Wave and Motion Measurements on Board the Semisubmersible Pipelaying Barge LB-200

ABSTRACT For a barge laying pipe under rough weather conditions at sea, pitch, roll, heave, surge, sway and yaw are important phenomena to be known and monitored. It is of vital importance that the static and dynamic stresses in the pipe are kept within prescribed limits. The static bending stress in the pipe at the upper bend, on the stinger, and at the lower bend, near the sea bottom touch point, are highly dependent on the deformation of the pipe. The horizontal tension of the barge mooring lines will effect the static pipe deflection. However pipe deflection does not depend only upon static forces. Dynamic forces caused by dynamic barge motions add an important part to the stresses in upper and lower pipe bends. Too much stress will cause pipe buckling in the bends and results in much delay time and costs as the pipe, already at the sea bottom, will have to be replaced. Usually extensive calculations and model studies are performed in an early stage of the pipe lay project. This paper gives a general description of the barge motion measuring system used on board of the pipelaying barge LB-200 owned by HcDermott International Inc., North Sea Division. In the Norwegian sector of the North Sea the LB-ZOO was contracted for the Statpipe Development Project requiring more than 300 miles of pipelines, which will bring gas from offshore fields to Norway. One of these rich sources is the Statfjord field, where oil as well as gas has been found. In 1983 the LB-ZOO layed the pipeline to shore at Karmoy Island and went on to lay 125 miles of 28-inch-diameter line to block 16/11, crossing the Norwegian Trench in water 300 m deep. Lay rates of 2.3 miles a day were made in the western slope of the trench toward Karmoy. All motions of the barge i.e. pitch, roll, heave, surge, sway and yaw had to be monitored continuously. The pipe had to be disconnected once those movements reached a certain magnitude due to weather circumstances. Another condition was that an historical record of all movements mentioned had to be kept on file. This paper describes the realization of the motion monitoring system. The following demands had to be fulfilled:automatic computer controlthe least possible maintenanceonline continuous monitoringcontinuous filing INTRODUCTION The lay barge LB-200 is roughly 160 m long and it has a width of 60 m. The deck is supported by six columns placed on two pontoons. Fig. 1 gives an overall view. A twelve mooring line system keeps the barge in position. Pitch motions are important for pipe laying because of the danger of pipe buckling. Plus or minus two degrees of pitch motion was set as the limit during operations. Beyond this limit operations were stopped.