New technologies reduce pre-commissioning time, cost

The increasing number of subsea and deepwater developments brings new challenges when there are no surface connections to the pipeline available for testing and pre-commissioning.

Once constructed/installed, such subsea and deepwater systems still must undergo certain pre-commissioning and commissioning operations, from initial flooding, gauging and testing, up to final start up.

While the provision of such services in shallow water and topside-to-topside developments is routine, the same services at water depths in excess of 1,000 m (approx. 3,300 ft) pose many challenges. These challenges, and the current/planned technologies to address them, include:

• Flooding and pigging subsea pipelines using a remote flooding module (RFM). This enables the use of available hydrostatic head to flood and pig subsea pipelines while meeting the project requirements in terms of pig speed, filtration, and chemical treatment.
• Use of ROV driven pumping units to complete flooding and pressurization. By using the hydraulic power from a work class ROV to power a custom built pump skid connected onto the RFM, all pigging and pipeline testing can be performed subsea.
• Use of smart gauge tools (SGT) to gauge pipelines without using aluminum gauge plates. This allows the gauging of lines with reduced bore PLETs at each end. Also, this gives the ability to communicate the result of the gauging run through-wall without the need to recover the gauge plate to surface. This allows testing without pig recovery.
• Use of subsea data loggers to record pressures and temperatures during subsea testing, and use of systems to transmit this data to surface in real-time during the test.

The need for new and improved pre-commissioning technologies is expected to be particularly acute in the Asia/Pacific market, where there has been a significant increase in deepwater pipeline projects over the past few years.

Pre-commissioning defined

This flow chart illustrates the pre-commissioning process as typically applied to oil pipelines. The process for gas lines is similar but involves additional steps prior to handover such as removal of hydrotest water (dewatering), drying, MEG swabbing, and nitrogen packing (not covered here).

Subsea pipeline flooding

The first subsea pigging units were conceived and developed to overcome problems associated with flooding and pigging pipelines in deepwater. The latest subsea flooding device is the BHI Remote Flooding Module, which essentially achieves the same objectives using the latest ROV and subsea technologies. The RFM is a subsea flow control and regulation system. Once positioned on the seabed and connected to the pipeline to be flooded or pigged via the HP loading arm, it is "operated" by the ROV opening the valves to the pipeline. The hydrostatic head of the sea then enters the pipeline through the RFM because of the differential pressure between the inside of the pipeline, which is at atmospheric pressure, and the sea.

The pre-commissioning process as typically applied to deepwater oil pipelines.

Seawater enters the RFM via a filter manifold with a specified filtration level, usually between 50 and 200 microns. It passes through a venturi device, which creates a small pressure drop in the onboard flexible RFM chemical tanks which connect to the water flow pipework. This small differential pressure induces anti-corrosion chemicals into the water flow at the desired rate. This is pre-set prior to deployment and adjusted subsea by ROV if necessary.

The chemically treated water is held to a pre-determined rate by a flow regulation system. This maintains the water flow at the desired speed to match specified or optimum pig speed or flooding rates. Again, this can be pre-set prior to deployment and because the rate is controlled at a steady level, the chemical inducement is assured throughout the entire "unassisted" operation. A boost pump is required to complete final pigging operations due to pressure equalization. This pump is ROV driven, usually operated when the ROV returns to disconnect and recover the RFM, and in deepwater is required only for a very short time.

 

The vessel and ROV can leave the unit in isolation on the seabed during the unassisted operations and go on to other tasks. There is no need for connection to anything other than the pipeline. Onboard batteries power data-logging instrumentation which logs flows and chemical rates. Visual readouts allow the ROV to check status before it leaves and when it returns.

The RFM is positioned on the seabed by the ROV and connected to the pipeline to be flooded via the innovative rigid loading arm pipe system. The ROV then positions itself on the unit's roof from where it can monitor instruments and operate valves to manage the initial stages of the operation and adjust chemical control valves as needed.

Filtration and chemical treatment specifications are met by onboard facilities. Chemicals are stored in flexible tanks and introduced by a venturi system regulated by detecting changes in the water flow through the unit, and automatically adjusts the chemical flow accordingly.

To summarize, the aims of subsea pipeline flooding are to:

• Reduce the size of vessel required for pre-commissioning
• Negate the need for the vessel to remain on station during the bulk of the operations
• Remove the need for an expensive down-line, which is prone to damage
• Reduce schedule by increasing possible pig speed
• Reduce schedule by use of seabed water removing thermal stabilization for hydrotest
• Reduce crew size, equipment spread size, and environmental impact by removal of diesel engines on pumps, and also to improve safety by taking operations off-deck.

Offshore vessel requirements

RFM loading arm stabbed in.

In the following, we look at the commercial drivers for using such a system. For example, experience suggests that we need to inject 3,420 lpm (903 gpm) of filtered, treated seawater into a pipeline at a water depth of 1,000 m. Looking for example at flooding a 8-km (5-mi), 16-in. line at 1,000 m (3,281 ft) water depth, we can draw the following conclusions:

• The down-line option requires almost 10 times the deck space of the RFM option – with the current shortage of DP vessels and with vessel rates of around $40,000 per day, this can have a major impact on project cost.
• As the RFM floods the line with ambient temperature water, there is no stabilization period – this could save two days.
• The deployment and recovery time for the RFM is far quicker than for a 4-in. down-line.

