A support letter naming the specific project, its scope, and a defined end date, plus a pattern of separate assignment letters if the same employer brings the engineer back later in the year.
Document each assignment as its own defined project
The strongest evidence that a repeated turnaround arrangement is still temporary entry, not an undisclosed permanent posting, is a paper trail that treats each assignment as its own separate project rather than one continuous engagement. A support letter for a given turnaround should name the specific facility, describe the scope of work tied to that outage, and give a defined start and end date driven by the project schedule rather than an open-ended employment term.
Where the same employer brings the same engineer back for a second or third turnaround later in the year, each occasion should get its own letter rather than a single blanket document covering an unspecified number of future entries. Keeping the assignments documented individually, with their own scopes and schedules, supports the position that the pattern is repeated temporary entry tied to actual project needs, not continuous residence dressed up as separate visits.