NORTH SEA MODULAR PIG RECEIVER DESIGNED FOR BOTH OIL, GAS

July 5, 1993
Neil Gibbons Seatech Consultants Ltd. London Design of subsea modular pig receivers for the North Sea's Tiffany field export system employed conventional technology to achieve operational simplicity for pipelines handling both gas and crude oil under pressure. Subsea facilities were designed to provide for diversion of the normal flow through a modular removable subsea pig receiver. Each pig receiver was designed to accept a pig train containing a magnetic cleaning pig followed by an
Neil Gibbons
Seatech Consultants Ltd.
London

Design of subsea modular pig receivers for the North Sea's Tiffany field export system employed conventional technology to achieve operational simplicity for pipelines handling both gas and crude oil under pressure.

Subsea facilities were designed to provide for diversion of the normal flow through a modular removable subsea pig receiver. Each pig receiver was designed to accept a pig train containing a magnetic cleaning pig followed by an intelligent pipeline inspection pig which could then be retrieved to the surface.

Installation of all subsea work--pipelines, subsea intervention valves (SSIVs), and pig launchers and receivers--was completed in summer 1992. The Tiffany jacket was installed in November 1992. Commissioning is under way.

T-BLOCK DEVELOPMENT

The Tiffany field development is located in Block 16/17 of the Central North Sea and has been developed by Agip U.K. Ltd. acting as operator on behalf of the T-Block partners Fina Exploration Ltd., British Gas Exploration & Production Ltd., Lasmo North Sea plc, and AGIP U.K. Ltd.

The development consists of the Tiffany platform which will produce and process oil and gas from Tiffany and Toni and the associated Thelma and SE Thelma fields currently under conceptual development.

Oil and gas are to be exported through two pipelines connected into third-party facilities at two subsea tee locations as follows:

  • The 4 km, 12-in. oil export pipeline runs between the Tiffany platform and a subsea tee on the Brae A to Forties 30-in. oil pipeline.

  • The 36.4 km, 10-in. gas export pipeline runs between the Tiffany platform and a subsea tee on the Brae A to Brae B 18-in. gas line.

    Both third-party pipelines are owned and operated by Marathon Oil U.K. Ltd. and the subsea tees are existing facilities within these pipelines. Each pipeline system consists of the pipeline and three major subsea facilities:

  • Close to the Tiffany platform lies a subsea intervention valve (SSIV) that consists of an hydraulic spring-return operated valve with a manually operated valve both upstream and downstream contained within a protective structure.

  • A predelivery facility close to the third-party subsea tee consisting of a pipework manifold contained within a protective structure designed to facilitate connection of a removable subsea pig receiver.

  • A delivery facility consisting of the existing subsea tee, pipework, and protective structure.

This arrangement is illustrated in Fig. 1.

In 1991, Agip (U.K.) Ltd. awarded a lump sum contract for design, procurement, and installation of the pipelines to Allseas Marine Contractors S.A.

Allseas subcontracted to Comex U.K. Ltd. for the design, procurement, and installation of the subsea pipeline facilities and their associated tie-ins.

Seatech Consultants Ltd. was employed by Comex U.K. Ltd. to undertake the design of the SSIV along with predelivery facilities and the associated pipeline tie-ins.

The pig receiver was designed to accept a pig train containing a magnetic cleaning pig followed by an intelligent pipeline inspection pig which could then be retrieved to the surface.

lt was a requirement to undertake this operation safely using a standard diving service vessel (DSV) and to minimize any interruption of normal production of either oil or gas.

Fabrication of all facilities was by Cromarty Firth Engineering (Scotland).

CONCEPT AND DESIGN

Agip expects that pigging the pipelines will be unnecessary during normal operation because the gas is processed on the Tiffany platform, and wax formation in the oil export pipeline seems unlikely.

Further considerations, however, including existing and future requirements of the (then) U.K. Department of Energy particularly on inspection of the pipelines dictated incorporation of subsea pig-receiving facilities.

The principle strategy adopted by Agip is to undertake inspection through a regime of wall-thickness measurements by ultrasonic readings at repeatable subsea locations supplemented by topsides corrosion monitoring.

The decision to incorporate pig-receiving facilities into the pipelines was taken to provide back-up to this policy with the additional benefit of isolation facilities independent from third parties for future construction or repair work.

Agip anticipates intelligent pig runs will have a minimum frequency of 5 years and possibly extend to 10 years depending on the results of the ultrasonic wall-thickness measurements. This infrequent use of the facilities was a major influence on the design decisions made during the project.

The original concept for the subsea pig-receiving facility was by removal of a section of the pipeline and installation of a receiver on the resulting open flange. A by-pass would allow normal production during the installation and removal phases.

The pig receiver would penetrate through the protection structure during hook-up and operation. This arrangement is illustrated in Fig. 2.

