The Articulated Stinger: A New Tool for Laying Offshore Pipelines

Abstract

Abstract A new form of pipe support structure, or stinger, has been designed for laying pipelines offshore from a pipe-laying barge. The new stinger consists of 4 to 12 segments, each about 50 feet in length, each stinger segment is adjustably buoyant and contains rollers for supporting the pipe. The stinger segments are connected in series by special hinge joints which provide a limited degree of vertical, lateral, and torsional flexibility. Vertical flexibility increases the water depth capability, and lateral flexibility increases the weather capability of the stinger. Introduction One of the major problems encountered in laying underwater pipelines concern the means of supporting the pipe span from where it leaves the lay barge to a point off bottom where the pipe can safely sag to bottom. In all expect very shallow water some form of support structure. Commonly called a stinger, is necessary to avoid excessive bending and possibly buckling a pipe. The conventional stinger is along straight, stiff consisting of two buoyant pontoons connected by numerous crossovers and containing rollers for supporting the pope as it passes to bottom. To prevent excessive bending of the pipe at the hitch, a conventional straight stinger must descend at an angle essentially the same as that of the barge ramp. To prevent excessive bending in the sag bend a straight stinger must discharge the pipe very near the bottom. Figure 1 show the normal configuration of a straight stinger during a typical pipe-laying operation. A straight stiff stinger has four main disadvantages:A straight stinger is limited to a narrow range of water depths; significantly different water depths require stinger of different lengths. The length of a straight stinger is typically 4 to 6 times the water depth.Deep water requires a stinger of extreme length. Stinger exceeding 1000 feet in length have been used to lay pipe. The maximum practical length of a straight stinger seems to be about 600 feet. A straight stinger tends to break frequently. The maximum depth capability of a 600-foot straight stinger is 100 to 150 feet.A straight stinger must withstand severe loads arising from water currents and relative motions between the stinger and thelay barge. To avoid damage straight stinger are constructed from high-strength steel which is expensive.High loads can be developed in the pipe at the tip end of the stinger. The pipe can be damaged if the stinger becomes misaligned with pipe already laid. References 1–3 discuss various problems related to laying underwater pipelines, especially those involving use of a straight, stiff stinger. The vulnerability of a straight stinger in heavy seas has been demonstrated by model tests at the Shell Pipe Lines Research and Development Laboratory and by contractor's experience with straight stingers. Reference 3 describes the laying of 35 miles of 3 ½-inch, 30-inch (double) pipelines in the North Sea. A hitch was reported to have failed at least four times during the job. A new form of stinger has been developed at the Shell Pipe Lines Research and Development Laboratory?which eliminates the main problems inherent in the straight stiff stinger.