The extraction of crude oil is, and always has been, a complex process in which many different production sequences intermesh. Noble Drilling Corporation, based in Houston Texas, carries out exploratory drillings and prepares the extraction of oil on behalf of oil companies operating all over the world. One important piece of equipment on every exploration rig is the blow-out preventer (BOP) – a special valve, 150t in weight and worth over $1.5m, that is anchored with millimetre accuracy over the well hole on the sea bed after the initial drilling has been completed.

To the blow out preventer are then connected the pipes that will later transport the crude oil to the extraction and storage rigs. A special crane on board the drilling rig is the key component for assembling and positioning the BOP before it starts on its dangerous journey to the sea bed. Maximum availability, 100% explosion protection, compact dimensions, simple controls and erection in arduous physical conditions – these are the application parameters for this unusual lifting equipment.

As explosion protection is absolutely essential on drilling and extraction rigs, BOP cranes have up to now been equipped with hydraulic drives. This type of crane did not fulfil Noble’s expectations. Therefore, in March 1998 an enquiry for an electrically driven BOP crane of explosion-proof design was placed at the Aberdeen bureau of R Stahl Fördertechnik’s UK subsidiary.

For the supplier, it was a question of designing an uncommon crane concept without knowing the exact dimensions of the installation point. On one occasion a Stahl engineer went into the supply shafts of the 25-year-old Ton van Langeveld drilling rig, moored in Scotland for overhauling, and ascertained the precise dimensions for crane runway and crane.

Together with their British colleagues, Stahl’s design engineers in Künzelsau, Germany, designed two twin hoists with a total lifting capacity of 150t. At the heart of the $580,000 crane are four explosion-proof EEx hoists. The two twin hoists, electrically coupled for operation, with 8/2-1 reeving, together lift the BOP on its special beam with the required accuracy.

R Stahl claims to be the only company in the world providing explosion-protected hoists from a single source.

The Switchgear division of the company manufactures explosion-protected controls, switch gear, bus systems and lights. In co-operation with the explosion protection experts, a contactor control was developed which is safe enough to be operated in Zone 1. Time and again during the construction phase it was necessary to make allowances for changed dimensions, difficult installation conditions and complicated erection procedures.

Before the crane was constructed all the design drawings had to be submitted to Lloyds Register for verification, which is usual for equipment on drilling rigs. Construction did not begin until final approval had been given by Loyds Register. The explosion-protected hoists on their special crabs, ready for installation, were subjected to their final inspection by the customer and Lloyds Register in Künzelsau in June 1999 and then freighted to their destination in Scotland.

This BOP crane must function with 100% reliability, as a breakdown at a critical moment could result in extremely high costs. Even during a slight swell (the Ton van Langeveld is a semi-submerged design which does not descend to the sea bed) the BOP crane must operate reliably with 150t load on the hook. For this reason, the crane travels on its 10m runway with a rack-and-pinion drive which functions even at an incline of 2%. If the rig lists more steeply, the travel and hoist motors switch off automatically.

The Stahl electric explosion-proof BOP crane weighs about 10t, lifts the BOP at 2m/min with its four 16kW powered hoists and traverses the load at 1.6/10 m/min. Meanwhile, the crane is working to the customer’s satisfaction and is awaiting its first deployment near the coast of Spain.