The Challenge
ExxonMobil faced decommissioning of 940km of pipelines and umbilicals across the Gippsland Basin with no clear understanding of optimal removal methods, or environmental impact – critical factors for regulatory approval and cost control.
Our Solution
Ternan Energy delivered a comprehensive site-specific decommissioning strategy that evaluated the feasibility of unburial and trenching methodologies across four distinct pipeline groups, calculated sediment disturbance volumes, and assessed pullout options for flexible flowlines and umbilical assets.
The Results
- Cost optimisation: Identified most cost-effective methodologies and equipment for each pipeline type
- Execution clarity: Provided expected unburial/trenching rates (km/day) and vessel recommendations for accurate project planning
- Environmental compliance: Calculated trench volumes and turbidity impacts to support regulatory submissions
- Stakeholder confidence: Created a clear workflow that enabled ExxonMobil to present technical findings to regulators and community stakeholders in accessible language
- Risk mitigation: Identified execution risks and data gaps before field operations began
Project Overview
ExxonMobil is decommissioning multiple assets across the Gippsland Basin, including a network of approximately 830km of rigid pipelines (with 11km of flexible flowlines) and 110km of umbilicals.
To meet regulatory requirements and optimize costs, ExxonMobil needed to assess feasible and cost-effective methodologies for:
- Unburial of pipelines and umbilicals requiring removal
- Trenching of pipelines and umbilicals to be left in-situ
The Ternan Approach
Ternan Energy was engaged to review the current burial status of the pipeline and umbilical assets and provide expert guidance on unburial and trenching options that would not only satisfy the decommissioning requirements and regulatory obligations but also give clarity on commercial implications.
Key Objectives:
- Identify feasible and cost-effective methodology and equipment for unburial and trenching operations
- Recommend vessels and equipment for unburial and trenching operations
- Calculate expected unburial/trenching rates (km/day)
- Determine volumes of seabed soils disturbed in unburial/trenching scenarios
- Identify execution risks and information gaps
Preliminary Assessment and Pipeline Grouping
Our team conducted a comprehensive data review and gap analysis, examining all available information provided by ExxonMobil. We presented a summary of existing data alongside recommendations for additional data acquisition that would further inform the decommissioning of their assets.
Available trenching records from multiple years were analyzed to assess seabed mobility and the current extent of burial across all pipeline and umbilical assets scheduled for decommissioning.
We grouped pipelines and umbilicals into four categories based on coating and composition, size, and use. For each pipeline group, we assessed:
- Bathymetry and seabed features
- Geological and geotechnical conditions
- Pipeline condition and burial status
- Trench status
- Suitability of Trenching technology
This analysis enabled us to define the most suitable decommissioning approach for each pipeline group.
Trench Volume and Sediment Disturbance Analysis
A critical component of the study involved quantifying soil disturbance during unburial or lowering (trenching) operations. For each pipeline, we calculated:
- Volume of material requiring removal in an unburial scenario
- Volume of sediment that would be disturbed in a lowering scenario
Our calculations accounted for how different equipment types affect trench profiles and wall angles. Trench wall angles and shapes are dependent on soil type and particle distribution. Given that the predominant seabed soils in Bass Strait are fine to coarse sand with varying shell content, we selected trench wall angles by evaluating these soil conditions against the expected angle of repose for submerged sand.
Trench volumes were calculated for both unburial scenarios and in-situ remediation to lower exposed sections to a depth of cover of 0.60m.
Additional Studies
We evaluated turbidity from seabed intervention and suspended sediment concentration, as understanding how far suspended sediment may travel is essential when assessing the potential environmental impact of decommissioning works.
Following completion of the main scope, ExxonMobil requested additional analysis of pullout loads for several umbilical lines and flexible flowlines. This investigation determined whether these assets could be removed without prior unburial, thereby minimizing seabed disturbance.
Decommissioning Workflow
Based on our findings, we developed a site-specific decommissioning workflow for each pipeline group. This workflow provides an easy-to-follow process for determining the most appropriate decommissioning option.
The workflow format enabled ExxonMobil to present our study findings to regulators and community stakeholders in a language that was accessible and understandable without requiring in-depth technical knowledge – facilitating smoother approval processes and stakeholder engagement.
Ternan Energy is a specialist marine geoscience consultancy providing expert technical support for offshore energy projects including offshore wind, oil & gas decommissioning and interconnectors. For more information about our decommissioning services, please contact James Deacon, Geoscience Manager, james.deacon@ternan-energy.com.