Subsea pipeline connections completed by welding at atmospheric pressure
Authors: Feller M.D. 
Published: December 2007

A system for completing subsea welded pipeline connections in one atmosphere conditions has been developed and tested. The connection system is designed to be used in conjunction with the Lockheed Petroleum Services one atmosphere diving system and Petroleum Services one atmosphere diving system and subsea connection chambers. It includes mechanical cut-and-bevel equipment for preparing the pipeline, measuring equipment for determining the configuration of the connection pup pieces, torch-type cutting equipment for pup preparation and gas shielded semi-automatic welding equipment for completing the connection. The ability to perform radiographic examination with a gamma ray source and ultrasonic examination was investigated. The system was developed with the aid of a pipeline riser chamber mock-up which simulated space pipeline riser chamber mock-up which simulated space limitations, welding environment and pipeline misalignment.

APPLICATION

Initial application of the system was to complete connections of 16 in diameter pipelines at the base of the Thistle 'A' platform in the North Sea. The first connections were made in early 1977.

RESULTS, CONCLUSIONS AND GENERAL OBSERVATIONS

The system permits completion of pipeline connections with butt welds which meet the requirements of API 1104 and ASME Section VIII. All subsea work is done in a dry, breathable one atmosphere pressure environment. pressure environment

Introduction

This paper discusses the welding system developed for use with the Lockheed Petroleum Services (LPS) service capsule and within subsea chambers, and specifically the work undertaken to develop and qualify the welding system for completing 16 in o.d. pipeline connections on the Thistle 'A' platform. This system differs from that described in the paper Subsea flowline connections in one atmosphere chambers, written by Mr. D. Woodlock of LPS, in that it deals with pull-in while the chamber is flooded, and welding in a pull-in while the chamber is flooded, and welding in a dry one atmosphere environment. During fabrication of the Thistle 'A' platform 14 chambers were installed on the platform's lower-most horizontal structural tubulars. Four of the chambers are 16 in pipeline riser chambers, two are 12 in. pipeline riser chambers, and the remaining eight are pipeline riser chambers, and the remaining eight are flowline riser chambers. At present, only three of the 16 in pipelines are pulled into the chambers. No definite schedule is set for completion of the remaining pipelines. pipelines. Pipeline riser chambers Pipeline riser chambers The pipeline riser chambers are man-rated pressure vessels complete with a riser bend, a swivel port for pipeline entry and a teacup for mating with the service pipeline entry and a teacup for mating with the service capsule. Refer to Fig. 1. The swivel ports are designed to accept the entry of the pipeline with as much as plus or minus 10 degrees of misalignment between the pipeline and port without inducing any bending loads into the pipeline - Fig. 2. pipeline - Fig. 2. The end of each pipeline is fitted with a bullnose which serves as an attachment point for the pull-in cable during the pipeline pull-in and pipeline end closure to sea water - Fig. 3. When pullin is complete, each bullnose is landed in the swivel port. Each bullnose is equipped with seals to prevent entry of sea water into the chamber through the annular space between the bullnose and swivel port once the chamber is dewatered. The bullnoses are fitted with internal plugs designed to prevent chamber flooding due to flooding of the pipeline during connection operations in the chamber. The swivel ports are equipped with snap retaining rings designed to engage the bullnose and lock it into the swivel port when the bullnose has fully entered the port. Temporary heavy wall pipes built into the riser port. Temporary heavy wall pipes built into the riser bends protect the riser from being worn by the pull-in cable.

Service capsule

The service capsule provides access to the chamber - Fig. 4. It is a positively buoyant diving capsule designed to transport men and equipment from the surface to the subsea chambers. The capsule atmosphere is kept pure for breathing, and at one atmosphere pressure, through an umbilical from the surface support vessel. The umbilical also conducts three phase 440-v. electrical power to the capsule, and provides telephone and television communication between the capsule and surface support vessel. Standard capsule equipment includes dewatering pumps, a hydraulic power supply, and downhaul winch for winching the buoyant power supply, and downhaul winch for winching the buoyant capsule to the subsea chamber.