What is a liftboat?

Liftboats are the preferred vessel for most shallow water jobs in the United States Gulf of Mexico (GOM) where there are about 200 such vessels. So, outside the US-GOM, why are they virtually unknown? This article will address the overall features of a liftboat, uses, and address the questions - why have they been unknown outside the US-GOM.

A liftboat is a self-propelled, multi-purpose, self-elevating vessel. Some have referred to liftboats as jack-up barges, lift barges, jack-boats. We will examine the distinguishing features of a liftboat as compared to its closest cousin, the jack-up drilling rig. Since, a liftboat spends most of its working life out of the water, it is felt it better to address what it does during its working life. Further, lets look at how it gets to the job site.

The overall liftboat design optimizes the interrelationship of the key features, i.e., one would not desire a big deck area without a comparable deck load, etc. Looking at the liftboat, there are certain key elements to focus on (1) Legs and pads, (2) Jacking System, (3) Working Water Depth, (4) Open Deck Area, (5) Net Deck Load, (6) Crane Capacity, (7) Personnel Accommodations, (8) Propulsion and (9) Classification Considerations.

Legs

Proven to be a cost effective design, the vast number of liftboats have three columnar legs (pipe legs). Columnar legs and pads provide added buoyancy for the liftboat as the legs are lowered to the sea floor. This buoyancy in both the legs and pads (spud cans) helps reduce the bottom bearing pressure on the pads. In combination with the buoyancy, larger liftboat pads are designed to reduce bottom bearing pressure. This has the effect of less penetration for the typical liftboat.

Working water depth also affects the type of leg. After about 220 feet (67 meters) of water, then lattice legs are the norm.

The vast majority of liftboats are three-legged. The four-legged liftboat will cost more than the cost of simply adding the cost of another leg, i.e., the jacking system to support two legs loaded diagonally is greater and the hull structure is different/heavier. But operationally, the setup time savings of a four-legged equates to money, so, one has to weigh the costs and benefits of three vs. four legs. Four-legged is faster. Just remember that history has shown that 95%+ of the existing liftboat fleet is three legs!

Jacking System

The jacking system for a liftboat is very different than the jacking system for a jack-up drilling rig. The two major differences center around speed and cycles.

Speed of the liftboat jacking system is essential. While a typical jack-up drilling rig elevates at two feet per minute a liftboat could elevate at four to six feet per minute and lower the legs at 14-18 feet per minute. This gives the liftboat the ability to get on and off location significantly faster.

The jacking system for a liftboat encounters a very different operational cycle. It would not be uncommon for a liftboat to jack up and down in one year the same number of times that a jack-up drilling rig would encounter in its entire lifetime. So, the wear factors, redundancy, material grades and shock loads are different between jack-ups and liftboats.

The jacking system for a liftboat encounters a very different operational cycle. It would not be uncommon for a liftboat to jack up and down in one year the same number of times that a jack-up drilling rig would encounter in its entire lifetime. So, the wear factors, redundancy, material grades and shock loads are different between jack-ups and liftboats.

Working Water Depth

The expected wind, wave and current (environmental conditions) in the maximum working water depth needs to be determined. Although, other variables will go into the liftboat design, such as, net deck load, the starting point is still working water depth. An area with moderate environmental conditions vs. harsh environmental conditions will be a significantly different liftboat design in all aspects, i.e., legs, jacking system and hull. As a general rule, as working water depths and severity of environmental conditions increase, then, costs of the vessel rise exponentially.

Deck Area

The deck area is the unobstructed, open area. The liftboat crane is used to move equipment and materials around and on to and off the platform/offshore location. There is a relationship among these variables – open deck area, net deck load and crane capacity. Further, you need a crane capacity that is commensurate with the expected deck loads and area.

Deck Load

The net deck load for a liftboat is what you have on your deck in transit and can then jack up with on the deck. Consumables (such as water, fuel, hydraulic fluid, etc.) are in addition to the net deck load. Liftboat operators are concerned with what they can carry and elevate.

There is a definite relationship among deck area, deck load and crane capacity. Liftboats are designed as multi-purpose vessels.

Also, one needs to consider the type of equipment and components that will be on the deck, so that, various areas can be designed to accommodate the various loadings, i.e., an area near the bow may have higher point loading than other areas.

Crane Capacity

The value of open deck space, deck load and crane capacity is what makes a liftboat a liftboat.

There are a variety of cranes for liftboats. However, there is one caveat – cranes for liftboats are designed by crane manufacturers specifically for liftboats. Weight is a key consideration in liftboat design, so, every consideration needs to be made in the overall design including the crane. Therefore, cranes for liftboats are typically structurally lighter than a comparable offshore crane.

Regarding types of liftboat cranes, there are lattice boom, box boom, telescopic boom, pedestal mounted, and leg encircling cranes. The main things to consider in crane selection is reach, load capacities and affect on deck area.

Accommodations

The accommodations are another feature of liftboats. The personnel accommodations are generally divided into two areas – navigation crew and customer/operational personnel. As liftboats have gotten bigger with increased capacities, so, has the requirements for more personnel. For example, a 100-man accommodation block would typically have about 20 navigation crew and the remaining 80 for customer/operational personnel.

The type of job that the liftboat is performing will dictate the accommodation requirement, i.e., an offshore construction job would generally require more personnel than a well intervention job. The construction job may even require additional modular living quarters on the deck.

Additionally, one needs to consider the area of operation in accommodation design.

Propulsion

A standard and key feature of a liftboat is that it is self-propelled. This alleviates the need for tugs to move the liftboat or to set up on location. This results in a significant cost advantage in that the liftboat can move to a new location without having to wait on tugs to move them. The liftboat can wait for a weather window and move. The larger liftboats have integrated propulsion and bow thrusters.

Remember the liftboat captain is lowering the legs and station keeping all at the same time – an operation that takes very little time compared to the jack-up drilling rig. Power design is all important when one in positioning near the platform.

Classification Societies

Most of the major Classification Societies class liftboats which is considered a requirement for international operations.

Advantages of Liftboats

Liftboats have historically proven to be a cost effective offshore service vessel. Major advantages include lower down time, no need for tug assists, stable work platform in the elevated position, negates the need for jack-up drilling rigs in a lot of cases, and negates the need for derrick barges in a lot of cases. They can be used for all types of offshore construction and maintenance to well intervention services like wire-line, coiled tubing, and nitrogen. They have even been used to build high-rise bridges. So in answer to the question that was poised at the start of this article “why have they been unknown outside the US-GOM.” Liftboats that are designed for the US-GOM are generally not suited for international operations due to increased working water depths, increased environmental conditions, and need for a recognized Classification Society approval. It is simply not enough to have a United States Coast Guard (USCG) approved liftboat for the international market place. Liftboats have gotten bigger and more capable - capable of international markets.

Levingston Liftboats