Offshore Pipeline Construction Stress Measurement

Abstract Stresses induced in offshore pipelines were measured during construction of a 24- and a 20-inch pipeline in 200 and 250 feet of water, respectively. A unique self-contained subsea package attached to an instrumented section of pipe recorded the pipe profile and bending stresses as the pipe traveled from the lay barge to the ocean floor. Barge motions, pipe tension, and pipe travel relative to the lay barge were recorded simultaneously on the barge. A total of fifteen pipe sections were tested during various sea and current conditions. This paper reviews instrumentation procedures, describes test results and compares results with calculated values. Static stresses and profiles determined from the measurements are compared with the theoretical solutions. Horizontal bending strains measured during cross currents up to 1.6 knots are presented. Wave induced dynamic stresses in seas up to 9 ft and vertical motion of the barge-stinger hitch computed from the measured pitch, roll, and heave are plotted versus time to illustrate the importance of dynamic stresses. Introduction Offshore pipelines are usually installed using the lay barge-stinger (or "stove pipe") method. Stresses induced in the pipeline as it is lowered to the ocean floor during construction control the selection of the pipe wall thickness, the stinger design, tensioned, and, indirectly, the mooring system required for the particular pipeline installation. An analytical procedure was developed by Esso Production Research Company to predict static construction stresses during pipe laying operations for given water depth, pipeline design and lay barge equipment parameters. This procedure has been used as an aid in specifying pipe wall thickness, weight coating, applied tension, and stinger length and deployment. It has also been adapted for evaluation of riser setting operations. Recognizing the need for accurate specification of pipeline design and pipe laying equipment par& meters, Esso Production Research Company and Esso Australia Ltd. considered it important to measure pipeline construction stresses during pipe laying operations in Bass Strait off Australia. The field measurement program consisted of two separate series of tests. The first was conducer in November-December 1969 from the Ingram DB-5 during construction in about 200 feet of water of a 24-inch diameter crude oil pipeline from the Halibut field to shore; the second was conducted in January-February 1971 during construction of a 20-inch diameter crude oil pipeline from Halibut to Kingfish in 248 feet of water, using the Santa Fe Choctaw. The objectives of the DB-S tests were to verify the static stress calculation procedure and to determine the significance of dynamic stresses in pipe laying operations. Since the DB-5 is a conventional, flat-bottomed barge (100' wide by 400' long and 28.5' deep), it was anticipated that barge motion in operational seas in the Bass Strait would introduce observable dynamic stresses. In the DB-5 program, 8 test runs were made using instrumented segments of pipe. In each test run, pipe strains, pipeline geometry, barge motions, and other critical variables were measured as the instrumented pipe section moved from the lay barge to the ocean floor during normal pipeline construction. Analysis of the test results indicated that measured static stresses agree closely with calculated values and that the motion-induced dynamic stresses were substantial (20 to 25 percent of yield stress).