognized as the most environmentally sensitive and rich marine resources. Therefore it is necessary to provide efficient means for oil recovery at sea, to prevent shorelines from being polluted. Experiences with various accidents proof that oil skimming operations are most successful if collecting operations are starting as early as possible, as a thick oil layer and a limited area of pollution are favorable. Tanker disasters often happen during bad weather situations in heavy seas. Consequently, a rough sea environment must be considered in designing offshore oil skimming systems with special regard to operation and survival conditions. Furthermore, transit conditions must be optimized separately as a short-term appearance of the vessel at the oil spill area is important, to prevent the spreading of the polluting oil. For fighting offshore oil spills, this paper presents three oil recovery systems which are in service in the Federal Republic of Germany.

Problems arise from the fact that large ocean areas must be skimmed off. As the recoverable oil layer is very thin, all three vessels have large intake widths. Operation in steep head seas results in severe difficulties, as waves are amplified in the intake area by multiple reflections at the hull walls. In addition to this wave-structure interaction problem the basic two-phase-flow problem haa also to be investigated in detail: Fig. 1 shows the comparison of a pure water flow and a oil-water flow in front of a structure. As the oil layer acts like a rigid wall, the stagnation point in front of the vessel is shifted to a deeply submerged position. It separates the translational flow region from a powerful vortex with horizontal axis. Which drains the surface oil off the wall. The oil is collected at the accumulation area, and sucked down underneath the vessel by in stationary eddies.

If an oil skimming vessel is compared to a sieve with a large funnel, it is evident that the total flow resistance should be reduced as much as possible to avoid vortices or displacement of the surface flow pushing the oil slick away. This follows from the fact that only the oil-water quantity which can pass the narrow skimming device will enter the intake area. As a second important design requirement, the walls of the vessel should be equipped with slender wave attenuation systems to reduce the wave height in the intake area.