NO20210938A1 - Mud/water soluble low friction paint - Google Patents

Description

Field of the Invention

The present invention relates to a low-friction paint used to minimize skirt friction during installation of a subsea structure, a method for installing the structure into the seabed with low penetration resistance, and a subsea structure provided with a layer of such paint.

Background

The offshore wind market is rapidly increasing and thereby a need for more costeffective subsea foundations to provide seabed support for bottom fixed or floating offshore wind turbines. Today’s foundation systems are a significant cost element of the offshore wind project development total cost.

Larger offshore wind turbines require higher static and dynamic load capacities than conventional subsea oil and gas foundation systems can cost-efficiently provide today, and the load capacity requirement for mooring is expected to further increase as the developers want larger wind turbines producing more power per unit, a trend that is expected to continue for many decades.

All gravity and/or suction installed subsea structures/foundations or parts of such structure foundations, such as jackets, monopiles, suction anchors, suction caissons, suction piles, suction buckets and suction cans are hereafter referred to as a subsea installed structure.

For shallow waters, bottom fixed structures such as monopiles and jacket structures using single, or several caissons together as a tripod are the preferred option. For deep water areas where floating offshore wind is the only viable solution, the softer soil conditions often make a single suction anchor the preferred type of foundation solution.

Today, subsea installed structures are used as seabed support for a wide range of foundation, anchoring and mooring applications. They are mostly designed as a simple caisson, as a suction anchor, which may also include some stiffener plates to prevent bucking. The top plate has hatches for expelling water during installation and for the connection of suction pumps. These pumps basically apply suction or under pressure to create a vacuum inside the suction chamber such that the outside hydrostatic pressure can create a net force that push and penetrate the subsea structure further into the soil to the target depth.

The following explanation of the geotechnical principles are described using a suction anchor as an example, but the principles and advantages of the invention would in principle be similar for any subsea suction or gravity-based structure.

The resistance against soil penetration for a suction anchor is a function of skirt tip bearing, skirt skin friction and the possible impact with boulders. The main component is most often the skirt skin friction which is basically the skirt surface area times the clays average undrained shear strength times a friction factor. This adhesion or friction factor, α, is often taken as 1/St, where St is the sensitivity of the clay. The friction factor may also be adjusted due to its over-consolidation ratio.

The sensitivity of clays is defined as the ratio of their undisturbed and remoulded strengths, and varies from about 1 for heavily over-consolidated clays to values of over 100 for the so-called extra sensitive or “quick” clays.

This friction factor or adhesion factor, α, can be reduced to lower the penetration resistance, by applying normal subsea paint to the inside and outside of the skirt surfaces. However, since the reduced friction of normal subsea paint is permanent, it will also reduce the suction anchors load capacity permanently.

The suction anchor design and geotechnical engineering pr today are to design for the required vertical, inclined or horizontal load capacity by sizing the caisson diameter and skirt depth, then estimate the penetration resistance and find the required suction to penetrate to target depth. The suction is then used to calculate the required skirt and top of suction anchor wall thickness to avoid buckling. The suction must also be lower than the allowable underpressure which is the pressure when large soil heave occurs inside the suction anchor.

To achieve the required load capacity and at the same time keep the suction (force/area) lower than both the buckling and soil heave limits, the diameter must hence be increased. However, this again increases the skirt area and the penetration resistance and limit the skirt depth. The soil strength normally increases with depth and hence the iteration process described about leads to large diameter suction anchors, with increased wall thickness, weight and increased cost as a result.

By having another option for lowering the suction, such as a low friction paint, the suction anchor can have smaller diameters, less wall thickness, and longer skirts, reaching depths with increased soil strength. This will reduce the overall material and fabrication cost of the subsea structure.

Even though the design principles may seem simple and the design process in mature, the geotechnical engineering of the suction anchor can be complex and very specific to the soil conditions where it shall be installed. There are often uncertainties and local variations with the soil strength parameters (remoulded shear strength and friction angle) even if extensive soil investigation has been done and both upper and lower bound soil parameters are used together with material/safety factors.

Once the suction anchor is installed, the soil surrounding the skirts starts to regain its original undrained strength after being remoulded during the installation. This is called the set-up effect, which is a time dependent increase in the soils shear strength. This will give the new friction factor or adhesion factor, α-out, which are used for load capacity calculations. The soil set up process normally takes 2 months for most type of clays.

In addition, there are sensitivity and uncertainty to dynamic/cyclic load capacity that will increase pore pressure and degrade, called cyclic degradation, the remoulded shear strength of the soil. The design load will hence be based on very low soil strength estimates and high safety factor will need to be applied.

