№ 5 (May 2009)
Offshore Structures in the Arctic
Keppel Meets the Challenge with New Materials and Designs
Over the last few years, Keppel Offshore & Marine (Keppel O&M) has invested its resources to investigate the unique challenges to offshore structures in Arctic regions because of the extreme cold temperatures and ice loads.
The results have been encouraging and the areas of research are being expanded to include the prototype design of structures in new cementitious composite and an ice-capable jack-up.
Ultra-high performance cementitious composite for offshore structures
The recent interest and focus on the Arctic regions have posed some unique challenges to offshore structures because of the extreme cold temperatures and ice loads.
Temperatures in the range between minus 20 degrees Celsius and minus 40 degrees Celsius are common in the Arctic. In terms of structural design, it restricts the range of economical steel available for use under such extreme climate. In addition, some of the offshore structures may be floating or may need to be transported over shallow waters of the Arctic shelf, hence, lightship draft is a concern. Over the last few years, Keppel O&M has invested resources to investigate the use of concrete to address these issues.
Applications of concrete in offshore structures is not new, some notable examples in recent times include: Troll A platform, Sakhalin II platforms, Adriatic LNG terminal, Nkossa Barge, Adjuna Sakti and Heidrun TLP. There are a total of about 50 such concrete structures in the world today. For various reasons, concrete has been selected for these bottom-founded as well as floating structures.
Some of the main advantages of using concrete are its low maintenance and waiver of dry docking requirements. Furthermore, the strength of concrete increases at lower temperatures making it suitable for use in Arctic structures. In addition, the fabrication method is common with widely available civil engineering techniques, thus allowing much of the structure to be constructed at site. However, concrete structures are notably heavier as compared to their steel counterpart.
In 2007, a breakthrough in concrete technology was achieved with the discovery of ultra high performance cementitious composite (UHPC). UHPC is differentiated from normal concrete in its composition which consists of specially selected fine aggregates. (Refer to Table 1 for a comparison of the typical material properties of UHPC and its counterparts.)
In researching suitable applications for this technology, Keppel O&M has developed a new hull form structure with a unique cell-form reinforced grid that enables the construction of large floating vessels. Pre-tension technology is also applied to enhance its capacity. The result is a highly durable and strong offshore structure that is also lighter in overall weight.
The almost impermeable characteristic of concrete makes it very suitable for offshore and marine applications. Moreover, a unique concrete-based structure combining UHPC properties and new design principles makes a highly viable solution for offshore and marine applications in harsh environments such as the artic.
To illustrate, a simple 3,500 deadweight deck barge was designed according to ABS LNG Terminal Rule, 2008 and was supplemented with the latest concrete design code, ACI 318, 2008. (A sketch of the barge is shown in Fig. 1.) Approval-in-principle was also granted by American Bureau of Shipping for this design.
A comparison was then made between a UHPC barge and a conventional steel barge of this design. The UHPC barge compared well with the steel barge, and has several added advantages. The UHPC barge is low in maintenance, does not require dry docking, has a longer service life and is comparable in capital cost to the steel barge. (Refer to Table 2 for a comparison between the steel and UHPC barges.)
The UHPC characteristics combined with cell-form grid network structure and pre-tensioning systems enable the barge to handle the hogging and sagging moments experienced by ocean-going barges.
Moreover, this barge design is suited for shallow Arctic conditions both in floating as well as a bottom-founded structures. The concrete structure will also have the advantage of being able to withstand low temperature effects, resistance to corrosion, as well as the ability to take high impact loads from the ice.
KOMtech (Keppel Offshore & Marine Technology Centre) and Keppel’s design and engineering arm Offshore Technology Development, are presently working with the Classification Society to explore the use of UHPC in various Arctic offshore structures. Other possible applications of UHPC include very large floating structures for storage, floating LNG terminals and LNG deck overlays to prevent spills.
An Ice-capable jackup rig for the Northern Caspian Sea
Ice in Caspian Sea is limited to the northern part of the lake between the months of December to March. Ice conditions during these months are relatively mild with only thin first year ice present. In soft winter, ice forms only in waters less than 5 meters deep. In severe winter, ice could cover the entire northern part of the lake. The typical level ice thickness in the North Caspian Sea is less than one meter. Ice ridges could easily ground due to the shallowness of the water and, hence, restrict mobility.
In view of the short benign winter and considering its economical value, it is not necessary to design and construct a fully ice-resistant platform for operations in the Northern Caspian Sea. A jackup rig by far, offers a more cost effective solution for drilling operations there.
A jackup rig would be proficient in operating on extended season mode in that region. Extended season mode means that rig operation is limited to several months of the year to avoid full winter ice conditions which could cause unpredictable consequences to the rig. In the North Caspian Sea, full winter does not come until January and last until mid March. Thus, this allows an operation window of, at least, eight months in a year. However, unlike typical jackup rigs, ice conditions would govern the design of an ice-capable unit since preliminary analyses have indicated that ice forces on the jackup rig are far greater than the wind, wave, and current forces.
Several conditions determine the winterisation extent of the rig and thorough observations on the ice variability is a prerequisite. The length of operation time in ice implies different ice scenarios that the jackup must endure. For example, in early winter, where only thin ice (less than 30 centimeters thick) exists in shallow waters of 3 meters and less, the jackup legs and the spud cans must be designed to withstand the force from continuous level ice sheet of thickness up to 30 centimeters. Whereas, in late winter or early spring, mobile ice floes of thickness 60-90 centimeters are driven by wind and occasionally form ice ridges. In this situation, the jackup rig must be able to withstand impact force from mobile ice floes and ice ridges.
Analyses have been performed on an existing KFELS B Class jackup rig to determine the necessary appurtenances. A number of requisite appurtenances have been incorporated to enable the rig to operate in minimal ice conditions which characterize the conditions in late autumn and spring. One of the crucial requisite appurtenances is the installation of ice shields around the legs. The purpose of the ice shield is to prevent ice rubble from building up within and between the legs which could lead to failure of several truss members. There are several types of shields and an example is illustrated in Fig. 2 where a composite material is used. However, the beneficial effects of having a shield are sometimes compromised due to the larger loads attracted by the shield. This may be mitigated by using a conical structure around the jackup’s legs.
The jackup leg cone (patent pending) can be lowered from the hull level and locked in place at the water level where ice loads are typically attracted. When ice floes interact with the leg, the conical structure breaks the ice with a bending action; this helps to reduce the impact loads inflicted by the typical crushing action that occurs with ice failure. The jackup leg cone may be jacked up and down the legs of the rig to prevent ice accumulation. The movement of the cone along the legs is a unique feature of the design and serves to reduce ice impact loads and prevent the accumulation of ice rubbles around the legs.
In conclusion, a jackup rig is technically feasibly and commercially viable to operate in the Northern Caspian Sea. Winterisation appurtenances, however, are necessary to extend the operation period of the jackup rig further into the subzero months.