SALM Installation in Santa Barbara Channel

Abstract

Oil and gas production has been underway in Exxon's Hondo Field since April 1981. The field is being developed using a 945-foot drilling and production platform, the Hondo Platform, and an production platform, the Hondo Platform, and an offshore storage and treating vessel, the Exxon Santa Ynez. The dedicated 50,000 dwt vessel is moored to a single anchor leg type production riser which provides the connection to the three subsea pipelines and a power cable from the Hondo Platform. This paper presents the installation Platform. This paper presents the installation techniques used to install the Single Anchor Leg Mooring. Introduction The installation of the first single anchor leg mooring (SALM) in United States waters was completed during October 1980 off the coast of California. This SALM forms the vital link between the Hondo Platform, located in 850 feet of water in the Santa Barbara Channel, and an offshore storage and treating vessel (OS and T) moored to the SALM 8,500 feet from the platform (Figure 1). The Hondo crude will undergo liquid/ gas separation on the platform and then be transported via submerged pipeline up through the SALM to the OS and T. The floating storage vessel will perform oil/water separation, oil treating, offloading to a tandem-moored shuttle tasker and electric power generation for the production platform. In addition to crude oil and fuel gas, platform. In addition to crude oil and fuel gas, the EOIN provides a link for returning produced water and transmitting electric power to the platform. The SALM is installed in 490-foot platform. The SALM is installed in 490-foot water depth and consists of three individual components — Base, Riser and Buoy. The Base was transported to the Santa Barbara Channel and installed in May 1980. After installing the Base, the Buoy and Riser are co-joined, towed to the Channel and installed in September 1980. The main emphasis of this paper is on the loadout and water phase construction of the SALM. A brief description of the physical features, design criteria and fabrication of the SALM components is also included. SALM PHYSICAL FEATURES STRUCTURE The overall configuration of the SALM is shown in Figure 2. The permanently moored OS and T vessel is connected to the SALM by a 160-foot long rigid yoke, hinged at the vessel end to allow relative pitch. At the top of the buoy, the triaxial universal joint provides the three (3) degrees of freedom required to allow the vessel to roll and pitch with respect to the Buoy and to weathervane around it. This structure, constructed of fabricated plate elements, is essentially a universal joint sitting on a rotating tearing table. The Buoy itself provides the buoyancy necessary to tension the riser below and thus generate a restoring force as the Buoy is displaced from the neutral position by the environmental forces acting on the vessel. The 820-ton, 234-foot long Buoy structure consists of an upper buoyancy section and a lower extension stem. The upper buoyancy section has a maximum diameter of 26 feet and the stiffened-plate structure is divided into 12 compartments for damage stability. The 16-foot diameter lower extension stem is a one-inch thick rolled cylinder with five (5) ring stiffeners. The use of conical transition sections and the absence of longitudinal stiffeners help avoid local stress concentrations that could lead to fatigue failures. The lower extension stem remains flooded after installation. The 425-ton, 265-foot Riser is similarly divided into an 18-foot diameter upper compartmented buoyancy chamber and a six-foot diameter lower extension stem. The Riser stem design again avoids any internal stiffening to help limit the potential for fatigue failure of the two-inch thick wall. Universal joints are employed at both the junction between the Buoy and Riser and between the Riser and Base.