Qualifying legacy platforms for CCS: the inspection data that decides it

The decision hiding inside a CCS repurposing

A recent SPE paper (SPE 225805) applies the structural-integrity-management (SIM) framework to an aging offshore complex being assessed for the largest carbon capture, utilisation and storage project in southeast Asia. The work pairs a series of structural-integrity evaluations with a sacrificial anode requalification study, looking at a wellhead platform – WHP-B in the paper – and the modifications needed to press it into CCUS service as it approaches the end of its design life. The target is a life extension in the order of 25 to 30 years.

The SIM framework the authors use is the familiar four-phase cycle: data management, evaluation, strategy development, and programme implementation. That structure is not decoration. It sets the order in which questions get answered, and the first phase – the systematic collection, organisation and review of everything known about the platform – is where most repurposing business cases are quietly won or lost.

We want to take that grounding and open it out into the decision a survey or integrity manager actually faces. When someone asks whether a 30-year-old jacket can carry CO2 injection service for another three decades, the honest first answer is a question about data: do you hold enough of the right inspection and condition information to build a defensible fitness-for-service case, and if not, what will it cost to acquire it. That is a client decision with real money and real programme risk attached, and it deserves to be treated as an engineering problem rather than a hope.

Why the data question dominates the economics

The reuse-versus-decommission choice is rarely settled by the condition of the steel. Well-maintained tubular jackets in benign-to-moderate metocean regimes routinely have structural reserve beyond their nominal design life. The binding constraint is almost always the assurance case: can you demonstrate, to a class or verification body and to your own duty of care, that the structure remains fit for its intended purpose under a load history it was never designed for.

That distinction matters because the two paths carry very different cost profiles. Decommissioning is a large, largely known liability. Repurposing looks cheaper on paper – you inherit an installed jacket, foundations and topsides footprint – but it carries an information liability that is easy to underestimate. Every gap in the as-built record, every missing inspection campaign, every undocumented repair becomes an assessment assumption, and conservative assumptions eat into structural reserve fast. A jacket that is physically sound can still fail its reuse case because nobody can prove it is sound to the standard a 25-to-30-year extension demands.

For CCS specifically, the exposure is sharper than a straight oil-and-gas life extension. The changed service usually means new topsides mass and centre-of-gravity shift from compression, dehydration and injection equipment, additional wind area, and in many cases new conductors, risers or pipeline tie-ins. Those changes feed straight into base shear, overturning moment and pile loads. You are not re-qualifying the platform for what it did; you are qualifying it for something new. The inspection data has to support that new duty, not the old one.

What actually drives whether the data supports reuse

The assessment standards set the yardstick. ISO 19901-9 and API RP 2SIM define the structural-integrity-management process for existing fixed structures, ISO 19902 governs the reassessment of steel jackets, and NORSOK N-006 gives detailed guidance on assessing existing structures where original design margins no longer apply. Cathodic-protection design and requalification sit under DNV-RP-B401. For the CO2 side of the system – dense-phase behaviour, pipeline and well integrity – DNV-RP-J203 and the CO2 pipeline guidance in DNV-RP-F104 come into play. These documents tell you what an assessment must demonstrate; the inspection scope exists to feed them.

Working backwards from the assessment, a repurposing decision turns on a handful of physical questions, each with a data requirement behind it.

Load path and global response. With new topsides loads and possibly climate-updated metocean criteria, the jacket needs a re-run of global in-place and, for extended life, spectral fatigue analysis. That model is only as good as its inputs: current marine-growth thickness and roughness (which drive hydrodynamic diameter and drag), member as-built dimensions, and any deformation or out-of-straightness in primary members. Marine growth is not a cosmetic survey item – a few extra centimetres of growth diameter across the wave zone changes global loading materially.

Member and joint integrity. Fatigue-sensitive tubular joints, corrosion at the splash zone, and the through-wall condition of primary members govern reserve strength. Flooded member detection (FMD), by ultrasonics or radiography, is the efficient screen here because it finds water ingress – the signature of a through-thickness crack – without full member cleaning. Where FMD or close visual inspection flags a defect, crack detection and sizing by MPI, ACFM or eddy current follows, and wall loss is quantified by ultrasonic thickness gauging. Optical methods have a place in capturing and communicating defect geometry, but they characterise surfaces rather than sub-surface flaws; the distinction between a visualisation aid and an NDT method is one we have argued matters in subsea weld repair inspection, and it applies with equal force here.

Cathodic protection. This is the item the SPE work singles out, and rightly so. Sacrificial anodes designed for the original life are, by definition, near or past depletion at the end-of-life mark. A requalification needs anode consumption data – visual and dimensional assessment of remaining anode mass – alongside structure-to-electrolyte potential readings taken by ROV contact probe or proximity cell at representative nodes. Adequate protection means potentials more negative than roughly −800 mV against Ag/AgCl, with −900 mV the working target where sulphate-reducing bacteria create anaerobic conditions. If the design current demand for a 25-to-30-year extension exceeds remaining anode capacity – and it usually does – the strategy phase has to specify a retrofit, typically anode sleds or clamped anodes, and that retrofit is itself a design and installation cost that belongs in the reuse business case.

