Overcoming Obstacles In Landing Pipelines

ABSTRACT Pipelines constructed from offshore areas often encounter physical barriers or obstacles as they come ashore. Obstacles encountered may be in the form of difficulties to construct through a heavy surf or unstable areas, environmental or recreational restrictions, construction difficulties from steep bluffs, or manmade developments such as roads, railroads or canals. Directional controlled horizontal drilling has been successfully used to install over 60 crossings of rivers and streams. These crossings have been installed under waterways up to 75 feet deep and 2,200 feet wide. It is a well-proven method for installing marine pipeline crossings with a maximum of security and a minimum of environmental damage. Use of this method to bring pipelines onshore in areas where obstacles make the use of alternative methods difficult, if not impossible, is discussed. The principles of the method and the tools used are covered. INTRODUCTION Waves at the shoreline generate powerful forces which continuously deform and erode the shoreline. They are particularly harsh on structures erected by man. The National Shore Line Study conducted by the U.S. Army Corps of Engineers completed in 1971 revealed that one-half of the some 37,000 miles of ocean and Great Lakes shoreline of the U.S. is subject to significant erosion from waves. Throughout the ages the waves approaching the shore attack and erode the shoreline, whether they consist of vulnerable bluffs or gentle beaches. Man through his ill considered or sometimes from products of his extensive engineering studies has often interfered and accelerated these erosion processes. (Fig. 1). The physical processes occurring at a shoreline can make the construction of a pipeline a very difficult and costly endeavor. In excavating a trench through a shoreline composed of soil, the wave action and long shore currents try to backfill the trench as fast as it is excavated. Excavation of the trench is often attempted first and then the pipeline placed in the open trench. In other areas the pipeline may be laid first, then lowered into the soil by use of a plow, jetting, fluidization or vibration conducted by use of special built sea floor machines. In shorelines underlain by rock, the trenching method is a function of the type and nature of the rock. In soft rock, excavation may be by use of barge mounted rock dredges, or by bottom crawling rippers or cutters. Hard rock requires the use of explosives to first break the rock so that it may be removed from the trench. Conditions encountered in installing pipeline river crossings are often similar to those that occur in landing offshore pipelines. It follows, then, that a method used successively to carry out one of these functions may be equally-adaptable to the other. Primary considerations in bringing a pipeline ashore or installing a pipeline river crossing are the same:maximum security for the pipelineminimum damage to the environment, andeconomical to construct.