13.3 Project Economics, Estimating, and Cost Control

Screening a project: payback, present worth, and rate of return

Once cost structures and discounting are in hand, the remaining question is how a project gets approved. Three measures dominate, and each answers a different question.

Simple payback

Payback period = Initial investment / Annual net cash inflow

A heat-recovery retrofit costs QAR 6,000,000 and saves QAR 1,500,000 per year:

Payback = 6,000,000 / 1,500,000 = 4.0 years

Payback is popular because it is instant and because it is a crude risk screen—the shorter the payback, the less time the assumptions have to be wrong. Its two weaknesses are that it ignores the time value of money and ignores everything after the payback point, so it silently favours short-lived projects over durable ones.

Discounted payback

Discount each year's saving before accumulating it. For the same retrofit at 10%:

The cumulative discounted saving passes QAR 6,000,000 during year 6, so discounted payback is about 5.4 years against a simple payback of 4.0. Discounting always lengthens payback, and the gap widens with the discount rate.

Net present worth

NPV = Σ [CF_t / (1 + i)ᵗ] − Initial investment, with the decision rule accept if NPV > 0.

For the retrofit over an 8-year life at 10%, the series present-worth factor is 5.335:

NPV = 1,500,000 × 5.335 − 6,000,000 = 8,002,000 − 6,000,000 = +QAR 2,002,000

Positive, so the project creates value at a 10% cost of capital. NPV is the theoretically sound measure because it uses every cash flow and states the answer in money.

Rate of return

The internal rate of return is the discount rate at which NPV equals zero, and the rule is accept if IRR exceeds the minimum acceptable rate of return (MARR), also called the hurdle rate. IRR is intuitive to managers because it is a percentage, but it can mislead when comparing projects of very different size—a 40% return on QAR 200,000 creates far less value than a 15% return on QAR 20 million. When NPV and IRR disagree on mutually exclusive projects, NPV wins.

Estimating capital cost

Estimate classes

Estimate accuracy improves as engineering definition improves. The class names and ranges vary by owner and by industry, but the progression is universal:

You are not expected to recite a specific class table. You are expected to know that early estimates carry wide ranges and generous contingency, that late estimates support baselines and commitments, and that quoting a concept estimate as if it were a definitive one is how projects acquire their first credibility problem.

The six-tenths rule

Capital cost scales sub-linearly with capacity, because equipment cost tracks surface area while capacity tracks volume:

C₂ = C₁ × (Q₂ / Q₁)ⁿ, with n ≈ 0.6 as the generic exponent

A 50,000 t/y unit cost QAR 200 million. Estimate a 100,000 t/y unit:

C₂ = 200 × (100,000 / 50,000)^0.6 = 200 × 2^0.6 = 200 × 1.516 = QAR 303 million

Doubling capacity costs about 52% more, not 100% more. That single result is the economy of scale that drives world-scale plant sizing in Qatar's petrochemical sector—and the exponent is an approximation valid only within a reasonable capacity range and for the same technology.

Escalating an old cost with an index

C_now = C_then × (Index_now / Index_then)

An equipment package quoted at QAR 4.0 million when the cost index stood at 550, re-priced at an index of 660:

C_now = 4.0 × (660 / 550) = QAR 4.8 million

Never reuse a historical cost without escalating it—a five-year-old quotation is not a current estimate.

From equipment cost to installed plant cost

Purchased equipment is only a fraction of a plant. A Lang-factor approach multiplies the delivered equipment cost by a single factor—commonly quoted around 4 to 5 for fluid-processing plants—to approximate total installed capital including piping, instrumentation, electrical, civils, engineering, and contingency. Treat the factor as an order-of-magnitude tool, not a substitute for a take-off.

Bills of quantities in the Qatar contract context

Qatari construction contracts commonly work from a bill of quantities (BOQ): a schedule of measured work items, each with a description, a unit, a quantity, and a rate. Three consequences a process engineer meets in practice:

• Rates are per unit of measured work, so a design change that adds quantity adds cost automatically, without needing a variation for the rate itself.
• Measurement follows a defined method tied to the project specification, which is why QCS language and BOQ items have to align.
• Re-measurement at completion means the final account reflects actual quantities—so sloppy quantity estimates in design surface as cost growth at handover, not as a surprise at tender.

Linking economics to project cost control

Chapter 14 covers the project-management side: the cost baseline, earned value, and variance analysis. The handshake between the two chapters is straightforward:

• This chapter decides whether to spend the money—break-even, depreciation, payback, NPV, IRR.
• Chapter 14 decides whether the spending is under control once approved—cost baseline, CPI and SPI, contingency versus management reserve.

An engineer who confuses the two will try to answer a "should we sanction this?" question with an earned-value index, or a "are we overspending?" question with an IRR. Read which side of the sanction decision the stem sits on before you pick a tool.

Traps to drill