Understanding PBA Root Cause Analysis in SHORESIM
What is PBA?
Production-Based Availability (PBA) is one of the key performance metrics calculated by SHORESIM. It answers a fundamental question: of all the energy this wind farm could theoretically have produced, how much did it actually deliver?
The formula is straightforward:
PBA = Actual Power Production ÷ Possible Power Production
A PBA of 95% means the wind farm delivered 95% of its theoretical maximum output. The remaining 5% was lost — and PBA Root Cause Analysis is how SHORESIM tells you exactly why that 5% was lost.
How Root Causes Are Attributed
During the simulation, every turbine is tracked continuously. Whenever a turbine is producing less power than it theoretically could, SHORESIM attributes that lost production to a specific root cause.
The attribution follows a strict priority order, checking each possible cause in sequence and assigning the first one that applies:
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Offshore substation at max capacity — the wind farm's export capacity is constrained at the substation level, limiting what can be transmitted regardless of turbine status
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Export cable at max capacity — similarly, an export cable is fully loaded, curtailing output
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Another turbine in the string is offline — a neighbouring turbine's downtime is causing a reduction in this turbine's effective output
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The turbine itself has a work order — the turbine is down for maintenance or repair, and the root cause is taken directly from that work order
Only turbines that meaningfully contribute to wind farm output are included — assets configured as non-contributing (e.g. structures modelled with zero rated power) are automatically excluded so they cannot distort the results.
The Order in Which Root Causes Are Checked
For each work order, SHORESIM splits the full downtime period into segments and methodically attributes each segment to a root cause. This is done by working through a fixed, prioritised checklist. Once a segment of time has been attributed to a root cause, it is removed from the remaining unattributed time, and the next check only applies to whatever time is left. This means the order matters — causes checked earlier take precedence over causes checked later.
Step 0 — Fixed Pre-Checks (Always Applied First)
Before the main checklist runs, two causes are always checked first if they apply to the work order type:
| Step | Check | Applies to |
|---|---|---|
| 0a | Weather during active process cycles — time where work was already underway but had to pause due to weather restrictions in the field (e.g. waiting on wave height limits mid-installation) | Towing orders and component replacement orders only |
| 0b | Towing time — time the turbine or component is physically being towed | Towing orders only |
Step 1–12 — The Main Priority Order
After the fixed pre-checks, SHORESIM works through the following 12 checks in strict order. Each check claims the time that belongs to it, and the remaining unexplained time is passed to the next check:
| Step | Root Cause Checked | What is being looked for |
|---|---|---|
| 1 | Personnel outside schedule | All required personnel are outside their availability schedule (e.g. rotation not yet started, contract period ended) |
| 2 | Vessels outside schedule | All allowed vessels are outside their operational availability schedule (e.g. seasonal contract, not yet mobilised to the region) |
| 3 | Personnel outside work shift | All required personnel are outside their daily/weekly work shift hours |
| 4 | Vessels outside work shift | All allowed vessels are outside their work shift hours |
| 5 | Waiting to be scheduled | The work order exists but the scheduler had not yet attempted to assign a vessel — the job was sitting in the scheduling queue |
| 6 | Work order lead time | The work order has a mandatory lead time configured (e.g. a spare part order time) — the period from work order creation until the scheduler first looked at it |
| 7 | All vessels have a lead time | Every allowed vessel simultaneously has a mobilisation/preparation lead time window that prevents departure |
| 8 | Vessel in transit | The selected vessel has been assigned and is physically travelling to the turbine |
| 9 | Bad weather (work order restrictions) | Weather exceeds the limits defined on the work order itself |
| 10 | Bad weather (all vessels) | Weather exceeds the limits of every allowed vessel simultaneously |
| 11 | Single vessel has a lead time | At least one allowed vessel has a mobilisation/preparation lead time window (less restrictive than step 7) |
| 12 | Bad weather (single vessel) | Weather exceeds the limits of at least one allowed vessel |
Step 13 — Fallback
Any downtime that has not been attributed after all 12 checks is assigned to "no dropoff and pickup combination" — a catch-all for remaining unexplained response time, typically where no valid vessel logistics combination could be identified.
Important Note on Order
The order in which these checks run has a meaningful impact on how production losses are attributed. For example, if both bad weather and a vessel lead time were occurring simultaneously, whichever check comes first in the list will claim that period. The default order above reflects the design intent that availability and scheduling constraints are considered before weather constraints, since they are generally more operationally controllable.
The Root Cause Categories
Root causes are organised in a two-level hierarchy: main categories and sub-categories. The main categories represent the broad operational reason for the loss; the sub-categories provide more granular detail within certain categories.