REVISED CONCEPT

This original concept allowed for routine pigging of the pipelines with spheres or foam pigs through to the main line. Subsequent discussions with third-party operators, however, concluded that such pigging was impossible.

On this basis, the conceptual design had at least four disadvantages:

  1. Operation of the pig receiver required the removal and replacement of an integral part of the pipeline.

  2. The protection cover's roof panels must be removed to allow installation and removal of the pig receiver.

  3. Alignment and tie-in of the pig receiver were difficult because the two closing flanges were at right angles to one another.

  4. Handling the pig receiver was awkward because of the length required to penetrate the protection cover.

Early in the project this layout was revised to overcome many of these problems and this revised concept (Fig. 3) remained essentially the same throughout detailed design.

By rearrangement of the manifold pipework, the tie-in point to the pig receiver pipework now ends in two parallel flanges, an arrangement which allows simpler connection and disconnection.

The revised arrangement is not of course piggable but, as discussed earlier, the design scope did not require pigging beyond the predelivery facility to the tie-in tee.

This new arrangement allowed the development of the pig receiver pipework as a separate module which could be readily handled, lowered to site, and connected to the permanent pipework.

It was decided that the pig receiver would be designed with standard piping components rather than as a pressure vessel: the frequency of use did not warrant the expense of a one-off manufactured item.

The manifold pipework provides the normal production path and tie-in for the pig receiving loop through double-isolated tie-in points which allow work to continue safely on the pig receiver during normal operation.

The pig recovery module contains the main pig-receiver length of pipe with two take-off points.

The first provides the normal production loop during pigging operations, but after passage of the pig it can be closed off gradually to allow pipeline pressure to push the pig to its final location.

In this way, the velocity of the pig is controlled and prevents damaging impact at the end of the pig receiver.

Magnetic pig signallers within the pipework indicate the pig's position during control of the final stages of receipt of the pig.

Isolation valves on the pig-receiver pipework allow containment of hydrocarbons after disconnection of the pig-recovery module and recovery to the surface.

One principal decision was whether to provide protection to the PRM module while in its operating position or only to the permanent PRM base skid and accept the very small risk of damage to the PRM module.

Methods of complete protection evaluated included a large structure to contain both manifold and PRM module and rock bunds to provide deflection to trawl-gear.

The alternative solution was to protect only the base skid using some form of removable mattresses.

This solution was in fact chosen as the most cost effective given that a DSV would generally be attending during the operation of the PRM.

The permanent base skid was therefore designed in conjunction with the mattress manufacturer to provide a small profile for protection by removable concrete mattresses and a protection structure designed to protect only the manifold pipework with a removable hatch for tie-in of the PRM to the permanent pipework.

This arrangement is shown in Fig. 4. The final arrangement for the gas export pipework appears in Fig. 3; the oil export facility is almost identical.

DETAILED DESIGN

Establishing the likely makeup of pig trains was a major consideration in the detailed design of the pig recovery module. This makeup would dictate the required length of pig receivers.

Five principal operations were studied to evaluate pig trains: construction, commissioning, cleaning and dewaxing, inspection, and emergency repair.

Of these, the pig trains for inspection were identified as likely to be the longest received. The normal pig trains to be run during an inspection operation are:

  • Cleaning and scraper pig train to remove any obstruction and ensure good contact between pig sensors and pipe wall

  • A dummy intelligent pig to ensure clear passage

  • The intelligent pig.

Given the expense of retrieving the subsea PRM after each pig run, discussions with the suppliers of intelligent pigs led to a decision to reduce these runs to two.

For each train, these are as follows:

  • Oil Train 1: A cleaning train of three bidirectional pigs (0.6 m each pig), a caliper pig (1 m), and assumed debris (1 m); total length of 3.8 m.

  • Oil Train 2: British Gas On-Line Inspection (OLI) pig (4.2 m)

  • Gas Train 1: Magnetic cleaning pig (1 m), British Gas OLI dummy pig (4.2 m), and assumed debris (0.8 m); total length of 6.0 m.

  • Gas Train 2: British Gas OLI pig (4.2 m).

The pig-receiver length is taken as the distance between the end of the receiver and the by-pass tee. A reducer is also incorporated in the pig-receiver length to provide a flow route around the cleaning pig's cups during arrival of the intelligent pig.

The receiver lengths chosen were 5.3 m on the oil PRM and 6.0 m for the gas PRM.

ADDED REQUIREMENTS

Additionally, the following design requirements were incorporated: an unobstructed path for the pigs; double isolation of the PRM from the main production path; isolation of the PRM; indication of pig position; dewatering, flushing, and test connections; and PRM handling and tie-in facilities.

The design shown in Fig. 3 incorporates the following details to achieve these requirements:

  • A sphere tee at the bypass tee to ensure a clear path for pigs.

  • Dual welded in-line valves for double isolation. The welded configuration was chosen to minimize the size of protection cover.

  • A single valve on each leg of the PRM.

