Technical improvements in exploiting existing oil and gas fields and the continued high demand for hydrocarbons very often requires existing offshore structures to be modified and used beyond their design life.
In conjunction with increased MetOcean and functional loading, this very often necessitates structural strengthening and rejuvenation in order to keep operating the facilities at an acceptable structural integrity level.
Extending the life of platforms which have been in service as much as 30 years can require strengthening of overstressed braces and fatigue exposed joints.
Traditionally, this is carried out by cutting and replacing structural elements or by installing mechanical clamps which is exceedingly expensive due to the amount of underwater work involved in those operations. Due to the quality of underwater work and in particular underwater welding these methods are also technically questionable.
A technically sound and very cost-effective alternative method of strengthening tubular braces and joints is the injection of ultra high performance cementitious (UHPC) materials into the tubulars and joints. Compared with other methods, the injection of cementitious materials is a swift operation with limited offshore work involved. Through the use of UHPC materials a very significant increase in the capacity of the structural element is obtained.
Densit is a provider of Engineered UHPC Solutions. From initial inspection through engineering to final implementation, Densit’s UHPC Solutions have proven very cost-effective in upgrading offshore facilities, solving fatigue life and structural over-stressing problems.
Increased structural capacity
Over the past 10 years, the static performance of Densit UHPC in-filled tubulars and joints has been studied thoroughly through physical model test campaigns and finite element modelling. From these studies, it is evident that the load bearing capacity of UHPC injected members is increased tremendously in comparison to hollow tubulars. From Fig.1 comparing the axial capacity of a hollow tubular and tubulars
in-filled with Densit Ducorit for varying D/t ratios it is seen that the in-filled tubulars exhibit an increased axial capacity of up to eight times the capacity of hollow tubulars. The bending stiffness of in-filled tubulars is increased by about 50percent and as the bending stiffness is an important parameter in the buckling strength of a tubular member, this increase represents an impressive gain in buckling capacity.
The static strength of tubular joints relates to the ability of the joint to withstand the imposed brace load. Ovalisation and final collapse of the chord through compressive or tensile loading in the brace is the most common static strength failure mode for welded joints.
For tensile brace load regimes, injecting the joint with Ducorit has proven to increase the static strength by up to a factor of four, whereas joints exposed to brace loads in compression, the joints are virtually indestructible. Using the crack initiation as a measure for the ultimate capacity, Ducorit in-filled joints exhibit a capacity that is greater than two times the capacity of non filled joints.
Swift Installation
Densit UHPC materials are characterised by their extreme strengths and durability (Fig.2) at the same time being pumpable up to several hundreds metres in 2- to 4-in flexible hoses allowing for a swift and easy installation offshore.
Due to the highly viscous behaviour of the mixed materials, there is no risk at all of washing out the cement particles or mixing with water when cast subsea.
The injection of Densit UHPC is normally ROV assisted. The ROV will descend to the member and drill or cut 1 to 2-in inlet and outlet holes after which the ROV inserts the valves and connects the hoses (Fig.3).
The UHPC material is then mixed on the work boat, barge or platform, and pumped through hoses into the member or joint. Upon complete filling of the member the valves are shut in and the operation is completed.
To inject Densit UHPC material into a typical jacket brace at say 60 metres of water depth takes in the order of 4–6 hours.
Extending the service life of your assets does not only release additional revenues and increase the ROI, it also defers the huge costs of abandonment.
Put into perspective, the cost of extending the service life is insignificant when it is compared with the potential abandonment costs and the extended production life easily covers all the costs – from Densit’s experience just a few thousand barrels will do it!
Brian Barker is Manager, Engineering & Operations, Densit Asia Pacific Sdn Bhd, Kota Kinabalu, Sabah, Malaysia. www.densit.com
