Design And Model Testing Of Large Submersible Work System

Abstract

ABSTRACT An industry group has sponsored a system design for an unmanned repair or connection of submarine pipelines in water depths to 1500 feet and in a North Sea environment with a single trip to the seafloor. The design seeks to reduce sensitivity to surface weather conditions by integrating all tools and the replacement pipe piece into a single package which is never lifted from the water and is, in fact, towed to the jobsite ready to work. The system has the potential of accomplishing the complete emergency repair of a buried, concrete jacketed pipeline (up to 36-inch diameter) in a three day stay on location. Model tests in the open ocean were used to verify structural design calculations and to help in estimating system behavior while under tow in heavy seas. INTRODUCTION Underwater work related to pipeline construction or repair has become increasingly costly and time consuming. The major problems have been the increasing pipe size (with corresponding increase in weight of components to be handled sub sea), the increasing water depths being encountered, and the influence of weather and sea conditions in more hostile regions such as the northern North Sea. The actual cost of a single pipeline connection or repair, even when accomplished during the favorable "construction season", has often been in the millions of dollars. Should the same incident occur outside the construction season, the political, economic, and environwental consequences of an extended interruption of a major transportation artery would greatly exceed the actual cost of repair. The state of the art in the repair or connection of submarine pipelines has evolved very slowly. Various underwater techniques have been demonstrated using repair connectors, split repair sleeves, or welding under ambient pressure - either dry (in a chamber), or in the wet. In all cases, the techniques proposed have been quite sensitive to weather conditions. In order to deve1op a viable solution to these problems, a group of international companies has sponsored a design program. The first phase of that program, completed in 1976, developed a concept and design for an integrated, remotely controlled submersible pipeline repair system (SPRS), described in References 1 and 2, for the connection or repair of large diameter pipelines in water depths to 3500 feet; all operations to be completed in a single excursion to the seafloor. A recently completed second phase developed the same concept for application to intermediate depths (400 to 1500 feet), pipelines buried to as much as 6.5 feet of cover, with concrete weight jacket. The approach was to first reduce weather exposure by separating the actual work, from the motions of ships on the surface - no loads are transferred at sea and no direct connection was permitted between surface and work except a power/communication cable to be suspended in a catenary, allowing large support vessel excursions without affecting the work. Secondly, the connection or repair was to be by insertion of an articulated spool piece carried on the vehicle.