How ships prove they’re scrubbing sulfur: Inside the CEMS-driven control plan ports now expect

The sulfur cap arrived on a fixed date — and with it, a new expectation: continuous, tamper‑proof evidence that exhaust gas cleaning systems are actually working. Under MARPOL Annex VI, all ships worldwide must use fuel ≤0.50% sulfur outside Emission Control Areas (ECAs) and ≤0.10% inside ECAs since 1 January 2020 (www.imo.org). Ships that continue burning higher‑sulfur fuel must use an approved exhaust gas cleaning system (EGCS, “scrubber”) to meet these limits (www.imo.org).

Flag Administrations — Australia is a typical example — require any EGCS be approved by the flag State/recognized organization and operated per IMO EGCS Guidelines (MEPC.259/340) (www.amsa.gov.au). In practice there are two compliance schemes: Scheme A (use compliant fuel, with only daily spot‑checks if a scrubber is onboard) and Scheme B (continuous monitoring via CEMS, a continuous emission monitoring system) (www.dnv.com). Under Scheme B, the ship’s SOx Emissions Compliance Plan must explicitly describe how continuous SO₂/CO₂ monitoring is done and recorded (www.isesassociation.com).

Indonesian flag context and port constraints

For Indonesian‑flagged ships, the Directorate General of Sea Transportation initially mandated ≤0.5% sulfur fuel in Indonesian waters from 1 January 2020, with approval of scrubbers for those using >0.5% fuel (www.transatlanticlaw.com). A July 2023 government notice indicated a temporary deferral of the IMO2020 fuel rule for domestic fleets (www.e-ports.com). In any case, Indonesian ships engaged in international voyages must still meet IMO limits (www.transatlanticlaw.com).

Indonesian regulations require logging of fuel changeovers and wreckage; for example, MARPOL VI Reg.14.6 requires records of tank volumes, date/time/position for any fuel switch at ECA boundaries (awpmarine.com). Port authorities also enforce local restrictions (several ports ban open‑loop washwater discharges), so the system must track mode changes in and out of port.

Scrubber control loops and setpoints

The scrubber is managed around performance targets — pH, flow, and removal efficiency — to keep the SO₂ down. A typical wet scrubber (open‑loop or hybrid) pumps seawater or caustic through the exhaust plume to absorb SO₂; “closed‑loop” units dose sodium hydroxide (NaOH) and recirculate liquid. Control parameters include water flow rate and chemicals, each regulated to keep scrubber outlet SO₂ low. Maintaining scrubber discharge pH around 6–8 is common for open‑loop systems.

Process sensors (inlet/outlet flows, pressure drop, temperature) feed a control loop. If the outlet SO₂/CO₂ ratio begins to rise, the controller increases water flow or additive. For chemical addition, operators typically rely on a precise dosing pump to meter NaOH. Routine actions — flushing pipelines, replacing filters, and cleaning nozzles — are scheduled based on run hours and performance trends. In‑situ monitoring data can also trigger maintenance: for example, a gradual increase in SO₂ concentration at fixed engine load would flag scaling or catalyst decommission.

Removal efficiency and design targets

To ensure compliance, the scrubber must remove on the order of 95–99% of SO₂. Laboratory and field data indicate well‑designed wet scrubbers routinely achieve ~98–99% SO₂ removal (www.mdpi.com). For example, burning 3.5% sulfur fuel (the highest HFO limit) would require ~98% removal to meet a 0.10% output. The control plan therefore maintains an emissions ratio safely below regulatory limits. Key scrubber parameters (pH, effluent turbidity, PAH, nitrate etc.) are monitored continuously per MEPC guidelines, but the core control loop targets SO₂ removal efficiency. Measurements such as exhaust pressure drop and liquid flow rate (often standard in scrubber designs) also feed into the control logic to maintain proper spray density.

