The Influence Of Internal Waves On Deepwater Drilling Operations

Abstract

ABSTRACT Shipboard observations during deepwater drilling operations by the Discoverer 534 and a simultaneous oceanographic measurement program have confirmed the presence of large internal waves in the Andaman Sea, offshore Thailand. The Discoverer 534 has provided measurements of dri11ship response to internal wave activity in water depths ranging from 1900 ft to greater than 3400 ft; internal wave currents as high as 2.6 knots have been measured. As a result of the analysis of internal wave and dri11ship data it is concluded that internal waves may have an impact on dri11ship operations, and knowledge of their forces on offshore structures will be required to design production facilities in some deep water areas of the world. INTRODUCTION The purpose of this paper is to bring to the attention of the offshore industry a new environmental loading factor which may influence future deepwater operations: internal waves. Exxon experience has verified that internal waves are present in the Andaman Sea, and a search of the literature indicates that internal waves exist in all the world's oceans (1). The Discoverer 534 under contract to Esso Exploration Inc., operated successfully in the Andaman Sea in water depths ranging from 1900 ft to greater than 3400 ft. While internal waves were present during much of the operating period, they had no significant impact on drillship operations. However, because internal waves of magnitude greater than those observed by the Discoverer 534 are possible, Exxon Production Research Co. initiated a measurement and modeling program to obtain a detailed engineering description of internal waves. The measurement program, conducted during October 1976, was designed to simultaneously measure internal waves and the corresponding dril1ship response. A subsequent internal wave modeling effort is now underway. In the sections which follow we (1) briefly review the properties of internal waves, (2) discuss the measurements of internal waves in the Andaman Sea, and (3) describe typical dril1ship response to moderately sized internal waves. CHARACTERISTICS OF INTERNAL WAVES Background Internal waves are water waves which propagate beneath the ocean's surface. Just as surface waves propagate along the boundary layer formed between the ocean and the atmosphere, internal waves propagate along the boundary layer between water of greater and lesser density. Such boundary layers are associated with the oceanic thermocline and are normally characterized by warm, less saline water lying over cold, more saline water. Internal waves may exist in any body of water which is stratified by temperature and/or mineral content. However, energy sources must be available to generate the waves. For example, current flow over uneven bathymetry, shear current flow, and atmospheric disturbances may cause internal waves to form. Internal waves may have a finite amplitude everywhere in the water column except at the bottom where it is zero. The internal wave amplitude on the ocean's surface is usually very small, preventing easy surface observation of them. The maximum amplitude is normally found near the average depth of the main thermocline.