Pipeline Construction in Cook Inlet by the Pulling Method

Abstract

Twin marine pipelines were installed in Alaska's Cook Inlet by the pull method. The procedure used a locally available barge equipped with an auxiliary anchoring and pulling system. This paper describes the planning, design, and construction methods for the operation. Introduction Amoco Production Co. operates four platforms in Cook Inlet, Alaska, for the Chakachatna Group consisting of Phillips Petroleum Co., Skelly Oil Co., Atlantic Phillips Petroleum Co., Skelly Oil Co., Atlantic Richfield Co., Standard Oil Co. of California, and Amoco Production Co. Two of these, Platforms Anna and Bruce, Production Co. Two of these, Platforms Anna and Bruce, were constructed in 1966 in the Granite Point field. Concurrent with platform construction, twin 10 3/4-in. concrete-jacketed pipelines were constructed from these platforms to a treating facility at East Foreland as shown platforms to a treating facility at East Foreland as shown in Fig. 1, an orientation map of Cook Inlet. Since that time, the gas line has suffered seven failures and the oil line has faded twice. The latest gas-line failure occurred on Jan. 11, 1974, and the second oil-line failure occurred on Feb. 22, 1974. Failure of the oil line required shutting in the platforms until May 1 (68 days) while awaiting ice-free conditions in Cook Inlet, when a barge could be moved on location to repair the lines. In an attempt to preclude future production loss and reduce pipeline preclude future production loss and reduce pipeline operating costs, new twin 6 5/8-in. lines were constructed between Platform Bruce and the Mobil Oil Corp. treating facility at Granite Point. Although the onshore lines were installed conventionally, the offshore lines were installed somewhat uniquely by the pull method using an anchored barge. This paper discusses the procedures used in constructing the offshore lines. Cook Inlet Environmental Factors Cook Inlet is a marine estuary about 160 miles long and covers some 7,000 sq miles. Near its mouth it is 60 miles wide and as much as 300 ft deep, with tides ranging to about 14 ft. In the area of Middle Ground Shoal, the tidal range is about 30 ft. Tidal currents north of East and West Foreland average 4 knots, but in local areas may be as high as 8 knots. The water is turbid, ranging to 440 ppm suspended solids, and is almost opaque. In the summer, the water temperature may be as high as 55 deg. F; however, in winter it drops to around 28 deg. F. Upper Cook Inlet in the winter months is characterized by large floating ice pans 3 1/2 to 4 ft thick that preclude operation of work barges during the ice season. The construction season is considered to be the ice-free season beginning around April 15–30 and ending between Nov. 15 and Dec. 15. Therefore, timing and logistics are extremely important factors in all Cook Inlet marine operations. Study of Alternatives After failure of the Granite Point pipelines in early 1974, a study was initiated to determine the best method of permanent repair or replacement of the existing lines. permanent repair or replacement of the existing lines. This study concluded that the prospect of permanently repairing the existing pipelines was remote, that twin 6 5/8-in. pipelines between Platform Bruce and Granite Point would handle future production from Platforms Point would handle future production from Platforms Anna and Bruce, and that the marine portion of the lines could be pulled from a beach site on Granite Point without the use of a lay barge. In addition, it was concluded that temporary repair of the existing lines was justified because the platforms could be returned to production at least 5 months before new lines could be constructed and to recover some 9,300 bbl of crude in the oil line that could cause a major pollution incident. JPT P. 242