Effect of Concrete Coating on the Rigidity of 12 3/4-in. Line Pipe
Authors: Jirsa J.O. , Wilhoit J.C. , Aguirre M. , Mogbo N.C. 
Published: October 2008

1. Introduction

Bending stresses involved when laying pipelines in deep water from a lay barge is a problem that will become more and more critical as new oil and gas fields are being discovered at greater distances from shore in deeper water. This trend toward deeper water and greater distances means that improvements in the design of the pipelines as well as in laying methods and equipment are being made. The minimum radius of curvature to which the pipe can be subjected in the laying operation depends on the behavior of the pipe and the concrete coating. Failure maybe due to the pipe and coating becoming fully plastic, to local buckling, to the spalling of the concrete, or to a combination of these. In any event, to insure successful deep-water pipe construction, the bending stresses involved must be determined. These stresses are due to the unsupported buoyed weight of the pipe and to currents, barge motion, etc. To estimate these stresses, for a given moment, the flexural rigidity of the coated pipe is required. During pipe-laying operations the pipe has two flexural rigidities. In the field joint length, the concrete coating is still fluid and makes no contribution to the rigidity of the pipe. However, in the length that has a solidified concrete coating the concrete could be expected to increase the flexural rigidity.

This paper reports the results of tests of four 12-3/4" by 0.406 -in. x-52 pipes in which the effect of the concrete coating on the flexural rigidity was studied. Moment-radius of curvature relationships determined from strain and deflection measurements were compared with analytical results computed for both bare and coated pipes. The tests also provide some insight into the effect of joints on the behavior of the pipe.

2. Experimental Program

Test Specimens: the test specimens were nominal 40 ft. pipe lengths coated in accordance with routine practices for laying operations in the Gulf of Mexico. Table 1 shows the properties of the four pipes tested. Two of the pipes were coated with concrete over somastic and two with concrete over wrap coating. The concrete thicknesses tended to vary along the length of the pipe. The coating thickness shown in Table 1 is an average value for the section sub-jected to constant moment. In specimen l2Sla a 30 in, section of the concrete was removed to simulate a field joint.

Concrete strengths were obtained using a Schmidt Concrete Test Hammer which provided an indication of the in-place strength of the concrete. Using this procedure, the compressive strength of the concrete can be determined to within approximately 15 percent according to the manufacturer's specifications. Since this is a quick and inexpensive test, a large number of readings were made along the pipe length to obtain a representative value for the concrete strength. As a result, the concrete strength (?c') listed in Table 1 should be considered only as an estimate of the strength.

The pertinent properties of the steel are listed in Table 1. The values for each pipe were obtained from three coupon tests which were machined out of a large steel section cut from the pipe.