Foundations and seabed. Pile capacity under revised loads, plus any scour, settlement or debris at the mudline, closes the load path. A seabed and scour survey around the footings is not optional for a life-extension case that increases base shear.

The through-line is that the inspection campaign is not a general health check. It is a targeted data-acquisition exercise scoped to answer the specific questions the reassessment model cannot answer from records alone.

Where reuse cases go wrong on data scope

1. Treating the last routine inspection as a baseline

Legacy platforms often carry years of periodic inspection reports written to demonstrate continued fitness for the original duty. Those reports were scoped against a risk picture that no longer holds once you change the service. Reusing them as a baseline imports old acceptance thresholds and old critical-member lists into a new problem. The right move is to derive the data requirement from the reassessment first, then check what the historical record already satisfies – not the other way round.

2. Underscoping the anode and CP requalification

CP requalification is frequently reduced to a handful of potential readings and a note that the structure is “adequately protected.” At end of design life that conclusion is fragile. Without anode consumption data and a recalculated current demand for the extension period, you cannot show that protection holds for another 25 to 30 years. This is the single most common gap between a survey that looks complete and an assessment that stands up.

3. Ignoring the change in the load path

CCS topsides are not the topsides the jacket was designed for. Teams that survey the structure thoroughly but never revise the global model for new mass, wind area and conductor or riser loads produce a beautifully documented answer to the wrong question. The condition data has to be married to an updated in-place and fatigue model, or it demonstrates nothing about the future duty.

4. Confusing visual condition with structural fitness

A clean, well-painted member in the splash zone tells you about the coating, not about sub-surface fatigue at the adjacent weld. Visual and 3D-optical records are necessary but not sufficient. Where members show dents, bows or out-of-straightness, the deformation itself becomes a strength-reduction input, and it needs to be measured to a defined tolerance rather than eyeballed – the same discipline that governs a structural out-of-straightness survey applies to deformed jacket members feeding a reassessment.

5. Leaving data quality unquantified

The costliest error is treating all inherited data as equally trustworthy. As-built drawings that were never reconciled against fabrication records, weld details recorded as “typical,” undocumented repairs – each is an assumption, and the assessment has to be explicit about which findings are measured, which are inferred, and what conservatism covers the gap. An integrity decision that cannot show its audit trail is not an integrity decision; it is an opinion.

How to decide: build the data requirement backwards

The practical route to a defensible reuse decision is to scope the survey from the assessment, not from habit. Concrete steps for the team:

• Run the reassessment model first, in draft. Build the updated in-place and spectral fatigue model with the intended CCS topsides loads and current metocean criteria to ISO 19902 / NORSOK N-006. Let the model identify the governing members, joints and load cases. Those become your inspection priority list.
• Do a formal data gap analysis against the as-built. Reconcile drawings, fabrication records and past inspection reports against what the assessment needs. Classify every input as measured, inferred or unknown. The unknowns define the campaign scope.
• Specify the ROV and NDT scope against that gap list, not a template. As a working baseline: FMD across all primary members and fatigue-critical joints; CP potential readings by contact probe at defined nodes across each level, plus anode dimensional and consumption survey; UT wall thickness at splash-zone and identified wastage locations; marine-growth thickness and roughness profiling through the wave zone; MPI or ACFM on any defect flagged by FMD or CVI; and a seabed and scour survey around the footings.
• Tie acceptance to a closed audit trail. Every anomaly gets dispositioned against the assessment criteria, with a documented decision. Adopt the SIM cycle as a live process – the SPE paper is explicit that SIM is not a one-time assessment at the start of the extension, and the strategy phase must define ongoing inspection intervals and monitoring for the full 25-to-30-year period.
• Budget the retrofits the data will trigger. Assume a CP retrofit and some member or joint remediation as the base case, not the exception, and carry those in the reuse-versus-decommission comparison. A repurposing case that only counts the survey cost and ignores the near-certain anode retrofit is not a like-for-like comparison.
• Set the go/no-go on assurance, not sentiment. If the data gap cannot be closed to the standard a life extension requires within an acceptable cost, that is a decision, not a failure. Knowing early that the assurance case will not close is worth more than a discounted survey that produces an ambiguous answer.

The recurring lesson from life-extension work is that the vehicle – the jacket – is rarely the hard part. The assurance case is. Scope the inspection data to close it, quantify what you do not know, and let the model tell you where to look. A CCS repurposing decision made on that basis will survive verification; one made on the last routine report and an optimistic read of the paintwork will not.

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Based on: Framework Enables Evaluation of Aging Offshore Platforms for CCS