APPLICATIONS
Deck Drains Discharge
Oil rig decks have drains which allows waste and run off water to run down into the slop tanks. This water must be separated and discharged.
Offshore platforms, oil rigs, FPSOs and drill rigs have open deck areas exposed to rain, sea spray and the residues of drilling and production operations. Deck drains collect this runoff water which may contain oil, drilling fluids, chemicals and other contaminants and route it to slop tanks or directly to a discharge point.
Because deck drain water typically contains hydrocarbons, it cannot be discharged overboard without treatment and monitoring. An oil-in-water monitor measures the oil content of the drain water before discharge, ensuring that environmental compliance is maintained at all times. Rivertrace provides oil-in-water monitoring solutions for deck drain applications across offshore platforms, FPSOs and drill rigs.
| Key thing to remember
Deck drain water on offshore installations is not the same as clean rainwater. It collects oil residues, drilling fluid, hydraulic oil, chemical spills and other contaminants from the deck surface. This water must be monitored for oil content before discharge and in many jurisdictions, it must meet a specific oil concentration limit set by the field permit or applicable environmental regulation. |
What Are Deck Drains and Why Do They Produce Oily Water?
Deck drains are drainage systems built into the open deck areas of offshore platforms, drill rigs and FPSOs. They collect water from several sources:
- Rainwater and sea spray which picks up oil and chemical residues from deck surfaces
- Wash-down water used to clean drilling equipment, pipe handling areas and production deck surfaces
- Spills and leaks from drilling fluids, hydraulic systems, lubricants and process pipework
- Condensate and process fluid runoff from topsides production equipment
The water collected by deck drains is therefore a highly variable mixture the oil content and composition depend on the type of operations taking place, weather conditions, maintenance activities and the state of deck containment systems. This variability is one of the key challenges in specifying deck drain monitoring equipment.
What Regulations Apply to Offshore Deck Drain Discharge?
Deck drain discharge from offshore installations is typically governed by the field permit issued by the national environmental authority in the jurisdiction where the installation operates. There is no single global standard equivalent to the MARPOL 15ppm bilge water limit that applies universally to offshore deck drain discharge.
Field permits and national legislation
In most offshore producing jurisdictions including the North Sea, Brazil, West Africa and the Gulf of Mexico deck drain discharge is subject to the environmental permit conditions set for the specific installation. These may specify:
- A maximum oil concentration limit for deck drain discharge expressed as a ppm limit or a monthly average
- A requirement for continuous monitoring or periodic sampling before discharge
- Reporting and record-keeping requirements for discharge operations
- Restrictions on discharge in specific sea areas or weather conditions
MARPOL Annex I
Where a floating installation is classified as a vessel and operates under MARPOL, Annex I requirements may also apply to certain discharge streams. The applicable regime depends on the vessel classification, flag state and the specific discharge point.
| Variable composition is a key challenge
Unlike bilge water from a machinery space which has a relatively stable composition deck drain water varies significantly depending on operations, weather and maintenance activity. An oil-in-water monitor for deck drain applications must be able to handle this variability and remain accurate across a range of oil types and concentrations. |
How Is Deck Drain Water Treated and Monitored Before Discharge?
Deck drain water typically passes through a treatment and monitoring sequence before it reaches the overboard discharge point:
Collection and routing
Deck drains route the collected water to a holding point typically a slop tank or a dedicated drain tank. This allows bulk solids and free oil to separate before treatment and monitoring.
Separation and treatment
Free oil and solids are separated from the drain water typically using gravity separation, skimming or hydrocyclone equipment. Where deck drain water is routed through a slop tank, the slop tank acts as a primary separation stage before the water reaches the monitoring point.
Oil-in-water monitoring
An oil-in-water analyser is installed at the outlet of the treatment stage. It continuously measures oil concentration in the drain water and confirms whether it is within the discharge limit set by the applicable permit. If oil concentration is above the limit, the monitor activates an alarm and where connected to process controls stops the discharge automatically.
Discharge or retention
If the monitored drain water is within the permitted limit, it is discharged overboard. If it is above the limit, it is retained in the drain tank or slop tank for further treatment or transfer to a waste management system.
What Makes Deck Drain Monitoring More Challenging Than Other Applications?
