On a flotel, a construction support vessel, or a DP-classed accommodation barge working a North Sea campaign, two manning regimes operate on the same deck at the same time. Marine crew hold STCW certificates of competency and work under the Master's command. The installation workforce, whether that's the host platform's own personnel or contractors berthed aboard the vessel, is managed under the OIM's authority and the installation's safety case. Both populations need to be accounted for in the same muster. Neither system was designed with the other in mind.
This isn't a theoretical overlap. Flotels and semi-submersible accommodation units routinely carry several hundred people, a mix of marine crew, construction personnel, catering and hotel services staff, and client representatives, all of whom need to appear on one evacuation muster list regardless of which framework certified them to be there.
What mixed-regime crewing means
Mixed-regime crewing describes an asset where STCW-certified marine crew, operating under the ship's Master, and installation-regulated offshore workforce, operating under the OIM or duty holder, work and live aboard the same vessel concurrently. Each population is governed by a different competency and command framework, but both must be jointly musterable and jointly verified for the asset to meet its safety case.
Where STCW competency and installation manning diverge
STCW sets internationally recognised standards for training, certification and watchkeeping for seafarers, given effect in UK law through Merchant Shipping legislation and enforced through the Maritime and Coastguard Agency. A marine crew member's competency record sits in a ship's crew list, tied to their certificate of competency, medical fitness, and the vessel's safe manning document. DP vessel manning requirements add a further layer again, with dynamic positioning operators typically holding Nautical Institute DP certification tracked against sea time and equipment class.
Installation manning runs on a separate track entirely. Safety Case Regulations 2015 place duty on the operator to demonstrate that personnel on the installation are competent and authorised for their role, and PFEER 1995 sets requirements for emergency response including the ability to account for everyone on board during a muster. The OIM's competency and authorisation records, and the installation's POB system, were built around platform and rig populations, not seafarers.
The result is two parallel, internally consistent systems that were never designed to talk to each other. A marine crew member's training record lives in one place. An installation worker's competency and permit authorisation lives in another. On a flotel, both groups are physically present on the same vessel at the same time, and a muster doesn't care which system someone's record sits in.
Where command authority actually sits
The Master retains command of the vessel, including navigation, DP operations, and marine safety matters such as stability and lifeboat readiness. The OIM holds authority over the installation's operational and emergency response activities, including permit to work and the safety case obligations that apply to the work being done aboard. On most flotels and DSVs this is set out in an interface agreement or bridging document between the vessel owner and the installation duty holder, but the detail of where one authority ends and the other begins is worth checking rather than assuming is settled by default.
The practical friction tends to show up at the edges of these authorities rather than in their core. During a crane lift involving both vessel DP capability and installation permit to work, who signs off the simultaneous operation? During an emergency muster, does the Master's evacuation command take precedence over an OIM-led shutdown sequence, or does it run alongside it? These aren't questions with a single universal answer, they depend on the specific bridging arrangement for that asset, and a system that assumes one authority structure for all mixed-regime vessels is likely to be wrong for some of them.
The POB reconciliation problem
Mixed crew POB management has two common failure directions, and neither is hypothetical in the sense of being unlikely, both are structural risks inherent to running two separate systems side by side. Someone can appear on both the marine crew list and the installation POB register under slightly different identifiers, double-counted in a muster. Or someone can fall into the gap between the two, present aboard but not clearly owned by either system's reconciliation process, particularly during crew changes when marine crew rotate on the vessel's own schedule rather than the installation's shift pattern.
For assets running continuous POB tracking, we've written before about why a person on board tracker matters offshore, and the same principle applies with an added layer of complexity here: the tracker needs to resolve two source systems into a single, de-duplicated muster list, not just count heads against one register.
Flotel manning compliance in particular depends on getting this reconciliation right, because flotels by design carry a higher proportion of transient and rotating personnel than a fixed installation, and the accommodation capacity itself is often the binding constraint during an emergency response.
Testing a system for mixed-regime readiness
Rather than taking a vendor's word for it, ask to see the specific mechanics. Request a demonstration of a single muster event that pulls from both a marine crew list and an installation POB register and produces one reconciled headcount, with any discrepancy flagged rather than silently resolved. Ask how the system handles someone transferring from marine crew status to installation workforce status mid-contract, which happens more often on long-term accommodation vessels than operators sometimes expect. Ask whether competency records from an STCW certificate and an installation competency and authorisation record can sit against the same individual without duplicating their identity across two profiles.
These are the same categories of evaluation we'd suggest for any control of work software being assessed against a genuinely demanding operational pattern, applied here to the specific case of dual command structures.
If your asset runs a mixed marine and installation workforce and you're reviewing how POB and competency data get reconciled across the Master/OIM interface, we can work through the existing muster and verification process with your operations team to identify where the two systems create double-counting or coverage gaps.