Forces Exerted on a Submerged Oil Storage Tank by Surface Waves
Authors: Garrison C.J. , Chow Philip Y. 
Published: October 2008

Wave forces exerted on large submerged bottom-mounted structures is a topic of current interest. Particularly in the design of submerged oil-storage vessels, where the structure generally has limited net negative buoyancy, the wave forces are of considerable importance in the design. This paper deals with the interaction of a train of regular surface waves with a large submerged oil-storage tank resting on the ocean floor in water of finite depth. Linear wave theory is used to describe the incident wave, and viscous effects are neglected on the basis that the size of the submerged object is large compared to the height of the incident wave. The problem is formulated in the form of a potential flow problem, and to solve this problem, point wave sources are distributed over the immersed surface. The strengths of these sources are then adjusted to satisfy the no-flow condition at the surface of the object. Results from a computer program based on these theoretical concepts are compared with experimental results from wave channel testing.

Reference

Garrison, C. J. and Chow, P. Y.: "Forces Exerted on a Submerged Oil storage Tank by Surface Waves", J. of the Waterways, Harbor and Coastal Engineering Division, ASCE.

Civil Engineering Abstract

Diffraction theory for submerged objects of arbitrary shape is applied to calculate the wave forces acting on a submerged oil-storage tank. The computer-generated numerical results for both the horizontal and vertical force components are compared with experimental results.

Introduction

A new concept in the production of oil offshore involves the use of large submerged storage tanks. Oil is stored in the tanks during production at the offshore site and at certain intervals the accumulated oil is loaded aboard a tanker. Since the tanks used in this concept are very large, relatively light when submerged, and usually deployed in relatively shallow water, the forces induced by surface waves are of considerable importance in their design. It is, therefore, the intent of this paper to describe a basic experimental and theoretical study of the horizontal and vertical components of the forces induced by surface waves on certain practical storage-tank configurations.

It is generally accepted that the Morison equation1 is a valid approximation for the calculation of wave forces acting on objects whose lineal dimensions are small in comparison to the wave length of the incident wave. This equation, which involves both a drag and inertial component of force, assumes that the object is so small as not to disturb the incident wave. However, as the size of the object in comparison to the incident wave length increases, scattering occurs and the Morison equation becomes invalid. Moreover, the object may not be deeply submerged and the effect of the free surface, also disregarded in the Morison equation, may be important. Thus, in the case of large submerged objects the simplified theory based on the Morison equation becomes invalid, and a more basic approach that includes these additional factors must be taken.