CEMS architecture and analyzers

A continuous emission monitoring system (CEMS — a continuous analyzer installed in the exhaust) is mounted in the stack, typically downstream of the scrubber, to sample and analyze gas continuously. A common configuration uses extractive sampling with a dilution probe feeding analyzers. The CEMS must measure SO₂ concentration and a reference gas (usually CO₂ or O₂%). Most marine CEMS use UV fluorescence for SO₂ and NDIR/NDIR‑laser for CO₂.

A DNV type‑approved system can be user‑set for 0–50 ppm up to 0–2000 ppm SO₂ and 0–12% CO₂ (greentech-marine.dk). Detection limits are on the order of 0.1 ppm for SO₂ (greentech-marine.dk), sufficient to detect even trace leaks. In practice, the SO₂ channel is calibrated and ranged to the expected exhaust concentration (often hundreds of ppm), and CO₂ to ~0–12%. Modern laser‑based analyzers (e.g., Fuji ZQS) require only annual calibration (www.axcoleman.com), while fractional checks (zero/span gas) should be performed monthly or quarterly per manufacturer guidance.

Recording frequency and tamper-proof logging

Data logging hardware collects SO₂ (ppm) and CO₂ (gas %) at a high rate. IMO MEPC.259(68) originally required ≥0.0035 Hz (≈ one sample every 4.8 minutes), but updated 2021 guidelines (MEPC.340(77)) raise this to ≥0.011 Hz (one sample every ~90 seconds) (www.dnv.com). The CEMS sampler must be heated/moisture‑stable to avoid condensate, and data must be “tamper‑proof” (for example, encrypted or locked with password/time‑stamp) (www.isesassociation.com).

The recorded set includes time, GPS position, SO₂, CO₂, plus auxiliary parameters (some systems also log O₂% if needed, and gas temperatures/pressures). According to IMO rules, CEMS should log at least the following (with UTC time and GPS): SO₂ (ppm) and CO₂ (%), washwater inlet pressure and flow rate, discharge pH, PAH, turbidity, precatalyst exhaust pressure/temperature, and engine power/load (www.isesassociation.com). In practice, the emissions ratio (SO₂/CO₂ by mass) is computed continuously. To protect the fluid side, many operators maintain intake protection with a robust strainer so that filters and nozzles withstand marine fouling between scheduled replacements.

Failure modes and mandatory changeover

If a sensor fails, the EGCS manual must specify a backup plan (www.dnv.com). For instance, a single‑channel SO₂ analyzer may be backed up by an independent channel or by manual fuel analysis as a temporary measure. Per IMO Circ.883 guidance, any failure of the CEMS that cannot be remedied within 1 hour requires notifying authorities and switching to compliant fuel (www.isesassociation.com).

The control plan includes an alarm hierarchy: sensor health checks (span/zero errors) trigger alerts, and repeated or prolonged malfunctions — or any “continuous non‑compliance” beyond allowed short‑term exceedance — initiate immediate reporting and fuel changeover as mandated (www.isesassociation.com).

Data analytics and performance metrics

All CEMS data are logged in a secure database. Custom software computes the instantaneous emissions ratio and compares it to the limits, which depend on whether the ship is inside an ECA or not. The data tool also tags each entry with “ECA” or “non‑ECA” status (by GPS) so it can apply the correct ER limit (www.dnv.com).

If a parameter is allowed to exceed its limit for short durations (per Circ.883), the logs mark those events separately; otherwise all exceedances are flagged as violations. Daily or voyage summaries are automatically generated showing average and peak SO₂, CO₂, ER, and stack removal efficiency. The captain’s watch receives real‑time displays and alarms if SO₂ rises unexpectedly, so crew can adjust engine loads or scrubbing rates as needed.

Periodic calibration logs are attached to the digital record to prove analyzer accuracy. For additional quality control, “redundant” sampling lines or duplicate CO₂ channels may be installed (some guidelines stipulate tuning multiple analyzers to minimize lag effects) (www.scribd.com) (www.dnv.com). In short, all compliance parameters are kept within manufacturer specs and regulatory criteria via continuous monitoring and spot cross‑checks.

