STEVMANTA®

VERSATILITY FOR TAUT LEG SYSTEMS

Marine operations related to offshore exploration and production are moving fast into deeper water, pushing the operational limit of existing equipment. On floating units the weight of the mooring system is a limiting factor and weight savings are achieved by using different configurations and lighter materials.

Taut leg mooring

One way to achieve weight savings in a mooring configuration in (ultra) deepwater is to use a taut leg mooring. The mooring lines are much shorter than in a conventional catenary mooring, thus saving weight. In the taut leg mooring the unit’s movements are absorbed by the flexibility in the mooring line, typically provided for by light weight polyester ropes, so that even further weight savings can be achieved. As the anchor line arrives at a 45 degrees angle at the seabed, a Vertical Load Anchor (VLA) is required. Vryhof’s STEVMANTA^® has in the last decade been applied around the world, used on permanent systems as well as semi-permanent and mobile.

Vertical Load Anchor (VLA)

The STEVMANTA^® was designed to be installed in the same way as a drag anchor. A unique and patented system makes the anchor change from installation mode to vertical mode which results in an immediate increase of holding capacity of up to 3.5 times the installation load. It is the lightest and smallest deep water anchor available on the market. A full VLA mooring system can be installed by one anchor handling vessel in a fraction of the time required for the installation of other anchored systems.

Permanent versus mobile

Mobile Drilling Units (MODUs) have different anchor requirements than permanently moored systems. The permanent STEVMANTA^® is designed to meet the strength and fatigue requirements of a project. The shank is replaced by 4 wires to ensure extreme low installation resistance and thus deep penetration. The anchor is usually not retrieved. Anchors for temporary moorings need to be more robust to withstand repetitious handling, installation and retrieval. The shank usually consists of chain instead of wire rope. For retrieval, pulling the anchors with a fraction of the installation load suffices. Once the chains have been connected again, the anchor is ready for re-installation.

The MODU advantage

The STEVMANTA^® is suitable for use on a rig and even allows the rigs to be tautleg moored. With a maximum of 100 mT winch pull required the STEVMANTA^® can be deployed from a rig’s existing mooring system and chain or wire inserts are no longer needed. The installation of the STEVMANTA^® is conventional and simple, as is its retrieval. The benefits are clear. These weight reductions directly benefit the rig’s payload capacity, extending its operational capacity to much deeper water.

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Eirini Papapanagiotou

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Peter Foxton

THE DEEP WATER ANCHOR
• Provides holding power of 3.5 times the pull-in load
• Vertical and omnidirectional loading
• Deploys like a conventional drag anchor
• Adjusts to soil characteristics
• Used on permanent systems as well as semi-permanent and mobile
• Light weight, easy to install from a single AHV

Features & specifications of the STEVMANTA®

The STEVMANTA^® was designed to allow uplift at the anchor point, which is required in semi-taut or taut leg mooring systems. The STEVMANTA^® installs as a normal drag embedment anchor but the patented angle adjuster system changes the anchor from installation mode to mooring mode, resulting in an instant 3,5 factor of increase in holding capacity. It is the most cost efficient, lightest and smallest deepwater anchor available in the market today and since its introduction in 1996 hundreds of permanent and mobile moorings have been successfully performed worldwide.

Vertical Load Anchor design

The STEVMANTA^® was designed to be installed in the same way as a drag anchor. A unique and patented system makes the anchor change from installation mode to vertical mode which results in an immediate increase of holding capacity of up to 3.5 times the installation load. It is the lightest and smallest deep water anchor available on the market. A full VLA mooring system can be installed by one anchor handling vessel in a fraction of the time required for the installation of other anchored systems.
Taut leg mooring

One way to achieve weight savings in a mooring configuration in (ultra) deepwater is to use a taut leg mooring. The mooring lines are much shorter than in a conventional catenary mooring, thus saving weight. In the taut leg mooring the unit’s movements are absorbed by the flexibility in the mooring line, typically provided for by light weight polyester ropes, so that even further weight savings can be achieved. As the anchor line arrives at a 45 degrees angle at the seabed, a Vertical Load Anchor (VLA) is required. Vryhof’s STEVMANTA^® has in the last decade been applied around the world, mooring both semi-permanent and mobile units.
Permanent versus mobile

Mobile Drilling Units (MODUs) have different anchor requirements than permanently moored systems. The permanent STEVMANTA^® is designed to meet the strength and fatigue requirements of a project. The shank is replaced by 4 wires to ensure extreme low installation resistance and thus deep penetration. The anchor is usually not retrieved. Anchors for temporary moorings need to be more robust to withstand repetitious handling, installation and retrieval. The shank usually consists of chain instead of wire rope. For retrieval, pulling the anchors with a fraction of the installation load suffices. Once the chains have been connected again, the anchor is ready for re-installation.
The MODU advantage

The STEVMANTA^® is suitable for use on a rig and even allows the rigs to be tautleg moored. With a maximum of 100 mT winch pull required the STEVMANTA^® can be deployed from a rig’s existing mooring system and chain or wire inserts are no longer needed. The installation of the STEVMANTA^® is conventional and simple, as is its retrieval. The benefits are clear. These weight reductions directly benefit the rig’s payload capacity, extending its operational capacity to much deeper water.

MODU applications
Due to the very high agreement between theory and practice the application of the STEVMANTA^® does not require extensive mooring design engineering or extensive soil data. Its installation does not require expensive rental equipment such as pilinghammers, ROVs, pumps and barges.
Installation methods

Over a decade of experience with STEVMANTA^® operations generated vast experience with regard to the ideal installation methods. They are all based on minimizing time and the number of AHVs required for installing a full spread.

Single line installation
The STEVMANTA^® is deployed from the AHV and tensioned to embed in the seabed. Once it reaches a pre-set load, a shear pin breaks to release the angle adjuster plate, adding length to the front wires whereby the STEVMANTA^® is set in vertical or mooring mode. Its holding power will have increased by a factor 3.5 of the pull-in load.

Double line installation
In projects where handling of polyester ropes dictates the use of a separate installation line, each line is deployed from a separate AHV. Both lines are connected to the angle adjuster and once the installation load is reached the installation line is disconnected for instance by means of a subsea connector. The AHV sails into the installation direction pulling the angle adjuster into mooring mode.
The economics of anchoring

In a cost comparison between the STEVMANTA^® and other deepwater anchoring alternatives, the STEVMANTA^® will prove to be extremely cost effective as a result of direct and indirect savings. The cost advantage provided by the STEVMANTA^® will be larger with the increase of the waterdepth and remoteness of the location. The vast field experience with the use of the STEVMANTA^® in deepwater projects makes it the safe choice.
Lease/purchase

Vryhof’s anchors can be supplied on a purchase or on a lease/ purchase basis. Within a strategic alliance with major mooring component manufacturers, Vryhof can design and supply complete mooring systems. Leasing a complete (pre-laid) mooring system could well be a consideration for your next project.
Pre-laid moorings

MODU operators can significantly reduce mob/demob costs by installing the mooring spread in advance of the rig’s arrival. The rig can hook-up in only a fraction of the time otherwise required to pay out a full spread. By relocating mooring spreads ahead of and independently from the rig’s movements, the operator can save expensive operational days within a drilling program to a percentage of the project costs that might well be very considerable.
Ultime Holding Capacity