This often leads to a very high theoretical design penetration resistance and a very low vertical and horizontal design load capacity when using conservative soil strength parameters and material/safety factors. High penetration resistance then requires a high design suction/suction construction (under pressure) to ensure penetration to target depth which again increase the size of the suction anchors size and wall thickness due to allowable suction and buckling effects. The size limitations also limit the load capacity as it is directly based on skirt friction area.

Summary of the Invention

It is an object of the present invention to significantly improve and optimize the subsea installed structure design, construction and performance by a new enhancing principle using a low friction degradable and soluble paint, film or coating. By applying a low friction paint/composition on the subsea structure skirts during penetration, the friction factor or adhesion factor, α, can be lowered much further than the clay characteristics and codes, standards and recommendations would else warrant.

The required design suction (vacuum) will be lower and hence there will be more margin before reaching the allowable suction limits, which are often the sizing limitation factor together with allowable suction and skirt buckling. This will also reduce the requirements to skirt wall thickness, ring stiffeners and suction system design pressure, which will reduce cost and weight.

The paint will then dissolve prior to loads being applied during operation and increase the friction back to as it would have been unpainted including the set-up effect. The subsea installed structure dimensions can hence also be significantly increased and give more load capacity if required.

There have been projects in the past where normal subsea paint is partly or fully used on subsea installed structures skirts to lower the penetration resistance to ensure that the subsea structures are penetrated to target depth. This would however typically be for mudmat based subsea protection structures and shallow gravity-based structures to improve the downwards load capacity, but the normal paint and lower friction is permanent, and the upwards and lateral load capacity is permanently reduced accordingly.

The scope of the invention is defined by the appended claims.

Brief Description of the Figures

The invention will now be described in detail with reference to the appended drawings, wherein

Fig. 1 is an illustration of subsea structure or suction anchor during installation.

Fig. 2 is an illustration of a subsea structure or suction anchor during operation.

Fig. 3 is a graph illustrating the soil skirt friction versus a timeline.

Detailed Description of the Invention

The present invention relates to an application of a soluble paint, coating or film composition to the inside and outside of a subsea structure skirt prior to installation to reduce the penetration resistance by lowering the friction between the skirt and the soil during installation. The paint, coating or film will then dissolve, and the friction will increase to similar or above as if it was unpainted prior to operation to give the subsea foundation structure increased load capacity for vertical, inclined, and horizontal static or dynamic loads. The same principles are used to reduce weight and dimensions, if load capacity increase is not required.

Fig. 1 shows a subsea structure, in this case a suction anchor (1), which is being installed into the seabed (2). The suction anchor (1) includes a cylindrical skirt (3) which is open in the lower end (4), the upper end being closed by a cap with hatches (5). Prior to the installation, the skirt or parts of the skirt (3) has been painted with a soluble paint, both on the inside and the outside thereof. During installation, the suction anchor is lowered to the seabed. When engaging the seabed, the hatches are kept open to expel water inside the suction anchor, the skirt will first penetrate the sediments (2) due to the self-weight of the anchor until equilibrium is met. The hatches (5) are then closed by ROV and may also be connected to an evacuation hose creating suction inside the anchor. Suction is then applied using pumps and the suction anchor is penetrated to final target depth.

When fully installed into the seabed, Fig. 2, the pump equipment removed and the pump opening on the hatches (5) are closed. After some time, the paint on the skirt (3) will by a reliable degradation mechanism dissolve or react with the surrounding sediment material (2) and the sediments will adhere to the bare metal of the anchor skirt to increase the friction.

Fig. 3 illustrate the friction used for penetration resistance and load capacity calculations versus timeline and for unpainted skirts, skirts painted with normal paint and painted with soluble paint. It shows how the soluble paint has low friction during installation and how it regains its load capacity friction assuming the dissolving degradation mechanism is linear versus time. Other degradation functions are also possible, such as exponential, however, the results should be the same after a few months.

To further improve the holding power of the installed anchor, the surface of the skirt may be roughened prior to the application of the paint, e.g. by sandblasting, etching or patterning. When the paint is applied it will cover the roughened surface forming a slick outer layer, which is dissolved later and exposing the roughened steel.

The technical and cost reducing benefits of this application and in particular the dissolving of the paint is reduction of required suction during installation, which reduce required wall thickness and welding of caisson skirt and top plate, avoid buckling and ring stiffeners.

In addition, the skirt dimensions can be maximized which will increase drained and undrained pull-out capacity, horizontal, inclined, and vertical load capacity. More capacity per unit will also reduce the number of installations which reduce cost.

