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Bigger and heavier topsides: Part One

Hydrocarbon Engineering,

Float-over installation has become an attractive alternative to heavy-lift crane installation for offshore construction over the past 30 years, as ever-increasing size and weight of offshore platform topsides constantly exceed the lifting capacity of cranes.

An offshore installation method that is more than three decades old is generating renewed interest from many operators who are challenged with designing lift installations. The float-over installation method is experiencing a steady surge in interest, particularly for platforms in remote locations with very heavy topsides.

Dockwise envisioned developing float-over installation capabilities as an extended service to its core business. The demand for cost-effective and more flexible alternative to crane installations was the driving force behind float-over installation using semi-submersible heavy transport vessels (HTVs).

Dockwise’s first float-over was conducted in 2003 for Total’s Amenam topside weighing 11,200 t located offshore Nigeria. Since then, the company has installed more than 15 topsides for fields throughout the world ranging in weight from 8000 to 30 000 t. In a recent offshore installation project, Dockwise installed two mega sized offshore structures. The SHWE project, consisting of engineering, transport and installation of the SHWE topside weighing 30 000 t and the SHWE jacket weighing 22 000 t, marks the company’s biggest installation project to date. Being one of the world’s heaviest topsides, the SHWE topside tops Dockwise’s previous installation record held at 21 000 t.

Bigger and heavier topsides on the horizon

Float-over architecture has not changed much from the traditional platform design. The steelwork is somewhat heftier to overcome the large free spans in the main structure, as are the jackets, but also to accommodate the increase size and weight of the platforms.

The need for larger and heavier offshore platforms is on the rise as well as demand for float-over installations. In the coming four years, an estimated 660 offshore platforms are expected to be constructed, of which 180 will weigh greater than 12 000 t. The need to develop fields in remote locations where floating and gravity based production units are required is driving the demand for larger offshore units.

The installation approach for topside installations heavily depend on weight and type. According to a recent study conducted by Dockwise investigating topside installation trends, offshore installations have significantly grown over the past decade and are expected to increase in the coming years for both lift and float-over installations. The study segmented structures according to type (fixed, floating, gravity based) and weight (type A – between 4000 and 12 000 t; type B – over 12 000 t).

An analysis was conducted on installations dating from 2005 to 2012. During this period, the offshore crane lift method took the lion’s share totaling 115 (44%) of the 264 installations. Yard lift and float-over installations totaled 112 (42%) and 37 (14%) respectively. Of the 264 topsides, 192 (73%) weighed between 4000 and 12 000 t (type A), and 72 (27%) weighed over 12 000 t (type B). Only 6% of the type A topsides were installed by float-over. However, more than one third of the type B topsides were installed by float-over. This factor of six difference between type A and B is a strong indication float-over installation is growing as a preferred installation method as topside weight reach 12 000 t and heavier.

Trends indicate steady growth for both heavy-lit crane and float-over installation methods between 2014 and 2018. An estimated 480 type A and 93 type B production platforms are expected to be installed during this period. Production platforms are increasing in size and weight as one out of six units weigh more than 12 000 t. For the majority of type B units, the offshore installation approach is known: 14 are expected to be installed by means of float-over, 12 by crane lift and 1 by the yard lift, while 19 are unknown or yet to be decided. A total of 47 FSPSOs fall under type B units, but these units require a different installation approach other than float-over or heavy-lift.

Of the 46 type B productions units within scope (non-FSPO and known installation), 52% of the installation will be conducted by float-over. The majority of the 19 production platforms whose installation approach is unknown or to yet to be decided are expected to be float-over installations.

Figure 1. Topside in the field.

Heavy-lift installation

Most lift installations with crane vessels require prefabricated topside modules to be assembled in the field. Impossible to lift in one piece, large topsides are built in modules with cranes that seldom reach a lift capacity at around 12 000 t (5000 to 7000 t are common). Multi-module fabrication is known to require extensive yard crane capacity and jacking operations. Crane installations are subject to experience long overall offshore exposure, which can range between six to eight months due to extensive offshore hookup and commissioning activities. Furthermore, crane vessel availability in the area is critical, exposing ‘rendez-vous’ risk with HTV. Lift installations generate mobilisation costs and higher crane vessel rates when compared to float-over installations. In general, crane vessel workability during different weather situations is similar to float-over operations, but more rigid. Crane vessel mooring spread is on average tripled to 12 anchors of which each requires a fivefold radius increase totaling 1000 m when compared to float-over installations.

While the smaller topsides can be fully installed by cranes, the larger and heavier topsides usually require a different installation approach. As the weight and size of topsides approach 12 000 t, HTVs and dual barge float-overs are considered the preferred installation approach. Float-over installations have seen an increase in demand as larger installation projects are on the horizon.

Part Two of this article will be available soon.

Written by Jonathan Martinez, Dockwise, the Netherlands. Edited by Ted Monroe.

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