The current experience for the installation of large production facilities is essentially limited to 5,000ft. Can the technology used today be extended to meet tomorrow's challenges for installing subsea facilities in 10,000ft? This paper will address this question, particularly from the point of view of the offshore installation contractor who will be required to perform the work at these extreme depths. The paper will consider the issues associated with dealing these depths, highlighting technological areas specifically relating to lifting and lowering, pipelay, survey and positioning, and equipment and support vessels.
Over the last 10 years offshore oil and gas exploitation has moved into deeper and deeper water. In the early and mid 1990's the deep water frontiers were in Brazil and Norway in water depths of around 300 m. Currently large field developments are coming on-stream in West Africa in water depths of the order of 1,500 m and in Brazil and the Gulf of Mexico in water depths down to 2,000 m. Offshore installation work in these water depths has been successfully carried out by several installation contractors, using existing ships and equipment which have been extended and upgraded to meet the current deep water challenges. Can this equipment be pushed further to meet the future challenges set by the ultra-deep water fields beyond 3,000m (10,000 ft) ? Before considering this question further and whether or not new concepts for equipment and techniques are required, it is first necessary to review the current state of the art. The following sections address the experiences gained in carrying out deep water field developments specifically offshore West Africa and highlight specific problems and limitations for deep water installation work.
Deep water field development work requires that a large amount of hardware is placed and positioned on the seabed. The hardware includes subsea structures such as manifolds, anchors, tie-in spools, and other assorted subsea equipment. Table 1 presents weights and approximate dimensions for typical subsea hardware, which form part of a deep water field development. This paper addresses the issues and challenges as seen from the point of view of an installation contractor and as such hardware such as Christmas trees and BOPs and not considered within the paper. It has been assumed that this type of equipment will be installed by the drilling contractor using a drilling derrick.
Table 1 : Typical Subsea Hardware (Available in full paper)
Subsea installation work is commonly performed using two different deployment methods :-
- For heavy equipment (300 Tonne and beyond) a crane barge is commonly used where the crane is used both for overboarding and lowering the equipment to the seabed. The Seaway Polaris is an example of a crane barge (Figure 1). This type of system is particularly appropriate in greater water depths where a soft landing on the seabed using active heave compensation is required. A more detailed description of this type of crane based lowering system is described in the Section below.