A common source of repair specification errors is the mistaken assumption that ships are identical twins. That is rarely correct. Sister ships are just that: sisters, but not identical twins.
When specifications and/or plans for a vessel’s modification are prepared for the owner, they are often based on the configuration of a sister vessel. During contract performance, the shipyard calls to the owner’s attention the fact that the specifications and/or plans do not match the ship, thereby leading to an unexpected extra cost.
In the construction of multiple ships under a single contract, the shipbuilder is obligated to ensure that the major features of the ships are the same. The contractor is given the right to determine all the lesser features as long as they are consistent with the contract documents. Due to production anomalies, design developments, and the passage of time, the secondary features of sister ships are often different from each other, but still consistent with the contract. For example:
the placement of auxiliary or minor components may be different
the brand name or model of equipment items may vary
the location of bulkhead penetrations for piping or cables may be altered
the routing of cabling, ducting, and/or piping may have been altered
secondary features of alarm, control and monitoring systems may not be consistent.
Consequently, to avoid unexpected extra costs, base your contract specifications only on the ship being modified, not on a sister ship for which such details probably are different.