🛠️ Minor Maintenance
These root causes relate to smaller corrective and scheduled maintenance tasks, typically carried out by crew transfer vessels (CTVs).
| Root Cause | What it means |
|---|---|
| Scheduled service work | Turbine is offline for planned, routine maintenance |
| Minor work | Turbine is offline while technicians carry out the repair |
| Minor weather delay | Bad weather is preventing the vessel or technicians from accessing the turbine |
| Minor response time | Time lost before work can begin — see sub-categories below |
Minor response time sub-categories:
| Sub-category | What it means |
|---|---|
| Waiting to be scheduled | The work order exists but the scheduler has not yet tried to assign a vessel — the job is in the queue waiting its turn |
| No available vessel | A vessel has been identified but is currently occupied, in transit, in its mobilisation window, or outside its operational schedule |
| No available personnel | Technicians with the required skills are unavailable during this period |
| No room on asset | The turbine's access limit has been reached — too many technicians are already on board |
| No dropoff and pickup combination | There is no logistical combination of vessel movements that allows technicians to be dropped off and picked up efficiently |
| Emergency response limit | An emergency response time constraint has been triggered |
| Other | Remaining minor response time losses not captured by the above |
🏗️ Major Maintenance
These root causes relate to heavy maintenance and component replacements, requiring Heavy Lift Vessels (HLVs) or dedicated support vessels.
| Root Cause | What it means |
|---|---|
| Major work | The turbine is offline while major repair or replacement work is being carried out |
| Major weather delay | Bad weather is blocking the HLV or support vessel from operating at the turbine |
| Major lead time | Time lost before work can begin — see sub-categories below |
Major lead time sub-categories:
| Sub-category | What it means |
|---|---|
| Waiting to be scheduled for HLV | The work order exists but the scheduler has not yet tried to assign an HLV — the job is in the queue waiting its turn |
| Waiting to be scheduled for support vessel | Same as above, but for support vessel operations |
| No available HLV | An HLV exists in the fleet, but it is currently occupied, in transit to site, in its mobilisation/demobilisation window, or outside its operational schedule |
| No available support vessel | Same as above, for support vessels |
| Other | Remaining major lead time losses not captured by the above |
Important distinction: "Waiting to be scheduled" means the scheduler has not yet looked at this job — it is a queue or prioritisation delay. "No available vessel" means the scheduler has considered it, but the vessel cannot yet service this turbine due to its current activity, transit, mobilisation window, or schedule.
⛵ Floating Wind Operations
These root causes apply specifically to floating offshore wind turbines, which require towing operations and specialised crane vessels.
| Root Cause | What it means |
|---|---|
| Floating work — Vessel | Time spent on vessel-based work at the turbine while it is floating or towed |
| Floating work — Crane | Time spent on crane-based work at the turbine |
| Floating weather delay | Bad weather preventing towing or floating operations |
| Floating towing time | Time the turbine is being physically towed (to or from port/quayside) |
| Floating response time | All lead time and scheduling delays specific to floating wind operations, equivalent to the response time categories above |
🔌 Grid & Electrical Constraints
These root causes apply when the wind farm's own electrical infrastructure limits how much power can be exported, independently of any turbine downtime.
| Root Cause | What it means |
|---|---|
| OSS at max capacity | The offshore substation is transmitting at its maximum rated capacity; any additional turbine output cannot be exported |
| Cable at max capacity | An export cable is at full load; additional production is curtailed |
| Other asset in string disconnected | A neighbouring turbine or asset in the same electrical string is offline, reducing the effective output of this turbine |
🔄 External Causes
Every category above — minor, major, floating, and all response time sub-categories — also has an "External" counterpart. An external root cause means that another turbine in the wind farm is experiencing one of the causes above, and this is causing a production loss to be registered on the turbine being analysed. For example:
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External minor work — a neighbouring turbine is undergoing minor repairs, and this indirectly affects this turbine's output
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External major lead time — no available HLV — the fleet's HLV is occupied servicing another turbine in the farm
External causes follow the same sub-category structure as their internal counterparts.
Summary Metrics
In addition to the individual root cause breakdown, SHORESIM reports three summary figures at the wind farm level:
| Metric | What it means |
|---|---|
| Lost production (PBA root causes) | Total energy lost across all root causes, in MWh |
| PBA Root causes total | Total lost production expressed as a fraction of possible production (0% = nothing lost, 100% = all production lost) |
| PBA | The overall production-based availability — the complement of the above (100% = full availability) |
How to Read the Results
Results are reported per wind farm and broken down per simulation year, as well as in total across the full simulation period. This allows you to identify not just the dominant cause of lost production overall, but also whether certain causes are more prominent in specific years — for example, whether major lead time losses are concentrated in years when a known component type is due for replacement.
Sub-categories roll up into their parent category, so you can view results at whichever level of detail is most useful: the high-level "Major lead time" figure for a quick overview, or the individual sub-cause breakdown to understand exactly where the bottleneck lies.