Foinaven, Schiehallion conditions test pull-in flowline connectors

In December 1995, SubSea Offshore, working within the Foinaven Subsea Alliance, completed the world's first guide lineless remotely operated vehicle (ROV) based flexible flowline pull-in and con nection. In August 1998, SubSea completed the 150th DMaC flexible flowline tie-in on the Schiehallion Field. The flowlines were connected for BP Exploration using the DMaC flowline and umbilical connection system in a five-year campaign requiring 200 connections to be undertaken as part of the

Standard interface sought by joint industry group

In December 1995, SubSea Offshore, working within the Foinaven Subsea Alliance, completed the world's first guide lineless remotely operated vehicle (ROV) based flexible flowline pull-in and con nection. In August 1998, SubSea completed the 150th DMaC flexible flowline tie-in on the Schiehallion Field.

The flowlines were connected for BP Exploration using the DMaC flowline and umbilical connection system in a five-year campaign requiring 200 connections to be undertaken as part of the Foinaven and Schiehallion field developments.

Foinaven and Schiehallion are located 190 km west of the Shetland Islands in the Faroes/Shetland Trough Area of the Atlantic Ocean. Water depths at the site range between 400 meters and 600 meters. Weather and current conditions consistently proved to be the worst ever encountered by SubSea Offshore in any offshore location in the world, with significant wave heights of 15 meters and surface currents of up to 5 knots being frequently recorded.

The Foinaven experience has been fundamental to the development of Schiehallion. The two fields, which are less than 15 kilometers apart, share similar challenges posed by water depth of between 400 meters (msw) and 600 msw and strong currents extending almost to the seabed.

Foinaven know-how has assisted in the development of Schiehallion. First oil was produced in July 1998, approximately five years after discovery, via one of the world's largest new build floating production, storage and offloading vessels (FPSO).

As on Foinaven, the subsea production concept is based around the DMaC field design model, with clusters of subsea wells producing via central gathering manifolds on the seabed. These, in turn, are connected via flexible jumpers to rigid pipelines and subsequently via flexible risers to the FPSO.

History

The DMaC subsea production system was developed by Fuel Subsea Engineering under contract to Esso Exploration and Production UK Limited. The initial development and testing program was completed in 1991.

Following further in-house development, Fuel Subsea Engineering undertook a joint industry programe sponsored by BHP (Americas), BP, Exxon. and Statoil to detail enhanced capability designs of the DMaC flowline and umbilical jumper connection systems and associated tooling - key components of the overall DMaC system.

In January 1994, a joint venture was formed between Fuel Subsea Engineering and SubSea Offshore Limited to participate in an operability evaluation of the DMaC tooling systems for BP Exploration's Atlantic Margin Developments. This joint venture is formally known as the Fuel SubSea Offshore Alliance (FSOA).

During the subsequent land and offshore trials, FSOA further enhanced and refined the DMaC systems to ensure optimum performance and reliability. In order to meet the Foinaven schedule, SubSea designed, built and successfully tested the first system in less than 10 months. Five complete systems are now built and operational.

Tooling systems

The two major components of the DMaC Tooling Systems are:
  • Flexible flowline pull-in and connection system
  • Umbilical installation system.
The systems are designed to pull-in and connect flexible flowlines and umbilical jumpers to a structure fitted with a DMaC Porch. The flexible flowline assembly consists of either a single flowline or a bundle of flowlines fitted at each end with a DMaC pullhead containing the outboard hub and seal. The umbilical jumper is fitted at both ends with a multi-function termination plate.

The pull-in and connection system comprises an ROV fitted with an interface skid, pull-in tool (PIT) and PIT deployment basket. Fitted to the interface skid is a suite of ancillary tooling which is used to perform various functions during the pull-in and connection operation. These tools are stored on the interface skid tool deployment unit (TDU) module and are deployed by a SubSea designed and built manipulator, also mounted on the TDU module.

