The major application of this drilling rig type is for development drilling from platforms (platforms rigs). Only the rig derrick set, including brake cooling, BOP accumulator, and drilling fluid treatment skid, are mounted on the platform. Then, we can install all other equipment on the drilling tender. The drilling tender is usually a barge-shaped structure with cable anchor winches mounted on each corner.. Since tender assist drilling rigs are displacement-type vessels, they are suitable only for calm seas such as those in Brunei, the Southern South China Sea, and the Gulf of Mexico, and lakes, notably Lake Maracaibo in Venezuela. However, oil and gas companies have drilled wells from the platforms in the Gannet field using semi-submersibles as a drilling tender, which demonstrates the possibility of tender-assisted drilling in more hostile environments like the North Sea (see Figure 1).
The deck load usually is not an operating limitation for a displacement vessel. Provided space on the deck, the tender usually carries all equipment required to drill to a rated 6,000 m, allowing regular supplies. They are limited to operating in rough water so some weather-related downtime can be expected.
Definition of Tender Assist Drilling Rig
A tender assist drilling rig, or a tender rig, is a specialized offshore drilling unit used in deepwater drilling operations. Its primary application is to work alongside fixed or wellhead platforms in deeper waters where conventional drilling rigs cannot operate efficiently. Unlike traditional drilling rigs, tender assist rigs have no drilling packages on board. Instead, they transport and install drilling equipment and personnel from a separate service vessel, known as a tender, providing necessary drilling support and services. This innovative approach increases drilling efficiency, flexibility, and cost savings, making tender assist drilling rigs popular in offshore drilling.
Tender in Seas
Even in small seas, the tender will pitch and roll. Given the appropriate wave period, the bow of a tender could be higher than the prevailing wave height. The primary reason for this phenomenon is the standard practice of mooring the tender down weather of the platform. As waves strike the bow of the tender, they cause it to rise with buoyancy forces. The wave runs down the length of the tender, and since the stern of the tender is relatively static at this time, the prevailing buoyancy force created by the wave gains a mechanical advantage in lifting the bow of the tender. This mechanical advantage, coupled with the inertia of the rising bow, means that the bow can heave up to double the prevailing wave height.
In practice, this creates difficulties in transferring rig personnel between the platform and the tender. The ‘widow maker’ is a set of steps with a ramp that we are hanging off from the platform at about 45 degrees to the vertical so that its lowest point is approximately 2 m above the boarding deck of the drilling tender. A set of steps is placed on the boarding deck of the drilling tender close to the widow maker, and access to the platform from the tender is made from these steps and onto the widow maker. In calm weather, this is a straightforward procedure. However, when the bow of the tender is heaving 2 -3 m, Even in small seas, the tender will pitch and roll. Given the appropriate wave widow maker method of platform access, it displays the characteristics for which it earned its name.
Types Of Tender Rigs
Two types of tender assist drilling rigs exist: the mono-hull (barge tender) and the semi-submersible (semi-tender). Both units are equipped identically, but the semi-tender can operate more profoundly in harsher environments.
Benefits of Tender Assist Drilling Rig
A Tender Assist Drilling Rig (TAD) offers numerous benefits and efficiencies in drilling operations. The ability of TAD rigs to self-raise, remove, and rig up drilling packages significantly reduces the time between wells. This feature enhances personnel and logistics planning synergies, improving operational efficiency. TAD rigs also contribute to reduced CAPEX and OPEX over the entire duration of fields. Shorter well durations and optimized maintenance plans result in cost savings throughout the drilling campaign.
One of the significant advantages of TAD rigs is their higher utilization rate. These rigs can be quickly mobilized to different locations, allowing drilling contractors to maximize their profitability by minimizing downtime. The mobility of TAD rigs makes them an ideal choice for exploring and drilling in deeper waters. Additionally, TAD rigs offer improved safety and security services, protecting personnel and equipment from potential online attacks and ensuring a secure working environment.
Tender Assist Drilling Rig Vs. Barge
The main change in the design of drilling tenders over the past years is to give them a self-erecting capability and a sufficiently large crane mounted on the tender deck. Therefore, it will allow the placement of the derrick onto the platform by the drilling tender. This approach is more cost-effective than using a crane barge for two reasons. Firstly, the crane barge day rates are many times more expensive than the drilling tender day rates. Secondly, we can only lift the derrick set off a platform after completing a drilling program or workover program.
As with all operations, the actual lift-off date can not be predicted accurately. To have an idle crane barge waiting for a lift-off is wasteful and expensive. Alternatively, we can use a spare derrick set, which we can pre-install before the arrival of the tender. However, giving the drilling tender sufficient crane and deck capacity to lift off the derrick set eliminates a dedicated crane barge and spare derrick sets once a platform drilling or workover program is finished.
Major Innovations in the Industry
In recent years, the tender assist drilling rig industry has witnessed significant innovations that have significantly improved drilling efficiency, reduced costs, and expanded the capabilities of these rigs.
One key innovation is the advancement in drilling equipment and technology. Drilling rigs now utilize more efficient and technologically advanced equipment, such as lightweight drilling equipment sets, which enable faster and more precise drilling operations. In addition, using self-elevating drilling platforms has enhanced the mobility and versatility of the rigs, allowing ease of movement to different locations, reducing downtime, and increasing operational efficiency.
Another major innovation is the concept of factory drilling for marginal fields. This approach involves bringing the drilling rig to the field instead of constructing a separate platform. Tender assist rigs have become the unit of choice in these conditions due to their cost-effectiveness and ability to quickly set up and drill multiple wells within a small radius. This practice has helped reduce costs and improve operational efficiency, particularly for smaller oil and gas fields that may not justify the expense of building a permanent drilling package.