Lunch Time Teaser – 14

Welcome to a new LTT question, brought to you exclusively from our new home at Commsrisk.

You work in the cost control and operational efficiency department at a submarine cable company. Demand for capacity has been growing over the last few months, so the company decides it will need to lay another ring of STM-256 cable around the globe to meet anticipated customer demand.

Your company does not own the expensive cable laying and supply ships, so they will need to be leased from a supplier. It is a well-known fact that the main bulk of the project costs come from the lease of these ships, so you will need to keep the number of ships on lease to the minimum to avoid unnecessarily high project costs.

Each ship can carry enough fuel to take it to half way around the globe. Each ship can refuel with each other, but will have to have enough fuel to return back to base at Porthcurno. Each ship can be resupplied with fuel back at Porthcurno as many times as necessary. To meet the growing demand in capacity, the project will have to be completed asap, without the main cable laying ship stopping to wait for the supply ships to return with fuel.

Your task is to calculate the minimum amount of supply ships that will be required to support the main cable laying ship to lay a ring of cable around the globe. You may assume that no time is lost refuelling each ship, and there are no land masses in the way.

Suppose the ships are leased, as per your calculations. When the project is nearing completion, unforeseen circumstances force the main cable laying ship to take a longer route than expected. As a supplementary question, how much further could the main ship travel, before you need to negotiate an additional lease for a supply ship?

LTT-14 comes with a prize: a hardback edition of our book, Revenue Assurance: Expert Opinions for Communications Providers. The prize will be awarded for the best answer received before 12 noon UTC, May 17th 2015. Please send your answer to

Lee Scargall
Lee Scargall
Lee is a senior risk management professional. He has extensive experience of managing both ERM and RAFM teams in telcos around the world, having worked for Ooredoo Group, Cable & Wireless and T‑Mobile UK.

Lee earned a PhD in Electrical and Electronic Engineering from the University of Newcastle upon Tyne, UK, for advanced research in to 3G video-telephony.