There is a wide range of applications for this invention, but it offers most benefits as load capacity improvement of conventional suction anchors or plate anchors in soft soil (very soft to firm clay). It can be used for bottom fixed offshore wind foundation such as suction buckets, monopiles, tripods and jackets. For floating wind, it can be used for anchoring and mooring of tension leg platforms (TLP), semi submersibles and SPARs.

Other applications are for foundation structures for subsea production systems and to improve other anchoring and mooring types. Theoretically this could also be used onshore to improve onshore construction piling and sheet piling.

The invention can also be retrofitted to existing suction anchors and reapplied multiple times to suction anchors that are used for temporary mooring.

The paint composition will need to have a special chemistry as this invention will need to have a relatively fast-dissolving paint property. This is the opposite to most paint compositions that are made to not degrade or dissolve. The paint binder, which is what holds the paint together and stick to the surface, could be polymer based, such as vinyl resin, polymerible monomer and PVA (polyvinyl alcohol), with a particle structure that enable rapid dissolution even in low temperatures and with limited oxygen present.

In general coatings such as paints may be removed by affecting its adhesion properties to the substrate onto which it has been applied, its cohesion properties internally in the coating/paint layer or both.

In the present method it is preferred that the cohesion properties of the coating/paint layer are affected when the coating/paint is to be removed. The reason for this preference is that the coating/layer should not coat the subsea structure permanently but should be removed after the structure has been installed for reestablishing the friction between the soil and the structure in a predictable manner.

A quantitative measure for the cohesive properties of a polymer is the cohesive energy, Ecoh. The cohesive energy per unit volume is called cohesive energy density (ecoh) and the square root of the cohesive energy density the solubility or Hildebrand parameter, δh. This parameter is frequently used in the coating industry to aid the selection of solvents and to predict the compatibility of polymers.

Solubility parameters are also used to predict the chemical resistance, the permeation rate, and the mechanical properties of polymers, to name only a few applications.

The cohesive energy (Ecoh) of a substance in a condensed state is defined as the increase in internal energy (U) per mole of substance if all intermolecular forces are eliminated. The correlations to the other two quantities are

ecoh (J/cm<3 >= MPa) = Ecoh/Vm

δh (MPa<1/2>) = (Ecoh/Vm)<1/2 >= ecoh<1/2>

In the case of low molecular weight compounds, the cohesive energy, Ecoh, is the energy required to evaporate the material:

Ecoh ≈ ΔHvap – RT

whereas the cohesive energy of a polymer can only indirectly be measured because a high molecular compound usually decomposes before it evaporates. For this reason, cohesive energies are often calculated with group contribution methods.

The Hildebrand parameters of some common polymers are shown below.

Hildebrand parameter of some polymers

The cohesive energy and the related parameters are of great importance in polymer physics, because many thermophysical and mechanical properties are directly related to the cohesive energy.

The table infra depicts the glass transition temperature and cohesive energy in some polymer compounds. In the absence of strong specific intermolecular interactions and side groups, the normalized cohesive energy is directly proportional to the glass transition temperature.

Glass transition temperature and cohesive energy

Hydro-biodegradable plastics, made from plant sources (such as cellulose) can also be used and degraded by hydrolysis.

For increased performance, additives to lower friction can be included in the paint chemistry.

By “paint” in the present context it is meant a layer of a composition not necessarily including any pigment, since the “paint” is not to give any appearancemodifying properties to the substrate, but rather a temporary and mechanically modifying property to the substrate. The “paint” may be based on different types of solidifying action. Examples of such solidifying properties are the creation of a paint matrix through polymerization of two or more of the constituents of the “paint”, through evaporation or diffusion of constituents of the “paint” leaving the temporarily non-soluble constituents behind in the “paint” matrix, or combinations thereof.

The “paint” of the present invention may comprise elements for liquefying the solidified paint after a selected or fixed time interval. This may be done by different modes of affecting the “paint” layer.

In one embodiment the “paint” may be self-degrading by a time-based chemical reaction within the paint layer itself for a rate of dissolution from when applied. One possible embodiment of such an action would be to include an enzyme into the “paint” when the “paint” layer is applied to the substrate. For example if the “paint” includes starch as the solidifying component, adding an enzyme such as maltogenic α-amylase from Bacillus stearothermophilus, α-amylase from Pseudomonas saccharophila or β-amylase from Clostridium thermosylfurogenes or combinations thereof will cleave chemical bonds within the starch molecule and soften the “paint” after a predictable period of time.

