Structuring STPA Loss Scenarios for Safety Assessment: A Delphi-Based Framework for Degraded MASS Operations

Maritime autonomous surface ships (MASSs) require methods for translating system-theoretic hazards into structured and traceable inputs for subsequent scenario-based safety assessment, particularly under degraded operating conditions. This study structured 92 loss scenarios (LSs) previously derived through System-Theoretic Process Analysis (STPA) into a representative scenario framework while preserving traceability to the source hazard analysis. A three-round Delphi process involving 10 domain experts was used to group the LSs, refine overlapping categories, and evaluate the resulting framework. Round 1 produced 13 provisional categories, which were consolidated in Round 2 into 10 Representative Scenario Domains (RSDs). Each LS was assigned to one primary RSD as a traceability anchor rather than as a claim of single causality, while cross-domain interactions were represented through a separate concurrent-degradation construction layer. In Round 3, all RSDs achieved median ratings of at least 4 for name appropriateness, definition clarity, and scenario fit, with agreement proportions of 0.80 to 1.00; framework-level agreement ranged from 0.80 to 0.90. The framework provides a structured intermediate layer between STPA-derived loss scenarios and subsequent safety-assessment scenario specification. The findings support expert-based content validation rather than empirical simulator effectiveness; independent reproducibility assessment and simulator-based validation therefore remain necessary subsequent steps.