ROV operating RFM.

As with all new technologies, there are circumstances where the RFM may not be suited to a deepwater project. These include:

• Where one or both ends of the line terminate at a platform/FPSO, as with SCRs
• Where a down-line will be deployed for other operations and can conveniently be used for flooding
• Where a large number of pigs are used
• Where the line has to be flooded with either fresh water or MEG
• Where one on of the line terminates in shallow water.

Subsea pigging equipment

The original subsea pigging unit was designed by pre-commissioning engineers with little input from ROV and subsea specialists (despite efforts to include them). While the device was successful in achieving its pre-commissioning objectives, it was not the optimum method of operation for the ROV or deployment vessel. Unwieldy HP flexible jumper hoses, relatively crude instrumentation, and new ways to use choke assemblies meant there were areas to improve. With this in mind, recent improvements on the RFM included:

• Holding more chemical than the original subsea unit, allowing less recovery and deployment cycles and use on longer and larger lines
• Using rigid loading arm technology to reduce subsea connection times and to reduce the risk of HP flexible jumper hose damage
• Being extremely ROV friendly – ROV specialists were involved in design to ensure minimum ROV interface issues.

Other improved features include:

• An on-board latching mechanism that allows fast ROV connection for boost pumping
• An on-board emergency release system means no risk of an ROV getting stuck on the RFM
• Advances in electronics mean more reliable instrumentation
• Deployment times are less than one hour in deepwater.

Subsea hydrotesting unit

Recent developments in subsea pumping systems have allowed ROV pump skids to carry out subsea hydrotesting and leak testing of pipeline systems, thus affording additional savings on vessel size and cost. When used in with the RFM, significant benefits can be achieved. Naturally, the systems that can be tested are limited by the maximum performance available from an ROV test pump skid. The BHI SHP (subsea hydrotesting unit) can produce over 40 lpm (10.5 gpm) pressurization rate from typical project ROVs.

Subsea hydrotesting unit.

Previously, we examined a down-line system that was needed to flood an 8-km, 16-in. line. Deepwater lines typically require hydrostatic testing at between 200 barg and 350 barg. A typical 4-in. downline would not be rated for such pressures (specialized down-lines that can handle such pressures often cost too much for such applications). Thus, a different down-line must be deployed to pressurize the line. Deployment times for the down-line are similar to those of the flooding down-line.

The SHP can be deployed with the RFM boost pump; hence there is no delay between completion of flooding and commencement of pressurization. It has been estimated that this saves a minimum of 24 hours per pipeline.

Smartgauge technology

We need to examine the gauging of the line. All offshore pipeline pre-commissioning operations include the proving of the internal bore of the line. This is achieved normally by fitting a segmented aluminum disk to one of the filling pigs, the disk having an outside diameter equal to between 95% and 97% of the minimum pipeline internal diameter. The principle is that any restriction in the line (buckle, dent, etc.) would cause one of the aluminum "petals" to bend, indicating a restriction in the line.

Gauge pig prior to launch

The gauge pig is then run as part of the pipeline filling pig train and most specifications require that the gauge plate be inspected visually prior to the hydrotest. This ensures there is no mechanical damage within the line that could be affected by the hydrostatic test.

Removing and inspecting the gauge plate is simple onshore (and for pipelines with above surface terminations); but requires additional work on pipelines terminating subsea and in deepwater. It was for this application that BJ developed the Smartgauge tool to meet the following needs of deepwater pipelines. This technology:

• Allows lines with restrictions (heavy wall bends, PLET hub restrictions, reduced bore valves) to be gauged.
• Permits gauging data to be reviewed and analyzed. This helps users pinpoint and identify any restrictions.
• Incorporates a system to remotely annunciate the result of the gauging run. This means that the hydrotest can start immediately upon completion of flooding without the need to recover the gauge plate to surface for visual inspection.

A standard mechanical gauge plate gives no indication of where damage occurred; this makes identification of location difficult, time consuming, and expensive. By using the multi-channel Smartgauge tool with a segmented flexible gauge plate, both the clock position and the location of multiple defects can be ascertained, reducing the time needed to find the problem.

Future developments

Improving ROV capabilities and advances in electronics will benefit remote flooding and pigging systems. Use of remote data transmission and signaling will allow associated tasks to be reduced in impact and cost, or taken completely off of project critical paths.

All future developments will be driven by these common objectives:

• Reduce the in-field time required to complete subsea pre-commissioning, hence saving on both the vessel costs and hire periods for pre-commissioning spreads.
• Remove or replace operational processes that have high risk (such as deployment of large diameter down-lines in deepwater).
• Minimize offshore vessel deck space for pre-commissioning equipment, allowing smaller and cheaper vessels to be used.

John Grover
Baker Hughes Process and Pipeline Services