API 650 Storage Tank Design in Saudi Arabia: Codes, Loads and Deliverables

Which code applies: API 650, API 620 or API 653?

API 650 covers welded steel tanks for oil storage at internal pressures approximating atmospheric pressure, and up to 2.5 psig (about 18 kPa) under its Annex F, with metal temperatures up to 260 °C under its high-temperature annex. It is the standard for most atmospheric crude, product, water and chemical tanks.

API 620 applies to large low-pressure tanks above the API 650 pressure range, up to 15 psig. API 653 governs inspection, repair, alteration and reconstruction of tanks already in service — so a change of service, a new nozzle in an existing shell or a bottom replacement is an API 653 job, even if the tank was built to API 650.

On Saudi projects the code is almost always combined with a client specification. Saudi Aramco projects add the relevant SAES and SAMSS documents; SABIC, Ma'aden and other owners have their own requirements. These must be identified at the start, because they often go beyond the code — for example on minimum thicknesses, corrosion allowance, nozzle reinforcement or foundation details.

Design data to fix before design starts

Loads that often govern in Saudi Arabia

Coastal sites in the Eastern Province and on the Red Sea coast combine high temperatures, wind and corrosive atmospheres. Typical governing checks are:

• Wind stability of tall, thin upper shell courses, which drives top and intermediate wind girders.
• Seismic overturning and sloshing where the site hazard requires API 650 Annex E checks, and the resulting freeboard for roof clearance.
• Uplift and anchorage from internal pressure combined with wind or seismic loads.
• Roof live loads and access — platforms, handrails, maintenance loads and occasional scaffold loads through roof manholes.
• Nozzle loads from thermal expansion of connected piping, especially on lower shell courses.

When FEA adds value

The code formulas cover the standard cases. FEA is worth the effort when something is unusual: a new opening in an old shell, a support or platform loading the roof, local settlement, a heavier liquid than the original design, or a repair that changes stiffness.

Two SLETEC examples show the type of answer FEA gives:

• Heavier liquid in an existing tank (Ref. 79). An existing Ø6,096 mm tank with a 5.3 mm shell had to store a 1.83 SG fluid. Static-structural FEA under the hydrostatic gradient established a safe filling height of 4,450 mm at a maximum shell stress of 48.02 MPa — a clear operating limit for the owner.
• Scaffold access through the roof (Ref. 77). FEA of the dome roof under gravity, wind and scaffold interface loads gave a maximum vertical deformation of +0.51 mm and a total support reaction of about 4.90 × 10⁴ N, and access was approved within serviceability limits.

What a complete tank design package contains

• Design basis with codes, client specifications, loads and assumptions.
• Design calculations: shell, bottom and annular plates, roof and roof structure, wind girders, seismic, anchorage, nozzle reinforcement and venting.
• General arrangement drawing with nozzle orientation and schedule.
• Detail and fabrication drawings: shell development, bottom and roof plate layouts, nozzles, manways, stairways, platforms and handrails.
• Foundation loads and, where included, foundation design drawings.
• Material take-off and bill of materials.
• FEA reports where the scope requires them, with acceptance criteria stated.

Common mistakes that cause rework

• Design basis agreed too late. Starting calculations before the specific gravity, corrosion allowance and seismic parameters are frozen almost always means recalculating shell courses.
• Client specification read after the code. Owner standards frequently set minimum thicknesses, nozzle details or bottom slopes that differ from API 650 defaults. They should be in the design basis from day one.
• Nozzle loads ignored. Piping designers and tank designers need to exchange allowable and actual nozzle loads early; late changes can force reinforcement or shell thickening.
• Access added without roof checks. New platforms, handrails or maintenance access on an existing roof need a structural check — the roof was not necessarily designed for them.
• Foundation and tank designed separately. Settlement limits, anchor bolt positions and ringwall dimensions must match between the two packages.

Typical design sequence and timeline

Duration depends on tank size, the number of nozzles and accessories, and the client review period. A realistic offer lists the inputs required and the review periods, not only a delivery date.

What to send for a quotation

To price a tank design accurately we need the data sheet or the items in the table above, any client specifications, the soil report if foundations are included, and the deliverable list and schedule. For an existing tank, add the original drawings, the latest inspection report and photographs. Send them to enquiry@sletec.com or see our API 650 tank design service.