Ridley chooses shapes from Giger's Necronomicon for the derelict

Leading from

The design of Giger's derelict 

Giger had drawn something up that looked almost like a musical instrument
( Work 280: Mordor IV)

a) If we look at the picture Mordor IV to the right, the central face at the top appears to be putting it's mouth to the opening of a croissant shaped thing.

Thus I take this to be the picture and upon this structure are a series of four valves, which could easily be seen as the entrances to the derelict interior. 

A decision was made to reduce the number of entrances to three (and then the number three appeared to come through as a repeating theme all over the alien pilot's chair and the body of the alien monster itself)

 Morder IV detail, 4 valves lower centre

b) Giger wanted to do some research for the alien ship and Ridley Scott realised that he had too much to do as it was.

Ridley's next movie was to take it onto himself to find something in Giger's Necronomicon that was going to serve as the basis for the design of a ship that he wanted.

He studied one of the paintings and found something like a musical instrument such as a saxophone, 

Ridley drew around it, saying "What about this, it looks like a giant croissant?" and then perhaps it looked like a boomerang. 

This was to be the basis of derelict ship that Giger would further develop. 

He first came up with a design that resembled a saxophone and Ridley Scott recalls that his response was "No, this one goes a little too far in the unusual." 

So a second version of the design was accomplished which Ridley accepted.

detail from Giger's Mordor IV

detail from Giger's Wreck Entrance: work 375

source quotes

1. Ridley Scott:" Au départ, Giger voulait se lancer dans des recherches de designs pour le vaisseau étranger, mais je lui ai dit “Non, tu es déjà beaucoup trop occupé par tout ce que tu as à faire. Si tu ajoutes cela au rest, tu ne finirais jamais ton travail à temps!“ J’ai décidé d’y réfléchir de mon côté. J’ai repris son album Necronomicon , et je me suis attaché à scruter les détails de ses grands dessins pour y trouver une forme basique qui pourrait être celle du vaisseau. J’ai pensé que c’etait une démarche logique, car elle nous permettrait de rester dans son universe graphique. J’ai déniché un détail intéressant que j’ai soumis à Giger et à mon équipe de décorateurs, et au début, ils m’ont présenté quelque chose qui avait le form d’un saxophone! Je leur ai dit “Non, là on va un peu trop loin dans l’insolite“. Ce n’est que dans un second temps qu’ils ont conçu ce superbe objet courbe. Une fois que cet aspect a été finalisé, j’ai donné au vaisseau étranger le surnom de “cuirassé“, parce qu’il a l’aspect d’un bateau de guerre."(Ecran Fantastique Hors-Serie #29, p13)
   Translation
   Ridley Scott: "Initially, Giger wanted to go into research designs for the alien ship , but I told him "No, you're already too busy with all that you do. If you add this to the rest, you never finish your work on time "I decided to reflect on my side. I picked up the album Necronomicon, and I am attached to scrutinize the details of his large drawings to find a basic form which could be that of the vessel. I thought it was a logical step, as it would allow us to stay in his graphic universe. I found a interesting detail that I submitted to Giger and my team of designers, and at first they showed me something that had the form of a saxophone! I said "No, this one goes a little too far in the unusual."It is only in a second time they have designed this beautiful curved object. Once this has been finalized, I gave the foreign vessel the nickname "battleship", because it has the appearance of a warship"
2. Ridley Scott : "I took the drawing of the space ship off a section of one of Giger's Necronomicon, and he'd drawn something up that almost looked like a musical instrument, so I kind of drew around that, and said "what about this? it looks like a giant croissant", but actually it worked, like a boomerang. paintings, 'cause we couldn't work out what the hell the spaceship was going to look like, and so I was staring at his book, the... " (Alien DVD commentary)