The ROV mates with the PIT on the seabed, using a specially designed connector located in the interface skid. This connector provides mechanical, electrical and hydraulic connections between the ROV and the PIT.

Pull-in and connection of the flowline to the DMaC Porch is achieved by the PIT. Final makeup and pressure testing of the hub seal is achieved using the ancillary tooling, which comprises : low torque tool, medium torque tool, hydraulic hotstab, seal plate changeout tool (emergency intervention only), and the high torque tool (emergency intervention only).

The umbilical installation system comprises an ROV fitted with an umbilical tool interface skid, and an umbilical deployment basket. With the exception of the PIT, which is not required for this operation, the umbilical installation system utilises the same equipment as the pull-in and connection system. The ROV and interface skid are the same, the only difference being the TDU module is replaced by the umbilical tool module.

For both tooling systems, a second workclass ROV system is required. It assists the primary ROV by monitoring all work tasks and, in the event of any equipment failures, operates the DMaC system failsafes.

The DMaC tooling is operated using computer based control software which displays mimic diagrams of each tool. The mimic diagrams are incorporated into a mouse-driven, screen-based display, which ensures both ease of use and precise operation.

Current status

Working from a wide variety of vessels in the field, ranging from monohulls to semisubmersibles, flowline jumpers have been deployed to the seabed for immediate pull-in and connection or for wet storage. Jumper diameters have ranged from 12 in. through to 6 in., including multi-bore flowlines and have varied in type from flexible risers, to production flowlines and control umbilicals.

In 1995, the first Foinaven jumper, a 30-meter long, 6-in. by 6-in. by 2-in. multi-bore flowline was connected between tree and manifold in a water depth of 540 meters. SubSea has now completed more than 150 other flowline and riser connections, including a 160-meter long 10-in. flowline jumper between a manifold and a rigid export line. As a result of the lessons learned and experience gained, reliability has increased and pull-in times for the system have significantly decreased.

In addition, the disconnection system has also been field proven, with flowline jumpers requiring to be disconnected from central gathering manifolds at both Foinaven and Schiehallion during various phases of their respective installation campaigns.

Five flowline pull-in and connection systems have now been built, three for the Foinaven Field and two for the Schiehallion Field. The two systems built for Schiehallion have been upgraded and enhanced as a result of the knowledge gained from Foinaven operations. However, the Schiehallion systems remain compatible with the Foinaven field architecture, in the same way as the Foinaven systems are compatible with the Schiehallion field architecture.

The future

Working continuously through the northern hemisphere summer and within brief weather windows during the winter, the operation to date is considered a success. This achievement has proven beyond doubt the ability of ROV-based tooling systems to cost effectively undertake difficult and complex work tasks in deep and shallow water.

Despite its success in developing a diverless connection systems, SubSea does not regard DMaC, or any of the other systems currently in the market place, as a solution to all diverless field developments. Each new development has to be evaluated on its merit, with a host of issues to be considered before a suitable system can be selected.

Many factors will determine the type of system to be used. These include cost, environmental factors such as seabed conditions and currents, the requirement for rigid or flexible flowline, horizontal or vertical connections and customer preferences for hardware, such as connectors. Therefore, standardization, while highly desirable, will not be as easy to achieve as some may think and will probably not be the result of one particular system being dominant in the market place.

However, the firm does believe that the future of diverless connection systems is in standardization, which relies on cooperation between both operators and contractors. A similar approach has already been successfully introduced in the North Sea by BP Exploration in the field of component replacement (insert chokes, control pods, etc).

To this end, SubSea in conjunction with several North Sea operators, has initiated a joint industry program to develop a standard interface that can be fitted by all manufacturers to their proprietary underwater hardware.

When complete, the interface will enable operators to not only choose what they believe to be the best pull-in and connection system for their particular project, but also their favored connector. In addition, it will provide the operator added flexibility when undertaking any remedial work following field commissioning.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

More in Subsea