Construction of the Hondo Field Pipelines by the Reverse J-Tube Procedure

Abstract

During the summer of 1980 three pipelines were constructed between the Hondo Platform, located in 835 feet of water, and the Hondo SALM, located approximately 1-1/2 miles away in 500 feet of water. Figure 1 shows the Hondo Platform, SALM, Offshore Storage and Treating (O.S.&T.) vessel, and pipelines. These pipelines, ranging from 6-inch to 12-inch in diameter, pipelines, ranging from 6-inch to 12-inch in diameter, were installed by the reverse J-tube procedure, a technique developed by Exxon especially for construction of deepwater pipeline systems. The technique may also be cost effective in certain shallow water applications. Introduction In the late 1960's Exxon Company, U.S.A. (then Humble) acquired several deep-water leases in the Santa Barbara Channel, offshore California. It was clear that the development of these leases wood require extension of existing offshore pipeline technology, including means for installing risers in any water depths where bottom-founded platforms could be used. To meet this objective, Exxon Production Research Company developed a new procedure (U.S. Patent 3,595,312 and foreign counterparts) for riser installation known as the reverse J-tube method. This method is based on construction of the pipeline from the deck structure of the platform. Pipe sections are aligned and welded in a vertical orientation. As the pipe string is assembled, it is lowered into a J-tube pipe string is assembled, it is lowered into a J-tube which has been previously installed in the platform during land fabrication. The J-tube provides an enclosed conduit which guides the pipeline from a vertical orientation at deck level to an orientation parallel and adjacent to the seafloor. The pipeline parallel and adjacent to the seafloor. The pipeline is pulled downwardly through the J-tube and along the seafloor to its destination by a moored barge or tug located some distance away. For short pipelines, the entire length may be installed through the J-tube without use of a laybarge. Longer lines may be initiated through a J-tube and subsequently retrieved aboard a laybarge to be completed by conventional pipelay operations. pipelay operations. This paper describes how the reverse J-tube method was used to in stall the three Hondo pipelines. Early model tests conducted to prove the technique and other full-scale reverse J-tube installations in the Gulf of Mexico and various locations overseas are briefly discussed. Finally, an assessment will be made concerning cost effective applications for this method. THEORY In the direct J-tube method the pipeline is pulled upward through a J-tube from the seafloor to pulled upward through a J-tube from the seafloor to the platform deck. This method has been used world- wide by numerous operators. In the reverse J-tube procedure, however, the direction of pipe movement is procedure, however, the direction of pipe movement is downward. This reversal of direction offers two advantages over a direct J-tube installation:Pulling forces and reaction forces on the platform structure are reduced to as little as 25percent of those characteristic of a direct J-tube pull. Referring to Figure 2, note that in a direct J-tube installation the pull force must overcome laybarge tension, pipe-to-soil friction, and pipe weight. Furthermore, the high levels of tension on both ends of a pipeline during direct J-tube pull produces a brake-band effect that further increases friction between pipeline and J-tube. On the other hand, two of the forces to be overcome in a direct pull, barge tension and pipe weight, act positively in are verse J-tube pull to move the pipeline in the desired direction and, although pipe-to-soil friction must be overcome, it does not contribute to reactions exerted on the J-tube and platform. The use of a non-polluting lubricant applied to the pipe further eases its passage through the conduit. The low installation forces characteristic of the reverse J-tube method make it suitable for significantly larger pipe diameters than the direct J-tube procedure. Depending on platform size and geometry, pipeline risers up to 24-inch can be installed by the reverse J-tube technique.