Deck drain water presents several measurement challenges that distinguish it from more stable water streams such as cooling water or bilge water:
- Highly variable oil content deck drain water can range from near-clean rainwater to heavily contaminated wash-down water within the same drainage event
- Multiple oil types the drain water may contain crude oil, diesel, hydraulic oil, drilling fluid and chemical residues often in combination
- Suspended solids sand, scale, drilling cuttings and other solids can interfere with optical oil measurement if sample conditioning is inadequate
- Variable flow rate rainfall intensity and wash-down operations can cause significant flow variation, which affects sample conditioning and monitor performance
- Batch rather than continuous flow deck drain discharge is often intermittent rather than continuous, requiring a monitor that performs reliably from a standing start
Which Rivertrace Product Is Right for Deck Drain Discharge Monitoring?
The right product depends on the composition and variability of the deck drain water, the discharge limit set by the applicable permit and the installation environment on the offshore platform.
| Product | Best-fit application | Key capability |
| OCD Xtra | Deck drain discharge monitoring on offshore platforms, FPSOs and drill rigs where calibration against specific oil types is required | Measures oil accurately across 0–200ppm; factory calibrated on up to six oil types; adjustable onsite against laboratory analysis; suitable for variable composition deck drainage |
| SMART 50M | Deck drain and process water monitoring in high-temperature or high-pressure offshore environments | Versatile oil-in-water monitor designed for demanding temperature and pressure conditions; suitable for marine, offshore and industrial deck drain applications |
What Should You Consider When Specifying a Deck Drain Monitor?
Before specifying an oil-in-water monitor for a deck drain discharge application, confirm:
- What is the discharge limit set by the field permit and how is it expressed (absolute ppm, monthly average, or combined)?
- What oil types are likely to be present in the deck drain water crude oil, diesel, hydraulic oil, drilling fluid?
- What solids loading is expected and is filtration required in the sample conditioning system?
- What is the expected flow rate range and does the monitor need to perform reliably at very low or zero flow between drainage events?
- Does the monitor need to interface with a discharge control valve for automatic shut-off?
- Is the installation location in a classified hazardous area requiring ATEX or IECEx certification?
- What are the calibration and maintenance access arrangements on the offshore installation?
| Multi-oil calibration for variable deck drain composition
Because deck drain water can contain several different oil types crude, diesel, hydraulic oil, drilling fluid a monitor that can be calibrated on multiple oil types provides more reliable and defensible measurements than one calibrated on a single reference oil. The OCD Xtra can be factory calibrated on up to six oil types, making it particularly well suited to the variable composition of offshore deck drain water. |
In Which Applications Is Deck Drain Monitoring Required?
Oil-in-water monitoring for deck drain discharge is required across a range of offshore installation types:
- Fixed offshore platforms production platforms with open process decks, pipe handling areas and wellhead areas
- Floating Production Storage and Offloading vessels (FPSOs) topsides deck areas, module areas and loading equipment decks
- Jack-up and semi-submersible drill rigs drill floor, pipe handling deck and process deck drainage
- Drillships drill floor and deck drainage systems
- Offshore support vessels deck areas used for cargo handling, chemical storage and equipment maintenance
- Loading platforms and buoys where crude oil transfer operations create a deck drain monitoring requirement
Frequently Asked Questions
What is deck drain discharge on an offshore platform?
Deck drain discharge is the controlled release of water collected from open deck areas on offshore platforms, drill rigs and FPSOs. This water which contains rainwater, sea spray, wash-down water and contamination from deck operations is treated to remove oil and solids before being discharged overboard within the limits set by the applicable environmental permit.
What regulations govern deck drain discharge from offshore installations?
Deck drain discharge is typically governed by the field permit issued by the national environmental authority in the jurisdiction where the installation operates. The applicable limit varies by country and permit there is no single global standard for offshore deck drain discharge equivalent to the MARPOL 15ppm bilge water limit.
Why is deck drain water difficult to monitor accurately?
Deck drain water has a highly variable composition; it can range from near-clean rainwater to heavily contaminated wash-down water within the same event, and may contain multiple oil types including crude oil, diesel, hydraulic oil and drilling fluid. This variability requires an oil-in-water monitor that can be calibrated against multiple oil types and perform reliably across a wide range of concentrations and flow conditions.
Which Rivertrace product is recommended for offshore deck drain monitoring?
The OCD Xtra is the primary recommendation for offshore deck drain discharge monitoring. It can be factory calibrated on up to six oil types and adjusted onsite against laboratory analysis addressing the variable composition challenge of deck drain water directly. The SMART 50M is suitable for high-temperature or high-pressure deck drain applications.
Can a deck drain monitor be connected to an automatic discharge shut-off?
Yes. Rivertrace oil-in-water monitors can interface with discharge valve controls via relay outputs, so that a confirmed high-oil alarm automatically closes the overboard discharge valve and retains the contaminated drain water for further treatment or transfer to the slop tank.