  • Two magnetic pig signallers to indicate entry of the pig into the PRM.

  • Valved branch connections (2 in.) at suitable locations to allow access for flushing, dewatering, and testing.

  • The manifold pipework designed to allow access of ultrasonic equipment supplied by AEA Sonomatic Ltd., Warrington (England), to particular locations for wall thickness measurement.

  • A tubular frame around the PRM for protection and handling during installation and retrieval. Guides and jacking points were added to allow alignment of the PRM tie-in flanges.

  • Because Agip decided not to build the PRM until it was required, a trial fit between PRM and the predelivery facility was impossible.

The design as a result ensures that the PRM flanges will be lower than the matching predelivery facility's flanges. The jacking points are therefore incorporated to raise the PRM into alignment.

The protection cover was designed as a gravity-based tubular steel frame clad with grating panels to provide protection against dropped objects and trawlboard impact.

Mudmats stiffened by skirt plates support the frame. Attaching grout bags to the outside members of the frame added more weight for on-bottom stability.

The piping skid base was also designed as a gravity structure supported on mudmats and independent of the protection cover.

Extending the piping base provided a landing area for the PRM. Tubular members formed this extension to provide a smooth profile for protection by mattresses while the PRM is not in use but connected to the manifold base by plate flanges.

The piping skid base also provided guide pins in two locations. The height of the pins engages cones on the cover before the cover is level with the highest point on the valves.

The arrangement is illustrated in Fig. 5.

INSTALLATION, OPERATION

The facilities were designed to be installed in two main lifts in a maximum design sea state of 3.0 m significant wave height.

The installation sequence was:

  1. Pipe skid and extension weighing 32 metric tons.

  2. Protection cover weighing 37.5 metric tons.

The pig-recovery module has a total design weight of 23 metric tons for the oil PRM and 18.5 metric tons for the gas PRM.

Operating procedures for both oil and gas PRMs were developed and subjected, together with the design, to hazardous-operations and diver-safety reviews.

The hazop review aimed primarily at the operation of the predelivery facility in terms of process effectiveness and safety. The following areas were therefore addressed:

  • Integrity. Pipeline contents must be contained at all times either by fully tested components or flanges or by double-blocking valves which have been tested for integrity before use.

    Any change in operating conditions within the pipeline must be contained at any given stage in operation of the predelivery facility and pig-recovery module.

  • Safety. Any operation to be carried out must not impair diver safety.

    All handling operations must be able to be undertaken easily and without risk to adjacent equipment.

    Any inadvertent valve operation must not in itself produce an unsafe situation.

  • Environment. Any discharge to sea during operation must be nonhazardous. Oil spillage of any size is unacceptable. Gas venting from the DSV must be of limited amount only. Water entry into the -as pipeline is unacceptable.

With these principles, the procedure for operation of the gas export pig-recover,y module is as follows (Fig. 3):

  1. Hydrotest the module on the surface to ensure its integrity.

  2. Reduce the pressure to seabed ambient pressure. For the gas-export unit, the pipework is drained, dried, and pressurized with nitrogen.

  3. Lower the module to the site and install on the skid base.

  4. Flood the connecting spools and makeup the connecting flanges. The connecting spools are then purged with nitrogen and tested to ensure the integrity of the connecting flanges.

  5. The valves are then configured to divert pipeline flow through the pig-recovery module for receiving the pipeline pig train.

  6. Pipeline flow is reinstated to its normal flow path and the module depressurized to seabed ambient pressure by venting of hydrocarbon gas to sea via a diffuser (gas export only).

  7. The connecting spools are flooded with seawater and the module disconnected from the main pipework. Blind flanges are then fitted to the main pipework, the connecting spool purged with nitrogen, filled with methanol, and tested to ensure integrity of the blind flanges.

  8. After retrieval of the module to the surface, the gas is vented in a controlled manner and finally purged with nitrogen before opening and removal of the pig train.

This procedure for the gas-export module was accepted by Agip but additional facilities were required for venting the oil-export pig-recovery module to ensure the contained gas is vented on depressurization without oil being spilled.

This was achieved by the addition of containment tanks with internal bladders (Fig. 6).

With this facility the oil-export module could be depressurized into these tanks and the remaining oil in the connecting spools flushed into these tanks before retrieval to the surface.

Nitrogen from the diving vessel is used to perform this flushing exercise and subsequently replaced with treated seawater for the final test of the pipework blind flanges.

On the deck of the deployment vessel, the pig-recovery module and each of its two oil-reception tanks are degassed then deoiled via the oil-transfer tank. Degassing is achieved by controlled depressurization of the module or oil-reception tank to atmospheric pressure while deoiling is achieved by displacing the oil by a nitrogen blanket above the oil surface and water flushing.

Further nitrogen purging of the module, its oil-reception tanks, and the oil-transfer tanks is performed to remove any last traces of hydrocarbon gas. This operation is illustrated in Fig. 7.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.