Stabilization of Pipelines in the Surf Zone, Pt. Pedernales, California

Abstract

ABSTRACT This paper describes stabilization measures implemented on three product lines in a high energy surf zone environment using a mobile surf zone deployment vehicle. The operational challenges of such a job are described, as are the means by which these challenges were overcome. Other surf zone remedial and abandonment work for which this type of vehicle is suited are also discussed. INTRODUCTION AND PROBLEM DESCRIPTION A pipeline bundle comprising one 20 inch and two 8 inch pipelines were pulled approximately eleven miles from shore in a southwesterly direction to Platform Irene, which is located 25 miles northwest of Point Conception, California. Please see Figure 1. Immediately after installation, it was evident that the bundled lines in the surf zone were resting on rock outcrops or ledges at certain locations. One free span length was estimated at 125 feet. Due to lack of existing information on the local sand transport regime, the inability to acquire shallow geophysical data through a wide surf zone, and various environmental restrictions, the selection of the pipeline route from outside the surf zone to the landfall was not, and could not have been, made on any in-depth technical basis. Diver inspections revealed midspan excursions of the pipeline bundle in excess of several inches, for the one large span, in depleted sand conditions. These midspan oscillations occurred about shoulder height with the diver standing upright on bedrock. Calculations indicated that fatigue cracking due to cyclic stresses, superposed Significant nominal bending stress under contents loading, was a probability if these displacements persisted for any significant period of time. Changes in the magnitudes of these vibrations and their anticipated durations were not known. The frequency and duration of expected episodes of depleted sand levels (and exposed rock outcrops) were also unknown. Further, diver inspections revealed that at some locations the steel straps that were used to bind the three pipelines together had broken. Bending stresses in the 8 inch concrete-coated, fluid-filled lines were of particular concern in the unsupported condition. Lack of hard data on the environmental loads and the characteristics of the system responding to these loads precluded the use of detailed analysis that might have been used to establish the need for and type of remedial measures. OPERATIONAL CHALLENGES Underwater construction operations in the surf zone present numerous challenges. The impact of waves prevents the safe transport of materials and tools by divers to the work location. Where strong littoral currents run parallel to the beach, significant lateral forces are generated on the diver's air supply hose and other lines in the water. If airlifts are used for excavation, they have limited effect because of the shallow depths and wave action. Using water pumps to dredge is usually prohibitive because there is no water supply available, except in the surf zone where sand and wave action tends to clog the suction screens.