Storage tanks in operating oil & gas, petrochemical and water facilities age, corrode and sometimes sustain damage during service. When a tank requires repair — whether it is corrosion in the shell plates, bottom plate thinning, nozzle leaks or roof damage — the engineering and documentation requirements for a safe, code-compliant repair are substantially more complex than for a new tank. This article covers what a tank repair engineering scope involves, what an effective Method Statement (MOS) for tank repair must contain, and how API 653 governs the process.
API 653 — The Governing Standard for Tank Repair
While API 650 covers new tank design and construction, API 653 (Tank Inspection, Repair, Alteration and Reconstruction) governs the inspection, assessment and repair of tanks already in service. API 653 requires that:
- An authorised inspector (AI) — a person certified to API 653 standard or equivalent — is responsible for tank inspection and for accepting repair procedures
- Fitness-for-service (FFS) assessment is performed when thickness measurements fall below the minimum acceptable thickness (MAT) calculated from operating conditions
- Repairs to pressure-containing components use qualified welding procedures (WPS/PQR) and qualified welders — as with new construction
- A repair design is reviewed and accepted by a responsible engineer before work commences
Engineering Scope for a Tank Repair
Before any repair work begins, the engineering team typically delivers the following:
Inspection Data Review and FFS Assessment
The engineer reviews UT thickness scan data, visual inspection reports, floor scan results and any previous repair records. Using API 653 Section 9 methods, a minimum acceptable thickness (MAT) is calculated for each shell course under the operating liquid head. Areas below MAT must be repaired or the safe fill height must be restricted.
Repair Design
The repair design defines exactly how identified defects will be corrected. Common repair types include:
- Shell plate replacement: removal and replacement of a full or partial shell strake section, with weld details and material specification
- Flush patch: cutting out a corroded area and welding in a flush plate insert per API 653 Annex B
- Bottom plate renewal: lifting or lap-welding new plate over existing corroded floor, with annular ring detail
- Nozzle replacement: removing a leaking or corroded nozzle and installing a new nozzle with full-penetration welds and reinforcing pad
- Shell insert plate: installing a reinforcing plate over a localised pitted area where full plate replacement is not warranted
Each repair detail is drawn on a sketch or formal drawing that references the applicable API 653 clause and specifies the weld procedure, material grade, preheat requirements and NDE method.
What a Tank Repair Method Statement (MOS) Must Contain
A Method Statement (MOS) for tank repair is the primary safety and execution document submitted to the operator for approval before work starts. A well-structured MOS typically contains the following sections:
- Scope of work: equipment tag, tank dimensions, repair description referencing the inspection findings, API 653 basis
- Isolation and de-energisation plan: confirmation that the tank is isolated from live lines, degassed, vapour-tested, and deemed safe for hot work — referencing the permit to work (PTW) system
- Sequence of work: step-by-step task sequence with hold points and witness points for the inspector
- Welding procedures: WPS references applicable to each joint, preheat requirements (typically per API 653 Table 9-1 or ASME IX), inter-pass temperature limits
- NDT plan: inspection type (VT, MT, PT, RT, UT), extent (100%, 10%, spot), timing (before and after PWHT if applicable), acceptance criteria standard referenced
- Post-weld heat treatment (PWHT): where required by code or material, furnace or local heating procedures, soak temperature and hold time, thermocouple positions
- Hydrotest or alternative leak test: test pressure, test fluid, hold time, acceptance criteria
- Resources: welders and their qualification references, inspector certification
- HSE considerations: hot work controls, confined space entry requirements if working inside the tank, fire watch arrangements
SLETEC supports EPC contractors and plant operators with both the engineering design and the method statement documentation for tank repairs. Our API 650 tank engineering team prepares repair design sketches and calculation support referenced to API 653, and our method statement service produces MOS documents formatted to Saudi Aramco, SABIC and independent operator requirements — ready for submission and approval without significant revision cycles.