If, alternatively or additionally the matrix of the “paint” comprises cellulose or cellulose derivatives, such cellulose may be liquefied by adding enzymes such as cellulases, e.g. endo-1,4-β-D-glucanase, exo-1,4-β-D-glucanase and/or βglucosidase to the “paint”.

As an alternative to this mechanism, the “paint” may be degraded as initiated by a chemical reaction starting when applying a second layer of a two-component solution. As in the examples supra, the relevant enzyme solution may be added to the “paint” layer as a second layer at a predetermined time for softening the starch or cellulose component in the “paint”.

An alternative degradation method could be the use of microorganisms that are added to the paint composition to dissolve the paint after a set time interval.

In an alternative embodiment degradation of the “paint” may be initiated by contact with seawater (salt water) or seabed soil (clays, silt, sand).

In this embodiment the salinity of the “paint” will affect the internal cohesion of the “paint” matrix.

If a “paint” is made and applied to the substrate under conditions of low salinity, introducing such a “paint” to conditions of high salinity may affect the cohesion of the matrix forming the “paint” or vice versa.

Since a subsea structure or suction anchor includes an inverted bowl forming the suction component of the suction anchor, and this may be considered as a closed environment once the suction anchor is placed on the bottom of the relevant location, it will be possible to adjust the salinity of the interior of the suction bowl by introducing either high salinity or low salinity compositions into the interior of the suction bowl.

The interior of the suction bowl may in certain embodiments be considered as a closed system once the suction bowl has been placed in contact with the bottom layer. Suction bowl construction details of the suction anchor may comprise at least one conduit for removing fluid from the interior of the suction bowl for creating a reduced pressure or vacuum inside the suction bowl. This reduced pressure or vacuum will drive the suction anchor into the bottom soil beneath the suction anchor. If a friction-reducing paint or composition according to the present invention is applied to the interior and/or exterior of the suction bowl of the suction anchor, the at least internal paint coating may in certain embodiments be removed by adjusting the composition of the fluid trapped internally under the suction anchor dome or suction anchor bowl for liquefying the paint as indicated supra. Such liquefied remains of the paint and possible added reagent composition may in one embodiment be removed through the suction conduit and collected to be removed or reused. From an environmental consideration, when the remains of the paint and possible added reagents are removed, such materials may be deposited, destroyed in a safe fashion or be separated into their constituents to be re-used if desired. Alternatively, the suction bowl or dome may include at least one separate conduit for introducing the relevant paint-removing composition into the interior of the bowl or dome.

Another way to remove applied paint or composition from the walls of the bowl of the suction anchor after it has been installed may be done by degradation of the paint initiated by soil pressure, pore pressure or erosion.

When a subsea structure has been placed in its permanent or semi-permanent location, the paint on its external and possibly its internal surfaces will be subject to mechanical degradation through the soil particles eroding away the paint layer. This erosion is a naturally occurring phenomenon and will remove the applied layer of paint after prolonged exposure of the skirts to natural forces such as fluid currents, wave exposure, debris carried by under-water currents, thermal currents, etc. Sub-surface silt may not be a solid substance but may carry fine particles that will erode away the paint layer on the skirts over a period of time. This will enhance the holding capacity of the subsea structure since the friction-reducing paint then will be slowly removed from the surfaces of the skirts.

Another way to remove the paint or change its properties could be through degradation initiated by change in pH values of the water (seawater). Again, the internal environment beneath the bowl of the suction anchor could be considered as a closed environment. This being the case, it is possible to effectuate the introduction of an acid, a base or a buffer into the interior of the suction anchor bowl. This will obviously affect the internal paint layer of the suction anchor, and may increase the friction properties between the mud and the internal surface of the suction anchor bowl, thus increasing the hold-fast capacity of the subsea structure by removing the internal paint layer of the suction anchor bowl.

Another and alternative or additional way to remove the applied “paint” layer of the dome or bowl of the suction anchor could be degradation initiated and dissolved by applying electricity or current to the suction anchor. In this embodiment the “paint” composition could include a component being sensitive to electrical variations in the substrate onto which it has been applied. Alternatively, the cohesion of the “paint” material/”paint” polymers could be affected by an applied electrical current in the metal of the subsea structure.

