Page 40: of Offshore Engineer Magazine (Mar/Apr 2015)

39 41

in consort. The design frame- when the horizontal loads (and 10 work developed at COFS shows therefore moments and torsion) 9 that provision of pin-piles can come in to play due to thermal 8 reduce footprint areas by up to expansion of the attached 7

EPIC 60% for typical deepwater ? eld pipelines. Consolidation of 6 conditions. The industry part- the soil in the vicinity of the

Increasing soil strength heterogeneity 5 ner on this project has adopted foundation will take place under 4 this innovative pin-pile hybrid the self-weight of the foundation 3 foundation system on Esso’s and structure it is supporting in 2

Erha North project offshore the period following set down 1

Nigeria as a cost-effective miti- and before operation. 0

Optimal number of iternal skirts gation solution against pipeline Further ef? ciencies in subsea 0.05 0.1 0.15 0.2 walking. foundation design can therefore

Equivalent embedment ratio, d/B or d/L

Internal shear keys Provision be realized if the consolidation-

Fig. 2: Load carrying bene? ts of a pin-piled hybrid subsea of suf? cient internal shear keys induced strength gains can be foundation. Image from COFS.

or “skirts” to prevent shearing banked, i.e. it may be possible within the con? ned soil plug of to rely on a higher value of a skirted foundation enhances undrained shear strength than 6000 mudmat capacity. Results from measured in situ. The time 5000 a program of finite element lapse between installation and analyses have been distilled operation may range from a 4000 into simple design charts few months to a year depend- y 3000 Pin-piled mudmat de? ning the optimal number of ing on the project, over which internal shear keys as a function time considerable gains in 2000 of equivalent embedment shear strength may be achieved,

Mudmat

Moment, M (kNm) 1000 ratio for intervals of strength depending on the consolidation

Pile heterogeneity index and vertical properties of the sediment.

group 0 0 -500-1000-1500-2000 load mobilization (Figure 3). A theoretical method for

It is seen that the commonly predicting consolidated

Horizontal load, H (kN) x adopted shear key interval of s/d strength gains in capacity of

Fig. 3: Optimal number of internal shear keys for subsea = 5 in engineering practice over- shallow foundations under foundation under 6 dof loading. Image from COFS.

estimates the critical number vertical preloading, based on of internal skirts for cases of a critical state framework has low vertical load mobilization, low soil prediction of the response of structures been developed at COFS. The theoretical

Custom heterogeneity index and low embedment laid on or shallowly embedded in the method has been applied to a range of ratio but becomes unconservative with seabed, such as subsea mudmats (and foundation and pipeline problems under

REPRINts increasing vertical load mobilization, pipelines). A number of novel tools multi-directional loading, providing a

Take Advantage of your Editorial Exposure soil heterogeneity index and foundation suited to near-surface strength charac- quick and easy method for predicting embedment ratio. The recommendation terization are being developed at COFS, consolidated undrained resistances.

E