The Influence of Internal Waves on Deep-Water Drilling

Abstract

Internal waves were observed during drilling by the drillship Discoverer 534 in the Andaman Sea, offshore Thailand, in water depths ranging from 1,900ft to more than 3,400ft. An engineering analysis was conducted on measured internal wave data and simultaneous measured drillship response. Knowledge of internal waves will be required for designing production facilities in deep water. Introduction This paper describes a new environmental loading factor that may influence future deep-water operations internal waves. Our experience has verified that internal waves are present in the Andaman Sea, and the literature indicates that internal waves exist in all oceans. The Discoverer 534, under contract to Esso Exploration Inc., operated successfully in the Andaman Sea in water depths ranging from 1,900 ft to more than 3,400 ft. While internal waves were present during much of the operating period, they had no significant impact on drillship 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. began a possible, Exxon Production Research Co. began a measuring and modeling program to obtain a detailed engineering description of internal waves. The measuring program, conducted during Oct. 1976, was designed to program, conducted during Oct. 1976, was designed to measure simultaneously internal waves and the corresponding drillship response. A subsequent internal wave-modeling effort now is under way. In the next sections, we briefly review the properties of internal waves, discuss the measurements of internal waves in the Andaman Sea, and describe typical drillship response to moderately sized internal waves. Characteristics of Internal Waves Background Internal waves are water waves that 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 normally are characterized by warm, less-saline water lying over cold, more-saline water. Internal waves may exist in any body of water 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 small, preventing easy surface observation. The maximum amplitude normally is found near the average depth of the main thermocline. Internal waves commonly are found everywhere in the world and range in size from small-scale microstructures to waves with measured amplitudes as great as 180 m. However, these waves cannot have amplitudes greater than the average thermocline depth and, just as surface waves, they are limited in height by water depth. Normally, internal waves in shallow water are moderately sized, only a few meters or tens of meters high, being limited primarily by available energy sources, thermocline depth, and water depth. JPT P. 1497