Singapore Shipyard belongs to the IMG Guard faction. If you can not dock, your reputation to them is probably quite close to hostile. If I am not mistaken, there exists no automatic reputation adjustment for CR towards the IMG Guard faction, so a bunch of bribes from IMG NPCs on bases may help you with that. If it already is quite high, try making IMG your IFF and then lower reputation with it through the /droprep command. Then get more bribes and eventually IMG Guard reputation should be good enough to let you in.
Alternatively, if you want to take the long way, shooting a lot of Outcast NPCs or doing missions for IMG on their bases can raise your guard reputation slowly.
Guard reputations exist for a limited number of factions only, they are usually used to restrict access to more secure areas of certain factions. The bases and NPCs in the Westerwald nebula in Rheinland for example, where secret military bunch is located at is a high-security zone that nobody but military is really supposed to enter. With the help of guard factions the NPCs in there can shoot intruders while not making them hostile to all NPCs of the military.