Measuring Construction Stresses In Offshore Pipeline
Authors: McPhail James F. , Finn Lyle D. , Rohmaller Paul L. , Okaro J.C. 
Published: December 2007

During construction, 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 laybarge to the ocean floor. Barge motions, pipe tension, and pipe travel relative to the laybarge were recorded pipe tension, and pipe travel relative to the laybarge were recorded simultaneously on the barge.

Introduction

Offshore pipelines are usually installed using the laybarge-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, the tensioner, pipe wall thickness, the stinger design, the tensioner, and, indirectly, the mooring system required for the particular pipeline installation. An analytical particular pipeline installation. An analytical procedure was developed' by Esso Production Research procedure was developed' by Esso Production Research Co. to predict static construction stresses during pipe-laying operations for given water depth, pipeline pipe-laying operations for given water depth, pipeline design, and laybarge equipment parameters. This procedure has been used as an aid in specifying pipe procedure has been used as an aid in specifying pipe wall thickness, coating weight, tension to be applied, and stinger length and deployment.Recognizing the need for accurate specification of pipeline design and pipelaying equipment parameters, pipeline design and pipelaying equipment parameters, Esso Production Research Co. and Esso Australia Ltd. considered it important to measure pipeline construction stresses during pipelaying operations in Bass Strait off Australia. The field measurement program consisted of two separate series of tests. The program consisted of two separate series of tests. The first was conducted in Nov.-Dec. 1969 from the Ingram DB-5 during construction, in about 200 ft of water, of a 24-in.-diameter crude-oil pipeline from the Halibut field to shore; the second was conducted in Jan.-Feb. 1971 during construction of a 20-in. diameter crude-oil pipeline from Halibut to Kingfish in 248 ft of water, using the Santa Fe Choctaw.

The objectives of the DB-5 tests were to verify the static stress calculation procedure and to determine the significance of dynamic stresses in pipelaying operations. Since the DB-5 is a conventional, flat-bottomed barge (100 ft wide, 400 ft long, and 28.5 ft deep), it was expected that barge motion in operational seas in the Bass Strait would introduce observable dynamic stresses. In the DB-5 program, eight test runs were made using instrumented segments of pipe. In each test run, pipe strains, pipeline geometry, 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 laybarge to the ocean floor during normal pipeline construction. Analysis of the test results 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).As a result of the DB-5 test, work was begun on analytical procedure to predict dynamic stresses during pipeline construction. In addition, it was decided that the Choctaw field measurement program should be conducted. The Choctaw, which is a semisubmersible barge 106 ft wide, 400 ft long, and 54 ft deep, was equipped with a unique, column-stabilized stinger 147 ft long, with columns as large as 48 in. in diameter rather than the more conventional buoyant stinger employed by the DB-5.

JPT

P. 261