Using a semi-submersible tender vessel for drilling from floating installations with dry wellheads in deepwater could be more cost-effective in benign climates such as in Southeast Asia, Brazil and West Africa, according to some well known drilling contractor.
For example, West Alliance, built by Keppel, delivered sometime in 2002. The design of this vessel, which is based on that of the West Menang, has been modified with larger columns and pontoons and incorporates a higher variable deckload and more storage capacity for liquid mud and brine.
The eight anchor winches are larger, compared to the wire currently used on West Menang and West Pelaut, which are also some of semisubmersible tender rigs owned by Seadrill. The mooring of tender rigs in deepwater has been facilitated by the development of lightweight mooring ropes such as Dyneema, which is made of high-modular polyethylene and polyester, combined with a pre-laid mooring system.
Two key questions arise in considering whether tender-assisted drilling is beneficial in these circumstances:
•Are there savings to be made compared with alternative solutions?
•Is it technically feasible, given that there is no experience of tender rigs working with deepwater floaters?
Cost savings
The major economic benefit of using a tender rig is that the cost of providing an integrated platform rig is avoided. Estimation made and was suggested that for a wellhead tension-leg platform (TLP), a saving of at least US$140 million (net present value) can be achieved.
This figure is made up of the cost of the platform rig itself and the reduction in the platform construction cost, which results from the fact that the TLP can be smaller. The operating weight of the self-erecting tender is only about one quarter that of a platform rig, while the size of the deck required for the derrick equipment set is also significantly smaller since mud systems, power, pipe deck, accommodation, and so on, are contained on the tender.
•Modec, which has developed the Moses wellhead TLP design
•Aker Maritime, which has delivered several Spar platforms.
The design criteria used in the studies were water depths of 6,000 ft for the Spar and 4,000 ft for the TLP. The key feasibility issue is whether the rig can be moored alongside a TLP or Spar in such a way as to maintain an optimal distance from the installation, neither approaching so close as to run the risk of colliding with it, nor moving so far that the lines carrying power, mud and other functions from the tender to the platform come under strain.
The design environmental conditions used were the 100-year storm conditions in Africa. Criteria for sea conditions were set at a significant wave height of 13.8 ft, a one-minute wind of 43 knots, and a surface current of three knots.
Mooring systems
For the TLP, the study considered a steel wire catenary system with the additional use of two pre-tensioned "stretcher" ropes between the tender and the platform. The stretcher ropes, the purpose of which is to further constrain the relative excursion between the tender and the platform, are 3 1/4-in. nylon rope pre-tensioned to a relatively low value - 50 kips. By maintaining positive tension in the stretchers it is possible to ensure that a minimum controllable distance is always maintained to the TLP. A Tension Leg Platform (TLP) is a vertically moored compliant platform. The floating platform with its excess buoyancy is vertically moored by taut mooring lines called tendons (or tethers). The structure is vertically restrained precluding motions vertically (heave) and rotationally (pitch and roll). It is compliant in the horizontal direction permitting lateral motions (surge and sway).
For the Spar platform study, a taut-leg mooring (TLM) system consisting of wire and polyester rope was used, which is similar to the mooring system adopted for the Spar platform. Studies showed that it is possible to moor the tenders in depths down to 6,000 ft by using a pre-installed mooring system combined with the semisub tender rig's own mooring system upgraded from 2 1/4-in. to 2 1/2-in. diameter wire rope. In the case of the TLP, the maximum relative distance was 84 ft and the minimum distance was 73 ft. In the Spar's case, the maximum relative distance was 105 ft when using 2 1/2-in. wire combined with 3-in. polyester rope and 92 ft when using 2 1/2-in. wire combined with 5-in polyester rope. The minimum distance was 36 ft. The Spar concept is a large deep draft, cylindrical floating Caisson designed to support
drilling and production operations. Its buoyancy is used to support facilities above the water surface [Glanville, et a1 1991; Halkyard 19961. It is, generally, anchored to the seafloor with multiple taut mooring lines. A third generation “cell” Spar was introduced in 2004. It performs similar to the other Spars, but it is constructed differently. The hull consists of multiple ring-stiffened tubes, or “cells”, which are connected by horizontal and vertical plates. This method of construction is cheaper than the traditional plate and frame methods. Because of the length of a Spar, the Spar hull cannot be towed upright. Therefore, it is towed offshore on its side, ballasted to a vertical attitude and then anchored in place. The topside is not taken with the hull and is mated offshore once the Spar is in place at its site. The mooring cables are connected with pre-deployed moorings.
Courtesy of Dietswell - Horse Shoe Tender Rig
The HS Tender
Tender Assist Drilling vessel erecting of DES onto platform. Courtesy Axon Energy, Youtube downloaded.
Courtesy of AXON Energy
