Given some ambiguity and dis-satisfaction with the current rules, I've written out the following for comment and discussion. I don't think the essentials change, but hopefully the grey areas are removed:
Modifying warships
There are a number of tasks that can be done to improve a warship. These are grouped into five categories, with increasing complexity, cost, and infrastructure requirements. The most complex task in a warship improvement project determines the overall cost and infrastructure requirement.
Cost is both the percentage of original building time required to do the work, and the percentage of the light displacement that must be spent in warship materials.
Infrastructure requirements are, in order of complexity:
None (N): The ship can anchor anywhere and do the task itself - provided it has miscellaneous weight to carry the materials needed.
Tender (T): The ship and a tender or repair ship can anchor anywhere to do the task - provided the ships have the miscellaneous weight to carry the materials needed.
Port (P): The ship must be anchored at a military facility containing at least one Port, Slip, or Drydock. The ship is considered to be tied up at a pier - it does not use a drydock.
Drydock (D): The ship must be in a drydock long enough to accommodate it.
Level 1: Manual Refits (Cost = 5%)
-Changes (number/quantity/type/location) to searchlights: N
-Changes to ship’s boats: N
-Changes to deck mounted guns served by ammo lockers or belts (that is, no hoist): N
-Cosmetic or temporary changes to superstructure (ie, a fake funnel): N
Level 2: Minor Refits (cost = 15%)
-Changes to radar: P
-Changes to deck mounted guns served by hoists: T
-Changes to depth charge racks and throwers: T
-Changes to torpedo carriages: T
-Changes to gun directors: T
-Minor changes to superstructure (enclosing a bridge, adding a searchlight platform): T
Level 3: Major Refits (cost = 25%)
-Changes to catapults and/or above-decks seaplane hangers: P
-Changes to deck mount or turret armor: P
-Changes to conning tower armor: P
-Changes to external armor belts: P (upper), D (ends, main)
-Changes to underwater torpedo tubes: D
-Changes to sonar: D
-Removal of secondary (or tertiary) gun turrets (barbettes plated over and possibly covered): P
-Refurbishment of internal fittings for life-extension purposes: P
Level 4: Partial Reconstruction (cost = 50%)
-Changes to bunkerage (type or quantity): P
-Replacement of superstructure: P
-Changes to internal belt armor: P (upper), D (ends, main)
-Changes to deck armor: P
-Replacement of secondary turrets with new secondary turrets: P
-Changes to secondary (or tertiary) barbettes (addition, change of caliber, change to armor or capacity, replacement with bunkerage): P
-Replacement of secondary barbettes with powerplant machinery: D
-Removal of main gun turret (barbette plated over and possibly covered): P
-Change to powerplant (type and output): D
-Change to bow form: D
-Change to trim of ship: D
-Change to torpedo bulkheads: D
Level 5: Total Reconstruction (cost = 75%)
-Replacement of main gun turrets with new main gun turrets: P
-Changes to main barbettes (addition, change of caliber, change to armor or capacity, replacement with bunkerage): P
-Replacement of main barbette with powerplant machinery: D
-Insertion or removal of section of hull amidships: D
-Reshaping of stern: D
Note
There may be a scenario in which the most complicated task in a job is something requiring the use of a port - yet the job also includes a lesser task that requires the use of a drydock. For example, removal of submerged torpedoes (a 25% task) and a change to internal armor (a 50%) task. In this case the ship would have to spend 25% of its original build time in drydock, and the remaining 25% could either be spent in drydock or just in the port.