The paint reacts with the steel surface to give an accelerated corrosion which increase the friction and surface roughness

• The paint contains particles that react with the steel surface to form a rough surface (either time dependent or by using current)

One consideration to consider when producing the “paint” according to the invention, is their adherence to the relevant substrate when applied. Since subsea structures include a metal or metal alloy this will be the substrate material onto which the “paint” should adhere. Of course, the state of the surface of the relevant metal or metal alloy also plays a role, meaning that a smooth and polished surface will provide a poorer adherence of the “paint” than a more rough surface condition. Both for leaving a better anchoring effect of the suction anchor after the “paint” has been removed, and also for providing a better adherence to the surface when applying the “paint”, it is preferred that the substrate condition is rough. With “rough” is meant a surface condition with protrusions or granules ranging from 0 up to 0,1 mm from the bottom or base surface.

The paint will typically also have the following functional specification:

• The paint shall meet applicable requirements, codes and standards for health, safety and environment

• The paint shall be nontoxic, environmentally friendly and free from any substance that could cause pollution in a marine environment

• Be quickly and efficiently applied to sand blasted steel surface with high roughness • Withstand storage for a few months in normal outdoor close to sea conditions • Withstand one week of rain with occasional splash from seawater without losing the low friction properties

• Be applied on rough steel surface in normal outdoor temperatures

• Have sufficient pull-off adhesion strength to withstand maximum suction and shear stress from of soil (clay, silty clay, sandy clay) during installation

The paint shall perform as follows and in accordance with principles shown in Fig. 3.

• Have reliable soil to paint friction at approximately 0.1 or typically 1/3 of the unpainted friction

• Start to dissolve prior to or once submerged subsea in clay, silty clay, sandy clay or mud, if necessary, an activation agent or catalyst may be added to the paint before submerging subsea

• Dissolve completely within 2 - 8 weeks

Give soil to steel friction back to at least as if the paint were not applied

The paint shall dissolve by a reliable degradation mechanism, some alternatives are listed below. It could also be a combination of several such mechanisms:

Self-degrade by a time-based chemical reaction within the paint layer itself for a rate of dissolution from when applied

Self-degrade initiated by a chemical reaction starting when applying the second layer of a two-component solution

Degradation by using microorganisms

Degradation initiated by contact with seawater (salt water) or seabed soil (clays, silt, sand)

Degradation initiated by soil pressure, pore pressure or erosion

Degradation initiated by change in pH values of the water (seawater) Degradation initiated and dissolved by applying electricity or current to the suction anchor

The paint reacts with the steel surface to give an accelerated corrosion which increase the friction and surface roughness

The paint contains particles that react with the steel surface to form a rough surface (either time dependent or by using current)

The dissolve or release mechanism should be such that the dissolved paint to soil friction is the either equal or even enhanced compared to bare steel soil.

Claims (12)

Claims

1. A low-friction paint, coating or film composition that are used to provide lower friction on any subsea installed structure skirt during soil penetration, characterized in that the paint, coat or film is adapted to chemically dissolve fast to increase the friction back as it would have been unpainted prior to operation and applied static and dynamic horizontal, vertical, and inclined loading.

2. A low-friction paint according to claim 1, that by also being soluble, can reduce the required dimension and/or wall thickness of subsea installed structure for the same load capacity as unpainted.

3. A low-friction paint according to claim 1, that by also being soluble, enable installation of larger subsea installed structure dimensions to increase the load capacity compared to unpainted.

4. A low-friction paint, coat or film according to claim 1, wherein the low friction paint binder is a water soluble polymer selected from the group including vinyl resin, polymerible monomer or a polyvinyl alcohol.

5. A low-friction paint, coat or film according to claim 1, wherein the paint includes a hydro-biodegradable plastic, such as starch or cellulose.

6. A low-friction paint, coat or film according to claim 5, further including an enzyme or microorganisms, or an additional layer containing an enzyme or microorganisms.

7. A low-friction soluble paint, coat or film according to any of the previous claims where degradation could be initiated by and the paint cohesion be affected by applying electricity or current to the subsea structure.

8. A method for installing a subsea structure into a seabed, including applying a layer on the subsea structure parts of a low-friction paint, coat or film according to any of the previous claims,

installing the gravity or suction based subsea structure into the seabed, and let the layer of low-friction paint, coat or film dissolve.

9. A method according to claim 8, including the steps of applying a first layer containing a hydro-biodegradable plastic, such as starch or cellulose, and then applying an additional layer containing an enzyme or microorganisms onto said first layer.

10. A method according to claim 8, including the step of roughening the surface of the suction anchor before the layer of low friction paint is applied.

11. A method according to claim 10, wherein the surface of the subsea structure is roughened by etching, sandblasting or patterning.

12. A subsea structure installed by gravity or suction which is provided with a layer of a low friction soluble paint according to any of the claims 1, 4, 5, 6 and 7 on the inside and outside of the skirt.