Liuhua 11-1 Development- New Pipeline Technologies for Diverless Connections

Abstract

This paper provides an overview of the Liuhua infield production and test pipelines, focuses on new technologies used to tie the pipelines into a subsea manifold, and reviews the basis for selection of flexible pipe in preference to steel pipelines. The infield pipelines consist of two 13.5 -in.-ID flexible pipes for production and one 6. O-in.-ID flexible pipe for well test. Each pipeline riser is approximately 10,300 ft (3, 130 m) long and runs from the subsea manifold below the FPS, Nanhai Tiao Zhan, to the FPSO tanker, Nanhai Sheng Li. The technologies used to tie the pipelines into the subsea manifold involved:A transition tie-in base into which the flexible pipe was pulled using ROV-assisted tooling to make up the first end connectorA rigid pipe long jumper from the manifold to the tie-in baseThe survey and measurement tools to set up the jumper welding jigs Each new pipeline tie-in technology was proven through field tests of actual components before completion of manufacturing and installation. Flexible vs. Rigid Pipelines An overview of the manifold-to-pipeline system is provided in Fig. 1. Amoco uses two subsea connection techniques for the Liuhua subsea connection system. To understand how this occurred, a brief history on the Liuhua pipelines is presented. A more detailed discussion of the history and economics of the flexible pipeline selection has been presented previously (Refs. 1 and 2). During the defining stage, the Liuhua pipeline system consisted of three pipelines - two production and one test. The pipelines were to be of all-steel construction, fabricated either onshore or in shallow water and towed to site prior to the arrival of the vessels. Once the FPS was on site, the pipelines and manifold would be connected via a long jumper that spanned up to 150 ft between the manifold and the lead tow sled. At Coflexip's (now Coflexip Stena Offshore) request, Amoco agreed to reconsider the use of flexible pipelines. An all-flexible pipeline system was previously rejected due to its perceived higher cost. Full-cycle economics - including the rental of riser reels, risk of damage to the pipelines and communication cables during tow, and cost of cable crossings, corrosion inhibition chemicals, maintenance, and corrosion monitoring - was performed after receipt of installation bids. The economic result favored two 13.5-in,-ID flexible pipes for production and one 6,0-in, ID-flexible pipe for well testing. In addition, this solution outperformed the rigid pipeline option with respect to risk of damage or failure. Once awarded the work, Coflexip Stena worked with Amoco to develop an acceptable method to tie the pipelines into the manifold. The designed for steel pipelines, did not permit significant horizontal loads on the manifoldduring a flexible pipe lay away. Pile-founded tie-in bases were selected to resist lay-away loads with rigid-pipe long jumpers connecting them to the manifold.