The Challenge Of Lowering A Live Gas Pipeline

Acteon companies combine on a world-first project, offshore Indonesia.

Major development projects often have implications for existing infrastructure and accommodating the necessary changes may require fresh thinking. This was certainly the case when a proposed expansion programme at Tanjung Perak Port in East Java called for seabed dredging in the shipping channel to provide access for deepdraught vessels.

To accommodate the dredging operation, a 16-in. gas pipeline, which provides fuel to Madura Island, needed lowering. The aim was to lower the pipeline by 6 m so that its top was 9 m below the original seabed where it crossed the shipping channel. The gas supply was uninterruptable, so the pipeline remained fully operational and at a pressure of 500 psi throughout the operation.

Offshore Installation Services (OIS) took overall responsibility for the commercial and operational aspects of the project by providing project management, including chartering an offshore supply vessel and managing all the subcontractors, from its base in Aberdeen, UK. It also provided in-country project teams throughout, both on- and offshore.

The project required meticulous planning and an innovative approach to the lowering process, as Tom Selwood, vice president – commercial and business development, OIS, explains, “This unique, multimilliondollar subsea project presented many technical and logistical challenges. We identified four issues that would be crucial to the project’s success:

  • ƒƒan effective, stable trench design based on the specific seabed conditions
  • ƒƒa reliable engineering system to create a deep trench and enable controlled pipeline lowering
  • ƒƒa system to monitor the progress of the trenching and the pipeline lowering operation in real time to ensure that the bespoke trench design was being achieved
  • ƒƒengineering stress assessment of the pipeline at each step of the lowering process to ensure that the pipeline was not overstressed.”

The first challenge was to ensure that the trench walls were stable and would remain intact throughout the lowering operation. Any soil collapse onto the pipeline might cause stress fracturing and a pipeline rupture, with potentially catastrophic results.

The seabed is a highly variable natural environment. To ensure stability, the project team had to devise a trench design that took account of the local seabed composition. Soil expert Dr Indrasenan Thusyanthan, engineering manager from sister Acteon company Cape Group, designed a stepped, V-shaped trench specifically for this operation. The plan involved moving more than a million cubic metres of soil to create a 9-m-deep trench.

The project team undertook an extensive engineering assessment of the pipeline lowering before work started. This ensured that the stresses in the pipeline would remain below safe limits, as Thusyanthan explains, “No one had any experience of a job like this and some people thought it would be impossible to lower a live gas pipeline without a serious risk of structural failure. Clearly, as the pipeline is lowered from two fixed end points its profile changes and it stretches. Our task was to ensure that the stretching stayed within safe limits. We decided the best and safest option was to lower the pipeline in steps of 0.5 m and monitor at each step to ensure that the bending stresses induced in the pipeline were within acceptable levels.”

OIS contracted Rotech Subsea to conduct the trench excavation work. Rotech provided its T8000 controlled-flow excavation system, which was key to the success of the trenching operation. The T8000 excavator is a state of-the-art T-shaped tool with two contra-rotating impellers, one at the end of each arm of the tee. It draws seawater in and directs a controllable column of water vertically to the seabed at high volume and high speed but low pressure, which is ideal for working on live targets. It also has a range of nozzles for variable seabed conditions.

In addition to the T8000 system, Rotech supplied a higher-pressure water pumping system that would cut the stiff clay while the controlled-flow water column fluidised the soil to create the trench. This combination of tools enabled safe and controlled lowering of the pipeline.

As the trench was to be lowered in increments, it was vital to monitor the profile in real time and to carefully control each of the lowering passes so that the pipeline was not left resting on free spans. A fourth-generation survey solution from Acteon company NCS Survey monitored the trenching progress and ensured that the Rotech operations matched the engineering design from CAPE.

This involved mounting NCS Survey’s equipment on the front and back of the T8000 excavating tool to provide real-time before and after analyses of the excavation work. This inspection method was necessary because there were strong currents in the area, which caused significant backfill and would have rendered a postexcavation survey ineffective. The NCS Survey monitoring technology made a crucial contribution: without being able to monitor the trenching operation in real time, the job would not have been so quick or so safe.

The trench was deepened in 14 separate excavation passes over a period of eight months. The project team conducted a detailed stress assessment of the pipeline after each pass to ensure its integrity was not compromised. The oil and gas operators involved and SKMIGAS, the upstream oil and gas regulator, were pleased with the result. The Indonesian authorities independently verified the new burial depth as 9 m below the original seabed.

The lowering operation was a complex challenge requiring innovation in several technical disciplines and a clear vision for project delivery. Seamless working together by the Acteon companies and the subcontractor involved helped with the project’s success.

Share this content