Key performance figures: a well‑operated EGCS should drive the fuel‑equivalent SO₂ emissions below 0.50% (by ensuring ER < (fuel‑S%/14.5) as per regulations). In practice this means output SO₂ is typically in the single‑digit ppm or lower. Historical data can be trended: for example, a certified installation might log a 99% reduction in SO₂/CO₂ ratio when the scrubber is on (vs off) (www.mdpi.com).

Industry statistics show that by mid‑2020 nearly 25% of boxships had scrubbers (www.statista.com), and today about 36% of the global container fleet is so equipped (www.worldports.org), reflecting their technical success in maintaining compliance under IMO 2020.

Flag and port reporting requirements

All CEMS and scrubber records are maintained onboard with onboard backup. Key records include: the SOx Emissions Compliance Plan (SECP) itself, the scrubber’s Type Approval certificate, daily logs of SO₂/CO₂ ratios or CEMS output, and an “EGCS Operational Log” recording dates/times when the scrubber was in use. While MARPOL does not mandate a special “Scrubber Record Book,” Port State Control (PSC) and flag auditors commonly expect equivalent documentation. In addition, standard fuel logs (per Reg.14.6) must note any fuel changeovers, which help verify when the EGCS came online (awpmarine.com). All data — CEMS output, washwater pH/turbidity, etc. — are backed up at least three years as required.

Flag State: The plan ensures compliance with flag requirements. Classification/flag surveys will inspect the EGCS type‑approval, check that the Onboard Monitoring Manual covers sensor failures, and review that the SECP details the continuous‑monitoring scheme (www.isesassociation.com) (www.dnv.com). In line with MEPC guidance, any extended scrubber malfunction (>1 hour) or repeated sensor failure is formally reported to the flag administration as well as nearby port authorities, including corrective action (www.isesassociation.com). Indonesian and other flags typically grant an EGCS Statement of Compliance or PSC certificate only when satisfied the monitoring system meets IMO standards (www.amsa.gov.au).

Port Authority: On entering a port, the master presents the certificates as usual (EPSC). If queried about sulfur compliance, the crew can show the fuel delivery receipts and scrubber logs. Many port inspectors now use remote sensing or boardings to check SOx compliance. In such cases, the vessel’s tamper‑proof CEMS records (recorded against GPS/time) serve as “proof” that the scrubber was active and effective on the voyage (www.isesassociation.com). If a port is in an area banning open‑loop discharge, the vessel can demonstrate via logs that it switched to closed‑loop mode inside the port. In short, this plan provides all required data on‑demand: continuous SO₂ and CO₂ readings, washwater parameters, calibration certificates, and fuel switch logs (www.isesassociation.com) (awpmarine.com).

Measurable outcomes and enforcement reality

With this plan, a vessel can target and document SOx emissions well below 0.50% fuel‑equivalent. The baseline for success is compliance at all times: for example, >98% of logged intervals should meet the limit, with zero excursions beyond allowed short‑term buffers. Trends in the data (graphs of daily maximum SO₂) provide early warning of performance drift.

Statistically, implementing continuous monitoring has been shown to greatly aid compliance — as one industry report notes, enforcement depends on “tamper‑proof data collection” so shipowners “can satisfy…port State control that [the] scrubber is fully operational and was in use” (www.isesassociation.com). By following this plan, ship’s management will have the technical evidence to achieve and demonstrate 100% production of legal SOx values, while minimizing downtime and avoiding fines from flag or port enforcers.

Sources: Authoritative IMO guidelines and analyses, classification society advisories, and industry reports (www.imo.org) (www.amsa.gov.au) (www.dnv.com) (www.dnv.com) (www.isesassociation.com) (www.isesassociation.com) (www.isesassociation.com) (www.mdpi.com) (greentech-marine.dk) (www.axcoleman.com) (www.axcoleman.com) (awpmarine.com) (www.transatlanticlaw.com) (www.e-ports.com) (www.worldports.org) (www.statista.com).