Decommissioning Studies & Advisory | VORYX Offshore Engineering

Overview

Decommissioning is where an operator's remaining liability is concentrated and where engineering assumptions made early become very expensive later. A study that ignores vessel availability, crane capability, cut technology limits or yard reception capacity produces a cost estimate that will not survive tender.

VORYX prepares decommissioning studies from the execution end backwards. Every option we carry forward is screened against a real removal method, a real vessel class, a real lift weight and a real disposal route — so the programme you submit to the regulator is the programme your contractor can bid.

We work across the full asset stack: fixed steel platforms and jackets, topsides and modules, conductors and wells, subsea production systems, flowlines, rigid and flexible pipelines, umbilicals, power cables, mattresses, grout bags and rock berms.

Scope of Work

01

Conceptual & Screening Studies

Asset inventory, data gap analysis, end-state definition and screening of credible decommissioning options for each structure, pipeline and well group.

02

Platform & Topsides Removal

Single-lift, reverse-installation and piece-small strategies, lift weight and centre-of-gravity control, sea-fastening, cut locations and pile severance below mudline.

03

Subsea & Pipeline Abandonment

Cleaning and flushing philosophy, disconnection sequence, full removal vs. trench-and-bury vs. leave-in-place, exposed-span and crossing remediation, and long-term seabed stability.

04

P&A Concept Development

Permanent barrier philosophy, well categorisation, rig vs. rigless and vessel-based execution concepts, conductor severance and interfaces with the platform removal sequence.

05

Removal vs. Leave-in-Place

Structured evaluation against safety, technical feasibility, environmental impact, societal impact and cost — the five criteria regulators expect to see argued, not asserted.

06

Waste & Disposal Strategy

Material inventory, NORM, asbestos and mercury handling, recycling-rate targets, onshore reception capacity and cradle-to-grave waste tracking.

07

Execution Planning & ITT

Campaign sequencing, weather-window and vessel-spread planning, Scope of Work, Basis of Design, tender packages and bid evaluation criteria ready to issue.

Methodology

1

Baseline the asset

Consolidate as-built drawings, weight reports, installation records, inspection and survey history, well status and production data. Where records are missing, we define the survey scope needed to close the gap rather than assuming a value and carrying the risk forward.
2

Define the regulatory end-state

Establish the removal obligation that actually applies — flag state and coastal state legislation, IMO A.672(16) removal standards, regional instruments such as OSPAR Decision 98/3, and any derogation or deviation pathway that is genuinely open.
3

Generate credible options

Build an option set per asset group. Options that no available vessel or cutting technology can deliver are screened out early and documented as screened out — an audit trail regulators and stakeholders can follow.
4

Engineer the shortlist

Lift and load analysis, structural checks in the removal condition, cut feasibility, pipeline stability and burial assessment, marine operations limits, and interface definition between wells, structures and subsea scopes.
5

Cost, schedule and risk

Bottom-up CAPEX build from vessel day-rates, spread composition and weather downtime, probabilistic schedule, and a risk register that carries quantified contingency rather than a flat percentage.
6

Select and document

Comparative assessment, recommendation, and the decommissioning programme narrative — written to be read by a regulator, a stakeholder and a tendering contractor without three different documents.

Deliverables

Decommissioning strategy and Basis of Design
Asset register, weight control report and data gap register
Option screening and comparative assessment report
Removal method statements and engineering feasibility studies
P&A concept select report and well cost model
Waste inventory, disposal and recycling strategy
CAPEX estimate (Class 5 → Class 3) and probabilistic schedule
Risk register, HAZID close-out and ALARP demonstration
Scope of Work, ITT package and bid evaluation matrix
Decommissioning programme narrative for regulatory submission

Standards & Codes

Work is delivered against the applicable international and regional framework. The codes below are those most commonly governing this scope — the controlling set is confirmed per project and jurisdiction.

IMO Res. A.672(16) Removal of offshore installations — 75 m / 4,000 t thresholds (100 m for installations emplaced on or after 1 Jan 1998) and 55 m clear water column
UNCLOS Art. 60 Coastal state removal duty for disused installations
OSPAR Decision 98/3 Prohibition on leaving installations in place, with candidate derogation categories; UK guidance updates expected 2026
London Protocol 2019 Revised Guidance for platforms and other man-made structures at sea
Petroleum Act 1998 UK decommissioning programmes and s.29 notices
OPGGS Act s.572 Australia — maintain and remove; full removal as base case
ISO 19902 Fixed steel offshore structures — removal condition assessment
DNV-ST-N001 Marine operations and marine warranty
NORSOK D-010 Well integrity and permanent barrier requirements

Frequently Asked

When should a decommissioning study start?

Earlier than most operators expect — typically five to ten years before cessation of production. The study drives regulatory engagement, provisioning, well P&A scheduling and the long-lead vessel market, and all four have lead times measured in years rather than months.

Can we leave a structure or pipeline in place?

Sometimes — but it must be earned, not assumed. Full removal is the default under most regimes, including the OPGGS Act base case and OSPAR Decision 98/3. A deviation or derogation only progresses where a structured assessment demonstrates that full removal is not reasonably achievable against safety, technical, environmental, societal and cost criteria. We build that evidence base.

How accurate is a decommissioning cost estimate at study stage?

A screening estimate is Class 5 (-30% / +100%). What tightens it is not more contingency but better definition — confirmed lift weights, a chosen cut technology, a defined vessel class and a secured disposal route. We state the class, the basis and the drivers rather than presenting a single number.

Do you cover well P&A as well as structures?

We develop P&A concepts, barrier philosophy, well categorisation and cost models, and we manage the interfaces between the well scope, conductor removal and platform removal. Detailed well engineering and execution is delivered with specialist well partners.

Can you take the study through to tender?

Yes. The output is a Scope of Work, Basis of Design and ITT package with an evaluation matrix — and we support technical clarification, bid evaluation and award.